Equal Loss Distribution in Duty-Cycle Controlled H-Bridge LLC Resonant Converters

The duty-cycle control is a popular option for current limiting in LLC resonant converters. Such current limiting is required to protect the converter either during the start-up or during an overload. Although this method proved to be effective, the switching and conduction losses of the converter are increased because of the modified current waveforms in the resonant tank. Hence, for the safe operation, it is vital to distribute the losses between the switches symmetrically. However, the conventional duty-cycle or phase-shift modulation methods do not provide this naturally. This letter elaborates on a method that was proposed for a different soft-switching topology more than a decade ago to solve this problem, and is adaptable for the emerging dc transformers. Furthermore, a simple implementation of the method is shown and the equal distribution of losses is validated experimentally. The VHDL code with the implementation of the modulator is attached to this paper available openly for reuse.