IGBT driver limitations - where are they from? - Page 1
Forum section: electronics / projects

author: daqq
author_rank: Super Contributor
timestamp: June 22, 2015, 11:48:33 am
content: Hi guys,I'm looking into IGBT drivers, have seen this:http://www.farnell.com/datasheets/1857191.pdfThe datasheet states that:QuoteWhere does this limit come from? The way I understand IGBTs is that from the GATE point of view it's pretty much a MOSFET transistor - a capacitor against all other electrodes. How does the IC rating (or VCE rating) reflect into the driver limitations? I would understand if there was a time limit, gate capacitance limit or some such, but this seems strange.Could someone enlighten me?Thank you,David

author: Psi
author_rank: Super Contributor
timestamp: June 22, 2015, 11:57:38 am
content: I think that maybe just regulatory issues.The device has a list of approvals. "UL1577, CSA, DIN EN 60747-5-2, IEC 60950-1 and 61010-1 Approved"But i suspect it only complies with all those if you use it on a IGBT under 600V and under 150A

author: daqq
author_rank: Super Contributor
timestamp: June 22, 2015, 12:58:08 pm
content: Thanks! That is the last place I'd think to look for such limitations.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 22, 2015, 01:10:47 pm
content: Probably more about the typical gate charge of such a large device. It'll never swing it worth a damn. That's the territory of +/-15V drivers with >8A peak capacity.I don't see why safety standards would be concerned about current, so that should be that side of it.Tim

author: rossmoffett
author_rank: Regular Contributor
timestamp: June 22, 2015, 02:47:00 pm
content: I think that the gate is capacitively coupled to the output and you can blow the driver with inductive kickbacks. With rising voltage in the ultimate load comes rise in voltage kickback. I have some buddies (in my sig) that build very high-power, high voltage Tesla coils using IGBT bricks. You can still use this driver, but you'll need to use an isolation transformer to keep it from blowing up all the time (note: it will still likely blow up some of the time until you perfect your transient surge protection). This is just a ferrite bead wound in a 1:1 fashion with twisted pair cables of about a dozen turns. Steve hasn't updated his website since he was 13, but you can find some description of the method of winding isolation transformers for the gate driversunder the DRSSTC pages. I believe you can also use them in parallel to increase the gate charge current capacity.I haven't ever seen this technique used in commercial devices, but it works really well for the Tesla coils and so would probably translate well to other applications with inductive reactance such as motor control.These days I think they're making their own custom drivers for some applications, because commercially available IGBT drivers aren't powerful enough for the monsters they're building. I'm pretty sure Steve's latest backpack coil is still using ICs though.

author: krish2487
author_rank: Frequent Contributor
timestamp: June 22, 2015, 09:01:57 pm
content: Because typically the gate charge is a function of the physical construction of the IGBTs and the Ic rating.Gate Charge 100A < Gate Charge 150A < Gate Charge 200A.. so onAs teslacoil mentioned, the larger the charge more the rise time and more power dissipated by the transistor acting as high power variable resistor. Lesser the charge, more rapis the transition time and lesser the power dissipation.The Vce rating, I suspect, is a bootstrap voltage rating for driving the high side Power devices. It makes sense tospecify the maximum voltage ratings. Generally IGBTs are available in 600, 1200, 2400V ratings or so.What does this have to do with the specs mentioned??Take the typical IGBT manufacturers, Semikron, Mitsubishi, Fuji they trade off the gate charge vs rise time in the internal construction.The specs for the driver are more of a envelop case for the drive ratings. It can a drive a 150A IGBT with the lowest rise time ( aka largest gate charge) to turn on rapidly (provided the gate emitter voltage swing is large enough and with in maximum limits).It is an interesting question with no correct answer. The IC in question can made to drive a 300A IGBT comfortably with a +15/-12V gate emitter swing, 10nF gate capacitance and still fail driving a 100A with a +8/-5V gate emitter swing and a 10nF gate capacitance.(Not exact figures but a ball park estimate)It all boils down to how best you can make it work for you

author: daqq
author_rank: Super Contributor
timestamp: June 23, 2015, 07:20:12 am
content: Thanks, that makes more sense. Though it's still written pretty confusingly - wouldn't it be simpler to just write:"100ns time for 10nF GATE capacitance (typical for xxx)"

author: daqq
author_rank: Super Contributor
timestamp: June 23, 2015, 09:59:12 am
content: While we're at, what do you think about this one:http://www.farnell.com/datasheets/28818.pdffor driving a 40nF max. gate capacitance IGBT, @2kHz max, 3us pulse duration? I have no experience with Diodes Inc. (or Zetex?), are they reputable, or dubious?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 23, 2015, 02:33:14 pm
content: 40nF equivalent, or 40nF Ciss?Tim

author: krish2487
author_rank: Frequent Contributor
timestamp: June 23, 2015, 05:26:22 pm
content: @daqqThe reason it is not written in the way you mentioned is that the rise time/ fall time of the IGBT is inversely proportional (to a limit) on the current dumped into the gate of the IGBT. The current being dumped is dependant on the output voltage swing. Hence the 100 ns is for a given set of operating conditions. It is possible to reduce that '100 ns' to 20ns or increase it to 200 ns.I am assuming that you are trying to drive a 200 - 300A IGBT (high input capacitance)The IC should work just fine for the 2Khz frequency mentioned.Just ensure that the drive voltage swing is about 75 - 80% of the maximum rating. You can also drive the IGBT below 0V to turn it off faster and properly. It has two fold advantages.1.Driving the IGBT below 0V will sort of avoid/compensate for stray turn on due to miller capacitance.2. The entire + and - voltage swing allows you to utilise the maximum current drive output of the driver thereby turning on the IGBT faster.@teslacoilI am assuming 40nF gate capacitance and not the equivalent.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 23, 2015, 06:51:32 pm
content: So, Ciss?Beware of capacitance equivalents. The small-signal equivalent capacitance at 0V (usually where it's measured) isn't representative of the total input required to swing the gate. For that, you want to look at gate charge, Qg(tot). The equivalent switching capacitance is C = Qg(tot)/(Vg(on) - Vg(off)). Because of nonlinear capacitance, and Miller effect, the charge equivalent is often several times the small signal value.Tim

author: krish2487
author_rank: Frequent Contributor
timestamp: June 24, 2015, 06:41:15 am
content: @teslacoilMy apologies,The gate capacitance specified is Cies or equivalent capacitance.

author: daqq
author_rank: Super Contributor
timestamp: June 24, 2015, 07:40:17 am
content: Thanks guys, sorry for the late reply.QuoteSlightly more than that, but yeah.QuoteTo avoid a negative voltage line, would it be reasonable to drive the E and G electrodes "differentially"? An H bridge like configuration? Since the driver is isolated anyway, it should work. But then there's the problem of timing it all properly...Teslacoil: Thanks!I'm going to study this further - I've just started looking into this really. I was hoping that it would be a simple matter of taking an off the shelf IC, slapping in some support components and being done with it with nice parameters... apparently not

author: krish2487
author_rank: Frequent Contributor
timestamp: June 24, 2015, 01:35:32 pm
content: @daqqAhhh!!!A fuji electric 600A / 600V Two pack IGBT..:-)Then chuck all the above ICs and use a proper IGBT driver. Mitsubishi and Avago have a really good line up.Google for either M57962- mitsubishi or ACPL 316 or ACPL 332 from avago.The Mitsubishi part is rather outdated. Try the ACPL332J - It has miller compensation built in.Or if you are still looking for something COTS then have a look at Semikron SKYPER driver boards. Literally a drop-in solution.

author: daqq
author_rank: Super Contributor
timestamp: June 25, 2015, 10:56:44 am
content: QuoteClose:http://www.farnell.com/datasheets/1926728.pdfI've looked into those ICs - they seem nice! Their delays are a bit of an anoyance and the current limits are not very high though. I'm also trying to make a discreet version of a driver device. I've started a new thread on it actually.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 25, 2015, 02:07:02 pm
content: Quote from: daqq on June 24, 2015, 07:40:17 amYes, I've seen it done before.But, now your emitter "ground reference" is moving, so your collector desat threshold is also moving.When I saw it done, they also were not using desat detectors...Yes, they spent a lot on replacement transistors, and yes even during servicing (in one case, a serviceman had forgotten to bolt down one connection... the cable was instead laying gently across a pair of terminals... it wasn't pretty when they hit 'START').Protip: desat detectors work for saving MOSFETs, too. They're worth it.QuoteSure --If you have a standard IGBT package, you can usually find a complete PCBA that solders down onto it. Not cheap, but ready to go, with isolated logic level inputs. (DC-DC isolators may still be needed.)There are wonders like these,http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_integrated_gate_drive_optocoupler/acpl-339j-000e/a little pricey but available, and doesn't need too much external junk. It provides drive for an external complementary pair, which means you can make it as big or small as you need. (BTW, don't go crazy -- the transistors only need Rds(on) as low as total gate resistance requires.)Or you can pair a mode modest gate drive - desat - UVLO (it needs to be user-set UVLO though) chip, followed by a driver likehttp://www.digikey.com/product-detail/en/IXDD614CI/CLA356-ND/2424696which has the voltage and current range needed.I found common offerings with UVLO to be unappealing, so I've also done it with a dual (bidirectional) isolator (opto, or magnetic e.g. ADUM2101), logic level regulator, one of those drivers, and a dual comparator (LM393) to handle UVLO and desat functions manually (i.e., "with a bunch of extra stuff", though it's just small R, C, D's).Tim