Phase-shifted full-bridge resonant converter based on single-cycle control-EEWORLD

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0 Introduction

Resonant converters have the advantages of easy soft switching, high switching frequency, and low electromagnetic interference. However, general resonant converters generally adjust output by frequency conversion when input and load change, which brings many inconveniences to other designs. Resonant converters designed on the basis of phase-shifted full-bridge can not only achieve soft switching but also adjust output by PWM. In order to improve the dynamic response of resonant converters, an improved single-cycle control method is adopted, which has good dynamic response to sudden changes in input and load.

1 Phase-shifted full-bridge resonant converter

Phase-shifted full-bridge DC/DC converter is a converter that adopts phase-shifted control technology. The conduction time of the two switch tubes in the same bridge arm of the converter is equal, but complementary. Compared with other control methods, phase-shifted full-bridge DC/DC converter has many advantages, such as soft switching, simple structure, converter suitable for high-frequency operation, and high efficiency. Phase-shifted full-bridge resonant DC/DC converter is a resonant converter with a resonant link added on the basis of phase-shifted full-bridge.

Phase-shifted full-bridge resonant DC/DC converter is shown in Figure 1a. In the figure, S1, S2, S3, S4 are switch tubes; D1, D2, D3, D4 and C1, C2, C3, C4 are parasitic components of S1, S2, S3, S4 respectively; n1 and n2 are the turns ratios of the two secondary and primary respectively; Lr is the resonant inductor; Cr is the resonant capacitor, and the output part adopts Class-E current doubler resonant rectifier, as shown in Figure 1b.

To simplify the analysis, the following assumptions are made:

1) The filter inductor is large enough and works in the current continuous mode; 2) The transformer excitation inductance and leakage inductance are converted to the primary side; 3) The switch parasitic capacitance is a constant and does not change with the voltage; 4) All switches and diodes are ideal.

2 Working principle

Assuming the duty cycle is D and the switching period is T, the working process of the converter is divided into 8 stages. Figure 2 shows the main voltage and current waveforms.

Figure 2 Main voltage and current waveforms
[page] Mode 1 At T0, the switch tube S1 in Figure 1 is turned off. Due to the continuous flow of the resonant inductor current, the capacitor C2 is discharged, and the voltage drops linearly; C1 is charged, and the voltage rises linearly. After the capacitor C2 is discharged, the inductor current flows through the anti-parallel diode. At this time, S2 is turned on, which is a zero voltage turn-on.
Mode 2 At T0-T1, S2 and S4 are turned on at the same time, and the resonant circuit continues to resonate. Mode
3 At T1, S4 is turned off. Due to the continuous flow of the resonant inductor current, the capacitor C3 is discharged, and the voltage drops linearly; C4 is charged, and the voltage rises linearly. After the capacitor C3 is discharged, the inductor current flows through the anti-parallel diode. At this time, S3 is turned on, which is a zero voltage turn-on.
Mode 4 At T1- T2, S2 and S3 are turned on at the same time, and the resonant circuit continues to resonate. After a period of time, the current begins to reverse.
Mode 5 At T2, the switch tube S2 is turned off. Due to the continuous flow of the resonant inductor current, the capacitor C1 is discharged, and the voltage drops linearly; C2 is charged, and the voltage rises linearly. After capacitor C1 is fully discharged, the inductor current flows through the anti-parallel diode. At this time, S1 is turned on, which is a zero voltage turn-on.
Mode 6 T2-T3, S1, S3 are turned on at the same time, and the resonant circuit continues to resonate. Mode
7 At T3, S3 is turned off. Due to the continuous flow of the resonant inductor current, capacitor C4 is discharged, and the voltage drops linearly; C3 is charged, and the voltage rises linearly. After capacitor C4 is fully discharged, the inductor current flows through the anti-parallel diode. At this time, S4 is turned on, which is a zero voltage turn-on.
Mode 8 T3-T4, S1, S4 are turned on at the same time, and the resonant circuit continues to resonate. After a period of time, the current begins to reverse.
The figure below shows the equivalent circuit diagram. The input is equivalent to a square wave voltage source, and the secondary side of the transformer is equivalent to a resistor.

Figure 3 Equivalent circuit
Another important parameter for the resonant network is the amplitude-frequency characteristic. Figure 4 shows the amplitude-frequency characteristic under different loads. As can be seen from the figure, under the fundamental wave conditions of the same frequency and amplitude, different loads obtain different outputs, that is, different output gains. Therefore, under different load conditions, changing the amplitude of the fundamental wave can obtain the same output voltage. This is the basis for the resonant converter to adjust the output by changing the duty cycle when the input and load change. At the same time, in order to achieve soft switching, the entire resonant circuit is inductive, and the current lags behind the voltage, creating conditions for soft switching.

3 Single-cycle control loop

Figure 6 Principle of improved single-cycle control method

This paper adopts an improved single-cycle control method, which has good dynamic response to load and input mutations. See the improved single-cycle control buck circuit shown in Figure 6. A pi adjustment link is added to the control method. When the input voltage changes, the improved single-cycle control method can be quickly adjusted within an open cycle, just like the ordinary single-cycle control method. When the load changes suddenly, the DC output will also change, and the output of the pi link will also change at this time, which is equivalent to a change in the reference of the single cycle. Since the input DC bus remains unchanged, the duty cycle of the control signal changes rapidly, and the output is adjusted quickly. In this experiment, the duty cycle is obtained by the single-cycle control module, and then the four control signals are obtained through 3875 phase shifting.

Figure 7 Single-cycle control loop

[page]4 Simulation circuit and its waveform

This paper uses SIMetrix simulation software to simulate and analyze the phase-shifted full-bridge resonant converter. The circuit parameters are set as: output Po = 200W, input voltage Vin = 400v, resonant inductor Lr = 150µH, resonant capacitor is 47n, and switching frequency Fs = 200kHZ. Figure 8 shows the simulation waveforms of the resonant inductor current and resonant capacitor voltage, and Figure 9 shows the driving waveform of S1 and the voltage waveform of the drain and source of S1 (verification of soft switching). The dynamic response when the input voltage suddenly changes and the dynamic response when the load suddenly changes are shown in Figures 9 and 10 respectively. The simulation output waveform is completely consistent with the theoretical analysis. The simulation results show that the above work analysis is correct.

5 Experimental results

Based on the above principle, a circuit was designed, using a single-cycle phase-shift control mode. The key waveforms are shown in the figure below. The left figure shows the voltage and resonant current at both ends of the resonant cavity. The waveform of the main circuit is sinusoidal, which can effectively reduce the electromagnetic interference problem. The right figure shows the drive signal of S1 and the voltage of the source and drain of S1. It can be seen from the experimental waveform that zero voltage turn-on can be achieved.

This paper introduces a phase-shifted full-bridge resonant converter topology with simple structure, low cost and high efficiency. Experimental results show that this topology can easily achieve soft switching while generating lower electromagnetic interference, and is suitable for communication power supply occasions with low voltage and high current output.

References:

[1] The fundamental of power electroninc, Robert w.erickson , kluwer academic publishers,1995
[2]Moschopoulos, G.; Jain, P.; A series-resonant DC/DC converter with asymmetrical PWM and synchronous rectification, Power Electronics Specialists Conference 2000, PESC00, 2000 IEEE 31st Annual Volume 3, 18-23 June 2000, Page(s):1522 – 1527
[3]Research on one-cycle control for switching converters Yong Wang; Songhua Shen
June 2004
[4]Zhang Zhansong, Cai Xuansan, Principle and Design of Switching Power Supply, First Edition, Publishing House of Electronics Industry, 1999
[5]Ruan Xinbo, Yan Yangguang, Soft Switching Technology of DC Switching Power Supply, First Edition, Beijing Science Press, 2000
[6]Liu Shengli, Practical Technology of Modern High Frequency Switching Power Supply, Publishing House of Electronics Industry, First Edition, 2001

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