# IGBTS vs MOSFETS



## Georgia Tech (Dec 5, 2008)

I understand how to drive a MOSFET but I don't have experience with IGBT driver circuits. MOSFETs in the switching application want to be turned on hard and turned off hard. How does that differ from IGBT's? Also looking at the data sheets on some of these IGBTs their Current drive capability seems awfully high! Has anyone had any experience using IGBTS in really high current switching applications? I have always found if it’s too good to be true it usually is...


----------



## Tesseract (Sep 27, 2008)

Georgia Tech said:


> I understand how to drive a MOSFET but I don't have experience with IGBT driver circuits. MOSFETs in the switching application want to be turned on hard and turned off hard. How does that differ from IGBT's? Also looking at the data sheets on some of these IGBTs their Current drive capability seems awfully high! Has anyone had any experience using IGBTS in really high current switching applications? I have always found if it’s too good to be true it usually is...


IGBTs are basically MOSFETs driving a PNP transistor, so you drive an IGBT just like a MOSFET. The big difference is that the intrinsic PNP bipolar in the IGBT gives it a much higher current density compared to a MOSFET with the same die size. This is because bipolar transistors use both minority and majority carriers. Thus, allowable current density in an IGBT is a lot higher than in a MOSFET (that, anyway, is not too good to be true). The voltage drop remains relatively constant across a wide range of currents, just like a bipolar, and so the conduction loss can actually beat a similarly priced MOSFET.

The downside (the catch, if you will) is that just like a pure bipolar transistor, the intrinsic PNP in an IGBT is *slow* to turn off. Practically speaking, the highest frequency you can hard-switch an IGBT is 20kHz max, and some of the highest power IGBTs are only good for *1kHz*.

FWIW, the Zilla controllers use IGBTs; everyone else uses MOSFETs.


----------



## Georgia Tech (Dec 5, 2008)

Tesseract said:


> IGBTs are basically MOSFETs driving a PNP transistor, so you drive an IGBT just like a MOSFET. The big difference is that the intrinsic PNP bipolar in the IGBT gives it a much higher current density compared to a MOSFET with the same die size. This is because bipolar transistors use both minority and majority carriers. Thus, allowable current density in an IGBT is a lot higher than in a MOSFET (that, anyway, is not too good to be true). The voltage drop remains relatively constant across a wide range of currents, just like a bipolar, and so the conduction loss can actually beat a similarly priced MOSFET.
> 
> The downside (the catch, if you will) is that just like a pure bipolar transistor, the intrinsic PNP in an IGBT is *slow* to turn off. Practically speaking, the highest frequency you can hard-switch an IGBT is 20kHz max, and some of the highest power IGBTs are only good for *1kHz*.
> 
> ...


----------



## Tesseract (Sep 27, 2008)

Georgia Tech said:


> What frequency does the zilla run at?
> I have this Massive 2500 amp IGBT that I bought for 100 bucks and thinking of using it. There has to be some drawbacks to this big module as opposed to paralleling a bunch of MOSFETs. But as you said the drawback is low turn off speed. But I guess 500 Hz of switching speed is not that big a issue, or is it?


The Zilla's switch at 18kHz, IIRC. 

500Hz is a perfectly fine switching frequency as long as you keep in mind two things: one, the motor will likely sing loudly at so low a frequency; and two, the current flowing through most motors for this application can hit their current limit in a single pulse's on-time. This depends on the motor inductance and battery pack voltage.


----------



## Georgia Tech (Dec 5, 2008)

How is the Zilla able to switch at such a high frequency?


----------



## Tesseract (Sep 27, 2008)

The latest generation IGBTs are quite capable of hard-switching at 20kHz up to about the 100A per die level. Above that current level speed progressively slows due to the usual culprits in bipolar transistor construction (current crowding, minority carrier transit time, etc...). 

I'm not aware of any 2500A IGBTs rated for less than 1.5kV so yours might have a very high Vce[sat]... 4-6V would be usual for IGBTs in this voltage class. Post a picture of it or at least the part number and I can tell for sure.


----------



## Georgia Tech (Dec 5, 2008)

Tesseract said:


> The latest generation IGBTs are quite capable of hard-switching at 20kHz up to about the 100A per die level. Above that current level speed progressively slows due to the usual culprits in bipolar transistor construction (current crowding, minority carrier transit time, etc...).
> 
> I'm not aware of any 2500A IGBTs rated for less than 1.5kV so yours might have a very high Vce[sat]... 4-6V would be usual for IGBTs in this voltage class. Post a picture of it or at least the part number and I can tell for sure.


I bought this thing off EBAY. The part number is FZ2400R17KF6C B2 if that helps. I have the Data sheet. The thing comes in the huge module about 4.5 inches by 3.5 inches or some wheres about there.

Again I have the Data sheet and can read any spec off.



Let me ask you this, Do I have to supply some sort of negetive voltage to switch this thing? Is it as simple as +15 Vge to about +1>Vge that seems all too easy...
thanks for you help


----------



## Tesseract (Sep 27, 2008)

Yep, much as I suspected this is a 1.7kV IGBT. The Vce[sat] is impressively low: only ~3V at 2400A, which is equivalent to a 1.25 milliohm on resistance.

Yes, you either need to apply negative voltage to the gate during the off time OR actively clamp the gate to the source. The Miller capacitance will couple transients at the Collector back to the Gate which can lead to spurious turn on. I use +15V/-5V but +/-15V is fine.

The gate charge for this device is 29uC which is specified at 15V. If you divide the gate charge by the transition time you get amps. A graph in the datasheet implies that 0.5 ohms is the lowest recommended gate resistor and if you use that with a 15V supply you can theoretically turn this thing on (or off) in just under 1uS. Storage and delay time are substantially longer than this so 1uS seems to be unrealistically fast. Also, with 0.5 ohms you will need to be able to supply 30A peak, no mean feat. I'd go with 1 to 1.2 ohm and drive it with a IXYS IXDD430 or 414 gate driver IC.

Practically speaking, this means 2.5kHz is your maximum switching frequency. 2kHz would give you a little more leeway.


----------



## Georgia Tech (Dec 5, 2008)

Tesseract said:


> Yep, much as I suspected this is a 1.7kV IGBT. The Vce[sat] is impressively low: only ~3V at 2400A, which is equivalent to a 1.25 milliohm on resistance.


Excuse my ignorance but is that a good thing or a bad thing? Would I be better off chucking this thing and buy $500.00 worth of MOSFETs and elaborately placing them on a slab of Aluminum? Or is this IGBT worth using?


Tesseract said:


> Yes, you either need to apply negative voltage to the gate during the off time OR actively clamp the gate to the source. The Miller capacitance will couple transients at the Collector back to the Gate which can lead to spurious turn on. I use +15V/-5V but +/-15V is fine.
> 
> The gate charge for this device is 29uC which is specified at 15V. If you divide the gate charge by the transition time you get amps. A graph in the datasheet implies that 0.5 ohms is the lowest recommended gate resistor and if you use that with a 15V supply you can theoretically turn this thing on (or off) in just under 1uS. Storage and delay time are substantially longer than this so 1uS seems to be unrealistically fast. Also, with 0.5 ohms you will need to be able to supply 30A peak, no mean feat. I'd go with 1 to 1.2 ohm and drive it with a IXYS IXDD430 or 414 gate driver IC.
> 
> Practically speaking, this means 2.5kHz is your maximum switching frequency. 2kHz would give you a little more leeway.


So I can put my pull down side of my totum pole on the Source and the pull up side to a 15V supply and I will be fine? Is it still better to generate a negative supply and drive it below 0V ie Vgs < -1V

Question these devices seem to be a no brainer, why are controllers still useing FETs and not these boogers?

thanks for a ton of help..


----------



## Tesseract (Sep 27, 2008)

Georgia Tech said:


> Excuse my ignorance but is that a good thing or a bad thing? Would I be better off chucking this thing and buy $500.00 worth of MOSFETs and elaborately placing them on a slab of Aluminum? Or is this IGBT worth using?


Well, let's see: does a 3V drop with 2400A flowing sound good to you? Granted, that's probably still a 2V drop at 600A, and, yes, you could do better with a lower-voltage, newer generation IGBT, but not for $100.




> So I can put my pull down side of my totum pole on the Source and the pull up side to a 15V supply and I will be fine? Is it still better to generate a negative supply and drive it below 0V ie Vgs < -1V


I wouldn't count on that, no. You are strongly suggested to use negative voltage to hold the gate off. 



> Question these devices seem to be a no brainer, why are controllers still useing FETs and not these boogers?
> 
> thanks for a ton of help..


I wouldn't say IGBTs are superior to MOSFETs in all respects. Above 150V and 100A I think they are, but mainly I advocate DIYers use modules, whether MOSFET or IGBT - like you have, only not so big - rather than trying to parallel lots of smaller pc board mount components to achieve a realistic current rating. Even paralleling "easy to parallel" MOSFETs is tough unless you are content to switch them very slowly (like in a Curtis).


----------



## Georgia Tech (Dec 5, 2008)

Tesseract said:


> Well, let's see: does a 3V drop with 2400A flowing sound good to you? Granted, that's probably still a 2V drop at 600A, and, yes, you could do better with a lower-voltage, newer generation IGBT, but not for $100.
> 
> 
> 
> ...


well, yeah now that you mention it that is a ton of heat!!! yeah, I don't know maybe I need to look at some fancy way of mounting a bunch of MOSFETS on a sheet of Aluminum. I think mouser or DigiKey has a package MOSFET that is in a very user friendly package with screws...


----------



## engineer_Bill (Jun 24, 2008)

That is a massive IGBT. Do you plan on building a racer?


----------



## Georgia Tech (Dec 5, 2008)

engineer_Bill said:


> That is a massive IGBT. Do you plan on building a racer?


Might be trying to sale the durn thing wanna make an offer?


----------



## ga2500ev (Apr 20, 2008)

I love these threads. Every time one comes up I learn something new.
Tesseract suggested holding the gate to a negative voltage. When turn off the gate does it have to be driven at the negative voltage and high amerage to shift the gate charge? Grounding a gate at 8-10A is one thing. Driving it to -5V volts at that amperage is something else. Is it possible to have two switches, where the first drives the gate to ground at high amperage, then the second holds the gate at a negative voltage at a much more reasonable drive current?

If not then how does one generate -5V at 10A from a main pack?

One final question: how much should the IGBT module be derated for safe operation? Voltage isn't a problem because most of the common IGBT's can handle upwards of 1200V and we're only talking about switching a pack less than 170V. But if the IGBT is rated for 600A, what is a reasonable amount of current to expect to run through it continuously? Half? Two thirds?

I find that the last question is critical because as Tesseract has pointed out, paralleling IGBT's is no easy task. So you only get to choose one module for your controller.

Any suggestions welcome. 

Finally Tesseract, I just wanted to say (possibly again), that I really appreciate your contributions in this discussion. Every time you post, I learn something useful. From your discussions I actually have a reasonable understanding of the drive requirements for a IGBT module. Thanks.

ga2500ev


----------



## Qer (May 7, 2008)

Hah! Questions I can answer! Whoppie! 



ga2500ev said:


> Grounding a gate at 8-10A is one thing. Driving it to -5V volts at that amperage is something else.


Well, it's not as bad as it sounds, really. Let's pretend we have a controller that runs at 10 kHz (which is 100 us pulse cycle) and the turn off is 5 us long, that means that 10 Ampere during 5 us is merely 1/20'th of that, or 0.5 Ampere, in average. The -5 Volt need a reasonably big cap to even out the current though.



ga2500ev said:


> Is it possible to have two switches, where the first drives the gate to ground at high amperage, then the second holds the gate at a negative voltage at a much more reasonable drive current?


Possible, yes. Practical, I doubt it. Depending on how you do your driver you might end up shorting the -5 Volt with GND unless you keep your tongue in the right mouth...

But yes, it's possible to do it that way. Although it sounds overly complicated, which is always an opening for screw ups and you really don't wanna screw up with these currents.



ga2500ev said:


> If not then how does one generate -5V at 10A from a main pack?


Well, since you need less than 1 Ampere there's loads of DC/DC converters that will do the job for you without the dealer robbing you in the process. No prob.



ga2500ev said:


> But if the IGBT is rated for 600A, what is a reasonable amount of current to expect to run through it continuously? Half? Two thirds?


I think I'll pass that one.


----------



## Tesseract (Sep 27, 2008)

The amount of current it takes to drive a big IGBT (or MOSFET or lots of either in a bank) can appear daunting, but it only needs to be delivered for a very brief period. You don't need a 10A continuous power supply, ga2500ev, just some good capacitors with very low ESR (think polypropylene film or MLC ceramic) and power transistors or MOSFETs with the ability to handle the peak current demanded.

The peak current, btw, is most easily calculated by dividing the total gate charge (Qg) by the switching transition time. IIRC, the IGBT in question has a Qg of 29uC so if you wanted to turn it on (or off) in 1uS you would need to be able to deliver a peak current of 29A. 2uS would require a peak current of 14.5A and so on. Notice, Qer, how the current goes *down* as the transition time goes *up* 

ga2500ev: how much current you can pump through a given IGBT (or MOSFET, etc.) depends a lot on the packaging, specifically the amount of area it has to get rid of heat and the size of the bondwires or leads or whatever. So, you typically need to derate pc board size components by 1/2 whereas the much larger modules can typically be run at 80% of their maximum continous current rating without resorting to heroic measures with the heat sink. That said, if you do use, say, a cold plate (i.e. - liquid cooling) with a module you can often *exceed* the continuous current rating. It basically comes down to the temperature of the semiconductor die inside with modules - if you can get rid of the heat it produces you can jam as much current as you like through it... (within reason, of course). So, you actually can run 2400A of current through Georgia Tech's IGBT... you just need to be able to get rid of 7200W of waste heat at the same time


----------



## Georgia Tech (Dec 5, 2008)

Tesseract said:


> So, you actually can run 2400A of current through Georgia Tech's IGBT... you just need to be able to get rid of 7200W of waste heat at the same time


These are all the more reasons for going to a MOSFET design. They seem WAY more effecient. The MORE you add the More Effecient the controller will be!!


----------



## ga2500ev (Apr 20, 2008)

I agree on the efficiency. The IGBT voltage drop represents a serious consumer of power consumption.

But the flip side is having to manage individual MOSFETs.

My primary objective is to get a simple, reliable, inexpensive controller done. An IGBT module represents all three of these criteria well.

So I plan to accept the inefficiency for the sake of expedience. 

And Tesseract and Qer, thanks for pointing out that the power supply for the controller can charge caps, and then you pop the caps when you switch.

ga2500ev


----------



## Tesseract (Sep 27, 2008)

Georgia Tech said:


> These are all the more reasons for going to a MOSFET design. They seem WAY more effecient. The MORE you add the More Effecient the controller will be!!


That's true, but you do not seem to appreciate just how difficult it is to get all of those MOSFETs to turn on and off at the same time. Each inch of pc board trace has approximately 20nH of inductance. This inductance not only delays the propogation of the gate drive signal, it also forms a very high Q resonant circuit with the gate capacitance resulting in ringing at every transition. Thus, you need to place the gate drive resistors in just the right place - frequently this requires a lot of trial and error to get right (and the "error" here means "lots of destroyed MOSFETs"). Then there is how to make a reliable high current path for all of the source connections. The copper foil, even on special-order 4oz pc boards, just can't carry more than 50A or so without using ridiculously wide traces. Here's a convenient online calculator you can use to see for yourself:

http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

Plug in 600A, go ahead and plug in 4oz Cu, even though most PCB manufacturers will balk at sourcing it for you and assume a trace length of 6" because you can cram enough TO-247 or TO-3P parts to carry 600A in that length. Hmm... seems you need a trace width of 262mm (that's over 10" folks) and you're still going to lose 30W...

Something else you appear to have not considered is that if the MOSFETs do not all turn on and turn off at the same time, that implies that *one* MOSFET ends up being the first to turn on and one ends up being the last to turn off. Granted, the inductance of the wiring, motor, etc., all help you in this instance by slowing the rate of rise of the current, but I suspect it is precisely this problem that is resulting in Curtis 1231Cs dying untimely deaths when mated to Warp11 and Warp13 motors.

Note that I have only mentioned *some* of the issues you will need to address in order to get lots of MOSFETs to work together reliably. It would take me hours to list them all and, frankly, y'all aren't paying me enough for that... 

ga2500ev - no problem and thanks for the thanks. If you need any help working out kinks along the way just ask.


----------



## Technologic (Jul 20, 2008)

Tesseract said:


> So, you actually can run 2400A of current through Georgia Tech's IGBT... you just need to be able to get rid of 7200W of waste heat at the same time


 pretty sure that's not possible... area wise 

Carry on though... happy to learn


----------



## Technologic (Jul 20, 2008)

Tesseract said:


> That's true, but you do not seem to appreciate just how difficult it is to get all of those MOSFETs to turn on and off at the same time. Each inch of pc board trace has approximately 20nH of inductance. This inductance not only delays the propogation of the gate drive signal, it also forms a very high Q resonant circuit with the gate capacitance resulting in ringing at every transition. Thus, you need to place the gate drive resistors in just the right place - frequently this requires a lot of trial and error to get right (and the "error" here means "lots of destroyed MOSFETs"). Then there is how to make a reliable high current path for all of the source connections. The copper foil, even on special-order 4oz pc boards, just can't carry more than 50A or so without using ridiculously wide traces. Here's a convenient online calculator you can use to see for yourself.


Within the output stage why can't someone put a large current resistor or snubber in line to test the gate resistor location? What I think would be a perfect solution (and something amplifier manufacturers have been doing for YEARS) would be to run the outputs outside of the board itself. In TOP3 cases they can be mounted directly to the large (as large as you want) heatsinks. This seems to me a bit overkill, at least anything larger than say, a 12"x12" heatsink (small fan if you feel like it).

You could then use not 4 oz foil traces, but effectively use something like a 1"x0.25" copper bar to eliminate any noise from inductive ringings (though I'm unsure why snubbers can't be used on the output stage itself to keep the MOSFET from pooling inductance and destroying itself).

1 IBGT IMO, is way more simple. If you can find one that fits the bill. However, there are certain Amperage ratings (say when you get into the larger 4000lb truck range) that it simply won't work anymore without attempting to parallel IGBTs (which requires precise matching from what I understand).

Thoughts?... I realize my knowledge here may be significantly lacking.


----------



## DIYguy (Sep 18, 2008)

Tesseract said:


> FWIW, the Zilla controllers use IGBTs; everyone else uses MOSFETs.


Not exactly correct. There is another 500 amp rated, IGBT driven DC motor controller. This unit has 680 amp IGBT rating and is limited by a 500 amp shunt. It is current controlled to 80 deg C. The really nice thing, is that is sells for about half of what the others sell for. Any guesses??? Oh yes, and of course it handles higher voltages,... up to 192 I believe.


----------



## ClintK (Apr 27, 2008)

DIYguy said:


> Not exactly correct. There is another 500 amp rated, IGBT driven DC motor controller. This unit has 680 amp IGBT rating and is limited by a 500 amp shunt. It is current controlled to 80 deg C. The really nice thing, is that is sells for about half of what the others sell for. Any guesses??? Oh yes, and of course it handles higher voltages,... up to 192 I believe.


The ElectroCraft motor controller I'm going to buy is about that.


----------



## Tesseract (Sep 27, 2008)

DIYguy said:


> Not exactly correct. There is another 500 amp rated, IGBT driven DC motor controller. ... Any guesses???


Well, Logisystems is attempting to get an IGBT-based controller working, but at least from what I've read here they are having some teething pains, so to speak.

(Note: I am currently designing a controller for EVs and the first prototype has just passed initial testing, so I'm basically a competitor to whomever this person/company is).


----------



## DIYguy (Sep 18, 2008)

ClintK said:


> The ElectroCraft motor controller I'm going to buy is about that.


U got it ClintK. I will be buying as well. I visited Darius yesterday. Will you be buying any of his other products? He offers a discount on package deals. He will also offer a 25% discount on the third package that can be split 8.3% each.


----------



## ClintK (Apr 27, 2008)

DIYguy said:


> U got it ClintK. I will be buying as well. I visited Darius yesterday. Will you be buying any of his other products? He offers a discount on package deals. He will also offer a 25% discount on the third package that can be split 8.3% each.


His DC-DC converter is a little pricey. I'm thinking about the charger though... I'm going with Lead initially - fully intending on LiFePo4s for the second set, and Darius said that his charger will do both. On the other hand I like the idea of a complete package (Batteries, Charger, BMS from Elite Power Solutions).

Do you know anyone who has used the controller? It's a great deal if it performs as advertised. The 1 year warranty convinced me to buy it.


----------



## RKM (Jun 9, 2008)

ClintK said:


> His DC-DC converter is a little pricey. I'm thinking about the charger though... I'm going with Lead initially - fully intending on LiFePo4s for the second set, and Darius said that his charger will do both. On the other hand I like the idea of a complete package (Batteries, Charger, BMS from Elite Power Solutions).
> 
> Do you know anyone who has used the controller? It's a great deal if it performs as advertised. The 1 year warranty convinced me to buy it.


 
ClintK,

His DC-DC is a bit high, I agree. Plus, it isn't as high a power as many. Is it enough in your opinion?

Darius had provided several references. I recieved pretty strong endorsements from them. I could forward them if you'd want to send your address by PM.

Like Gary, I've been pretty impressed with the corespondence I've had with Darius. I'm planning on lithium, so will need a good BMS able to protect individual cells. His looks excellent but is pricey. I'll be contacting him soon to discuss the BMS further.

I like the charger and am considering dual chargers to shorten charge time.

Your USD makes this a very affordable supplier!

Rob


----------



## ClintK (Apr 27, 2008)

RKM said:


> ClintK,
> 
> His DC-DC is a bit high, I agree. Plus, it isn't as high a power as many. Is it enough in your opinion?
> 
> ...


PM Sent

I've seen higher power DC-DC converters for lower cost, so I'll probably just go with one of those.

I will be ordering the controller in early January though (just spacing out my purchases). I'll keep everyone posted on my experiences with it.


----------



## Georgia Tech (Dec 5, 2008)

Tesseract said:


> Yep, much as I suspected this is a 1.7kV IGBT. The Vce[sat] is impressively low: only ~3V at 2400A, which is equivalent to a 1.25 milliohm on resistance.
> 
> Yes, you either need to apply negative voltage to the gate during the off time OR actively clamp the gate to the source. The Miller capacitance will couple transients at the Collector back to the Gate which can lead to spurious turn on. I use +15V/-5V but +/-15V is fine.
> 
> ...


 
So let me ask what is the maximum safe current range you would opperate this thing in?


----------

