ATX PSU question. Why 3.3 and 5 volts share the same output diode?

[dog80]

All computer PSUs I've seen have a dual common cathode schottky diode for the 3.3 and 5 volt rails. The 12V rail on the other hand has a single diode all for itself.

Why not separate diodes for 5 and 3.3?

[mariush]

Some power supplies may work like you say, but it's not the rule.

Most don't, and a lot of new power supplies don't even have separate outputs, just a 12v and then they use 12v -> 5v and 12v->3.3v dc-dc converters.

[SeanB]

Cheap power supplies take the 5V rail and use a further switching regulator ( or a magnetic buck converter in some) to make the 3V3 rail from it. You will see in the ratings that he 3V3 and 5v rails share a power rating ( if the datasheet is even close to correct) while the 12V rail has a separate rating.

[amyk]

Cost savings, mostly, and the fact that ATX evolved from AT which only had 12 and 5, so it must've been easier to add another regulator on the 5V rail to produce 3.3 than another winding on the transformer. This post-regulator is usually a saturable reactor.