Influence of parasitic components on MHz-level frequency LLC resonant DC-DC converter

In this paper, the influence of parasitic components of LLC resonant dc-dc converters operated at MHz-level switching frequency is investigated. In MHz-level switching frequency, value of the parasitic components are getting closer to value of circuit parameters. Therefore, influence of the parasitic components cannot be neglected even in the initial step of the circuit design. In particular, undesirable boost conversion is happened at light load condition with parasitic components, which disturbs the output voltage control. Therefore, it is very important to reveal what happened with the parasitic components. In this paper, to verify the influence of the parasitic components, some analyses have been done. With the analysis results, the influence of the parasitic components has been revealed. Some experimental results have been conducted with an isolated step-down dc-dc converter with Gallium Nitride High Electron Mobility Transistors (GaN-HEMTs); the input voltage is 53V, the load current range is less than 1A, and the maximum switching frequency is about from 3 to 5.5MHz. From the experimental results, it revealed that secondary-side current transformer LCT2 and secondary-side junction capacitance Cj greatly affect the static characteristics at MHz-level switching frequency. Therefore, by minimum load current setting and number of parallel of secondary-side diode, using LCT2 for measuring secondary-side winding current is decided.