[Elecraft] Anyone know this info about switching supplies?

[Matt Palmer]

Since building switchers for the military is my profession these days
i would like to think i have some authority on the subject. Failures
can occur almost anywhere in a switcher and the most typical type
depends on the topology of the supply. Typically there is a pass
transistor on the input that will turn off power to the whole supply
(at least in my designs) if anything goes "out of spec" however i
would bet in cheap commercial supplies this is not always the case. My
most common failure in lab tests for me is the primary side
transistor, what happens depends on whether the transistor fails short
or open. If it fails open you typically have no voltage on the output
(again this is dependent on topology) but if it fails closed, you can
get runaway on the output in certain topologies. I would like to think
any 'smart' design has a pass transistor and enough bit and
supervision circuitry as to shut the supply down if something goes
wrong, Component failure in the feedback loop is going to be rare, as
these are typically passive components, either a resistive divider, a
sense winding on the transformer, these parts typically don't fail,
and if they do, they fail in spectacular fashion which usually gives
you a clue something is going wrong.


Matt
KD8DAO