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Old 7th September 2004, 08:26 AM   #1
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Default 20A power supply

I need to make up a regulated dc supply, 12V 20A for a fanless switch mode computer supply.

Any advice as to use parallel series pass transistors, or just parallel 7805s?

If you know of a good published circuit, please point me to the source.

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Old 7th September 2004, 08:48 AM   #2
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12v 20 amps for a switching computer power supply?
I'm lost here
Are you wanting to create the power supply for the computer itself (hence the need for a 12v and 5v rail) or are you powering a 12v device that runs a computer?

Either way you probably wouldn't want to linear regulate it. You can parallel nationals simple switching regulators for 20 amps with a fair amount of ease.
Otherwise, in the 78xx datasheets theres a schematic that allows for a greater current capability using discrete transistors. This will make alot more heat than a switching regulator.

cheers
Matt
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Old 7th September 2004, 10:48 AM   #3
MikeB is offline MikeB  Germany
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Hmm, if you want a fanless regulated powersupply giving 20A,
you shouldn't go without switchmodetopology. But switchmode
doesnt use any topology like 7812.
Let's say you feed the powersupply with 15volt unregulated DC,
you have to dissipate 3v*20A = 60watts. Unless you use a seperate
case just for the supply, you need a big fan to get rid of that much heat.
And i'm not sure if 15volts is enough.
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Old 7th September 2004, 11:03 AM   #4
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[QUOTE]Originally posted by fr0st
[B]12v 20 amps for a switching computer power supply?
I'm lost here
Are you wanting to create the power supply for the computer itself (hence the need for a 12v and 5v rail) or are you powering a 12v device that runs a computer?

Either way you probably wouldn't want to linear regulate it. You can parallel nationals simple switching regulators for 20 amps with a fair amount of ease.
------------------------------------------------------------------------------
The PS for the computer is a 12/5/3/3V compact unit that sits on the power supply socket. It is dc-dc convertor requiring 12V input. Hence 20A needed for peak 200W spec of the convertor. Might as well over engineer.

Other people seem to use 60W swtching units to popwer a 200W dc convertor!
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Old 7th September 2004, 11:06 AM   #5
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Quote:
Originally posted by MikeB
Hmm, if you want a fanless regulated powersupply giving 20A,
you shouldn't go without switchmodetopology. But switchmode
doesnt use any topology like 7812.
Let's say you feed the powersupply with 15volt unregulated DC,
you have to dissipate 3v*20A = 60watts. Unless you use a seperate
case just for the supply, you need a big fan to get rid of that much heat.
And i'm not sure if 15volts is enough.
------------------------------------------------------------------------
Yes separate case and big heat sink; I already have these.

I have silenced the dc-dc convertor that the PS will supply; don't want another switch mode unit to complicate matters.

My experience is that computer audio sound can be imp[roved considerably by doctoring and improving the PS. Getting rid ofn fans is another issue.
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Old 7th September 2004, 11:16 AM   #6
MikeB is offline MikeB  Germany
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Hmm, in this case i would suggest a 12volt carbattery ?
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Old 7th September 2004, 12:55 PM   #7
Eva is offline Eva  Spain
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That computer SMPS powered from 12V may not require a 12V regulated input and thus a simple transformer plus diodes and capacitor would do the job

Anyway, never forget that the 12V powered SMPS uses higher peak currents, uses switching MOSFETs instead of bipolar transistors and uses more EMI-likely topologies so it will surely generate more EMI than any standard 120V-220V powered ATX PSU

Also I think 12V powered equipment does not have to pass as serious EMI tests as mains powered equipment, so your sound card is likely to be getting more supply line ripple

The only way to go with computers is galvanically isolated SPDIF outputs [transformer or optical coupling] and using external DACs. Computer expansion cards were never intended for precision analog circuits, only for digital circuits

The insides of any computer is an EMI hell!!

And I'm not talking about vague audiophile stuff, I talk about listening to your hard disk/CR-ROM/DVD operations, video card refresh pulses, bus activity, fan ripple, even keystrokes... from the analog audio output of the sound card
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Old 7th September 2004, 01:43 PM   #8
MikeB is offline MikeB  Germany
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"The insides of any computer is an EMI hell!!"

Hey, thats funny !

Then, is a mobilephone the "lord of darkness" ?
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Old 7th September 2004, 03:04 PM   #9
Eva is offline Eva  Spain
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Old 7th September 2004, 03:55 PM   #10
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Quote:
Originally posted by MikeB
Hmm, in this case i would suggest a 12volt carbattery ?
---------------------------------------------------------------------------
Too heavy too troublesome

Doesn't last long enough for me to forget the thing.

Don't want to implement auto charger etc
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