Linked from
The 9 pages that link to Switch-mode power supply, each with the reason it gives.
TransistorRelated: Power transistors switch efficiently to convert voltage and current.
MOSFETRelated: MOSFETs serve as efficient high-speed switches in many such converters.
InductanceRelated: Inductors store and release energy during its switching cycles.
Pulse-width modulationRelated: PWM controls switch timing to regulate voltage or current.