One of the problems with using diodes is that they cause a voltage drop (some diodes it is higher than others) which is generally undesirable.
Also, due to the large quantities of current running through your typical psu, it would require quite a hefty diode (it also would more than likely get pretty hot).
Sorry that the mosfet solution did not work in practicality bigben2k, too bad that working in theory is not good enough
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