Over voltage protection circuit? - Page 1
Forum section: electronics / projects

author: Starlord
author_rank: Frequent Contributor
timestamp: February 06, 2017, 04:01:34 am
content: I recently had someone blow up one of my boards by connecting a 20V supply to it when it was designed for 12V with a max of 16V, and I was wondering... what's the ideal way to protect against this?When I say ideal, I mean something requiring a minimum number of components, which cuts the power instantly, is compact, and doesn't involve a fuse or diode that is designed to blow and would need to be replaced, and doesn't cause a significant voltage drop.For example, I have reverse voltage protection on my circuit already. Use the P-FET method outlined here:www.ti.com/lit/an/slva139/slva139.pdfI have heard of using crowbar circuits for this, but that could potentially destroy the diode. And a diode in series causes a voltage drop and dissipates heat. So this method I consider ideal as it's simple and cheap to implement and fairly compact. Though it's not the cheapest of course and a simple diode in series might be better in many cases.I should also say it doesn't have to handle an insane amount of voltage, but an ideal maximum would be in the 40V range as that's what might be experienced if you were connected to a 12V system in a vehicle and there was a load dump.I was thinking that the best solution might take advantage of that P-FET I've already got there. I'm just not sure of how best to go about making use of it.My gut tells me that what I might want to do is put a voltage divider and a diode from the input to the gate, and put a resistor on the gate to ground. So like, if I wanted to limit the voltage to around 12V, maybe make a 1/10 divider, and stick a diode with a 1.2V drop between the output of the divider and the gate, and then have that resistor from the gate to the ground which would be weaker than the resistor in series with the diode for the divider. That would be pretty simple.But maybe one could make it even simpler and do away with the diode entirely and just make sure the voltage output from the divider is low enough that it's below the gate threshold? Well below it I guess because you don't want the FET to only turn on partially?Then again, maybe there is a cheap part specifically designed for this purpose which handles both reverse and over voltage protection with the voltage settable by a voltage divider?I don't have any specific project I need this for right now, so I don't have specs to give you for what I need. I'm just thinking about the future if I ever design any more boards that draw a couple amps and require a 12V supply, or will work in a vehicle. I did some google searches but mostly I just saw people suggesting crowbar type circuits.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2017, 04:30:18 am
content: I have been using Linear LT4356 which absolutely limits voltage above the threshold you program. It turns itself into a linear regulator for a short time to deal with a transient voltage. If the over voltage persists, it shuts off.I recently had a thread on this part. It was struggling with stability because of a bank of very low ESR caps on the output. Easy fix. A previous design with the same part has been saving my butt for the past year.What is your application?Sent from my horrible mobile....

author: Unknown
author_rank: Unknown
timestamp: February 06, 2017, 04:38:35 am
content: You could also look at polyfuses if you want the protection of a fuse, but don't want to have to replace it.

author: amitchell
author_rank: Regular Contributor
timestamp: February 06, 2017, 04:49:52 am
content: I have used the LTC4365, it needs a dual Channel mosfet and three resistors. I find the linear stuff to be very nice but it comes with a higher price.Search for hot swap controllers with OVP.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2017, 05:11:13 am
content: Quote from: Elf on February 06, 2017, 04:38:35 amPolyfuses protect from overcurrent, not voltage.Sent from my horrible mobile....

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2017, 05:16:42 am
content: Quote from: amitchell on February 06, 2017, 04:49:52 amThe OP did not specify cheap :-)You only need a single N-FET unless there is a requirement for reverse polarity protection as well.I like them, but my application is not particularly cost sensitive. The 4356 also has an over-current option so it becomes an e-fuse as well.Does this solution need to be ultra-cheap or high-performance?Sent from my horrible mobile....

author: amitchell
author_rank: Regular Contributor
timestamp: February 06, 2017, 05:22:49 am
content: Quote from: rx8pilot on February 06, 2017, 05:16:42 amI actually moved away from them to spend more money, I implemented USB-C Power Delivery on my current project.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2017, 05:25:33 am
content: USB-C is super flexible and a standard - nice choice.My application is up to 28v/10A with protection up to 60v input.Sent from my horrible mobile....

author: Unknown
author_rank: Unknown
timestamp: February 06, 2017, 05:37:09 am
content: Quote from: rx8pilot on February 06, 2017, 05:11:13 amUnderstood, but in conjunction with something like a crowbar it can be a last ditch level of protection against high voltages. Of course the ICs listed above are a better fit to the asked question. I just thought it was worth mentioning given the bit about having to replace fuses in the original post.

author: Niklas
author_rank: Frequent Contributor
timestamp: February 06, 2017, 07:48:45 am
content: You will need two MOSFETs in a back to back configuration to implement protection against both wrong polarity and overvoltage. Have a look at the direction of the body diodes.TI has an app note describing a overvoltage disconnect circuit based on feed forward:http://www.ti.com/lit/pdf/snva190Have a look at the zener+resistor Vgs clamp for the MOSFET in this appnote:http://www.ti.com/lit/SNVA717

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 06, 2017, 07:57:32 am
content: Quote from: rx8pilot on February 06, 2017, 05:11:13 amCombined with a beefy Zener, it will work just fine as overvoltage protection.

author: sagnikbasu
author_rank: Contributor
timestamp: February 06, 2017, 12:56:34 pm
content: Can isolation using optocouplers help in your project ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2017, 01:10:57 pm
content: Note that the TVS/zener will short out long before the Polyfuse opens. Those fuses are ponderously slow: on the order of ~100ms even under fault conditions (let alone for a gentle overload, as would be the case here, which might take seconds).What to do? Well, it just so happens there is a combo Polyfuse+TVS device out there. TVS heats up the fuse, pushing it to "open" that much faster. Give it a look!You can also use an LDO to limit the internal supply voltage: normally it's saturated (in dropout), because input is below setpoint/nominal voltage. As supply goes up, the LDO kicks in and starts regulating. The input is allowed to rise, but the output stays nominal. This gives you more headroom, so you can absorb the overshoot with a TVS, or MOV even. (MOVs have a much sloppier knee than TVS: I recommend a circuit capable of withstanding triple the MOV nominal rating. So for a 12V system, you use maybe a 16V MOV, and a 48V withstand. MOVs have way more energy capacity, making this a practical approach to automotive load dump!)Tim

author: Starlord
author_rank: Frequent Contributor
timestamp: February 06, 2017, 04:58:22 pm
content: > Does this solution need to be ultra-cheap or high-performance?Not sure what you mean by high performance. Inexpensive is always nice! I don't have a specific application in mind for this, but the application I used my reverse voltage protection circuit for was a 20W stereo amplifier that required a 12V 2A supply. I've just had this on my mind for a while and wanted to see what solutions people might offer. Maybe I'll learn something!I liked that Linear chip, though at $2.50 each @ qty 100, plus a dual fet, it's a bit on the pricey side to just toss in to most circuits. It'd probably be cheaper to just replace the occasional board that blows up.

author: tszaboo
author_rank: Super Contributor
timestamp: February 06, 2017, 05:17:37 pm
content: I usually protect my circuits, by writing the maximum voltage on it. I think first you should decide what is "reasonable" to protect to, because every now and then, there is someone who cannot read. I had 12V battery connected to a max 5V 50A output charger. It blow the ass out of it. The protection would have increased the product's price by say 30%, it is not reasonable.So set your goals. I usually limit the stupidness by +100/-200% meaning inverse polarity or double the input voltage.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2017, 06:22:45 pm
content: Quote from: Starlord on February 06, 2017, 04:58:22 pmBy high-performance, I mean that an over voltage transient is a non-event with no over shoot at all. The over-voltage threshold is very precise and predictable. The response time is very fast. It can hold the output at the specified voltage even as the input voltage is rapidly rising. After a pre-determined amount of time, it will simply shut down. Reset can be automatic or by the user with no cool down and no cooked TVS devices. It will also issue an early warning to an MCU to facilitate the shutdown, ie saving data, issuing e-stop to moving systems, etc before total loss of power.You have to pay for that in both hard cost and part count so it is not appropriate for every application. In my situation, it is quite common to see big transients and persistent over-voltage conditions. This is a no-worries solution overall - mistakes are a non-event every time. Having over-current detection as well is a sweet bonus too.On the scope trace you can see a test I recently did. The blue trace is a rising input voltage that goes up to 30v. The yellow trace is the output voltage that is set to hold at 18.2v which it does for about 15ms. Since the input voltage is still high, it shuts down completely. If the over voltage lasted less than 15ms (or whatever time your MOSFET can handle being a linear regulator) - it would just go back to driving the FET fully on so the loss is extremely low. The output can be very precisely controlled to whatever voltage you need.The FET I am using prioritized a low Rds(on) so I can run it at 15A with no need for thermal management at all. It is dissipating about 90mW at that current - vastly better than any passive solution. The absence of thermal management and tiny size makes up for the cost quickly.Do you need reverse polarity as well?Quote from: NANDBlog on February 06, 2017, 05:17:37 pmI tried that but out in the wild - it did not work. Users just don't pay attention. I decided to make a feature out of it for my products. I print a voltage range that is expected on the enclosure, but protect for about 5x that value, plus reverse polarity. There are a LOT less frantic calls and emails that things stopped working and they could smell burning PCB

author: james_s
author_rank: Super Contributor
timestamp: February 06, 2017, 06:33:30 pm
content: The power supplies used in Gottlieb arcade games have a crowbar circuit consisting of a zener diode triggered SCR, when triggered it clamps the supply and blows the fuse, quite effective in an early-80s brute force sort of way. A similar circuit ought to still be effective and low cost, although the above suggested polyfuse may be a superior option to a traditional disposable fuse.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2017, 07:00:25 pm
content: Quote from: rx8pilot on February 06, 2017, 06:22:45 pm*snickers*(Certainly, fast enough for load dump -- but simply by shorting the input to a higher voltage supply, the current surge can be hundreds of amperes, and the dV/dt well into the V/us range.)Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2017, 07:21:15 pm
content: Quote from: T3sl4co1l on February 06, 2017, 07:00:25 pmThe LT4356 does not short anything, are your referring to the crow-bar concept?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2017, 09:10:35 pm
content: I mean if you hot-plug it to a high voltage source -- mechanical contacts generate ~nanosecond edges, so it would be sure to test the dV/dt and dI/dt capability of that circuit.Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2017, 09:30:06 pm
content: Quote from: T3sl4co1l on February 06, 2017, 09:10:35 pmOh, I see. Those kinds of edges are slowed by some low-ESR capacitance to give the MOSFET/driver some time to get a hold of the issue but that is only if the voltage suddenly changes during operation. I only had 700Mhz probes, but I did not see anything scary sneak through when hot-patching 30v into the circuit. The MOSFET gate is discharged on power up and output side has some low ESR MLCC's to catch anything that gets past the MOSFET.Maybe I should take a closer look although I feel like I am past the point of what would be dangerous down stream. In my case, the output goes down a cable into another SMPS that is further filtered so nanosecond edges will not survive for long.

author: DBecker
author_rank: Frequent Contributor
timestamp: February 06, 2017, 09:42:46 pm
content: Quote from: Elf on February 06, 2017, 04:38:35 amPolyfuses have very, very limited applicability. They are great at limiting USB port current draw, but disappointingly few other uses.They have two bad features that makes them a poor input protection device: they are slow, and they only work with low voltages.They also take an unpredictable time to recover, and are very lossy during the recovery, and only gradually drop (over days) to their specified resistance.I have a partial reel that I was expecting to use on prototypes, but I've never found a good use for them. The problem is always brief current spikes or higher voltage.

author: james_s
author_rank: Super Contributor
timestamp: February 07, 2017, 05:15:47 am
content: They're commonly used in NiMH battery packs as short circuit protection. The batteries for my Roomba vacuums all have them.

author: Wilksey
author_rank: Super Contributor
timestamp: February 07, 2017, 11:12:39 am
content: Most manufacturers do overvoltage protection by a simple means of telling you that you over volted the board and you're an idiot and it'll cost you!I have used a Zener with a crowbar thyristor with a polyfuse, seems to work OK for me!

author: max_torque
author_rank: Super Contributor
timestamp: February 07, 2017, 01:25:54 pm
content: Quote from: Starlord on February 06, 2017, 04:01:34 amer, you're going to have to deal with rather more than 40V!ISO 16750-2-2010 defines automotive 12v system Load Dump as a voltage pulse of between 79 and 101 volts! (depending on input impedance)

author: SvanGool
author_rank: Regular Contributor
timestamp: February 07, 2017, 01:41:52 pm
content: I agree with @Wilksey, a TVS based circuit is too difficult to dimension (12V nominal, max 16V), not enough margin.You could take the circuit from here:https://en.wikibooks.org/wiki/Practical_Electronics/Crowbar_circuitReplace SD1 for a wire and put SD2 in parallel with Q1 (cathode up, another crowbar) and use following components:R1=470 OhmC1,C2=100nFZD1=1N5245BSD2=MBR5200VPTR-E1Q1=MCR8SMGF1=PPTC RKEF250Which costs in total, single piece, less than USD 2.The circuit will provide: over-voltage (>16V), over-current (>5A) and reverse polarity (<-1V) protection.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 07, 2017, 05:46:10 pm
content: How much voltage drop / dissipation is tolerable?Sent from my horrible mobile....

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2017, 06:30:42 pm
content: Personally, I'd use two over voltage protection circuits:1) Something very fast, such as a transient suppression diode, which kicks in around at around 150% of the nominal voltage but doesn't latch on.2) A crowbar, set to 120% of the maximum nominal voltage, with a low pass filter (to prevent it from tripping on transients) and a PTC resistor for over current protection.An RLC filter on the supply can also help to remove fast transients.

author: tszaboo
author_rank: Super Contributor
timestamp: February 08, 2017, 03:37:59 pm
content: Quote from: max_torque on February 07, 2017, 01:25:54 pmlol. Such 20 century. A modern hybrid has the 12V battery charged from the main battery, by a DC-DC, so the voltage is up to 14V. It really feels like people drive in their ancient cars with advanced features, like "self-starting system" so you dont even have to get out and crank it up to start your car!

author: richard.cs
author_rank: Super Contributor
timestamp: February 08, 2017, 04:17:32 pm
content: I have used this circuit to protect the DC power supply input to my homemade metcal PSU:http://www.randomfunprojects.co.uk/bigfiles/metcal010.pngProvides reverse polarity and overvoltage protection, latching off until the supply is removed. Spice model attached, will probably need tweaking to suit your application. Whether it's fast enough for you depends largely on the supply impedance and downstream capacitance (and therefore rate of voltage rise), and how far from the limits you set the protection to fire.