Why are Magnetic Amplifiers not used as Pre-Regulators in Bench Supplies? - Page 1
Forum section: electronics / projects

author: martys
author_rank: Regular Contributor
timestamp: February 28, 2017, 03:02:13 pm
content: I am amazed by the circuit design in a desktop ATX power supply. I found that ATX supplies regulate the 3.3V output using a magnetic amplifier.This is one thing I have rarely seen discussed on this forum and that is the use and design of magnetic amplifiers in power supplies.Why aren't experimenters using magnetic amplifiers to make efficient pre-regulators for their supply designs? A magnetic amplifier seems to be a very easy way to efficiently do this.And the idea becomes much more attractive when you know that almost anyone can find a discarded PC ATX power supply with the magnetic amplifier cores available for salvage for free!I don't quite understand how the magnetic amplifier in an ATX supply works and my first question is which of the two large toroidal wound cores is the one used for magnetic amplifier control?I would like very much to experiment with magnetic amplifiers to build my own regulator, but I don't know how to begin design using the free materials I already have.Can anyone offer some design insight?BTW, the green rectangle in the attached schematic shows the magnetic amplifier circuitry.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2017, 03:56:16 pm
content: Regulation by magnetic amplifier usually has a limited control range, unless one uses a kind of push / pull version with 2 transductors. So it is not that suitable for coarse pre-regulation for a lab supply. It also works best with a high frequency AC supply. With just one channel it is easier to control the switched mode transformer directly by PWM instead of an extra magnetic amplifier. It is only if you need a second channel that the magnetic control makes a lot of sense.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2017, 10:07:48 pm
content: First you must ask yourself: why did they choose that type of device in the ATX power supply design?Second, you must ask yourself: is that applicable to the design of a typical bench supply?The answer is, probably not.Bench supplies have few outputs (usually up to three). They are usually linear supplies, not switching. Those that are switching, are usually buck regulated from an unregulated (linear) low voltage source (and rarely from a mains voltage source). The outputs are usually independent (though in different ranges, but, you can't generally assume that one output willalwaysbe greater than another).So the downsides are:1. For a linear supply: the magamp has to work at line frequency. The magnetic part itself is huge (several in^3 and hundreds of turns, just for <100W power levels). The loop response will be slow, and output regulation poor. Huge capacitance is required to stabilize it. (This works out almost identically to the old SCR + heavy iron power supplies: very heavy, but very robust, too.)2. For a switching (hybrid) supply: you need an AC source at all times, otherwise there's nothing to regulate from. This doesn't make sense if you have multiple independent outputs...2a. Sure, you could tack a magamp onto a single buck channel, but why not regulate the buck channel in the first place?3. For a postreg design, you could maybe hack it, like... you have a pre-reg buck switcher, 5-20V output, which gets linear-reg'd down to the 0-15V rail. It has a minimum 5V output, so can be magamp regulated for the 0-5V (at more amps) rail. (It wouldn't actually go all the way to zero, nor all the way to 5V; the real margins would need to be widened.)And as Kleinstein notes, the control range is kinda stinky. A narrow range is best.And just to close the circle, what is it about ATX supplies, then? Well...They have two main outputs (+5V and +12V, at ~amps). It would be convenient to add another winding to obtain the +3.3V, but it would never be satisfactory (the transformer would have to be heavily interleaved, to keep leakage inductance low enough, to get useful voltage stability between the outputs). And you incur a diode drop anyway, which already blows the voltage tolerance on this supply.So, instead, they half-wave rectify the 5V winding. But that's only 2.5V (give or take diode drop). Full-wave rectifying would obviously get 5V again, which is no good. So, wouldn't it be great if we could get a 1-1/2-pulse rectifier? Aha, we can use a magamp to cut one pulse in half! And make it adjustable, to regulate.It's really quite a wonderful situation: the other output voltages are fixed, and AC is plentiful (just tap it off the transformer). Perfect use case for a magamp!Unfortunately, it's a lot harder to justify such an approach in most applications. So, between that, and the general lack of knowledge, magamps aren't used that much.I hope this design/overview exercise inspires a few readers, though.Tim

author: prasimix
author_rank: Unknown
timestamp: November 02, 2017, 10:39:36 am
content: By many means I came to this topic too late, but never mind, maybe that encourage OP or someone else to try to do something with mag-amp (pre)regulation in the future. So, it seems that TTi is using mag-amps regularly in their power supplies what easily provides 20 A outputs and few hundreds watts. Take for example CPX400A which repair is posted on this forumhere(thanks ovnr!). If you check schematics available in the same post you can see that power board deliver up to 20 A and after mag-amponlyLC filter (no post-regulator!). I borrowed one of ovnr's picture to put my remarks:Of course, this mag-amp does not works on line frequency (50/60 Hz) that is quite obvious in schematic and switching transformer. Hence its compact dimension/size.EDIT: it has post-regulation with Q9 power mosfet.

author: jaycee
author_rank: Regular Contributor
timestamp: November 03, 2017, 02:51:25 am
content: It's more likely that these cheaper, older ATX power supplies used a magamp for the 3.3v supply, because they already had the transformers designed and wound for the original AT standard. Pretty much all of the modern ones use a tap on the main transformer. Some even more modern ones produce 24V from the transformer and then use DC-DC conversion.