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Switched Mode Power Supplies

Switched Mode Power Supplies

Of necessity this is only a broad overview of switched mode power supplies or SMPS. The switched mode power supplies or SMPS owes its origins back to the days when valve or tube car radios needed the large HT supply, e.g. 150V DC to be generated from an automobile power system of nominally 12V DC.
The end I provide a link to a 200W ATX PC Power Supply including schematic. This may prove helpful in troubleshooting PC power supplies.

In those days the switched mode power supply took the form of a “vibrating reed” or vibrator which “chopped” up the 12V DC by electro-mechanical means and was then applied to a transformer, rectifier and filtering circuit to produce the much higher DC output.

Switched mode power supplies or SMPS as DC to DC Converters

Essentially switched mode power supplies or SMPS act as DC to DC converters. In these applications the switched mode power supply acting as a DC to DC converter first rectifes an AC input voltage (110V / 240V), converts it to DC and depending upon the design considerations chosen, chops this DC in a “chopper” and converts it to a higher or lower level of DC or perhaps both a higher AND lower level of DC. Typical modern applications include your computer power supply and the power supplies in your TV and Video sets.


Older televisions sets had power transformers which were bulky, heavy and quite expensive. Although modern solid state electronics have considerably reduced the power consumption of TV sets, the switched mode power supply has entirely replaced the power transformer leading to a substantial reduction in cost, size, weight and vastly improved efficiency in the use of power in a modern television set. Similar improvements and efficiencies brought about by switched mode power supplies can be seen in modern computers. As one example I serviced a video for my daughter – actually removed pieces of toast – it was a TEAC video and its power consumption was 240V 50 Hz 14 Watts.

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