MOSFET battery switch consistently frying - Page 1
Forum section: electronics / general

author: felixer
author_rank: Newbie
timestamp: September 13, 2021, 10:42:39 pm
content: I need to connect a 12S LiPo (50.4V fully charged) to a power bus. This bus has over 6,000uF of capacitance on it and can be at higher voltage than the battery itself. Ideally this would be done as quickly as possible, in the 1-2 mS range. The switch needs to support about 300w steady state.I currently have an Hbridge driver abused into making an isolated 10v for an isolated gate driver.The attached circuit has been my most robust thus far but only stays working when the turn on time is up in the 200+ ms range. When I reduce the gate resistance and capacitor to the shown values it turns on in ~32ms on the bench. Then when connect to the battery and about half the capacitance is still works. But when finally connected to the full level of capacitance it fails instantly (FETs short and stay closed). I'm really confused because the measured inrush current is FAR below the rated amount for the FETs, I have tried P ch, Nch, IGBTs, GaN everything. It does get more robust as the FETs current capacity goes up but nothing has just worked. I have looked for dedicated current controlled drivers but haven't found one that works with a bidirectional switch AND at these voltage levels.I have killed A LOT of silicon trying to get this working and i'm at my wits end, I would really apricate any advice. Maybe I need to be turning the FETs on faster not slower?FET PN: AUIRF7749L2TRDriver PN: ADUM4121-1ARIZ-RLHbridge driver for the isolated 10v: MAX13256ATB+T10v LDO: S-1142BA0I-E6T1U

author: uer166
author_rank: Super Contributor
timestamp: September 13, 2021, 11:04:41 pm
content: 1-2 ms is infinity in terms of blowing up MOSFETs. Most likely you're violating the SOA of the FET since it spends so much time in linear range, you don't have to exceed the peak current rating to blow up a switch with that. E.g. the AUIRF7749L2TR SOA curve says at 10ms it can handle only 1A at 10V Vds. You're running it at 50V, which is so useless it's not even on the curves.You'd do well turning those modern FETsas quickly as possibleat the correct speed. What I would do is: switch the FETs in under 10 microseconds with a real gate driver. Of course that may create > 1kA inrush current, depending on impedances, so you can also add a little bit of dedicated resistance in series to limit inrush to some guaranteed level, probably in the couple hundred amps range. Generally the actual silicon doesn't matter here, your current approach wouldn't work with any FETs since it doesn't take into account the safe operating area ratings.

author: felixer
author_rank: Newbie
timestamp: September 13, 2021, 11:10:26 pm
content: That is what I am thinking. does the ADUM4121-1ARIZ-RL part count as a real gate driver? I think it is going to be easily capable of switching these FETs on within a few microseconds.Are you recommending series resistance with the gate drive or with the battery?Also thank you so much for taking the time to look at this for me, I really appreciate it.

author: uer166
author_rank: Super Contributor
timestamp: September 13, 2021, 11:18:51 pm
content: Quote from: felixer on September 13, 2021, 11:10:26 pmYes and yes, at 2A it's plenty to switch even large FETs in us range. A gate resistor is a good idea to control the speed, you don't want it too fast, especially for turn-off (in case the harness is inductive). You'll need to generate a floating rail as you did before, like with a charge pump or a transofmer based DCDC, but obviously connect to gate driver VCC instead of directly to the gates.

author: f4eru
author_rank: Super Contributor
timestamp: September 15, 2021, 04:53:41 pm
content: Use a pre-charging resistor, with a separate switch, switching on first.This way, the energy is not dissipated in mosfets.Why 1-2ms, and not a bit more ?

author: felixer
author_rank: Newbie
timestamp: September 15, 2021, 07:00:15 pm
content: uer166's suggestion worked, the resistors inline with the load gross me out a little but i'll work on reducing that over time and fully utilizing the FETs limits. Now i'm blowing the 10v power suppliesbut that is a much more straightforward issue.Considered the second switch with a charging resistor but in situations when turn on times are important it doesn't help at all. 1-2ms is the time between the main power source shutting off and the loads browning out. Honestly even that is an optimistic number, faster is better in this case.

author: f4eru
author_rank: Super Contributor
timestamp: September 17, 2021, 06:18:58 am
content: Quote from: felixer on September 15, 2021, 07:00:15 pmOf course it helps. A whole lot.You try to dissipate 0.3 joules into an extremely small mosfet, nearly instantly (probably 30us ?). That is very bad because the chip die is small, and the cooling is too slow, so all the copper around is not effective at cooling it in time before it melts. Just a perfect recipe for disaster, not even accounting for the inductive spikes and all that crap.Dissipate the same energy in 2ms into a 0.2Ohm power resistor instead, which is capable of handling this pulse power...Of course the mosfet should then switch very fast, to avoid the linear region.

author: felixer
author_rank: Newbie
timestamp: September 17, 2021, 01:38:38 pm
content: Yup, much of the inrush managing advice i found online pointed me towards controlling the turn on time of the mosfet rather than reducing the current through other means. But thanks to you guys this is all working-ish now. again, thanks so much.