An isolated quasi-resonant step-up converter and a 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 but operates in a completely different manner. With modifications to this topology’s parameters and a newly introduced control scheme, load-independent soft-switching is achieved on all the switches on the secondary side and volage stress across the primary switch has been reduced, for any 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 technique that ensures proper timing. With significant reduction in switching losses, the converter is well-suited for applications where switching losses would otherwise be dominant. The operation of the converter is experimentally verified using a discrete prototype with input voltage of 3V, output voltages of 60V, 90V and 120V, and load currents ranging from 1 mA to 15 mA.
