LLC synchronous rectification using homopolarity cycle modulation

In order to enhance the efficiency of LLC resonant converters, a new synchronous rectification modulation technique, called homopolarity cycle modulation, is proposed. The proposed modulation technique answers the main challenge in synchronizing the LLC converter's rectifier, which is detecting the conduction angle. By observing the polarities of the inverter voltage and secondary voltage of the transformer and mapping them in the homopolarity plane, the conduction angle of the synchronous rectifiers (SRs) is obtained. The homopolarity cycle is defined and used in the LLC converter's time domain equations to mathematically calculate the gain and the SRs' conduction angle. The theoretical analyses have resulted in a very simple control law with low-cost implementation requirements. Since the amplitudes of the signals used in the proposed control law are not important, the modulation technique's noise immunity is considerable. The maximum efficiency of the LLC converter using the proposed homopolarity cycle modulation technique is measured as 97.9%, which is 4% higher than the efficiency rate of a conventional LLC converter.