What's blowing my FETs - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: August 29, 2016, 04:27:32 pm
content: I have a "battery monitor" module which uses a pass FET to control overcurrent situations at around 1.5A. The schematic is attached.The module inserts itself of the return path (V-) and tests the current on a 0.1R current sense resistor, and if needed closes the MOSFET. V+ is 20V-25V and there is a LM7805 regulator to feed the MCU (and Arduino Pro Mini).The MCU can switch the MOSFET on and off by applying 5V to the Gate (on) or 0V (off).I have made 10 such modules and so far 3 of them had blown FETs. I do not know when the FETs blew. When they go they become closed circuit between Drain and Source, so the circuit always appear to be working even though the pass MOSFET is busted.The part number is NDPL070N10B if that is helpful.

author: klr5205
author_rank: Regular Contributor
timestamp: August 29, 2016, 04:48:07 pm
content: Check to see whether 5V is enough to turn that FET on all the way. I suspect that it might not be... Does the FET get hot during normal operation? If it's fully on, it shouldn't.Two other things, either of which may also contribute:Gate resistor is pretty high, resulting in slow turn-on/turn-off of the FETNo pullup/pulldown resistor to default the FET into cutoff or saturation.Also, there is no reverse polarity protection so if the supply was hooked up backwards it will probably blow the body diode.

author: step_s
author_rank: Regular Contributor
timestamp: August 29, 2016, 05:21:38 pm
content: Seems like a little overkill to use a 100V, 70A mosfet to drive a protection circuit for 25V, 1.5A?Also, as other state, the plateau voltage of the gate is 6V, meaning that anything below will not really charge the gate fully. Also charging a 15nC gate through a 100K resistor, is going to make the MOSFET open veeeery slowly, and combined with the insufficient voltage, it's not going to open fully.Have you tried some other MOSFET?

author: Edsoncan
author_rank: Newbie
timestamp: August 29, 2016, 05:50:47 pm
content: I am not sure, most of the time I am wrong but the Mosfet is not logical:1) The Gate Threshold voltage is 10V and you use 5 Volts;2) The maximum Gate to Source Voltage is 20 Volts and is your circuit you have a 24 Volts.3) The ID is 1mA and you use 5V/100K.http://www.onsemi.com/pub_link/Collateral/NDPL070N10B-D.PDF

author: akis
author_rank: Frequent Contributor
timestamp: August 29, 2016, 06:48:41 pm
content: Thanks guysIn the beginning I was using the PSMN017-80PS which has a <5V threshold voltage. I ran out and started using the NDPL070N10B which has lower Rdson but did not read other important parameters! So it may well be running hot with 10V-15V gate threshold.In order to make minimal changes, I will reduce the gate resistor to 10K and go back to using the PSMN017-80PS which works at 5V.

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 29, 2016, 06:59:02 pm
content: Why would you use even a 10k gate resistor? They are used mainly to reduce ringing on the gate, I would consider a 100 ohm resistor "big"?

author: akis
author_rank: Frequent Contributor
timestamp: August 29, 2016, 08:03:06 pm
content: Quote from: Ice-Tea on August 29, 2016, 06:59:02 pmTo limit inrush currents as the Arduino output pins have limits (touted to be 50mA per pin but why risk it). I think with a 10K it will be OK. The FET is always on and on exceptional conditions it switches off, if ever, there is no hurry, the currents are small (comparatively). The most likely candidates for them burning are (a) getting hot because not fully open (b) weird capacitive coupling between drain and gate bringing the gate to over 20V which is the max.Quote from: AcHmed99 on August 29, 2016, 07:23:44 pmThere is a connector to the top left, GND is battery V-, V+ is battery and load V+, and V- RTRN is the return from the load under control (so I can sense the current).

author: Ice-Tea
author_rank: Super Contributor
timestamp: August 29, 2016, 08:14:57 pm
content: Quote from: akis on August 29, 2016, 08:03:06 pmYour funeral, but: like this, it will take the FET several ms, probably tens to turn off and on. If you plot 25V, 1.5A and tens of ms on the SOA graph, you may be in for a surprise.

author: void_error
author_rank: Frequent Contributor
timestamp: August 29, 2016, 10:21:19 pm
content: Quote from: akis on August 29, 2016, 08:03:06 pmA 1k load can be easily handled by pretty much any micro. Or, alternatively, you can use a totem-pole push-pull output stage between the micro and the MOSFET and reduce the value of the gate resistor even more but in your case switching speed doesn't matter that much unless you don't want your MOSFET spending too much time in its linear region when switching.

author: step_s
author_rank: Regular Contributor
timestamp: August 30, 2016, 01:32:06 am
content: 5V / 0.05A = 100ohms. Anything below would be "too much", but the outputs are likely to be protected from drawing too much current anyway. But just in case, the resistor is a good ideaPSMN017-80PS is better, but I must admit it's still not suited for the job. Its 80V 50A, for a 25V 1.8A circuit. The gate plateau is just barely under 5V, which is cutting it very close, and you will most likely not even have 5V at the output pin when drawing current.Any MOSFET rated at 30V would be more appropriate.How have the models with PSMN017-80PS worked out so far? Have they had the same problems with blowing up, or was it just the others?

author: akis
author_rank: Frequent Contributor
timestamp: August 30, 2016, 07:08:35 am
content: Quote from: step_s on August 30, 2016, 01:32:06 amNone of the PSMN017-80PS modules has had issues, it's just the NDPL070N10B . I am looking at your suggestion to go for lower gate threshold transistors, and I have chosen this PSMN4R3-30PL with a 1K resistor.

author: bson
author_rank: Unknown
timestamp: August 30, 2016, 07:48:58 pm
content: Are we looking at the same schematic here? I see 100k on G not 10k.Either way, I'd make that 100R and use the 100k for a ground pull-down. This way if the Arduino output goes floating (e.g. if it goes to sleep, or the software crashes after scribbling port control registers) the FET gate will be bled and it will shut off. Makes for a nicer failure mode than letting it build up charge until it turns on, or stays on.

author: rx8pilot
author_rank: Super Contributor
timestamp: August 30, 2016, 08:02:04 pm
content: Quote from: Ice-Tea on August 29, 2016, 08:14:57 pmYes, very slowly transiting the linear region of the FET will heat it up and then it is remaining in the linear region at 5V Vgs. Use the smallest resistor you can to limit the uC pin current capability. 10k slows the FET down.You should consider a logic level FET where the Vgs(th) is low and you can get reasonable Rds(on) at only 5v. There are many that can handle this. They key is to switch as fast as you possibly can to keep the FET out of its linear region where heat can destroy it before you can blink.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 31, 2016, 04:09:59 pm
content: Quote from: rx8pilot on August 30, 2016, 08:02:04 pm... OTOH, at the same time, limiting the switching speeda bitto help reduce parasitic LC ringing.In practice, MCU pin is rather limiting already (it's not a proper FET driver capable of 2A peak after all!) - so the resistor is there to mostly prevent the gate from ringing. Any value between about 22 ohms and 220 ohms would probably do this job just fine, without slowing the transition down too much.There is no way this could work with a 100k gate series resistor. It's a completely insane value. 100k should be there as a pulldown because the MCU doesn't start up with the pin as output; it's floating.

author: rx8pilot
author_rank: Super Contributor
timestamp: August 31, 2016, 04:51:55 pm
content: Quote from: Siwastaja on August 31, 2016, 04:09:59 pmTrue although as you pointed out, this system will never switch so fast to make ringing a problem even if there was a significant inductance. I do have some DC switch circuits that I have some rather slow switching to cope with inrush current. It requires careful selection of the FET and understanding of the Safe Operating Area (SOA).Aside from the giant gate resistor, I would be most worried that the FET chosen would never be fully on at 5v Vgs and get very hot.

author: akis
author_rank: Frequent Contributor
timestamp: September 01, 2016, 05:58:02 pm
content: As a summary1) inrush currents (charging a 2.2mF cap) only limiting factor is the 0.1R sense resistor and the wiring. The current would be tens of amps, in the first ms at least. say 50A avg. (100A at 0.1ms and 12A at 1ms - I could use the scope to verify I suppose).2) low threshold voltage keeping the FET hot and eventually burning it3) weird capacitive charges exceeding the +/-20V and destroying the gate (no safety zener on the gate)Because none of the PSMN017-80PS got destroyed, and it was only the NDPL070N10B that suffered, I suppose it must be (2) that's caused the issues.

author: tatus1969
author_rank: Super Contributor
timestamp: September 01, 2016, 07:08:45 pm
content: that is most probably thermal damage during the switch (opening) process. I had posted some thoughts about this in a different thread:https://www.eevblog.com/forum/projects/relay-latching-and-getting-stuck/msg994307/#msg994307and below. That will also answer why some of your transistors blow up and some don't. I bet that they have bigger die sizes.