Flying Capacitor Multilevel Converter at Constant Resonant Frequency With Negative-Voltage-Blocking GaN Switch

The resonant flying capacitor multilevel (RFCML) converters in switched capacitor (SC) operations have gained attention for their high efficiency and high power density. The RFCML converter transfers charge to the flying capacitors by changing their connection, resulting in various resonant frequencies in each phase. These multiple resonant frequencies increase both the peak and rms inductor current, leading to an larger inductor size. This article proposes a new RFCML converter that uses a compact rerouted capacitor to maintain a constant resonant frequency, reducing the required inductor volume. In the proposed topology, a switch capable of blocking a reverse voltage larger than the charged voltage of the rerouted capacitor is required and is implemented with a GaN switch and a negative voltage gate driver. The proposed topology, incorporating two additional switches for a rerouted capacitor, has lower switch resistance compared to the conventional topology, resulting in reduced conduction loss and improved efficiency. The proposed RFCML also applies a zero-voltage switching (ZVS), improving light-load efficiency. It is validated through an RFCML converter prototype for 48-to-12 V conversion and its experimental results. It verifies the constant resonant frequency operation and achieves 98.9% peak efficiency and 96.7% full-load efficiency with 2169 W/in³ power density.