99% Peak Efficiency MHz Cellular DCX Considering the Nonlinear Parasitic Capacitance for Different Output Voltages

The dc transformer (DCX) with series resonant converter (SRC) has been adopted in DC bus conversion. However, in applications with wide output voltage range, as the output voltage decreases, the nonlinear characteristics of the switch output junction capacitance and the resonant capacitor (X7R MLCC) cause both capacitances to increase. Under a fixed dead time, zero voltage switching (ZVS) of the switches cannot be achieved due to insufficient magnetizing current. Furthermore, nonlinear characteristics of the intrinsic capacitance causes a mismatch between the switching frequency and the resonant frequency, which may reduce the overall converter efficiency. This paper proposes a variable frequency SRC-DCX strategy considering nonlinear capacitance, which solves the problems of insufficient magnetizing current and mismatch between switching frequency and resonant frequency at the same time under fixed voltage gain and wide voltage range. By utilizing the nonlinearity of the resonant capacitance (X7R MLCC), the switching frequency is reduced with the output voltage decrease, the magnetizing current can be enlarged. Then, ZVS in a wide voltage range is realized, and the overall efficiency of the converter is improved. Finally, a 400-800V /16.7-33.3V/2.5kW prototype is optimized by achieving a peak efficiency of 99% and 50% loss reduction at minimum output voltage compared to fixed frequency control.