Using two power supplies in PC (a proper way)

power supply

I would like to know, whether it's possible to use two ordinary PC/ATX power supplies safely in one machine.

I've heard about Add2Psu board, but I have a very severe objections about it.
(The power on molex from 1st PSU, shorts PWR_ON on 2nd PSU via relay.)

I would like to build something like that, but maybe in a much safer manner.

I'd really appreciate if someone could check the following scenario and fix it if it's needed. I refer to pin numbers on 24pin ATX connector.

  1. When power button on the casing is pressed, the motherboard sends signal to 1st PSU by shorting its 16th (green) PWR_ON pin with ground.
  2. I assume the 1st PSU is completly ready if its 8th (grey) PWR_OK pin is pulled up to +5V. (There is the main difference from the Add2Psu approach in which they only check the voltage on molex connector.)
    I use the 8th (grey) PWR_OK pin from 1st PSU to short 16th (green) PWR_ON pin on 2nd PSU, but they're galvanically separated.
  3. Then I wait until the 2nd PSU is ready, which is denoted by +5V on its 16th (grey) PWR_OK pin. The feedback signal returning to motherboard by its 8th (grey) PWR_OK pin is a logical conjunction of PWR_OK signals from both supplies, but electrically flows only from the 1st PSU, so the second PSU is still galvanically isolated from the first as well as the motherboard.

Could you comment on the correctness of this approach above and a few concerns about it:

  1. In my approach, I slow down the feedback signal that returns to motherboard twice as in the case of single PSU. Does such a delay can affect the waking up process, e.g. the motherboard and CPU would get power first and the graphic cards (via additional PEG connectors) must wait a bit. Could the motherboard detect this delay by measure time between sending PWR_ON and returning PWR_OK and rise an error?

  2. Should I bother with a situation when graphic cards gets power from 2 differet supplies and for instance can have two different grounds.
    I would feed it via PEGs from 2nd PSU, but get always get a minimal amount of power from PCI-Express connector and I cannot avoid that.

  3. How should look like the shutting down process? Should I literally revere it by triggering the shutting down process on 2nd PSU and waiting for an indicator that it's completed and after that turning down the second or simply shut down the first and the second would turn off anyhow?

  4. Are there any aspetcs that I may not be aware of?

Thank you in advance for your reply.

Best Answer

There may be issues because power supplies are regulated. If two were paralleled, and one line had, for example, 4.99 V and the other had 5.00 V, both well within tolerance, one PS would be trying to force current backward, while the other tries to limit the voltage. At the best, the full load would be carried by one PS and the other would do nothing. A worse scenario is where the driven supply has a "crowbar" to short the bus on over-voltage.

While it would be possible to use two PS by inserting two series resistors and moving the sensing point to the PCB, it seems more effort than it's worth.