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From: "Da Man"
Subject: Re: Multi-voltage PC switchers -- how?
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Date: Tue, 19 Nov 2002 01:11:48 GMT
NNTP-Posting-Date: Mon, 18 Nov 2002 21:11:48 AST
"David Lesher" wrote in message
> PC supplies are getting up there. I wonder how they are now
> being designed...
> The 150w ones [i.e. bottom of the line] regulated on the 5v line,
> and just let the 12v and others tag along. [Maybe it was the 12v;
> but in any case..]
> But a friend has a new one that's something like 20A at 3.3v, 40A
> at 5v, 15A at 12v, and a little -5/12 etc. (I expect twist-lock 30
> amp plugs al-la PDP-11's Real Soon...)
Doubt that! Actually, the problem is well solved already with many redundant
wires only capable of supplying 8 -15A each. The 3.3V is sensed right at the
plug to account for the voltage drop in the wiring.
>The first three claim
> independent regulation.
OK, the typical ~250W ATX supply has 20A @ 5V, 30A @ 3.3 V, 12V @ 8 A, 0.5 -
1A @ -5 and -12V, and a standby ~1A @ 5V supply.
The 5V standby supply is a seperate switcher used just for that purpose. The
rest is generated by a big switching transformer (usually in a half bridge
configuration, but flybacks are not uncommon) regulated by the 5V line.
Tapped into the 5V transformer output is a mag amp used to cut the voltage
down to 3.3 V. By saturating the mag amp's core when the voltage rises above
3.3 V regulation is achived. Older, cheap models just used a honk'in power
MOSFET to linearly regulate the 5V down to 3.3V. The 12 V is regulated by
using a common filter inductor for both the 5 V line and the 12V line. With
carful filter inductor and transformer design the 12V line will be well
regulated, as long as the 5V line is loaded down a bit. No load on the 5V
volt line will cause the power supply's 12V to sag. The -5V and -12V are
generated by an extra set of small diodes in reverse from the ones that
generate the 5V and the 12V.
Now for the scary part, How much energy is wasted in heating up those nice
large fan cooled heat sinks used on the rectifiers and switching
transistors? Now multiply that by the 100's of millions of computers in use
> Pondering same, I was wonder how it's done. You can have one
> rectifier/filter; that's easy. But do you have 3 separate
> oscillators/three separate PWM regulators; or do you make
> high frequency (well, in THIS context..) AC and distribute
> and use that....or?
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