Forward converter magnetics

The procedure presented in this section applies explicitly to the single-switch forward converter. However, the general procedure remains unchanged for the two-switch forward converter as well. 

If foil is 14 mils thick, its cross sectional area is 14 x 530 = 7420 sq.mils 

The magnetization current component is not coupled by transformer action to the secondary. In that sense, it is like a parallel leakage inductance. We need to subtract this component from the total switch current, and only then will we find that the primary and secondary currents scale according to the turns ratio. In other words, the magnetization current does not scale — it stays confined to the primary side. 

For various subtle reasons, like being able to ensure the transformer resets predictably under all conditions, and also for various production-related reasons, the number of turns of the tertiary winding is usually kept exactly the same as the primary winding. Therefore by transformer action, the voltage at the primary-side switching node (drain of the mosfet) must rise to 2 x Vin when the switch turns OFF. Therefore, in a universal-input off-line single-ended (i.e. single-switch) forward converter, we need a switch rated for at least 800 V. 

Note that even prior to transformer reset, the secondary winding has not been conducting for a while — simply because the output diode (i.e. the one connected to swinging end of the secondary winding) has been reverse-biased during the time the tertiary winding was conducting. 

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The flyback inductor actually behaves both as an inductor and a transformer. It stores magnetic energy as any inductor would, but it also provides mains isolation (mandated for safety reasons), just like any transformer would. In the forward converter, the energy storage function is fulfilled by the choke, whereas its transformer provides the necessary mains isolation. 

The most basic question in design invariably is — what input voltage represents the worst-case point at which we need to start the design of the magnetics (from the viewpoint of core saturation) For the forward converter choke, this should be obvious — as for any buck converter, we need to set its current ripple ratio at around 0.4 at Vinmax- But coming to the transformer, we need some analysis before we can make a proper conclusion. 

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