An Accurate Voltage Gain Model Considering Diode Effect for LLC Resonant Converter in Wide Gain Range Applications

For wide gain range applications with LLC resonant converter, variable frequency modulation is widely used to achieve different output voltage. The first harmonics approximation (FHA) is one of the most popular adopted methods to calculate voltage gain with different switching frequency. However, it is not accurate when the switching frequency is far away from the resonant frequency and the diode effect on voltage gain makes FHA deviate from the actual results even more. Several models have been proposed to solve the drawback of FHA method that only first harmonic of the resonant current is examined. Few of them takes the diode effect on voltage gain into consideration. In this paper, detailed analysis of diode effect on voltage gain is presented and based on the analysis, an accurate voltage gain model for LLC converter is proposed. To verify the proposed model, a 1.6 kW GaN based LLC converter prototype is built and tested at 700 kHz and 1 MHz.