I have a 8-port poe switch with a dc requirements of 54V and 3A.
I'd like to use a redundant atx power supply (from an HPE rack server) so my highest voltage is 12V.
My idea is to use two DC booster in parallel each setted at 54V 3A so of one fails the switch is powered by the other.
These boosters are made with TL494C IC and a NCE8580 mosfet.
Is it doable?

 

I would think it doable – but wiring the 54V outputs together may upset them (especially if one of them fails). Therefore I’d recommend you place a suitably rated diode in the positive output of each booster.

Don't forget that your 12V from the ATX PSU will have to supply at least 14A (and probably a bit more) to power the boosters.

 

I would think it doable – but wiring the 54V outputs together may upset them (especially if one of them fails). Therefore I’d recommend you place a suitably rated diode in the positive output of each booster.

Don't forget that your 12V from the ATX PSU will have to supply at least 14A (and probably a bit more) to power the boosters.

This the specs of the power supply:

 

Therefore I’d recommend you place a suitably rated diode in the positive output of each booster.

What kind of diode can I use? Is 10A10 (10A 1000V) diode fine?

 

I don’t see the rating of the ATX PSU 12Vdc output on your screen shot.

Also consider that you will need wiring of suitable CSA to handle the 14A (at least 1mm2 / 16AWG).

 

The blocking diodes require a rating of at least 3A and a breakdown voltage of 54V.

Allowing for headroom on the specs, I’d select a diode with a minimum of 100V, 3.5A.

 

I don’t see the rating of the ATX PSU 12Vdc output on your screen shot.

Found it. 30A max @ 12V. All 12V cables are 18AWG. If I use two of them for each booster, they should carry the needed current without issues, right?

 

If you take four 18AWG wires to each of the boosters (two in each of the supply lines +/-) you should be OK.