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 supply, this has provided another opportunity for the development of power converters. For half-bridge converters, if designed properly, 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 the two converters
1.1 Half-bridge flyback converter
Figure 1 and Figure 2 show the circuit diagram and operating waveform of the half-bridge flyback converter respectively.
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, the voltage on which serves 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 spike 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.
Figure 1: Half-bridge flyback converter
Figure 2 Working principle of half-bridge flyback converter
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; and the transformer stores energy;
2）当S2导通S1关断时，隔直电容Cr上的电压加在变压器的原边，副边Ns2工作，二极管D1导通。
In Figure 2, n1=Np/Ns, and n1=n. By analyzing the circuit, the calculation formula for the converter duty cycle D of the half-bridge flyback converter can be obtained:
1.2 Half-bridge resonant converter
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.
Figure 3: Half-bridge resonant converter
图3中，包括两个功率MOSFET（S1和S2），其占空比都为0.5；谐振电容Cr，副边匝数相等的中心抽头变压器Tr，Tr的漏感Lk，激磁电感Lm，Lm在某个时间段也是一个谐振电感，因此，在半桥谐振变换器中的谐振元件主要由以上3个谐振元件构成，即谐振电容Cr，电感Lk和激磁电感Lm；半桥全波整流二极管D1和D2，输出电容Cout。
Figure 4: Working principle of half-bridge resonant converter
Figure2 : The steady-state operating principle of the LLC converter is as follows.
1）〔t1，t2〕当t=t1时，S2关断，谐振电流给S1 的寄生电容放电，一直到S1上的电压为零，然后S1的体二级管导通。此阶段D1 导通，Lm上的电压被输出电压钳位，因此，只有Lk和Cr参与谐振。
2）〔t2，t3〕当t=t2时，S1在零电压的条件下导通，变压器原边承受正向电压；D1继续导通，S2及D2截止。此时Cr和Lk参与谐振，而Lm不参与谐振。
3）〔t3，t4〕当t=t3时，S1仍然导通，而D1与D2处于关断状态，Tr副边与电路脱开，此时Lm，Lk和Cr一起参与谐振。实际电路中Lm远远大于Lk，因此，在这个阶段可以认为激磁电流和谐振电流都保持不变。
4）〔t4，t5〕当t=t4 时，S1关断，谐振电流给S2的寄生电容放电，一直到S2 上的电压为零，然后S2的体二级管导通。此阶段D2导通，Lm上的电压被输出电压钳位，因此，只有Lk和Cr参与谐振。
5）〔t5，t6〕当t=t5时，S2在零电压的条件下导通，Tr原边承受反向电压；D2 继续导通，而S1和D1截止。此时仅Cr和Lk参与谐振，Lm上的电压被输出电压箝位，而不参与谐振。
6）〔t6，t7〕当t=t6时，S2仍然导通，而D1和D2处于关断状态，Tr副边与电路脱开，此时Lm，Lk和Cr一起参与谐振。实际电路中Lm远远大于Lk，因此，在这个阶段可以认为激磁电流和谐振电流都保持不变。
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 both the half-bridge flyback converter and the half-bridge resonant converter are 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-type. Therefore, they have great differences in control methods, voltage stress of the secondary rectifier tube, current stress of the primary side, etc. 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 changes in a wide range, the duty cycle of the switch tube also changes in a wide range. In theory, the duty cycle of the half-bridge flyback converter can exceed 0.5, thus adapting to a wider input voltage range. Therefore, the half-bridge flyback converter has a better power-off maintenance time characteristic and can be widely used in occasions with 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 voltage stress of secondary rectifier tube
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 voltage variation on the secondary rectifier when the output voltage is 48 V. When the input voltage is high, the voltage on D1 is high, so D1 must use a diode with a higher withstand voltage rating, which will increase the circuit loss and material cost.
Figure 5: Voltage stress diagram of the secondary diode of a half-bridge flyback converter
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 secondary diode turn-on
From the analysis of the half-bridge flyback converter, we know that its secondary diode is hard-on, with relatively large losses; while from the analysis of the half-bridge resonant converter, we know that its secondary diode is a zero-current switch, with relatively small losses, 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 has no DC bias phenomenon.
Secondly, the half-bridge resonant converter adjusts the output voltage by adjusting the operating frequency of the switching tube. Therefore, it is relatively complicated to implement synchronous rectification control for the half-bridge resonant converter. The half-bridge flyback converter adjusts the output voltage by adjusting the duty cycle of the switching tube. Therefore, it is relatively simple to implement synchronous rectification control for the half-bridge flyback converter.
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 can save a DC/DC converter.
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 side opening, it can be known that the half-bridge resonant converter is more suitable for the development demand of high efficiency of power supply; while the half-bridge flyback converter is more suitable for the PD power supply field.
