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Soft-Switching Interleaved Boost Converter with High Voltage Gain

R.N.A.L. Silva, G.A.L. Henn, P.P. Praça, L.H.S.C. Barreto, D.S. Oliveira Jr., F.L.M. Antunes Federal University of Ceará/Department of Electrical Engineering/Energy and Control Processing Group - GPEC,

Fortaleza-Ce, Brazil

Abstract— In this paper a soft-switching interleaved boost converter with high voltage gain is presented. The high voltage gain converter is far suitable for applications where a high step-up voltage is required, as in some renewable energy systems, which use, for example, photovoltaic panels and/or fuel cells. Besides, in order to guarantee small switching losses and, consequently, a high efficiency, a non-dissipative soft-switching cell with auxiliary commutation circuit is used. Thus, a large step-up voltage, low switching stress, small switching losses, and high efficiency are expected from this topology. In order to verify its effectiveness, experimental waveforms from the high voltage gain converter operating with hard-switching and soft-switching are compared. Also, waveforms from the soft-switching cell are presented and analyzed. Theoretical analysis, operation principle and topology details are also presented and studied.

I. INTRODUCTION

Renewable energy systems are being more and more common to help providing electric energy. However, conventional photovoltaic panels and fuel cells produce low voltage levels, requiring a large step-up voltage DC/DC converter to feed conventional 110 VRMS AC systems. The conventional boost converter is not suitable for this purpose because, in order to obtain such high gain, it would operate with duty cycle greater than 0.95, which is very hard to achieve due to operational limitations.

To solve this drawback, some topologies were suggested, as in [1-13]. In [3] and [4], the use of an interleaved boost converter associated with an isolated transformer was introduced, using a high frequency AC link. Despite of the good performance, this topology uses three magnetic cores. In [5], the converter presents low input current ripple and low voltage stress across the switches. However, high current flows through the series capacitors at high power levels. In [6-8], converters with high static gain based on the boost-flyback topology are introduced, which presents low voltage stress across the switches, but the input current is pulsed, as it needs an LC input filter. The step-up switching-mode converter with high voltage gain using a switched-capacitor circuit was proposed in [9]. This idea is only adequate for low power converters as it results in a high voltage stress across the switches and many capacitors are necessary. In [10-12] the three-state switching cell is shown. In [12] a voltage doubler rectifier is employed as the output stage of an interleaved boost converter with coupled inductors.

The converter presented in [13] has some advantages compared to the others: possibility to operate in large voltage range, high efficiency, and high power capability.

Figure 1 shows the high voltage gain boost converter from [13]. It can be seen from Figure 1 that the number of semiconductor devices is the same as in the traditional interleaved boost arrangement, though two coupled inductors L1 and L2 are added, resulting in higher output voltage. The main drawback of this topology is the hard switching mode, which causes power losses. Because of this drawback, it is important to analyze the most suitable soft-switching cell to reduce the power losses in both switches S1 and S2 [14-20].

The soft-switching cell proposed in [20] presents some advantages compared to the others: it is non-dissipative, uses auxiliary commutation circuits, and do not present load limitations, as it can deal with high nominal power systems. Thus, the topology presented in Figure 1 will be adapted to work with a non-dissipative snubber, as seen in Figure 2. Its operation and main waveforms will be detailed and discussed in this paper.

Figure 1. High voltage gain boost topology

Figure 2. Converter with soft-switching cells

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II. OPERATION PRINCIPLE

This section presents the operation principle from the high voltage gain boost converter with the commutation cells. For the theoretical analysis, it will be considered that the input voltage (Vi) and output current (Io) are ripple free and all devices are ideal.

First Stage [t0 – t1] - Prior to the first stage S1 and D3were turned-on, Lb1 is charged, and VCr1 and VCr2 are equal to zero. This stage (Figure 3) begins when Sa2 and Dr2 are turned-on in ZCS mode. During this stage, the resonant current through the inductor (ILr2) increases linearly from zero to the input current ILb2. This stage ends when ILr2 = ILb2.

Second Stage [t1 – t2] - During this stage (Figure 4), ILr2 and the resonant capacitors Cr3 and Cr4 starts to resonate, discharging Cr3 and charging Cr4. This stage ends when VCr3 reaches zero.

Third Stage [t2 – t3] - During this stage (Figure 5), ILr2and Cr4 resonate. This stage ends when ILr2 reaches zero. At this stage, S2 is turned-on in ZVS mode.

Fourth Stage [t3 – t4] - After ILr2 reaches zero, the auxiliary switch Sa2 is turned-off in ZCS mode. The switch S2 and S1 remains turned on. The energy keeps being stored in LB1, without being transferred to the load and LB2 starts to be charged. Besides, Cr4 is linearly discharged to zero by the current ILb2. As the voltage through Cr1 is zero, the switch S1 will be turned-off in ZVS mode, ending this stage.

Fifth Stage [t4 – t5] - At t4, as VCr4 is zero, the diode D4is turned-on in ZVS mode. As ILb1 cannot reach zero instantly, it will flow through Cr1, until VCr1 reaches VCf.Thus, the diode D3 turns-off in ZVS mode. This stage, represented in Figure 7, ends when Db1 turns-on in ZVS mode.

Sixth Stage [t5 – t6] - As Db1 starts to conduct, the energy previously stored in the inductor LB1 is now transferred to the capacitor CF1 through the circuit showed in Figure 8.

Figure 3. First Stage

Figure 4. Second Stage

Figure 5. Third Stage

Figure 6. Fourth Stage

Figure 7. Fifth Stage

Figure 8. Sixth Stage

Figures 9 to 14 present the next stages, which are similar to the six stages presented above, although stages 7 to 12 describe the principle operation for the boost switch S1 and for its soft-switching cell (Dr1, Cr1, Cr2, Lr1, and Sa1).

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Figure 9. Seventh Stage

Figure 10. Eighth Stage

Figure 11. Ninth Stage

Figure 12. Tenth Stage

Figure 13. Eleventh Stage

Figure 14. Twelfth Stage

III. SIMULATION AND EXPERIMENTAL RESULTS

This topic presents the simulation (figures 15 and 16), and experimental (figures 17 to 21) results from the non-dissipative snubber commutation cell applied to the high voltage gain converter.

The system has an input voltage of 28 Vdc and output voltage of 180 Vdc, making possible to feed a 110 VRMSAC system. The prototype was assembled to supply a linear 500 W load.

Figure 15 presents the voltage and the current during the turn-on period through the switch S1, while figure 16 presents the voltage and the current during the turn-on period through the auxiliary switch Sa1. It must be observed from these figures that the switch S1 start to conduct in ZVS mode, while the auxiliary switch Sa1operates in ZCS mode. The same waveforms are valid to switches S2 and Sa2. It is also important to emphasize that the same occurs during the turn-off period (ZVS for S1and S2, and ZCS for Sa1 and Sa2).

Figures 17 and 18 present, respectively, the experimental results from the input and output voltage and current. From them, it can be verified the effectiveness from the converter, which highly step-up the input voltage to the desired output voltage.

Figure 19 presents the voltage through each output capacitor, which are equilibrated. Measured Vcf1 is 52.5V, while Vcf2 is 53.9V, and Vcf is 60,3V.

Figure 20 presents the voltage and the current waveforms through switch S1. It can be observed from that figure that the main switch only starts to conduct in ZVS mode, as expected from simulation results. It is important to emphasize that the voltage stress through that switch is only Vout/3.

Figure 21 presents the voltage and the current waveforms through switch Sa1. It can be noticed that the auxiliary switch only starts to conduct in ZCS mode as expected from simulation results, which verifies the non-dissipative characteristic from the used soft-switching cell.

Figure 15. V and I through S1 during the turn-on period (simulated)

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Figure 16. V and I through Sa1 during the turn-on period (simulated)

Figure 17. Input voltage and current

Figure 18. Output voltage and current

Figure 19. Vcf1, Vcf2 e Vcf

Figure 20. V and I through S1 during the turn-on period

Figure 21. V and I through Sa1 during the turn-on period

IV. CONCLUSION

Interleaved boost converter with high voltage gain and non-dissipative soft-switching snubber cell was presented. The main advantages of the topology are: low switching losses, high efficiency, and the converter’s suitability in applications where large voltage step-up is demanded, such as renewable energy systems.

The model validation from the high voltage gain boost through experimental results is presented, as the experimental results from the non-dissipative snubber cell, verifying the expected characteristics from the converter.

ACKNOWLEDGMENT

To the FUNCAP that supports the technologic development of Ceara’s state, and to the GPEC members, for the friendship and diary support and knowledge exchange.

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