Buck-flyback DC-DC converter

It is difficult to obtain a large input/output voltage ratio with a DC-DC converter, because the duty factor d may not reach very small values. For the same reason, it is difficult to obtain an output voltage that is adjustable in a large range. A DC-DC converter circuit is proposed that overcomes this limitation by performing a voltage ratio d²/(1-d) in the best operating mode. Circuit operation is analyzed, operating modes are evidenced, and the voltage ratio is deduced in each operating mode as a function of output current, duty factor, and circuit component values. Boundary conditions between different operating modes are obtained; consequently, it is concluded that these conditions do not occur for some operating modes. Component ratings are summarized, to facilitate circuit design. The buck-flyback DC-DC converter is very suitable for low-voltage (e.g. computer) power supplies and for power supplies with the output voltage (adjustable in a large range) supplied from the mains without a mains voltage transformer