Samsung LE37B550A5wW Inverter MOSFETs dead short

[trigger]

Mosfets on board BN44-00262A have gone dead short. Replaced the two FDPF7N5OU (QI820 QI821) with other mosfets as these were not available. I used FDPF8N50NZF with a better spec and these blew after about 1 sec and took the main 6.3A fuse out as well as the 3,15A fuse which had blown originally. Have i lost drives or something? Any help would be appreciated.

[tom66]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

You should also replace the components near them which drive the MOSFETs - diodes, resistors, etc. They will connect to the gate pins.

[tom66]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Also get BN44 number -- I may have a schematic you can use.

[budm]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

From TOM website.
the fusible resistor that feed the PFC B+ to those two MOSFETs is OK?

[trigger]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Is that PFC out? Cant find PFC B+ on schematic.Resistors round the two Mosfets test Ok.

[tom66]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Yes that is the same as PFC B+.

[trigger]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Thanks for that. I'm trying to source the 4 diodes KDS184, would BAV70 be OK? They are 85V 200mA High Speed switching diodes.
Will post when Ive replaced the diodes and trannies.

[tom66]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

I can't see why that wouldn't work. Should be an acceptable substitution, given the MOSFET gate is typically driven up to around 10 to 15V.

[tw2005]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

looks bang on target using NXP cross reference.

http://www.nxp.com/xref/nxp?typenumber=kds184

[trigger]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Thanks again. Just removed all the diodes and transistors found one diode and one transistor open cct. Had weird readings around that bit of the cct.Hopefully new bits arrive tomorrow and that will fix it ????

[trigger]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Replaced the transistors,diodes and MOSFETs .Powered up and the 6.3A fuse blew and the NTC NT811S. Checked for shorts and can't find any. Components I've replaced are OK and the 3.15A fuse. No shorts on the two MOSFETs Q1820,1821. Shall I change the bridge rectifier and the 150uF cap CP815. Strange no shorts? Unless something blew when I first replaced Q1820,Q1821 ?????

[tom66]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Do you have a multimeter with frequency and VDC capability?
Try powering up with MOSFETs removed, see if the fuse blows.

[budm]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Or remove that fuse (FM802S 3.15A) for the b+ that feeds the two MOSFETs. I have a feeling he may have bad PFC MOSFET (QP801S), or the bridge, or the PFC IC.

[trigger]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Thanks
I do have a multimeter and a scope. The Mosfet QP801S does not appear to be short cct or the bridge rectifier. I will replace the bridge and the NTC, remove fuse FM802S and power up and see what happens.

[trigger]

Re: Samsung LE37B550A5wW Inverter MOSFETs dead short

Sorry about delay in replying for all your helpful advice. Kept on going round in circles with this one. Replaced bridge rectifier and the NTC. Didn't blow the 6.3 amp fuse this time. Still blew the 3.15 amp fuse. Getting expensive, bought another board £40.00. Now working fine.
Thanks everyone for your help. Will stay with this forum. Brilliant.