SWITCHED MODE POWER SUPPLIES

Switching power supplies require the use of high permeability ferrite cores for the switching transformer. High permeability iron powder cores would be too lossy. Type 77 ferrite is ideal for switched mode supplies operating at 20 - 500 kHz. Although the toroid core is the most efficient shape, the pot-core and E-core are often used for ease of winding. Other components in the power supply must of course be suitable for operation at the switching frequency. As mentioned before, the T-( )-26 cores are ideal for the storage and filter inductors in supplies operating up to 250kHz. Above 250kHz use T-( )-52 cores if available.

There are three basic circuit types for smps, and the one chosen will depend upon the application. The most efficient use of the ferrite switching transformer occurs with the push-pull circuit, and the following information applies to this circuit. It is generally accepted that smps transformers operating at 20 kHz can have a max. core flux density (Bmax) of 2000 gauss/200 mT, but at higher frequencies Bmax must be reduced. A plot of Bmax vs Frequency gives a good idea of the relationship. Excessive temperature rise must be avoided either by using a larger size core and/or wire or reducing the output power. As a guide, the following table shows power capabilities of some different core shapes/sizes at three switching frequencies: