Low voltage DC

I want to power a ATX PC that runs 24/7 and draws about 100 watts from the mains power supply from a 24V DC battery supply. Being a battery supply, the voltage can vary from a low of 18V up to a high of 28V. The 24V system has mains (grid supply) but also can be supplied by wind and solar when available.

This means minimal losses in the power supply are desirable. Using a 24V to 230VAC inverter and then putting that through the standard PC supply to get the ATX voltages is just another layer of losses.

There are a few DC-DC ATX/ITX power supply modules available. I already have a couple in use on smaller 40W PCs.

The small DC-DC-ATX modules claiming 120-160 watts look nice and simple but also a bit dodgy. Most need almost exactly 12V DC input, probably because they pass it straight through to the 12V output and therefore are not suitable for battery operation.

They also lack any sort of reasonable heatsinking. They’d have to be 98%+ efficient to survive. I did test one of these on a 40W mini-itx board and it got too hot to be trusted. I’d expect them not to be long-term reliable running anything larger than 30-40 watts. For smaller mini-itx systems they are probably great.

This module claims 150 watts and better than 92% efficiency for about US$40 on Ebay. If the 7A at 12V is just passing through and not being regulated, that’s 84 watts of output that is not contributing to losses.

The larger 160-250 watt units are better but still seem to lack heatsinking. Running a 100W PC, at about 90% efficiency means losses of about 10 watts. Considering the larger size of the board, probably not unreasonable, but 10 watts is going to make this board very hot without any forced air cooling. Worse still if it’s in a fan-less case.

The wider input voltage range (8-28V DC) means they must be regulating the 12V output as well.

This module is rated 200 watts and costs about US$60 on Ebay.

I already have two similar units running two 40 watt mini-itx systems, one with a large HDD, one with a small SSD. These have been reliable.

Another possibility

Update: I have designed a circuit board to carry 4 of these modules for another purpose; here. But it can be assembled to operate as a ATX power supply, accepting 20-30V DC input. Not yet tested.

So, I’m looking at assembling a one-off supply from 4 separate step-down modules. These are 12A rated and accept up to 32V input. The output voltage and maximum-current is adjustable.

I can mount four modules on a carrier PCB and combine the outputs to provide motherboard and HDD power. 3.3V, 5V and two at 12V.

Without getting clever it’s difficult to combine the outputs of 2 modules to provide one 12V rail at more than 12A without stressing one of the modules.

I can put the power-good, on-off and fan control on the carrier board. Input under-voltage shut-off would be handy to protect the 24V battery. The easiest would be to use a small micro to monitor the supply and output voltages, provide power-good and on-off control. Don’t want to make it too complex though.

These modules cost about US$7 ea. on Ebay : here.

I have tested one of these modules and found the inductor gets a bit hot with no fan at higher power. They are likely wound for low-cost rather than efficiency. I expect I can re-wind the inductor of the 12V module for lower loss if needed.

I’m expecting my all up cost excluding my time to be about NZ$70, for this spec:

• equivalent of a 200+ watt PC power supply
• Input: 15-30V DC input
• output: 12V 16A, 5V, 12A, 3.3V 12A
• power-good and on/off control (solid state input power switch)
• temperature controlled fan

There will be some time in assembling the unit; assembly of the carrier board and some minor modification of the dc-dc modules.

I’m not sure yet how to arrange 4 modules to benefit from one small fan.

Next step

I have ordered some more of these modules and will design the carrier board.