Multi-Ratio Operation of Flying Capacitor Multilevel Converters At and Above Resonance

The flying capacitor multilevel (FCML) converter is an attractive candidate for dc/dc step-up and step-down applications due to its relatively low switch stress and small inductor volume. It can be operated in both PWM regulating or fixed-ratio resonant modes. However, there is limited prior analysis of the FCML converter for higher level-count (N>3) and alternate conversion ratios when operating at and above resonance. The FCML is capable of achieving different conversion ratios (up to a maximum of N:1) with the same switch-capacitor network, simply by changing the gating signals for its switches. Furthermore, when operating at or above resonance, the inductance requirement can be significantly decreased compared to regulated operation, and switching losses can be mitigated at resonance due to the presence of soft-switching. This work therefore presents a full analytical method to calculate phase durations for the resonant FCML across all possible conversion ratio modes, while operating at or above resonance. Hardware results from a nominal 5:1 FCML converter are presented for conversion ratios 5:1, 5:2, 5:3 and 5:4, highlighting the trade off of efficiency and maximum load as a function of conversion ratio and switching frequency.