I repaired a blown DVD's SMPSU by inspection of the components
(No Schematic). The following problems found:
1) The 240v input terminated on a dry cracked joint - presumably the root
cause of the failure.
2) A power MOS FET was short circuit everywhere. This took out the BR
and a 15v zener and a fast recovery diode.
3) A C3112 NPN high voltage switching transistor popped. It looks like it is a single transistor oscillator to drive the MOS FET (primary side).
The nearest but uncomfortable equiv I could find is a BC547CG.
This main diff is the C3112 has an amazing high DC gain. Somewhere
between 600 and 3600 while the BC547CG has a respectable typical 600
and nowhere near the max of 3600.
It all works, but audibly buzzes the transformer.
(If I put a plastic ruler on the
transformer, It travels up the ruler.) I'd guess it to be 700 Hz or so.
Questions:
Is the lower HFE likely to be causing this?
Is there a better substitute or a reliable supplier for the C3112?
Could I use two BC547CGs in a darlington pair arrangement to get
the gain without losing any significant characteristics? I guess I need
to know if there are any particular disadvantages of using a darlington
pair under these circumstances. I should add that I know that the
DP will have a lower speed than an equivalent single transistor,
and possibly temperature instability which can be solved with a
resistor but this affect the turn-on voltage. But will this matter?
Or would a Sziklai pair be more beneficial?
Thanks!
(No Schematic). The following problems found:
1) The 240v input terminated on a dry cracked joint - presumably the root
cause of the failure.
2) A power MOS FET was short circuit everywhere. This took out the BR
and a 15v zener and a fast recovery diode.
3) A C3112 NPN high voltage switching transistor popped. It looks like it is a single transistor oscillator to drive the MOS FET (primary side).
The nearest but uncomfortable equiv I could find is a BC547CG.
This main diff is the C3112 has an amazing high DC gain. Somewhere
between 600 and 3600 while the BC547CG has a respectable typical 600
and nowhere near the max of 3600.
It all works, but audibly buzzes the transformer.
(If I put a plastic ruler on the
transformer, It travels up the ruler.) I'd guess it to be 700 Hz or so.
Questions:
Is the lower HFE likely to be causing this?
Is there a better substitute or a reliable supplier for the C3112?
Could I use two BC547CGs in a darlington pair arrangement to get
the gain without losing any significant characteristics? I guess I need
to know if there are any particular disadvantages of using a darlington
pair under these circumstances. I should add that I know that the
DP will have a lower speed than an equivalent single transistor,
and possibly temperature instability which can be solved with a
resistor but this affect the turn-on voltage. But will this matter?
Or would a Sziklai pair be more beneficial?
Thanks!
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