LED TV - power circuit damaged after lightning strike

[HellasTechn]

Yes. remove it from the circuit and it should read infinite resistance. like an open circuit.
Chances that it is faulty are fairly low.

If i where you i would order a new mosfet controller and i would also replace the bridge rectifier and the surrounding capacitors as a precaution (caps and bridge cost very little)

You may as well find it on e-bay if not at your local electronics shop

 

[HellasTechn]

Could be the MOSFET destroyed during soldering / using multimeter ?

Hardly. unless you shorted it with the meter's probe or kept the solder gun on it for way too long.

 

[michalw]

I've tested the MOSFET one more time.

Before desoldering the MOSFET there is short-circuit between the Drain and the Source
After de-soldering the MOSFET there is high resistance between the Drain and the Source.

I've checked connections on the PCB and the Source is connected with the GND through 0.15 Ohms resistor. With the GND is connected also the radiator, which is connected with the Drain(flange) connector of the MOSFET.
So, without any isolator between the radiator and the Drain(flange) there is short-circuit between the Drain and the Source.
There was the thermal grease between the original MOSFET and the radiator, but after replacing the MOSFET I haven't refilled the thermal grease, so there's Drain-Source connection through the radiator.

I'm going to buy the thermal grease ASAP and let you know what is the result of that change.

 

[HellasTechn]

I've tested the MOSFET one more time.

Before desoldering the MOSFET there is short-circuit between the Drain and the Source
After de-soldering the MOSFET there is high impedance between the Drain and the Source.

I've checked connections on the PCB and the Source is connected with the GND through 0.15 Ohms resistor. With the GND is connected also the radiator, which is connected with the Drain(flange) connector of the MOSFET.
So, without any isolator between the radiator and the Drain(flange) there is short-circuit between the Drain and the Source.
There was the thermal grease between the original MOSFET and the radiator, but after replacing the MOSFET I haven't refilled the thermal grease, so there's Drain-Source connection through the radiator.

I'm going to buy the thermal grease ASAP and let you know what is the result of that change.

i dont think that will change anything though. thermal compound is non conductive as far as i know.

 

[davenn]

i dont think that will change anything though. thermal compound is non conductive as far as i know.

agreed ... and it isn't going to provide isolation. it should have a mica or other spacer and also for the screw if its going through a metal flange on the MOSFET

 

[michalw]

Hi again,

I'm still struggling with that TV set.
I've fixed all problems related with that mentioned MOSFET and changed all not working elements, but there's no output voltage.
I've found that there's no voltage on the Source of another MOSFET. Please see the scheme below:

According to the scheme there should be 16.6 V on the Source of the Q603, but there's zero.
I've replaced that MOSFET with the new one, but still no voltage there.
Do you have any ideas what could be the problem?
As R602 there's used 1.5 Ohm instead of 1.2 Ohm, could this be a problem?
As far as I know Q603 it is an enhancement MOSFET transistor, so to have any voltage on the Source there should be some voltage on the Gate, but according to the scheme, there's zero. A mistake or I don't understand that circuit?

Regards,
Michał

 

[davenn]

is jumper link JS608 in or out ? ... the diagram says open, inferring no link

if it's in, you cannot get a voltage on the source as it is at 0V

 

[(*steve*)]

Is it possible the original mosfet was plastic or had an isolated tab?

 

[michalw]

Hi,

is jumper link JS608 in or out ? ... the diagram says open, inferring no link

if it's in, you cannot get a voltage on the source as it is at 0V

Sorry, I forgot to mention that I attached only a part of the scheme, the Source of Q603 is connected to the Gate of another MOSFET. Full scheme attached here.

Is it possible the original mosfet was plastic or had an isolated tab?

It's SC-62 SMD MOSFET, so I guess it's plastic.

 

[davenn]

According to the scheme there should be 16.6 V on the Source of the Q603, but there's zero.
I've replaced that MOSFET with the new one, but still no voltage there.

infers that the MOSFET isn't turning on

As R602 there's used 1.5 Ohm instead of 1.2 Ohm, could this be a problem?

0.3 of an Ohm isn't going to make any difference ... the tolerance of the resistor is probably 10 x that anyway

As far as I know Q603 it is an enhancement MOSFET transistor, so to have any voltage on the Source there should be some voltage on the Gate, but according to the scheme, there's zero. A mistake or I don't understand that circuit?

I would be trying to figure out why there is 0V on the Gate ... which one of those components prior to the Gate is possibly faulty

Dave

 

[KrisBlueNZ]

Thanks for posting the schematic in post #29.

The circuit in post #26 is only a small part of the power supply circuit, and that MOSFET, Q603, is not the main switching MOSFET (Q601). I'm not sure what it's even there for; it seems to be connected as some kind of overvoltage detector, but its action would be to force the main MOSFET (Q601) permanently ON, which would make it go bang!

Here's the relevant part of the power supply:


I haven't been able to find a full data sheet on the driver IC, a Fuji FA8A00N, but here's a block diagram. It implies that the MOSFET gate driver is powered from VCC, which is initially powered via the "start up block" from VH, so Q603 isn't needed as any kind of pullup.



Based on all of this, I'd say that the voltage that's supposed on Q603's source isn't supposed to come from Q603 (its gate voltage is supposed to be zero, after all) and is actually supposed to come from the driver IC, IC602.

So I suggest you check for voltage on pin 8 (VH) of IC602, and if it's missing, suspect R615.

Any other suggestion are welcomed.

 

[michalw]

Hi,

thanks for great explanation.

I've measured the voltage on pins of IC602 and the results are:
- pin 8, VH: 224 V
- pin 6, VCC: 0 V
- pin 5, OUT: 0 V.

So, the next step is to replace IC602?

 

[KrisBlueNZ]

Yes, possibly. I don't know if you'll be able to get it though.

First, have you checked R610,1,2,3? If they're open, they'll stop the controller IC from generating any drive.

 

[michalw]

R610-3 has been replaced.

As I googled, there is possible to buy FA8A00, but I can't see anything in my country:

https://www.google.pl/search?q=fa8a...org.mozilla:en-US:official&channel=sb&start=0

 

[KrisBlueNZ]

What's the story with D612? If R610~613 were fried, D612 would have been damaged as well. Have you replaced it?

Does it try to start up when you first apply power? If so, check the VCC circuit - D608, D618, D613. If D613 is short, it will prevent the converter from starting up properly. Also, is C615 really 1000 pF? If so, you could try putting a 1 µF, 50V electrolytic across it (positive side to the side that connects to D608 cathode).

Where are you located? Poland? Can you put that in your profile please, because it's important.

It seems that aliexpress will ship to Poland. You can get a pack of two FA8A00s for a reasonable price through aliexpress at the first link that comes up with that Google search.

 

[michalw]

D612 has been replaced too.
I've checked D608, D618 and D613 and are fine.

 

[KrisBlueNZ]

OK, I guess the FA8A00s are the next thing to try.

 

[michalw]

Hi,
I replaced FA8A00 and it's working.
Thanks for your help

There's one, small thing left - Q601 is connected with a radiator, which is connected to the GND. There should be an isolation between Q601 and the radiator, but after replacing damaged MOSFET, there's always short circuit. I have no idea what should be used between the MOSFET and the radiator to avoid short circuit - I tried thermal paste, but it didn't work with a screw.
I wonder if it is a good idea to desolder radiator from the GND - does it matter?

BR,
Michał