How to select a PTCC and TVS combo?  - Page 1
Forum section: electronics / beginners

author: jnz
author_rank: Frequent Contributor
timestamp: October 22, 2018, 11:18:16 pm
content: Automotive 12V circuit, shouldn't be plugged in during transient generating events but could be.My 10uF capacitor on the input to the LDO regulator is 25-35V MLCC because of space issues, output could be MLCC, tant, niobium. Plan was to put in a TVS at 20-24V, and a PTCC/polyfuse for it. LDO is good up to 40V. 220mA is peak worst case usage on the 5V rail. Please ignore the nonsense with optional resistors.So what I don't know...It seems like without a fuse/polyfuse the TVS is useless, at least after more than one big transient event, could I expect a TVS to live on it's own without a fuse for automotive transients?I'm protecting the LDO over 40V and the capacitor from over voltage transients, I haven't found many references for quality automotive circuits, is this a bad plan I have?Not certain on polyfuses or how to pair with the TVS. I THINK I want the TVS to clamp at say 20V, and a polyfuse at maybe 300mA, the idea being the TVS will short at over 20V and the polyfuse will open the path before it burns itself? If that's the case I need a polyfuse that will go to my max expected voltage, is there another important factor I might be missing?Can I do this cheaper/better/smaller? The idea here was small and cheap, and I keep adding parts. Thanks!

author: floobydust
author_rank: Super Contributor
timestamp: October 23, 2018, 01:30:16 am
content: A naked TVS will get killed by typical automotive transients, due to the energy.The only TVS I know of that can withstand (12V) ISO7637-2 including load-dump is the massiveVishay SM8A27 TVS6600W.A smaller, low cost approach is to have some series resistance upstream of the TVS. This could be from a resistor, diode, polyfuse or regular fuse - or any combination of these.I wouldn't use a uni-directional TVS here, as -ve going transients will damage your vreg. A 1kV series-diode works and adds some series resistance help to limit spike current to the TVS.edit: ensure your MLCC capacitors are oversized, as over temperature and with their voltage coefficient they can end up being much smaller and the LDO goes unstable.Also note TVS's have a large temperature coefficient, so a 20V TVS could have high leakage current at 14V in cold weather.

author: jnz
author_rank: Frequent Contributor
timestamp: October 23, 2018, 04:41:38 am
content: I did know that about MLCC, but not about the polyfuse.As it were on the reverse protection diode, my LDO has one built in, but I may switch to one that doesn’t. I guess that’s something else I need to figure out, with just a TVS alone, it would snap itself instantly having only the resistance of the source (.03ohm) so it makes sense to put something in there.I guess more or less I’m not sure about values the TVS and PPTC together should share.

author: Ian.M
author_rank: Super Contributor
timestamp: October 23, 2018, 06:07:16 am
content: If its got to survive a worst case ISO7637-2 unclamped load dump, the PTCC is going to need a fairly high voltage rating - probably one intended for 115V AC use would be suitable. Then you can use its minimum on resistance to estimate the peak clamping current, and hope there's enough data in the form of I2t curves for both, or power vs pulse duration curves for the TVS diode to estimate if the energy let through will exceed the TVS diode pulse rating. Fortunately most modern vehicles will have central load dump protection at the alternator so you can get away with a lower voltage rating PTCC.

author: max_torque
author_rank: Super Contributor
timestamp: October 23, 2018, 11:43:48 am
content: With "surviving automotive" load dump, the key to survival is to 'blink last'What i mean by this is that a modern car has a lot of control modules connected to it's power system, and typically all those modules include a degree of load dump / over voltage protection. And the first module to trigger it's protection is the one that ends up trying to swallow the most energy! If you say have an input voltage architecture that can survive 60v, then you could set your clamping diode to conduct at say 50v, which means any other modules that may start to clamp at say 40v get most of the energy. so, Last to blink, wins!Typical automotive rated linear regulators often withstand around 40v continuously, and perhaps 60v transiently, but you could consider buffering in front of that regulator with a simple transistor first stage, and you can easily install a 100v rated transistor, meaning your clamping circuit doesn't have to blink till say 80v.Alternatively, as most digital electronics runs down on 3.3 or 5v, and your supply is 12v nominal you could drop a few volts across a series input resistor, that acts as protection for overvoltage. You do need to decide on what the lowest voltage you can operate with is (cars need critical modules to run down to 8v (sometimes 6v to protect for cold cranking, ie engine ecu)) but you may let your module brown out gracefully at say 10V. Normally with a running engine and alternator, you'll have around 14v available, much more than the 6v or so your LDO needs to regulate correctly)Regarding PTCs, there physical size is broadly related to the voltage they can interrupt, and of course there power handling. For a device that is an "accessory" and "Non critical" ie is not present in the car all the time, and if it fails does not lead to secondary issues you don't have to make it as robust to full load dumps. Your user manual can just say "device must not be plugged in during jump starting or during battery disconnection etc"As a reference, although operating well outside of the datasheet limits, this 1206 polyfuse:1206L050/15YRsurvives a single full 24v iso load dump pulse (212v peak!!) when followed (in series) by a 33ohm power resistor and clamped by a SM30T33AY TVS diode after that resistor.( a bit of smoke comes out, but it resets ok without any significant increase in resistance) It would almost certainty survive many pulses at the 12v test rating.It's also worth noting that the upstream resistance of the supply to your device very much depends where that device is connected! Usually, low current power feeds have significant resistance and are fused accordingly. A direct connection to the battery or alternator is of course much, much worse!

author: floobydust
author_rank: Super Contributor
timestamp: October 23, 2018, 11:47:53 am
content: A linear Vreg at 220mA will make heat, 2W is a fair bit.I think once you work out how much pcb copper area you need, factoring in little air flow inside an enclosure and high ambient (automotive) temperatures, the (linear) approach will be difficult.A buck-converter is preferred and if very low quiescent current is required, there are parts that switch over to an LDO for low output currents, or do pulse-skipping etc.The reason I mention heat is the polyfuse will be located near the Vreg thus also be hot, so you'd have to spec a high trip-current part to avoid nuisance trips. This means using a low resistance part, so transients stress the TVS more, and it also has to be larger.I find higher voltage (safely under the vreg's spec) TVS did better than using low clamping voltage TVS.Something about the part (low clamping V) being on longer for the transient pulse duration vs peak energy dissipated (high clamping V).Load Dump is extremely hard on vehicle electronics, ECM's and other modules are spec'd to only withstand a handful of those transients. I get product management to remove the load dump requirement because it adds significant size/cost to the product, and it's only for the one (open battery connection) failure mode.Your LDO is spec'd to withstand -ve input voltages, but the enable pin is probably not. Watch for that trap.I rent a transient generator for ISO 7637-2 tests, but you will blow up a lot of parts as it is destructive testing.

author: max_torque
author_rank: Super Contributor
timestamp: October 23, 2018, 12:01:28 pm
content: The following voltage need to be considered, and ideally simulated and tested (even partially) for all automotive powered systems0) Lowest transient voltage (negative) at which the module will be undamaged, but will not operate1) Lowest transient voltage (positive) at which normal operation will be maintained, time limited, set a sensible time limit2) Lowest voltage at which continuous operation is possible (usually brown out limit for LDO)3) highest voltage at which continuous operation is possible (usually thermal limit, due to large vdrop)4) Highest transient voltage through which normal operation will be maintained - time limited5) Highest transient voltage at which the module will be undamaged, but operation will be suspendedmy suggestions, for an "aftermarket" 12v system would be as follows0) -15v fully charged battery connected backwards1) 0v for 100ms if possible as a starting point (as voltage climbs between 1 & 2 time limit increases)2) 8v (unless operation during cranking is necessary, in which case 6v)3) 16v in highest ambient expected (include solar load if in direct sunlight)4) >30V5) >60VFor any device that is "handled" also consider HV ESD, as static shock can be an issue, especially if people can touch pins etc ("big" load dump protection devices may not be fast or low inductive enough to protect against HV ESD!)

author: floobydust
author_rank: Super Contributor
timestamp: October 23, 2018, 12:08:58 pm
content: -15V is a fail, inductive loads switching easily generate -200V transients. It depends on the vehicle.One engineer overlooked this and we had a ton of warranty claims, his product died once outside the lab and installed on real vehicles. It was a disaster.

author: max_torque
author_rank: Super Contributor
timestamp: October 23, 2018, 01:28:18 pm
content: note i listed those voltages for "aftermarket" systems. The protection levels i'd recommend for production or safety critical systems are way different!It's also actually quite unusually to get significant negative excursions on a passenger car because most loads are high side switched and ground referenced (through the body), so whilst individual circuits that are switched do see negative voltages at turn off, the supply doesn't. (note unusual, not impossible, especially for systems powered via low current individual power feeds from the battery, like accessory sockets etc)A lot of aftermarket stuff doesn't include negative protection, often not even for reverse polarity connection, because the warranty work or replacement sales are a nice little earner! I've used plenty of high end automotive test equipment that is completely foobar'd by just being plugged in backwards..........The other fact, often missed is that automotive load dumps are simulated with a system "battery" which is worst case, In reality the esr and capacitance of a large lead acid battery make real in-vehicle load dumps a loss less severe in my experience.

author: jnz
author_rank: Frequent Contributor
timestamp: October 23, 2018, 04:26:06 pm
content: Quote from: Ian.M on October 23, 2018, 06:07:16 amThis is exactly what I'm planning on, as it's new vehicles only. Do you have any estimation or reference to what a modern vehicle would need for specs / what kind of load dumping numbers I could expect from/at the alternator?

author: jnz
author_rank: Frequent Contributor
timestamp: October 23, 2018, 06:16:02 pm
content: Excellent information everyone! Thanks!Turns out a physical fuse will work better here than a PTCC. But it's the same / similar problem.I'm still not entirely clear how to select the TVS.My typical drain is 40mA for the whole module with a worst case of 220mA. Basically some wiring has to be shorted to get that to happen. The fuse at 10A is going to be approx 5.8mOhm. At 20A blows between .15S and 5S, at 60A blows between .03S and .1S. But the fuse datasheet doesn't say what voltage that is at, just that it's "rated" for 58V which is absurdly low for a generic automotive 'mini' fuse.So I'm trying to figure out what kind of load the TVS would see. I should probably consider that it practically shorts to ground so that's the max voltage spike I'd expect and 5.8mOhm of source resistance right? Which is like all the amps the battery can source and thus the fuse should blow instantly? I guess I'm not sure if there is a scenario the TVS could overheat because the fuse is taking too long?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2018, 06:42:22 pm
content: Two things:1. There are polyfuse+TVS combos on the market. Though I don't know how common they are, if you can find stock anywhere. Obviously(?), the TVS heats the polyfuse gunk, making it trip so much faster than, say, placing them side by side. This is a good application to consider them for... there's just that matter of availability. Worth looking for, at least.2. TVSs don't "turn on", they clamp. Unless you intend it to fail shorted, in which case, you're replacing components anyway, so I guess it doesn't really matter how many things die in the process, as long as it doesn't start a fire. Why bother with a TVS at all, y'know?......Unless you get a thyristor (SIDAC or other names) type device, which clamps on until current stops. These are common for telecom circuits, where the idle current is low, but, they're not recommended anywhere the current is large and maintained (e.g., DC circuits, mains AC circuits*..). Doing that with a polyfuse might be okay, but you need adequate I^2t rating on the device to clear the fuse (polyfuses have much more let-through energy than fast-acting conventional fuses do!), and adequate "open" current through the fuse to keep the clamp active until power can be cycled to reset it. (If it's too low, it may oscillate during the transient, which is probably less good.)*Similarly, GDTs latch on; they are used for induced lightning protection in telecom and mains applications. For mains, you put a MOV in series with the GDT to ground, so the GDT keeps ground leakage low, while the MOV ensures a minimum voltage drop which allows the GDT to extinguish once the transient is over. (You don't use a GDT for line-to-line; a MOV alone is better there. You do use it for line-to-ground, because a MOV won't have good ground leakage, and the increased peak surge voltage is acceptable for most applications, that expect higher line-to-ground voltages than line-to-line voltages.)Tim

author: floobydust
author_rank: Super Contributor
timestamp: October 23, 2018, 10:39:30 pm
content: A 10-20A fuse for a 0.22A load is far off, the wiring or PCB traces or TVS will melt and burn first taking the car with it. A 1-2A fuse is safer. I use in-line mini-blade fuse holders.Littelfuse Mini 2Apops 3.5A in 1 second. The fuse will only blow when a hard failure occurs, such as a shorted TVS.For 12V automotive systems, a TVS with standoff voltage of 24V-27V is typical.If you look at those TVS datasheets, a small SMC-sized 3000W TVS requires a series resistance of several ohms to lower the(load dump) surge current the TVS tries to clamp. Otherwise a small TVS will fail shorted with load-dump.SM30TYneeds up to 4 ohms.If you forgo withstanding load dump, or have the high series resistance, a smaller SMB-SMC TVS is fine.Newer alternators have "avalanche diodes" or "avalanche rectifiers" as zeners to limit load-dump transients. It depends on the car maker, but 24V or 40V or nothing is used for limiting.edit: added Littelfuse graph

author: jnz
author_rank: Frequent Contributor
timestamp: October 23, 2018, 10:42:26 pm
content: Quote from: T3sl4co1l on October 23, 2018, 06:42:22 pmTim,1. I'll look, but now that a 10A physical fuse is in play (albeit a slow fuse), that's a lot better. I need fast, small, and cheap here. Special unicorn part doesn't appeal too much to me.2. Yea, IDK. I'm just trying to keep the 60-100V spikes from seeing my capacitors and LDO. That's all. I figure if there is an event of 60+ V and the TVS clamps, it'll blow the 10A fuse pretty quickly as correct me if I'm wrong, but that would basically be SpikeV - ClampV / source resistance right? Which is a lot and should blow the fuse quickly, maybe with in the 600W surge/spike my TVS says it can handle. I'm almost out of board space, so a large series resistor on the input might fit, but it's getting down there in terms of space. I'm open to recommendations!

author: jnz
author_rank: Frequent Contributor
timestamp: October 23, 2018, 11:15:20 pm
content: Quote from: floobydust on October 23, 2018, 10:39:30 pmYea, I had considered the same thing about the traces at 10A even briefly, although this is on the input to the board, so very small wide traces.I wasn't aware TVS really called out for series resistance. That's my mistake, I'll read on that next. Makes sense, although I'm running out of room for things like that. Plus I'll need to see how 4ohms or so will effect my LDO's input. The goal here was to run this thing from 12V to 3.3V where the micro will still run albeit in a handicapped mode from it's normal 5V. I can take a few volts loss, but actually not sure how to figure that out yet.Ugh... This seems a lot harder than it should be. I just want to add some quick protection for my caps because I can't get higher rated ones that physically fit in the enclosure!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2018, 01:32:06 am
content: Quote from: jnz on October 23, 2018, 10:42:26 pmHeh...Once again: the semiconductor protects the fuse, not the other way around.Load dump can deliverhundreds of joules, mainly thanks to the lengthy duration, 100s of ms. This is a time scale where heatsinking is relevant! Or at least partially so.That "600W" diode is only rated for that, on a 1ms surge. It's rated about 1W continuous (depending on what's attached to the leads for heatsinking). Somewhere between 1ms and infinity s, the rating is less than 600W, but more than 1W. Unfortunately it's not obvious where that is...If we assume a square pulse, 600W * 1ms = 0.6J, quite a respectable amount of energy for a semiconductor to dissipate; but again, we expect the energy capability to be higher, thanks to the long duration giving more time for heat to spread out. (The continuous rating implies infinite joules at infinite time. Somewhere between, ... you know.)All this to say: if you have the thermal resistance curves for the device, with heatsink (if applicable), you can calculate the power rating of, well, a square pulse at least, if not an arbitrary curve necessarily. (Load dump is done with an exponential curve, of which you're clamping a roughly square-sided segment, but the power still varies a lot over that segment; so it's not obvious how to calculate the square-wave equivalent, except with the help of calculus.)So, given that I can't offer any conclusive calculation or data supporting this claim: I can safely say a 600W diode will blow up in a couple of milliseconds, let alone a hundred or more. And not just that it will fail shorted in that time, but I wouldn't be surprised if it melts open during the transient, too!There's really only three ways of dealing with this:1. Brute force. Use abig fuckingabsorber. Even MOVs are bad at this, actually, because low-voltage types are thin -- there just isn't much material to absorb a lot of energy -- besides having quite sloppy clamping voltage, especially in the lower voltage ratings. TVS diodes are great, but you needbigones to do it, like $20+ worth to do it.So, this is really only a feasible method when the load is already very large, so that other methods aren't really applicable (or the load is mission-critical such that it can't be switched off, even under such intensive conditions), and the expense of this method is therefore tolerable.2. Shut off. LT makes transient protector controllers, or if you don't want to pay for LT, there are a few discrete approaches (e.g.,SNVA681A). Downside: you need a big enough transistor to handle load current plus peak input voltage, making this annoying for heavy loads. Also keep in mind the tradeoff for different circuit designs: P-ch MOSFETs have 2.5 times worse performance than N, making an N-ch based circuit that much more attractive for high power loads (thus justifying the added complexity of the charge-pump or bootstrap gate drive thus required).3. Ride through. Get an even higher voltage LDO, or use a high voltage HDO with a bypass transistor somehow to save the operating voltage drop (maybe like #2?), or anything equivalent. This is perfect for light loads, where the dissipated power during the transient is quite manageable. A DPAK transistor will easily handle a 60V-peak load dump with a load up to, say, 0.15A or so. Don't use P-ch MOSFETs: you're basically reinventing a drain-output LDO in that case, and you'll have no end of problems getting compensation right. N-ch is seemingly harder to use, but don't overlook the possibility of depletion-mode MOSFETs, which are still modestly available today, and for just these sorts of reasons! They're also great for generally limiting current flow, and have good switching performance, too (though most controllers won't be suited to their peculiar gate voltage range, of course).3a. You can get the best of both worlds, by switching the input on and off rapidly enough that it averages out. *Cough*, in other words, just another buck converter. Well, as usual, it doesn't need to be pretty -- it doesn't need to meet EMI requirements, it only needs to be quiet enough that it doesn't trip itself up. It doesn't need to regulate load voltage or current very well, nor deliver low ripple voltage. A crude hysteretic controller will suffice. The filter inductor can be cheap and crappy, maybe a powdered iron toroid or something. Often you'll have one of these in the circuit anyway, for EMI purposes (whether that's primarily against EMI inside or outside the device!). Switching doesn't even need to be sharp, as it's not doing continuous duty.Basically, by switching at modest (say >80%) efficiency, you -- obviously enough -- pentuple (or more) the power capacity of the switch. Instead of a DPAK, you can use a SOT-89, or even less. Or that same DPAK can protect a load of 1A+ instead of less than 0.2A.That's what I did here:https://www.seventransistorlabs.com/Images/SwitchingCurrentLimitUnits.jpghttps://www.seventransistorlabs.com/Images/SwitchingCurrentLimitUnits2.jpgA pair of D2PAK transistors handles a +/-30V, 20A (i.e., up to 600W) load, for 150ms. In this case, excess power is dumped into a couple TVSs when a return path is not available. The three SMDJ (3kW) series TVSs excel at this, absorbing about a dozen events before tripping the onboard thermal limit, which cools down and is ready to go again after about a minute.Tim

author: kellogs
author_rank: Regular Contributor
timestamp: October 07, 2022, 10:21:46 am
content: Quote from: floobydust on October 23, 2018, 11:47:53 amWhat would you say to two series 20V clamps of the same 6.6kW series instead of one 40V clamp ?What would you say to two parallel 25- 30V clamps of the same 6.6kW series instead of one 40V clamp ?