Samsung pn50b860y2f smps (BN44-000277a) problem

[sberri]

I have a Samsung pn50b860y2f plasma TV that runs perfectly until the smps board (part # bn44-00277a) warms up. All voltages are dialed in as per the label and are stable until the smps warms up (roughly 5 to 10 minutes after power up) Vs and Va measure a stable 207v and 56v respectively. Once the smps is warm however, the two voltages drop to 182v and 50v and the screen becomes dark and has temporary ghosting/image retetion caused from the drop in Vs and Va voltages. If left on, the TV will display ever darkening/smuged images and will shutdown after 10 to 15 minutes of operation. If left to cool off, the entire process repeats. X and Y boards and both buffers have been swapped out with verified working boards and they are eliminated as possible faults. There are no visibly bad components on the smps and all caps test good on my ESR meter.

I discovered that mosfet QS801 is overheating. If I keep it cool by a high CFM fan or by freezing it, VS and VA are stable and the picture is wonderful. If cooling is removed, VS and VA begin dropping within a minute or two and my picture quickly becomes bad again. I've cleaned the heatsink and applied new thermal pads and have tried artic silver thermal paste ( without the pads or course) , neither solves the problem. I also replaced MOSFET QS801 with an identical brand new MOSFET ( tried this at least six times) and got the same overheating results. I'm at a complete loss here. I've attached the schematic and have circled the MOSFET that is overheating. It is on page two, titled VS BLOCK. Any and all help is greatly appreciated.

[R_J]

Re: Samsung pn50b860y2f smps (BN44-000277a) problem

I was looking at the wrong power supply before so I'm not sure, try replacing qs802, it might be leaky putting more strain on qs801

[jetadm123]

Re: Samsung pn50b860y2f smps (BN44-000277a) problem

Since it's temp related, maybe a bad solder joint? Try touching up all the solder joints around QS801

[budm]

Re: Samsung pn50b860y2f smps (BN44-000277a) problem

That is the Half-bridge drive setup. QS802 did not get hot?
Check the caps CS806, CS807 capacitance and also leakage resistance after you heat them up with hair dryer first, you need to remove them to test.
Also check the Gate discharging diode DS820, it is connected in parallel with low Ohms resistor so you need to remove it, if this diode is bad then the Gate capacitance will not be discharged fast enough so the QS801 will be on when it should be off when QS802 is turned on, vice versa for the DS821, so you need to verify for sure that QS802 is not also over heated.