2 Dec
2007
2 Dec
'07
2:25 p.m.
One of the trickiest things about switchers is that
they often
use the main output to provide the rails for the regulation feedback.
This causes a chicken/egg problem. We can see that there is
no output so no feedback to tell the high side to deliver more
If there is no feedback -- that is the feedback detects no output
voltage, then the power supply controller should try to increase the
output voltage. That's why a fault in the feedback circuit -- even
unconencted sense terminals in some poorly-designed supplies -- will get
the outputs to go sky-high. Hopefully the overvoltage protection circuit
(crowbar or whatever) trips before any real damage is done.
You might be confusing this issue with another one. That is, the power
supply control circuitry needs to be powered. Once the supply is
running, it; can be powered from one of the outputs of said supply, but
at swich-on, the power supply outputs are all 0, to the power control
circuitry is not powered, so the supply can't do anything, so the outputs
remain at 0.
Some larger supplies -- even things like the VT100 -- have a little linear
supply to get things going. Other times there's a dropping resistor
(startup resistor) to power some bits of the control circuit directly
from the recifiied mians, just to get it all going.
Once it's started up,
it carries on running from the main supply outputs.
DEC took this to extremes in the 11/44. The main PSU circuits (and there
are 2 of them) in that machine start from a little SMPSU circuit. That in
turn has a startup resistor.
-tony