Characteristic Analysis and Selection of Resonant Capacitors for LLC Resonant Converters

LLC resonant converters are widely used in high-frequency and high-efficiency power converter applications. In the design of an LLC resonant converter, the selection of the resonant capacitor is critical because the entire primary current flows through the series-connected capacitor. Thus, the capacitor must have sufficient ripple-current capability to satisfy efficiency and thermal requirements. Additionally, sufficient voltage rating is required to withstand high DC bias voltages and ensure safe and reliable converter operation. This paper examines the proper selection of resonant capacitors for a half-bridge LLC resonant converter. Key capacitor parameters affecting converter performance, including dissipation factor and equivalent series resistance, are investigated for multilayer ceramic capacitors and film capacitors suitable for high-frequency resonant converters. Based on these characteristics, the capacitor power loss, temperature rise, and allowable voltage rating are analyzed to identify most appropriate capacitor types. Experimental results obtained from an LLC resonant converter employing MLCCs and film capacitors are presented to validate the theoretical analysis.