What is the difference between low and high frequency transformer?

For a given supply voltage, the flux density in a transformer core is:

• Inversely proportional to supply frequency

• Inversely proportional to the cross-sectional area of the core.

So as the operating frequency of a transformer increases, for the same voltage and power, we can use less turns and a smaller core cross-sectional area. So a high-frequency transformer is smaller than a low frequency transformer of the same power rating.

However there is one problem running at higher frequencies - the hysteresis losses in the core will increase with frequency if the flux density is kept constant. So for high frequency transformers (say operating at higher than 500Hz ) we ditch the laminated iron core and use a ferrite material. This needs to be operated at a lower flux density than iron but exhibits zero hysteresis losses.

In summary, when comparing a high frequency transformer eith a low frequency transformer of the same power and voltage ratings:

• The high frequency transformer is smaller and lighter.

• The high frequency transformer has fewer turns on primary and secondary.

• The high frequency transformer has a ferrite core rather than laminated iron.

• High current windings on the high frequency transformer are likely to be “multifilar” i.e. a quantity of smaller wires rather than one large wire. This is to reduce the “skin effect” which becomes a problem at high frequency.