Analysis of PWM Control Strategy for Soft-Switching Half-Bridge DC/DC Converter

0 Introduction

The half-bridge DC/DC converter has a simple structure and is easy to control, making it very suitable for small and medium power applications. The switching loss of the hard-switching converter is large at high frequencies, which seriously affects its efficiency. Soft switching technology can reduce switching losses and line EMI, improve efficiency and power density, and increase switching frequency to reduce the size and weight of the converter. There are two control methods for traditional half-bridge converters, one is symmetrical control and the other is asymmetrical complementary control. This article mainly analyzes the PWM control strategy for realizing soft switching of half-bridge DC/DC converters.

1 Control type soft switch PWM control strategy

The controlled soft-switching half-bridge DC/DC converter does not add any main circuit components (inductor and capacitor components can be added to achieve soft switching conditions), and achieves soft switching by properly designing the control circuit. Figure 1 shows four PWM control strategies for controlled soft-switching half-bridge DC/DC converters.

Figure 1 Controlled soft-switch PWM control strategy

1.1 Asymmetric complementary pulse PWM control

The control pulses of the switch tubes are asymmetric and complementary. The traditional asymmetric half-bridge converter using this control strategy has been widely used in small and medium power applications. There are two ways to achieve ZVS for the primary switch tube: load current ZVS method and excitation current ZVS method [1]. Its advantages are: both switch tubes can achieve ZVS; some measures that can improve the soft switching conditions of the lagging arm of the phase-shifted full-bridge converter can also be used for the asymmetric half-bridge converter; there is no oscillation problem in hard switching; compared with the phase-shifted full-bridge converter, there is no circulating energy. Its disadvantages are: the voltage stress of the switch tube is inconsistent with the soft switching conditions of the switch tube, and it is difficult to achieve soft switching on the upper tube; the voltage stress of the rectifier tube is inconsistent and changes with the duty cycle. In some applications, the voltage of one rectifier tube is very high, and the device is difficult to select; the soft switching condition will be lost when the load is light; the transformer is DC biased, the heavier the load, the smaller the duty cycle, and the more serious the bias; it is very unsuitable for applications with wide input or output voltages.

1.2 Phase-shifted pulse PWM control

The half-bridge using this control strategy is also called a dual-active half-bridge [2,3].

This control strategy is similar to the traditional phase-shifted full-bridge topology, except that the two phase-shifted bridge arms are distributed on the primary and secondary sides of the transformer. In this topology, the leakage inductance of the transformer is the intermediate energy storage element. The primary and secondary half-bridges each generate a square wave with a duty cycle of 50%. By adjusting the phase shift between the two output bridges, the energy of the transformer leakage inductance is controlled to adjust the output voltage. This topology can achieve soft switching over the full load range, while the output can also obtain synchronous rectification. Its disadvantages are: the circulating energy is very large and the output current ripple is large. In order to improve the disadvantage of large output current ripple, the phase-shifted ZVS half-bridge circuit was proposed [4].

1.3 Pulse Shift PWM Control

Reference [5] proposed a pulse shift PWM control strategy. The falling edge of the upper tube is complementary to the leading edge of the lower tube, and the pulse width is the same. ZVS can be achieved for the lower tube, and the upper tube is still a hard switch. Its advantages are: it can reduce some switching losses; there is no DC bias in the transformer; the voltage stress of the rectifier tube is symmetrical; it is better than the asymmetric half bridge in a wide range of inputs. Adding an auxiliary circuit can achieve ZVS for the upper tube [6].

1.4 Asymmetric Pulse PWM Control

Reference [7] proposed asymmetric pulse PWM control, in which the falling edge of the lower tube complements the leading edge of the upper tube, and the upper tube can achieve ZVS. As long as the designed duty cycle is small, no other measures are needed, and the switching loss is small even when working at a higher frequency. The transformer DC bias magnetization, except for the duty cycle end point, the bias current is smaller than the asymmetric half bridge. The wide range of applicability is better than the traditional asymmetric half bridge. It has certain advantages in low voltage and high current applications.

2 Buffered soft-switching symmetrical PWM control strategy

Symmetrically controlled half-bridge converter has bidirectional core magnetization, high utilization rate, and no bias. It is easy to control and has linear control characteristics. The voltage stress on the power tube is low, which is suitable for high input voltage occasions. However, this half-bridge converter is difficult to achieve soft switching, and the converter efficiency is difficult to improve.

2.1 Symmetrical PWM Control ZVS Half-Bridge Converter

Reference [8] proposed a symmetrical PWM controlled ZVS half-bridge converter (see Figure 2). Compared with the traditional half-bridge circuit, the symmetrical PWM controlled ZVS DC converter adds a branch consisting of an auxiliary switch tube and a diode. Its main switch tube not only works in a symmetrical state, but also the lower tube and the auxiliary switch tube can achieve ZVS in the full load range, and the upper tube can also achieve ZVS in a wide load range, causing very little additional loss. The converter device is less stressed and has high reliability. It is more suitable for using MOSFET as the switch tube and is rarely used in high voltage and high power applications. The converter needs to use the energy storage of the resonant inductor to achieve ZVS of the switch tube. Increasing the resonant inductor can expand the ZVS range of the upper tube, but it will cause serious duty cycle loss. When designing the resonant inductor, it is necessary to balance the realization of ZVS of the upper tube and the reduction of duty cycle loss [9].

Figure 2 Symmetrical PWM controlled ZVS half-bridge converter

2.2 Symmetrical PWM Control ZCS Half-Bridge Converter

Reference [10] proposed a symmetrical PWM controlled ZCS half-bridge converter (see Figure 3), which adds an auxiliary branch consisting of an auxiliary switch tube, a resonant capacitor and a resonant inductor in series to the secondary side of the traditional asymmetrical half-bridge circuit transformer. Its main switch tube not only works in a symmetrical state, but also the converter can achieve ZCS of all switches and ZVS of all diodes in the entire load range. Like the symmetrically controlled half-bridge, the auxiliary switch tube is turned on once every half cycle, and the resonance of the resonant capacitor and the transformer leakage inductance creates conditions for ZCS of all switches and ZVS of all diodes in the full load range. It is difficult to achieve soft switching under heavy load.

Figure 3 Symmetrical PWM controlled ZCS half-bridge converter

3 Conclusion

This paper clarifies the definition of controlled soft-switching half-bridge DC/DC converter, and mainly summarizes and generalizes the PWM control strategies of four controlled soft-switching half-bridge DC/DC converters and the symmetrical PWM control strategies of two buffered soft-switching half-bridge DC/DC converters. It conducts an in-depth analysis and comprehensive comparison of the above PWM control strategies, providing a basis for selecting specific application scenarios.

References:

［1］ Zhang Youjun, Ruan Xinbo. Comparison of two ZVS AHB DC converters［J］. Electric Power Automation Equipment, 2009, 29(10): 69-73.

［2］ IONel Dan Jitaru.A 3KW switching DC-DC converter［C］. New Orleans, LA, USA: Proceedings of IEEEAPEC, 2000.86-92.

［3］ Zhang JM, Xu DM, Qian Zhaoming. An improved dualactive bridge DC/DC converter［C］. Vancouver, Canada: Proceedings of IEEE-PESC, 2001.232-236.

[4] Zhang JM, Zhang F, Xie XG, et al. A novel ZVS DC/DC converter for high power applications [C]. Dallas, USA: Proceedings of IEEE-APEC, 2002.635-640.

[5] Hong Mao, Jaber Abu-Qahouq, Songquan Deng, et al. A new duty-cycle-shifted PWM control scheme for halfbridgeDC-DC converters to achieve zero-voltage-switching［C］. Miami, FL, USA: Proceedings of IEEEAPEC, 2003 (2): 629-634.

[6] Hong Mao, Jaber Abu-Qahouq, Shiguo Luo, et al. Amodified ZVS half-bridge DC-DC converter [C]. Anaheim, Cal, USA: Proceedings of IEEE-APEC, 2004.1436-1441.

［7］Xie Xiaogao, Zhang Junming, Cai Yongjun, et al. Soft switching control strategy for half-bridge converter［J］. Proceedings of the CSEE. 2006, 26(3): 48-52.

[8] Hong Mao, Jaber Abu-Qahouq, Luo Shiguo, et al. Zerovoltage-swiching half-bridge DC-DC converter with modifiedPWM control method [C]. IEEE Trans on PE, 2004, 19 (4): 947-958 .

［9］ Yang Haiying, Xie Shaojun. Research on symmetrical PWM controlled ZVS half-bridge converter［J］. Transactions of China Electrotechnical Society. 2006, 21(6): 29-34.

[10] Qin Ling, Wang Yafang, Zhang Hang, et al. Study on symmetrical control ZCS-PWM asymmetrical half-bridge converter [J]. Electrical Automation. 2007, 29 (5): 39-42.