A switched mode power supply, switch-mode power supply, or SMPS, is an electronic power supply unit that incorporates a switching regulator. In linear power supply uses a linear regulator. Linear regulators uses a transistor baised in its active region to specify an output voltage, but in SMPS actively switches a transistor between full saturation and full cutoff at a high rate.
In SMPS, the active device that provides regulation is always operated in a switched mode, it is operated either in cutoff or in saturation. The input DC is chopped at a high frequency(10 kHz to 100 kHz) using an active device and the converter transformer.
A sample of the output voltage is used as feedback signal for drive circuit for the switching transistor to archive the regulation. The us of feedback mechanism is to alter the output voltages as per the requierment of the load. Hence SMPS is more efficient than linear power supply.