Isolated Soft-Switching Flying-Capacitor based Quasi-Resonant Step-Up Converter

An isolated quasi-resonant step-up converter, and a complementary mixed-signal controller suitable for isolated low-power ultra-high step-up applications, with voltage conversion ratios exceeding 60x and sub-one-watt to a few watts load, are introduced. The converter has the same structure as the previously reported isolated flying capacitor multilevel flyback converter (FCMFC), but operates in a completely different manner. With modification to this typology’s component parameters, and a newly introduced control scheme, load-independent soft-switching is achieved on all the switches on the secondary side and voltage stress across the primary switch has been reduced to the input voltage, for any input-to-output voltage conversion ratio. These improvements are achieved by fully charging and discharging a small flying capacitor on the secondary side, as a resonant element, through a specific technique that ensures proper gating timing for achieving soft-switching. The operation of the converter is experimentally verified using a discrete prototype with input voltage of 3 V and common output voltages of 90 V, 120 V and 180 V, and load currents ranging from 1 mA to 15 mA, demonstrating efficiencies up to 81%.