MOSFET failure detection - Page 1
Forum section: electronics / beginners

author: ElectronKing
author_rank: Newbie
timestamp: January 31, 2020, 02:41:53 pm
content: I currently have a circuit setup on the breadboard to drive 12v 9W solenoid from a 3.3v PWM output of a micro via a bog-standard A03400A MOSFET. The solenoid will be opening and closing a value controlling flow of a liquid. I'm under the impression after doing some research that when a MOSFET fails, it usually fails closed. In my case, this would not be an ideal situation since the liquid would continue to flow until someone detected the failure.My question: Is there a common way to design for a MOSFET failure.One solution I've thought of is to use a small shunt, inline with the solenoid, and tie it back to the micro's A/D utilizing some routines to determine if the solenoid is still drawing power when it's supposed to be off. This seems really clunky and uses a lot of extra resources (components and firmware).Another idea is to have two MOSFETs in series. If one fails, the other will still be open. I'm not really sure how this would play out with PWM since each MOSFET would have to be very closely matched. Also, this doesn't guard against the failure of both MOSFETs.Any thoughts?

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 31, 2020, 03:19:55 pm
content: Very best way is to use an autoprotected Fet, made by many vendors. These are extensively used by the automotive industry, and have thermal protections, diagnostics, protections against inductive spikes, etc.I have used STMicro's OmniFets very successfully in all sorts of very hostile environments.But many vendors produce similar devices.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2020, 04:21:18 pm
content: You can certainly monitor for failure by detecting whether the gate terminal is producing erroneous voltage, or current draw (drain-gate breakdown -- often, transistors fail as three-way shorts), and whether the drain voltage is rising while the gate is low and vice versa, and whether load current is nominal or not. The latter two somewhat depending on the load of course, but thresholds can be designed for typical load ranges, which also gives you diagnostics about the load.Planning for failure is one thing, but designing to avoid failure is the higher priority. Indeed, protected FETs, as mentioned, are the way to go. They're still killable, but they stand a quite good chance of survival in common applications like this, and are very easy to use.Somewhat more advanced load switches offer high side switching, current sensing or limiting, fault detection, latching or auto resetting, monitoring, etc. Many are rated for voltages that are crazy, like everything upside down and all, to deal with automotive transients and cross-wiring. Some of these do get more expensive of course, but you can also find cost-sensitive automotive parts that have a surprising amount of functionality in them.There are also controller chips, you add an external FET to dissipate more power than a monolithic version can handle. Or you can go for broke and make your own, bonus being you can design in whatever limiting method you like, downside of course being it's a bit of a PITA for all the parts needed to implement it. I've made a discrete switching limiter before, which uses a single D2PAK transistor (among other things), that's able to dissipate 600W for up to 150ms per shot.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: January 31, 2020, 05:55:39 pm
content: Use a load switch IC with integrated and specified short circuit protection, overtemperature protection, and ability of driving inductive loads. To add margin, use one with higher ratings than you need, both voltage and current.For safety-critical loads that normally run for just a second or two at low duty cycle, I have sometimes used a carefully chosen traditional fuse, so that it blows in a few minutes of runtime. Fuses, while extremely reliable and simple, are not precision devices so this only works in situations where there exists a large difference between expected average operating current and failure current, and when the on-time is short enough (a few seconds max) so the fuse works as an averaging device.