Toshiba 40L3433DG - very hot Q301 on PSU board

[Sebenicz]

Sorry for my english.

Toshiba 40L3433DG, Vestel 17IPS20R6.
TV not working, only led blinking. I find shorted Q301 on psu. I change it to similar. I don't find any other damage. Now works fine, picture are very good, but Q301 is extremely very hot.
Where look for other damage?

[TechGeek]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Can you check the temperature it's running at? If so, post it here. Also, can you post the manufacturer/part number of the replacement Q301 item?

[Sebenicz]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

I don't know what exactly temperature is, but I can't touch this. Original was FQP7N80C and now is STP20NM60.
Also I add pictures before repair.

[TechGeek]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Try to get something like a temperature measuring 'gun' or a thermocouple or something like that. The replacement device is rated for a max of 150C operating temperature.

Also, do you have another FQP7N80C you could try?

[Sebenicz]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Q301 is very hot because other damage is.

[Sebenicz]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Voltage on C105 is 390V.

[TechGeek]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Voltage on C105 is 390V.

That's good. That means that the voltage booster circuit is working. If this circuit wasn't working, then the TV wouldn't work.

[Sebenicz]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

But why Q301 is very hot?

[TechGeek]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Well, I can think of a couple reasons:
1) The replacement part is different, and thus will run hotter
2) The original part operated hot in the first place
I think it's more of reason 2, because it has a heatsink on it to help dissipate heat. Could you try adding a fan to blow some air onto that heatsink?

[Sebenicz]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

1) no.
2) the original part is damage because is damage in other place.

[ducky29]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

1) no.
2) the original part is damage because is damage in other place.

Check the pm that I sent you!

[gabiz_ro]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

A big heatsink will help.
You may see there is enough room for bigger one.
Encountered few times same problem,MOSFET short on G-S,replaced with original but very hot in short time.
So I remove old heatsink and replace with one that fit in that area.
After few hours of working heatsink was at ~45-50 °C
On PCB there is footprint for bigger heatsink but they use a little very small one.

[tom66]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Check the gate resistor and gate pulldown resistor are not damaged. If the MOSFET failed these can be damaged causing poor performance of the new MOSFET.

[Sebenicz]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Gate resistors are good.

[Sebenicz]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Vcc on U300 (NCP1380D) is 18,3V.
Vgs is max 3,5V.
Voltage drop on R320 (0,47R) is max 0,08V.
Any ideas? :/

[R_J]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Does this power supply use D512 and D513? They might be shorted, you would need to remove R320 to check them.
I see the schematic is marked wrong for the value of R320 (56K) not likely

[Sebenicz]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

D512 and D513 is not use in this psu.

[Andrew F. Ali]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

Look for even the smallest leak on any associated semiconductors in the Transistor circuitry.Even the replacement could be leaky, though rare it could happen. Let replacing the original heatsink with a bigger one.

[R_J]

Re: Toshiba 40L3433DG - very hot Q301 on PSU board

It could be that the replacement fet is the problem, your replacement has a Rds(on) of 0.29Ω the original was Rds(on) of 1.9Ω which might change the circuit operation enough to run hot.