Re: Re: Heads Up for Power Supply deal...

beer@... wrote:

> From: beer@...
>
> On 31 Jan, CAD_CAM_EDM_DRO@onelist.com wrote:
> >
> > Yes, in a linear regulated supply, this could indeed happen. In a switching
> > regulated supply, there is just a transformer winding and a diode, so there
> > will be no harm to it.
> >
>
> Not strictly true .. in most switching supply designs, there is
> circuitry on the output side as well, even if it's only voltage
> feedback. Add the diode, as described.
>
> Add a SINGLE diode. Paralleling diodes ( unless equalized by balancing
> resistors ) will not result in a doubling of the current rating. There
> will always be SOME slight difference in Vf between the diodes, meaning
> that one will do almost all the work and the other will do almost none.
>
> Diodes this size are relatively cheap.

Yes, this is true, but I still say it is unnecessary. There are several
techniques used to do the feedback across the isolation barrier. Most
of them use a voltage reference chip hooked to a variety of optoisolator
circuits, and won't be harmed from being powered from the external
source. This is pretty much what happens any time the power supply
is shut off under light load conditions. And, the internal diode will
prevent more than 0.7 reverse volts from developing across it when
powered by the other power supply, only.

Yes, true, they are cheap. I guess I'm just picking nits, here.

Jon