Detail of switched mode power supply?

Answer 1

A switched mode (or switching) power supply is one that uses pulse-width modulation technology, rather than linear technology, to provide the voltage required to a load. Often, it requires no power transformer, nor is a great deal of power developed across the power supply, which reduces size, cost, and heat. It works by the fact that inductors resist a change in current. There is a high voltage power supply, often 160VDC running directly off the AC line. A transistor or other switching device supplies power to an inductor, which is in series with the load. There is also a Schottky diode from the input side of the inductor to ground. When the transistor is on, voltage is supplied to the inductor, which supplies current to the load. When the transistor is off, back voltage from the inductor is shunted to ground through the diode, and the inductor continues to supply current to the load. A pulse-width modulator monitors the load voltage, and adjusts the transistor on time at a high frequency. The average current flow, and thus the average load voltage, remain constant. Since the transistor turns on hard - it is in saturation - very little voltage drop is across it. As a result, very little power is developed, resulting in less overall heat.