Flyback transformer testing and replacement

[(*steve*)]

Best of luck

 

[gkmaia]

Hi Steve,

O got a question. I just want to understand better the flyback topology. I've been reading quite a few articles and what you said. And that is what I understood.

Current flows down R41 and stops at ZD1 and C25 but flows to Q9 base. That allows the primary (2-4) and induce a current on the feedback (1-5) and on the secondary (7-9).

The feedback current will trigger the transistor to stop conducting and as the magnetic field collapses it flows into the secondary? If that is right what phenomenon (or components and its characteristics) makes the current stops flowing to Q9’s base?

Also the wave generated by this oscillation wont look like a traditional AC sine wave. It will resemble a square wave more likely?

 

[(*steve*)]

The operation is probably something like this:

1. If Q10 is turned off, the circuit is enabled.
2. Current through R41 turns on Q9.
3. The current through Q9 and the transformer (pins 2 and 4) storing energy in the increasing magnetic field
   
4. As the transformer saturates due to the current in the pin 2-4 winding, voltage across the winding 1-5 starts to act to reduce the base current of Q9.
5. As Q9 begins to turn off, this increases the feedback, causing Q9 to be turned off harder and harder.
   
6. As Q9 turns off (very quickly), most of the energy in the magnetic field is dumped into the secondary winding (pins 7-9). (very little energy is required to turn off Q9)
7. As the current in the transformer falls toward zero the winding 1-5 can no longer hold the Q9 off, and R41 can now start to supply current to the base of Q9, resuming the the process from step 2.

 

[gkmaia]

Thanks for the explanation!

The saturation of 2-4 and Q9 allows more current to flow past ZD1 (zener 4v) and when current is allowed past ZD1 it is pulled hard by 1-5 and that is what drops the current on Q9 base?

 

[(*steve*)]

I have been flip flopping about the operation of this part of the circuit. The overall operation is very likely how I have described, but this part of the circuit may do a number of things.

One purpose may be to protect the base-emitter junction of Q9. Another may be to prevent Q9 from being switched off too early.

 

[gkmaia]

Ok, that sounds complicated.

I got some great news! Repalced Q10 and the DMM function works 100%.