Question

1. What is the type of transformer used in a SMPS power supply? Why is it necessary to use such type of a transformer?

Answer 1

The most usual transformers that form a part of the majority of SMPS circuits are pulse transformers with hard magnetic material based core (usually ferrite) owing to their low hysteresis and ability to work with high frequency signals.

SMPS transformers, working at around 500kHz,like in a TV receiver, use ferrite instead of laminated cores as losses in the ferrite are much less at high frequencies than in laminated cores.

The waveforms handled by transformers in SMPS, in addition to being high frequency, generally have a square wave shape. Because of this, they will contain many harmonics at even higher frequencies. This creates a problem due to 'skin effect'; high frequency currents flowing in wires tend to flow only along the outer skin of the wires, which makes the normal calculations of wire cross sectional area more complex. Because the effective cross sectional area changes with frequency, then so will the effective inductance of the winding. Also, the layout of components in relation to SMPS transformers needs careful design, as electromagnetic interference at high frequencies is greater.