Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!zaphod.mps.ohio-state.edu!sdd.hp.com!ucsd!ucbvax!hplabs!hpfcso!hpfcdj!myers From: myers@hpfcdj.HP.COM (Bob Myers) Newsgroups: sci.electronics Subject: Re: Switched power supplies, how do they actually work ? Message-ID: <17660122@hpfcdj.HP.COM> Date: 17 Oct 90 18:58:51 GMT References: Organization: Hewlett Packard -- Fort Collins, CO Lines: 27 >I don't understand how a switcher "replaces" the bridge, >since the AC mains still have to be converted to DC. >I suppose that the transformer can be eliminated since no >voltage step-down is required (but then the pass transistor has In fact, there IS a rectifier after the AC mains in switchers - I think Henry got a bit carried away with that one. Also, one will very often find a transformer included in a switching supply - but located just "after" the switch, and operating at the switching frequency (so it's considerably different than the typical 60Hz power transformer of linear designs). For that matter, there's really no *need* for a power transformer in a linear design - but it'd take a helluva pass transistor to get from the 300 VDC rail down to, say, a +5 VDC output! :-) A true "transformerless" design in either case usually only comes about when the desired output voltage is within a reasonable fraction (up or down) of the DC rails existing immediately after the rectifier. One other *disadvantage of switchers that I believe Henry forgot to mention is a typically atrocious power factor, as the switching action plays merry hell with the input current waveform. This has become quite a concern at sites which have a large number of independent switchers (such as a bunch of PCs or workstations) on their AC lines. Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers@fc.hp.com | sentient life-form on this planet.