Resonant Converter Having Variable Frequency Control and Valley Skipping Time-Shift for Efficient Soft Switching Operation

A method of operating a DC-DC converter circuit to maintain an output current to charge a battery is disclosed. The method includes receiving an output voltage of the DC-DC converter circuit and receiving a desired charging current to charge a battery. When the output voltage is greater or equal to the predefined output voltage, the method includes adjusting an operational frequency to a synchronous second gate signal that is synchronous with a first gate signal, where the first gate signal and the second gate signal are each operating at 50% duty cycle and causing the output current to be adjusted to the desired output charging current. When the output voltage is less than the predefined output voltage, the method includes applying an operational time shift to a second gate signal (operating at 50% duty cycle and ƒo) producing the second voltage waveform in the bridge circuit.