Vizio D40FG9 - No power/standby

[anupkg1985]

Hi,

I'm working on Vizio D40F-G9 tv, there is no voltage present on the secondary side of SMPS. Fuse was blown out and found two diodes on the bridge were shorted. So I replaced it with bridge as I was not having replacement diodes. Then found the mosfet QB101 is shorted. Original mosfet was SW7N65 which I replaced with TK10P60W. Again it shorted and fuse blown. I could measure the continuity between legs of main caps with shorted mosfet so I removed mosfet. Then found the diodes marked in red (R12K J0) color are shorted, could this be an issue to make mosfet and whole circuit shorted? Or what else do I need to check?

[R_J]

Re: Vizio D40FG9 - No power/standby

The diodes are not likely shorted, they are parallel to the 0.27Ω resistor. If the mosfet was shorted drain to gate, I suspect the gate drive circuit or driver ic UB101 is bad aswell

[anupkg1985]

Re: Vizio D40FG9 - No power/standby

Thanks RJ for quick reply, yes those are in parallel so looks like diodes not shorted. However mosfet is shorted on source to drain and not on drain to gate. Also what IC do you suspect?

[R_J]

Re: Vizio D40FG9 - No power/standby

Thanks RJ for quick reply, yes those are in parallel so looks like diodes not shorted. However mosfet is shorted on source to drain and not on drain to gate. Also what IC do you suspect?

The driver ic, it's on the other side of the board. If at any time during the mosfet failing the +300vdc may have found its way to the gate and took out the ic. If the ic is damaged it might just turn the mosfet on hard so it just shorts again.
Shopjimmy has 40 of them in stock. $37.95

[anupkg1985]

Re: Vizio D40FG9 - No power/standby

Thanks, I have ordered one from ebay for $19, hope that will work. However I would like to repair this board as well. I could see the voltage is jumping on cap EB105 from 10v to 15V. It's connected to the Vcc of the driver IC. May be IC is trying to come on but failing as it's damaged. I could see IC labelled as RIP 37T, couldn't find anything on internet. Any replacement or alternate for the same?

[anupkg1985]

Re: Vizio D40FG9 - No power/standby

One thing I want to understand, as it looks weird to me is continuity exists between -ve of main cap and source of mosfet as well as there is continuity between +ve of main cap and drain of mosfet. Is this expected? Doesn't it become a short when mosfet starts working?

[R_J]

Re: Vizio D40FG9 - No power/standby

One thing I want to understand, as it looks weird to me is continuity exists between -ve of main cap and source of mosfet as well as there is continuity between +ve of main cap and drain of mosfet. Is this expected? Doesn't it become a short when mosfet starts working?

The negative lead of the main capacitor is connected to HOT ground. The source of the mosfet will be connected to hot ground via 0.27Ω resistor, The positive lead of the main capacitor will be connected to one end of the primary winding of the transformer, the other end will connect to the mosfet drain. The transformer DC resistance will be very low so it will seem like the cap positive has a short to the fet drain.
If the mosfet is just turned ON, this will cause excess current in the primary blowing the fuse.
Under normal operation, the mosfet switches at around 60kHz, this is how the supply operates.
Here is a similar power supply primary circuit

[anupkg1985]

Re: Vizio D40FG9 - No power/standby

Thanks for the explanation, I got it now how it should work. Also I don't see pfc section in this board, so does that mean main caps will never have voltage around 360V on it? Or am I missing anything?

[R_J]

Re: Vizio D40FG9 - No power/standby

The main cap is only rated at 250vdc so the answer is no there is likely no pfc and the voltage should be around 160v on the cap.