Power Electronics
The forward converter is the isolated version of the buck — unlike the flyback, energy is not stored in the transformer: it is transferred directly to the secondary while the switch conducts (hence the name). The secondary carries a classic buck output stage: series diode D1, freewheeling diode D2 and an LC filter. Output: Vo = D·Vin/n. Because the transformer is a true ratio converter rather than an energy store, the same core moves far more power than a flyback — the classic choice for the 100–500 W band.
There is a catch: every on-time pumps magnetizing current Im into the transformer. If that current has nowhere to go, it accumulates cycle after cycle and the core saturates (= short circuit, smoke). The core must be reset every cycle. The classic fix is a third reset winding; the more elegant one is the two-switch forward: two MOSFETs (high + low) on the primary ends plus two crossed reset diodes. When both switches open, Im flows back into the source through the diodes — the primary sees exactly −Vin and the energy is recycled.
Since reset happens at −Vin, demagnetizing takes as long as magnetizing: duty can never exceed 50% (volt-second balance). As a bonus, each MOSFET in the two-switch version sees at most Vin (instead of the flyback's Vin + n·Vo) — which is why professional off-line designs love it. In our ready-made circuit, watch the reset clamp live on CH2 (the P+ node): +48 V while conducting, ~−0.9 V during reset.
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