High frequency transformers don't need a large core or many windings to get a lot of power, so they can be made small and switching power supplies cost little. The oscillator duty cycle can be adjusted through feedback, allowing it to be adjusted without losing a lot of power. So you get good regulation and good efficiency at the same time. Switching Power Supplies This article will introduce line-operated flyback power supplies. Other topology converters exist and are popular when no wiring is required, but when you look at what's going on between the AC input and the DC power supply of some electronic device today, this is the basic topology used, then filtered through a pair of capacitors . This will generate two high voltage power supplies. One positive, the other negative. These high voltage supplies are then switched on the primary winding of the transformer using a pair of transistors. The switching action is relatively fast. Typical switching speeds are around 40,000 cycles per second or 40KHz.
Often integrated circuits are used to control transistors. The output voltage of the source is determined by the "on" time of the transistor. If the transistor remains on for a longer time, the output voltage of the power supply will rise, while a shorter time will reduce the output voltage. This is called "pulse width modulation".
The output of the transformer, which is now AC, is rectified through a high-speed diode to turn it back into DC. However, this output is not pure DC and requires general filtering to clean up the high frequency "noise" created by the speed switching action of the transistors. Filtering is accomplished by using a combination of coils (also called "chokes") and capacitors.
The output voltage of the power supply can be regulated by feeding some output back to the integrated circuit that controls the switching transistors. If the output voltage is too low, the IC will allow the transistor to remain powered on for a long time, increasing the voltage. An excessively high output voltage will signal the IC.