Rule of thumb for using TVS to continuously sink current - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: May 03, 2014, 10:32:28 pm
content: Does anyone know a way to realistically estimate how much continuous power i can put through a GSOT05C-GS08 TVS diode? (Basically using it like a zener)I'm using a mcu output as a switchable high current pullup (30mA).However in some situations there maybe a 2nd 14V pullup external to my circuit, so i need to add some protection to stop this 14V backfeeding.A resistor and a 5V zener would work fine to clamp it at the input, then i can feed my pullup signal to the zener through another resistor. 220R for both resistors should work.My question is, i already have 5v TVS diodes in the BOM so it would be nice to use that instead of adding a new zener. The problem is that TVS datasheets never say anything about the continuous power handling.What do you reckon a GSOT05C-GS08 could handle steady state power wise?http://www.vishay.com/docs/85824/gsot03c.pdfWith my current circuit I would need the diode to handle 43mA continuously at 5V so 216mWand all the SOT-23 zeners ive looked at say 300-350mA for their max power handling.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2014, 06:49:56 am
content: Use a series blocking diode insteadAlso, you shouldn't be drawing 30mA, and I doubt it's a good "current pullup". MCU pins are usually rated around 20mA. Yours might even be rated for... 50 or 100mA, but it's probably better to keep it under 10mA. An external transistor, say 2N3906 with a base voltage divider and emitter resistor, adds only four components and isolates the MCU from the nasty world beyond, while having excellent compliance (a 30mA CCS will be within 5% of nominal for any voltage more than, say, 0.5-1V from the rail; and probably +/- 20% over temperature and manufacturing variation, for a typical example). Depending on how this all goes together, you could even add some diodes so the current source always pulls to the highest supply, or is triggered by either, or whatever kind of logic operation.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 04, 2014, 08:30:39 am
content: and there is also this:http://www.analog.com/en/interface-isolation/protection-products/products/index.htmla bit more pricy then a zener diode... I like your creativity, the chinese are creaming their pants right now.this sounds like a interesting experiment at the very least, can you hook up a TVS to a lm317 current source and forget about it for a few months?I thought that TVS were primarily damaged by over heating (my rudimentary knowledge of diodes), so if its cool you are fine. The only thing I would worry about is spot heating, its possible (but i would imagine unlikely) that there is little attention paid to spot heating problems during the design of these diodes, so a low continuous current could damage it... i wonder if you can decapsulate it and look at it with a thermal imaging microscope (i wish right?)There might be some design benefit towards making it in a way that leads to dangerous spot heating, but hell if i know what that could be. (fear mongering)or that it being hot could cause over voltage conditions to damage the material more then if it is cold..perhaps you should see how the reliability of the TVS changes over temperature conditions, picking a high temperature version of your part might be a good idea.the problem is that, even if you measure case temperature, the junction temperature might be much higher, so if yoou measure the case at 40c, you might actually need to look at the temperature reliability of a diode running at 80c (just example numbers here).. this is where decapsulation and thermal imaging would be helpful..because if the ambient is 40c, the junction will be 40c (the whole part is uniform temperature). under a load the junction might be heated to 60c while the case is 40c,trap for young players., you should look up the reliability at your actualhighestjunction temperature, not case temperature. I would imagine the whole diode structure would be of a similar temperature, but I don't know jack about doped junctions loli think that determining the reliability of this is not trivial. cost savings like this might cost you alot more money (and reputation) if failure happens.on a personal note I have a beef with shitty input protection schemes, like PTC's in shitty switch mode power supplies burning out (i have seen this many many times). I think that many designers share the same sentiment of not being cheap with protection circuitry.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2014, 12:09:33 pm
content: But yeah, as for the TVS, it's just a zener diode, and I don't think there's anything special relative to the generic small ones. If anything they should be better, as they are made to withstand large currents (small ones are not, and may fail due to hotspots!). Keep it within thermal limits.The immense capacitance of the junction may be a problem for switching speed.Tim

author: SeanB
author_rank: Super Contributor
timestamp: May 04, 2014, 01:07:45 pm
content: You can use it as a clamp, just keep current below the constant power rating, and it will act as a somewhat poorly defined zener diode. If you look in the data sheet you will see the breakdown voltage specified at some low current, so run it at around that current and you will have a reasonably well controlled voltage. Power rating should handle it as well, while leaving plenty of reserve capacity for the surges.

author: Psi
author_rank: Super Contributor
timestamp: May 05, 2014, 06:59:00 am
content: Thanks guysQuote from: SeanB on May 04, 2014, 01:07:45 pm1mA is too low, a SOT23 sized diode should be able to handle much more than that.

author: SeanB
author_rank: Super Contributor
timestamp: May 05, 2014, 07:29:06 pm
content: That just is the max voltage at which they will be off, you can drive it at a higher current but then zener voltage will be higher by some unspecified amount. You probably could do 30-40mA with little issue provided you use both devices in parallel and have a copper pad on each lead for heatsinking the device. If you are worried use 2 devices so you have a higher rating, but they will have to be from the same batch do they are similar in breakdown.

author: Niklas
author_rank: Frequent Contributor
timestamp: May 05, 2014, 09:53:51 pm
content: Quote from: Psi on May 05, 2014, 06:59:00 amThere can be large differences in thermal resistance between different manufacturers and products. I had an issue with a TL431 in SOT23-3 package a while ago, where the BOM specified two options from Diodes and Texas. The one from Diodes had more field claims and most of them had that small typical burn mark on the package. According to the datasheets, the Texas option had a Tja of 206 C/W and the Diodes option had 380 C/W.Same package and product type, but with large differences in the thermal handling. Something to check when looking for second source components. As the joke about the chinese: same, same but different

author: David Hess
author_rank: Super Contributor
timestamp: May 06, 2014, 03:03:46 pm
content: I agree with T3sl4co1l; the difference between a TVS and zener diode (Note that zener diodes above about 6.2 volts rely on the avalanche effect.) is that it is avalanche rated just like an avalanche rated rectifier. Stay within the derated continuous power limit and it will be fine.