SMPSU goes BUZZZZZ

Thread Starter

iamspook

Joined Aug 6, 2008
27
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!
 
Last edited:

bertus

Joined Apr 5, 2008
20,042
Hello,

The 2sc3112 is a darlington transistor.
You should try to find one with about the same specs.

Greetings,
Bertus
 

nanovate

Joined May 7, 2007
665
The MPSA18 is closer to what you are looking for.

The 2SC3112 is a "super-beta" NPN not a Darlington. There are 2 grades A:600-1800 and B:1200-3600. If it is just driving the FET then the beta is not the reason for the buzz. You may have a damaged transformer or there is some current noise getting coupled to it or there is just plain undesired DC current running through it.
 

Thread Starter

iamspook

Joined Aug 6, 2008
27
Thanks for the advice. I changed the transistor for a trusty robust little 2n2222 and turned it on. No buzzing. Go figure! I'll have to soak-test it
to see if it is reliable. It should be ok for breakdown voltage and power and Ic and so on. Beta is about 100 on this one. (I measured it) I guess the
high beta requirement was a furfy. Maybe the BC547CG was not stable in the circuit.
 

nanovate

Joined May 7, 2007
665
One thing to double check is the pinouts of the transistors... it is easy to overlook.
The P2n222a is CBE and the PN222A is EBC and the 2SC3112 is ECB
 

Thread Starter

iamspook

Joined Aug 6, 2008
27
You are right. I had to insulate and cross one of the legs to make it fit. It all works perfectly.
 
Top