Switch Mode Power Supply

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Double Insulation Symbol (Class 2)

A switching (or Switch Mode) power supply (SMPS) is a more modern version of the linear power supply.

One of the main users of this type of power supply was the computer industry. They have been available commercially since about 1970. Hewlett Packard used one in a calculator in the early 1970s to provide power for the LED display and timing signals.

Switching power supplies are preferred when small size and high efficiency is desired. The typical linear power supply is very inefficient, losing a lot of energy to heat and noise in the transformer and filters. A switch mode power supply rectifies the incoming Alternating Current directly, and this is fed to a chopper. The chopper is a high frequency switch. The resulting waveform is fed to a transformer to be boosted or reduced, as needed. The output is then rectified and filtered to provide a very smooth and stable Direct Current output. By controlling the chopper, the SMPS is capable of very tight regulation of the output voltage. Since the SMPS works with high frequencies, it is easier to filter and smooth the second rectifier's output using small chokes and filter capacitors. A linear power supply would use large chokes and capacitors to filter the 120 Hz output from the rectifier. The SMPS uses frequencies which can be 50 kHz or more. This allows for a small package with little loss to heat, and almost silent operation.

While a linear power supply is simple to construct, a SMPS is smaller and more efficient. Their design also allows it to operate on a wide range of voltages and line frequencies, with quiet operation and good regulation. The linear power supply, while simpler, wastes a lot of energy in the transformer, the filters, and the regulation circuits. It can also be much larger compared to a similar SMPS.

Both designs incorporate transformers to isolate the line (mains) side from the low voltage (LV) outputs.

Many power supplies are double insulated, indicated by the box within a box symbol. For safety, never connect the GND terminal to your earth ground.


  • Regulation is the ability of the power supply to maintain the output under changing load conditions
  • A choke is a coil of wire wound around an iron core, also known as an inductor. It stores energy (current) in a magnetic field, resisting any change in the current.
  • A filter capacitor stores energy (voltage), and resists any change in the voltage