A switched-mode power supply (SMPS) can be thought of as an electronic power supply that integrates a changing controller to efficiently convert electrical energy. Like other power sources, SMPS transfers power from a DC or AC source (usually mains) to a DC load, such as a PC, converting both voltage and current quality. Completely different from linear power supplies, switch-mode power supplies have pass transistors that constantly toggle between low-power, fully-on and fully-off states, with little time to devote to high-power transitions, limiting wasted energy. Voltage regulation is achieved by changing the ratio of the switching time (also known as the duty cycle).
Conversely, linear power supplies regulate voltage by constantly consuming power in the pass transistors. This greater conversion capability is a significant advantage of switching power supplies. Switch-mode power supplies may also be smaller and lighter than linear power supplies due to the smaller size and weight of the transformer. Switching controllers are used as an alternative to linear controllers when higher efficiency, smaller size, or lighter weight are required. However, they are more complicated. If their switching currents are not deliberately suppressed, electrical noise problems can result, and simple designs can have poor power components.