How to clamp transients on LDO outputs - Page 1
Forum section: electronics / projects

author: AndersJ
author_rank: Frequent Contributor
timestamp: January 28, 2021, 01:39:21 am
content: Input overvoltages aren often clamped to power supply rails with diodes.I do this with diodes to 5V, on the input to protect receiving 5V powered schmitt triggers.I use schottky diodes (0,2V) to not rely on the internal protection diodes (0,6V) of the schmitt trigger.Normally, with at least ”some” load on the 5V supply the overvoltages/currents are absorbed by the load.But with low loads, the LDO needs to absorb (sink) the current, which it cannot, since it only sources current.The straightforward solution is a 5V zener diode.These are however not very precise.They either conduct too early or too late.I could add a permanent dummy load to absorb the transient overcurrent.I do however dislike such inelegant brute force power inefficient compromises.How would you design a precise input protection without lifting the 5V rail?

author: rtm
author_rank: Contributor
timestamp: January 28, 2021, 04:11:47 am
content: Possibly a TL431 and two resistors. This can create very accurate “zener” since it is a shunt regulator. Set it just above your 5V rail so that it won’t conduct unless the 5V rises due to a transient. It can shunt up to 100 ma, which may be enough depending on the transients you are seeing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2021, 04:44:02 am
content: Well, how sharp does it really need to be? If the 5V load is all 74HC logic, it's rated for 7V continuous, and who knows what, transient. It'll survive. Use a TVS and it'll be fine.6.5, 5.5V stuff? I wouldn't recommend running it at 5V in the first place, if it can be avoided. It might actually break down around 7, 8V or more, but you'd have to test some samples and see (and presumably it won't perform in spec at those higher voltages, but nevermind that, gotta survive first!).Can also make a more precise transient clamp with a schottky diode and bulk capacitor, or a shunt regulator with some active circuitry. (Active circuitry will respond more slowly, and may draw more leakage current. A transient clamp is a good complement for that, filling in over shorter timescales.) Or if shutdown is acceptable, a crowbar to dump the whole thing if it goes above threshold.As for magnitude, what kind of transients are you dealing with? ESD isn't much charge (~1uC), easily absorbed by a few uF of bypass caps which you probably have in circuit already. If surge (e.g. IEC 61000-4-5), consider isolating the surge current as best you can, either with series resistance (clamp the bulk with a TVS, then a series resistor to interior circuitry -- circuitry is not exposed to direct surge currents), or with literal isolation (a good idea on long lines that are susceptible to surge).Tim

author: AndersJ
author_rank: Frequent Contributor
timestamp: January 28, 2021, 05:14:50 am
content: Quote from: T3sl4co1l on January 28, 2021, 04:44:02 am”All” transients, within reason,ANDalso permanent overloads when 24V is connected to the input.I do have a series resistance to take the voltage drop, but the current still needs to go somewhere.I also have EMI filters for HF transients such as ESD and FTB.Slower transients can be handled by rail capacitance.It is the longer/permanent overvoltages I need to clamp.The logic is 74 and CD4nnn, with Vcc max you mention.I’m still uncomfortable seeing the rails overshooting.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2021, 05:40:28 am
content: I mean... direct lightning is withinsomereason, just depends on the setting... If you're flying in a plane, that might be a direct requirement!So, should I assume direct lightning strike here? Or what?As for sustained overvoltage or current, a series current limiter/foldback might be used, or a crowbar to shunt the current. Crowbars are popular for telecom lines: SIDACs respond relatively slowly (fractional us) but thanks to their low voltage drop, they can divert huge surges in small sizes. Downside, they stay latched under bias, so can't be used on DC power lines. Example:https://www.littelfuse.com/~/media/electronics/datasheets/sidactors/littelfuse_sidactor_fixed_voltage_do_214_datasheet.pdf.pdfFor a series limiter, sooner or later you'll have to clamp the voltage, as it's only going to do its limiting up to some rated voltage. A MOV may be a good choice then.And if you're seeing large surge or fault currents, don't forget to provide ample copper around all the protection components and associated wiring. Don't want that fusing open and flashing arcs everywhere.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: January 28, 2021, 09:49:05 am
content: What is the expected transient exposue and environment eg lab bench, house mains 240V, industrial, railroad? All have different transient energy and voltage.The standard solution is a Transient Absorbing Zener, NOT a plain Zener.https://www.littelfuse.com/~/media/electronics/trainings/littelfuse_diode_technology_comparison_high_power_tvs.pdf.pdfhttps://www.vishay.com/docs/49337/49337.pdfTheir large die is designed for a rated energy.We combine a carefully selected TAZ with LC filter and catch diodes for the highest protection levels.Kind Regards,Jon

author: AndersJ
author_rank: Frequent Contributor
timestamp: January 28, 2021, 12:10:43 pm
content: Lightning strike immunity is probably overkill.This concerns industrial systems.I do not have a specific application in mind.I’m primarily hoping for a general discussion on how to clamp non sinking LDO’s.Especially when I dump overvoltages to the rails through diodes.Perhaps better to have a dedicated 4,8V ”dump rail” built with a TL431or a rail referenced op-amp buffered with a two transistor totempole output, and a cap?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2021, 09:01:37 pm
content: So industrial systems, like control boxes and stuff? Lots of electrical noise, switching transients (EFT, ESD), maybe induced surge (long runs, poor grounding)?TL431 is only rated to 100mA, but can be boosted with a transistor or two. Beware of the increased gain, it may oscillate. Don't forget to add compensation components, and test performance.Heh, this is a situation you might get away with some op-amp abuse: tie the output to +V or GND, and set inputs so it sinks supply current (OUT to GND or +V to OUT, as the case may be) when sensed +V rises above VREF. "Might get away with" = amps might not behave well when shorted to a rail, and without any wiggle room at the output; YMMV. Alleviation includes series resistance (so there's a proportional voltage drop), voltage offset (say a stack of diodes or a small zener, so the output is biased in the middle -- maybe relevant for non-RRO types), or using a transistor to convert output voltage into load current. Note that this is only able to draw limited current.But that's basically what the TL431 is, an op-amp with a suspiciously stable input offset voltage and a single combined supply and output pin. So it's a good start.And again, concerning the tradeoff between sharpness and robustness -- you could do a zener into a BJT or MOSFET, which can be alright precision (maybe within 5% for the zener, plus a Vbe or Vgs(th)), and which can sink much more current (10s of amperes, even 100s, though I don't know that I'd go that far with it). That might be preferable in some cases over a plain TVS.Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2021, 01:33:27 am
content: At low voltages where base-emitter breakdown is not an issue, A PNP transistor with its base connected to the positive rail can act as a diode with its current gain directing the excessive input current to ground instead of the positive rail. If the 0.6 volt Vbe is unacceptable and must be smaller, then a resistor and diode can create a voltage which is one Vbe lower than the positive rail for the base connection cancelling out the PNP transistor's Vbe. Higher currents will require either a stiffer Vcc-Vbe reference or a pair of PNP transistors in series and compensation for 2 x Vbe.Alternatively, use a current driven diode bridge for input protection although this requires higher voltage bias supplies and a low impedance source.

author: AndersJ
author_rank: Frequent Contributor
timestamp: February 14, 2021, 07:34:42 am
content: A schematic would clarify the PNP suggestion.David?Anyone?

author: Ian.M
author_rank: Super Contributor
timestamp: February 14, 2021, 08:55:05 am
content: LTspice sim of a variety of PNP clamp circuits attached.The first is the simplest, and is basically David's second suggestion of a PNP transistor with its base held one diode drop below Vcc.The second uses a current mirror to increase the current through the bias diode in proportion to the clamping current to get a very flat precision clamping voltage. The disadvantage is it draws the same current from Vcc as its clamping.The third modifies the current mirror to be ratiometric (approx 30mA/A) by adding emitter resistors, and adds a resistor in series with the bias diode connected transistor. This means the compensation is no longer near perfect, but over a limited range it can be acceptably flat. If the bias resistor (R7) is too small, it undercompensates and is closer to the basic clamp with the clamping voltage rising excessively. If you make it too big, you get foldback clamping. The current draw from Vcc while clamping is vastly reduced.In all cases you can clamp multiple signals by connecting multiple PNP clamping transistors to the circuit with their bases and collectors in parallel. If you do so there will be some disturbance to the clamping voltage of other signals while one is sinking a significant clamping current.The input leakage current of the third circuit can be reduced by increasing the startup resistor R6. However it must *NOT* be omitted, as there must be enough base current available for the clamping transistor to initially start conducting to activate the current mirror.Ideally the PNP clamping transistor(s) and the bias diode connected transistor should be reasonably well matched and thermally coupled. For the current mirror compensated versions, the mirror transistors should also be a matched and thermally coupled pair.

author: gnuarm
author_rank: Super Contributor
timestamp: February 14, 2021, 09:38:57 am
content: Quote from: AndersJ on January 28, 2021, 05:14:50 amHow long is a piece of string?ESD protection circuitry would be hard to design if it weren't for standard that mostly define the nature of the ESD surge to protect against. As someone said ESD charge is small, but the currents can be very large. The standard defined a source and the characteristics of the current and voltage and more importantly, a test method.Until you define all that for your "surges", no one will be able to tell you how to protect your circuit.But you did throw us a bone by talking about 24V continuous protected with a series resistor. You seem to be saying you are working with 5V logic rather than analog, so that is good info as well. So what value resistor did you pick? What delays can you tolerate to the signals? Adding series resistance creates RC edges and delays on the inputs, so delays must be accounted for.I typically protect circuits from ESD with Schottky diodes to the rails with no series resistance. Large capacitance on the rails helps a lot. These inputs are typically from cables and I'm relying on the cables to provide some filtering of fast edges. I can't add series resistance they are analog I/O circuits with as low as 50 ohm impedance. Fast digital circuits also can't tolerate the RC delays.If you are running TTL level signals off board you need to worry about signal integrity as much as protection. If these signals are just between boards in a chassis, then what surges are you talking about? If they are between boxes, you probably should consider something other than direct logic level interfaces. RS-232 devices are great for shorter distances between chassis and have lots of built in protection. I want to say they survive up to ±25V, so just over your number.I see RS422 receivers that are also protected to high voltages, as high as ±60V! These will run much faster, but are intended for double ended (differential) use although they can be used single ended with one input grounded... or tied to a reference I mean. Not sure how single ended use impacts the speed but it messes with the noise sensitivity. I think RS-423 might be the single ended equivalent of RS422 and I see data sheets for these with input voltages of ±25V and higher.I hope this makes some sense. It should be a lot easier to run a non-TTL signal through a cable, one that is intended to be run through a cable, rather than TTL which can have trouble running through a backplane. Why add all that extra circuitry to protect TTL signals when you can just use a proper interface standard and be done with it?