Ian Field wrote in message
...


"Arfa Daily" wrote in message
...


"Ian Field" wrote in message
...


"Michael Karas" wrote in message
...
[This followup was posted to sci.electronics.repair and a copy was
sent
to the cited author.]

In article , says...



Is there a "bleed-through" route for start-up?

I do not really know what you are asking about here.

Its common practice for the SMPSU control chip Vcc to come from an
extra
winding on the chopper transformer. Before the PSU starts up - there
has
to be some source of Vcc to get it kick started.

Very often there's a high resistance resistor (or 2 in series) from the
+ve of the mains bridge to provide the initial start up supply for the
control chip - these resistors are very prone to going high/OC.


+1

And the cap in the C6 position on Reinhard's schematic is the one
component that I change more of on every design of switcher that I work
on, than any other component for no or intermittent start up. As the
startup resistor is such a high value, that cap has to be in
near-perfect
condition to buffer that startup supply down to a low impedance for the
control chip, until the supply gets going, and the low impedance
maintenance supply takes over. An ESR meter is your friend here ...

I don't have that particular schematic, but in general terms such
capacitors
are usually quite small ( 1 - 47uF) such values are easy to get hold of in
multilayer ceramic chip capacitors - and the electrolyte will never dry
out.



But MLCCs then have their own problems, excess heat from PbF fixing, lead to
cracks and then metal migration into the cracks due to the tiny geometries
http://web.archive.org/web/200805120.../news/rfa_feat
ure_07b.asp
I don't know the relative frequency of each failure process though