"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