Analysis and Mitigation of Oscillations in Bi-directional CLLC Resonant Converters

CLLC resonant converters are commonly used for isolated bidirectional DC-DC power conversion systems due to the soft-switching characteristics over whole load range. However, the output capacitance of the high-voltage side switching devices can interact with the resonant tank and cause unexpected oscillations in the discharging mode. As a result, an output capacitance that is too large lead to extra power losses. This paper analyzes the influences of the output capacitance of the switching devices on the operation of the converter and derives an approximate expression for the oscillation. The extra power losses that the oscillations cause are analyzed in detail, and mitigation methods are proposed. Simulation and experimental results verify the theoretical analysis.