"Phil Allison" wrote in message
...

"Arfa Daily"

Further circuitry damage can often be a consequence of failing smps
secondary-side electrolytics. If the supply monitors say the 12v rail for
regulation feedback, and the filter cap on that rail goes bad, the
resulting hash and ripple can appear to the sensing circuit as a low
output. This causes the m/s ratio of the chopper drive to open up in an
effort to restore the rail to the correct value. As all the other rails
are tightly magnetically coupled to the bad rail as a consequence of them
all sharing the same transformer core, the end result is that the 3.3v
and 5v rails can go sky-high, causing a trail of catastrophic damage to
various LSIs in the equipment.


** Over-voltage protection is essential for devices with such SMPSs - a
sacrificial zener bridging the regulated voltage will do the job for a
single output supply.

Multiple output supplies need something a bit more complex that monitors a
sum off all the DC outputs and reacts to any significant increase.

Many cheap and some expensive products do not have anything.



..... Phil



Yes, agreed. Most LCD TV switchers have sophisticated shutdown circuitry for
the main control IC, and sometimes for the pfc supply as well. These
circuits measure under and over voltage, as well as over-current conditions,
and can represent a fault-finding nightmare, as you try to over-ride them
with the supply in isolation from the TV, to see which supply or protection
circuit, is causing the shutdown ...

But as you say, most cheapo switchers as found in DVD players and home
cinemas and the like, have absolutely nothing south of the rectifier.

Arfa