An educational tool for fundamental DCDC ... - Wiley Online Library

An Educational Tool for Fundamental DC–DC Converter Circuits and Active Power Factor Correction Applications KORHAN KAYISLI,1 SERVET TUNCER,1 MUSTAFA POYRAZ2 1

Faculty of Technical Education, Department of Electronics & Computer Education, Firat University, Elazig, Turkey

2

Faculty of Engineering, Department of Electrical-Electronics Engineering, Firat University, Elazig, Turkey

Received 24 March 2009; accepted 3 May 2010 ABSTRACT: Computer-aided education has become popular thanks to its flexible and useful structure in classroom environment. In this study, a computer-based educational tool for DC–DC converters and its active power factor correction (PFC) applications are presented for effective education of these circuits at power electronics courses and reinforcement of the theoretical instruction given at these courses. Firstly, simulations were generated by MATLAB/Simulink toolbox. After then, graphical user interfaces (GUIs) were designed for visual approach to specifying all input and output parameters. The results of the educational tool developed are illustrated with screenshots and graphics. © 2010 Wiley Periodicals, Inc. Comput Appl Eng Educ 21: 113–134, 2013; View this article online at wileyonlinelibrary.com/journal/cae; DOI 10.1002/cae.20455 ´ converter Keywords: educational tool; DC–DC converter; power factor correction; boost converter; Cuk

INTRODUCTION Computer-aided education is an interactive instrument in the classroom that makes the students continuously active, the learning much more stable and it is also more concrete and more flexible as a reinforcement and has a function of giving the appropriate ability to serve the programmed teaching, and these advantages makes, it more popular today. In view of these advantageous properties, computer-aided educational tools have been increasingly used in educating students of traditionally hard engineering subjects such as electrical, computer and mechanical engineering of the past years [1]. These subjects should be thought with a convenient combination of theory, exercises, and laboratory experimentation. It is important to attract the student’s attention during the lessons, and therefore the modernization of the classroom environment is essential. The recent technological trends in engineering education have evolved computers and software tools. The use of software simulation tools in classroom environments have become an integral part of modern curriculum [2]. Recently, computeraided education in classroom and laboratory environments has been increasingly investigated and can widely seen in literature. In Ref. [3], a software tool offering students a quick and easy introduction to simulation and laboratory experiments related to electrical Correspondence to K. Kayisli (kkayisli@firat.edu.tr). © 2010 Wiley Periodicals, Inc.

machines and power electronic, Avouris et al. [1] described a computer-based laboratory teaching tool as a support to laboratory course, and in [4–8] where internet-based learning methods are discussed, an attractive and useful tool for teaching fuzzy logic controller design steps by using MATLAB environment was described. Nowadays, graphical user interfaces (GUIs) are being increasingly used in the classroom environment to provide users of computer simulations with a friendly and visual approach [9,10]. The most important benefit of a GUI is that it can post-process the results of the simulation providing the user with instant feedback. This is especially important when performing parametric studies where variables are changed over a certain range [9]. Especially, DC–DC converters have many circuits and control parameters. After parameter calculations, students can see the difference of the values with GUI based educational tools. This article describes an educational tool which can be used in power electronics courses. With this aim, a GUI-based educational tool was prepared by using MATLAB for the DC–DC converter circuits and active PFC applications. At the same time, one of the main characteristics of MATLAB is the availability of a set of toolboxes almost ready to be used in the design of these circuits. MATLAB/Simpower toolbox is useful and powerful for simulations of power electronic circuits because the models of passive and active components are available. A prepared example simula-

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Figure 1 A DC–DC converter system structure.

tion of a DC–DC converter circuit can be seen in Figures 7 and 8. In the first section of this study, the circuit diagrams of DC–DC converters investigated in two main titles as non-isolated and isolated, and operating principles and mathematical models are also presented. In the second section, controlling the input currents and output voltages of these circuits are explained with block diagrams and information about harmonic standards and PF is given. Boost converter is used for explaining active PFC and results are illustrated with graphics by using MATLAB/Simulink and Simpower toolbox and the simulation blocks of this application is shown at the end of this section. In the next section, a GUI prepared for an educational tool of DC–DC converters and active PFC applications is presented by using graphical interfaces and screenshots in addition that this educational tool is constituted as a reinforcement material for specific topics on power electronics course.

FUNDAMENTAL DC–DC CONVERTER CIRCUITS DC–DC conversion technology is a major research area in the field of power electronics and has been under development for decades. DC–DC converters are commonly used in applications requiring regulated DC power such as computers, communication devices, battery chargers, switched mode power supplies, and DC machine drive applications [11]. Generally, unregulated DC voltage obtained from rectifying the line voltage is used at the input of these converters. Therefore, step down and step up are occurring at the input of these converters as a result of changing the line voltage. These converters are being used for converting unregulated DC voltage level to needed regulated DC voltage. A standard DC–DC converter system is shown in Figure 1. DC–DC converters can be investigated under two parts which are non-isolated and isolated DC–DC converters. The classifications of these converters are shown as follows [12]: Non-isolated fundamental DC–DC converters: (1) (2) (3) (4) (5)

Buck converter. Boost converter. Buck–boost converter. Cúk converter. SEPIC converter.

Isolated fundamental DC–DC converters: (1) Flyback converter. (2) Forward converter. (3) ZETA converter.

Buck, boost, and buck–boost converters are the main structures of DC–DC converter circuits. When these three main converters are investigated, operating principles, basic functions, circuit structures of these circuits, and the topologies coming along the developing duration of these can be understood. The other converters such as Cúk and SEPIC are the first developed examples from the three main topologies. For example, Cúk converter circuit is derived from duality principle on the buck–boost converter. The situation is not different at isolated structures, either. In many DC– DC applications, multiple outputs are required and output isolation may need to be implemented depending on the application [13– 15]. In addition, input to output isolation may be required to meet safety standards and/or provide impedance matching [16,17]. Fundamental non-isolated DC–DC converter circuits are introduced briefly below.

Buck Converter The buck or step-down converter regulates the average DC output voltage at a level lower than the input voltage. The circuit diagram of the buck converter is shown in Figure 2. After obtaining the current and voltage equations according to the S switch’s on–off position, state space equations of buck converter at matrix form are as in the following equation [11].

dIL (t) dt dVC (t) dt

=

0 1 C

1 L 1 RC

IL (t) + VC (t)

␤ L

0

VS

(1)

where ␤ is defining the state of switch (␤ ∈ {0,1}); R, L, C represents ohmic load, inductor, and capacitor values of the circuit, respectively. IL is the inductor current and VC is the capacitor voltage.

Figure 2

Circuit diagram of buck converter.

FUNDAMENTAL DC-DC CONVERTERS AND ACTIVE PFC

Figure 3

Circuit diagram of boost converter.

IL VC dVC = − dt C RC

Boost Converter The boost converter regulates a higher DC voltage than input voltage. Circuit diagram of the boost converter is shown Figure 3. The circuit operation can be divided into two modes. Mode 1 begins when switch is turned-on at t = 0. The input current, which rises, flows through inductor L and switch S. Mode 2 begins when switch is turned-off at t = t1 . The current flowing through the switch would now flow through L, C, load, and diode. The inductor current falls until switch is turned on again in the next cycle. The energy stored in inductor L is transferred to the load. During the turn-on time of switch S (S-on, D-off), characteristic equations of the circuit can be defined as [18], VL = VS

(2)

dIL = VS dt

(3)

L

VS dIL = dt L

(4)

IC = −IO

(5)

VC dVC =− dt RC

(6)

where VL , VS are inductance voltage and rectified AC line voltage, respectively. Equation (4) shows the slope of the current as positive. In the same way, during the turn-off time of switch S (S-off, D-on) following equations are validated VL = VS − VC

(7)

V S − VC dIL = dt L

(8)

IC = IL −

VC R

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where VC is equal to the output voltage. Equation (5) shows the slope of the current as negative because the output voltage is bigger than rectified source voltage (VO > VS ). The following equation determining the characteristic of inductance current is found by using Equations (4) and (8); VS VC dIL = − (1− ␤) dt L L

(11)

where ␤ ∈ {0,1} and when the switch is turned-on (␤ = 1), Equation (11) is equal to Equation (4). At the other state of switch (␤ = 0), it is equal to Equation (8). If the same operation is performed for Equations (6) and (10), Equation (12) determining the characteristic of capacitance voltage is found IL VC dVC = (1− ␤) − dt C RC

(12)

If Equations (11) and (12) are written with the matrix form, the state space equation of the circuit is like the following equation


=

0 1−␤ C

1−␤ L 1 RC

IL (t) + VC (t)

1 L

0

VS

(13)

Buck–Boost Converter The buck–boost converter regulates the average output DC voltage both lower and higher than the input voltage. The circuit diagram of the buck–boost converter is shown in Figure 4. In Equation (14) the current and voltage state equations of buck–boost converter at matrix form are given

(9)

(10)


=

0 1−␤ C

Figure 4 Circuit diagram of buck–boost converter.

− 1−␤ L 1 − RC

IL (t) + VC (t)

␤ L

0

VS

(14)

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Figure 5 Circuit diagram of Cúk converter.

´ Converter Cuk Similar to the buck–boost converter, the Cúk converter provides an output voltage less than or greater than the input voltage, but the output voltage polarity is opposite to that of the input voltage. It is named after its inventor. The circuit diagram of the buck–boost converter is shown in Figure 5.

CONTROL OF DC–DC CONVERTERS A DC–DC converter must provide a regulated DC output voltage under load variation and input voltage fluctuation. In addition, the converter component values can also change with time due to temperature, pressure and etc. Hence, the control of the output voltage should be performed in a closed-loop manner by using negative feedback. The two most common closed-loop control methods for PWM DC–DC converters are the voltage-mode control and the current-mode control.

Voltage-Mode Control Voltage-mode control is probably the most common way to control a power supply. In essence, an error voltage obtained from the difference between a reference voltage and a portion of the output voltage is permanently compared to a fixed frequency and amplitude sawtooth. The crossing point between these two signals generates a transition on the comparator’s output. When the output voltage deviates from its natural target, the error voltage e (error) increases. In DC–DC converters, the voltage-mode control scheme is shown in Figure 6. Here, the converter output voltage is sensed and subtracted from a reference voltage. The error amplifier produces a control voltage that is compared to a sawtooth waveform. The

Figure 6

comparator produces a Pulse Width Modulation (PWM) signal fed to drivers of controllable switches in the DC–DC converter. The duty ratio of the PWM signal depends on the value of the control voltage. The frequency of the PWM signal is the same as the frequency of the sawtooth waveform. An important advantage of the voltage-mode control is in its simple hardware implementation and flexibility [19]. In order to understand the voltage-mode control of DC–DC converters, a simulation of boost converter by using MATLAB was prepared. The main Simulink/MATLAB block of the simulation is shown in Figure 7. At the main block, power circuit of boost converter and voltage-mode controller are constituted and the subblocks of these can be seen clearly in Figure 8. The output voltage and the inductance current can be seen in Figure 9. Inductance current value is changing between 5 and 20 A. The parameters of power circuit components are VS = 311 V, VO = 400 V, L = 4.4 mH, C = 1,500 ␮F, and R = 40 , respectively. The voltage PI controller parameters are Kp = 0.0003 and Ki = 0.02. The advantages of voltage-mode control are [20]: (1) A single feedback loop is easier to design and analyze. (2) A large-amplitude ramp waveform provides good noise margin for a stable modulation process. (3) A low impedance power output provides better crossregulation for multiple output supplies.

Voltage-mode’s disadvantages can be listed as: (1) Any change in line or load must first be sensed as an output change and then corrected by the feedback loop. This usually means slow response. (2) The output filter adds two poles to the control loop requiring either a dominant-pole low frequency roll-off at the error amplifier or an added zero in the compensation.

Voltage-mode control scheme.


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Figure 7 Simulation blocks of voltage-mode control with MATLAB.

Figure 8

Control and power circuit blocks of simulation.

(3) Compensation is further complicated by the fact that the loop gain varies with input voltage.

Current-Mode Control The output current in PWM DC–DC converters is either equal to the average value of the output inductor current or is a product of an average inductor current and a function of the duty ratio. In practical implementations of the current-mode control, it is feasible to

sense the peak inductor current instead of the average value. As the peak inductor current is equal to the peak switch current, the latter can be used in the inner loop, which often simplifies the current sensor. The current-mode control scheme is also presented in Figure 10. Note that the peak inductor (switch) current is proportional to the input voltage. Hence, the inner loop of the current-mode control naturally accomplishes the input voltage-feedforward technique. The inner control loop feedbacks an inductor current signal. After then, a voltage converted from this current signal is compared to the

Figure 9 Inductance current and output voltage of boost converter controlled with voltage-mode in steady state.

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Figure 10

Current-mode control scheme.

control voltage. This modification of replacing the triangular waveform of the voltage-mode control scheme by a converter current signal significantly alters the dynamic behavior of the converter which takes on some characteristics of a current source. Among several current-mode control versions, the most popular is the constant-frequency one that requires a clock signal. Advantages of the current-mode control are the input voltage feedforward, the limit on the peak switch current, the equal current sharing in modular converters and the reduction in the converter dynamic order. The main disadvantage of the current-mode control is its complicated hardware, which includes a need to compensate the control voltage [19]. In order to understand the current-mode control of DC–DC converters, a simulation of boost converter by using MATLAB was prepared. The main Simulink/MATLAB block of the simulation is shown in Figure 11. At the main block, power circuit of boost converter and current-mode controller are constituted and the subblocks of these can be seen clearly in Figure 12.

Figure 11

The output voltage and the inductance current for currentmode control can be seen in Figure 13. Inductance current value is nearly constant by 14 A. If current-mode control is compared with voltage-mode control, it can seen clearly that the inductance current and output voltage is nearly constant. The signals have very small ripples. The parameters of power circuit components are VS = 311 V, VO = 400 V, L = 4.4 mH, C = 1,500 ␮F, and R = 40 , respectively. The voltage PI controller parameters are Kp = 2, Ki = 20 and current PI controller parameters are Kp = 50, Ki = 100. The advantages which the control technique offers include the following [20]: (1) Since inductor current rises with a slope determined by VS –VO , this waveform will respond immediately to line voltage changes, eliminating both the delayed response and gain variation with changes in input voltage.

Simulation blocks of current-mode control with MATLAB.

Figure 12

Control and power circuit blocks of simulation.


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Figure 13 Inductance current and output voltage of boost converter controlled with current-mode in steady state.

(2) Since the error amplifier is now used to command an output current rather than voltage, the effect of the output inductor is minimized and the filter now offers only a single pole to the feedback loop. This allows both simpler compensation and a higher gain bandwidth over a comparable voltage-mode circuit. (3) Additional benefits with current-mode circuits include inherent pulse-by-pulse current limiting by merely clamping the command from the error amplifier, and the ease of providing load sharing when multiple power units are paralleled.

While the improvements offered by current mode are impressive, this technology also comes with its own unique set of problems which must be solved in the design process. A list of some of these is outlined below:

Figure 14

(1) There are now two feedback loops, making circuit analysis more difficult. (2) The control loop becomes unstable at duty cycles above 50% unless slope compensation is added. (3) Since the control modulation is based on a signal derived from output current, resonances in the power stage can insert noise into the control loop. (4) A particularly troublesome noise source is the leading edge current spike typically caused by transformer winding capacitance and output rectifier recovery current. (5) With the control loop forcing a current drive, load regulation is worse and coupled inductors are required to get acceptable cross regulation with multiple outputs.

Power electronic loads especially front-end rectifiers; inject harmonics to the alternating current grid. Thus, DC–DC converters

Input current of boost converter without PFC.

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Figure 15

Harmonic contents of input current without PFC.

are used as intermediate stages, just after a rectifier and before the load, for shaping the input ac current to improve PF and decrease the harmonic content. The boost converter is especially popular in such PFC applications.

POWER FACTOR CORRECTION AND APPLICATIONS Power factor is a relation between active power and apparent power. At the same time, PF is also known as the phase difference between sinusoidal input voltage and input current. PF is a number between 0 and 1 showing the rate of reactive power to active power. The active power of traditional ac circuits and PF expression is shown in Equation (15). Because of the non-sinusoidal input current at rectifier circuits, PF is defined as,

PF = displacement factor × distortion factor

(15)

PF =

VI1 cos ␾ I1 = cos ␾ VI I

(16)

where I is the rms value of input current; I1 is the rms value of input current’s fundamental harmonic, and V is the input voltage of converter, respectively. Traditional power supplies draw pulsating ac line current, resulting in low power factor and high rms line current. For singlephase electronics applications, passive power filters, active one and two stage PFC rectifiers are typical approaches used to achieve high PF and low total harmonic distortion (THD). High harmonic contents at the input current causes stress for the power wiring, circuit breakers, and electric utility. In addition, these harmonics may effect other electronic equipment connected to the same power line. The DC–DC converter systems without PFC have nearly between 0.6 and 0.7 PF values. Because of the low PF values at the rectifier circuits, some active and passive methods have been developed and the generally named these methods as PFC [16,18,21–24]. For the active methods, single-stage and two-stage PFC approaches

Figure 16 (a) Experimental setup of boost converter. (b) Input current of boost converter without PFC.


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Table 1 EN 61000-3-2 Classifications of Equipment With Limited Harmonic Content Original EN 61000-3-2 classification

Amendment A14 classification

Class A

Balanced three-phase equipment, and all other equipment, except that stated in one of the following classes

Class B

Portable tools, arc welding equipment which is not professional equipment Lighting equipment PC, PC monitors, radio or TV receivers. Input power P ≤ 600 W (not class B or C, single phase, not motor driven and possessing special wave shape)

Balanced three-phase equipment; household appliances excluding equipment identified as Class D; tools (except portables), dimmers for incandescent lamps (but not other lighting equipment), audio equipment, anything not otherwise classified No changes

Class C Class D

can be used. Active PFC that reshapes the input current before its application to the switch-mode supply solves this problem. As a result of a full-wave rectifier followed by a capacitor, switched mode power supplies present nonlinear impedance to the ac grid. The capacitor maintains a voltage of approximately the peak voltage of the input sine wave until the next peak comes along to recharge it. In this case, current is drawn from the input only at the peaks of the input waveform, and this pulse of current must contain enough energy to sustain the load until the next peak. This can be seen at Figure 14. Input current without PFC and its FFT analysis are shown in Figures 14 and 15, respectively. As can be seen in Figure 15, PF value is nearly 0,695. Here, the system parameters of the PFC circuit are as follows: R = 330 , L = 4.4 mH, C = 1,500 ␮F, VS = |311Sinwt| V, VO = 400 V, and fS = 10 kHz. Figure 16a shows the experimental setup of boost converter including voltage and current measurement modules, calibration circuit, driver circuit, EMI filter circuits and isolated DC sources. Experimental results related to input current and output voltage of boost converter circuit without PFC are presented in Figure 16b. PFC has become important since the European Union established the EN 61000-3-2 Standard and Amendment A14 for electronic equipment. This standard limits allowable ac line current harmonics. Limits depend on the input power, type of product and specific harmonic. See Table 1 for a list of the original equipment classifications and the Amendment A14 classifications [25,26].

Figure 17

All lighting equipment except incandescent lamp dimmers PC’s computer monitors and television receivers between 75 and 600 W

Current Control Techniques for PFC Many PFCs based on the boost topology have been proposed in the literature. Various control strategies have also been implemented. In the following, the most popular control techniques are reviewed. • • • • •

Peak current control. Hysteresis control. Borderline control. Discontinuous PWM control. Average current control.

Power Factor Correction Application for Boost Converter Many DC–DC converter circuits and topologies are used at the PFC applications, but boost converter is widely used as active PFC pre-regulator. These PFC applications generally have two control loops. The first loop of these is a voltage control loop for regulated output voltage (outer loop). The second loop is also a current control loop for high PF value and active shaping the input current (inner loop) [27]. This structure is named as average current mode. A PFC application with boost converter is shown in Figure 17. In basic form, a switch controls the boost converter circuit. When the switch is closed, current flows through the inductance. After then, the switch is opened and the current is forced to flow through the diode to the output. Multiple cycles of this switching

Boost converter circuit with PFC.

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Figure 18

Main simulation screen of PFC application for boost converter.

cause the output capacitor voltage to build due to the charge it stores from the inductor current. The result is a higher output voltage than the source voltage. Generally, voltage control of boost converter can be fulfilled via traditional control methods such as PI controller. The advantages of average current-mode control: • constant switching frequency, • no need of compensation ramp, • control is less sensitive to commutation noises, due to current filtering, • better input current waveforms than for the peak current control since, near the zero crossing of the line voltage, the duty cycle is close to one, so reducing the dead angle in the input current. In the following, disadvantages of average current-mode control can be written as • inductor current must be sensed, • a current error amplifier is needed and its compensation network design must take into account the different converter operating

Figure 19

points during the line cycle [28].

When operating PFC or active current shaping, it is wanted to be the same phase of current and voltage and minimum total harmonic distortion at source current. Because of low losses and filtering of the high level harmonics from inductance, input and output powers can be accepted equal (Pi = PO ) and following equations are written

V2 1 (Vm Im ) = O 2 RY

(17)

ILref = |Im sinwt|

(18)

Simulation blocks of PFC control with MATLAB.

Figure 20

Boost converter power circuit of simulation.


Figure 21 The waveform of input current with PFC.

Figure 22

Figure 23

The harmonic spectra of input current with PFC.

Input currents with–without PFC and source voltage of boost converter.

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Figure 24

Output voltage of boost converter under load variations.

When Im is obtained from Equation (17) and substituted in Equation (18), reference current is found by the following equation

2VO2 ILref = sinwt Vm RY

Figure 25

Inductor reference current of the boost converter depends on output and input voltages of the circuit. For the PFC, firstly a reference is calculated for the source current at control operation. For this, output voltage controller is compared with a sample of

(19)

Input current of the converter under load variations.


Figure 26

Figure 27

Cúk converter circuit with PFC.

Main simulation screen of PFC application for Cúk converter.

wave rectified voltage signal and written as Equation (21)

source voltage. It can be obtained from Equation (20)

VOref − VO )(Kp v +

Ki v s

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= |IL |

ILref = |IL | · |sin 314t|

(ILref − IL ) Kp i + (20)

(21)

After then, output of the voltage controller providing the amplitude of the reference current is multiplied with the sample of the full-

Figure 28

Ki s

i

=d

(22)

Obtained current reference is compared with the actual value of the current and founded error from this comparison is used as input of the current controller and duty cycle is founded as Equation (22). At last, the output of the current controller is compared with a carrier signal and the gate signal of the switch is found. Generally, the current and voltage can be in phase in spite of the severe distortion of the current waveform. Applying the

Simulation blocks of PFC control with MATLAB.

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Figure 29

Main simulation screen of PFC application for Cúk converter.

Figure 30

The waveform of input current with PFC.

“cosine of the phase angle” definition would lead to the erroneous conclusion that this power supply has a power factor of 1.0. Boost converter is comprised of DC input voltage source VS , boost inductor L, controlled switch S, diode D, filter capacitor C and load resistance RY . When the switch S is in the on state, the current of the boost inductor linearly increases. The diode D is off at the time. When the switch S is turned off, the energy stored in the inductor is released through the diode to the input RC circuit.

Figure 31

Using the Faraday’s law for the boost inductor VS DT = (VO − VS )(1 − D)T

(23)

From which the DC voltage transfer function turns out to be MV =

The harmonic spectra of input current with PFC.

VO 1 = VS 1−D

(24)


Figure 32

The main GUI screen of the educational tool.

As the name of the converter suggests, the output voltage is always greater than the input voltage. The boost converter operates in the CCM for L > Lb where Lb =

(1 − D)2 DRY 2fs

(25)

For D = 0.223, RY = 330 , and fS = 10 kHz, the boundary value of the inductance is Lb = 2.2 mH. The peak-to-peak ripple current can be found 3.15 A by using Equation (26) I =

VS D fs L

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(26)

But wanted I value of this circuit is nearly a half of founded value. If the L is chosen 4.4 mH, I is been nearly 1.57 A. Boost converter and PFC controller was simulated and MATLAB main screen of this simulation is shown in Figure 18. VS =|311Sin314t|V, VO = 400 V, L = 4.4 mH, C = 1500 ␮F, and RY = 330 are selected in this simulation. In main screen of this simulation, PFC controller and boost converter power circuit was prepared as two parts. In Figure 19, PFC controller part of the simulation and in Figure 20, power circuit of boost converter is shown. Waveform of the input current shown in Figure 21 is obtained with active PFC that is the boost converter placed between the

input rectifier and the output capacitor. Here, the boost converter controlled for shaping the input current to match the input voltage waveform. In this simulation, PF is approximately 0.99 and THD is 3.51%. As illustrated in Figure 22, the magnitude of the current is nearly 4.525 A at fundamental frequency (50 Hz). And other current harmonic contents are also shown in this figure. If fundamental current is compared with the other harmonics, maximum 10% value of harmonics can be seen. The biggest harmonic except for the fundamental component is the third harmonic which is nearly 0.1 A. In Figure 23, scaled (1/15) input voltage and input current with and without PFC are shown on same graphics. As can be seen in figure, the input current with PFC and input voltage are same phase and the current is nearly sinusoidal. The dynamic performance of the proposed PFC circuit was tested under load variations. The state of output voltage can be seen under load variations in Figure 24. Firstly, simulation was run between 0 and 2 s at 330 , secondly between 2 and 3 s at 250 and after then between 3 and 5 s at 200 . It is zoomed that the variations and transitions of the output voltage can be easily seen in this figure. When the simulation was performed under load variations, the input current of the circuit shown the variations and transitions which are seen in Figure 25. The figure is examined

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Figure 33

The Sub-GUI screen of the boost converter and PFC application.


Figure 34 The sub-GUI screen of the boost converter when the start with PFC button is clicked.

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Figure 35

The sub-GUI screen of the Cúk converter when the start with PFC button is clicked.


Figure 36

131

The screens of MATLAB powergui for boost converter and FFT analysis tool.

with dividing it to four sections. Between 0 and 2 s is shown as section 1, 2 and 3 s is shown as section 2, 3–5 s is shown as section 3 and the transition and variation of the current is shown in section 4. At section 1, load RY = 330 and THD value is 3.68%, at section 2, RY = 250 —THD = 2.44% and at section 3, RY = 200 —THD = 2.20%. The zoomed figures of the sections presented under the current signal in Figure 25. It can be seen by the help of section 4 that the current have not any deterioration at transition.

´ Converter Power Factor Correction Application for Cuk The Cúk converter is a switched-mode power supply named after the inventor Dr. Slobodan Cúk. The basic non-isolated Cúk converter and PFC application shown in Figure 26 is designed based on the principle of using two buck–boost converters to provide an inverted DC output voltage. The advantage of the basic nonisolated Cúk converter over the standard buck–boost converter is to provide regulated DC output voltage at higher efficiency with identical components due to an integrated magnetic structure, reduced ripple currents, and reduced switching losses [11]. In the first state when the power switch is off, the inductor currents flow through the diode and energy is stored in the transfer capacitor from the input and the inductor L1 . The energy stored in the inductor L2 is transferred to the output. As a result, both of the inductor currents are linearly decreasing in the off-state. In the

second state when the power switch is on, the inductor currents flow through the transistor and the transfer capacitor discharges while energy is stored in the inductor L1 . As the transfer capacitor discharges through the switch, energy is stored in the inductor L2 . Consequently, both of the inductor currents are linearly increasing in the on-state. The voltage and current ratio for the non-isolated Cúk converter can be derived by assuming the inductor currents, which correspond to the input current and output current, are ripple-free. This results in an equal charging and discharging of the transfer capacitor during the off-state and the on-state. The charging and discharging are defined in Equation (27) in terms of the product of current and time. IL1 toff = IL2 ton

(27)

The resulting current ratio is expressed in Equation (28) by substituting IL1 = Is , IL2 = Io , toff = (1 − D)Ts and ton = DTs into Equation (27) Io 1−D = IS D

(28)

If the input power is equal to the output power for the ideal case, the voltage ratio in Equation (29) is determined as the inverse of

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Figure 37

The screens of MATLAB powergui for Cúk converter and FFT analysis tool.

THE EDUCATIONAL TOOL FOR DC–DC CONVERTERS AND PFC APPLICATIONS

the current ratio using the analysis of an ideal transformer Vo D =− VS 1−D

(29)

Cúk converter and PFC controller was simulated like boost converter and MATLAB main screen of this simulation is shown in Figure 27. VS = |311Sin314t| V, VO = 400 V, L1 = 4.4 mH, L2 = 350 ␮H, C1 = 2.5 ␮F, C2 = 1,500 ␮F, and RY = 330 are selected in this simulation. In main screen of this simulation, PFC controller and Cúk converter power circuit was prepared as two parts. In Figure 28, PFC controller part of the simulation and in Figure 29, power circuit of Cúk converter is shown. Waveform of the input current shown in Figure 30 is obtained with active PFC that is the Cúk converter placed between the input rectifier and the output capacitor. Here, the Cúk converter controlled for shaping the input current to match the input voltage waveform. In this study, PF is approximately 0.99 and THD is 3.27%. As illustrated at the Figure 31, the magnitude of the current is nearly 4.407 A at fundamental frequency (50 Hz). And other current harmonic contents are also shown in this figure. If fundamental current is compared with the other harmonics, maximum 10% value of harmonics can be seen. The biggest harmonic except for the fundamental component is the third harmonic which is nearly 0.1 A.

An educational tool is presented for the teaching of DC–DC converters and PFC applications by using MATLAB environment. MATLAB is one of the software languages with many useful toolboxes that assist a wide-range of fields in engineering including control, analysis and design. Compared with C or Fortran languages, MATLAB is easier to understand and use in integrates computation, visualization and programming. In MATLAB version 2008a, there is an aided tool for constructing GUIs called GUIDE (Graphical User Interface Design Environment). Instead of writing MATLAB codes in an M-file editor, GUIs can easily be generated in a simple manner. The educational tool has nine GUI pages; main GUI page and the sub-GUIs containing converter circuits. At the left side of main GUI page, name of the GUI, university, department and the designers names are placed that is shown in Figure 32. The right side of main GUI page is including an explanatory info and simulations links of the converter circuits. If the students click any converter link, a sub-GUI page of the selected converter will come to the screen as shown in Figure 33. The sub-GUI given in the Figure 33 can be investigated with four parts. These are menu bar, information part, parameters part and graphics part. Open, save, print, zoom in, zoom out, back to main GUI and extra information (when clicked, an


Table 2 The Buttons of Menu Bar Open button

Save button

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was prepared by using MATLAB. The students can use this GUI at their projects about DC–DC converter circuits as a simulator. In addition, they can find a chance to compare theoretical instruction with simulation results and find the similarities and differences. The designed GUI is also suitable in analysis and simulations of all of the DC–DC topologies.

Print button

ACKNOWLEDGMENTS Zoom in–zoom out buttons (for graphics)

Back button (to main GUI)

This work is funded by FUBAP with project titled “FUBAP-1580 Design and implementation of converter circuits with unity power factor which using sliding mode-fuzzy and space vector pulse width modulation techniques”.

More info button (adobe PDF file about subject)

REFERENCES adobe .PDF book is opened) respectively placed at the menu bar (Table 2). When students open any sub-GUI, they firstly revise their learning about the subject being studied with the help of information part. At the same time, circuit diagram can aid the students about the operation of converter because polarities of the voltages and current flows are shown. Student can select the circuit parameters by using the PDF file before starting the simulation. The parameters must be entered at the required places of the sub-GUI before operating. Then, the parameters are entered and the start with PFC button placed at left part is clicked, the screen of the sub-GUI is seen like at Figure 34 for boost converter and Figure 35 for Cúk converter. At these figures, output voltage of the boost and Cúk converters are shown at the top graphic and the input currents are also shown at the bottom graphic with zoom in steady state. When FFT button placed at the bottom of the sub-GUI is clicked, the powergui toolbox of the MATLAB is opened. The screenshot of this toolbox for boost and Cúk converter is shown in Figures 36 and 37, respectively. The powergui toolbox parameters are placed at the left side of this screenshot. This toolbox has many properties such as steady-state voltages and currents, FFT Analysis, Hysteresis Design Tool, etc. In order to understand better of the PFC application, FFT Analysis is necessary for the input current. The FFT Analysis toolbox has six parts and these parts are menu bar, signal to analyze, FFT analysis, available signals, FFT window and FFT settings. If this toolbox is wanted to use, at first step the signal can be chosen from the available signals place at the GUI. At second step, FFT window and FFT settings menus must be set. At last step, display button is clicked and then FFT analysis of the selected signal is shown.

CONCLUSION In this paper, an educational tool for fundamental DC–DC converters and PFC applications within MATLAB environment is presented. This tool provides effective and perpetual training for power electronic courses in classroom environment. The proposed tool has flexible structure and graphical interface, and enables users to change converter and controller parameters easily for different conditions. With this aim, a GUI

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BIOGRAPHIES Korhan Kayisli received the BSc degree in electronics education from Sakarya University, Sakarya, Turkey, in 2001, and the MSc degree in Electronics and Computer Science from Firat University, Elazig, Turkey, in 2004. He is a PhD student in the area of power electronics in electric and electronics engineering at Firat University. He is currently a research assistant in the Department of Electronics and Computer Science, Technical Education Faculty, Firat University, Elazig, Turkey. His main research interest is in power electronics and robust control. Servet Tuncer received the BSc and MSc degrees in electrical and electronic engineering from Firat University, Elazig, Turkey, in 1993 and 1999, respectively, and the PhD degree in the area of power electronics at the same University, in 2004. He is currently an assistant professor in the Department of Electronics and Computer Science, Technical Education Faculty, Firat University, Elazig, Turkey. His main research interest is in power electronics, electrical machines and drives, intelligent control and digital signal processing. Mustafa Poyraz received the BSc and MSc degrees in electrical and electronic engineering from Karadeniz Technical University, Trabzon, Turkey, in 1974 and 1976, respectively, and the PhD degree in the area of circuits and systems at Firat University, in 1981. He is currently a professor in the Department of Electric and Electronics Engineering, Engineering Faculty, Firat University, Elazig, Turkey. His main research interest is in circuit and systems, digital control.