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tillin9 · 06-04-2006, 08:54 PM

Howdy, I'm another late comer to this thread. Basically I'm doing a similar build, trying to make a redundant PSU for a fileserver. When I went seraching, I found Yo_DUH's article and later this thread. First off, I'm amazed at how long this has been going and that you'd been able to stick with it. Second, I have a couple of questions:

1) What are you doing about the negative voltages? ATX connectors have a -5 and -12 rail? I know these are low current so you could just use the lines from one PSU, but that kills the redundancy.

2) Why do you need the complicated FET drivers and the headache of SMD? I know many ways you can drive high current 12V from just a TLL (3V) source. There are nicer examples in Horowitz and Hill (page 167 C in the 2nd revision is the best I found, it has a circuit to use a filter capacitor with short circuit protection which would remove any ripple, I'll scan it if you want) but here are some (simpler, though crudly drawn) ones I could find online: http://www.tim.woodburn.btinternet.co.uk/ttl2fet.htm

3) How exactly does the reverse protection work? I'm probably just not seeing something. Power FETs generally have very high off resistance and low bidirectional on resistance. Meaning this offers no reverse protection when on, and a single FET would offer roughly the same protection when off. The regular diode in the circuit also doesn't seem to offer any reverse protection as its role seems to just stiffen the ground for, and with the resistor protect, the zener so as to make it work with the driver.

Finally, the only surefire way I can see to have reverse protection is to use an array of diodes, then using the pots to adjust the voltage rails to compensate. Using extra diodes so the current per diode was well over spec would prevent a forward fail (and probably lower the amount you would have to boost them), and since diodes don't have much resistance, very little heat would be dissipated. Though the ones with 0.3V drops are generally in the 5-10A range, so you'd need an array of three or four diodes per FET to be safe. This diode: http://www.st.com/stonline/books/pdf/docs/3619.pdf is available from digikey for about $1.00 and would probably do the job. It has around 0.3V forward drop at 10A and has 25V reverse protection with less than 0.01mA reverse current. Just a suggestion.

06-04-2006, 08:54 PM	#214
tillin9 Cooling Neophyte Join Date: Jun 2006 Location: Middletown, CT Posts: 1	Re: 300w + 300w = 600w? Howdy, I'm another late comer to this thread. Basically I'm doing a similar build, trying to make a redundant PSU for a fileserver. When I went seraching, I found Yo_DUH's article and later this thread. First off, I'm amazed at how long this has been going and that you'd been able to stick with it. Second, I have a couple of questions: 1) What are you doing about the negative voltages? ATX connectors have a -5 and -12 rail? I know these are low current so you could just use the lines from one PSU, but that kills the redundancy. 2) Why do you need the complicated FET drivers and the headache of SMD? I know many ways you can drive high current 12V from just a TLL (3V) source. There are nicer examples in Horowitz and Hill (page 167 C in the 2nd revision is the best I found, it has a circuit to use a filter capacitor with short circuit protection which would remove any ripple, I'll scan it if you want) but here are some (simpler, though crudly drawn) ones I could find online: http://www.tim.woodburn.btinternet.co.uk/ttl2fet.htm 3) How exactly does the reverse protection work? I'm probably just not seeing something. Power FETs generally have very high off resistance and low bidirectional on resistance. Meaning this offers no reverse protection when on, and a single FET would offer roughly the same protection when off. The regular diode in the circuit also doesn't seem to offer any reverse protection as its role seems to just stiffen the ground for, and with the resistor protect, the zener so as to make it work with the driver. Finally, the only surefire way I can see to have reverse protection is to use an array of diodes, then using the pots to adjust the voltage rails to compensate. Using extra diodes so the current per diode was well over spec would prevent a forward fail (and probably lower the amount you would have to boost them), and since diodes don't have much resistance, very little heat would be dissipated. Though the ones with 0.3V drops are generally in the 5-10A range, so you'd need an array of three or four diodes per FET to be safe. This diode: http://www.st.com/stonline/books/pdf/docs/3619.pdf is available from digikey for about $1.00 and would probably do the job. It has around 0.3V forward drop at 10A and has 25V reverse protection with less than 0.01mA reverse current. Just a suggestion.