A ripple-free input and continuous output forward-flyback DC-DC converter with galvanic isolation

This paper presents a forward-flyback DC-DC converter that achieves ripple-free input current and continuous output current while ensuring galvanic isolation. To eliminate input current ripple, a coupled inductor and an auxiliary inductor are employed for effective ripple cancellation. The energy stored in the leakage inductance of the second coupled inductor, which provides galvanic isolation, is transferred to a DC-link capacitor and subsequently recycled in the following switching cycle. Continuous output current is ensured by incorporating an additional output inductor on the secondary side. During the switch-on period, a portion of the input energy is directly delivered to the output inductor, while the remaining energy is stored in the magnetising inductance of the second coupled inductor. When the switch is turned off, the stored energy in the magnetising inductance is transferred to a secondary-side capacitor. By operating in critical conduction mode (CRM), the proposed converter significantly reduces switching losses. A 200 W prototype is implemented to step up a 48 V input to a 200 V output, demonstrating the feasibility and effectiveness of the proposed design.