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 greatly reduced inductor volume. It can be operated in either a regulating pulse width modulated (PWM) or fixed-ratio resonant mode. For fixed-ratio operation, the FCML converter is capable of achieving multiple rational conversion ratios (up to a maximum of $N:1$ ) with the same switch-capacitor network simply by changing the gating signals of its switches. In addition, the inductance requirement can be further reduced compared to regulated operation, while switching losses at resonance can be mitigated due to the presence of soft-switching. However, there is limited prior analysis of higher level-count $(N\geq 3)$ fixed-ratio FCML converters operating at resonance or above resonance, where the derivation of appropriate phase durations becomes significantly more involved. This work presents a full analytical method to calculate optimal phase durations for the fixed-ratio FCML converter across all rational conversion ratios while operating at or above resonance. Hardware results for an $N=5$ FCML converter are presented for conversion ratios 5: 1, 5: 2, 5:3 and 5:4, illustrating the ability to use above-resonance operation to optimize the distribution of switching losses and conduction losses in the converter.