A Novel ZVS-PWM-MR Boost Converter

1. Overview

Small size and light weight are the current goals of switching power supply products. The main means to achieve this goal is to increase the switching frequency. However, the increase in switching frequency will lead to increased switching losses and serious electromagnetic interference (EMI), which limit the further increase of switching frequency. The emergence of soft switching technology can solve these problems well, but the development of soft switching technology is not perfect yet. There are many problems, such as the realization of soft switching of all switches, the reduction of voltage and current stress, and the realization of soft switching in a wider input and load range. There is still room for further development [1,3].

This paper proposes a soft switching implementation method for Boost converter, which realizes the soft switching of all switches, which can not only reduce switching losses, but also significantly reduce electromagnetic interference (EMI). At the same time, it has the same voltage and current stress as the Boost hard switching converter, and can realize soft switching in the input and load regulation range that is approximately the same as the Boost hard switching converter.

2. Working Principle

The schematic diagram of the converter is shown in Figure 1, where D1, D2 and C1, C2 are the parasitic diodes and parasitic capacitors of the switch tubes S1 and S2 respectively. On the basis of the traditional hard-switching Boost converter, the switch tube S2, diodes D3, D4 and inductor L are added, which together with C1 and C2 form a resonant network.

Figure 1 Converter circuit schematic
For the convenience of analysis, assuming that Lf and Cf are large enough, the input can be equivalent to a current source and the output can be equivalent to a voltage source.

Figure 2 Main working waveforms of the converter

(a) Before t0, S1 is turned off, S2 is turned on, and the inductor L continues to flow through S2 and D3 to maintain energy. The equivalent circuit diagram is shown in Figure 3 (g). At t0, S2 is turned off, and the energy stored in the inductor L charges C2. The voltage on C2 continues to rise until it reaches V0. After that, D4 is naturally turned on and Vc2 is clamped to V0. In this stage, S2 is turned off with zero voltage and D4 is turned on with zero voltage.

(b) In this stage, L continues to discharge through D4, the load and D3. At t2, the inductor current drops to zero, and D3 and D4 are naturally turned off. In this stage, D3 and D4 are turned off with zero current. In stages (a) and (b), the input current all flows through the rectifier diode Do.

(c) After the inductor current drops to zero, L resonates with C1 and C2, and the inductor current changes direction. At this time, the current through the rectifier diode Do will be the sum of the input current Ii and the resonant current IL, and its peak value is IDomax=Ii+Vo/Ze, where . This is the only overshoot in the entire cycle, but due to the small value of the parasitic capacitance, it can be made much smaller than L by taking an appropriate value, so that IDomax≈Ii, which is the same as the corresponding value of the traditional hard-switching converter.

(d) Through the multi-resonance of the previous stage, the energy stored on C1 and C2 is discharged. At t3, the voltage on C1 and C2, that is, the voltage between the drain and source of the switch tubes S1 and S2, drops to zero. Therefore, S1 and S2 can be turned on under ZVS conditions at this time. Due to the existence of L, the current through S1 and S2 increases linearly, so S1 and S2 are also in a zero current state when they are turned on. At the same time, the current through Do decreases linearly. At t4, IL rises to I0, IDo drops to zero, and Do is naturally turned off.

(e) In this stage, the input current flows entirely through the switch tubes S1 and S2, which is similar to the boost inductor energy storage stage in the traditional hard-switching Boost converter. The implementation of PWM control is also achieved by adjusting the length of this stage.

(f) When S1 is turned off at t5, its parasitic capacitance is charged and the voltage rises linearly, so S1 is turned off under ZVS. At the same time, the voltage across Do decreases linearly. At t6, VC1 rises to Vo, and VDo drops to zero, so Do is turned on under ZVS conditions.

(g) This stage is similar to the stage in which the boost inductor releases energy to the load in the traditional hard-switched Boost converter. All input current flows to the load side through the rectifier diode Do. Unlike the traditional hard-switched converter, the resonant inductor L forms a freewheeling path through S2 and D3 to maintain energy. At t7, S2 is turned off, completing a cycle.

3. Soft switching implementation conditions

1) Time conditions

From the analysis of the above working principle, it can be easily obtained that the time condition for realizing soft switching is: the turn-off time of S2 should satisfy , the opening time of S1 should meet It should also be noted that due to the small value of the resonant capacitor, the resonant phase accounts for a very small proportion of the entire switching cycle. When the resonant phase is ignored, the output-input regulation ratio of the converter is ,in , which is the same as the traditional hard-switched Boost converter, so this converter has the same input and load regulation range as the traditional hard-switched Boost converter.

2) Energy condition

The energy condition for this converter to achieve soft switching is that at the end of the [t0-t1] time period, C2 should be able to charge to Vo. Since the parasitic capacitance of the switch tube is used as the resonant capacitor, for a specific converter, the values ​​of C1 and C2 are determined, and what needs to be selected is the value of the resonant inductor L. In the [t0-t1] stage, there is and initial conditions

4. Simulation verification

In order to verify the working condition of this soft switching converter, a Boost converter with a switching frequency of 100 kHz and a power of 50 W is designed. The voltage and current waveforms of each switch tube are shown in Figure 4. It can be seen that all the switch tubes have achieved soft switching, so the waveforms are relatively clean and there is basically no switching noise, which also means lower electromagnetic interference (EMI).

5. Conclusion

This paper proposes a new type of ZVS-PWM Boost converter, analyzes its working principle in detail, and verifies it. The results show that it can realize soft switching of all switch tubes, effectively reduce electromagnetic interference, and has lower voltage and current stress. References


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1] BP Divakar and Ioinovici, A. PWM converter with low stress and zero capacitive turn-on losses. IEEE Trans on Aerospace and Electronic System, vol. 33, No. 3. pp. 913-920, July 1997.
[2] Yungtaek Jang and Milan M. Jovanovic. A new, soft-switched, high-power-factor Boost converter with IGBTs. IEEE Trans on Power Electronics, vol. 17, No. 4. pp. 469-476, July 2002.
[3] Ruan Xinbo, Yan Yangguang. Soft switching technology of DC switching power supply. Beijing: Science Press