Switch Mode Power Supply

Switching Power Supplies By Darryl Smith, VK2TDS

By Darryl Smith, VK2TDS

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.

Conventional power supplies have the
following components.

  • Transformer
  • Diode(s)
  • Capacitor
  • 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.

    Once DC has been generated the transformer
    is powered by rapidly turning on and off the DC supply to the transformer. The
    effect of this is to create a very high frequency AC signal from the DC signal
    in a similar way to an audio amplifier.

    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.

    The capacitor and the diode at the end act
    like a normal power supply. However as the capacitor is being charged more
    often, a smaller capacitor may be used.

    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.