Overvoltage protection using TL431 and mosfets - Page 1
Forum section: electronics / beginners

author: derGoldstein
author_rank: Regular Contributor
timestamp: November 24, 2017, 06:27:29 pm
content: I'm trying to implement over-voltage protection using a TL431 and either an N-channel or P-channel mosfet. I've been simulating the two circuits in ltspice, which I'm not very proficient at. I'm attaching the circuits.The n-channel mosfet one is much simpler, but disrupts ground.The p-channel one requires a few more components, I found it in this TI doc:http://www.ti.com/lit/ug/tiduay1/tiduay1.pdf(page 7)They both prevent more than ~18V from reaching the load (the 10ohm resistor). The TL431 starts conducting at ~18V, which in the case of the n-channel mosfet circuit decreases the mosfet's gate voltage to about 2V, shutting it down.In the case of the p-channel mosfet circuit it shuts down the small-signal (n-channel) mosfet, which pulls the gate of the p-channel mosfet high, shutting it down.I have three questions:First, is there anything wrong with these circuits that I might not be noticing? Are there any vulnerabilities that might come up when I construct them? (I ask so that I don't fry components unnecessarily...)Second, is there anything wrong with the way that I'm simulating them? I haven't used ltspice much yet.Third, in the p-channel circuit, I can't use a "common" 2N7002 as M2. Is this because it doesn't turn on at 2V?

author: jmelson
author_rank: Super Contributor
timestamp: November 24, 2017, 06:40:56 pm
content: Note that a soft turn-off of a power FET under load may cause the FET to fail shorted. This can happen in milliseconds, so unless you have a way to force the gate to cutoff quickly, it may not work.Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2017, 06:58:44 pm
content: Needs hysteresis and better Vgs(off) -- note that TL431 can't pull down to below about 1.8V for obvious* reasons.*I don't know if the LTSpice model models this. The datasheet (usually) shows this.As supply voltage falls, you also have a startup transient, charging whatever capacitance the load has. And any time there's a load switching on and off, there's worry about whether it's safe to do so or not (what if the load failed shorted in the mean time, whether due to overvoltage that wasn't quite well enough handled by this circuit, or due to an unrelated -- if that much more unlikely -- failure?).Tim

author: derGoldstein
author_rank: Regular Contributor
timestamp: November 25, 2017, 04:59:33 pm
content: Quote from: T3sl4co1l on November 24, 2017, 06:58:44 pmThis will be used to protect a circuit if a higher-voltage supply is connected (DC), so hysteresis isn't required (I think). It's going to have an XT60 input plug, and accept input from 4S battery packs (16.8V max). Since battery packs with much higher voltage have XT60 connectors, I don't want the wrong battery pack to kill the circuit if connected. So I'm not expecting the circuit to "clip" an AC input (although that will be an interesting thing to implement for other circuits...), just reject a DC input over a certain voltage.When you say better Vgs(off), you mean to completely shut down the mosfet by (in the case of an n-channel) drop the gate voltage to near 0?Quote from: T3sl4co1l on November 24, 2017, 06:58:44 pmYeah the ltspice model shows this -- the attached images show the gate voltage of the n-channel mosfet circuit and the p-channel mosfet circuit. The "shut down" state for the p-channel is almost the input voltage, so I think it should be fine (correct me if I'm wrong though...). The "shut down" of the n-channel mosfet is just under 2V. Can this be a problem even if I use a mosfet that requires 10V to fully turn on?Quote from: T3sl4co1l on November 24, 2017, 06:58:44 pmWould some input capacitance (~10uF) be able to block some (or most) transients, or should I be looking to add a TVS diode to catch those?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2017, 07:08:01 pm
content: Quote from: derGoldstein on November 25, 2017, 04:59:33 pmAh, cool. So the threat model is a one-time sort of thing: either it's connected to the wrong voltage, or it's fine, or disconnected completely.Note that mechanical connections are very fast: nanoseconds. In practice, your circuit will be limited by bypass capacitance (load end) times stray inductance (battery cable length). This can be mere microseconds.QuoteYes. Suppose it's a hot day, the device was just being used, heavily -- so the transistor is hot too -- and the battery pack is hot swapped to the wrong one. Now Vgs(th) is depressed (because hot), it doesn't shut off fully, and the transistor cooks at some intermediate current -- maybe 0.1 to 10A, at whatever voltage drop it gets.0V is much better, and tossing in a couple little BJTs, to ensure quick switching and correct gate levels, is a cheap performance boost!FYI, MOSFETs are always slower than BJTs -- 2N7000 is sort of like the MOS equivalent of a TIP29, in TO-92 format. It's good for way more voltage and current than a jellybean needs to be. Even the smallest MOSFETs you can find are more like 2N4401s than 2N3904s, and they're way more ESD sensitive (even with built in gate protection zeners). You can't find new production small signal RF MOSFETs (3 terminal) anymore (and there are very few dual-gate RF MOSFETs left). But small BJTs abound (MPSH10/81 are cheap, fast and useful, in case you need something faster than 2N3904/6). So, BJTs are where it's at, even if it's the general purpose kind (like 2N3904/6).QuoteYes, but only to the extent that you're willing to add enough filtering to guarantee that delay.Suppose you want to extend it to a 10us delay. This is more than enough time for the TL431 to wake up and respond, and enough time for a BJT level shift + driver to turn off the MOSFET. To get a 10us delay, you need a cutoff frequency in the 30kHz range. Assuming this is a low impedance load (a few ohms), this requires, say, 2uH and 22uF. Which, really, that's probably not bad at all. You can guess the battery wires will have some 0.2uH or so, more if long, and you can loop some extra wire inside your box to add more. (Note: avoid cored inductors for this purpose: the inrush surge will be near a hundred amperes, or more. You need that inductance available under pulsed conditions, which means air core action.)Any resistance you can afford, also helps, particularly in reducing the peak inrush surge current. Said inductor doesn't need to be high quality; if it has a Q of 1 under this pulsed condition, it's fine (not that that really means anything, but for the sake of argument, let's say at the peak, it's dropping half and half through inductance and DCR, thus -- informally -- giving a Q of ~1).Now, using huge resistors to drive gates, you're looking at 10~100us, which is an eternity, and also enough to toast the FET (if it's not toasted already from inrush -- again, mind where you put capacitors, and how much of 'em, after this poor transistor!). Insert some drive in there, and you'll have a much better start, at least!Tim

author: Zero999
author_rank: Super Contributor
timestamp: November 25, 2017, 09:23:04 pm
content: I designed one awhile ago, for someone else on this forum. It used a P-channel MOSFET and a BJT to switch it. Because the circuit suffers from a delay in switching the load off, a faster transient suppressor, such as a zener diode, needs to be connected across the load.Hysteresis is generally not necessary because the power supply voltage rises slightly, when the load is disconnected, causing it to cut the load of more. This isn't the same as an undervoltage lock-out circuit which will oscillate, unless there's some hysteresis.https://www.eevblog.com/forum/beginners/2n7000-strange-behaviour/msg1186642/#msg1186642

author: derGoldstein
author_rank: Regular Contributor
timestamp: November 28, 2017, 04:54:01 pm
content: Quote from: T3sl4co1l on November 25, 2017, 07:08:01 pmYeah, switched to a BJT in this last one. I'll have to assemble and test a few circuits to see how they respond to different connection types (mechanical switch, XT60, digital switch-on, etc.).Quote from: T3sl4co1l on November 25, 2017, 07:08:01 pmThere are more variables here than I imagined... Putting an inductor in series with the circuit is too much for this implementation, but maybe some low resistance at the input is possible.Quote from: Hero999 on November 25, 2017, 09:23:04 pmThanks!I implemented the circuit in ltspice, attached for reference.I'm wondering if there would be a super-safe way to do this by having a system that's default-off until a subsystem turns it on. In the n-channel version of this circuit, if the tl431 fails open (or just responds too slowly), the protection's gone. In the p-channel version, it would depend on how fast the PNP turns on.What if I were to use a voltage reference, like a LM4040, and a comparator. Have the mosfet, whichever type it is, biased off. Until the LM4040 and the comparator start working, the mosfet stays off. It's more expensive and more complex, but if the system being protected is super-sensitive, it could be worth it.

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2017, 08:27:22 pm
content: Quote from: derGoldstein on November 28, 2017, 04:54:01 pmWhat does C2 do, other than slow it down and make it more dangerous, by allowing a high voltage pulse through? There will already be some delay in this circuit working. Don't make it any worse!Get rid of C2 and add a zener, with a higher voltage than the trip point, to the output, to get rid of any spikes. Refer to the schematic I originally posted and the link underneath it.There is no safe way to implement this circuit. A MOSFET can fail short circuit, causing the output to jam on. The only way to guard against this would be an overvoltage crowbar, consisting of an SCR and zener or another TL431, to short circuit the supply, if the output voltage rises above a safe level, causing a fuse or the MOSFET's bond wires to blow.QuoteWell if the MOSFET is off, then I don't see how it can be a problem. If undervoltage is an issue, then what you need is a window comparator.There's also the LTC4365, an IC specially designed for under/over voltage lock-out and reverse polarity protection.https://www.eevblog.com/forum/projects/ltc4365-resistor-values/msg1194789/#msg1194789

author: xavier60
author_rank: Super Contributor
timestamp: November 28, 2017, 09:10:10 pm
content: From my experience, A TL431 can draw upto 0.5ma of Cathode current before the input threshold voltage is reached.

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2017, 11:16:31 pm
content: Quote from: xavier60 on November 28, 2017, 09:10:10 pmIt shouldn't do that. The data sheet specifies off state cathode current <1µA, so the voltage across R3 should be 10mV, worst case, which isn't enough to switch on the transistor.http://www.ti.com/lit/ds/symlink/tl431b.pdf

author: xavier60
author_rank: Super Contributor
timestamp: November 29, 2017, 12:21:40 am
content: Figure 4 in section 7.14 shows the Cathode current rising to 400ua before regulation is reached.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2017, 06:36:11 am
content: The transition between "leakage" and regulation is a modest curve: still, sharper than a real zener diode. It's not exactly 2.50V where it first turns on, but it can be useful nonetheless!Anyway, that's just changing resistors around.A little REF-GND capacitance might not be a bad idea, to the extent that it prevents erroneous behavior from ambient RF. Remember TL431 is just a funny bipolar opamp, and will rectify RF to DC. (Important to remember in SMPSs, too!)Any short term overvoltage needs to be handled with filtering and/or clamping.Quote from: Hero999 on November 28, 2017, 08:27:22 pmWell, sure. Nothing's ever safe.If you're not going to put probabilities on those failures, you're spreading nothing but hearsay.Prevent MOSFET failure with SOA protection (turn-off is fine, turn-on should be done very carefully however), and clamp excessive input voltage with a TVS. Or MOV for that matter -- you can afford to use a 100V+ MOSFET here, giving enough excess operating range where a MOV is applicable. You can then not only ride out a load dump, but clamp it as well. Good news for something deep inside an automobile.Note that the NCH version is better, because PCH have about half the performance, and every milliohm matters here. LM4040 is a "PNP" TL431, though it's even less specified on that nature than TL431 is (which is truthfully an NPN with precision Vbe and very high hFE, or an op-amp with OC output and precision offset).Tim

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2017, 05:54:46 pm
content: If the off current of the TL431 is an issue, reduce the value of the base-emitter resistor (R3 in my circuit) to 470R and the base resistor (R4 in my circuit) to 1k.The LM4040 is fixed. The LM4041-N ADJ is adjustable and is probably a better candidate for this circuit, than the TL431. It will work down to 100µA and having a PNP input, rather than an NPN input like the TL431, makes designing a crowbar with an exact tripping voltage easier.