Series-Connected Power Devices in a CLLC Resonant Converter for DC Transformer Applications

This paper proposes a voltage balancing method for series-connected devices in a bidirectional CLLC resonant converter. Series connection of power devices can allow the CLLC resonant converter working in high or medium voltage applications. The key problem of series-connected power devices is the voltage imbalance between devices caused by device parameter spread and gate drive delays. An active clamping module (ACM) is in parallel with each of the series-connected power device. Each ACM consists of a clamping capacitor and an auxiliary switch with an anti-parallel diode. During the switching period, the power device voltage will be clamped at the voltage of ACM capacitor. By balancing the voltages of ACM capacitors, we can balance the voltages of series-connected power devices as well. The charging and discharging of ACM capacitor are introduced. The design process of discharge time is given. A balancing control method for ACM capacitors is proposed. Experimental results from a CLLC prototype with 4 power Schottky silicon carbide diode series-connected are provided to verify the effectiveness and feasibility of the proposed method. The imbalance degree of these series-connected devices is less than 3% under different voltage and load conditions. The peak efficiency reaches 97.7%.