A power supply is a device that supplies energy to electric loads and converts one form of energy to another. Power supplies are capable of converting one form of electrical energy to another. A power supply transfers power from a source like the electrical power grid to a load such as a personal computer, while converting voltage and current characteristics. It is usually employed to provide a regulated output voltage, different from the input voltage. There are many different types of power supplies. Broadly, they can be divided into linear and switching power supplies.

Linear power supplies rely on the use of a mains transformer to obtain direct current voltage from alternating current. Linear power supplies are bulky and heavy and tend to consume energy, thus making efficiency low. Switched mode power supply (switching mode power supply, switch power supply or simply a switcher) is an electronic power supply that works on a different principle. Alternating current is converted to direct current without the use of a mains transformer. The switch in voltage is regulated by an electronic switching circuitry, which helps this form of power supply be a highly efficient converter. This is because the pass transistor switches very quickly between full on and full off states, making it more efficient than a linear power supply, which must dissipate excess voltage to regulate the output.

The input power slicing in switching power supplies occurs at extremely high speeds such as 10 kHz - 1 MHz. The high frequency and high voltages means that smaller transformers and smoothing capacitors can be used. This is different from linear power supplies, which deal with supplies operating at mains frequency, which requires larger transformers and capacitors. This superior efficiency of switch power supplies enables these power supplies to be lighter in weight and smaller in size than linear power supplies. This is crucial to lower shipping costs, and improved distribution.

Switching power supplies include safety features such as current limiting, or a crowbar circuit to help protect the device and user from harm. If there is a high current power draw detected, the switch power supply will assume that it is a direct short circuit and will shut itself down to prevent damage from being done to the device. Switch mode power supplies with protection circuits will shut down if no load has been detected to protect the device.