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Interior view of an ATX switched-mode power supply.
A - bridge rectifier
B - input filter capacitors
C - transformer
D - output filter coil
E - output filter capacitors

A switching-mode power supply for laboratory use.

A switched-mode power supply, switching-mode power supply or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator. While a linear regulator maintains the desired output voltage by dissipating excess power in a "pass" power transistor, the SMPS rapidly switches a power transistor between saturation (full on) and cutoff (completely off) with a variable duty cycle whose average is the desired output voltage. The resulting rectangular waveform is low-pass filtered with an inductor and capacitor. The main advantage of this method is greater efficiency because the switching transistor dissipates little power in the saturated state and the off state compared to the semiconducting state (active region). Other advantages include smaller size and lighter weight from the elimination of low frequency transformers which have a high weight and lower heat generation from the higher efficiency.