A Light Load Efficiency Optimal Analysis Method Applied to Phase-Shift-Modulation Synchronous Rectification Resonant Converters
Chen Mengying1, Chen Bo1, Liu Chao2, Wang Yifeng1, Wang Ping1
1. School of Electrical and Information Engineering Tianjin University Tianjin 300072 China; 2. State Grid Jining Power Supply Company Jining 272000 China
Abstract：The characteristics of high efficiency and wide gain are the key indicators of the operational performance of resonant converters. Traditionally, pulse frequency modulation (PFM) is used to adjust the voltage gain of resonant converters. However, due to inherent structural limitations, the classical resonant converter has a flat voltage gain curve in the over-resonant region (fs>fr), which makes it difficult to adapt to wide output voltage applications, such as the LLC and LCLC resonant converters. Therefore, phase shift modulation (PSM) can be employed. It can effectively widen the voltage gain range. Unfortunately, the traditional PSM control method faces the problem of missing soft switching in the lagging bridge arm under light load conditions. Thus, this paper proposes a light load efficiency optimal analysis method for phase-shift- modulated synchronous rectification resonant converters (LLEOAM). Firstly, the idea that the synchronous rectifier is conducted in advance is adopted in the LLEOAM. By adjusting the advance conduction time of the synchronous rectifier, some energy is fed back from the secondary side to the primary side. Interestingly, in the process of increasing the advance conduction time of the synchronous rectifier, there is a dynamic game between the reduced loss (turning on loss and output capacitance loss of the primary lagging bridge arm switch) and the increased loss (turning off loss of the primary lagging bridge arm switch and loop conduction loss), which has an important impact on the overall efficiency. Secondly, by constructing the mathematical model between the controlled variable (the synchronous rectifier advance conduction time TR) and the loss (Ploss), the basic relationship diagram between the loss derivative () and TR under different working conditions is put forward and summarized. Finally, the optimized advance conduction time (TR_op) and the corresponding optimization efficiency (ηop) are obtained. Overall, LLEOAM adopts the idea of a loss game to propose a general and universal TR calculation method instead of simply and roughly setting a TR that makes the lagging bridge arm switching tube realize ZVS. A 500 W experimental prototype is built based on an LCLC resonant converter. Simulation analysis and power experiments are conducted. Besides, a concept of incomplete ZVS efficiency optimization is proposed. According to the value of TR, three cases can be obtained: Case 1 (TR=0), Case 2 (TR=TR_op=Tcoss), and Case 3 (TR=TZVS). The test results show that in the condition of P=100 W and Vout=35 V, the efficiency of the three different cases is 93.757%, 95.821%, and 90.379%, respectively. Compared with traditional PSM control (Case 1), the efficiency of light load (20% load) is improved by more than 2% (93.757% to 95.821%) by using the efficiency optimization analysis method. Compared with traditional synchronous rectification advanced on-time control (Case 3), the efficiency of light load (20% load) is improved by more than 5% (90.379% to 95.821%). The following conclusions can be drawn. (1) Regarding low efficiency under light load conditions, a light load efficiency optimal analysis method is proposed for LLEOAM. (2) Instead of traditional synchronous rectification advanced on-time control’s setting TR=TZVS, LLEOAM adopts the idea of loss tradeoff to propose a universal and effective calculation method of TR. (3) It can be extended to all synchronous rectification resonant converters and has good universality.
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