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Posted to sci.electronics.design,sci.electronics.repair,aus.electronics
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Understanding a split-mode power supply.
N_Cook wrote:
Sylvia Else wrote in message
u...
After my airconditioner failed the other day, and being reluctant to pay
a technician to come and fix it, I've been taking a look at its
electronics board.
It's clear that its power supply circuit has failed. From the board
itself I've inferred this partial circuit:
http://members.optusnet.com.au/sylviae/smps.jpg
I am pretty sure there are no other components connected to the
transistor labelled Q1, and it is this transistor that has failed. The
failure mode is a short (a few ohms, polarity insensitive) from base to
emitter. The collector is open circuit. The transistor is thus unable to
sink enough current to prevent the switching transistor from turning on,
and as a result the 8.2 Ohm fusible resistor has also failed.
It seems moderately likely that by replacing these two components I can
get the board working again.
The circuit nevertheless puzzles me. The function of Q1 appears to be to
bias the switching transistor. But this seems to rely somewhat on the
characteristics of the two transistors, which I would have thought was
asking for trouble. In particular, it looks to me as if Q1 could simply
prevent the switching transistor from ever conducting, and nothing would
happen.
Is this an accepted technique? Or have I misunderstood the purpose of Q1?
BTW, this is from the external unit of a nine year old Daikin split
system.
Sylvia.
Someone else , last month, asked a similar question here.I have now found
the text book that I found useful for understanding SMPSs
Simplified design of Switching Power Supplies
by John D Lenk, 1995
Thanks for the reference.
Sylvia.
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