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. Disadvantages include greater complexity, the generation of high amplitude, high frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof.
SMPS can be classified into four types according to the input and output waveforms, as follows.
|Linear power supply||Switching power supply||Notes|
|Size and weight||If a transformer is used, large due to low operating frequency (mains power frequency is at 50 or 60 Hz). Small if transformerless.||Smaller due to higher operating frequency (typically 50 kHz - 1 MHz)||A transformer's power handling capacity of given size and weight increases with frequency provided that hysteresis losses can be kept down. Therefore, higher operating frequency means either higher capacity or smaller transformer.|
|Output voltage||With transformer used, any voltages available; if transformerless, not exceeding input. If unregulated, voltage varies significantly with load.||Any voltages available. Voltage varies little with load.||A SMPS can usually cope with wider variation of input before the output voltage changes.|
|Efficiency, heat, and power dissipation||If regulated, output voltage is regulated by dissipating excess power as heat, which is inefficient; if unregulated, transformer iron and copper losses significant.||Output is regulated using duty cycle control, which draws only the power required by the load. In all SMPS topologies, the transistors are always switched fully on or fully off.||The only heat generated is in the non-ideal aspects of the components. Switching losses in the transistors, on-resistance of the switching transistors, equivalent series resistance in the inductor and capacitors, and rectifier voltage drop will lower SMPS efficiency. However, by optimizing SMPS design, the amount of power loss and heat can be minimized. A good design can have an efficiency of 95%.|
|Complexity||Unregulated may be diode and capacitor; regulated has a voltage regulating IC or discrete circuit and a noise filtering capacitor.||Consists of a controller IC, one or several power transistors and diodes as well as a power transformer, inductors, and filter capacitors.||Multiple voltages can be generated by one transformer core. For this SMPSs have to use duty cycle control. Both need a careful selection of their transformers. Due to the high operating frequencies in SMPSs, the stray inductance and capacitance of the printed circuit board traces become important.|
|Radio frequency interference||No high-frequency interference. Some mains hum induction into unshielded cables, problematical for low-signal audio.||EMI/RFI produced due to the current being switched on and off sharply. Therefore, EMI filters and RF shielding are needed to reduce the disruptive interference.||Long wires between the components may reduce the high frequency filter efficiency provided by the capacitors at the inlet and outlet.|
|Electronic noise at the output terminals||Unregulated PSUs may have a little AC ripple superimposed upon the DC component at twice mains frequency (100-120 Hz). Can cause