Comparison between LLC resonant converter and asymmetric half-bridge converter
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Abstract: Two different types of soft switching topologies, LLC resonant converter and asymmetric half-bridge converter, are introduced . Their working principles are analyzed, and their control methods, voltage stress of the secondary rectifier and secondary turn-on are compared. The analysis results show that LLC resonant converter is more suitable for high frequency and high efficiency requirements. Keywords: LLC resonant converter; asymmetric half-bridge converter; voltage stress 0 Introduction 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 resonant soft switching topologies and PWM soft switching topologies. In recent years, with the development of semiconductor device manufacturing technology, the on-resistance, parasitic capacitance and reverse recovery time of the switch tube have become smaller and smaller, which provides another opportunity for the development of resonant converters. For resonant converters, if they are properly designed, soft switching conversion can be achieved, so that the switching power supply has a higher efficiency. 1 Working principles of the two converters 1.1 Asymmetric Half-Bridge Converter Figures 1 and 2 show the circuit diagram and working waveform of the traditional asymmetric half-bridge 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 Cb , 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 Ns1 and Ns2 respectively; half - bridge full - wave rectifier diodes D1 and D2 ; output filter inductor Ld and capacitor Cf. 
Figure 1 Asymmetric half-bridge converter 
Figure 2 Working principle of asymmetric half-bridge converter The steady-state operating principle of the asymmetric half-bridge (AHB) 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 Ns1 is working; the diode D1 is turned on and the diode D2 is turned off ; 2) When S2 is turned on and S1 is turned off, the voltage on the DC blocking capacitor Cb is applied to the primary side of the transformer, the secondary side Ns2 works, and the diode D1 is cut off. In Figure 2, n 1 = N p / N s1 , n 2 = N p / N s2 , and n 1 = n 2 = n . By analyzing the circuit, the calculation formula for the duty cycle D of the traditional asymmetric half-bridge converter can be obtained. D =  (1) 1.2 LLC resonant converter Figures 3 and 4 show the circuit diagram and working waveform of LLC resonant converter respectively. Figure 3 includes two power MOSFETs (S1 and S2 ) , both of which have a duty cycle of 0.5; resonant capacitor Cs , center- tapped transformer Tr with equal turns on the secondary side , leakage inductance Ls of Tr , magnetizing inductance Lm , Lm is also a resonant inductor in a certain period of time, therefore, the resonant elements in LLC resonant converter are mainly composed of the above three resonant elements, namely resonant capacitor Cs , inductor Ls and magnetizing inductor Lm ; half - bridge full-wave rectifier diodes D1 and D2 , and output capacitor Cf. 
Figure 3 LLC resonant converter 
Figure 4: Working principle of LLC resonant converter The steady-state operating principle of the LLC converter is as follows. 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 Ls and Cs participate in the resonance. 2)〔t 2 , t 3〕When t = t 2 , S 1 is turned on under the condition of zero voltage, and the primary side of the transformer is subjected to positive voltage; D 1 continues to be turned on, and S 2 and D 2 are turned off. At this time , C s and L s participate in the resonance, while L m does not participate in the resonance. 3)〔t3 , t4 〕 When t = t3 , S1 is still on, while D1 and D2 are off , and the secondary side of Tr is disconnected from the circuit. At this time, Lm , Ls and Cs participate in the resonance together . In the actual circuit, Lm >> Ls , so at this stage it can be considered that the excitation current and the resonant current remain unchanged. 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, so only Ls and Cs participate in the resonance. 5 ) 〔t5 , t6〕When t = t5 , S2 is turned on under the condition of zero voltage, 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 Cs and Ls 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 on, while D1 and D2 are off , and the secondary side of Tr is disconnected from the circuit. At this time, Lm , Ls and Cs participate in the resonance together . In the actual circuit, Lm >> Ls , so 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 both the asymmetric half-bridge converter and the LLC resonant converter are soft-switching converters, there are essential differences between the two. The asymmetric half-bridge converter is a PWM type, while the LLC resonant converter is a resonant 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 asymmetric half-bridge converter adjusts the output voltage by adjusting the duty cycle of the switch tube. FIG5 shows the duty cycle variation under different input voltages. It can be seen from FIG5 that when the input voltage variation range is relatively large, the duty cycle variation range of the switch tube is also relatively large. Therefore, the power-off holding time characteristic of the asymmetric half-bridge converter is relatively poor. 
Figure 5 Asymmetric half-bridge duty cycle variation diagram Compared with the asymmetric half-bridge converter, the LLC resonant converter adjusts the output voltage by adjusting the switching frequency, that is, its duty cycle remains unchanged under different input voltages. Therefore, compared with the asymmetric half-bridge, its power-off holding time characteristic is better and can be widely used in occasions with high requirements for power-off holding time. 2.2 Comparison of voltage stress of secondary rectifier tube By analyzing the working principle of the asymmetric half-bridge converter, the calculation method of the voltage stress on the secondary diode can be obtained as shown in equations (2) and (3). In this way, when the input voltage changes, the change of the secondary diode voltage can be understood. Figure 6 shows the voltage change on the secondary rectifier when the output voltage is 48V. When the input voltage is relatively high, the voltage on D2 is relatively high. Therefore, D2 must use a diode with a relatively high voltage rating, which will increase the loss of the circuit. V D1 =  (2) V D2 =  (3) 
Figure 6 Voltage stress diagram of the secondary diode in an asymmetric half-bridge Under the same conditions, the voltage stress on the secondary diode in the LLC resonant converter is much smaller than that in the asymmetric half-bridge converter, because the voltage stress on the secondary diode in the LLC resonant converter is twice the output voltage, as shown in Figure 7. Therefore, a diode with a relatively low withstand voltage can be selected in the LLC resonant converter, thereby improving the efficiency of the circuit. 
Figure 7. Voltage stress diagram of the secondary diode in LLC converter 2.3 Comparison of secondary diode turn-on From the analysis of the asymmetric half-bridge converter, it can be seen that its secondary diode is hard-on and has relatively large losses; while from the analysis of the LLC resonant converter, it can be seen that its secondary diode is a zero-current switch and has relatively small losses, which can improve the efficiency of the converter. 2.4 Other aspects First, in the asymmetric half-bridge converter, the duty ratios of the upper and lower switches are complementary, so the transformer in the asymmetric half-bridge converter has a DC bias phenomenon; while in the LLC resonant converter, the duty ratios of the upper and lower switches are equal, so the transformer in the LLC resonant converter has no DC bias phenomenon. Secondly, the LLC resonant converter adjusts the output voltage by adjusting the operating frequency of the switch tube. Therefore, it is more complicated to implement synchronous rectification control for the LLC resonant converter; while the asymmetric half-bridge converter adjusts the output voltage by adjusting the duty cycle of the switch tube. Therefore, it is relatively simple to implement synchronous rectification control for the asymmetric half-bridge converter. In addition, through the analysis of the LLC resonant converter, it can be seen that its current stress is relatively high; while in the asymmetric half-bridge converter, the current stress is relatively low. 3 Conclusion Through the analysis and research of the asymmetric half-bridge converter and the LLC resonant converter, and the comparison of their control methods, secondary rectifier voltage stress and secondary side opening, it can be seen that the LLC resonant converter is more suitable for the development needs of power supply for high frequency and high efficiency.
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