"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 ...

Arfa