A control method includes the following operations: combining multiple output voltages into a total output voltage; generating a first switching frequency based on a first voltage difference, and generating at least one offset frequency based on at least one second voltage difference; generating at least one second switching frequency according to the at least one offset frequency and the first switching frequency; and generating a first set of switching signals according to the first switching frequency, and generating at least one second set of switching signals according to the at least one second switching frequency to respectively control a switching element of the resonant converter circuits.