Frequency control of self-sustained push-pull resonant converter using effective parasitic capacitances

A method to control the frequency of a self-sustained push-pull converter at megahertz (MHz) level is proposed by utilising the parasitic capacitances of the circuit. As the equivalent parasitic capacitance and the frequency of the converter can be controlled by a DC voltage, the converter works as a voltage-controlled oscillator and a phase locked loop (PLL) controller is developed to lock in the zero-voltage switching frequency under circuit parameter variations such as the load, the coupling coefficient etc. The effect of the parasitic capacitances on the operating frequency and why the DC voltage can control it are analysed in details with equivalent circuit analysis and simulation study. Practical experiments demonstrate that the PLL controller can lock the operating frequency within 20–30 MHz with relatively good waveforms.