Protecting a MOSFET from overcurrent - Page 1
Forum section: electronics / beginners

author: bonzer35
author_rank: Contributor
timestamp: March 30, 2025, 06:47:42 pm
content: Some time ago, I put together a PWM-controlled circuit with current-sensing to vary the speed of a pump (shown in attached image).I need to protect the MOSFET from an overcurrent condition caused by accidently shorting the pump output wires together (I have done this once already). I tried to use the current sensing in the software but it is not quick enough to protect the MOSFET. I was wondering if a fuse would be the best thing but it needs to allow at least 10A before blowing and this takes up a lot of space on a small PCB.Is there another more elegant solution?Thanks in advance.

author: BillyO
author_rank: Super Contributor
timestamp: March 30, 2025, 07:19:01 pm
content: Agood10A 250V fast blow fuse should do it. That MOSFET will pass over 40A so should pop the fuse quickly. But use a good fuse like LittleFuse, Eaton or Bourns. Don't go with an unknown cheapie.However, a good fuse that will be guaranteed to protect the MOSFET will cost considerably more than the MOSFET.I have found the IRLZ44 MOSFETs you can get from AliEx to be not too bad. Apparently they can be made properly for next to nothing. However, the cheap oriental fuses are useless and will cause things to burst into flames. A decent fuse will cost ~ $5. An IRLZ44, even from Mouser, will cost ~$2 or less than $0.50 from AliExpress.

author: PGPG
author_rank: Frequent Contributor
timestamp: March 30, 2025, 11:56:23 pm
content: Quote from: bonzer35 on March 30, 2025, 06:47:42 pmMay be ST:OmniFET or Diodes:IntelliFET having integrated protection are what you need.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2025, 12:00:42 pm
content: A fuse can work but what about including a current sensing part of the circuit which trips and clamps the gate to the source to shut off the MOSFET until reset?I would put a current shunt in series with the source, monitor it with a single supply comparator, and trip an SCR or otherwise latch the output of the comparator to clamp the gate to the source.

author: Peabody
author_rank: Super Contributor
timestamp: March 31, 2025, 02:44:17 pm
content: Possibly a current limiting circuit would work. So instead of blowing a fuse, it would just limit the maximum current. A foldback circuit would be even better.https://www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/current-limiter-circuit.php

author: wraper
author_rank: Unknown
timestamp: March 31, 2025, 02:51:24 pm
content: High side current sensing is a waste of parts in this circuit. You can easily sense it on the low side the same way as in cheap electronic load. You also can add a comparator that shuts down the MOSFET if current is exceeded (doing it in MCU should be fine if firmware is done properly). Or you can use a part with built in protection instead of simple MOSFET.

author: wraper
author_rank: Unknown
timestamp: March 31, 2025, 03:09:09 pm
content: Quote from: BillyO on March 30, 2025, 07:19:01 pmHow about no? You need MCU firmware to be really shit for it to be slower than a fuse. Fuse is good for protecting things from catching flames but is quite worthless for protecting semiconductors from overcurrent.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2025, 05:02:47 pm
content: Quote from: Peabody on March 31, 2025, 02:44:17 pmIf current limiting is implemented, then it *must* be foldback current limiting. Otherwise a short will destroy the MOSFET even faster because of excessive power.Better would be to trip and shut the MOSFET off, with a short delay before applying power again.