Powering Your PC: Truths and Misconceptions
A Guide To Power Supplies

Switching Power Supplies:

All modern PCs use what is known as a "switching power supply". While the process is more complex, it yields much greater efficiency & power density. A switching power supply works by controlling the average voltage supplied to the load. This is done by opening and closing a switch (usually a high powered field transistor) in a rapid fashion (hence the name "switching") and is commonly known as Pulse Width Modulation (PWM). PWM is the holy grail of what makes this type of power supply work, so if you happen to forget everything else, at least try to remember PWM.

The graph below illustrates pulse width modulation and is fairly simple to figure out: V = Voltage, T = Time, o = out, i = in. The average voltage seen by the load can be explained in the following equation: V_o(avg) = (t_(on)/T) x V_i. The pulses happen so quickly (in the kHz range) and capacitors help keep things constant so that the load doesn't see the rapid on / off cycle, to the load it is a constant voltage. Reducing t_(on) (time on) reduces V_o(avg) (Average voltage out), and the opposite is also true, increasing the pulse time increases the voltage.

Example of Pulse Width Modulation¹

In a nutshell, a switching power supply takes the incoming AC voltage (generally 120V or 220V) and it is rectified (using a high-voltage / low current bridge rectifier), filtered (to remove AC line noise), and optionally can have PFC (power factor correction) circuitry at this stage. A pair of large capacitors are used for smoothing the voltage before it is switched via a field-effect transistor (usually a MOSFET).

A metal-oxide semiconductor field-effect transistor (MOSFET) is connected in series with the primary side of a transformer and serves as the on-off switch. The power coming out of the secondary side of the transformer is then full-wave-rectified and reconstructed into the proper DC level. Feedback (V_o, I_o, etc.) can be sent back to the primary side to serve as input to the PWM circuit. The PWM circuit then modifies the length of time that the MOSFET is switched on in order to maintain output regulation.

Simple Switching Power Supply Example¹

Here's an example, say our switching power supply was producing 12Vdc out powering a 6A load. Now, if the load increased to 8A, the voltage would drop slightly. Feedback being sent back to our PWM circuit would show the drop in voltage, which in turn would turn the MOSFET on for a slightly longer period of time t_(on) allowing more power to go through the transformer which will bring the voltage back up to the preset level (12Vdc).

Switching frequency for a switching power supply usually ranges between 30kHz and 150kHz, but it can be much higher. Frequency for linear power supplies is the same as the incoming line frequency (typically 50/60Hz). Switching frequency selection depends on the application for which the power supply is designed. The selection of the frequency has to be such that none of these harmonics will interfere with the load.²