Reliability and efficiency improvement in LLC resonant converter by adopting GaN transistor

LLC resonant converter may operate in zero current switching (ZCS) region under overload or short circuits condition. It will destroy devices using traditional high voltage silicon MOSFETs as primary switches due to the poor reverse recovery characteristics of body diode. Gallium nitride(GaN) power transistor has smaller reverse recovery energy, therefore there is possibility to increase the system reliability. High efficiency is another requirement for LLC resonant converter and using low voltage enhancement GaN transistor as synchronous rectifiers (SR) can contribute to that. A 1MHz/300W, 400V/12V LLC resonant converter prototype is built to verify that. However, SR turns on after the current has flowed from source to drain through its reverse conduction mechanism and turns off before the current decreases to zero, which means there exists time intervals when the source to drain current flows through GaN´s reverse conduction mechanism. This will result in high power loss during these time intervals due to the high reverse conduction voltage drop of low voltage GaN transistor, especially in high switching frequency. This paper proposes a novel driving method for SR and decreases the voltage drop, increasing the efficiency. Finally, simulation results are given and verify the proposed driving method.