Analysis and Characterization of the LLC Resonant Converter in Sub-Resonant ZCS Operation Region

Abstract:
The LLC resonant converter is typically operated around the resonant frequency, which is characterized by the series inductance and series capacitance of the resonant circuit. Around this switching frequency, zero-voltage switching (ZVS) of the primary-side inverter stage is possible. This is essential when using high-voltage silicon MOSFETs, as these would be destroyed by hard (on-)switching. The DC-voltage transfer characteristic and the ZVS-operation region are usually described using the fundamental harmonic approximation (FHA). However, if the converter is operated well below the resonant frequency in order to achieve zero-current switching (ZCS), the converter behaviour deviates significantly from the description of the FHA. ZCS operation in conjunction with an IGBT-based inverter stage represents a robust and cost-effective alternative to the conventional MOSFET-based ZVS-operated LLC. This paper analyses the transfer characteristic of the LLC resonant converter for switching frequencies below the resonant frequency for a wide load range in order to enable a simple design of a ZCS LLC. Several methods are shown and compared. The analysis using a switched-model simulation shows the best results. Based on this, mathematical equations are derived which describe the borders between ZCS and ZVS. With these easy-to-use equations, the operating range can be described very precisely, regardless of the component values. Verification using a 2 kW prototype shows maximum deviations of around 3 %, thus confirming its suitability for practical use.