TV SMPS and mysterious zener diode

[miwamiwa]

Hello,

The problem : TV does not start at all.
TV model :THOMSON 40FA3113
PCB : MT31EPA-MAC2HG

The fault is located in the primary of the SMPS.


The shorted power MOSFET appears to be the cause of the problem. So I changed the transistor (QW1 which is a 10N65B), the bypass capacitor (C409 which was a ceramic 220pF before exploding), and the fuse.
But now I'm stuck because I spotted a defective small signal zener diode (Z402)  that I can't replace because I have no way of knowing its value. There is no marking on the PCB, the package is SOD-80(miniMELF) with a blue stripe.

Here is the schematic I drew based on my PCB :




I haven't found the schematic of the PCB so I can't identify it in this way either, however I managed to find the schematic of another version of this card (MPLE31N75-2B-T, see attachment below, page 15) where this diode does not appear, this which makes me think that its role is not crucial and that the card could work without it. But before doing anything I would like your opinion on its role in the circuit. Perhaps we can guess the order of magnitude of its zener voltage?

Finally, i have tested all the diodes and transistors in this area and it's all good but i don't know if the flyback controller (RT7736B) is dead or not.

Thank you.

[fzabkar]

https://www.richtek.com/assets/product_file/RT7736/DS7736-06QV.pdf

The maximum voltage rating for the CS pin is 6.5V. The set-point is 1.1V.

Your CS resistors should probably be 0.68R, not 680R, so the effective resistance of the array is 0.34R. That sets the current limit at 3A.

ISTM that, as long as current sensing/limiting is working properly, the voltage at the CS pin will never exceed its rating. Therefore, the zener seems unnecessary, except to sacrificially protect the IC if the chopper fails.

[miwamiwa]

​
Sorry for the long period of no response from me but I have little free time at the moment.

Your CS resistors should probably be 0.68R, not 680R, so the effective resistance of the array is 0.34R. That sets the current limit at 3A.

Yes you are right, i misread the resistors marking!

After replacing the MOSFET and the zener diode (I used a 5.6V one) the TV still does not start. Now I have current in the primary but nothing happens (so I imagine that the MOSFET is not working at all). Apparently the flyback controller doesn't even seem to be powered.

Now i would like to understand how this thing is powered.
In the datasheet we can see the application circuit :


So i understand that the chip is powerd by a dedicated winding of the transformer and rectified by a simple diode. But why is it connected to the mains before the diode bridge (through a resistor divider, which is not the case on my PCB) ?

[Circlotron]

​So i understand that the chip is powerd by a dedicated winding of the transformer and rectified by a simple diode. But why is it connected to the mains before the diode bridge (through a resistor divider, which is not the case on my PCB) ?

The voltage divider charges the capacitor which holds enough charge to kick the chip into action and then the transformer winding and diode take over. The resistor is too high a value to supply the chip continuously. If it were a low enough value to supply the chip continuously it would dissipate a LOT of heat.

[miwamiwa]

Ok, so briefly after starting the TV, the RT7736B is powering himself. So if there is no power at IC Vcc pin, it means the chip died along with the MOSFET, right ?