Switched mode power supplies.

Switched mode supplies.
A switched mode power supply works by switching a DC supply on and off.
As any active components are always either on or off, rather than any level between the two, less energy is wasted as heat. Also, as these supplies are generally run at higher much frequencies than the mains supply, smoothing is much easier and there will be no mains 'hum' present.



Input rectifier and smoothing.
The rectifier converts the mains AC input to DC. The output from the rectifier will be rising and falling at either mains frequency or twice mains frequency, depending upon the type of rectifier circuit used.
The smoothing reduces the ripple from the rectifier stage, to produce a more steady voltage output to the chopper circuit.

Oscillator and chopper.
The oscillator creates a square wave to drive the chopper circuit, which then switches the power to the transformer on and off. These circuits run at a higher frequency than the mains supply, and the timing of the pulses will affect the final supply output voltage.

Transformer.
This converts the input voltage to the required output voltage and isolates the input and output circuits. For supplies with a high voltage input, this isolation can be important for safety reasons.

Output rectifier and smoothing.
As the DC has, once again, been converted to AC, this must be changed back to DC and smoothed as before. However, this time the frequency is higher, so smoothing the output is much easier than for the mains frequency input.

Feedback.
To regulate the output voltage, a feedback circuit controls the oscillator. To ensure isolation between input and output, this may come from another winding on the transformer or use some other component to isolate the two circuits.

Further information.
  Requirements.
  Linear supplies.
  Regulation.