How to combine OVP with Surge Protection without burning out TVS? - Page 1
Forum section: electronics / general

author: OleMi
author_rank: Contributor
timestamp: August 01, 2019, 01:41:41 pm
content: Looks like a trivial task from a text-book, but not really.Provided:A device which is rated for 12V (DC) application with desirable limit up to 16.7V.Current 4.5A.The first IC on the device withstands only 21V according to AMRs.Looking for OVP 18V.Looking for a Surge Protection as well.Desirable reasonable AMRs for the device, say, 28V (for the case if integrator will connect 24V truck battery).So, according to text-books:TVS + fuse (5A) + "electronic relay" for OVP (protections with MOSFET that will be off for OV conditions). Well, let's take a look.The core problem is: TVS for what voltage?Say, OVP's components (MOSFET) withstand DS Breakdown 50V. We take TVS for 36V. Surges above 36V will not damage OVP, but surges above 36V will reach the IC and damage it, because it is rated for 21V and is not protected from surges. Why OVP will not help? Because it is a surge (an impulse of short duration) and OVP is not that fast. There is no fast OVP exist, because TVSs are designed for protection from fast short-duration impulses and OVP must be protected from them as well. The OVP itself can't provide Surge Protection. So, doesn't work.OK, obviously we want to move TVS Breakdown/Clamping close to OVP's value 18V then. The maximum limit we allow for the user is 16.7V. Let's take 15's series of TVSs which start opening at about 16.4 and clamped at about 18-19v. Wonderful, protection from surges of 18V and more is guaranteed. The IC is protected from Surges. Now let's look what happens in over voltage conditions of close to 18V, say 17.8V. Clamped TVS will burn out, before the fuse will burn! Why? Because TVS is for short-duration impulses and not for continuous over voltage conditions. Why fuse will not burn out? It's slower than TVS and the fuse is installed not for that case, but for reverse polarity, when the TVS will short pretty any voltage and the fuse will blow earlier than TVS.So, how not to burn TVS?

author: richard.cs
author_rank: Super Contributor
timestamp: August 01, 2019, 04:21:09 pm
content: The simple answer is that your 21V rated IC is marginal for a device where the user is expected to apply up to 16.7V.The complex answer is of course you can but the design is more complex, for example a precision OVP circuit that cuts in around 17V before the TVS current gets too high, placed before the TVS, and then a higher voltage TVS before that to protect the OVP circuit.You then have a TVS at 36V that protects the device from surge if the OVP switch is off and protects the OVP switch, another that protects the device to 19V if the OVP switch is on, and the OVP switch which protects both the device and the lower voltage TVS from overvoltage.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 01, 2019, 05:29:17 pm
content: https://www.analog.com/en/products/ltc4367.htmlThese are claimed fast enough for your kind of protection.

author: Marco
author_rank: Super Contributor
timestamp: August 01, 2019, 05:53:46 pm
content: Use a smarter OVP ... when voltage is higher than 16.7 for x amount of time, crowbar. It's slower than the TVS, but it's fast enough to limit the energy the TVS can see.

author: OleMi
author_rank: Contributor
timestamp: August 01, 2019, 08:37:37 pm
content: @richard.csThe reason of OVP because the user is expected to connect up to 28V. I'm not talking about voltage regulation here, but about protection from users' mistakes.Second TVS doesn't solve the problem at all. Just draw it and simulate say 30V impulse, 40V impulse, 24V over-voltage and you will see all the same problems.@capt bullshotI don't consider to use specific ICs, especially those with two external MOSFETs, especially overprices Linear's stuff... I also don't believe that such solutions are really surge protected. They are OVP, not Surge Protection.@MarcoI don't understand how OVP, which is slower than TVS can help to protect TVS in over-voltage conditions. Do you say that I should use crowbar OVP as a surge protection in the same time and remove TVS from the design? One thing that I don't want neither is to have fuse blown in OV or surge conditions, only in the reverse polarity case. Crowbar in my knowledge will blow the fuse - the same like OVP based on zener clamp with transistor buffer. The last, btw, is not surge protected, because transistor will not survive the same surges as TVS. Crowbar also looks like a bit unreasonable for DC application.

author: thm_w
author_rank: Super Contributor
timestamp: August 01, 2019, 10:08:23 pm
content: Crowbar or a monster TVS combined with SRF would work. Self resetting circuit breaker is an option too (or even manual reset if the user can access it).richard.cs solution is valid, the first TVS is going to be a very high voltage, say 60-75V to protect the overvoltage circuit/IC. Note the protection IC suggested above can handle up to 100V.I think you are avoiding solutions here because you don't like the added cost/complexity, but its going to be necessary.

author: richard.cs
author_rank: Super Contributor
timestamp: August 01, 2019, 10:22:45 pm
content: First of all decide the highest overvoltage you want to protect from and put a TVS at the input that does not conduct at that voltage. Let's say you want 50 V to cause no damage so you pick a 60 V holdoff TVS that clamps a surge to 80 V.Next comes the overvoltage protection, it needs to survive the surge which is clamped to 80 V, so choose overvoltage protection with a 100 V rated switch. Set it to open at 17 V so as to protect your downstream devices.Finally, fit a 17 V holdoff TVS after the OVP. It clamps surges to 19 V, and never sees more than 17 V sustained because of the OVP circuit.Other approaches are also valid, oversized TVS and fuse or thermal cutout, thyristor crowbars, etc. A high voltage TVS, OVP and then a lower voltage TVS is what I use on rail industry designs that have these kinds of requirements.

author: Marco
author_rank: Super Contributor
timestamp: August 01, 2019, 10:23:30 pm
content: Your drawing shows the OVP in crowbar configuration, I took that at face value. The OVP doesn't need to be fast, it just needs to trigger at a lower voltage than the TVS and be fast enough to limit the energy to below what the TVS is rated to absorb.You seem to want a series OVP though, not a crowbar one ... so just do that then. Since you want to do it discrete use a P channel MOSFET and a shunt regulator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 01, 2019, 11:44:49 pm
content: How much space do you have for energy or power dissipation? Budget?Have you considered a pass type OVP for ride-through or turn-off? (LT Surge Stopper has been mentioned already; there are simpler solutions too, of course.) 4.5A might be a bit much for ride-through during load dump, but could be just tolerable at 28V input (min 31.5W dissipated?).Oh, and is that 4.5A nominal at 12V? Would it be less at 18-20V?I'm not a big fan of fuse+TVS because the let-thru energy is humongous. A crowbar or shutoff helps with that. There's also combo PTC+TVS devices; they do exist, but they are unfortunately hard to find in stock, so if you're only making a few 10k of something, you may be in need of an alternative.Tim

author: OleMi
author_rank: Contributor
timestamp: August 02, 2019, 07:34:01 am
content: @MarcoSorry, that is correct that the draw is misleading on that point. I do use the series OVP with P-channel MOSFET to cut off supply on over-voltage conditions.@thm_wIt's mass production and user-do-mistakes default (including they will not install the fuse or will not re-install the correct one). A big pre-installed fuse (or terminals for it) is also is not in harmony with the whole design of PCB.You and richard.cs encouraged me to re-think why my two-TVS protection doesn't work. I put the second TVS mistakenly after reverse-polarity protection (I use a MOSFET for this too), instead of put it after OVP protection. So stupid fault. Moving to the right place seems will solve the problem. Thanks.The only weak point is that MOSFETs for series OVP and reverse-polarity protections I use are rated for 30V Break-Down voltage only and it's hard to find any better for that current in a suitable SMD case, so with the first TVS I'm balancing at around 25V-28V of the maximum for users mistakes.Oversized TVS... yep, I though it might help to blow the fuse.

author: thm_w
author_rank: Super Contributor
timestamp: August 02, 2019, 09:13:13 pm
content: Quote from: OleMi on August 02, 2019, 07:34:01 amI didn't mention a user replaceable fuse, but if user cannot access it then SRF/self resetting breaker are still options. They are expensive but simple. Google self resetting fuse if you don't know what part I am referring to.What package? Surely there are mosfets rated for more than 30V in almost any size.

author: OleMi
author_rank: Contributor
timestamp: August 03, 2019, 01:18:58 pm
content: SRBs for these parameters are not better options than breakable fuses and they are not in harmony with "tiny" SMD PCB in the same way as fuse holders are not.SRF PTCs for these parameters are all TH, moreover their hold current is always half of trip current, which is useless point to protect TVS.Two TVS is the correct solution when the second one installed on the correct location.The only issue is balancing at the edge of limits for both TVS because of the MOSFET rated 30V, desirable maximum 16.7V and AMR for the IC 21V. The gap between break-down and clamping for TVS is relatively high. Package for MOSFET is power package 3x3mm , AEQ is mandatory, ESD gate protection is mandatory, gate protected without zener (i.e. Vgs at least 25V) is mandatory, price for reel below 0.3usd is mandatory, good avalanche characteristics tested indicated in the datasheet (and yes, I know it's tricky part...), Rdson below 15mOhm. The one with BD 30V is the only option in this size and Rdson, believe me.When I research for a component for mass production, I check all (and here I mean literally all) parts from all (again) major semiconductor suppliers. If omit ESD and Vgs requirements, there will be couple more options with better BD, but I don't like an idea to rely on additional component to protect gate in handling and normal working conditions. Finally, this will not solve the problem of small gap for the second TVS, where MOSFET is not involved.