Power MOSFET N-Channel 600V Overheating | TV PSU - Page 1
Forum section: electronics / beginners

author: Haruki
author_rank: Contributor
timestamp: June 13, 2025, 07:58:26 am
content: Hello, i'm trying to fix the Power Supply board (BN44-00473B) of a 2012/13 Samsung tv (modelUE40D5000). The TV starts fine but turns off after a while.I found that an area of the PSU is for sureoverheatingjust after 10-15 seconds. The "hotspot" seems to be the heatsink of thePower MOSFET N-Channel 600V (MDF11N60)in the primary side.Can't see any burned marks / component or visual damage anywhere. The solder joints seems fine, but i re-solder again some stuffs just to be sure (also on the back full of smd).I "tested" the mosfet desolder from the board, diode mode:-Drain-toSource+= 0.500v |Drain+to -Source-= OL-Drain-toGate+= OL-Drain-toSource+= drop to 0.200v- short Gate & Drain, testing againDrain-toSource+= back to 0.500vOthers people on the internet complain about that issue but can't find a real solution.Some videos on yt advice to change the capacitor near that mosfet (marked151 1kv) for one marked101k/2KW. Tried this but same issue. Another video says to replace it with a more low value (47pf) but...seems odd to go off-spec like that...so i didn't tried this.On the back side, thePower Supply Control ICit's markedFA5591. I don't know if this component is working as it should be, it's possible to test it off-board ?I have others tv PSU but sadly all different, and also the mosfet / ic are not the same.Why the TV works (for a while...) but that mosfet has abnormal temps just after few seconds on? What should i check more?At the end that poor mosfet will just explode right?For tests, i only have a digital multimeter...Datasheet here, with also the board image. Thanks!

author: Antonio90
author_rank: Frequent Contributor
timestamp: June 13, 2025, 08:33:33 am
content: Do you have any way of looking at the VGSwaveform? The MOSFET might not be turning fully on, or staying too long on the linear or active region and dissipating too much heat.

author: Haruki
author_rank: Contributor
timestamp: June 13, 2025, 09:06:27 am
content: I don't have an oscilloscope so i think i can't check what you asked...sadly :c

author: AlexRG
author_rank: Regular Contributor
timestamp: June 13, 2025, 09:21:23 am
content: Use a special tester (like FNIRSI LCR-ST1) to check the capacity and ESR(!!) of all electrolytic capacitors, yellow arrows. Especially those marked with blue arrows.

author: Haruki
author_rank: Contributor
timestamp: June 13, 2025, 09:42:33 am
content: Ugh...i don't have an ESR tester also 😅 (should have to buy one i know xD). I will update the first post... only have a digital multimeter 😅.But i already checked the 3 caps near the mosfet off-board with the multimeter and at least the uF values are ok.Didn't focus a lot about the caps to be honest...the tv is not so old...I thought the problem must be deeper and more particular concerning something else ... also because in the past, whenever the solution for others was just “replace the caps,” it was never the case for me...Well at this point i can just try to replace the caps on the primary side, just to be sure, not a problem. Thanks 🙂.

author: fourtytwo42
author_rank: Super Contributor
timestamp: June 13, 2025, 09:52:33 am
content: The pcb in the area of the MOSFET appears blackened, is this from your soldering or was it there before ?It could be an open circuit snubber component causing the MOSFET to constantly avalanche, usually the snubber is a capacitor, resistor & diode connected usually across the transformer primary, you are looking for a ceramic disk of a few nF, the resistor would be 10-500R at maybe a watt.Does it still get hot if you turn the display brightness as dim as possible ?

author: AlexRG
author_rank: Regular Contributor
timestamp: June 13, 2025, 09:58:08 am
content: Quote from: Haruki on June 13, 2025, 09:42:33 amA normal capacitance value does not mean that the capacitor is in good condition. Just as it does not mean that the capacitor is in good condition if it is not swollen. A capacitor has a much more important parameter - equivalent resistance to alternating current, ESR. A capacitor can have a normal capacitance, but the ESR is already very large, which leads to the TV turning off and other problems.It is very easy to make a durable capacitor that will work for 30-50 years. But it is very difficult to make a capacitor that will work for 1-2 year of warranty and fail. Capacitor manufacturers invest millions of dollars in developing capacitors that will work for as many years as the manufacturer needs.It is not profitable for the equipment manufacturer for you to use one TV for 20-30 years. It is profitable for him for you to use it for 1-2 years, your TV fails, and you buy a new one. Capitalism.I recommend a very simple tester for ESR - FNIRSI LCR-ST1For example, if the capacitor is at 47u, the normal ESR is less than 0.5-1 ohms, if above 0.5-1 ohms, the capacitor is defective. If the capacity is 10u, the normal ESR is up to 1-2 ohms, if higher, the capacitor is defective. The blue arrow shows the ESR

author: Haruki
author_rank: Contributor
timestamp: June 13, 2025, 10:07:38 am
content: Quote from: fourtytwo42 on June 13, 2025, 09:52:33 amThe image is taken from internet, not my photo. I just post it to show where the "overheating mosfet" is located on the board.About my board i can only say that it had a bit of rusty around some components, that i cleaned, taked out and did again some solder joints just to be sure, but nothing else was wrong.No black spot, no visible damage, no broken traces or things like that. The mosfet itself and the board under it, was clean.The TV wasn't mine so i don't know where and how much was used 🤣.About the display brightness, i can say that with the "no signal" text (so the backlight is very low, screen almost all dark), the heat is for sure less in theshort time(less than 1 min) but still there and only in that area (i can feel it from the metal backplate of the tv). No heat spot from others area or from the tv mainboard.Will become super hot FAST (less than a minute) when the display has just a normal static image with full brightness (like the guide or media menu).

author: fourtytwo42
author_rank: Super Contributor
timestamp: June 13, 2025, 10:15:06 am
content: Quote from: Haruki on June 13, 2025, 10:07:38 amDo you actualy know how hot ? Have you measured it with an infra-red thermometer ?It may be a cheap as chips design and just runs hot!! Don't forget semiconductor junctions can run at 120degC and some are rated over 150degCWhen the tv stop working have you confirmed no output from the psu ?

author: Haruki
author_rank: Contributor
timestamp: June 13, 2025, 10:23:41 am
content: Quote from: fourtytwo42 on June 13, 2025, 10:15:06 amThey said that it turns off after a while so for sure something is faulty 🤣. And that abnormal high temp seems to be the issue for me...like it goes on protection for overheating...I don't know how hot in value...but i think is not normal that you can't TOUCH themetal backplate(not the internal heatsink!the back of the TV itself!) just in thatlocalized areaafter few seconds that a simple static image is ON with full backlight.Is so strange. I could understand after hours of use that it heats up, but not "burn" after few seconds! If the TV backplate is so hot, the mosfet itself under the heatsink is just more hot ready to explode. 🤣When the TV is off / standby, no heat. I didn't leave the tv turn itself off. I am worried that it might blow the mosfet or create more damage. The TV is not mine and I'm just trying to repair it as a favor, so I have to be cautious. 🤣Anyways @AlexRG i know that just the uF values is not enough, i read it everytime everywhere 🤣. About all the other text, yes, maybe I am too optimistic on how long things will last.... or because for me the problems are always elsewhere and never the capacitors, even in much older things.Will try to replace them later and write here. Thanks!

author: MariuszD
author_rank: Regular Contributor
timestamp: June 13, 2025, 12:37:12 pm
content: Quote from: Haruki on June 13, 2025, 07:58:26 amThe MOSFET has a very high gate resistance. Out of the multimeter's range. The multimeter should show OL on the highest ohmmeter range. The remaining circuits connected to the gate will interfere with the measurement. They need to be disconnected. You can desolder the transistor, but it would probably be easier to desolder one lead of the resistor that connects the gate to the controller. The same goes for the pull-down resistor. If there is one.I saw partially damaged transistors. That's why I think you need to check it thoroughly.Is there any snubber connected to drain?

author: Haruki
author_rank: Contributor
timestamp: June 13, 2025, 01:22:53 pm
content: Quote from: MariuszD on June 13, 2025, 12:37:12 pmHello, i wrote again the test i did, maybe was not so clear sorry...the mosfet was desolder sure...

author: MariuszD
author_rank: Regular Contributor
timestamp: June 13, 2025, 02:33:34 pm
content: Quote from: Haruki on June 13, 2025, 01:22:53 pmThat means it is broken. A good one should not conduct between S and G

author: Haruki
author_rank: Contributor
timestamp: June 13, 2025, 06:41:12 pm
content: Quote from: MariuszD on June 13, 2025, 12:37:12 pmSorry but i'm more confused than before....i wrote nothing about probes between S and G. I wrote diode values between Drain and Source... I followed some videos on yt and I think I did everything correctly.Now the mosfet is again soldered on the board, if you can write exactly what and how i should test it, i can try again. I also read that for a "good" test i should give voltage to the mosfet and that maybe the multimeter alone can't give enough voltage to "active" the mosfet. On the datasheet of my mosfet the "Gate Threshold Voltage" is min 3v. Not sure if this can mean anything...Anyway, i will see again later or in the next days and sorry if I don't understand much, first time dealing with a mosfet "half alive" problem. Other times was just burned -> replace and finished 🤣.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 13, 2025, 08:14:27 pm
content: Quote from: Haruki on June 13, 2025, 06:41:12 pmNow I understand. I misunderstood what you wrote earlier. 0.2V is not related to G-S.So I have no reason to claim that MOSFET is damaged.

author: Haruki
author_rank: Contributor
timestamp: June 13, 2025, 08:22:40 pm
content: Quote from: MariuszD on June 13, 2025, 12:37:12 pmSo... something else is overheating the mosfet, i will put more photos tomorrow, maybe will help to point what near components i should check better...

author: Haruki
author_rank: Contributor
timestamp: June 20, 2025, 10:43:50 am
content: Quote from: AlexRG on June 13, 2025, 09:21:23 amSo...i replaced the four little caps on the primary side....and now the tv doesn't turn on anymoreStandby power led is working. If try to power on, 2 fast red blink then nothing, then again 2 fast blink, etc.Capacitors are soldered fine, same uf / voltage values, right polarity and taken from different working psu...First image is not my board, others yes (with still the old caps). As i said there was a bit of rust in some components or jumper wire, so i put new wires / solder just to be sure that they are connected..jumpers with still a bit of rust are connected from the other side of the board, all this was done since the first post, now i just changed the 4 caps and i have a "new issue". I should trash all?Edit: i will also add another photo (n5) of how was the board when i opened the tv, with rust etc around some components (that i cleaned, resoldered etc..).