Newsgroups: sci.electronics Path: utzoo!henry From: henry@zoo.toronto.edu (Henry Spencer) Subject: Re: Switched power supplies, how do they actually work ? Message-ID: <1990Oct15.151720.28380@zoo.toronto.edu> Organization: U of Toronto Zoology References: Date: Mon, 15 Oct 90 15:17:20 GMT In article otto@tukki.jyu.fi (Otto J. Makela) writes: >... talking about power supplies and stuff. Suddenly I find out >that I don't know how a switching power supply works. Could someone with the >necessary knowledge give me the baby-talk -type explanation... Here's one I posted some time ago: > ... can anyone take the time to explain the theory > behind how these supplies actually work, and why they are used rather than > the traditional transformer/diode bridge type? A switching supply replaces not just the transformer and diode bridge, but also the regulators that supply precise output voltages. That's important. The standard sort of regulator for the old linear (transformer/diode) supply holds its output at a given voltage by imposing enough resistance internally to drop the input voltage to the desired output voltage. The trouble is that passing high currents through a resistance generates a lot of heat and wastes a lot of power. A switching supply, on the other hand, turns the flow on and off at a high frequency such that the *average* output voltage is the desired one, and then filters the result to get a (more or less) smooth DC voltage again. The key point here is that the control transistors are either fully off (no current flowing, so no power loss) or fully on (minimum voltage drop through the transistor, so minimal power loss), not halfway between as they usually are in a linear supply. This is much more efficient. As a side issue, if one does want to use a transformer -- usually desirable, partly for isolation from the AC line and partly to make design easier -- the switching can be done at high frequencies where physically-small transformers are efficient. This eliminates the great lump of metal in the corner. :-) Both types of supplies have their advantages. Switchers are smaller, lighter, and more efficient. On the other hand, their output tends to be noisier, they have to be carefully designed to minimize radiated interference, they are more complex, and they often regulate poorly when asked to run at much less than full output. Generally, switchers come out better for powering big pieces of digital equipment (which have semi-predictable power demands and don't mind a bit of noise) and linears come out better for lab work (highly variable demands), analog work (noise sensitivity), and small equipment (lower complexity). -- "...the i860 is a wonderful source | Henry Spencer at U of Toronto Zoology of thesis topics." --Preston Briggs | henry@zoo.toronto.edu utzoo!henry