Samsung 32" inverter MOSFET Overheating problem

[suprex]

I have Samsung LE32C450E1W with backlight inverter problem, so everything works just no backlight. Inverter is VIT70097.00 REV 2, and it's powering four CCFL's that are in the form of elongated letter U.
The inverter itself hadn't MOSFET's on it, all four of them were removed by someone before me. So I googled and found out it uses two N channel and two P channel fet's, P2904BD as N channel and P4004ED as P channel, as a replacement I use AOD 4184a and IRFR 5305 respectively, and everything works, but one P channel FET is geting way to hot (three others transistors are cold). I followed the gate line of the P channel FET transistors and all resistors and diodes are fine, but I noticed that the parts are not the same, ie. different resistors values. They hadn't been changed I know that because I compared with other images on the internet and everything is the same as on my inverter, but I do not understand why. By the way, the inverter has two IC: OZ 964AGN and CP 1099.

So my main question is why this transistor heats up?
Thanks in advance and Happy new year.

[dick_barton]

Re: Samsung 32" inverter MOSFET Overheating problem

Have you compared the secondary and primary resistances of all the 8 transformers. Perhaps one of them is faulty and causing the heating up.

[suprex]

Re: Samsung 32" inverter MOSFET Overheating problem

Have you compared the secondary and primary resistances of all the 8 transformers. Perhaps one of them is faulty and causing the heating up.

Yes, i did check all them, all eight primers are connected in series and each one have 0,1 Ohm resistance, secondaries are about 384 Ohms +-5 Ohm.

[CapLeaker]

Re: Samsung 32" inverter MOSFET Overheating problem

maybe the lamp is drawing too much current?

[suprex]

Re: Samsung 32" inverter MOSFET Overheating problem

maybe the lamp is drawing too much current?

Can I measure it with regular DMM? I'm asking this because of high voltage.

[CapLeaker]

Re: Samsung 32" inverter MOSFET Overheating problem

nope. all you can do is change lamps around and see if there is a different mosfet is getting hot.

[ReeceyBurger123]

Re: Samsung 32" inverter MOSFET Overheating problem

Could also be dry inverter caps. Test them for esr.

[jetadm123]

Re: Samsung 32" inverter MOSFET Overheating problem

tors are cold). I followed the gate line of the P channel FET transistors and all resistors and diodes are fine, but I noticed that the parts are not the same, ie. different resistors values.

Thanks in advance and Happy new year.

When you say the resistors and diodes are fine, did you actually measure their values or do they just "look" okay?

[suprex]

Re: Samsung 32" inverter MOSFET Overheating problem

all you can do is change lamps around and see if there is a different mosfet is getting hot.

I will try switching lamp places.

Could also be dry inverter caps. Test them for esr.

I do not have ESR meter. Is there other way to test them, except with ohm meter, for short?

When you say the resistors and diodes are fine, did you actually measure their values or do they just "look" okay?

I measured all the resistors, and for the diode first i unsolder it, and then test them.

[suprex]

Re: Samsung 32" inverter MOSFET Overheating problem

I found solution for overheating. The problem was in P channel replacement FET's. When a low voltage is applied to gate, the drain- source resistance is a bit high, causing FET to heat abnormally. Anyway thanks for the help, I hope this will help someone with similar problem.

Cheers.

[petehall347]

Re: Samsung 32" inverter MOSFET Overheating problem

why was the voltage low ?

[ReeceyBurger123]

Re: Samsung 32" inverter MOSFET Overheating problem

So how did you fix that ?

[suprex]

Re: Samsung 32" inverter MOSFET Overheating problem

why was the voltage low ?

The gate voltage which comes from ic goes trough resistors and diodes is ok, but is insufficient to fully turn on the replacement P type FET so the rds-on is a couple of Ohms, and from there the temperature goes high.

So how did you fix that ?

Sorry, I forgot to write that although that was my intention. I replaced those P channel FET's with P4404ED (very hard to find in my country), and everything works great without overheating. Other option was to change values of the resistors to match the IRFR5305 FET, but the first option was easier and safer.

[petehall347]

Re: Samsung 32" inverter MOSFET Overheating problem

so the conclusion is you chose poor substitutes to start with ? i am learning about fets too myself . even though i have replaced many in the past with like for like .. my head is at bjt mainly .

[suprex]

Re: Samsung 32" inverter MOSFET Overheating problem

so the conclusion is you chose poor substitutes to start with ? i am learning about fets too myself . even though i have replaced many in the past with like for like .. my head is at bjt mainly .

Yes, the first substitute was not equal...
I also replaced a couple of fets in past with similar ones and most of those was easy fix, this was a little to think over.

[petehall347]

Re: Samsung 32" inverter MOSFET Overheating problem

well done .. i must have read same article as you recently as i am just learning about these power supplies . i remember same thing about fets needs to be switched on hard to work properly .

[budm]

Re: Samsung 32" inverter MOSFET Overheating problem

http://www.microsemi.com/document-po...osfet-tutorial

https://cdn.badcaps-static.com/pdfs/...19d1be1cc7.pdf