An asymmetrical cancellation method for common mode voltage in full-bridge LLC converter utilizing nonideal transformer

This paper proposes a common mode voltage (CMV) reduction method for power full-bridge LLC converter. A detailed derivation of asymmetrical CMV source in energy transmission (ET) and dead time resonance (DR) is presented, and a common mode (CM) equivalent circuit model considering stray parameters of power devices and transformer for the high-voltage power LLC converter is constructed. This study assesses the influence of asymmetric junction capacitances of power devices and stray capacitances of transformer on both the CMV source and the CM model. It also examines the impact of power level on the constituent elements of the CMV source. Additionally, a theory of asymmetric CMV source cancellation is deduced based on the nonideal power LLC converter, where asymmetric noise source and coupling pathway are taken into account, which helps mitigate it through matching circuit structure without extra components. Finally, a 30 kHz 2 x 50 kW prototype is constructed and validated to reduce the CMV where the experiments show that the output CMV within high-frequency band could be reduced by 12.15 dB at rated operating conditions.