A New Isolated SEPIC Converter with Coupled Inductors for Photovoltaic Applications Ali Ghasemi , Student Member IEEE *, Ehsan Adib, Member IEEE **, and Mohammad Reza Mohammadi*** * Department of electrical and computer engineering, Isfahan University of Technology,
[email protected] **Department of electrical and computer engineering, Isfahan University of Technology,
[email protected] *** Department of electrical and computer engineering, Isfahan University of Technology,
[email protected]
Abstract: In this paper a new isolated SEPIC converter which is a proper choice for PV applications, is introduced and analyzed. The proposed converter has the advantage of high voltage gain while the switch voltage stress is same as a regular SEPIC converter. The converter operating modes are discussed and design considerations are presented. Also simulation results are illustrated which justifies the theoretical analysis. Finally the proposed converter is improved using active clamp technique.
Keywords: High step-up converter, Isolated SEPIC converter, Coupled inductors, Photovoltaic system, Active clamp. 1.
Introduction
Today, high step up converters play an important role in the industry. Applications such as Uninterrupted Power Supplies (UPS), electric traction, fuel cell systems and photovoltaic systems rely on such converters [1]-[5]. In photovoltaic applications, due to the limitations in increasing the series connected panels (because of the shadow effect and PV panel parasitic capacitance), the panels final terminal voltage cannot exceed a certain amount which is normally inadequate for an inverter unit to use as a DC link supply [6]. For this purpose, applying high step-up converters is inevitable. A boost converter is normally used for this purpose but high voltage gain along with high efficiency is hard to achieve in this converter [7]. For example, the efficiency can drop to 40% in high voltage gains [8]. Isolated types of boost converter such as push-pull, full bridge and half bridge can be used [9],[10], but these converters also require large transformer turns ratio to achieve high voltage gain. This will lead to high amount of leakage inductance, parasite capacitors of transformer, which reduce the converter effective duty cycle and efficiency.
Also these converters have a complex control system [11]. Buck-boost converters are another type of converters that can be used in this application. But these converters have a pulsating input current which is not suitable in applications such as photovoltaic that needs low amount of current noise in order to have a proper maximum power point tracking and stable operation point [12],[13]. Isolated Single Ended Primary Inductance converters (Isolated SEPIC) are also a good choice that can be used in high step-up photovoltaic applications. This type of converters has advantages such as: 1- same input and output voltage polarity, 2- low input current ripple, 3Possibility of having multiple outputs, 4- Possibility of working in both step-up and step-down modes, 5- Low amount of EMI due to low input current ripple and 6Electrical isolation between input and output [12],[14],[15],[16],[17]. However, increasing the output voltage gain in this type of converters is performed by increasing the secondary to primary ratio of the isolating transformer which has shortcomings mentioned before. In this paper, coupled inductors will be used in the simple isolated SEPIC converter to maintain a high output voltage gain without having large transformer turn ratio which will finally leads to higher operation efficiency compared to a normal isolated SEPIC converter. The paper is organized as following: First the proposed circuit topology is described in the second section. The converter operating modes during a single period are presented in the third section. Design guidelines are discussed in the fourth and fifth section. The performance and efficiency is improved using an active clamp circuit
in the sixth section. Finally the simulation results and conclusions are presented. 2.
Topology Derivation
A simple isolated SEPIC converter scheme is shown in Fig.1. The equations for voltage gain and switch voltage stress are as following.
M SEPIC =
VSW − SEPIC
nD 1− D v = in 1− D
(1) (2)
Where D is the Duty cycle of the switch and n is the ratio of the secondary winding turns to the primary (n2/n1).
3.
Operation Principles
The following assumptions have been taken into account in order to perform the analysis. 1) The converter is at the steady state condition and is operating in Continuous Current Mode (CCM). 2) The switch and the diode are assumed ideal. 3) The coupled inductors and converter main transformer used in the converter are modelled with magnetizing inductor and an ideal transformer with ratios of m and n respectively. 4) The input voltage (V1) and the dc blocking capacitor voltage (VC1) are considered constant. Similar to the normal isolated SEPIC converter, a single operation period of this converter consists of two modes. The schematic of each mode is shown in Fig.3.
Fig. 1: A simple isolated SEPIC converter schematic
The proposed idea in this paper is based on placing an auxiliary DC voltage source between the secondary winding of the transformer and the rectifying diode, which will allow the output voltage gain to be higher without additional switch voltage stress or significant change in the standard circuit. This auxiliary voltage source is implemented using an inductor coupled with the input inductor of the converter. In the proposed isolated SEPIC converter which is shown in Fig. 2, the switch voltage stress equation is similar to (2) but the output voltage gain will be
M proposed _ isolated _ SEPIC =
D (m + n) 1− D
(3)
Where m is the ratio of the coupled inductances winding turns (n4/n3) and n is the ratio of the transformer’s secondary winding turns to the primary (n2/n1). Equation (3) shows an increase in the output voltage gain by the use of mentioned coupled inductors, which makes it a better choice in high step up application in comparison to the normal isolated SEPIC converter.
Fig. 3. Operating modes of the proposed converter. (a) Mode 1.(b) Mode 2.
Mode 1 (t0 – t1) [Fig. 3 (a)] In this mode, the switch S1 is on and the output diode is reverse biased and won’t conduct any current. The input voltage is applied to the magnetizing inductor LM1 and its current will begin to increase by the following equation.
I LM 1 (t ) =
V1 (t − t 0 ) + I LM 1 (0) LM 1
(4)
Also VC1 is applied to the magnetizing inductor LM2 and its current will begin to increase by the following equation.
I LM 2 (t ) =
VC1 (t − t 0 ) + I LM 2 (0) LM 2
(5)
In this mode, the output capacitor provides the load current.
Fig. 2: Schematic of proposed isolated SEPIC converter
Mode 2 (t1 – t2) [Fig. 3 (b)] This mode starts when the switch is turned off. The energy stored in the LM1 and LM2 will be transformed to
the output through the ideal transformers and the rectifying diode. The voltage polarity on the LM1 and LM2 is reversed and their current will begin to decrease according to the following equations.
(Vswitch − V1 ) (t − t1 ) LM 1 (V − VC1 ) I LM 2 (t ) = I LM 2 (t1 ) − switch (t − t1 ) LM 2
I LM 1 (t ) = I LM 1 (t1 ) −
(6) (7)
In t2, a single operating period ends and a similar period begins. Key waveforms of the proposed SEPIC converter are shown in Fig.4. 4.
Converter Analysis
In this section, the converter operating duty cycle, input voltage, output voltage, output power and switching period are considered as D, V1, Vout, P and T respectively. represent the average value of *. By writing the voltage-second balance equation for the LM1, the switch voltage stress will be
Vswitch _ proposed _ isolated _ SEPIC =
V1 1− D
(8)
Also the voltage-second balance equation for the LM2 is as following: (9) VC1 DT = (Vswitch − VC1 )(1 − D)T
Fig.4. Key waveforms of the proposed SEPIC converter
The diode voltage stress is as
V D = Vout + ( m + n )V1
From (8) and (9)
VC1 = V1
(10)
5.
(16)
Design Guidelines
Besides, by rewriting the voltage-second balance equation for the LM2, the converter voltage gain will be
By considering the desirable current ripple of LM 1 and LM2, their values can be calculated using the following equations.
M proposed _ isolated _ SEPIC =
LM 1 =
(17)
LM 2
(18)
D ( m + n) 1− D
(11)
It can be observed that the voltage stress of the main switch is similar to regular SEPIC converter, but the voltage gain is boosted. By considering the capacitor C1 current balance, the average current of ILM2 will be obtained as
< I Lm 2 >=
n.P vout
(12)
V1 DT ΔI LM 1 V DT = C1 ΔI LM 2
Or:
LM 2 = (
Vout (1 − D ) − mDV1 )T ΔI LM 2
(19)
Also by considering the converter output power, the average input current can be calculated as
By considering the desirable voltage ripple of C1 and C2, their values can be calculated as
P 〈 I in 〉 = V1
C1 =
(13)
By using average input current, the average current of ILM1 will be
< I LM 1 >=
mP(1 − D) P + (m + n) DV1 V1
(14)
Also, the peak current through the output diode when this diode is conducting is equal to
P ID = vout (1 − D )
(15)
I LM 2 DT ΔV C 1
(20)
and
C2 = 6.
PDT Vout ΔVC2
(21)
Improving the Performance Using an Active Clamp Circuit and simulation results
In the proposed model, the transformers were considered ideal and the leakage inductances were neglected. In real operation, the leakage inductances will cause voltage spikes during switch turn off which
increases the switch voltage stress. This problem can be solved by adding an active clamp circuit to the proposed converter which will improve the performance and efficiency of the proposed converter. The circuit scheme of the proposed converter with active clamp is shown in Fig.5. In this circuit, the snubber capacitor of the main switch will keep the switch voltage near zero at turn off instant to achieve ZVS condition at turn off instant. Also, the leakage inductance energy is absorbed by the clamp capacitor. Then by turning the clamp switch on, the energy stored in the clamp capacitor will be transferred to the leakage inductance. After that, by turning the active clamp switch off, the transformer leakage inductance will discharge the snubber capacitor of S1 and ZVS condition is achieved at turn on instant. The simulated converter parameters are shown in table 1. For the proposed converter without and with the active clamp circuit, the wave forms of the main switch voltage and current are shown in Fig.6 and Fig.8. Also in Fig 7 and Fig.9, the output diode voltage and current waveforms for the proposed converter without and with active clamp circuit are shown. The efficiency of the regular converter and active clamped converter are 89% and 93% respectively.
Fig. 6: The main switch Voltage (100 volt/div) (top) and current (20 ampere/div) (bottom) waveforms – without using active clamp circuit.
Fig. 7: The output Diode Voltage (500 volt/div) (top) and current (5 ampere/div) (bottom) waveforms- without using active clamp circuit.
Fig. 5: Circuit scheme of the proposed converter with active clamp TABLE I: The values of the simulated converter parameters
Part
Value
Input voltage Vin Output voltage Vout
50V 450V
Switching frequency f
100kHz
Lm1 Lm2
500µH 500µH
n
3
m
3
C1 C2
10µF 30µF
Duty cycle
0.6
Main switch(S1) Auxiliary switch(S2)
IRFP260 IRFP260
C3
1nF
Diode Cc
D-break 10CF
ROUT
300Ω
Fig. 8: The main switch Voltage (50 volt/div) (top) and current (25 ampere/div) (bottom) waveforms – with using active clamp circuit.
Fig. 9: The output Diode Voltage (500 volt/div) (top) and current (10 ampere/div) (bottom) waveforms – with using active clamp circuit.
7. Conclusion In this paper a new isolated SEPIC converter with coupled inductors is introduced. The proposed converter will provide a high output voltage gain while keeping the switch voltage stress equal to a regular isolated SEPIC converter. An active clamp circuit is used to absorb the voltage spikes due to the leakage inductances of the transformers. Active clamp circuit also provides zero voltage switching condition for the main switch and improves the performance to a good extent. References [1] T.-F. Wu, Y.-S. Lai, J.-C. Hung, and Y.-M. Chen, “Boost converter with coupled inductors and buck-boost type of active clamp,” IEEE Trans. Ind.Electron., vol. 55, no. 1, pp. 154–162, Jan. 2008. [2] Q. Zhao and F. C. Lee, “High-efficiency, high step-up dc-dc converters,” IEEE Trans. Power Electron., vol. 18, no. 1, pp. 65– 73, Jan.2003. [3] R.-JWai and R.-Y.Duan, “High step-up converter with coupledinductor,” IEEE Trans. Power Electron., vol. 20, no. 5, pp. 1025– 1035, Sep. 2005. [4] M. Prudente, L. L. Pfitscher, G. Emmendoerfer, E. F. Romaneli, and R. Gules, “Voltage multiplier cells applied to non-isolated DC-DC converters,” IEEE Trans. Power Electron., vol. 23, no. 2, pp. 871–887, Mar. 2008. [5] E. H. Ismail, M. A. Al-Saffar, A. J. Sabzali, and A. A. Fardoun, “A family of single-switch PWM converters with high step-up conversion ratio,” IEEE Trans. Circuit Syst. I, vol. 55, no. 4, pp. 1159–1171, May 2008. [6] M. Meinhardt, G. Cramer, B. Burger, and P. Zacharias, “MultiString-Converter with Reduced Specific Costs and Enhanced Functionality” ELSEVIER SCIENCE on Solar Energy, vol 69, nos. 1-6 , pp. 217–227, 2000. [7] Ki-Bum Park, Gum-Woo Moon, and Myung-Joong Youn, “Nonisolated High Step-up Boost Converter Integrated With Sepic Converter,” IEEE Trans. Power Electron., vol. 25, no. 9, pp. 2266–2275, September. 2010. [8] Sung-Sae Lee; Seong-Wook Choi; Gun-Woo Moon, “High Efficiency Active-Clamp Forward Converter With Transient Current Build-Up (TCB) ZVS Technique,” IEEE Trans. Ind. Electron., Vol. 54, No. 1, pp. 310-318, Feb. 2007 [9] Y.-K. Lo, T.-S. Kao, and J.-Y. Lin, “Analysis and design of an interleaved active-clamping forward converter,” IEEE Trans. Ind. Electronics, Vol. 54, No. 4, pp. 2323–2332, Aug. 2007. [10] EG&G Technical Services, Inc. Science Applications International Corporation, Fuel Cell Handbook, 7th ed. Morgantown, WV: U.S. Dept. of Energy, Office of Fossil Energy, National Energy Technology Laboratory, Nov. 2004. [11] S. Inoue and H. Akagi, “A bidirectioanl isolated dc–dc converter as a core circuit of the next-generation medium-voltage power conversion system,” IEEE Trans. Power Electronics, Vol. 22, No. 2, pp. 535–542, Mar. 2007. [12] Mummadi Veerachary, “Power Tracking For Nonlinear PV Sources With Coupled Inductor SEPIC Converter,” IEEE Trans. Aerospace And Electronic Systems, Vol. 41, No. 3, pp. 1019–1029, July. 2005. [13] E. Duran, J. M. Enrique, M. A. Bohorquez, M. Sidrach-deCardona, J. E. carretero, and J. M. Andujar, “A New Application of the Coupled-Inductors SEPIC Converter to obtain I-V and P-V Curves of Photovoltaic Modules,” in Proc. 2005 IEEE Power Electronics and Applications European Conf., pp. 10pp. – p.10. [14] A. F. Witulski, “Introduction to modelling of transformliers and coupled inductors,”. IEEE Tranlsac. on Power Electronics, May 1995. [15] A. Chih-Chiang Hua, and B. Cheng-you Tsai “Design of a Wide Input Range DC/DC Converter Based on SEPIC Topology for Fuel Cell Power Conversion,” in Proc. 2010 IEEE Power Electronics Conf. (IPEC), pp. 311– 316. [16] A. Kavttha, G. Indira, G. Uma, S. Inoue and H. Akagi, “Analysis and control of Chaos in SEPIC dc – dc Converter using Sliding Mode Control,” in Proc. 2008 IEEE Industry Applications Society Annual Meeting, pp. 1–6.
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