Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

A [[switched-mode power supply]] (SMPS) works on a different principle. AC mains input is directly rectified without the use of a transformer, to obtain a DC voltage. This voltage is then sliced into small pieces by a high-speed electronic switch. The size of these slices grows larger as power output requirements increase.

The input power slicing occurs at a very high speed (typically 10 kHz — 1 MHz). High frequency and high voltages in this first stage permit much smaller [[step down transformer]]s than are in a linear power supply. After the transformer secondary, the AC is again rectified to DC. To keep output voltage constant, the power supply needs a sophisticated feedback controller to monitor current draw by the load.

Modern switched-mode power supplies often include additional safety features such as the [[crowbar (circuit)|crowbar circuit]] to help protect the device and the user from harm. ''High efficiency direct coupled switched mode power supply'': ''The power supply can also include crowbar circuit protecting it against damage by clamping the output to ground if it exceeds a particular voltage.'' > They are only able to output above a certain power level and cannot function below that point. In a no-load condition the frequency of the power slicing circuit increases to great speed, causing the isolation transformer to act as a tesla coil, causing damage due to the resulting very high voltage power spikes. Switched-mode supplies with protection circuits may briefly turn on but then shut down when no load has been detected. A very small low-power [[dummy load]] such as a ceramic power resistor or 10 watt light bulb can be attached to the supply to allow it to run with no primary load attached.

[[Power factor]] has become a recent issue of concern for computer manufacturers. Switched mode power supplies have traditionally been a source of [[power system harmonics|power line harmonics]] and have a very poor power factor. Many computer power supplies built in the last few years now include power factor correction built right into the switched-mode supply, and may advertise the fact that they offer ''1.0 power factor''.

By slicing up the sinusoidal AC wave into very small discrete pieces, the portion of the alternating current not used stays in the power line as very small spikes of power that cannot be utilized by AC motors and results in waste heating of power line transformers. Hundreds of switched mode power supplies in a building can result in poor power quality for other customers surrounding that building, and high electric bills for the company if they are billed according to their power factor in addition to the actual power used. Filtering capacitor banks may be needed on the building power mains to suppress and absorb these negative power factor effects.