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Old 06-25-2022, 11:56 AM   #1
alltvrepairs
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Default Bn44-00623B pfc mosfet very hot

Hi,
Found pfc mosfet Qp802c shorted. Replaced together with Pfc ic and blu ceramic caps.
Now pfc is working at 388v. However the heatsink is too hot to the touch. Have compared most values with a scrap o
Pcb and consulted circuit.

Any tips on this common board?
I have the 46 model.
The 42 is Bn4400622b with same pfc circuit.

Thanks.
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Old 06-25-2022, 12:12 PM   #2
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Default Re: Bn44-00623B pfc mosfet very hot

How was QP802C shorted? was it shorted D~G ? if it was you may have damaged the gate drive components, likely a shorted diode. Also check if there is a zener diode off the IS pin of the drive ic that may be shorted
On the BN44-00622b schematic, check dp806,rp804,rp805 and zdp801, Check that dp803 or dp805 are not shorted.
Did you replace the mosfet with the same part number?

Last edited by R_J; 06-25-2022 at 12:25 PM..
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Old 06-25-2022, 03:34 PM   #3
alltvrepairs
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Default Re: Bn44-00623B pfc mosfet very hot

I have swapped twice mosfet with an identical from another board. Mosfet was shorted drain to source. Have already checked those components but will try sgain desoldering diodes just in case.
Tks
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Old 06-25-2022, 03:36 PM   #4
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Default Re: Bn44-00623B pfc mosfet very hot

20220625_222256.jpg
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Old 06-25-2022, 03:50 PM   #5
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Default Re: Bn44-00623B pfc mosfet very hot

Do you have an oscilloscope to check gate drive is turning gate off?
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Old 06-26-2022, 03:23 AM   #6
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Default Re: Bn44-00623B pfc mosfet very hot

Yes i have oscilloscope. Tbh its been a while since i last used it.
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Old 06-26-2022, 10:09 AM   #7
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Default Re: Bn44-00623B pfc mosfet very hot

I would have to think one of the components on gate drive or CS is out of spec. Not allowing fet to completely turn off and on. You do have to isolate your ground to check gate drive with oscilloscope.

Last edited by dskall; 06-26-2022 at 10:11 AM..
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Old 06-28-2022, 04:24 AM   #8
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Default Re: Bn44-00623B pfc mosfet very hot

HI,

I have replaced most of the Gate driver compoinents and others as highlighted in pdf attached.
I have scoped both the bad board and a good one I have as spare. There is a slight frequency difference as both settings where the same.

Offset 0V
base.jpg
Bad board Gate pin
bad.jpg
Good board Gate pin
good.jpg
Highlighted schematic
samsung_bn44-00622b_power_supply_sch.pdf

Still no progress.
I am trying the board on its own now on the bench. Still heats up, a bit less due to less load but still more than the good one.
Any clues?
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