Replacing relay with solid state DC switch with an eye towards safety - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: September 15, 2023, 04:55:11 am
content: I'm looking at replacing a mechanical relay with a semiconductor switch, probably just two back to back N-Fets with switchmode or chargepump boost to drive them.What I have questions about is how to do this while keeping the system somewhat safe from a malfunction that turns it on in error.Like maybe if water got into the circuit it should never turn on, or if one SMT part vibrates off the PCB, that sort of thing.The thought of having the two connections needed to power on the system only separated by a few 1000 atoms inside a transistor instead of 3mm apart inside a relay is a little worrying. Is that rational?This is not medical/life-support levels of safety or anything, but having it turn on when it hasn't been commanded too could start a chain of events that might causes injury.Anyone have any comments or tips for making a semiconductor based DC switch safer?

author: jonpaul
author_rank: Super Contributor
timestamp: September 15, 2023, 05:44:09 am
content: relay spec? Contact, coil?Voltage , current, load on contacts?AC/DC?j

author: BrianHG
author_rank: Super Contributor
timestamp: September 15, 2023, 05:54:01 am
content: something like this?https://www.digikey.com/en/products/detail/ixys-integrated-circuits-division/CPC1907B/3908816

author: Psi
author_rank: Super Contributor
timestamp: September 15, 2023, 10:09:49 am
content: na, big relay, 100A 12V DC.Designing and building a semiconductor replacement is not the issue, just looking for tips for how to add safety to it.

author: BrianHG
author_rank: Super Contributor
timestamp: September 15, 2023, 12:35:20 pm
content: Just design it to handle 10, 000 amps and fuse it to 100 amps. Your circuit will survive.There is always electronic fusing, you can break an overload within a microsecond or 2.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2023, 12:42:33 pm
content: Semiconductors have the tendency to fail as a short. So a big part of the problem is to avoid it from failing at all.At least the gate drive part is the lesser part to worry about: a SMPS hardly even fail in way to accidently produce the isolated voltage - they usually fail off in on one way or the other as it needs the working active circuit to create an output.Depending on the use one may even want the mechanical relay in addition to the FET switch. With DC one often can get away with just 1 FET as the polarity is usually fixed / known.

author: TimFox
author_rank: Super Contributor
timestamp: September 15, 2023, 02:19:29 pm
content: An amusing anecdote about relay safety:A traditional US safety requirement for x-ray interlocks is that opening the door breaks a switch that removes power from the x-ray generator without further mechanical action (i.e., no coil-operated relays in the chain).Therefore, a large physical switch (we used to call it a "BRH" switch) is mounted on the shield door, connected to the power feed to the HV supply.We had a borrowed x-ray power supply in the lab that used solid-state relays at the interlock, which we connected to normal door safety switches, meeting the letter of the specification (no further mechanical action).However, that supply had horrible conducted EMI, and the leads to the switch picked up enough crap to hold the solid-state relays closed: opening the door did not stop x rays.Guess if we learned that one the easy or hard way?

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 15, 2023, 03:05:43 pm
content: You will be switching a significantly large current. The Mosfet switches very fast. Therefore the very large dI/dt coupled to the wiring inductance will produce a large Voltage spike. You will require to suppress it somehow.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 15, 2023, 08:49:05 pm
content: Quote from: Psi on September 15, 2023, 10:09:49 amLook at how they design Power Distribution Switches, Load Drivers / High Side switches.There is a bit more to them, than just FET+Chargepumps.They have inrush control, and many include current sense so they can also act as a circuit breaker.Does it need to carry 100A continuous ?A quick look shows you you can get hi-side switches for 12V at 0.6mOhms rated for 65A continuous.If you are safety focused, you could add a crowbar and second enable so that of the switch element ever fails, the illegal output detection fires the crowbar.Addit: TI have high side controllers, 12V & 48V systems, for external N-FETs. If you really need 100A carry, you need well under 1mOhm, so multiple FETs would be indicated.These have overcurrent sense and external temperature sense and under voltage lockout and re-try timers.https://www.ti.com/power-management/power-switches/high-side-switch-controllers/products.html

author: zapta
author_rank: Super Contributor
timestamp: September 15, 2023, 10:05:52 pm
content: When I needed a similar switch I went with a mechanical relay because SSR may fail in the short circuit. Just saying. I am not an expert.

author: uer166
author_rank: Super Contributor
timestamp: September 15, 2023, 11:28:32 pm
content: Quote from: zapta on September 15, 2023, 10:05:52 pmI've seen about as many large mains relays with huge gaps, fail short, as I've seen eFuses/solid state fail short. Bottom line is both fail short readily due to a variety of external factors, and if you require true safety, you need some kind of redundant system, or an inherently fail-open device (which a relay is not). Most of the thread seems to imply that relays don't fail short, which is warm fuzzies but not reality.Re: solid state: that's a really difficult subject, and making an eFuse/HSD requires a lot of care. The automotive world has successfully done that many times in safety critical systems, but it's a lot of work/testing/validation, and even with large budgets they sometimes get it wrong. I like the HSD+regular series fuse+crowbar failsafe idea. I also like relay + solid state redundancy, since you can avoid wearing the relay down almost entirely by only opening it in case the solid state section fails short.Solid state provides some inherent safety benefit that relays don't; namely that you can react to overload conditions so fast, that you have a much better control of let-through/short circuit current.

author: uer166
author_rank: Super Contributor
timestamp: September 15, 2023, 11:31:46 pm
content: Quote from: TimFox on September 15, 2023, 02:19:29 pmThe only thing you learned is that your specific solid state solution was shittily designed, and it had nothing to do with it beinginherentlysolid state. If a cockroach got caught in the mechanical interlock, keeping it closed, you can just as easily have claimed that the ss solution is superior since it doesn't have that failure mode. They both have unique issues and both require tailored design work..

author: uer166
author_rank: Super Contributor
timestamp: September 15, 2023, 11:34:29 pm
content: Quote from: Psi on September 15, 2023, 10:09:49 amIf you feel like reverse engineering a successful commercial item of similar rating (IIRC it's actually multiple channels of >100A), tear apart this:https://www.ebay.com/itm/143609918825

author: bdunham7
author_rank: Super Contributor
timestamp: September 16, 2023, 12:11:52 am
content: Quote from: uer166 on September 15, 2023, 11:31:46 pmPerhaps, but microwave ovens have an extremely cheap and very effective way of dealing with this, if the first mechanical interlock does fail, there's a secondary one that blows up and turns it off permanently, or at least until it gets repaired. No need for a sophisticated solution!

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2023, 03:36:38 am
content: Quote from: uer166 on September 15, 2023, 11:31:46 pmThat's an unnecessarily nasty response.Note that the EMI generated by that crummy supply would not have held an electromechanical relay.It's an object lesson.