These are notes of an impromptu lecture given to the Fisher’s Ghost Amateur Radio Club on 18 March 1997. They are not meant to be an introduction into switching power supplies only. One note of warning. Switching or switch-mode power supplies often operate with high voltage DC, with large capacitors. These capacitors may take weeks to discharge if their discharge resistor has blown, and will kill you easily.

Switching Power Supplies have some things backward. They have Diode Capacitor switch Transformer

The Diode and the capacitor rectify the incoming mains voltage generating high voltage direct current. This is done without using a transformer, so a device is added to reduce the turn on current to the capacitor.

But by turning the switch on and off very fast the transformer does not need to be as large. Think of a transformer as a couple of inductors as shown in this circuit diagram. As the frequency increases more signal can get through the inductors. In effect if you ignore the voltage change a transformer is not much more than a high pass filter. And once the inductive losses are reduced as frequency increases, the transformer can get smaller.

Voltage regulation is performed by changing the duty cycle of the switch, such that the voltage goes up or down. By turning on the switch longer we have a higher voltage at the output.

Turning a switch on and off fast creates harmonics which cause problems to other equipment nearby. To remove this interference, inductors are added on the input and the output.

 

Since Switching power supplies have no normal voltage regulator they become up to 90% efficient. Normal linear power supplies are about 60% efficient. That means 40% of the power going into your power supply is being lost as heat.

 

In summary Switching power supplies are cheaper because

Transformers are smaller containing less copper They have no series voltage regulators as in normal power supplies (Such as a 3 pin voltage regulator) Output capacitors are much smaller They are more efficient They are physically smaller

Of course these power supplies are more complex, but with mass production they become much cheaper.

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