Accurate ZVS boundary in high switching frequency LLC converter

Due to the realization of zero voltage switching (ZVS) under the full load range, LLC resonant converter is widely adopted in the telecom, battery charger and several applications, characterized with high efficiency, high frequency and high power density, to realize DC conversion. Recently, by using Gallium Nitride (GaN) HFETS, switching frequency of LLC converters is further increased. However, ZVS failure cannot be predicted accurately in the high switching frequency condition by only considering traditional constraints generally applied in the low frequency design. The traditional constraints result in a too optimistic estimation of the dead time to obtain ZVS without considering the reverse resonance under the dead time and the design of resonant parameters at high resonant frequency and high load condition. The experiment shows the LLC converter loses ZVS even through the converter satisfies the ZVS constraints proposed by previous paper. In this paper, the failure mode will be investigated in detail and an accurate ZVS boundary is proposed for high frequency LLC converter design. The proposed theory was verified on a 1 MHz, 1500 W LLC prototype.