Substitute for NTP18N06L mosfet - Page 1
Forum section: electronics / beginners

author: giosif
author_rank: Frequent Contributor
timestamp: July 02, 2023, 10:08:16 pm
content: Hi,I am looking for a suitable replacement to an OnSemi NTP18N06L mosfet.So far, I came up with the following candidates:-Diodes DMT6009LCT-ST Micro STP80NF55-06-Vishay IRLZ24However, I am not that confident in my abilities to identify the main parameters when looking for substitutes for mosfets, so I would appreciate it if someone could validate the above list or suggest a more appropriate replacement.Thanks!

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 03, 2023, 12:39:25 am
content: What it's used for?The very low QGand capacitances look like it's used for switching, and probably fairly high frequency at that. In that case STP80NF55-06 is probably not fit as replacement.

author: bdunham7
author_rank: Super Contributor
timestamp: July 03, 2023, 12:51:50 am
content: What package is it?

author: liaifat85
author_rank: Regular Contributor
timestamp: July 03, 2023, 07:41:54 am
content: I think it depends on your requirement.

author: giosif
author_rank: Frequent Contributor
timestamp: July 03, 2023, 07:56:57 am
content: Quote from: ArdWar on July 03, 2023, 12:39:25 amThis is used in combination with an op-amp to generate the -15V power rail in a Fluke 8846A bench meter.One thing I noticed was the lower gate threshold volage for turning the mosfet on: 1-2V.I suspect that's the reason for the "L" suffix in the name of the model.Quote from: bdunham7 on July 03, 2023, 12:51:50 amIt's a TO-220.This is for the same meter we've been discussing in the repair thread.Quote from: liaifat85 on July 03, 2023, 07:41:54 amAgree. As mentioned at the beginning of this message, this is part of the circuitry providing the -15V power rail in a Fluke 8846A meter.

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 03, 2023, 09:03:50 am
content: Sounds there's no need to be of particularly specific performance tbh. Although for close match should be IRLZ24

author: bdunham7
author_rank: Super Contributor
timestamp: July 03, 2023, 05:06:09 pm
content: Quote from: giosif on July 03, 2023, 07:56:57 amI dont have mine open, so I didn't know the package type. I think most of the specs aren't critical in a linear application like this. Probably all the ones you listed will work OK, I only looked at the IRLZ24 and I don't see why it wouldn't work. Because it is linear, I don't think RDS (ON)matters either, so even something like IRF830 would probably work.How did this fail--open, short, something else? What voltages do you see on the -15V rail now, both before and after the MOSFET? Is something else shorted on the -15V rail that caused this to burn up? And am I right to presume the -15V rail in inguard?

author: giosif
author_rank: Frequent Contributor
timestamp: July 03, 2023, 07:35:14 pm
content: Quote from: ArdWar on July 03, 2023, 09:03:50 amOk. Thanks for the confirming!

author: giosif
author_rank: Frequent Contributor
timestamp: July 03, 2023, 07:36:50 pm
content: Quote from: bdunham7 on July 03, 2023, 05:06:09 pmSorry, I didn't mean it that way.I only wanted to make you aware this is on the same overall topic (i.e. Fluke 8846A repair), so you know more specifically which part of circuitry this belongs to.Quote from: bdunham7 on July 03, 2023, 05:06:09 pmI see. In this case, I will give it a try with a STP80NF55 I already have in my parts box.Quote from: bdunham7 on July 03, 2023, 05:06:09 pmAs mentioned in the other thread, I got the Fluke meter as non-working and, right from the beginning, the -15V rail was sitting at -18V.First, I checked the associated op-amp, but that turned out to be functional.I then desoldered the mosfet and confirmed it was dead; to be specific, the mosfet wasn't shorted, but it was showing something in the 100s Ohms between gate and drain, as well as between gate and source (and in the kOhms between source and drain). Or something like that.Also, testing with a Peak Atlas DCA55 component analyzer, it could not detect it as a mosfet.The mosfet is removed from the board now and I threw it away, so I cannot provide measurements with it installed anymore.Quote from: bdunham7 on July 03, 2023, 05:06:09 pmYes, this is the inguard -15V power rail.And, yes: U24, an LF353M op-amp (which feeds off both -15V and +15V rails), was damaged, although not shorted.As I mentioned above, the -15V rail wasn't pulled (up?) to GND; it was just sitting at -18V.

author: giosif
author_rank: Frequent Contributor
timestamp: July 08, 2023, 11:55:09 am
content: Just to update everyone that I installed a STP80NF55 MOSFET and the -15V rail is looking good now.Thank you all for the help!Now, on to troubleshooting other areas of the meter.