hovan@bgsuvax.UUCP (John Hovan) (10/21/87)
While working with various high quality electronic components, I have noticed the utilization of switching power supplies. I know these supplies dissipate less heat and work by somehow increasing the input frequency, but are these supplies considered new technology to replace humming transformers? More importantly 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? John Hovan BGSU Hardware Support hovan@bgsuvax.uucp
robert@csustan.UUCP (Robert Zeff) (10/26/87)
In article <1340@bgsuvax.UUCP>, hovan@bgsuvax.UUCP (John Hovan) writes: > > > > While working with various high quality electronic components, I have > noticed the utilization of switching power supplies. I know these supplies > dissipate less heat and work by somehow increasing the input frequency, > but are these supplies considered new technology to replace humming > transformers? More importantly 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? The switching power supply has been developed in response to demand for lightweight, efficient & regulated power supplies. Transformer core size is related to the operating frequency due to their saturation characteristics. 60Hz transformer cores are huge, with a lot of wire, and are heavy and lossy. Linear regulators are also very lossy. Switching supplies operate in the frequency range of 20KHz to 1MHz with lightweight cores, using pulse modulation techniques for efficient regulation. Essentially switchers they work like this: The ac line voltage is directly rectified and filtered to produce a raw dc. This dc is fed to a switching element, ie., transistor, FET, SCR. This device will chop the raw dc at the operating frequency resulting in a high voltage chopped waveform. This is waveform is fed to a transformer, rectified, and integrated to a smooth dc. Regulation is accomplished by monitoring the output voltage and varying the duty cycle of the high voltage chopped waveform. (hence the term PWM, Pulse Width Modulator). An analogy might be to switch a light off and on very fast, so that one can't see the switch cycles. The thermal mass and the eye would be the integrator, varying the duty cycle would control the intensity. Many types of switchers exist: Half and full bridge, push pull, Cuk, flyback, series-resonant, forward converter, etc. A good book is: High Frequency Switching Power Supplies, George Chryssis, McGraw-Hill #ISBN 0-07-010949-4 -- Robert Zeff (209) 577-4268 voice, FAX: (209) 577-8548 ZAPCO {lll-lcc,lll-crg}!csustan!zapco!robert 2549 Yosemite Blvd Ste. E {lll-lcc,lll-crg}!csustan!zhome!robert Modesto, Ca. 95354 {lll-lcc,lll-crg}!csustan!robert
henry@utzoo.UUCP (Henry Spencer) (10/28/87)
> ... switching power supplies. I know these supplies > dissipate less heat and work by somehow increasing the input frequency, > but are these supplies considered new technology to replace humming > transformers? More importantly 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). I think the above is relatively accurate, although I'm not an expert on this and may be wrong on some details. -- PS/2: Yesterday's hardware today. | Henry Spencer @ U of Toronto Zoology OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry
blarson@skat.usc.edu (Bob Larson) (05/29/89)
Anyone seriously contemplating building such a switching power supply
should proably look at the article in a recent Circuit Celler magazine.
(May/June or April/May 1989 I think.) It describes step up, step down,
inverting, and up/down converters. Getting regulated 12vdc out of a 12v
battery is by far the hardest. The final design for a multi-voltage
converter is probably close to what is needed for a PC clone.
Bob Larson Arpa: blarson@skat.usc.edu
Uucp: {uunet,cit-vax}!usc!skat!blarson
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