PSU switch on moment

[Martin Czech]

I know that chips with several supplies (like +-15V in our usual OP-AMP)
need a specific power supply turn on order, just to avoid forward bias
of junctions and thus overcurrent or even worse latch up.

A logic chip with only +5V supply doesn't care of course, as long
as all the signal pins don't get forward biased...

Though I know this and I have destroyed many chips in the lab
by toying with the supply in wrong order, I never cared about that
topic in my diy activities.

So, what happens if the mains switch is toggled on or off?

On: It should take several cycles of mains power to reach the final
voltage level, we could assume a linear ramp (it is not), but we don't
know the charging speed, this is load dependend and the loads are voltage
dependend. This strange moment in between should be ok for most of the
chips, as long as inputs don't get higher or lower due to other faster
supplies. (eg. +12V and -3V).  This means that inputs to other modules
etc. should have protection diodes to supply and series resistance,
as often recommended by the manufacturers.

Off: well there is energy storage in the electrolytics, we don't really
know which supply goes down first. Same situation as ON, the heavier
load goes down first.

Some questions:

1. Do you know chips that are sensible for this situation?
2. What happens to pins connected direcly to ground?
3. Is a startup circuit required? Is such a fast turn on
   desireable?
4. Is a tracking regulator recommended?


m.c.