Is it safe to replace a AP2761I-A power MOSFET with a ST-12NM50FP on a PSU? - Page 1
Forum section: electronics / repair

author: SparkyTD
author_rank: Contributor
timestamp: March 10, 2024, 12:42:21 pm
content: A couple days ago my Dell SE198WFPV monitor started turning itself off and on at random time intervals. While this was happening, I also started hearing a strange buzzing sound coming from near the power connector. So I took the thing apart to see if there's anything obvious like a failing cap or other burn marks, but I didn't see much. But I did notice that the buzzing sound came from the yellow SMPS transformer, and one particular heatsink got so hot that I burned my finger on it.After some digging, I found the source of the heat to be a AP2761I-A power MOSFET on the heatsink, and I'm hoping that this component is the root cause of the problem. The closest donor I have is a ST-12NM50FP that I salvaged from an old projector's PSU board. When comparing the two sheets side by side, most of the important specs seem close enough, with the biggest difference being the zener type diode. Other than that, the donor has a slightly higher current rating (Id=12A instead of 10A), and a lower gate resistance (RDS=0.35Ω instead of 1.0Ω).I think the overheating MOSFET is definitely the main problem here, because if I turn on the monitor, it does work just fine for about 30 minutes, and I don't even hear the buzzing at the beginning. But then the PSU overheats, and the monitor goes into a reboot-loop.Is it safe to replace this component as a troubleshooting step? Or is it more likely that the faulty component is the SMPS transformer and the whole monitor is basically useless?AP2761I-A:https://alltransistors.com/adv/pdfview.php?doc=ap2761i-a.pdf&dire=_apeST-12NM50FP:https://www.mouser.com/datasheet/2/389/stb12nm50-955456.pdfA pic of the PSU:

author: denimdragon
author_rank: Regular Contributor
timestamp: March 10, 2024, 01:12:03 pm
content: Hi,The first think I would check on monitors/Flat screen TV power supplies is cracksin solder joints from thermal expansion (heat cycling). This would explain intermitentconditions. As for as mosfet substitution goes, compare the data sheets. RDS needsto be close to the same. The other parameters need to be close as well but RDS isimporant for switching on off conditions.

author: wraper
author_rank: Unknown
timestamp: March 10, 2024, 01:24:26 pm
content: Issue has absolutely zero chance to be MOSFET related. It either works or fails short, with very small chance failing non destructively. Check small electrolytic capacitor on PWM controller VCC. Large high voltage electrolytic capacitor may be cause of the issue too but they don't fail nearly as often.BTW this monitor may actually not turn off but have just a backlight shutting down. In such a case green electrolytic capacitors on the secondary side are are the most likely culprits.Also as this monitor was made in the time of widespread capacitor plague, I'd just do a full recap, maybe leaving only high voltage capacitor due to its cost and generally long life.

author: SparkyTD
author_rank: Contributor
timestamp: March 10, 2024, 04:21:57 pm
content: Quote from: denimdragon on March 10, 2024, 01:12:03 pmI inspected all the solder joints, but I didn't see anything out of the ordinary. Also, I did try the donor mosfet just to see if it would blow up, but it made no difference so I put the old one back in. At this point I think my safest bet is to just buy a new one.

author: SparkyTD
author_rank: Contributor
timestamp: March 10, 2024, 04:26:28 pm
content: Quote from: wraper on March 10, 2024, 01:24:26 pmUnfortunately I don't have any matching replacement caps, and I'm not sure if I would even be confident enough to do a full recap. I've been using this monitor for over 15 years so this was expected at some point. I think my safest bet is to buy a new one. But I appreciate the help regardless.