Load Dump Circuit : Review/Improvements Suggestions - Page 1
Forum section: electronics / projects

author: Saboteur94
author_rank: Contributor
timestamp: August 08, 2021, 09:44:36 am
content: Hi Guys,The following input protection circuit(Attached) forms part of an automotive product as such it meant to provide load dump protection. The product is meant to work on vehicles that operate using 12-36V batteries.I would like some feedback to identify any glaring flaws in the circuit and any improvements thereof.Any assistance in this regard would be greatly appreciated.Kind Regards,

author: jonpaul
author_rank: Super Contributor
timestamp: August 08, 2021, 01:18:20 pm
content: Hello faced this exact issue long ago, a transient absorbing diode can work.We used the Motorola MR2525 diode, specifically designed for alt load dump protection, a very large die transient protective Zener, specified for that use.This was circi 1986-1989, modern equivalents existhttps://datasheet.datasheetarchive.com/originals/distributors/Datasheets-3/DSA-44077.pdfBon ChanceJon

author: Saboteur94
author_rank: Contributor
timestamp: August 09, 2021, 08:23:29 am
content: Hi Jon Paul,Thanks for the feedback much appreciated.However,would it be possible to view the attached circuit and see if there's anything thats glaring at you that may lead to failure.Kind Regards,

author: jonpaul
author_rank: Super Contributor
timestamp: August 09, 2021, 02:40:35 pm
content: Saboteur94:As an Engineer I always seek the simplest solution. Here it is the properly rated TAZ.To evaluate your circuit I would need the detailed BOM, parts spec sheets, and other info.I am incredibly jammed on many urgent projects, this needs some hours.the 50 V caps are underrated, alll parts must run industrial tem -25....+85C and the TAZ is underrated.LDT = 75V 400 ms specific wave.https://www.researchgate.net/figure/Simulated-alternator-voltage-and-load-current-during-load-dump_fig6_251920846https://electronics.stackexchange.com/questions/130368/simulating-a-load-dump-circuitVery simple test fixtures exist for alternator/gen load dump.https://www.jstor.org/stable/44473230Strongly suggest to research the problem and long known solutions , redesign, Build , make a simulator (cap/relay/RLC) and test it out.Bon Chance,Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2021, 06:25:54 pm
content: What load? Shall I assume 3A, given the S3M there? Or 2A given the ferrite bead?- C22-C24 are more or less superfluous; the TVS has some capacitance by itself, but more importantly the FB will be well saturated anywhere near rated current. (How much is not clear; Murata does not publish DC bias characteristics. A comparable part from Laird saturates at ~100mA.*) The effect is the caps are effectively shorted together, so you have as much effect by using one bulk cap. (Parts savings, yay!)*They give a family of curves, 500mA apart; the 500mA curve is already well below (~12dB) the 0mA curve, so it's not clear where in the low-100s mA range saturation really is. Based on similar parts, it's safe to assume it's rolling off as low as 50mA, and well saturated above 200 or 300mA. Judging "saturation" in terms of impedance change under bias, e.g. Z(100MHz, 500mA) / Z(100MHz, 0mA).- PTC1, isn't. It's not a PTC, the listed part is a conventional fuse. With low I^2t (0.1 @ 50A, goddamn, that's only 40µs!), expect it to open while shunting load dump.On the upside, this protects the TVS, and downstream stuff. Likely the TVS survives, saving cost on the 5.0SMDJ size part, versus a proper beefy one. Downside, the board fails regardless: the fuse is not user serviceable and the only viable maintenance option is whole board replacement. (The board could be easily reworked and refurbished, but whether it's worthwhile to do so is another matter. Or a cartridge fuse could be chosen, even an SMD "brick" style one -- there are holders for them -- but that adds cost, and maybe still isn't feasible due to environmental reasons e.g. conformal coating, potting, location etc.)If this meets your failure criteria (fails, safely), then that's fine.- TVS1 isn't rated for load dump. At a 36V rating, it will dissipate a huge amount of power during a surge. This isn't a problem per se because of the previous note, but if normal operation is expected following the surge, a slow-blow fuse will be required, and either a much higher voltage TVS, or a much larger one, will be required.- Why is L3 between Q2-E and Q1-S?- SQ2361ES is rated barely over 2A, and it's a pretty optimistic rating at that. At Rds(on,max,175C) = 0.32 ohms, and say a more reasonable Pd = 400mW, is a maximum of merely 1.1A.Note that this circuit, by itself, provides no current limiting, or inrush or anything (just a modest slew rate, afforded by C95+R99), so if the load has much more capacitance, or is prone to momentary higher currents, or faults, this transistor is toast (and burns out well before the fuse, mind).The overvoltage function should work more or less as intended. I recommend some hysteresis, which can be implemented by feeding back some VIN to Q2-B.D8 can be smaller, anything 5-10V will suffice. Over 12V is excessive, not that it really does anything, but 15V is kinda close to the 20V rating.I would recommend some resistance in the loop between D8 and Q1-G. Say 100 in series with Q1-G. This loop can form a tuned-gate or Colpitts, or something like that, oscillator, which activates in the linear region (Vgs < Vgs(th), Vds < Id * Rds(on)). The resonant tank thus formed can have an Fo ~ 100s MHz (e.g., 20nH from component and trace inductances, ~50pF from zener junction --> ~160MHz is in the right ballpark). The gate "stopper" resistance merely quashes this mode, keeping it from squealing.- Note that, if L3 is for reducing dI/dt (which might help with inrush, though it's not much so YMMV), it should probably be placed in series with Q1-D, and a reaction diode provided, since this starts to look very much like a buck converter (i.e. a diode from GND to Q1-D, maybe another S3M -- it seems unlikely this will cycle very often at all, so the slow recovery is fine).If for filtering, it's better used as L2 instead, and then the two bulk caps will be relevant (indeed, choose their values so that the turn-off peak voltage is within ratings, avoiding the need for a second TVS).So with that out of the way -- what do you really want from this circuit?You didn't give an operating criteria, nor the expected load dump conditions. I'm not familiar with 36V systems, but I'm guessing it'll be something like, 2 ohm and 150V peak?As for operating criteria:1. Fails, safe.The fuse and TVS will suffice. Filtering is whatever is needed by the load. The polarity diode is whatever; probably worth. You may consider a fast-recovery diode, to help out with fast (switching) transients. No need for disable circuit, it's crowbarred anyway.Speaking of crowbar, a SIDAC type TVS might be a cheaper option? Or a smaller zener TVS like a SMAJ36A, relying on its typically fail-shorted behavior to help blow the fuse. Or none at all, let it fuck up the load; the fuse is there, all the same.2. Must survive; disabled during surge.TVS only needed up to the overvoltage switch's rating. If a relatively high rating is acceptable, an MOV can even be used here, or perhaps even nothing at all.This isn't so great with a PMOS, as the performance is so terrible (~2.5x worse than NMOS, i.e. higher capacitance, larger die, higher cost for the same V, I ratings). At 2A it's not such a big deal, you could put in a, say, D2PAK, maybe even DPAK, rated say 200V 5A+, and maybe not break the bank. 200V is enough that a MOV might even suffice, but if nothing else a 120-150V TVS should keep it safe. (Note that, if your surge voltage is much beyond the TVS rating, it's still likely to burn up.)Alternate is use NMOS, which needs either a low side switch (not an option in a device which shares ground connections elsewhere) or a bootstrap supply (typically supplied by a charge pump). This is a bit of a pain, either in complexity (needs an oscillator, cap, diodes, etc.) or in cost (e.g. using one of those LT surge protection ICs -- charge pump, comparator and gate driver all in one, nice but $$).Slight variation, depletion NMOS. With gate pulled up to drain, it's "always on" (Rds(on) is specified at Vgs=0). Gate has to be pulled all the way to GND, actually a bit negative to fully disable the load, because it's a source follower configuration, and Vgs(th) is negative; but this is feasible with the mirror of a circuit like yours, for example.3. Ride through.If the load is light, or it must remain on, it's reasonable to simply regulate the surge away. Simply tack on an LDO -- it can be a crude one -- and you're set. An excellent example is using a depletion NMOS source follower, with the gate biased with a pullup to POWER and a zener from GND. Vg is limited to Vz, and Vs is at most a couple volts above that. 200V ~1A types are reasonably affordable, and give a huge input range for very little complexity. (Using an actual as-such LDO, good luck finding one with the voltage range -- but if you do, and it meets your requirements, there you go!)If the load is heavy, it's less reasonable to use a depletion NMOS. They aren't available in a huge range of sizes, and the power dissipation stacks up quickly. Nor are LDOs available in large ratings (though there are a few oddball regulatorcontrollerchips, that can be used with external transistors for much higher ratings), for which otherwise you're basically stuck making an old fashioned linear power supply, with a bootstrap (charge pump or switching converter) pullup rail so it can have low dropout in normal operation.As load goes up, at some point, you'd end up dissipating more power in dropping the surge, than you would in simply shunting it.In fact, that crossover point is given by the surge's series resistance:Say the surge is 2 ohm and 150V peak, and the load is 50V max.If the load is 10A continuous, then the peak drops to 150 - 2*10 = 130V under load, and shunting the difference (80V) will draw an additional 40A, dissipating 2kW; dropping the difference will draw only the 10A and dissipate 800W.If the load is 30A instead, then the peak is 90V, a shunt would dissipate 1kW and a dropper would dissipate 1.2kW.If the load is 50A, then the peak drops to 150 - 2*50 = 50V under load by itself, and theoretically, nothing is needed!Of course, this assumes the load is continuous; any current the loadisn'tdrawing at time of surge, is current the shuntmusttake on, or additional peak voltage the dropper must tolerate.For many cases, the load isn't always at full current, or might be near zero much of the time, so keep that in mind, too. The default / easy solution is to just shunt it, with a huge ass zener (TVS), and grumble about the cost.(There is a third option, for completeness' sake -- use a switching converter, not a linear dropper. Basically, this is extending the input range of whatever converters you're already using. The trouble is, going from say 50V to 5V is already quite a low ratio, and going from 200V would be ridiculous; it can be worthwhile using two stages in cascade like this. As for the other pass cases, of course, the switch needs to handle the full load current, and do it quickly so the switching can be reasonably efficient. Though as it should only be active for a brief time, it can be on the slow side, 100s of ns say, which greatly eases driving and layout issues. Also the inductor can be lossy e.g. powdered iron.)Tim

author: jonpaul
author_rank: Super Contributor
timestamp: August 09, 2021, 07:18:22 pm
content: We used the MR2525 large area TAZdiode in 1980s on manufacture of 7000 PSUs used in flight hardware, 28V avionic bus, to met FAA DO-160.We Never had a failure.Jon

author: Niklas
author_rank: Frequent Contributor
timestamp: August 10, 2021, 10:07:05 am
content: When load dump is mentioned, I assume the application is automotive. There are two cases listed for Pulse 5 in ISO 16750-2 and ISO 7637, clamped and unclamped, with either 65 V or 202 V as peak voltage. Based on the component ratings, I would guess that it is the clamped pulse with 65 V peak that applies. If you need 202 V, then the proposed input circuitry will fail.With 24 V battery voltage, have you considered the jump start condition? Typically 48 V for 1 minute from a low impedance source. That could destroy the TVS.The wrong polarity protection diode is too slow if you want to pass the other automotive pulses, for instance pulse 1 with -600V and 50 ohm series resistance. Go for a faster alternative, for instance RS3*.Also check the response at power on and off when the battery voltage is unstable if supplied through the key ignition switch (KL15).

author: jonpaul
author_rank: Super Contributor
timestamp: August 10, 2021, 10:45:00 am
content: Rebonjour a tous:Your notes re cost of the diodes: There are several, in the series. 2525, 2520, 2535.Typ specs: 23V 600 amp surge 25 A fwd currentWe found MR2525 and MR2535 Load dump protect diodes at ON Semi, and American Semi.Also MR 2520 at Diotek.http://www.datasheet.hk/view_download.php?id=1307085&file=0168%5Cdr2535l_4143893.pdfEbay has many listing for ~ $1..$6 each.This standard solution is much Cheaper, smaller PCBG area and more reliable that the complicated circuit being proposed and analyzed.Hopefully this solves the OP questions.Bon Chance,Jon

author: Niklas
author_rank: Frequent Contributor
timestamp: August 10, 2021, 07:42:49 pm
content: The MR**** type of TVS seems to be something of the past, with no new production. The reverse withstand voltage is also too low to handle 28V without drawing current, especially if connected directly to the battery with requirements for low sleep current.I don't have access to the DO-160 standard, but it seems that the transients in section 17 are much shorter (us instead of ms) and with higher series resistance (50 ohms instead of 1-2 ohms) compared to the automotive load dump.The TVS listed in the schematics matches several parts that are recommended for clamped load dump protection, so that should be fine both for technical specs and multiple source. The 36V stated in the part number is the reverse withstand voltage and the clamping voltage is just below 60 V.There might be cases where the combination of TVS and a MOSFET are necessary, for instance to get a better defined cutoff voltage to protect a voltage regulator. There are many automotive Buck converters that are rated max 36 V in, with possibility to withstand 40 V transient. The TVS alone is not able to fulfil the requirements of both jump start and the max input to the regulator.

author: jonpaul
author_rank: Super Contributor
timestamp: August 10, 2021, 08:48:41 pm
content: Hello again sorry forgot , the Avionic 28V used 2X MR2525 in series. For a 12V application its fine.Check the site of ProTek Devices for app notes, selection guides and similar parts, very wide range.http://www.protekdevices.com/xyz/documents/kb/tech/gl1123.pdfEnjoy,Jon

author: floobydust
author_rank: Super Contributor
timestamp: August 10, 2021, 09:20:44 pm
content: I have successfully used VishaySM8A27TVS for automotive (load dump), one for 12V and two in series for 24V systems. I know of car makers using the part as well, it's commonly used.Any load dump solution that put the mosfet into the linear region for "ride through" does not work well.The SOA requirements are huge and so the physical size/cost are high. Easier to just shut it off until voltage comes down.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2021, 01:17:45 am
content: Quote from: floobydust on August 10, 2021, 09:20:44 pmYeah, that's the TVS that came to mind. It's the modern SMT equivalent of the old MR mentioned.Yes, not to oversimplify, linear ride-thru is a whole design process -- fail the SOA and the thing shorts out and blows up all the same, so do your due diligence!In my experience, a typical D2PAK is adequate to ride through say, 30V * 10A * 7ms; make of that what you will. (Older e.g. 50N06 are better than newer e.g. PSMN4R6-60PS, I think?) For a pulse of 60V x 100ms, you might get up to an ampere this way; more will need bigger and more devices. So a small control module with under 100mA load, will use a DPAK depletion MOS just fine like this, but a 10A load, consider pushing back on the always-on requirement.Tim