Protecting MOSFET gate (ESD) - Page 1
Forum section: electronics / projects

author: papo
author_rank: Regular Contributor
timestamp: September 23, 2015, 09:15:48 pm
content: Hi guysI came acrossthischarging cradle for LiPo batteries that has a bit of a design flaw: There are three lugs connecting to the battery; one is ground, one is supplying power to the battery and one picks up some sort of feedback from the battery. Unfortunately, the nature of this feedback is unknown, although I have seen thermistors and series resistors used there in other designs. I managed to reverse engineer the part of the circuit surrounding this pin (edit:This shows one connection to the battery along with the external 12 V supply. The two other connections to the battery as described above are not shown):The lug connecting to the battery is the one with the little lightning symbol. Apparently, the voltage on that feedback pin is used to switch some logic inside the charger. Now sadly, the designers decided to use a p-channel MOSFET in order to achieve that. As you can see, the gate is totally exposed to the user and the FET inside the unit got zapped (check out all the "Green light always on" failures on the page above -- that's exactly what I got and this seems to be the cause). I replaced it with a jelly-bean logic-level FET and the unit seems to be functional again.Now I want to put together a write-up on the issue in order to help people repair and/or protect their units. At first I thought I'd replace the FET with a PNP to avoid the issue altogether. But since I don't know exatly if they are relying on a very high-impedance node there, I'm not quite sure whether I should do that. I'm already worried that the Vth of that FET might have something to do with the detection of the end of the charging cycle. Probably the exact Vth doesn't matter anyhow, but the original part with a HI5 marking has a zapped gate and I can't recover the original Vth.So I thought I'd add some protective TVS diodes (breakdown maybe 10 V or so) to clamp the voltage applied to the transistor. I have three suggestions and I'm not sure which one to pick as I would never ever even think of doing something like that. The first one is a single TVS between the gate and the source:The second one adds another TVS between the source and the drain to prevent the issue shifting to there:The third one adds a series gate resistor. I could probably even increase the resistance a bit more than what I'm showing here:What's your advice? I'm particularly interested in the added benefit of the series resistor as that would involve cutting traces which I'd avoid if the resistor doesn't help.Regards and thank youMatt

author: SeanB
author_rank: Super Contributor
timestamp: September 24, 2015, 12:50:41 pm
content: Series resistor of around 100k will do the trick, along with a 470p to 1n foil or NPO ceramic capacitor from gate to the junction of D1 and R1 will be adequate. Does not need a TVS, which in any case would have too much leakage, and the series resistor and capacitor will do the job cheaply and with good protection. The built in protection zeners of the FET will clamp the residual spike in this case. The capacitor is optional, will help with RF pickup and the floating input picking up AC mains induced when the unit is not in use but powered.

author: mzzj
author_rank: Super Contributor
timestamp: September 24, 2015, 08:14:09 pm
content: Quote from: SeanB on September 24, 2015, 12:50:41 pmWhat zeners?

author: SeanB
author_rank: Super Contributor
timestamp: September 24, 2015, 08:31:42 pm
content: Most mosfets have a pair of 18V zeners on the gate as part of the device. Not rated for much, but will handle a slight amount of use.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 24, 2015, 08:46:52 pm
content: Quote from: SeanB on September 24, 2015, 08:31:42 pmI find this surprising too. Do you have some reference link for this? I am aware of the reverse-biased Body Diode between Drain and Source that results from the manufacturing process, but this is the first I've heard about Gate Protection Zeners installed as part of "most mosfets". Always willing to learn, myself. So do you have a link?

author: Marco
author_rank: Super Contributor
timestamp: September 24, 2015, 08:56:06 pm
content: Quote from: SeanB on September 24, 2015, 08:31:42 pmA BSS84 doesn't (ESD class 0).

author: SeanB
author_rank: Super Contributor
timestamp: September 24, 2015, 09:01:29 pm
content: First hit for Mosfet, showing the ESD protection diodes.https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdfTi P channel with the ESD diode built in.http://www.ti.com/lit/ds/symlink/csd75208w1015.pdfThere is very little in the shortform data sheets on the ESd protection, but most devices do have them, aside from certain ones designed for ultra high input impedance, and ultra low gate leakage current.

author: Marco
author_rank: Super Contributor
timestamp: September 24, 2015, 09:09:15 pm
content: The lowest cost jellybeans are ancient designs, you can get ESD protected small signal MOSFETs though.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 24, 2015, 09:22:42 pm
content: Quote from: SeanB on September 24, 2015, 09:01:29 pmThanks, that's news to me. I guess I'm having trouble with the "most have them" part, since none of the mosfets I've ever used have had them. I use mostly IRF and IRFP series N-ch types in my hobby and repair work.

author: SeanB
author_rank: Super Contributor
timestamp: September 25, 2015, 08:53:02 am
content: Big ones almost always have them, just that the datasheet will not show them or characterise them. The best bet for them is to look in the family datasheet for the series to get the common parameters, which might include the ESD diodes with some specs as to what they will survive using a human body simulator. That they are there but not specced is just how fragile they are, and how easy they are to damage, as most will barely survive 2 ESD events. 18V and low leakage means they are really small.http://www.vishay.com/mosfets/related/#appnotStart reading, there is a lot of info in that pile of application notes, which will help.

author: papo
author_rank: Regular Contributor
timestamp: September 25, 2015, 09:19:35 am
content: Hello SeanB and thanks a lot for your reply!I had indeed overlooked leakage effects and the TVS I was going to use would have had too high a leakage for it to work. I am now considering the series resistor you suggested along with aFDV304Pas I happen to have a few of those laying around. I would have preferred a transistor with two Zeners back to back because the voltage at the gate can apparently reach up to 7.4 V or more (I measure a low impedance path between the "+" terminal of the battery and the terminal tht connects to the gate). Do you think I can get away with it? In this particular circuit, I don't see any way a forward biased internal Zener could mess up anything but maybe I'm overlooking something.If that is not viable, I'll try to find a suitable FET with two protection diodes back to back. I tried this briefly already, but couldn't find anything suitable (SOT-23, low gate leakage, low |Vth| plus the diodes) at first glance.Regards and thanks again for your insightsMatt

author: newbrain
author_rank: Super Contributor
timestamp: September 25, 2015, 09:29:28 am
content: Could it be H15 rather than HI5?And yes, p-channel, with protection diode:http://www.smdcode.com/en/smd/code/h15-2sj204-metal-oxide-semiconductor-p-channel-fet-enhancement-type-with-integrated-gate-protection-/http://documentation.renesas.com/doc/DocumentServer/D17904EJ3V0DS00.pdfFor the protection, I'll leave it to the experts...

author: papo
author_rank: Regular Contributor
timestamp: September 25, 2015, 09:44:17 am
content: HelloQuoteOh wow nice find. I looked for both HI5 as well as H15 but couldn't find anything on either. I attacked a close-up of the transistor. Looks more like an I than a 1 to me but what do I know... Otherwise the transistor would be a fairly good match maybe apart from a slightly high gate leakage current which is specified as 1 uA max. I'd expect it to be a bit lower in this circuit though, so they might have not cared about that indeed.As for the "why did it break even though there is protection", my understanding of the experts is that the diode cannot turn on sufficiently fast once it's blasted directly by the ESD; hence the call for that series resistor.Matt

author: SeanB
author_rank: Super Contributor
timestamp: September 25, 2015, 09:52:19 am
content: Series resistor would be the best, so the built in ESD protection will have only to deal with limited energy. as the source of the input is unknown ( probably another P channel device pulling up from ground to 7V of the 2S battery pack) just the resistor and the new FEt would work, it already has the 5M pull down, so the pull up must be pretty weak. Thresholds are not too similar, but it is being operated well into the fully on or fully off range in any case. Any added protection would be an issue, so 100k to 200k of series resistance right by the gate ( leave the 5M resistor across the input) would work fine and only slow down the switching time slightly, which in this case is almost DC.Gate leakage spec would only be approached if you are running it at at the full rated voltage, with the device being at 150C die temperature. Otherwise the leakage is not a worry, it will be under the specified by quite a bit, as the test machine is just looking that leakage is 0, and the resolution at high speed is 0.1uA, so any leakage that flips the LSD is a failure, most are going to be a lot less.

author: mzzj
author_rank: Super Contributor
timestamp: September 25, 2015, 08:11:38 pm
content: Quote from: SeanB on September 25, 2015, 08:53:02 amI have done destructive testing to some mosfet gates and with my limited sampling haven't found any protection diodes unless specified so.For example IR jellybean IRF540(N) gate will start to leak nanoamps at around 60 to 100 volts.

author: mzzj
author_rank: Super Contributor
timestamp: September 26, 2015, 06:19:08 am
content: Quote from: SeanB on September 25, 2015, 08:53:02 amSo I had to do some actual testing:List of mosfet without gate protection zeners:FDP8441 fairchildOnsemi:MTP3055VL75N06MTP52N06VL60N06L50P03NMTP23P06V14N50EMPT10N10EIR:IRFP450G4PC50W (IGBT)ST:IRF740P120NF10List of mosfets WITH gate protection diodes:-found none in my stocks.

author: Marco
author_rank: Super Contributor
timestamp: September 26, 2015, 07:04:49 am
content: Quote from: papo on September 25, 2015, 09:44:17 amThat's probably 7 orders of magnitude out of whack.

author: eneuro
author_rank: Super Contributor
timestamp: September 26, 2015, 01:16:34 pm
content: Quote from: mzzj on September 25, 2015, 08:11:38 pmInstead of killing mosfets, I've used maybe overkill solution, but i like symetric +/-12.5Vgs and it works to protect IRFZ44N gate in one of latest SMD designs-two 12Vz diodes in anti-series between mosfet gate and source seams to work in simualtion and it works also in my latest ZVS driverThat is interesting, could I remove Zener diode from this design-probably not, but maybe I will test this in spare time, but it is a pity to kill my favourite IRFZ44N

author: SeanB
author_rank: Super Contributor
timestamp: September 27, 2015, 12:30:48 pm
content: Adding the extra zeners is fine if your source impedance is low enough that leakage is not a problem. The OP issue is the impedances there are high, the pull down resistor is 5M6, so whatever is driving must provide a very low current, and diode leakage might be just enough to stop it working. Series high value resistor and relying on the ESD diodes built in will probably be enough in this case.