Normally closed SSR gate driver - Page 1
Forum section: electronics / projects

author: fredboivin
author_rank: Regular Contributor
timestamp: October 19, 2019, 10:57:05 pm
content: Hi,I want to replace a 1 form C (SPDT) mechanical relay that keeps failing due to vibrations with a solid-state relay. It must fit in a tight and hard to access part of a machine I can't modify. The specs are: isolated 36VDC input, 50VAC, 15A load at an ambient temperature of 70C. There is no fan in the enclosure and I need to fit 4 of them in a volume of 60x80x70mm. That's some beefy specs for a SSR, so I'd like to hear your inputs on that. The NO part does not pose a problem.For the NC contact, I think the best choice is to use depletion n-mosfets (altough UnitesSic makes an expensive power normally-on JFET). I found the IXYS IXTH16N10D2, which is a nice big 100V, 16A, 64mR Rdson. I plan to drive it with one of those photovoltaic gate drivers like the IXYS FDA217.All that is nice, but even with such a low Rdson I am still looking at around 20W dissipation per transistor. The datasheet says that I can half that if I drive Vgs to 5V instead of the usual 0V.Now my problem: How can I generate this +5V gate drive when the input is off in order to minimize Rdson? I can't use a DCDC from the input as it is off. I thought of generating it from the load voltage, but I only have access to the common and NC terminals and there is only around 1V drop between them. A coin cell battery is not a good idea as it would be a huge pain and waste to change it twice a year.What do you think?I'd love to hear your ideas on this problem. Thanks!Edit: the load is bidirectional DC, not AC. Sorry.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2019, 09:57:40 am
content: To be fair, a coin cell in series with the gate, would probably never run out (5+ yr?), even at that ambient. (Or if not regular Li/MnO2 coin cells, then a formula more comfortable with high temperatures.) Downside, you probably need to stack two PV drivers in series to turn it off.More concerned with heat, how are you going to remove the <= 80W without water cooling?May also want faster drive than those PV arrays can deliver -- you might be looking at 10s of ms rise and fall time, which is very much into the danger zone at that voltage and current. A 555 timer, pulse transformer and rectifier would be fine for drive.Can always find bigger transistors or stack more in parallel to save heat, but there's only so many (in standard packaging) you can fit in there. Still doesn't mean it's impossible, but the number of methods quickly drops to "beg the manufacturer for bare dies and make a custom ceramic hybrid stack"...Ed: on another note, what about mercury displacement relays? If there's too much vibration, I suppose they'll just slosh around, but FYI.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 20, 2019, 10:26:01 am
content: Quote from: T3sl4co1l on October 20, 2019, 09:57:40 amThat's definitely an orthogonal yet excellent solution. High current capacity, NO or NC operation as long as excessive tilt doesn't occur, practically infinite contact life - I've seen Hg relays that were 50+ years old still chugging away, and very high surge current/overload capability.The bad thing is that mercury displacement relays are getting harder to find these days - you can special order them from Grainger but don't look too close at the price - but a quick duckduckgo search turned up a site that specializes in them and the prices seem pretty reasonable, especially considering the hackery involved with the alternatives:https://mercuryrelays.com/60amp_relays.html

author: Zero999
author_rank: Super Contributor
timestamp: October 20, 2019, 11:48:45 am
content: How about using an ordinary enhancement MOSFET with a small battery to bias it off?

author: duak
author_rank: Super Contributor
timestamp: October 20, 2019, 06:40:41 pm
content: To get around the rise time limitation of a PV coupler, could the initial energy for the gate drive be derived from the drain to source voltage? ie., during the first few AC cycles, could a cap be charged up and when it gets to the appropriate level local the driver snaps the FET on? After that the PV coupler needs to supply leakage current to keep the FET on. When shutting the FET off, the driver can discharge the gate without needing much power from the PV coupler.What is the failure mode of the relays? Contact chatter or structural failure or ??How about a triac or back-to-back SCRs driven by an NC reed relay? I think a small resistor is needed to limit the relay's contact current and the thyristors' gate current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2019, 07:18:46 pm
content: The electrical equivalent of a boosted / self-powered pneumatic valve? Sure, why not. Downside is more electrical leakage though. If the off state doesn't need to be very off, it's an option.Tim

author: fredboivin
author_rank: Regular Contributor
timestamp: October 20, 2019, 07:32:29 pm
content: Thanks all for your great replies! The current relay fails due to mechanical failure. It's controlling a DC motor, but the supply can come from either the common or NC terminal as the motor is bidirectional. The rise time didn't really bother me because the relay is only switched every 5min. Or am I missing something?The power dissipation will definitely be a problem, which is why I'm trying to minimize the power dissipation. I was already planning on using an aluminium PCB with a heatsink as large as I could fit.The Hg relays are nice, but they don't fare well in a high shock and vibration environment. I'll keep them in mind though.The battery could be acceptable if they last 10 years, so I did some math. The gate charge of the IXYS depletion mosfet is around 250nC. A typical coin cell holds 250mA capacity. Considering an activation every 5min of 8 transistor, it would take 1000's of years to discharge that battery. Of course, there's leakage but unless I'm completely wrong, it seems like a viable solution?I'm not sure using an enhancement mosfet with a battery to bias it on is a good solution, as the failure mode as the battery depletes would put the transistor in linear region and dissipate a ton of heat until if finally opens and I have a non-conducting NC contact. Mind you, the depletion mosfet with a battery would do the same, but would never open...Thanks,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2019, 09:09:07 pm
content: What are the chances the DC motor gets put in a locked-rotor condition? How much fault (or also inrush) current can it draw? Or is the supply current limited in any way?Is there any snubbing or clamping on the motor's voltage? It has considerable inductance and that may be a problem for MOSFETs (even with a ~ms fall time).Are these relays being used in an H-bridge, by any chance (reversing switch)? Could you replace the circuit with an equivalent function (a MOSFET H-bridge), and save considerably (~fourfold?) on transistors? (Put another way, is this an X-Y problem? -- you asked about replacing relay contacts verbatim (problem Y), but the underlying problem is switching or reversing a motor (problem X), an easier task.)The problem with switching time is, if it's dissipating roughly 50V and 10A for some milliseconds, well, that's 500W in a transistor rated for maybe 100Wwhen it's bolted to a refrigerated block of metal. Check the SOA curves -- you can only spend ms, maybe 10s of ms, in the linear region. (The motor inductance will help with turn-on switching to some extent, but worsens turn-off switching.)Gate charge doesn't matter to a bias battery as it's returned on the next cycle. Even non-rechargeable types are rechargeable on a small enough scale. Or they might not recover all the charge put back in, but they'll keep some, and that still helps greatly with lifetime.Tim

author: fredboivin
author_rank: Regular Contributor
timestamp: October 20, 2019, 09:45:23 pm
content: I'm still on the proof of concept phase, so I don't have much info on the motor yet, but I know it draws about 10A. I hadn't considered the inrush during the turn-on and the inductance at turn-off, but it is a very valid point.To be fair, I was given the relay and asked: "Hey, can you make me this so that it doesn't break?" But you're right, depending on the application I could design something more custom that would be more optimized. I'll have to do some more research.The series battery to bias the gate is new to me, so please bear with me. You're saying that appart from leakage, this is essentially a no current situation? So the battery lifetime would come down to it's shelf life?Thanks,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2019, 11:30:16 pm
content: Yup!And if "machine I can't modify" is really just another order coming down from on high, thenreallywhat they mean is, "we expect [that course of action] will cost more than [this presumably simpler course of action]". They may be committing an even higher level X-Y problem.The costs of each option may not be obvious. For example, a modified machine may no longer be covered by mfg warranty, or it requires extensive safety approvals for the change, or something. (Neither seems all that likely to me, but who knows, I have no idea what kind of machine this is.)Maybe the existing process (replacing relays, or the board they're on, when they inevitably fail) is costing more in downtime than in actual repair cost. Maybe the repair is even covered by mfg warranty, and they're eating the cost, but they still don't give enough of a crap to actually-fix it.Maybe thereisenough free space in the machine, nearby; or a heavy metal bulkhead that can be used for heatsinking, but that isn't included in the immediate 6 x 7 x 8 cm assumed region of interest. Maybe you need to drill some holes for screws but that's all. Maybe there really isn't any room for anything more, anywhere, at all.Maybe the alternatives you come up with, really aren't cheaper overall, and the best option really is just to keep repairing it, as shitty as it is. (Probably unlikely given what effort it sounds like repair takes; but it's at least possible that your labor might be better spent doing something else?)Or maybe there really isn't a physical cost, and it's just a manager's ego getting stuck on the one solution they brainstormed.But in any case, the only thing thatshouldbe of interest to management, is which option has the least overall cost.And if they aren't actually interested in that, well, there are plenty of other places to work, where insightful questions and carefully considered repairs (or whatever else your regular duties include) are rewarded rather than punished.Just some ideas of the kinds of questions to think about. Don't be afraid to think outside the box, even to very high levels; just keep it grounded. The most extreme options are generally also the least likely to be of direct use, but even if they aren't, there can still be of benefit by forcing you to consider the problem from a new perspective.Tim

author: fredboivin
author_rank: Regular Contributor
timestamp: October 21, 2019, 12:37:37 am
content: That's very good insight. This is not for work, only a side project for a friend where I saw an opportunity. I really only have the relay and a vague idea of what the machine does. As always, I took a bite of an elephant without looking at it first.I'll see with him what are the stakes and the possibilities for tackling the problem from the sides. From what I understood, this is a part related to a safety mechanism of a heavily regulated machine. The manufacturer can't be bothered by fixing the part and the clients eat the cost of regular maintenance because there is no other choice. Downtime for maintenance is a big deal for them, which is why there is an opportunity for a more reliable part.I can see why a replacement with same form/fit/function is attractive to them, as there is nothing to modify and this relay is used all over the place. On the other side, maybe it only fails in one particular use case and a customized solution would be acceptable.I have some homework to do. For now, the series battery seems viable.How hard is it to get and mount bare die transistors to an aluminium/ceramic substrate? Is it even cost-effectice in 10000's volume?

author: james_s
author_rank: Super Contributor
timestamp: October 21, 2019, 12:42:13 am
content: What about mounting a mechanical relay on a vibration damper rather than rigidly attached as I assume it is now. There may also be relays that are specifically designed to tolerate vibration, maybe something intended for aircraft or military use?

author: duak
author_rank: Super Contributor
timestamp: October 21, 2019, 01:19:02 am
content: fredbolvin, I'm confused here - you initially say the load is in a 50 V AC circuit, but later on say it's a DC motor that's being reversed. If so, I think the problem you're having is contact erosion from switching DC. When switching DC if an arc is produced between the contacts where the electrons go one way and the vaporized metal ions go the other way and deposit on the other electrode. Depending on the contact material, an oxide film can build up that is particularly bad for NC contacts because they don't move relative to each other to break through any oxide layer.So when the relay fails are its contacts open or intermittant or welded closed or does something actually come apart?I had a problem with a machine that used a reversible AC motor to drive an indexer using a Geneva drive. When the machine got older the friction was reduced, the motor wouldn't stop as quickly and then slide part way in to the next position. The motor was controlled by a PLC using relays. The NC contacts were free so I applied 12 VDC to brake the motor when it wasn't driven by the AC. It worked great for about 6 months and then the NC contacts would foul and the motor would sometimes slide to the next stop. I tried different relays - the last were silver-nickel - but no luck, the NC contacts would foul and not consistently close. The final solution was to replace the motor with a brakemotor.I don't know if a more refractory contact metal like tungsten is better - that was used in automotive point style ignition systems. Even so, they face significan contact erosion.

author: fredboivin
author_rank: Regular Contributor
timestamp: October 21, 2019, 01:34:22 am
content: Yes, my mistake, it's reversible DC. I don't know enough about the failure mode to be sure. You could very much be right. As I said, I need to do some homework before committing to a solution.If the failure mode is contact erosion, it's nice to know that a beefier mechanical relay is not a solution. Your experience prevents me from going down a rabbit hole.If the failure really is due to vibration, a rubber mount would be very simple and kind of a slap on the forehead solution. I like it!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2019, 09:17:42 am
content: Quote from: fredboivin on October 21, 2019, 12:37:37 amNo idea, but I would guess if you can show a good ROI, then a ~$1M investment would be adequate cash to move things along. You'd contact mfg FAEs about getting bare dice, and shop around for hybrid fabs; they're like PCB fabs but fewer and more expensive. Evidently, the price can't betoobad in quantity (plenty of hybrids find use in automotive components), and that should be adequate.Don't know what a minimum viable project would be. May be on the order of $10k's to get engagement and a round or two of prototypes. Ensuring that the equipment is safe with this modification, will be another challenge; if there are standards bodies or laboratories that approve various aspects of the machinery, you can look into that. (Check nameplates on the machine?) (Expect similar or greater budgets for testing.)Tim

author: HCT203
author_rank: Newbie
timestamp: July 23, 2022, 07:49:51 pm
content: You may want to take a look at the Supertex HT0440 and TI SM74611 Smart Bypass Diode, have been attempting to come with a similar SSR for quite a long time without much success , after working with those two parts and well aware of what PowerMosfet ability driven by a Micro and proper software, it is quite doable, not requiring brain science, I think that two thing stand in the way of such a SSR , cost and RIO, the initial small production will be crazy expensive, resulting in a very small market in the beginning, quite sure within a few years the volume will bring the price down to less than mechanical relay, and far greater MTBF, here in USA nobody want to consider a ROI longer a few quarters. VERY SAD .Be interested in keeping in touch since we have similar needs .