The forward converter family, which includes the push-pull and half-bridge circuits, evolved from the stepdown or “buck” type of regulator. A typical forward converter circuit is illustrated in Figure 6-5.
When the transistor switch is turned on the transformer delivers power to the load through diode D1 and the LC filter. When the switch is turned off diode D2 is forward biased and maintains current to the load.
Without the third winding, and diode D3, the converter would lose efficiency at higher frequencies. The function of this winding is to return energy stored in the transformer to the line and reset the transformer core after each cycle of operation.
This is a popular low power (up to about 200 W) converter and is almost immune to transformer saturation problems.