Comparison between Half-bridge Resonant Converter and Half-bridge Flyback Converter

Comparison between Half-bridge Resonant Converter and Half-bridge Flyback Converter

With the development of switching power supplies, soft switching technology has been widely developed and applied, and many high-efficiency circuit topologies have been studied, mainly PFM-type soft switching topologies and PWM-type soft switching topologies.

In recent years, with the widespread application of the third-generation semiconductor device GAN and the continuous development of PD power supplies, this has provided another opportunity for the development of power converters. For half-bridge converters, if properly designed, soft switching conversion can be achieved, so that the switching power supply has higher efficiency and greatly reduces the size of the power supply.

1 Working principles of two converters

1.1 Half-bridge flyback converter

Figure 1 and Figure 2 respectively show the circuit diagram and working waveform of the half-bridge flyback converter.

Figure 1 includes two complementary controlled power MOSFETs (S1 and S2), where the duty cycle of S1 is D and the duty cycle of S2 is (1-D); a DC blocking capacitor Cr, whose voltage is used as the power supply when S2 is turned on; a center-tapped transformer Tr, whose primary turns are Np and secondary turns are Ns; an output rectifier diode D1; an output filter capacitor Cout; and output rectifier tube peak absorption resistors and capacitors R1 and C1.

As can be seen from the schematic diagram, the primary part of the half-bridge flyback converter is the same as the traditional asymmetric half-bridge (AHB) converter, and the secondary part is the same as the flyback converter. The steady-state working principle of the half-bridge flyback converter is as follows.

1) When S1 is turned on and S2 is turned off, the primary side of the transformer is subjected to a forward voltage, and the secondary side Ns does not work; the diode D1 is cut off; the transformer stores energy;

2) When S2 is turned on and S1 is turned off, the voltage on the DC blocking capacitor Cr is applied to the primary side of the transformer, the secondary side Ns2 works, and the diode D1 is turned on.

In Figure 2, n1=Np/Ns, and n1=n. By analyzing the circuit, the calculation formula for the duty cycle D of the half-bridge flyback converter can be obtained:

The half-bridge resonant converter is commonly referred to as the LLC resonant converter. Figures 3 and 4 show the circuit diagram and operating waveform of the half-bridge resonant converter respectively.

In Figure 3, there are two power MOSFETs (S1 and S2), both of which have a duty cycle of 0.5; a resonant capacitor Cr, a center-tapped transformer Tr with equal turns on the secondary side, a leakage inductance Lk of Tr, and an exciting inductance Lm. Lm is also a resonant inductor in a certain period of time. Therefore, the resonant elements in the half-bridge resonant converter are mainly composed of the above three resonant elements, namely, the resonant capacitor Cr, the inductor Lk and the exciting inductor Lm; the half-bridge full-wave rectifier diodes D1 and D2, and the output capacitor Cout.

1)〔t1, t2〕When t=t1, S2 is turned off, and the resonant current discharges the parasitic capacitance of S1 until the voltage on S1 is zero, and then the body diode of S1 is turned on. In this stage, D1 is turned on, and the voltage on Lm is clamped by the output voltage, so only Lk and Cr participate in the resonance.

2)〔t2, t3〕When t=t2, S1 is turned on under zero voltage conditions, and the primary side of the transformer is subjected to a forward voltage; D1 continues to be turned on, and S2 and D2 are turned off. At this time, Cr and Lk participate in the resonance, while Lm does not participate in the resonance.

3)〔t3, t4〕When t=t3, S1 is still turned on, while D1 and D2 are in the off state, and the secondary side of Tr is disconnected from the circuit. At this time, Lm, Lk and Cr participate in the resonance together. In the actual circuit, Lm is much larger than Lk, so it can be considered that the excitation current and the resonant current remain unchanged at this stage.

4)〔t4, t5〕When t=t4, S1 is turned off, and the resonant current discharges the parasitic capacitance of S2 until the voltage on S2 is zero, and then the body diode of S2 is turned on. In this stage, D2 is turned on, and the voltage on Lm is clamped by the output voltage. Therefore, only Lk and Cr participate in the resonance.

5)〔t5, t6〕When t=t5, S2 is turned on under zero voltage conditions, and the primary side of Tr is subjected to reverse voltage; D2 continues to be turned on, while S1 and D1 are turned off. At this time, only Cr and Lk participate in the resonance, and the voltage on Lm is clamped by the output voltage and does not participate in the resonance.

6)〔t6, t7〕When t=t6, S2 is still turned on, while D1 and D2 are in the off state, and the secondary side of Tr is disconnected from the circuit. At this time, Lm, Lk and Cr participate in the resonance together. In the actual circuit, Lm is much larger than Lk. Therefore, it can be considered that the excitation current and the resonant current remain unchanged at this stage.

Through the above detailed analysis, we have a certain understanding of the working principles and characteristics of these two types of soft-switching converters. The following will compare the differences between them to further deepen our understanding of them.

2 Comparison of the differences between the two converters

Although the half-bridge flyback converter and the half-bridge resonant converter are both soft-switching converters, there are essential differences between the two. The half-bridge flyback converter is PWM-type, while the half-bridge resonant converter is PFM. Therefore, they have great differences in control methods, voltage stress of the secondary rectifier tube, and current stress of the primary side. These differences will be analyzed in detail below.

2.1 Comparison of control methods

The half-bridge flyback converter adjusts the output voltage by adjusting the duty cycle of the switch tube. When the input voltage variation range is relatively large, the duty cycle variation range of the switch tube is also relatively large. In theory, the duty cycle of the half-bridge flyback converter can exceed 0.5, thereby adapting to a wider input voltage range. Therefore, the power-off maintenance time characteristics of the half-bridge flyback converter are relatively good, and can be widely used in occasions with relatively high requirements for power-off maintenance time.

Compared with the half-bridge flyback converter, the half-bridge resonant converter adjusts the output voltage by adjusting the switching frequency, that is, its duty cycle remains unchanged under different input voltages. Theoretically, the duty cycle of the half-bridge resonant converter will not exceed 0.5. Therefore, compared with the half-bridge flyback converter, its input voltage range is relatively narrow and the power-off maintenance time characteristics are relatively poor.

2.2 Comparison of secondary rectifier voltage stress

By analyzing the working principle of the half-bridge flyback converter, the calculation method of the voltage stress on the secondary diode can be obtained as shown in the following formula:

In this way, when the input voltage changes, the change of the secondary diode voltage can be understood.

Figure 5 shows the change of the voltage on the secondary rectifier when the output voltage is 48V. When the input voltage is relatively high, the voltage on D1 is relatively high. Therefore, D1 must use a diode with a relatively high withstand voltage rating, which will increase the circuit loss and material cost.

Under the same conditions, the voltage stress on the secondary diode in the half-bridge resonant converter is much smaller than that in the half-bridge flyback converter, because the voltage stress on the secondary diode in the half-bridge resonant converter is twice the output voltage. Therefore, a diode with a relatively low withstand voltage can be selected in the half-bridge resonant converter, thereby improving the efficiency of the circuit and reducing the material cost.

2.3 Comparison of the turn-on of the secondary diode

From the analysis of the half-bridge flyback converter, it can be seen that its secondary diode is hard-on, and the loss is relatively large; while from the analysis of the half-bridge resonant converter, it can be seen that its secondary diode is a zero-current switch, and the loss is relatively small, which can improve the efficiency of the converter. Therefore, in theory, the overall efficiency of the half-bridge flyback converter is slightly worse than that of the half-bridge resonant converter (but still far better than other converters).

2.4 Other aspects

First, in the half-bridge flyback converter, the duty cycles of the upper and lower switches are complementary, so the transformer in the half-bridge flyback converter has a DC bias phenomenon; while in the half-bridge resonant converter, the duty cycles of the upper and lower switches are equal, so the transformer in the half-bridge resonant converter does not have a DC bias phenomenon.

Secondly, the half-bridge resonant converter adjusts the output voltage by adjusting the operating frequency of the switch tube, so for the half-bridge resonant converter, it is more complicated to achieve synchronous rectification control; while the half-bridge flyback converter adjusts the output voltage by adjusting the duty cycle of the switch tube, so for the half-bridge flyback converter, it is relatively simple to achieve synchronous rectification control.

2.5 Current stress

Through the analysis of the half-bridge resonant converter, it can be seen that its current stress is relatively high and the output current ripple is relatively large; while in the half-bridge flyback converter, the current stress is relatively low and the output current ripple is relatively small.

2.6 Output voltage range

Through the analysis of the control principle of the half-bridge flyback converter, it can be seen that the output voltage range of the half-bridge flyback converter is wider, while the output voltage range of the half-bridge resonant converter is very narrow. Therefore, in the field of PD power supply with multiple output voltages, the half-bridge flyback converter is more suitable, and a DC/DC converter can be omitted.

3 Conclusion

Through the analysis and research of the half-bridge flyback converter and the half-bridge resonant converter, and the comparison of their control methods, secondary rectifier voltage stress and secondary opening, it can be known that the half-bridge resonant converter is more suitable for the development demand of power supply for high efficiency; while the half-bridge flyback converter is more suitable for the PD power supply field.