Re: Can Low Voltage kill a power supply?

Rod Speed wrote:
David Maynard <nospam@xxxxxxxxxxx> wrote

Rod Speed wrote

Sjouke Burry <burrynulnulfour@xxxxxxxxxxxxxxxxxx> wrote

Ray Cassick (Home) wrote

Rod Speed <rod.speed.aaa@xxxxxxxxx> wrote

n33ck0@xxxxxxxxx wrote


Hi, everybody. 2 weeks ago ther was a case of low voltage in the
neighborhood. After calling the utility co. it was determined there was too much demand on the grid. not anything specific in the
house. So here is my question can a low voltage situation kill a
computer power supply.


A properly designed power supply should shut down in that situation.


A badly designed power supply can be killed in that situation.


Just a bit curious as to how you think a power supply can be damaged by a low voltage situation?


Since all the regulation in a PS is designed to convert AC to DC and then regulate those output voltages down to useable levels within a
specified tolerance I don't see how a low voltage could result in a
dead PS. I DO see how it can cause flakiness on the low voltage side since all outputs are based upon a properly regulated input voltage
tolerance, but all that should result from a low input voltage is
proportionally low output voltages. Personally I would think that
most of the damage would have been
caused by a backlash of higher voltage that can often occur after
low voltage situations. This higher than normal voltage inrush
could happen faster than the regulators are prepared to handle and cause a very quick spike to get through the filters and fry the lower voltage side of the regulation circuit perhaps.


A switching supply will draw a bigger current from the
mains to compensate for less voltage, the output voltage
wont drop ,power wont drop, so you need more current.
When input is low enough ,and current becomes big enough,
either the safety cuts in, or the supply blows.


That increased current isnt what kills a badly designed supply.


Very well can.


Nope, not when it didnt drop enough to cause the other system to turn a hair.

According to that kind of logic it didn't fail at all, yet it did.


The only current that goes up is from the mains and the diodes
that rectify the mains arent that marginal current capacity wise.


The problem is the flyback regulator. Low voltage on the filter caps means an increased PWM duty cycle to compensate,


Yes.


stressing everything: drive transistors, flyback transformer, flyback diodes, etc.


That shouldnt kill anything in a properly designed power supply.

In theory nothing should... but it failed.


What actually kills a poorly designed power supply in that
situation is the inevitable associated mains surges as loads
trip out due to the low mains voltage, particularly motor loads.


Of course, that shouldn't make it past the input suppressors and filter caps.


Shouldnt and didnt are too entirely separate matters.

That's funny considering you've just been singing the "properly designed power supply" mantra.


MUCH more likely than your scenario killing the power supply.

Frankly, no. Surge suppression of the type you're arguing is relatively simple and fairly static from design to design. I.E. the power mains don't 'change' when one goes from, say, a 400W design to a 450W. The flyback circuit, however, is a whole different situation. Get that off so the core goes into saturation with an unpredicted scenario and you've got fried flyback. And it's no very hard to do, especially when plagiarizing, 'revising', or designing to power line 'specs' (meaning normal tolerances).


And that depends on the quality of the supply.




.

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