Forward dc-dc converter is actually transformer isolated buck dc-dc converter topology. To see this, we need to start form buck converter scheme.
If we want to do galvanic isolation of the load, we can insert transformation just behind main switch and add another switch just behind transformer secondary. It is the same as if we spitted main switch and keep “original” on transformer primary and “copy” on transformer secondary.
Original switch on the transformer primary and “copy” switch on the transformer secondary operate simultaneously, so “copy” switch can be replaced with a diode. This is how forward converter is obtained.
When transformer ratio is one to one, transfer characteristic of forward dc-dc converter is identical to buck converter. Transformer is wound in such a way that during “ON” time of the main switch, energy is transferred directly from primary side to secondary side and no energy is stored inside transformer itself. It is opposite from flyback transformer, where energy is stored in magnetic core, and released later, during “OFF” time. Since energy is released immediately, forward transformer is harder to saturate then flyback transformer. Not to be confused, the same transformer can be utilized in both flyback or forward converter, but it is harder to saturate if it is used in forward topology. Forward converters are typically used for loads up to 500W. During “OFF” time, transformer voltage on secondary side is negative, left hand side diode is clamped and current path for output inductor is provided. Output voltage dependency from input voltage and duty cycle is the same as for buck converter, but additionally scaled with transformer turns ratio. According to figure, if primary has N times more winding then the secondary, in continuous conduction mode CCM it is:
Transformer can increase input voltage, but in practice forward converter is used for stepping down input voltage, together with galvanic isolation, so transformer is used to additionally step down input voltage. For higher power processing with galvanic isolation, full or half bridge is used.