Switching Mode Power Supply Circuits

A switching mode power supply (SMPS) converts electrical power from one voltage to another by rapidly switching a transistor on and off, rather than dissipating excess energy as heat in a series regulator. Because the switching transistor is either fully on or fully off, it wastes very little power, so an SMPS is far more efficient and runs cooler than a linear supply of the same rating. This efficiency, along with smaller and lighter magnetics, is why switching supplies are used in computers, phone chargers, and most modern electronic equipment.

The core idea is to chop the input into a high-frequency square wave, pass it through an inductor or transformer, and then rectify and smooth it back into a steady direct current. A feedback loop continuously adjusts the proportion of time the switch is on — the duty cycle — to hold the output voltage constant despite changes in load or input. Common DC-DC converter topologies include the buck converter, which steps voltage down, the boost converter, which steps it up, and the buck-boost, which can do either.

Operating at high frequency, often tens or hundreds of kilohertz, lets the inductors and transformers be small, but it also generates electrical noise that must be filtered to avoid interfering with other equipment. Offline supplies that run from the mains add a transformer for isolation and safety. Design involves choosing the switching frequency, the magnetic components, and the control method, and managing switching losses and electromagnetic interference.

Frequently asked questions

Why is a switching supply more efficient than a linear one?

Its switching transistor is either fully on or fully off, so it dissipates little power. A linear regulator instead drops the unwanted voltage across a transistor, turning the difference into heat.

What is the difference between buck and boost converters?

A buck converter steps the input voltage down to a lower output, while a boost converter steps it up to a higher output. A buck-boost can produce a voltage above or below the input.

Why do switching supplies need filtering?

The rapid switching produces high-frequency noise and electromagnetic interference. Input and output filters are added to keep this noise from disturbing the load and other nearby equipment.