short circuit protection of IGBT? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 11:21:30 am
content: So I read if a IGBT has a short circuit current and even if it manages to turn off, you can still have failures.Assuming you are running something like 300A igbt's, is there some other component you can put ahead of the H-bridge to break the circuit fast enough without this problem?It would have to work within a microsecond or so of the detection. The idea would not be to turn the h bridge off but to cut off power to the H-bridge.I thought maybe some kind of SCR or whatever other weirdness there is that would not be appropriate for power conversion (it would not have to work frequently). And maybe something on the bus bar connected to ground that would short it out while its in the middle of turning off?

author: max_torque
author_rank: Super Contributor
timestamp: December 28, 2018, 12:11:30 pm
content: google "desaturation detection"

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 12:26:06 pm
content: Even the short-circuit current is successfully turned-off,short-circuit failure could still happen after severalmicroseconds, which is called delayed failure in . It isverified by experiments and numerical simulation that largeleakage current leads to the thermal runaway . A “criticalenergy (EC)” is proposed to explain the mechanisms ofcatastrophic failure and wear out failure under repetitive short-circuit operations . When short-circuit energy is belowEC,IGBT may survive for more than 10^4 times repetitive short-circuit operations before failed. However, when short-circuit energy is far beyond EC, IGBT may fail after first short-circuit due to thermal runawaythats with the detector I am pretty surewon't cutting power be better then trying to power it down while its over loading? thats what i think. I get the feeling messing with the gate is a bad idea if its really bogged down? it says something about 'local overheating' too.

author: jbb
author_rank: Super Contributor
timestamp: December 28, 2018, 06:37:25 pm
content: And what kind of switch would you use for removing power from the H Bridge?

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 28, 2018, 07:07:23 pm
content: Forget it, the least expensive way to protect an igbt is to let the igbt break, that is besides designing a system with enough tolerances.If there was a switch fast enough to cut the supply it would cost 10x the igbt and add additional inductance to the whole setup, besides weight and size. So in power switching no one does that.

author: mk_
author_rank: Regular Contributor
timestamp: December 28, 2018, 07:34:18 pm
content: Quote from: coppercone2 on December 28, 2018, 11:21:30 amIt is a solvable problem to measure current, compare the measured voltage against a reference and shut down the Gate within 150ns. It needs some more time to shut down the IGBT so a hard short without any inductivity isn`t covered with this systemWe do this regulary with 50 - 300A and well... it`s working well enough for us. Of course we have some µH in the wiring which helps a lot.

author: David Hess
author_rank: Super Contributor
timestamp: December 28, 2018, 07:39:49 pm
content: I do not remember where now but I saw IGBT short circuit protection implemented using a low voltage high current power MOSFET in series with the emitter of the IGBT. Under normal conditions, the IGBT gate controlled switching but under fault conditions, the gate of the power MOSFET was clamped low very quickly disconnecting the emitter of the IGBT and driving the gate voltage of the IGBT negative.

author: Benta
author_rank: Super Contributor
timestamp: December 28, 2018, 07:43:04 pm
content: Most high power IGBTs are specified to withstand a few microseconds short circuit current, which should leave you time to turn it off. Apart from that, adding a small inductance in either the emitter or collector will limit di/dt, which will give you extra time.

author: duak
author_rank: Super Contributor
timestamp: December 28, 2018, 08:20:57 pm
content: Fuji Electric uses temperature and current trips to protect the IGBTs in some of their IPMs:https://www.fujielectric.com/products/semiconductor/model/igbt/.../REH983a.pdfsee fig. 3-2. Making your own semiconductors allows you to add temperature sensors and an emitter connection just for current sensing right on the die. Without the affordances of bespoke dice, (Pardon me, I just had to use these words), it's harder to do with off the shelf devices.I'd think about a simple current limit plus a short term energy limit based on the product of the voltage across the IGBT and the current thru it. Basically this would prevent the IGBT from being operated outside of its SOA on a cycle by cycle basis. Analog Devices makes a fast multiplier but it's way to expensive for something like this. It should be possible to come up with a simple multiplier good enough to do this.Cheers,

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 08:21:48 pm
content: Quote from: blueskull on December 28, 2018, 07:44:51 pmWhich one of these does the "energy shock" described by the paper I linked fit into?It lists like 6. on page 5/8https://www.tf.uni-kiel.de/etit/LEA-download/publications/2013/Catastrophic_Failure_and_Fault_Tolerant_Design_of_IGBT_Power_Electronic_Converters_An_Overview.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 08:26:57 pm
content: "thermal runaway after successful turned off"will this be prevented if you use aridiculous non cost effectivedevice to block the current flow rather then messing with the gate if it was about to happen?say its connected to the IGBT right up against it with the least amount of inductance possible.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 08:37:29 pm
content: Quote from: David Hess on December 28, 2018, 07:39:49 pmbased on your understanding does this like specifically target one of the failure modes described by the picture?

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 09:03:35 pm
content: Quote from: blueskull on December 28, 2018, 08:50:25 pmso energy pulse is completely synonymous with the definition of energy shock used in the paper if I wanted to do further research?the paper does not seem to put energy shock into the same category as over voltage.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 09:13:55 pm
content: so it kinda sounds like, if this FBSOA breakdown is going to happen, it means the gate is no longer in true control of the current going through the circuit, and some kind of external device is required to block the current.How does the 1uS time constant work described in the paper as the time after device turn off and device breaking work in terms of FBSOA?so if t=0 at turnoff, and you literary teleport the device out of the circuit, does it still break or are they saying its not really turned off just the gate is 0V and it still draws current? its not like internally stored energy in the silicon that goes and focuses in the wrong places or something like that right, it shows a impedance if you had a meter on it proportional to how much energy it would need to consume in 1uS to break itself? or if you had a ammeter on the power rail it would still draw current despite its definition of being turned off?am i being confused by latch up?

author: max_torque
author_rank: Super Contributor
timestamp: December 28, 2018, 11:03:16 pm
content: An important question is really "what actually am i trying to protect against"??Most high power switching systems are optimised to run under a defined condition, and generally, they accept (for non life critical applications) that some failure modes do exist that could result in mortality of the switching element. The question then becomes "What is the probability of those failure modes occuring" and "what happens if the switching element does fail".This is why, you never really see any separate switching element used to "protect" the primary switch itself....

author: David Hess
author_rank: Super Contributor
timestamp: December 29, 2018, 12:41:59 am
content: Quote from: coppercone2 on December 28, 2018, 08:37:29 pmMy recollection is that it was done to provide the fastest possible turn off under overload conditions; it could be considerably faster than driving the gate of the IGBT because of the lack of Miller capacitance. I think it was implemented by monitoring some function of current, time, and collector voltage so it would help in all cases except excessive collector voltage; maybe it only monitored current and time.It could not have been a safety feature because failure of the IGBT would destroy the power MOSFET anyway. What I do not know is why the power MOSFET was not driven as the switching device instead of the IGBT; maybe it was added to improve the reliability of an existing design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2018, 01:38:35 am
content: BTDT, measured the transient waveform myself. 'Twas a ramp up to about 4500A, a few microseconds long.Desat protection works.In 2.5 years of designing induction heaters (of tabletop and floor-standing size), I only blew up two handfuls worth of transistors -- two only because one hand is needed to hold the one 600A module that died.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 03:23:00 am
content: i still don't understand why it fails 1uS later??is it not turning off impedance wise or is something happening inside.does it overload and get messed up, turn off on the gate, but still conduct enough energy in some kind of area inside of it thats irregular to break in around 1uS or is it more complicated and weird? if you put a ammeter on the power supply for it would it draw current during this 1uS period?to help define what energy shock is.

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 03:48:43 am
content: ok so its not some bizarre shit with stored energy some how.but there is already some internal damage if that happen at t=0 of that 1uS? even if you magically got rid of all current flow. once whatever preconditions happen before t=0 its fucked?before t=0 of energy shock, there is a punch through (damage layers)?

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 03:54:37 am
content: I don't understand because it says it will fail due to thermal runaway, not that there is punch through damage. when talking about energy shock.does punch through cause migration? i thought it was a transient state.PunchthroughWhen the base–collector voltage reaches a certain (device-specific) value, the base–collector depletion region boundary meets the base–emitter depletion region boundary. When in this state the transistor effectively has no base. The device thus loses all gain when in this state.why is it already dead then? punch through doesn ot sound fatal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2018, 04:40:39 am
content: If it's not due to latch-up (device continues carrying fault current despite Vge << Vge(th)), then it's due to thermal runaway alone (for all the various mechanisms that have been mentioned).Typically the critical time frame is ~10us, so you want to be much faster than that in control response. It is somewhat crazy for a thermal phenomenon to be quite so fast, but we are talking hundreds or thousands of amperes, and hundreds of volts, across a die much smaller than a typical MOSFET, say. There's a metric shitload of heat (10s, 100s kW) across not much silicon here!The desat detector I designed for the industrial projects, responded in about two microseconds, the slowness being intentional to avoid nuisance faults under hard switching.The same protection strategy can be used on MOSFETs of course, but the expected voltage drop is usually higher (I set it at 50V for the industrial system), and the allowable fault duration is usually longer (less short-circuit current density, much bigger die).SuperJunction MOSFETs weren't around back then; now that they're mainstream, I wonder what the allowable fault duration is. They don't seem to be rating them [yet?] so it must not be too critical (over 20us? over 100us?). They're much smaller dies than the previous generation, in high voltage ratings (I used 800 to 1200V FETs there), so the fault duration should be correspondingly shorter.Interestingly, SJMOS also has an intrinsic current limiting characteristic, typically experiencing current saturation for any Vgs > 7V or so. (This is true of ST's MDmesh II at least.) This is comparable to small-die IGBTs offering modest current capacities. Large-die IGBTs do seem to offer considerable short-circuit current capacity though, so beware.Example: compare a 30A 1200V, TO-3P packaged IGBT being good for maybe 100A short circuit, to a 300A module being capable of >3kA short circuit.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2018, 05:01:00 am
content: Here's an example:https://www.seventransistorlabs.com/Images/STP19NM50N%20Drain%20Output%20Curves.pngThe curves for Vgs > 7V are pretty much identical so I didn't bother to measure them.I haven't played with SiC JFETs so that's good to know. Apparently there are two-ish styles of MOSFETs: one make has a small tempco, the other has a tempco massive enough to trigger thermal runaway (in switching applications). I forget which is which (Cree/Infineon/etc.)...Tim

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 05:39:43 am
content: i am confused because blueskull says its damaged by the time this happens (what mechanism) while teslacoil is saying that there is something like 10uS to respond.do you have enough thermal energy built up in there by the time its considered off that it just breaks after? why is it not broken then, are the atoms vibrating loose ?does anyone have a GIF of what is happening in the transistor with fields, carriers and heat? when it breaks by energy shock. and whatever actually breaks (i guess it cracks or expands or oxidizes or diffuses or something else?) like the motion of the structure as its its being destroyed

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 06:04:01 am
content: i cant find it free\but with what tesla says, if there is a fault current thats very high, can you run a current limited SCR to shoot the rail down so the IGBT gets something manageable?maybe not all the way but at least take the edge off.

author: BravoV
author_rank: Super Contributor
timestamp: December 29, 2018, 07:08:29 am
content: Quote from: coppercone2 on December 29, 2018, 06:04:01 amNot my stuff, as its way too academic for this noob hobbyst, a sneak peek.

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 07:10:10 am
content: do you just put a rogawski coil right before the h-bridge and trigger a SCR with it?there are hocky puck scr that look like they can work.what would you use as a current limiter? do they have little carbon pucks or something?would a mov be a better choice off the shelf? it has some resistance.

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 07:11:25 am
content: Quote from: BravoV on December 29, 2018, 07:08:29 amthat graph actually makes me want to buy it

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 07:17:07 am
content: i used those chips before but it cant hurt to have secondary protectionthe iso5500 has it built ini think it would be cool to experiment with the scr. and it seems a project that could justify from a cheaper inaccurate pcb rogawski coil

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2018, 08:03:17 am
content: I wouldn't put a crowbar on there. The fault currents available from large capacitors (film or electrolytic, as they happen to be) plus the mains (if we're talking industrial 480 here) is ridiculous. You'll spend as much on the crowbar as the inverter.Parts are dying either way. Replacing a fuse plus SCR plus IGBTs isn't any better than replacing just the fuse and IGBTs.Better to prevent failure from happening in the first place. Monitor operating conditions, use redundant power stages or switches (series connected..?) if necessary, respond to faults quickly.That's good enough for industry. Heh. Well, maybe I shouldn't reference industry here, the old designs I saw were discrete logic and no inverter protection whatsoever...You may find it more interesting to look up what they do for managing power on board the ISS. It's a public project, freely accessible (if a bit hard to find your way into the design resources). Many kW of solar power, distributed safely through vacuum as well as interior spaces.Tim

author: Zero999
author_rank: Super Contributor
timestamp: December 29, 2018, 02:58:22 pm
content: Quote from: David Hess on December 28, 2018, 07:39:49 pmThat's a good idea. The MOSFET can also double as a current sense resistor.

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 03:30:50 pm
content: the ISS uses shunt regulation it seems.