Vizio E601i-A3 Mofset and fuse keeps burning out

[b4hr4m]

Hi all,

I have this Vizio 60" E601i-A3 with no power on issue. After I opened the tv, found out the fuse was blown. Replaced the fuse and blown again. Checked all the mofsets and found out the Q103 (FDPF16N50) was shorted. Replaced the mofset, without the mainboard connected, had 3.3v on first pin on CN201. But when I connected just the mainboard back again, the fuse and mofset went bad again. Thought it's a bad mainboard, so I replaced the mofset again, with no mainboard connected, shorted the PS_ON to ground and boom, same problem, fuse and mofset went bad again. I don't want to blow that mofset again because I just only have one more good mofset. Any idea what's the problem and where to start to diagnose the board?
Thanks

[R_J]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

Usually when a mosfet blows, it also takes out the drive ic U101, also check the resistors and diode in the gate circuit. That looks like the pFC circuit, you may be able to leave the mosfet out of circuit and see if you get any voltage on the 12v circut (voltage would likely be low around 6v)

[b4hr4m]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

checked the diod D107, multimeter on diod mode, get .023v both directions! R107 5ohms, R108 47ohms. on IC U101, ground pin and ready pin is shorted! Is that normal?

Will take out the mofset to check the 12v circuit but as I'm a newbie, where exactly should I check for 12v?

[CapLeaker]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

This is the PFC circuit (FAN7930C). Take that diode out and check it again out of circuit. Replace that FAN7930. Chances are it is shot. It doesn't need to fail short to gnd. It can just fail latched on as well. Mental Note: If a Fet is shorted to its gate, full voltage went to the gate drive circuit, blowing that too.

[R_J]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

The ready pin has a 0Ω resistor to ground. The pfc circuit is used to boost the voltage on the main filter caps from 160v (in standby) to around 340v when the power supply is on.
So with the main board disconnected, and the PS_ON taken low the rest of the power supply section that supplies the 12v REG (CN201) should turn on. This is just a test and may or may not work but if it does work and you get some voltage on the 12v line it is likely just the pfc circuit has a fault. and since it is a fairly simple circuit I suspect u101 is defective.
You can also check the voltages on U101 to see what voltage is on pin 7

[b4hr4m]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

diod D107 is fine.
No voltage on pin 7.
Took out the fet but no 12v on CN201, however 12v is present before Q202 and Q204.
Will change the 7930C and see if it works.
Since I'm ordering it online, do I need to order any other components that may cause this problem just in case?

question: does the mosfet make the ic goes bad or other way? if it's the mosfet, even if I changed the ic, same thing would happen again right? or am I wrong?

[CapLeaker]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

that is why we have a little jig (put a 60W or 100W incandescent light bulb in series trick). Basically you take the hot wire and put it through the light bulb. If there is a short (high current draw) then the light bulb will glow nice and bright and thus limiting the power to the PSU (hopefully saving it).
Take the MOSFET out and check on the PCB for shorts again. Also check those diodes after the MOSFET.
With a blown gate drive circuit you will blow good MOSFETS all day long. You have to make sure that the gate drive circuit works first! That is why we said before: if the source / drain pins on the MOSFET is / are shorted to the gate pin, full power went to the gate drive circuit (IC101 output pin, and that small diode). Working on the hot side of the PSU is not easy. One wrong move and BANG!

[b4hr4m]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

Ok I'm trying to learn how things work and now I'm so confused!

The source pin of mosfet is connected to - pin of rectifier ic (ground) through the R110 big 0.1 ohm resistor right?

And the drain pin of mosfet is connected to + pin of rectifier ic through T101 transformer.
I checked the drain pin and 160v was present.

So my question is, when the gate pin of mosfet gets voltage to allow current between drain and source, isn't that basically short the +160v to ground?

Sorry if it's a stupid question but it doesn’t make sense to me.

[b4hr4m]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

Forget my last stupid question lol

Replaced the ic and tv is working now

Thanks for your help

[CapLeaker]

Re: Vizio E601i-A3 Mofset and fuse keeps burning out

I figured that the IC is bad. It can't handle the high voltage that got dumped on to the MOSFET's gate when the MOSFET failed.

Glad you got it sorted out.