3.4 Basic Circuit Diagram of the ZVS-PWM Three Phase Current-Fed ..... Fuel cells have a slow response due to the natural electrochemical reactions required ...
A project report on
A ZVS-PWM THREE-PHASE CURRENT-FED PUSH–PULL DC–DC CONVERTER WITH FUEL CELL INPUT Submitted in partial fulfillment of the requirement for the award of the degree of
Master of Technology in
ELECTRICAL AND ELECTRONICS ENGINEERING (POWER ELECTRONICS) Submitted by
Bandi Mallikarjuna Reddy (12A81D4301) Under the esteemed guidance of
Sri Ch. Narendra kumar, M.Tech, (Ph. D), MISTE Senior Assistant Professor
Department of Electrical and Electronics Engineering
SRI VASAVI ENGINEERING COLLEGE (Affiliated to Jawaharlal Nehru Technological University, Kakinada, A.P) Pedatadepalli, Tadepalligudem, West Godavari District, Andhra Pradesh, Pin-Code: 534101, INDIA.
SRI VASAVI ENGINEERING COLLEGE (Affiliated to Jawaharlal Nehru Technological University, Kakinada, A.P) Pedatadepalli, Tadepalligudem, West Godavari District, Andhra Pradesh, Pin-Code: 534101, INDIA.
Department of Electrical and Electronics Engineering CERTIFICATE This is to certify that the project report entitled “A ZVS-PWM THREE-PHASE CURRENT-FED PUSH–PULL DC–DC CONVERTER WITH FUEL CELL INPUT” is a bonafide work done by Bandi Mallikarjuna Reddy (12A81D4301) submitted in partial fulfillment of the requirement for the award of the degree of master of technology in electrical engineering with specialization in power electronics during 2012-2014.the results of investigation enclosed in this report have been verified and found satisfactory. The results embodied in this project report have not been submitted to any university or institute for the award of any other degree or diploma.
PROJECT GUIDE
HEAD OF THE DEPARTMENT
Ch.Narendra Kumar
Ch. Rambabu
Sr.Assistant professor
Professor
EXTERNAL EXAMINER
DECLARATION
I, Bandi Mallikarjuna Reddy (12A81D4301) hereby declare that the project report title “A ZVS-PWM THREE-PHASE CURRENT-FED PUSH–PULL DC–DC CONVERTER WITH FUEL CELL INPUT “is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which has been accepted for the award of any other degree or diploma of the university or other institute of higher learning, except where due acknowledgment has been made in the text.
Place: Sri Vasavi engineering college
Signature:
Tadepalligudem
Name: Bandi. Mallikarjuna Reddy
Date:
Reg. No: 12A81D4301
ACKNOWLEDGEMENT First of all I would like to thanks the almighty, who has always guided me to work on the right path of the life. I owe a profound gratitude to my advisor, project guide, Sri Ch. NARENDRA KUMAR
M.Tech, (Ph. D), MISTE
Sr. Assistant Professor who has been a constant source of
inspiration to me throughout the period of this project. It was his competent guidance, constant encouragement, critical evaluation and editorial assistance that helped me to develop a new insight into my project. I express our sincere thanks to Sri Ch. RAMBABU,
M. Tech (Ph. D)
HOD of
electrical and electronics engineering department for his sincere co-operation and arranging necessary facilities for completing this project successfully. I am most indepted to Dr. J. SRIHARI RAO, B.E, Ph. D principal for providing me all the facilities to carry out this project.
Bandi Mallikarjuna Reddy (12A81D4301)
ABSTRACT A ZVS-PWM three-phase current-fed push–pull dc–dc converter is proposed. When compared to single-phase topologies, the three-phase dc–dc conversion increases the power density, uses the magnetic core of the transformer more efficiently, reduces the stress on switches, and requires smaller filters since the frequency for its design is higher. The proposed converter employs an active clamping technique by connecting the primary side of the transformer to a three-phase full bridge of switches and a clamping capacitor. This circuit allows the energy from the leakage inductances to be reused, increasing the efficiency of the converter. If appropriate parameters are chosen, soft-commutation of the switches (ZVS) can also be achieved. The use of this new rectifier improves the efficiency of the converter because only three diodes are responsible for the conduction losses in the secondary side. The current in the secondary side of the transformer is half the output current. In addition to this, all the advantages of the three-phase dc/dc converter, i.e., the increased frequency of the output and input currents, the improved distribution of the losses, as well as the soft commutation for a wide load range are preserved. The ZVS-PWM three phase current-fed push-pull DC-DC converters with fuel cell input is simulated by the MATLAB/simulink software for the pre-estimation and performance of the circuit for the customer requirement. This project will give the good efficiency than the basic three phase dc-dc converter and give less conduction losses as well as the less effect of the electromagnetic effect. The output voltage and current is enhances the efficiency to the DC-DC converter. A new family of dc-to-dc converters featuring clamping action, PWM modulation and soft-switching (ZVS) in both active and passive switches is proposed to overcome the limitations of clamped mode dc-to-dc converters. The new family of converters is generated and the new circuits are presented. As the resonant circuits absorb all parasitic reactances, including transistor output capacitance and diode junction capacitance, these converters are suitable for high-frequency operation. Besides operating at constant frequency and with reduced commutation losses there is no significant increasing on circulating reactive energy that would cause large conduction losses.
i
CONTENTS Page No Abstract
i
List of figures
v
List of Tables
vii
Nomenclature
viii
List of publications
x
1
Introduction
1
1.1 Introduction
2
1.2 Resonant Converter Advantages
3
1.2.1 Advantages and Disadvantages of the Resonant
4
Converter
2
1.3 Concept of Resonant Converters
5
1.4 Classifying Resonant Converters
6
1.4.1 Series or Parallel Loading
6
1.4.2 Fixed or Variable Frequency
7
1.4.3 Discontinuous Resonance
8
1.4.4 Zero-Current or Zero-Voltage Switching
9
1.4.5 Half or Full Cycle Conduction
10
1.5 Resonant Converter Basic Operation
11
1.6 Transformer Coupling
12
1.7 Alternate Resonant Operating Modes
12
1.8 The Series Resonant Converter
13
1.9 Difference between the ZVS and ZCS
15
1.10 Applications of the zero voltage switch (ZVS) technique
16
1.11 Description of the distributed power generation
16
1.12 Transportation System Application
16
1.13 Organization of Thesis
17
Literature Review
18
ii
3
ZVS-PWM DC-DC Converter Topologies and Methodology
32
3.1 Introduction
33
3.2 Power Electronics Devices
34
3.2.1 Power Converters
34
3.3 Power Electronic Switches
35
3.3.1 Thyristor
36
3.3.2 Insulated Gate Bipolar Transistor (IGBT)
37
3.4 Proposed ZVS-PWM Three-Phase Dc–Dc Converter
38
3.4.1 Overview
38
3.4.2 History
38
3.4.3 Applications
39
3.5 Circuit Analysis
40
3.5.1 Circuit Description
40
3.5.2 Operating Region R3
41
3.6 Voltage gain
43
3.6.1 Clamping Voltage and Duty Cycle
45
3.6.2 Turns Ratio of the Transformer, Effective Duty
45
Cycle and Commutation Inductance 3.6.3 Boost Inductance
45
3.7 Advanced Active Clamping (AAC) topologies
4
45
3.7.1 Advanced Active Clamping (AAC) Concept
45
3.7.2 Effectiveness of AAC in 3-level Topologies
46
3.7.3 DC-DC Converter with Asymmetrical Duty Cycle
46
3.8 DC-DC Converters For Distributed Power Generation
48
Simulation Diagrams and Simulation Results
50
4.1 Introduction
51
4.2 Simulation diagrams of proposed a ZVS-PWM three phase
52
current-fed push-pull technology 4.3 Simulation Diagram of a ZVS-PWM Three Phase Current-
56
Fed Push-Pull Dc-Dc Converter with Fuel Cell Input 4.4 Simulation Results Comparison iii
63
5
Conclusion & Future Scope
64
5.1 Conclusion
65
5.2 Future scope
65
Reference
66
Appendix
68
iv
LIST OF FIGURES Fig.
Page no
1.1
Power System Supply Frequencies
3
1.2
PWM Vs. Resonant Switching
4
1.3
Switching Stress and Switching Loss
4
1.4
Resonant Switches
5
1.5
Resonant Mode Loading
6
1.6
Fixed Frequency Resonance
7
1.7
Variable Frequency Continuous Resonance
8
1.8
Quasi-Resonant Control
9
1.9
Switch Activation
10
1.10
Switching Techniques Compared
10
1.11
Half and Full Cycle Wave Forms With Unloaded Resonant Switch
11
1.12
Transformer Coupling
12
1.13
Series-Loaded, Half-Bridge Converter
14
3.1
Symbol of Thyrister
36
3.2
Structure on the Physical and Electronic Level, and the Thyristor
36
Symbol 3.3
Symbol and Circuit Diagram Of IGBT
37
3.4
Basic Circuit Diagram of the ZVS-PWM Three Phase Current-Fed
40
Technology 3.5
Operating Principle in Region R1,R2,R3
42
3.6
Wave Forms of the Operating Region R3
44
3.7
Three-Phase LCC Resonant DC-DC Converter
47
3.8
ZVS DC-DC Three-Phase Converter
47
3.9
Typical Structure of the Interface Converter for Residential FC-
48
Powered Systems 3.10
FC Power Systems
49
4.1
A ZVS-PWM Three Phase Current-Fed Push-Pull DC-DC Converter
52
4.2
Output Voltage
53
v
4.3
Input Current
54
4.4
Voltages across The Lp1, Lp2 And D1
54
4.5
Currents through Ls1, Ls2 And Ls3
55
4.6
Gate Signals
56
4.7
A ZVS-PWM Three Phase Current-Fed Push-Pull DC-DC Converter
57
with Fuel Cell Input 4.8
Input Voltage
58
4.9
Input Current
58
4.10
Output Voltage
59
4.11
Current through Ls1, Ls2 And Ls3
59
4.12
Voltage and Current Through Ls1
60
4.13
Current through Ls1, Ls2 And Ls3
60
4.14
Voltage and Current Through Ls3
61
4.15
Gating Signals to the Converter
61
4.16
Current and Voltages Comparison In Ls1
62
4.17
Saw-Tooth Signals of PWM Generator
62
4.18
Current through Lp1
63
vi
LIST OF TABLES Table
Page No
3.1
Operating Regions
41
3.2
Time Intervals of Operation
42
3.3
Main Voltages During Each Topological Stage
43
4.1
Simulation Results Comparison
63
vii
NOMENCLATURE AAC
Advanced Active Clamping
AC
Alternating Current
DAB
Dual Active Bridge
DC
Direct Current
DG
Distributed Generations
EMC
Electromagnetic Compatibility
EMI
Electromagnetic Interference
FC
Fuel Cell
HEV
Hybrid Electric Vehicles
HF
High Frequency
HVDC
High Voltage Direct Current
ID
Diode Current
IEEE
Institute of Electrical & Electronics Engineering
II
Input Current
IL
Load Current
IPR
Inter Phase Reactor
ITS
Information Technologies
KW
Kilo Watt
MOSFET
Metal Oxide Semiconductor Field Effect Transistor
PSM
Phase Shift Modulation
PSU
Public Sector Union
PV
Photo Voltaic
PWM
Pulse Width Modulation
RMS
Root Mean Square
SDCS
Separate Dc Source
SMPS
Switched Mode Power Supply
SVC
Static Var Compensator
TVS
Transient Voltage Suppressors
UPS
Uninterrupted Power Supply
viii
VDC
Dc Voltage
VI
Input Voltage
VO
Output Voltage
VS
Source Voltage
ZL
Load Impedance
ZS
Source Impedance
ZCS
Zero Current Switch
ZVS
Zero Voltage Switch
ix
LIST OF PUBLICATIONS 1. Bandi Mallikarjuna Reddy, Ch. Narendra Kumar and Ch. Rambabu, “A ZVS-PWM THREE-PHASE CURRENT-FEDPUSH–PULL DC–DC CONVERTER WITH FUEL CELL INPUT” International Journal Of Science, Engineering and Technological Research(IJSETR), vol. 3, pp 1-6, December-2014.
x
Chapter-1 INTRODUCTION
1
Chapter-1 INTRODUCTION 1.1 Introduction Over the years we have seen power conditioning move from simple but extravagant linear regulators, through early low frequency pulse-width modulated systems, to high frequency square wave converters which pack the same power handling capabilities of earlier designs into a fraction of their size and weight. Today, a new approach is upon us the resonant mode converter and while offering new benefits in performance, size, and cost, this new technology brings with it an added dimension of complexity. The purpose of this project is to offer a means of categorizing and defining the various topologies and operating modes of resonant mode converters with the hope of enhancing the capability for design, analysis, and evaluation of these new power systems. 5-20 kHz
-audible noise -slow bipolar switches -large L’s and C’s
20-100 kHz
-above audible range -fast bipolar range -magnetic become important -small sizes
100-500 kHz
-power MOSFET switches -losses in L’s and C’s -diode recovery time -RFI and EMI -packing problems
A resonant converter as a power conditioning system which utilizes a resonant LC circuit as a part of the power conversion process. All resonant converters operate in essentially the same way a square pulse of voltage or current is generated by the power switches and this is applied to a resonant circuit. Energy circulates in the resonant circuit and some or all of it is then tapped off to supply the output. While basically simple, this principle can be applied in a wide variety of ways, creating a bewildering array of 2
possible circuits and operating modes. Resonant Converter Advantages before getting into these, however, let's pause to review why we are even interested in resonant mode power conversion. With the earliest switched-mode power converters, it became obvious that higher frequencies allow smaller L's and C's and this, in turn, should lead to smaller, lighter, and (hopefully) less costly systems.
1.2 Resonant Converter Advantages The down side to moving to higher frequencies, however, is the problems of greater susceptibility to parasitic capacitance and leakage inductance, greater stress in the switching devices, and increased EMI and RFI. [4]
Fig. 1: Basic Resonant Converter A resonant mode system offers the potential of achieving the benefits while sidestepping many of the disadvantages of higher frequencies. With a resonant circuit in the power path, the switches can be configured to operate at either zero current or voltage points in the waveform, greatly reducing their stress levels; the resonant sine wave minimizes higher frequency harmonics reducing noise levels; and since the circuit now requires inductance and capacitance, parasitic elements may enhance rather than detract from circuit performance. With these benefits, power systems operating in the range of 3
500 kHz to 2.0 MHz are now practical and in fact are already being produced by a few leading edge manufacturers. 1.2.1 Advantages and Disadvantages of the Resonant Converter Advantages: The advantages of the resonant converters are 1. Zero current switching 2. Low component stress 3. Low EMI 4. Useful parasitic elements 5. Improved diode recovery Disadvantages: The disadvantages of the resonant converters are 1. Greater complexity 2. Higher peak currents 3. New technology learning curve
Fig. 2: PWM vs. Resonant Switching
Fig. 3: Switching Stress and Switching Loss 4
1.3 Concept of Resonant Converters Before attempting to classify resonant converter topologies, it might be helpful to introduce the concept of Resonant Switches. [5] A resonant switch consists of a switching device (a transistor with a steering diode, for example) in combination with a twoelement resonant circuit. This resonant switch may be configured in several different ways, some of which are shown in Fig. 4, but they always perform the same function as the conventional switch in a square wave converter. It is a useful concept as most resonant mode circuit topologies can be visualized as a conventional PWM circuit with the power switch replaced with a resonant switch. We will discuss the operation of the various switch configurations in greater detail as we get into the circuit topologies but first, let's take an overview of some of the circuit options. To bring some order and ease in understanding the broad range of circuit choices which are possible as we move to resonant mode operation, it helps to establish a system to classify resonant topologies by defining the following operating characteristics.
Fig. 4: Resonant Switches 1. Is the load in series or in parallel with the resonant circuit elements? 2. is the control system a fixed or variable frequency type? 5
3. Does current (or voltage) in the resonant circuit flow continuously or is it equal to zero for some portion of the switching cycle? For discontinuous operation, it is also important to know: 4. Is the switching designed for zero current or zero voltage activation, and 5. Does the energy in the resonant circuit flow in only one direction or is there a full cycle before it returns to stop at zero?
1.4 Classifying Resonant Converters The general properties of any resonant converter are completely dependent on these options, so they are a good basis to use as a starting point in understanding the principles involved. 1.4.1 Series or Parallel Loading Since resonant converters operate by putting energy into a resonant circuit and then transferring some or all of it into the load, we need to know that there are two ways this may be Accomplished as shown in Figure 8. If the load is in series with the resonant circuit elements, as in Fig. 5A, we call it a series loaded converter and the operating characteristics tend toward a current source with a high impedance Output. Parallel loading is the opposite, with a low impedance voltage source output as shown in Fig. 5B. Both modes have application to power systems with high voltage outputs usually using series loaded current source drive and low voltage supplies using parallel loading.
Fig. 5: Resonant Mode Loading 6
1.4.2 Fixed or Variable Frequency Resonant converters may be configured for either constant or variable frequency operation, but these choices infer significant differences in their operation. Fixed frequency control systems Use conventional pulse width modulation to change the output in response to a control input, as shown in Fig. 7. This forces a fixed-frequency system to have at least one non-zero switching transition and possibly two, thereby voiding one of the more significant reasons for choosing to use a resonant mode topology. This would usually preclude its use unless system considerations required a synchronized frequency operation.
Fig. 6: Fixed Frequency Resonance Variable frequency operation, however, needs to be subdivided by the third classification: whether the resonant circuit current is continuous or discontinuous. A circuit operating in the continuous resonant mode uses the slope of the resonant circuit impedance curve to control the output. As shown in Fig. 8, the circuit can operate either above or below resonance but the principle is the same: that the control circuit changes the frequency to move either toward or away from resonance, and thereby controls the amount of energy which is transferred into the resonant circuit - and therefore to the load. While many practical systems have used continuous conduction, variable frequency operation, there are several disadvantages: 7
1. The non-zero switching adds stress to the transistors. 2. As the frequency approaches resonance, peak currents or voltages can get very high, adding stress to the resonant components. 3. The control transfer function is very nonlinear following the resonant impedance curve. The major advantage of the continuous mode of operation is that the frequency varies over a much smaller range than with the discontinuous mode.
Fig. 7: Variable Frequency Continuous Resonance 1.4.3 Discontinuous Resonance The discontinuous operating mode works by supplying constant packets of energy to the load with the rate, i.e. Frequency, determined by load power demand. Perhaps the most popular and important Class of resonant converters with variable frequency and discontinuous current is often called Quasi-Resonance. [11] Most of the remaining portion of this discussion will be oriented toward this Quasi- Resonant converter category but even within this class there are still many variations in circuit operation. Quasi-resonant circuit waveforms are not sinusoidal, but have two essentially linear portions interspersed with two sinusoidal portions.
8
A quasi-resonant converter control loop is usually configured as shown in Fig. 8, with a pulse generator driving the resonant circuit at a repetition rate defined by the control circuit. The pulse generator may be set for constant pulse width -defined by the resonant circuit -or set to sense zero crossing of either current or voltage. With maximum loading and low line voltage, a quasi-resonant converter can approach continuous resonance as a limit when the individual pulses run together. Within the variable frequency, discontinuous mode of operation there are two remaining decisions a designer must make which will have significant effect on the characteristics of his power supply:
Fig. 8: Quasi-Resonant Control 1.4.4 Zero-Current or Zero-Voltage Switching Since reducing the stress on the switching components is a major incentive for resonant operation, we need to understand ways in which that might be accomplished. The most common approach, and the one to which most of this paper will address, is to switch at zero current so that the dynamic load line stays very close to the V-I axes. With a sine wave current shape, it should be clear that the peak current will be close to twice the value of an equivalent square wave system, and although this adds to the I2R losses, most semiconductor devices are much more comfortable with this than the high peak power levels reached with square wave switching. Of course, low switching stress may also be achieved by switching at zero voltage and one should realize that this approach is merely a dual of zero current as shown in Fig. 9. 9
The choice of which approach is best is usually determined by whether the parasitic inductance of the load or the capacitance of the switch is the bigger problem. [6-8] Zero voltage switching is primarily appropriate with very high frequency operation where rapid charging and discharging of the semiconductor switch capacitance could represent substantial power loss. Note from the duality of characteristics shown in Fig. 10 that zero voltage switching, with its sine voltage waveform, would force high peak voltages on the switch and, although it loses control at light loads, is unaffected by a short circuit.
Fig. 9: Switch Activation
Fig. 10: Switching Techniques Compared 1.4.5 Half or Full Cycle Conduction In a quasi-resonant circuit, energy transmission begins and ends at zero followed by a wait defined by the needs of the load. Full or half cycle conduction relates to whether each pulse will allow current to flow only from source to load, or ring in the 10
resonant circuit allowing surplus energy to return to the source. The waveforms shown in Fig. 13 describe the operation which is controlled by the placement of diodes either in series or anti parallel with the switch. [19] Effective power supplies can be implemented with either approach but, as one would expect, there are significant differences in their characteristics.
Fig. 11: Half and Full Cycle Waveforms with Unloaded Resonant Switch While half cycle operation may be easier to implement, the pulse repetition rate is a direct function of the loading and, when coupled with input voltage variations, can result in huge swings in switching frequency. A full cycle configuration usually requires a diode in series with the switch as well as the anti parallel diode in order to prevent any reverse conduction through the slow switch body diode. Additionally, with current flowing in both directions, conduction losses tend to be greater. The advantage is that by returning surplus energy to the source, switching frequency is independent of load.
1.5 Resonant Converter Basic Operation After having defined the various classifications of resonant mode topologies, we will now describe the detailed operation of one such circuit. The circuit is a single-ended, buck-derived, parallel loaded, half-cycle, zero-current switching, quasi resonant converter. We will later extrapolate this operation to other circuit topologies. A resonant converter, like all switching regulators, requires an output filter to smooth the power delivered to the load. This output filter must have a break frequency less than one fifth the lowest switching frequencies. Therefore we assume the current
11
through Lo and the voltage across Co are both essentially constant at the switching frequency.
1.6 Transformer Coupling The introduction of a transformer into the -power path does not change anything from a topology standpoint but it adds some interesting and useful features. Fig. 12 shows some possible extensions of the simple buck regulator discussed above. In Fig. 12, the resonant circuit is in the primary and the transformer provides merely an impedance match to a parallel load. This approach has the advantage that the transformer passes only load current and therefore current sensing is easily done in the primary side.[17] Fig. 12 moves the resonating capacitor to the secondary with two benefits: the leakage inductance of the transformer is no longer a parasitic but adds directly to the resonating inductance, and with sine wave current flow, secondary diode switching is soft with less tendency for transient ringing. The Circuit of Fig. 12 moves the capacitor further to the other side of the rectifying diode and while transformer reset must now be accomplished by some other means, the lack of reverse current frees the primary side of the need for high-voltage, high-speed blocking diodes. In addition, the transformer now sees only the square voltage waveform from the primary switch and thus has to support fewer volts -seconds. The practical implementation of this circuit also builds enough leakage inductance into the transformer such that it becomes the total resonating inductance, eliminating a separate component.
Fig. 12: Transformer Coupling
1.7 Alternate Resonant Operating Modes Without going into the same depth that was used above in the description of the single ended, buck-derived circuit, we will now examine and compare the whole range of 12
operating modes for both series and parallel resonant converters. For this comparison, we will use a classic half-bridge topology as shown in Fig. 13. These circuits have been normalized with all waveforms drawn to the same scale and the following definitions apply. V.: the switched input voltage into the resonant circuit Ir: the resonant current in the inductor Vr: the voltage on the resonating capacitor Vd: the voltage at the transformer secondary Id: the current through each leg of the output diodes In all cases, Vo is assumed constant and the primary diodes defining full-cycle or halfcycle operation are not shown.
1.8 The Series Resonant Converter Figure 13; Variable-frequency, Half-cycle, Discontinuous Mode: In this mode, current is allowed to flow in only one direction through each switch as can be seen from the Ir waveform. This power stage has a constant power output characteristic where the output power is given by 1/2 Cr V2 Fs. [3] An increase in either the input voltage or the switching frequency will proportionately increase the power delivered to the load, irrespective of the load impedance. In the ideal case, the output voltage can rise almost infinitely and some method must be used to limit it under no load conditions.The slope of the output curve changes with the load impedance which affects the small signal gain of the power stage and makes it more difficult to include inside a feedback loop. The switching frequency is also dependent on both the input voltage and the load current and so may have a very wide switching frequency range. This circuit and its derivatives are among the most simple and least costly to produce converter circuits available but the resultant dynamic performance has been traded for this low cost. Fig. 13; Variable-frequency, Full-cycle, Discontinuous Mode: As the Ir waveform shows, in this mode the switches carry current in both directions which gives the circuit a constant current output characteristic. The low frequency model is simply a voltage controlled current source feeding the output capacitor. The switching frequency variation in this converter mode is directly related to the output current. If a wide output current range is needed, the switching frequency range will also be wide. This will restrict the 13
control loop bandwidth when used for a constant voltage output. This circuit is most commonly used for high voltage outputs because peak voltage on the secondary is simply equal to the output voltage. An interesting aspect of this converter is the way the resonant current waveform changes slope abruptly on each half cycle as the current crosses through zero.
Fig. 13: Series-Loaded, Half-Bridge Converter The output voltage is reflected into the resonant circuit through the diode bridge and when the current reverses, the reflected output voltage changes sign. This lowers the effective voltage across the resonant circuit and hence the resonant current is lower. Note that although the current through the switches and diodes is sinusoidal, the voltage waveforms are square. This behavior is characteristic of all series resonant converters. Fig. 13, Variable-frequency, Continuous, and Below Resonance: Notice that in this mode the resonant voltage and current are both partial sine waves. Because the current in the inductor is continuous, each switch transition must force commutate the anti parallel diode of the opposite switch. Also, as noted before, the diode voltage is still square although the current is sinusoidal. Phase shift is occurring between the switch voltage and the resonant current. [5]The transfer function of the power stage is non linear since its gain is dependent on the impedance slope of the resonant circuit. This makes this circuit somewhat difficult to stabilize but reasonably wide control bandwidths can be maintained as long as the output load is able to keep the circuit in continuous resonance. If the load goes open the switching frequency will go to zero.
14
Fig. 13; Variable-frequency, Continuous, Above Resonance: As was noted in the preceding paragraph, below resonance each switch must force the opposite anti parallel diode off and carry the current which was flowing through it. Above resonance the switch turns on naturally at zero current because its anti parallel diode conducts first, but the switch must turn off with current through it. The anti parallel diode of the opposite switch will conduct immediately and is then naturally commutated by the resonant circuit in its turn. Above resonance, the resonant current resembles a Saw-tooth wave more than a sinusoid even though it is made up of sinusoidal sections. The capacitor voltage is the integral of the current and it more closely resembles a sine wave. The frequency range of this mode of operation is generally low and it operates on the slope of the resonant circuit impedance curve the same way it does below resonance. A required limit on minimum switching frequency allows the control loop bandwidth to be wide although the transfer function of the circuit in this mode is still very nonlinear .Also note that with a large load current variation, the switching frequency range will be wide and if the load goes to an open circuit the switching frequency will go to Infinity. Fig. 13; Fixed-frequency, Continuous, At Resonance: The waveforms in this mode are similar to the ones of variable frequency continuous resonance mode above resonance. The switch voltage is different because the resonant circuit begins to ring after the current goes to zero. This shows up as the funny looking blip in Vs which is the start of ringing. Obviously, the switching frequency does not vary with load or input voltage but the pulse width may vary over the whole range. The operation of the circuit is similar to an amplitude modulation system. The square wave coming from the switches has a fundamental frequency and many harmonics. A change to the pulse width produces a similar change in the amplitude of the fundamental frequency component. The action of the resonant circuit eliminates the harmonics and passes a sinusoidal current at the fundamental frequency to the output where it is rectified and filtered.
1.9 Difference between the ZVS and ZCS 1. In zcs, the switch is required to handle A peak current of load current plus VS/Z0.For natural turn off , VS/Z0 must be more than I0 .There is , therefore , an upper limit to the value of the load current in zcs converter.
15
2. in zvs, the switch is required to with stand a peak voltage of VS +I0*ZS. This shows that peak switch voltage is dependent on the load current IO .A wide variation of load current would need large voltage across the switch. As peak voltage across the switch is a dominating factor, zvs converters are used only for the constant load applications. 3. In general, zvs is preferred over zcs at the high switching frequencies, primarily due to internal capacitances associated with the switch. 4. It is effectively reduce the input harmonics in this project. 5. It will excellently maintain the very low voltage across the switches. 6. Lower transformer turns ratio leads to the smaller duty cycle loss and transformer losses. 7. Direct and precise control of the input current is also possible with the current fed technology. 8. The current fed dc-dc converter is better suited to low voltage high current application.
1.10 Applications of the zero voltage switch (ZVS) technique The applications of zero voltage switches are: 1. Energy processing of the renewable energy sources. 2. Uninterrupted power supply. 3. Distributed power supply. 4. Transportation system.
1.11 Description of the distributed power generation 1. It is used in distribution side. In the distribution side, we have more problems compare to the other sector in the network. 2. End consumers face more shortage problems. 3.Specially this application is excellently used for the end consumer’s purpose. 4. Power losses are less with this project compare to the other schemes. 5. Reduces congestion . 6. Improves power quality.
1.12 Transportation System Application Transportation system applications are listed below: 1. Manned light air craft’s. 2. Two and three wheels vehicles such as scooters. 16
3. Light duty vehicles. 4. Buses and trucks. 5. Ferries and small boats 6. Unmanned aerial vehicles. 7. Portable fuel cells. 1.12.1 Energy Processing of the Fuel Cells 1. Energy processing of the renewable energy sources is the main application. 2. The output of the each and every renewable energy is direct current. It is not direct connect to the grid. 3. The output of the renewable energy is first connecting to the active clamping of the circuit. 4. Active clamping of the circuit is excellently work under the three phase is good compare to the single phase. 5. The clamping circuit is connecting to the transformer through coupled inductor. 6. The dc is first converted into the variable dc and then required frequency of the grid.
1.13 Organization of Thesis Chapter-2 deals literature survey of the project. Chapter-3 deals ZVS-PWM topologies and methodology. Chapter-4 deals simulation diagrams and simulation results. Chapter-5 deals conclusion and future scope.
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Chapter-2 LITERATURE SURVEY
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Chapter-2 LITERATURE SURVEY Danwei Liu, Hui Li [1] fuel cells are considered to be one of the most promising power generation devices because of their environmentally friendly energy conversion. Fuel cells have a slow response due to the natural electrochemical reactions required for the balance of enthalpy. Therefore bidirectional energy storage is required as an external leveling system to sink/source the power difference. A multi-port dc-dc converter, one of which is bidirectional, is promising in terms of cost, power density, efficiency and simplicity. Since each cell in a fuel cell stack has a low output voltage (0.6 V at full load), it is necessary to stack many cells in series to obtain a reasonable output voltage. Stacking many cells in series adds to the complexity of the system including a complicated plumbing to uniformly distribute the fuel and difficult water/thermal management. Due to these limitations, a lower output voltage fuel cell is preferred. For an example, fuel cell with 22 V nominal output voltages was adopted for 2003 International Future Energy Challenge sponsored by Department of Energy, USA. An ultra capacitor (UC) has an advantage over batteries in terms of transient energy storage. It has other advantage that it can be recharged and charged virtually an unlimited number of times and thus much less maintenance is required. The SOC of UC is a simple function of voltage so a sophisticated indictor is not required. The UC supports very low voltages (1-2.5V). A stack of series-connected UCS is required to hold up tens of Volts. Stacking many cells in series to support higher voltage requires extra dc voltage balance control that results in the adding cost and Complexity. The lower output voltage UC is thereby also preferred in the application. The low output voltage of fuel cell and UC making the design of multi-port dc-dc converter very challenging. For example, an isolated dc/dc converter is needed to convert low voltage dc to a dc voltage higher than 400 V for a 240 V ac output at 5 KW continuous and 10 kW peak. This dc/dc converter will see high current at both fuel cell and UC side. Although the power from fuel cell is unidirectional, a bidirectional power flow is required between UC and the load. In addition, UC has a widely changing dc operating voltage that may affect the
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converter operation. Most of current bidirectional multi-port dc-dc converter deals with the low output power and/or low input current applications. This paper proposed a threeport three-phase dc-dc converter to address the above design issues. Major features of the converter include: 1) provide an integrated energy conversion solution with combined power flow from fuel cell and UC; 2) draws and injects smooth current from the fuel cell and UC; 3) increase input current rating by interleaving three phases, not by paralleling components; 4) reduce core number and current ripple with improved efficiency by integrating three input inductors into a single core with coupling; 5) achieve zero-voltageswitching (ZVS) over a wide range by hybrid pulse-width-modulation (PWM) and phaseshift-modulation (PSM) control. This converter has a great potential for application where an interface between low voltage fuel cells, low voltage energy storage element and high voltage dc bus is required. Examples include distributed power generation systems and fuel cell vehicle. A new design of three-phase inductor is presented with minimum current ripple. The power flow among three ports is analyzed as well as the soft switching conditions. The optimized operation of converter is achieved by adding controllable duty cycle to traditional phase shift control. The component stress of three-phase topology is compared with single-phase circuit at a specified operating condition. Hanju Cha, Jungwan Choi, Prasad N. Enjeti, [2] fuel cells are identified as a future source of generating energy due to their efficient and clean energy characteristics; furthermore, they produce low-varying dc voltage in the range of 30–60 V for static power application such as residential use. For static fuel cells, the power conditioning system usually consists of a low-voltage fuel cell as the primary source, a dc/dc converter to obtain isolated high voltage, and a dc/ac inverter to connect commercial ac voltage. Since a dc/ac inverter supplies power into a 220-V ac utility, an isolated dc/dc converter has to convert low varying dc voltage to high constant dc voltage at around 370 V. Therefore, a dc/dc converter with a high voltage ratio is needed, and a transformer is usually employed for boosting voltage as well as isolation. However, high leakage inductance in the transformer leads to trouble such as voltage spikes and electromagnetic noise. In order to achieve a high voltage ratio while limiting the overshoot in the turn-off voltage caused by the leakage inductance, a current-fed dc/dc converter with an active 20
clamp has been introduced in the push–pull topology and full-bridge topology for all single–phase application. In addition, a soft-switching active clamp scheme has been proposed to minimize turn-off losses in the clamp switch. These converters have been shown to perform quite well, but the single-phase circuits face severe components stress and degraded efficiency for higher power levels. In high-power applications, research has been focused on the three-phase dc/dc converter due to the benefits it can offer, such as high power density and high-quality waveforms. Ziogas introduced three-phase converter topology in the high-frequency dc/dc conversion area, and showed that three-phase structure had superior potentials in power density, RMS current through switches, size of reactive components, and efficiency compared to a relevant single-phase structure. However, negligible leakage inductance is required in the three-phase transformer implementation and switches commutate in the hard switching condition. A series resonant converter comprised of resonant capacitors, stray inductances of the transformer, and equivalent resistances was proposed for the three-phase converter, and showed variable switching frequency with limited parameter tolerance. However, the series resonant converter results in an increase at the volume of reactive components. Zero-voltage switching (ZVS) commutation using leakage inductances in the transformer and intrinsic capacitances in the switches was introduced in three-phase dc/dc converter and achieved high power density with simple power structure. Since ZVS commutations are occurred in the same way as a single-phase phase shift- modulated full-bridge converter, so ZVS actions are limited in higher load condition. A six-leg three-phase converter was introduced along with phase-shift modulation and increased voltage transfer ratio by Y connection of secondary side of three single-phase transformers. Although the six-leg converter showed very high efficiency without auxiliary snubber circuits, the six-leg converter requires more switches and control circuit complexity. Development of a three-phase current-fed dc/dc converter with an active clamp is proposed. Major features of the proposed converter include: 1) increased power transfer for switches with the same current and voltage rate, when compared to single-phase solutions; 2) an achievement of ZVS in active switches through a single common active 21
clamp branch and zero current switching in rectifier diodes; 3) lowering of the transformer turns ratio by using boost characteristics inherited by current-fed boost type; 4) a reduction in the size of the input dc inductor and elimination of the output filter inductor by increased effective switching frequency; and 5) a reduction in conduction loss through distribution of RMS current among per-phase switches and transformer windings, when compared to a single-phase converter for the same power ratings. Due to these advantages, the converter is highly suitable for the interface between a low-voltage high-power fuel cell source and an inverter load. It may also be extended to other lowvoltage sources, such as batteries and photovoltaic array that need high voltage, highpower dc/dc conversion. However, the proposed converter also exhibits disadvantages such as higher count of switches, increased power and control circuit complexity, and thus reduced application reliability. Sergio Vidal Garcia Oliveira, Ivo Barbi, [3] the great necessity in several areas, particularly the industrial Sector, for switch mode power converters with Larger power ratings in the end of the 80s was the starting point For the appearance of high-frequency (HF) three-phase dc–dc Converter. Since the first three-phase dc–dc isolated converter. In the input stage, the most common configuration is this stage to operate as a voltage source, while the output stage has current source characteristics. The advantages of three-phase dc–dc isolated solutions are as follows: 1) reduction of the input and output filters volume, as well as the reduction of weight and size of the isolation transformers; 2) lower RMS current levels through the power components, when compared to single-phase solutions for the same power ratings. With the increasing threat of the fast depletion of resources such as petroleum, coal, and natural gas forces, people seek renewable energy sources, such as solar, wind, geothermal, and hydraulic energies. In recent years, fuel cell (FC) research and development have received special attention for their higher energy-conversion efficiency and lower CO2 emissions than thermal engines in the processes of converting fuel into usable energies. For these systems, different power converter topologies can be used for the power electronic interface between them and the load. Basically, low-voltage high-current structures are needed because of its electrical characteristics. In these systems, a classical boost converter is often selected as a possible converter. However, a classical boost converter 22
will be limited when the power increases (greater than 4.5 kW) or for higher step-up ratios (greater than two times). The major problems of using a single dc/dc converter connected with FC in high-power applications are the difficulty of the design of magnetic component and high FC ripple current, which may lead to the reduction of its stack lifetime. To overcome these limitations, the use of paralleling power converters with interleaved technique is also applied and may offer some better performances. In industrial applications, the inverter systems, together with dc–dc high-voltage-ratio converters, that feed electrical power from low-voltage-level systems into the grid must convert their direct current into alternating current for the grid. A study focusing on this, offering a comparative view of some dc–dc converters applied in systems in the medium power range of 20 kW and higher, is presented. The three-phase step-up dc–dc isolated converter presented in this paper, has all of the main advantages of the three-phase solutions presented up until now. Moreover, the reduced number of switches, if compared with other topologies, such as presented and the voltage step-up characteristic improve the efficiency and reduce, along with a high switching frequency, the output filter volume, respectively. Furthermore, due to the input-current-source characteristic or non pulsed input current, this topology can be adopted in all types of applications supplied by alternative energy sources, such as battery or photovoltaic (PV) arrays and, the more recent, FC systems. Jacobo Aguillon-Garcia, Gun-Woo Moon, [4] the broad resources provided by information technologies (ITS) have created a remarkable trend in human history. Global networks reach almost every aspect of our daily life at virtually any time. Naturally, those resources are backed up by enormous machinery (i.e., data centers), which, in turn, has the need of energy to attain their respective duties. In fact, Those IT resources account for approximately 2% of all global CO2 emissions. Computer servers are the workhorse at the Center of the whole infrastructure in an outstanding increasing Rate. Therefore, to deal with the carbon dioxide footprint, there are several standards that the servers’ power supply unit (PSU) should accomplish and they are mainly based on the Efficiency at 20%, 50%, and 100% of power loading. The required Values are presented for the most significant.
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Several circuit topologies have been used mostly in medium power level (600– 800 W) for low-voltage high-current dc–dc conversion in the PSU such as: single-phase full-bridge and half-bridge converters and multilevel converters matrix converters and lately, parallelism of components or even converters. However, at a higher power range (beyond 1.5 kW), previously mentioned architectures for PSU applications are scarce. An architecture that is commonly used for higher power ranks with relative low losses in a dc–dc conversion is the zero voltage switching (ZVS) PWM three-phase inverter with galvanic isolation coupled to a three-phase full-bridge rectifier where the main characteristic is the low output current ripple, the reduced RMS current over the switching components, and the decrease of the transformer volumes. Even though the previous characteristics make the three phase converter a perfect candidate for a PSU, a soft commutation with low losses has not been fully achieved for a full load range. Numerous remedial strategies have been proposed to cope up with this problem. Zheng utilized LCC-type resonance in three-phase dc–dc converters. Nevertheless, that architecture is prone to voltage and current stresses on the semiconductors and a substantial increase of the reactive element dimensions. Afterward, in order to provide a ZVS commutation of all switches, the use of an asymmetrical duty cycle was proposed with a notable high efficiency, even if higher conduction losses in the rectifier stage were present. Consequently, a three phase dc–dc converter that is capable of achieving a soft commutation requires a high-efficiency rectifier to attain the optimal Arrangement for low-voltage high-current applications. In order to improve the efficiency required at the rectifier Stage, a three-phase version of the hybridge rectifier was utilized, obtaining a soft commutation on a wide load range and a modest enhancement in the overall circuit efficiency. The penalty is still an increased number and volume of the output inductors, which are prone to current unbalance. To alleviate such disequilibrium, a zigzag connection in the output filter can be implemented with the consequence of an enlarged volume and the fact that the core for the zigzag filter is not commercially available. Another suggested architecture is the double-wye converter with an inter phase reactor (IPR). The IPR is implemented by two identical inductors (LO1, LO2) mounted in one or two separated cores. This IPR cannot play its role as a voltage divider unless an Alternating current I flow in it. 24
The rapid decrease in the rectifier voltage when io increases from zero to the critical current are a problem in many applications. It is approximately 13.33% or 1.17 V of the voltage that would be produced during a normal operation at no load. Therefore, it is often necessary to reduce IO critical to a very low value, increasing the reactance of the IPR. Anything that increases the impedance of the circuit followed by the circulating current I reduces the amplitude of the latter. The core losses of the IPR, supplied at a frequency of 3fs, and the resistance of its winding increase the overall equivalent resistance and, consequently, decrease the critical current. Some efforts have been made to avoid this situation. Nevertheless, the resulting architecture includes extra active elements or magnetic components that are susceptible to current unbalance, an increase in the total volume of the rectifier stage, and the utilization of a complex control strategy. Demercil de Souza Oliveira, Ivo Barbi, [5] nowadays, the main topology used in high power dc/dc conversion is the zero-voltage switching (ZVS) pulse-width modulation (PWM) full-bridge converter. It is characterized by four switches operating in high frequency. The soft commutation can be obtained by using phase shift modulation, which preserves the simplicity and achieves high power density. However, for higher power levels, the components face several stresses. As possible solutions, the parallelism of components or even converters can be applied. The former choice increases the complexity of the compromise between the layout circuit and the thermal design. Besides that, one should consider that the dynamic and static current sharing problem limits its application. The other alternative causes redundancy in the control circuits as well as in the number of power components and drivers, increasing the global cost and size of the equipment. A prominent alternative was proposed by Ziogas. It uses a three-phase inverter coupled to a three-phase high frequency transformer and to a three-phase high frequency rectifier. The resulting advantages are as follows. 1) Increase of the input current and output current frequency, by a factor of three compared to the full-bridge converter; 2) Lower RMS current through power components 3) Reduction in the transformer size. The disadvantages are: 25
1) Control circuit complexity. 2) Commutation losses. Considering that digital signal processors (DSPs) can overcome the control aspect, several researchers have investigated techniques to achieve soft commutation. Divan has proposed a circuit in which soft commutation can be achieved for a wide load range. Nevertheless, it is only viable in applications where bi-directional power flow is needed, because six additional switches are included. Bhat and Ziogas applied the resonance concept in three phase dc/dc converters, reaching soft commutation. However, the resulting topologies suffer high voltage and current stresses, and also a considerable increase in the volume of reactive power elements. Dmitri Vinnikov, Indrek Roasto [6] distributed power generation, when fully implemented, can provide reliable, high-quality, and low-cost electric power. As a modular electric power generation close to the end user, It offers savings in the cost of grid expansion and line losses. If connected to the power grid, the bidirectional transactions between the grid and the local generation result in grid capacity enhancement, virtually uninterrupted power supply, and optimum energy cost due to the availability of use/purchase/sales options. Distributed power is a concept that covers a wide spectrum of schemes used for local electric power generation from renewable and nonrenewable sources of energy in an environmentally responsible way. Basic schemes are mainly based on solar energy, wind energy, fuel cells (FCS), and micro turbines. An FC is potentially the most efficient modern approach to distributed power generation. The efficiency of the conversion, i.e., the ratio of the electrical output to the heat content of the fuel, could be as high as 65%–70%. In fact, its electrical efficiency could be greater than 70% in theory. Current technologies have only been capable of reaching efficiencies of around 45%. Combined cycles are intended to raise electrical efficiency up to 60% for plants based on high-temperature cells. To interconnect a low-dc-voltage-producing FC (typically 40–80 Vdc) to residential loads (typically 230-Vac single phase or 3 × 400 Vac), a special voltage matching converter is required. Due to safety and dynamic performance requirements, the 26
interface converter should be realized within the dc/dc/ concept. This means that low voltage from the FC first passes through the front-end step-up dc/dc converter with the galvanic isolation; subsequently, the output dc voltage is inverted in the three-phase inverter and filtered to comply with the imposed standards and requirements (second dc/ac stage). The design of the front-end isolated dc/dc converter is most challenging because this stage is the main contributor of interface converter efficiency, weight, and overall dimensions. The low voltage provided by the FC is always associated with high currents in the primary part of the dc/dc converter (switching transistors and primary winding of the isolation transformer). These high currents lead to high conduction and switching losses in the semiconductors and therefore reduce the efficiency. Moreover, the large voltage boost factor requirement presents a unique challenge to the dc/dc converter design. This specific requirement could be fulfilled in different ways: by use of an auxiliary boost converter before the isolated dc/dc converter or by use of an isolation transformer with a large turn’s ratio for effective voltage step-up. Claudio Manuel C. Duarte, Ivo Barbi, [7]
the objective of high frequency
operation in dc–dc converters is the reduction of reactive components size and cost. As in any power application, high efficiency is essential, and hence the increasing of frequency can be problematic because of the direct dependence of switching losses on frequency. The use of soft-switching techniques ZVS and ZCS is an attempt to substantially reduce switching losses, and hence attain high efficiency at increased frequency. Different techniques have been proposed to operate dc-dc converters in high frequency. The active clamping technique has the advantages of PWM modulation, softcommutation (ZVS) on main switches and low voltage stresses due to the clamping action. Besides operating at constant frequency and with reduced commutation losses there is no significant increasing on circulating reactive energy that would cause large conduction losses. The parasitic ringing caused by the interaction of the junction capacitance of the rectifier, in the clamped mode Boost converter and the resonant inductor are eliminated by the inclusion of an auxiliary clamping diode limiting the voltage stress on the rectifier to the output voltage. It is important to note that to simplify the analysis, the input filter 27
inductance is assumed large enough to be considered as a current source and the capacitor is selected to have a large capacitance so that the voltage across the capacitor could be considered as a constant one. Although the voltage stress on the rectifier has been eliminated, by this approach, both and diodes still present hard switching commutation and the voltages across these devices still rise in a high rate, which means compatibility electromagnetic problems and switching losses. This paper presents an improved family of dc-dc converters featuring clamping action, PWM modulation and soft-switching (ZVS) in both active and passive switches. The inclusion of capacitor and clamping diode, in the Boost converter, results in reduced voltage and soft-switching conditions for all switching devices, including diodes and . Therefore all parasitic reactance are absorbed, including transistor output capacitance and diode junction capacitance, resulting in high efficiency at high frequency operation without significant increasing in voltage and current stresses on switches. Dharmraj V. Ghodke, Kishore Chatterjee, B. G. Fernandes [8] over the years single phase full-bridge (FB) and three-phase FB pulse width modulation (PWM) dc to dc soft switched converters have become popular in the field of dc to dc conversion system. For these converters metal oxide semiconductor field effect transistors (MOSFETS) are generally preferred over insulated gate bipolar transistors (IGBTS), because they can be operated at higher switching frequency and they do not have the problem of long tail current. However, these FB PWM soft switched converters are not suitable for switch mode power supply applications, where the input voltage is high. This is because the MOSFETS have to sustain high input dc link voltage. Moreover, service of auxiliary circuits is required to operate devices in soft switched mode. This requires extra components, devices and hence it leads to incurring additional cost while reducing the system reliability. In order to reduce the voltage stress to half of the input dc voltage, a three-level topology has been considered for inverter application and it has been used for realizing a dc to dc converter. The soft commutation is achieved by using phase shift PWM modulation which is having simple control structure and high power density can be achieved. However at high power levels, these components experience considerable current stress. In order to overcome this problem, topologies consisting of three-phase inverter coupled to a three28
phase high frequency transformer followed by three-phase high frequency bridge-rectifier have been proposed. This results in an increase in the input current and output current frequencies by a factor of three as compared to the full bridge converter. This also results in lower current rating for the components and also a considerable reduction in size for the isolation transformer. However, the devices experience high voltage stress and the control structure is also quite involved. In an effort to overcome the aforementioned limitations a new converter topology involving three-phase, three-level, (TPTL) phase shifted PWM converter involving six switches operating as zero voltage switching (ZVS) and six switches operating as zero current switching (ZCS) has been presented in this paper. It should be mentioned that in this case soft switching of the semiconductor devices is achieved without taking help from any additional auxiliary circuitry comprising of active or passive elements. In the proposed Topology the output rectifier is a center tapped full wave current producing either two or three-level output voltage depending on the operating duty cycle. This leads to considerable reduction in size of the output filter compared to that of the conventional full bridge topology. The principle of operation and exhaustive analytical studies for the proposed converter is presented. In order to obtain behavioral and performance characteristics of the converter, detailed simulation studies are carried out. Finally the viability of the scheme is confirmed through detailed experimental studies on a 6.6 kW laboratory prototype developed for the purpose. Finally, extension of the proposed converter for different output voltage and multiphase configurations has been illustrated. Hyungjoon Kim, Changwoo Yoon, Sewan Choi, [9] since a dc voltage generated from fuel cells is usually low and unregulated, it should be boosted and regulated by a dc–dc converter and converted to an ac voltage by a dc– ac inverter. Highfrequency transformers are usually involved in the dc–dc converter for boost as well as galvanic isolation and safety purpose. The single-phase dc–dc converter based on the push–pull or full-bridge topology has been used as an isolated boost dc–dc converter for less than several kilowatt power levels. For higher power level, the single-phase converter could suffer from severe current stresses of the power components. The three-phase dc–dc converter has been proposed as an alternative for highpower application. The three phase dc–dc converter has several advantages over the 29
single phase dc–dc converter: (1) easy MOSFETS selection due to reduced current rating; (2) reduction of the input and output filters’ volume due to increased effective switching frequency by a factor of three compared to single-phase dc–dc converter; and (3) reduction in transformer size due to better transformer utilization. The three-phase isolated boost dc–dc converter can be classified to dual active bridge (DAB) converters current-fed converters and voltage-fed converters. The DAB can achieve ZVS on both high- and low-side switches and has no inductors involved in the power circuit. However, the DAB has many active switches and high ripple currents. Also, the VA rating of the transformer is comparably large, and manufacturing of the high-frequency transformer with large leakage inductance is a challenging issue. The current-fed converter, in general, exhibits lower transformer turns ratio, smaller input current ripple, lower switch current rating, and lower diode voltage rating. However, higher switch voltage rating of the current-fed converter implying larger Rds (ON) of MOSFET switches is a major disadvantage since switch conduction loss at the primary side is actually a dominant factor in determining overall efficiency of the dc– dc converter for low-voltage high-current application such as fuel cells. A clamping or snubber circuit is usually required for the current-fed converter to limit the transient voltage caused by transformer leakage inductance. The current-fed converter is also lack of self-starting capability and, therefore, it necessitates an additional start-up circuitry. The three-phase current-fed dc– dc converter proposed for step-up applications has only three active switches, but the active switches are hard switched and the passive clamping circuit on the high-current side may cause large amount of losses. The three-phase current-fed dc–dc converter with an active clamping circuit not only clamps the surge voltage but also offers ZVS on the active switches. However, this scheme suffers from the high ripple current imposed on the clamp capacitor located at the high-current side. The voltage-fed dc–dc converter has also been used in fuel cell applications. An important advantage of the voltage-fed type is lower switch voltage rating since the switch voltage is fixed to input voltage, and therefore MOSFETs with lower Rds (ON) can be selected. This is critically beneficial in the fuel cell application where more than 50% of the power loss is lost as a switch conduction loss at the low-voltage side. Also, the 30
voltage-fed converter does not have a self-start problem unlike the current-fed converter. However, the voltage-fed converter suffers from a high transformer turns ratio, which causes large leakage inductance resulting in large duty cycle loss, increased switch current rating, and increased surge voltage on the rectifier diode. The three-phase voltage-fed dc–dc converter, so-called V6 converter, proposed for step-up applications, significantly mitigates the problem associated with high transformer turn ratio of the voltage-fed type by utilizing the open Δ-Y type transformer connection, which reduces the required turn ratio to half. Also, the size of the input filter capacitor to reduce the input current ripple is reduced, since the effective switching frequency is increased by three times due to the interleaved operation. In this paper, a three-phase voltage-fed dc–dc converter for isolated boost application such as fuel cells is proposed. The turn ratio of the high-frequency transformer is reduced to half by employing the Δ-Y connection. A clamp circuit that is located at low-current, highvoltage side not only clamps the surge voltage But significantly reduces the circulating current flowing through high-current side, resulting in reduced switch conduction losses and transformer copper losses. Further, with the help of the clamp circuit zero-voltage and zero-current switching (ZVZCS) for all switches over wide load range is achieved. The duty cycle loss can also be compensated by the clamp switch. The operating principles and features of the proposed converter are illustrated and experimental results on a 1.5kwprototype are also provided to validate the proposed concept.
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Chapter-3 ZVS-PWM DC-DC CONVERTER TOPOLOGIES AND METHADOLOGY
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Chapter-3 ZVS-PWM DC-DC CONVERTER TOPOLOGIES AND METHADOLOGY 3.1 Introduction Three-phase systems are well known by their use in electric power generation transmission and distribution. The cost saving that they provide by employing less material than single-phase systems assured success in these areas and led to three-phase rectifiers, inverters, and also dc–dc converters. Industrial environments have an increasing need for high efficiency dc–dc converters. Applications including distributed generation and uninterruptable power supplies generally count on single-phase dc–dc converters with big and heavy transformers. The high volume associated with these converters makes them an expensive choice, and their use in the transportation area is sometimes impossible. The introduction of high-frequency three-phase transformers on dc–dc converters brought the possibility of increasing power density, using the magnetic cores more efficiently and reducing the current stress on power switches. In addition, the increase in the high-frequency component seen by the filters allowed the use of much smaller inductors and capacitors. After this, other three-phase dc–dc converter topologies were developed and compared, and techniques to increase the efficiency even more using soft-commutation and reducing the number of semiconductors in the output rectifier bridge were studied. Most studies conclude that the three-phase structures perform better than their single-phase counterparts. However, depending on the topology, the voltage across the switches is not naturally clamped; requiring passive voltage clampers that dissipate energy stored in the leakage inductances to prevent overvoltage. This energy loss reduces the efficiency of the converter. In order to avoid this problem, active clamping techniques have already been presented for single-phase converters and have successfully reused the energy that would be dissipated both in non-isolated and isolated topologies. To sum up, soft-commutation (ZVS) was also achieved with a correct parametric combination. A zvs-pwm three phase current-fed push-pull dc-dc converter with fuel cell input is excellent compare to the basic concept of the dc-dc converter. 33
3.2 Power Electronics Devices Power Electronics is a field which combines Power (electric power), Electronics and Control systems. Power engineering deals with the static and rotating power equipment for the generation, transmission and distribution of electric power. Electronics deals with the study of solid state semiconductor power devices and circuits for Power conversion to meet the desired control objectives (to control the output voltage and output power). Power electronics may be defined as the subject of applications of solid state power semiconductor devices (Thyristors) for the control and conversion of electric power. Power electronics deals with the study and design of Thyristorised power controllers for variety of application like Heat control, Light/Illumination control, and motor control – AC/DC motor drives used in industries, High voltage power supplies, Vehicle propulsion systems, High voltage direct current (HVDC) transmission. 3.2.1 Differences between Electronics and Power Electronics This question can be answered in number of ways. The simplest way to say all the electronics devices which deal with power are covered in Power Electronics. There are devices which you do use in general purpose electronics like: Diode, BJT, MFETs etc. You will find these devices in electronics & power electronics both. The devices used in Power electronics may differ in term of construction & behavior from those used in electronics. There are types of devices which you will find only IN PE, ex: IGBT. 1. The only difference is that power devices (e.g. MOSFETs) are made to handle much larger power requirements. 2. Ordinary devices are low current and low voltage devices. 3. Power devices are high current and/or high voltage devices. 3.2.2 Power Converters A power converter is an electrical or electro-mechanical device for converting electrical energy. It may be converting AC to or from DC, or the voltage or frequency, or some combination of these. Amongst the many devices that are used for this purpose are;
Rectifier 34
Inverter
DC - DC converter
AC - AC converter
AC - DC converter (rectifier): A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. DC - AC converter (inverter): An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. DC - DC converter (choppers): A dc chopper is a dc-to-dc voltage converter. It is a static switching electrical appliance that in one Electrical conversion, changes an input fixed dc voltage to an adjustable dc output voltage without Inductive or capacitive intermediate energy storage. The name chopper is connected with the fact that the output voltage is a „chopped up‟ quasi-rectangular version of the input dc voltage. AC - AC converter: An AC/AC converter converts an AC waveform such as the mains supply, to another AC waveform, where the output voltage and frequency can be set arbitrarily. AC/AC converters can be categorized into 1. Cyclo converters 2. Matrix Converters
3.3 Power Electronic Switches 35
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. A power electronic switch integrates a combination of power electronic components or power semiconductors and a driver for the actively switchable power semiconductors. The internal functional correlations and interactions of this integrated system determine several 3.3.1 Thyristor A thyristor is a solid-state semiconductor device with four layers of alternating n and p-type material. They act as bi-stable switches, conducting when their gate receives a current pulse, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed).Some sources define silicon controlled rectifiers and Thyristors as synonymous.
Fig. 3.1: Symbol of Thyrister The thyristor is a four-layer, three terminal semiconducting devices, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to the cathode. (A variant called an (SCS)Silicon Controlled Switch brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action:
Fig. 3.2: Structure on the physical and electronic level, and the thyristor symbol. Thyristors have three states: 36
1. Reverse blocking mode : Voltage is applied in the direction that would be blocked by a diode 2. Forward blocking mode : Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction 3. Forward conducting mode : The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current" 3.3.2 Insulated Gate Bipolar Transistor (IGBT) The insulated-gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. It switches electric power in many modern appliances: electric cars, variable speed refrigerators, airconditioners, and even stereo systems with digital amplifiers. Since it is designed to rapidly turn on and off, amplifiers that use it often synthesize complex waveforms with pulse width modulation and low-pass filters. An examination of reveals that if we move vertically up from collector to emitter. We come across p+, n- , p layer s. Thus, IGBT can be thought of as the combination of MOSFET and p+ n- p layer s. Thus, IGBT can be thought of as the combination of MOSFET and p+ n- p transistor Q1 .Here Rd is resistance offered by n Approximate equivalent circuit of an IGBT.
Fig. 3.3: Symbol and Circuit Diagram of IGBT
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–
drift region.
3.4 Proposed ZVS-PWM Three-Phase Dc–Dc Converter 3.4.1 Overview Buck regulator key requirements are typically based on size and efficiency. Printed circuit board area is a precious commodity and no designer is willing to give up more space than what is needed for their power design or scheme. Also, as existing designs get upgraded with the latest and greatest processors, DSP, etc. The power increases but the footprint cannot increase in size. Thus, high density regulators have evolved leveraging the latest in IC integration, MOSFETS, and packaging. These high density regulators still cannot keep pace with the demands put on them by new systems with ever increasing higher density. The primary reason for this is the switching losses hindering performance within the regulator MOSFETS. Without addressing these losses head on, only incremental performance increases can be expected. The PI33XX with its ZVS topology allows for operation at a higher frequency and at higher input voltages without sacrificing efficiency. The ZVS topology is a soft switching topology in contrast to a hard switching topology deployed in conventional regulators. The soft switching technology of the PI33XX provides the higher efficiency and higher density performance than conventional regulators. ZVS topology is typically associated with high performance isolated power supplies. Integrating a ZVS topology within the PI33XX is an industry first. 3.4.2 History Switches have been used for power conversion since the late 1800‟s, first with mechanical devices and later with electronic switches. The advantages of controlling or at least modifying the turn-on and turn-off voltage and/or current waveforms associated with a switch have been recognized from the beginning. The capacitor turn-off snubber, for example, shows up as a capacitor across the contacts of the mechanical vibrator used by Heinrich Hertz to demonstrate the existence of electromagnetic waves. The idea was to retard the rate of rise of voltage across the switch as the switch opened which in turn, suppressed arcing and limited the life of the contacts. Because switching of inductive loads is intrinsic to most power conversion, a great many schemes have been advanced for “commutation aids”– circuits that reduce the 38
loss or stress on a switch while turning on or off. This range from a wide variety of snubber circuits, soft switching using resonant transitions, zero current switching (ZCS) resonant converters to ZCS and zero voltage switching (ZVS) quasi-resonant circuits. There are many possible routes to achieve reduced switching stress and/or loss. In this discussion we will take a look at the history of one approach. We will cover other approaches in subsequent articles. The present renaissance in soft switching for power converters appears in 1981. In April of that year, a paper by Goldfarb demonstrated the use of the transformer magnetizing inductance to “recover the charge on the snubber capacitors” in a full bridge switching converter. Dead-time for resonant transitions usage was shown. In September of 1981, Carsten presented a paper using an active clamp in a forward converter which used both the magnetizing and leakage inductances for soft switching. These papers seemed to spark interest in the technique. Since that time, there have been many, many more papers on the subject. 3.4.3 Applications Commercial applications: Heating Systems Ventilating, Air Conditioners, Central Refrigeration, Lighting, Computers and Office equipments, Uninterruptible Power Supplies (UPS), Elevators, and Emergency Lamps. Domestic applications: Cooking Equipments, Lighting, Heating, Air Conditioners, Refrigerators & Freezers, Personal Computers, Entertainment Equipments, UPS. Industrial applications: Pumps, compressors, blowers and fans Machine tools, arc furnaces, induction furnaces, lighting control circuits, industrial lasers, induction heating, welding equipments. Aerospace applications: Space shuttle power supply systems, satellite power systems, aircraft power systems. Telecommunications:
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Battery chargers, power supplies (DC and UPS), mobile cell phone battery chargers. Transportation: Traction control of electric vehicles, battery chargers for electric vehicles, electric locomotives, street cars, trolley buses, automobile electronics including engine controls. Utility systems: High voltage DC transmission (HVDC), static VAR compensation (SVC), Alternative energy sources (wind, photovoltaic), fuel cells, energy storage systems, induced draft fans and boiler feed water pumps.
3.5 Circuit Analysis 3.5.1 Circuit Description The circuit of the proposed ZVS-PWM three-phase current-fed push–pull dc–dc converter is shown in Fig. 3.4. Switches S1, S2, and S3 and the capacitor Cg were added to the converter in order to achieve active clamping. Inductances Ld1, Ld2, and Ld3 are responsible for maintaining the current during the commutation intervals. They represent the sum of the leakage inductance of the transformer and an external inductance, which is added to each phase if needed. The addition of capacitors C1, C‟1, C2, C‟2, C3, and C‟3 and appropriate dead time between main and complementary gate signals provide the Possibility to operate with soft-commutation.
Fig. 3.4: Basic Circuit Diagram of the ZVS-PWM Three Phase Current-Fed Technologies
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Table-3.1: Operating regions region
Duty cycle
Switches simultaneously ON
R1
0
