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Old 25th July 2010, 09:48 PM   #1
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Default Quick Q - 2 SMPS supplies in parallel

This isn't audio related but i've received a lot of great info from you SMPS guys here, so here goes: I have a dual Pentium III system which is *extremely* 5v heavy. It is powered by a Xilence 450W PSU which is fine otherwise, but being that practically only 5v is used (all the fans are either on 5v or 7v as well...), it drops to 4.65v, and 12v is up to 12.8v.

The system runs fine with the Gigabyte 6BXDS motherboard, but that limits my 1400MHz Tualatins to 1000MHz since the board only supports FSB 100. I have a fully modded Asus P2B-DS that could run the processors at their rated speed, but for some reason it will only work stable with one Tualatin. Lower CPUs such as a pair of 700MHz CPUs work fine. I'm thinking the instability reason is the huge 5v power requirement - on the P2B-DS 5v drops even lower, to 4.5v, when i try to run both Tualatins at 1400MHz.

I was thinking of solving this the quick and dirty way - running a buck regulator off the 12v rail and paralleling its output with the 5v rails in the ATX connector on the motherboard. That would also have the benefit of bringing the 12v rail down to safer levels. The power supply has 30 or 35A on the 5v rail (can't remember exactly, will have to check), and it isn't enough. I haven't really seen ATX power supplies with more than that on the 5v rail. So, would my idea work?
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Old 26th July 2010, 03:05 AM   #2
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How do you enforce current sharing?

The best way would be to sense the main supply's current flow (with a low resistance high-side shunt, I suppose?) and pump in a proportional amount of current to help it out. You have to compensate it, so it doesn't buzz in weird ways, which is always an interesting problem when connecting systems to other systems. Minus a small amount of energy stored in the buck inductor, the total load will be constant as seen by the supply, and 5 and 12V are jointly regulated, so it shouldn't notice anything unusual. Then it's just a matter of keeping current stable, which is first-order, so you'll have a nice, simple 2nd order control loop. The problem is then measuring current, which is high side, requiring a differential amp. With a 12V rail available, this isn't as hard as it could be.

It would be cheaper to buy a new supply, or more practical to buy a new motherboard. Man, I'm not even running 100 FSB, and my motherboard is a hunk of junk...

Tim
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Old 26th July 2010, 04:27 PM   #3
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I can't find a higher capacity supply for one, as everything now is designed for lots of amps on 12v. Secondly the P2B-DS (the board i am trying to get working) does 133 FSB (up to 150). The P2B-DS along with the CPUs i bought from someone in the USA because it was next to impossible to find another one here (i used to have another P2B 5 years ago but a crappy power supply fried it).

Mind you, this is not my primary computer, i have it at my grandparents' place and it's enough for what i do there. Here at home i have a Core 2 Duo with 8GB RAM, enough said. However the dual PIII is a very reliable machine, and it's a dream of mine to get everything out of it.

It's a project that has been in the works for over 4 years and it's still not completed yet, and it's far from what you'd think. It looks like this: Gigabyte 6BXDS, dual PIII-1400MHz on s370->slot1 adapters unfortunately running at 1000 due to board limitations, 1GB SDRAM, voltmodded Radeon 9800 Pro 256MB/256bit, Silicon Image SATA card + 320GB HDD (a second one coming up for RAID 0), Via USB 2.0 card, Creative AWE64 ISA (coz all PCI slots are used up). Displays are a 21" IBM Trinitron monitor and a 24" Thomson TV, and it's got a nice lighted case with temperature display.

It has been rocking the same XP install for over 3 years even though practically everything has been changed over time. It even took the IDE->SATA hard drive change without requiring a reinstall. Even without the CPUs at full potential it wipes the floor with any Pentium 4, and keeps up with the low-end Core 2 Duos at video encoding, i'm not lying here. Hint: The Core 2 Duo is based on the Pentium M which was based on the PIII Tualatin. And the most important feature: It's the most stable system i have ever had. It has never locked up or crashed. The only bad thing i can say about it is that the 9800 Pro doesn't have Pixel Shader 3.0. Other than that it handles anything i throw at it, Photoshop, Dreamweaver, Flash, you name it.
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Last edited by Th3 uN1Qu3; 26th July 2010 at 04:29 PM.
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Old 26th July 2010, 06:45 PM   #4
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Sweet!

AWE64 rocks. I carried one around for as long as I could. Now I don't have any PCI slots, so I have to settle for an SB Live!. Fortunately, it has synth MIDI, which is why I like it; onboard audio is only wavetable these days, so synth MIDIs don't sound anything like as written!

As for the power mod, you might want to add a crowbar just to make sure that, if your transistor fries, you don't take out the rest of the computer. The power supply should safely go into current limiting under fault conditions (but if it burns, well, that's its problem). Speaking of which, the other part about compensation is over/undershoot, and you'll want it critically damped. Squarewave testing, it's a good thing.

Tim
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Old 26th July 2010, 07:17 PM   #5
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Yeah MIDIs rock. One trick i like is to turn both the FM synth and wavetable on at the same time, it gives an interesting sound. Also the Creative wavetable owns the crappy Roland set in Windows. For computers where you can't get a half-decent soundcard, you can try the Yamaha XG Softsynth, it sounds awesome. It's quite hard to find, so if you need it drop me a PM.

A crowbar circuit sounds like a good idea, taking all those rare parts out is definitely something that i DON'T want. As for the power supply, it won't burn, i've tested all its protections thoroughly.
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