Zero-voltage-switching conditions for half-controlled secondary-side LLC converter using phase-shift modulation

In this article, the half-controlled secondary-side (SS) LLC converter is presented and analysed, along with the boundary conditions for achieving zero-voltage-switching (ZVS). Resonant converters present high-efficiency and power density, minimizing the losses and increasing the overall system efficiency, as well as reducing the electromagnetic interference. LLCs are the most widely used converters of this family and many variations have been proposed in literature for optimizing the operation of specific applications, where usually the switching frequency regulates the output voltage. One of the proposed variations, the SS-LLC, presents a unique topology where the secondary-side bridge is half-controlled, consisting of two diodes and two transistors. While operating at a constant frequency, equal to the resonant, the secondary-side voltage is regulated by the phase difference of the pulses between the two bridges. The proposed topology acts as a boost converter, capable of increasing the output voltage, when necessary, while maintaining commutation under zero-voltage for a wide range of power and voltage output. From the time-domain analysis that is conducted, the boundary conditions for achieving ZVS are derived, based only on the passive components of the converter. The accuracy of the analysis is compared to numerous simulation scenarios.