An efficient control method for an isolated multilevel DC/DC resonant converter (700, 800, 1000, 1400, 1600. 1650, 1700, 1750) achieves a wide output voltage range with a narrow device switching frequency range, relative to the output voltage range and the device switching frequency range of the prior art. At any given time, a control circuit (801, 1401, 1601, 1651, 1701, 1751) selects one of two different modulation schemes to operate the primary-side switching devices of the resonant converter (700, 800, 1000, 1400, 1600. 1650, 1700, 1750) based on at least one of output voltage ( V o ), output current, input signal, and one or more external control signals. Together with a selected device switching frequency, the two modulation schemes generate different voltage waveforms to a primary-side transformer, which are coupled to the secondary-side to provide different output voltages ( V o ).