Analysis and design for LLC resonant converter considering output capacitance of MOSFET

Abstract

The LLC resonant converter is widely used for high efficiency and high power density application because of its advantages. However, it has a critical problem that the output voltage is unregulated at very light load or no load condition, although the switching frequency is widely changed. To find out the reason of problem, conventional analysis such as First Harmonic Approximation can be considered. However, since the voltage gain characteristic at very light load condition resulted from FHA is different from the experimental results, FHA cannot explain the problem. This is because it considers only fundamental components of all the voltages and currents and ideal components of devices like MOSFET, diode and transformer. Therefore, to analyze the voltage gain characteristic at very light load condition precisely, time domain analysis at very light load condition considering parasitic components such as junction capacitance of rectifier diodes and output capacitance of primary MOSFET is proposed. Proposed analysis explains the reason of the regulation problem of LLC resonant converter and resolves the regulation problem simply. To confirm the validity of proposed analysis, experimental results with 200W prototype are demonstrated.