12V UPS system design questions

[unhappy_mage]

I'd like to investigate creating a battery-backed 12V source for my server, so that I don't need to go up to 120AC and back down to 12V. I know how to go about building a regulated 12V source, and I can use my existing Enermax Revolution 1050 as a solid 12V source when the power's on, but the next question is how I can power my server off 12V alone.

It looks like the PicoPSU can only supply ~8A on the 5V and 3V3 rails. The Revolution is designed with 12V to 5V and 3V3 converters, would it be feasible to feed my external 12V source into it?

FWIW, I'm planning on doing my 12V regulator using something like the TI UC2837, which controls an N-FET in linear mode to drop the ~14 volts of the car battery to exactly 12, which is nice for PC use. This means you only get ~85% efficiency, and a computer drawing 100W will dissipate about 16W in the FET. I will actually be using a fleet of FETs in parallel to avoid melting components, and I may look into throwing a switching regulator between the battery and the linear stage to avoid some of those losses.

 

[plugwash]

It looks like the PicoPSU can only supply ~8A on the 5V and 3V3 rails

Still i'd think that would be sufficiant for most systems. If you have a lot of drives you could always use a seperate 5V regulator to feed them.

The Revolution is designed with 12V to 5V and 3V3 converters, would it be feasible to feed my external 12V source into it?

Afaict it really comes down to whether the 12V side of the PSU can tolerate being backfed and I have no idea what PC PSUs make of being backfed.

I'm planning on doing my 12V regulator using something like the TI UC2837, which controls an N-FET in linear mode to drop the ~14 volts of the car battery to exactly 12, which is nice for PC use.

What will you do when the battery voltage drops below (12V+regulator's dropout voltage)?

 

[unhappy_mage]

Still i'd think that would be sufficiant for most systems. If you have a lot of drives you could always use a seperate 5V regulator to feed them.

Looking around, I found this, which takes a wide-range input and delivers 15A on both the 5 and 3V3 rails.

I have 6 5k3000s and 6 7k2000s. Both of the drives' spec sheets claim a maximum draw of 1.2A from the 5V rail, but StorageReview's coverage of the 7k3000 (which should be a reasonable upper ceiling on either of the drives I have) says they draw about 2 watts from the 5V rail at startup, and as many as 3 while active.

So the 12 drives I have will draw about 24W = 5A at startup, and 36W = 7A when I read from the disks.

Afaict it really comes down to whether the 12V side of the PSU can tolerate being backfed and I have no idea what PC PSUs make of being backfed.

Ooh, good point. I suppose I could put diodes inline to prevent it from backfeeding.

What will you do when the battery voltage drops below (12V+regulator's dropout voltage)?

I'd try putting an AVR or something like it in the circuit to send a USB message to the computer to shut down when a certain voltage is reached.

 

[tdg]

Ooh, good point. I suppose I could put diodes inline to prevent it from backfeeding.

Traditional diodes would prevent backfeeding, but they will introduce a voltage drop across them large enough for the output to fall out of spec (up to ~1 volt drop). You would have to use Schottky diodes with a low iF voltage drop, but still expect ~ 400mv of drop on your output.

 

[unhappy_mage]

Traditional diodes would prevent backfeeding, but they will introduce a voltage drop across them large enough for the output to fall out of spec (up to ~1 volt drop). You would have to use Schottky diodes with a low iF voltage drop, but still expect ~ 400mv of drop on your output.

Oh yeah, voltage drop. Somehow I always forget about that... These only have 230mV of drop, but only 16A each means I'd probably need a couple (especially since I'd need at least one per rail). At five bucks each, that seems unnecessary.

Maybe I could just switch everything with FETs? I dunno, I'll have to sit down at some point and lay out a circuit and see what looks reasonable.

 

[extide]

I have an M2-ATX psu in my carpc. Says it can work from 6-24v input. Seems like something like that would be nice. Would account for sag in battery supply voltage too. I'd probably get something that output~14v as a source though, or even a proper (intelligent) 12v battery charger instead of your psu. Charging a 12v battery with 12v will take a really long time.