Non-Zero Crossing AC SSR: Use with DC? - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: October 20, 2019, 02:28:43 am
content: Hi all,I'm looking for a relay to use in an SMPS inrush tester I am designing. After pushing a button (or driving the input with a square wave signal), the circuit fires the relay at thepeakof the next AC cycle. The circuit uses a zero-crossing detector and then applies a delay to turn the relay on exactly at the peak of the AC wave.I was planning on using an solid state relay (SSR) because I think it should have cleaner turn on/off characteristics. It's important that there isn't a crazy amount of ringing on the measurement.Many SSRs are zero-crossing-switching, meaning they will only turn on or off at a zero-crossing. But, many manufacturers will also offer an instantaneous-switching version. Here is one I am considering:http://www.crydom.com/en/products/pcb-mount/ac-output/pf-series/Now, if I want to use this same tester to switch DC, is it possible? From my research, most people say AC SSRs cannot be used with DC because there is no zero-crossing. However, if I use an instantaneous-switching SSR, is this still an issue? Any other issues?Thanks!TimP.S. Current schematic attached, for reference.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 20, 2019, 09:56:40 am
content: Quote from: TimNJ on October 20, 2019, 02:28:43 amIf the SSR uses a triac or back-to-back SCRs you can't use it on DC unless you are fine with cycling the incoming power to turn it off (or load current drops below the holding current of the thyristor(s)). If the SSR uses MOSFETs for the output switch then you can use it on AC or DC (note that for AC there needs to be two MOSFETs wired in inverse series - that is, source to source).

author: TimNJ
author_rank: Super Contributor
timestamp: October 20, 2019, 02:04:02 pm
content: Quote from: MagicSmoker on October 20, 2019, 09:56:40 amAh. Thanks. I knew they had back-to-back SCRs but I don't really understand SCRs very much, so that limitation didn't really make sense to me.On that note, another thing I noticed is the relatively high minimum/holding current rating for these devices. For the Crydom PF, it says 60mA minimum. I'd like to make sure the relay stays on even if the SMPS is in a light load condition. To satisfy 60mA @ 90VAC, need about a 1.5K resistor. But, since I'd like this to also be compatible with high AC line voltage (264V), then the dissipation of the 1.5K resistor @ 264V will be about 50W. Yikes!It's only a test fixture, so maybe it's okay, but seems a bit crazy.Thanks again.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 20, 2019, 02:24:26 pm
content: Quote from: TimNJ on October 20, 2019, 02:04:02 pmThere's a couple of solutions to the minimum holding current issue that don't require wasting any heat in an external minimum load resistance. The first is to wire a decent quality film capacitor rated for use across the mains in parallel with the SMPS-under-test; 2.2uF should do the trick. The second is to simply provide a continuous trigger signal to the SSR for the entire time you need the load to be on.EDIT - added the wink at the end.

author: TimNJ
author_rank: Super Contributor
timestamp: October 20, 2019, 03:16:48 pm
content: Oh...well that's interesting. Do I not understand how an SSRs is typically triggered? My plan was to provide continual 5V or 12V to the "coil" terminals just like a normal non-latching relay.Regarding the capacitor, I thought of that, but thought I wouldn't want it because it would throw off the inrush measurement. However, I realize now that you can avoid measurement of the X-cap charging by simply measuring the current on the output of the tester, instead of on the input side.Thanks once again.

author: TimNJ
author_rank: Super Contributor
timestamp: October 20, 2019, 04:23:05 pm
content: Some more discoveries for anyone playing along at home.After reading a few Wikipedia pages for about 10 minutes, I learned the following:1. An SSR (using SCRs) can be turned on quite easily (and at any point in the AC cycle) by simply applying a gate voltage.2. An SSR (using SCRs) will inherently turn off when the current drops to 0 after the gate signal is removed.This is why the random/instantaneous/non-ZCD SSRs have a turn on spec of about 0.1ms, but a turn off spec of up to 1/2cycle (max). You cannot turn off the SCR at an arbitrary point in the AC line, and more specifically at any point on the current waveform.That's a good thing for me, since I can delete my ZCD shut off circuit.However, I wonder if that will affect whether my 50/60Hz SSR will shut off with a 400Hz source. I guess I'll have to check..Thanks.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 20, 2019, 07:50:38 pm
content: Quote from: TimNJ on October 20, 2019, 03:16:48 pmNo, I just forgot to put theat then end of my earlier reply because you normally provide the trigger signal for as long as you the load to be on with an SSR.As for operation at 400 Hz, you may want to check with the manufacturer because SCRs - but especially triacs - are not known for being especially fast. You also need to be very careful to not exceed the max dI/dt spec during turn on, and this may already be an issue given the type of test you are trying to perform here (ie - inrush measurement).

author: TimNJ
author_rank: Super Contributor
timestamp: October 21, 2019, 03:39:02 pm
content: Thanks again.Honestly, I think the joke went right over my head. (I think I still don't quite get what you are saying.) Does holding the trigger help to keep the SCRs on or not?Regarding 400Hz operation, I understand your points.The I2t rating for the Crydom PF is 260A2/sec, rated at 8.3msec. Sounds like that will be fine. I2t = 1/2 *(Ip)2*t, where Ipis the peak current and t is the width of the half sine pulse.Based on the 260A2/sec rating, I believe that means the peak current can be 245A at 60Hz. That said, there are typically a few subsequent half-sine pulse, but I guess we can (sort of) disregard those. Factor in a little extra margin to account for them. The switch mode power supplies I deal with usually have inrush currents <100A.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 21, 2019, 04:07:08 pm
content: Quote from: TimNJ on October 21, 2019, 03:39:02 pmFor the purpose of this discussion, if the supply is AC then then a triac or back-to-back wired SCRs will turn off every zero crossing unless continuously triggered. If continuously triggered then there will still be a brief period of non-conduction around every zero crossing (so some distortion of the current waveform) when the voltage is insufficient to overcome the forward drop (typically given as Vtmin the datasheet).Quote from: TimNJ on October 21, 2019, 03:39:02 pmNope, that is the fusing energy rating - basically, if you want a fuse to protect the triac/scr from overload it needs to have a lower I2t rating than the triac. The spec you are interested in is the max dI/dt rating... which apparently isn't given in that datasheet, I see. So, again, better confirm with the manufacturer. FWIW, back-to-back SCRs are superior to triacs when it comes to dI/dt, dV/dt and I2t at the cost of being more difficult to trigger (there isn't a common reference for the trigger signal to each SCR).

author: TimNJ
author_rank: Super Contributor
timestamp: October 21, 2019, 05:23:50 pm
content: Quote from: MagicSmoker on October 21, 2019, 04:07:08 pmI see. I thought "fusing" energy, in this context, was the point at which the SSR "fuses" (aka fails), kind of like how fusible resistors are specified. Maybe that's still true, but I see that the fusing energy is still separate idea from the dI/dt rating. Let's see if Crydom will talk to me.Thanks for your help.

author: james_s
author_rank: Super Contributor
timestamp: October 21, 2019, 07:59:58 pm
content: The zero crossing switching offered by some SSRs refers to the turn-on point, a zero cross SSR will always turn on at the zero cross. All AC SSRs will turn off at the zero cross, this is part of the standard behavior of a thyristor. Once triggered they latch on until the current through them falls below a minimum threshold.

author: TimNJ
author_rank: Super Contributor
timestamp: October 22, 2019, 02:43:29 am
content: Quote from: james_s on October 21, 2019, 07:59:58 pmThanks. Yes, I just learned (re-learned?) that. Never worked on a project with SCRs before so didn't have that engrained in my memory yet!I think I'm getting the itch to feature-creep in a DIY MOSFET based solid state relay. Seems like a MOSFET based relay would be superior in this application. Plus it'll work with DC.Why are the high power MOSFET based SSR options so limited? Is there something I'm missing? Why are SCR based relays still king? Inherent zero current switching?

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2019, 03:28:29 am
content: SCRs (and triacs) are inexpensive, robust, easy to trigger, and they have a low forward drop so the dissipation is reasonable.SSR's are typically used to control AC line powered loads from low voltage DC control systems. When one needs to control a largish DC load it's usually easier to just use low side switching with a single transistor or use a mechanical relay. The advantages offered by a SSR aren't really needed.

author: TimNJ
author_rank: Super Contributor
timestamp: October 22, 2019, 04:22:16 am
content: Quote from: james_s on October 22, 2019, 03:28:29 amThanks. I see that the price of 2 MOSFETs with comparable combined voltage drop (at rated current) is about $10-15USD, whereas the cost of the Crydom PF module is about $30. Crydom part is obviously very thoroughly tested and easy to use, without much fuss. Need to add in isolated DC-DC converter, optocoupler, etc. for MOSFET version, so in the end, it seems like the price might come out about the same.I can see how the latching action of an SCR could be advantageous in some applications.In terms of robustness, are you just saying they are "generally" robust? It seems like MOSFETs are also pretty robust these days.

author: Circlotron
author_rank: Super Contributor
timestamp: October 22, 2019, 04:25:23 am
content: Quote from: TimNJ on October 20, 2019, 02:04:02 pmYou could always put a normal mechanical relay across the SSR and trigger them both at the same instant. The SSR will turn on almost immediately and the mechanical relay with its contacts across the SSR will follow several milliseconds later.

author: beduino
author_rank: Regular Contributor
timestamp: October 22, 2019, 07:45:06 am
content: Quote from: TimNJ on October 22, 2019, 02:43:29 amA few AC mosftes switches in parallel should do the job depending on how low resistance needed and voltage levels

author: Zero999
author_rank: Super Contributor
timestamp: October 22, 2019, 12:15:15 pm
content: Quote from: TimNJ on October 20, 2019, 02:28:43 amWhy not design a separate tester for DC? That would be much easier, rather than something which will need to work with both AC and DC.If it doesn't need to turn off, a simple SCR will do. Once triggered it will turn on and not switch off again, until the power is interrupted.It does need to turn off, a MOSFET can be used.Isolation can be provided with an opto-TRIAC for the SCR and an opto-photovoltaic cell for the MOSFET.

author: TimNJ
author_rank: Super Contributor
timestamp: October 22, 2019, 01:30:14 pm
content: Quote from: Zero999 on October 22, 2019, 12:15:15 pmI agree with you that often times it's much better to make a single purpose device instead of making something that "does it all". However, if it's not too much extra work, it might be less work to tack on DC function to the AC tester. However, as I think about it more, I'll probably wind up making two separate testers because I might want to put some chunky caps on the DC input to get the source impedance down as low as possible.I think I'll try designing a MOSFET SSR first (as a standalone module) and then integrate that into the full tester. There are some cool opto-PV gate drivers like Vishay's VOM1271 which seems reasonable. I don't really think switching speed is that big of a deal in this case, but they still are pretty slow...I'm thinking a little 1W isolated DC-DC converter + regular opto-coupler might have better performance without being overly complex or expensive.Thanks!

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 22, 2019, 02:16:39 pm
content: Quote from: TimNJ on October 22, 2019, 01:30:14 pmThe last sentence outlines the best approach - go with inverse series connected MOSFETs driven by a proper gate driver and a small isolated dc-dc converter to supply it because you do need decently fast switching speeds given that you are trying to maximize inrush current for testing purposes. You really don't want the MOSFETs taking >1ms to turn on while also trying to conduct a huge peak current.

author: TimNJ
author_rank: Super Contributor
timestamp: October 28, 2019, 01:00:49 am
content: As I'm work on this MOSFET SSR design, I am beginning to understand why SCR based SSRs tend to dominate in the high power segment.For one, zero-crossing switching, a freebie for SCRs, has to beaddedto a MOSFET design. Some preliminary modeling shows that switching off a 1KW load with 1 or 2mH of inductance will produce crazy ringing with 1-2KV peaks! That's not surprising, but still a problem.My best solution so far is a current transformer + comparator which detects when the current is at or near zero. But of course, that makes the relay physically bigger extra and adds a few dollars to the BOM..I'm curious: Is there any "real" application where a MOSFET based (high power) SSR would be advantageous? Any application where you need to control the turn on of something even faster, or with greater precision?Thanks.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 28, 2019, 07:26:07 pm
content: Quote from: TimNJ on October 28, 2019, 01:00:49 amMost MOSFET SSRs switch much more quickly than any thyristor type; the possible exception is when a photovoltaic isolator is used (literally an LED shining on a PV stack that generates a feeble current at enough voltage to turn on the MOSFET... usually over a period of hundreds of microseconds).Insufficiently snubbed thyristor SSRs may fail to turn off at the next zero crossing with very low power factor loads whereas AC/DC MOSFET SSRs retain positive control of turn-off for DC loads, so won't care about the power factor of an AC load.That said, thyristors are far more rugged than MOSFETs for a given die area and nominal current rating. A 20A thyristor will happily switch tungsten lamps on and off all day long which can demand up to 10x nominal current during inrush, whereas a MOSFET will likely need to be much more conservatively spec'ed; perhaps needing a 100A-rated device for the same application.Rarely is there a one-size-fits-all answer, as seems to always be the case in engineering.

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2019, 07:57:11 pm
content: Yes, the first DC power switch semiconductor was the SCR, though there you always had at least 2 of them, some really high value inductors, high power resistors and big capacitors to allow one SCR to be used to force commutate the other. But you could have a 200A SCR controlling a DC motor at around 100Hz PWM rate to allow variable speed control, which was a massive jump in efficiency over the previous method, which involved both multiple tappings off the battery bank and some serious power resistors to give stepped control. SCR you could use a single battery voltage, have control from near 0% to 100% with near stepless control, along with feedback so your motor speed was constant despite massive load variations. High voltage withstand when off, high current when on, and pretty much almost no switching loss in operation until you got into the audible frequency ranges, still the preferred device for ultra high power use, where IGBT devices are only now starting to get serious attention.

author: Circlotron
author_rank: Super Contributor
timestamp: October 28, 2019, 09:02:33 pm
content: Yep, force commutated SCRs are a discipline all their own. About as sophisticated as a steam locomotive, but also just as reliable. Huge overload capacity, as has been mentioned. Lots of interesting resonant circuits involved, and really getting to grips with basic fundamentals. Kinda sad in a way that they are gone from general use.

author: TimNJ
author_rank: Super Contributor
timestamp: October 28, 2019, 10:10:16 pm
content: Quote from: MagicSmoker on October 28, 2019, 07:26:07 pmThanks for all of that information. It's clear I'm lacking in my knowledge of thyristors, and I definitely want to learn more.I guess my question still remains. What might be some applications where you need to turn on a big AC load in the matter of microseconds? It seems like most (industrial?) loads of that nature will probably be fine to turn on within a few 10s of milliseconds.I'm just confused because before itseemedlike people were suggesting that an SCR based solution (for the inrush tester) would NOT be as robust as a MOSFET solution. But maybe I misconstrued that idea. MagicSmoker suggested that I might exceed the dI/dt rating of an SCR SSR if used for inrush testing. Are MOSFETs any better at that?To me, it seems like I should just use a beefy SCR at this point. Or maybe even drop the whole DIY SSR concept completley and use an off the shelf SSR as originally planned.

author: Circlotron
author_rank: Super Contributor
timestamp: October 28, 2019, 10:41:33 pm
content: Quote from: TimNJ on October 28, 2019, 10:10:16 pmIt's not so much that the load needs to be turned on in microseconds but the switching device, mosfet, SCR etc needs to fully turn on quickly. When it is transitioning from off to on, depending on the nature of the load, it can be dissipating a LOT of power internally. Best to get through that transition as quick as possible.

author: Circlotron
author_rank: Super Contributor
timestamp: October 28, 2019, 10:49:07 pm
content: Quote from: TimNJ on October 28, 2019, 10:10:16 pmWith an SCR you can simply put a small value inductor in series with the SCR to limit the di/dt. Doesn't need to be much. I have even seen several turns of cable looped in a circle on a big inverter. And there is no problem with inductive kick from the inductor because the SCR turns off at zero current so there is no energy left in the inductor to cause problems. SCRs used correctly are great!https://electricalbaba.com/di-dt-protection-scr-explained/

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 28, 2019, 11:00:13 pm
content: Quote from: TimNJ on October 28, 2019, 10:10:16 pmIf you are doing phase angle control of power to an AC load (think "light dimmer") then you need to be able to control turn-on within a matter of microseconds. Also, don't confuse response time to a control signal with the time it takes to transition states (ie - switching time). Faster switching time is always better... up until it excites resonances that cause destructive ringing, anyway.Quote from: TimNJ on October 28, 2019, 10:10:16 pmThe answer was already given: very low power factor loads are more difficult for thyristors to switch, and intentionally provoking high inrush current into a capacitive input filter definitely qualifies as such (the current greatly leads the voltage). Thyristors naturally turn off when the current drops below the holding current value, or the voltage drops below the forward threshold, but in a low power factor load the current waveform is displaced so far from the voltage that voltage crosses zero when current is still high or vice versa. The thyristor tries to naturally commutate off, but instead just goes through a high dissipation quasi-off state every half-cycle, often ending in its failure within a few tens of cycles.MOSFETs are far more robust in this respect - as long as gate drive is present they will be on regardless of any phase angle displacement between voltage and current - but they have a far smaller overload capability compared to thyristors, and you kinda need both here. However, it is easier to solve the overload capability problem with brute force, hence MOSFETs are the overall better choice.EDIT - agree with Circlotron completely.

author: TimNJ
author_rank: Super Contributor
timestamp: October 29, 2019, 03:40:35 am
content: Thank you to both of you. I think I understand the issue more clearly now. And, as you both suggested, I think my misconception was thinking the issue was related to signal propagation instead of an actual switching delay, and the associated period of high power dissipation. Though, now that I think of it, why would a discrete semiconductor device have a propagation delay on the order of milliseconds??I am sorry if you already answered these questions before; As you can tell, my understanding of thyristors is not fully developed. (I'm pretty good with MOSFETS though!)Now the Crydom spec sheet makes a lot more sense. "Minimum Power Factor (at Full Rated Load) = 0.5". That was a meaningless spec to me before, but understood now.Thanks again. I'll keep plugging along on this MOSFET SSR. So far pretty straightforward, just will need to make sure the ZCS circuit works reliably.