# IGBT snubbers, What do I need to know?



## david85 (Nov 12, 2007)

Some of you may know that the controller I am running had a minor fire from a loose IGBT terminal screw. The IGBTs have 2 types of MKP snubbers on them.

One type is 50uF, the other are smaller 5uF. 2 of the larger ones, and 3 of the smaller.

My question is what role to they play in the controller?
and Is there a formula to sizing them correctly? I can't seem to find any that are an exact match and may have to replace all of them to run the same type.

I'm guessing it would not be a good idea to run it without all of them installed and working.

Thanks for any info, I'm still pretty green at this.


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## jackbauer (Jan 12, 2008)

The idea of a snubber is to quench (snub) any nasty voltage transients that may appear across a power transistor. Ideally they are specified by measuring the frequency of the noise and sizing the capacitor to have a low impedance at that frequency thus effectively shorting it but leaving the desired waveforms alone. Have a look at my experiences here with my dc controller:
http://www.diyelectriccar.com/forums/showthread.php/snubbers-37937.html


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## david85 (Nov 12, 2007)

Its too late at night for me to understand some of the info in the links on your thread, but I think you gave me what I need. I'll read over those links later,

Thanks


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## Tesseract (Sep 27, 2008)

david85 said:


> Some of you may know that the controller I am running had a minor fire from a loose IGBT terminal screw. The IGBTs have 2 types of MKP snubbers on them.


The IGBTs are shot, then. Don't try to reuse any of them even if some/all still appear to switch normally.




david85 said:


> One type is 50uF, the other are smaller 5uF. 2 of the larger ones, and 3 of the smaller.
> 
> My question is what role to they play in the controller?


A critical role, especially in a DC motor controller (yes, I realize yours is not such).

What these film caps do is eliminate or reduce, at least, the effect of any stray inductance in the battery cables and, especially, between the electrolytic "input" capacitors and the IGBTs. If they aren't present then when the IGBTs turn *off* the stray inductance will cause a huge voltage spike that is invariably fatal.




david85 said:


> and Is there a formula to sizing them correctly?


Unfortunately, no. They are selected more or less empirically. That is, the designer makes a guess and then the converter waveforms are observed to see if any adjustments need to be made. Someone experienced in this sort of stuff can usually nail the size of the snubber cap on the first or second try.

One thing is known up front, though, and that's the worst case ripple current. For a 3ph. inverter (where it can be presumed all phases are balanced because you are driving a single motor) the ripple is 12.5% of the maximum phase current. For a DC converter ("controller"), though, the worst case ripple is 50% of the motor current! Clearly, life is harder for the input capacitor(s) in a DC motor controller!

At any rate, the most important capacitor is the one mounted right on top of the IGBT module (assuming such was used - if it's an array of little IGBTs in parallel then who knows which one is most important?!?!). So that would be the 2.2uF caps in this case. Now, the 50uF cap(s) are mainly for reducing heating inside the electrolytics (assuming there are any?!) from the reflected ripple current. As mentioned before, the ripple current will be 12.5% of the phase current, so if the phase current is 100A then the ripple current will be 12.5A. This current flows through the ESR of the input capacitors and causes heating, just like it would in a "regular" resistor. If the ESR is, say, 0.1 ohm, then the power lost inside the capacitor will be 12.5 x 12.5 x 0.1, or 15.6W. Doesn't sound like much, but keep in mind that the cylindrical shape of the typical electrolytic isn't so hot at radiating heat (pardon the awful pun...).

In contrast, film capacitors - like the big beast used inside the Soliton1, or these smaller MKP jobbers - have ESRs down in the 10's of milliohms range. The same amount of ripple current through them causes a nearly negligible 1-2W of loss. And there's no electrolyte to dry out, so any heating that does occur is less injurious.




david85 said:


> I can't seem to find any that are an exact match and may have to replace all of them to run the same type.
> 
> I'm guessing it would not be a good idea to run it without all of them installed and working.


It would be an awful idea to run the controller without those caps! My guess is that it won't last more than a few switching cycles before letting out the magic smoke.

As for picking replacements, I wouldn't change the 2.2uF cap at all - try to get a replacement of the same physical size, as well as the value (oh, and the same type - MKP). The 50uF caps are a little more flexible. You could replace them with a couple of 33uF, or maybe one 47uF, or a 68uF, etc... Post a picture of each and I can give you better advice.


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## major (Apr 4, 2008)

Tesseract said:


> For a 3ph. inverter (where it can be presumed all phases are balanced because you are driving a single motor) the ripple is 12.5% of the maximum phase current. For a DC converter ("controller"), though, the worst case ripple is 50% of the motor current!


Hi Tess,

Is this just a rule of thumb or is there some math basis behind it? I have often wondered how the bus caps were sized.

Thanks,

major


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## david85 (Nov 12, 2007)

Forgive me, but could you please elaborate on why you think the other IGBTs are junk? Considering the cost, I would like to know why you think they cannot be reused.


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## Guest (Jan 4, 2010)

I think he said the one is toast that had the caps crap out. Not the others. That is what I got from his post. 

Pete


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## Tesseract (Sep 27, 2008)

major said:


> Hi Tess,
> 
> Is this just a rule of thumb or is there some math basis behind it? I have often wondered how the bus caps were sized.
> 
> ...


As you've probably guessed by now I am fond of "thumb rules" - they do tend to become such for a reason, though. 

There is math basis behind it, but the equation is too difficult to attempt to reproduce in a text-based medium which is probably why the rule of thumb is so popular!  

Suffice it so say, though, that much of the reduction in ripple current owes to the reduction in ripple voltage. To wit, ripple is about 10x less in the 3 phase full wave rectifier compared to it's single phase counterpart. There are some subtle differences in behavior between the two, but suffice it to say that in the end the dominant factor in determining the amount of capacitance in the front end of an inverter has more to do with providing a low ESR/ESL source of current for the phase leg half-bridges (just like with the buck).


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## Tesseract (Sep 27, 2008)

david85 said:


> Forgive me, but could you please elaborate on why you think the other IGBTs are junk? Considering the cost, I would like to know why you think they cannot be reused.


No problem... you can certainly try to reuse them, just keep in mind that if they blow they might take out the rest of the inverter and I imagine it's a lot more expensive to replace than the IGBT module(s) 

Anyway, if the snubber caps become disconnected/go bad/otherwise stop doing their job, then this subjects the C-E junction of the IGBT immediately after turning off to a huge over-voltage - a spike, if you will - that if it doesn't outright cause it to turn back on (called "avalanching") it certainly weakens what is called the "heterostructure" of the semiconductor.

In less technical terms, it zaps the $hit out of it 

Some devices are designed to avalanche safely - smaller power MOSFETs in particular - but avalanching a semiconductor is ALWAYS hard on it, regardless of it's supposed tolerance level, and it is therefore prudent to avoid it.

You're snubber got disconnected; some black soot/carbon tracking surrounds the terminals; ergo, some arcing occurred; ergo, the IGBTs were almost certainly exposed to a much higher voltage than they are rated for.

Hey - wanna know one way we found to destroy a Soliton1 (and it is, I assure you, extremely hard to destroy) - forget to tighten the bolts that connect the film capacitor to the laminated bus structure. I don't think we got more than a single full switching cycle out of it before it blew up, and there was a huge spark between the bus plates, too. Plasma city. Needless to say, we double-check the tightening of all bolts as a result


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## david85 (Nov 12, 2007)

These particular IGBTs are rated at 400V, and my car is running 160V (resting voltage). Could the surge really be that high? Also, all the other snubbers are still in tact so there was still something left to control the spikes.

Here is the inner layout of the controller. The two snubbers on the right hand side of the photo are damaged. The tiny 5uF snubber is fried to a crisp and not recognizable anymore. The larger 50uF snubber has surface damage and may still be viable but would like to replace it anyway. Other than the outside of the IGBT on that end being damaged, everything else is clean.


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## david85 (Nov 12, 2007)

Rrrr! got the uF rating of the smaller ones wrong. They are 3uF + - 5%, not 5uF like I posted earlier.


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## major (Apr 4, 2008)

david85 said:


> These particular IGBTs are rated at 400V, and my car is running 160V (resting voltage). Could the surge really be that high? Also, all the other snubbers are still in tact so there was still something left to control the spikes.


Hi david,

Can't say as I think much of the design/workmanship from that photo. It looks more like a DIY (no offense to anyone here) project than a "professional" inverter. And don't know I have ever seen a 400V rated IGBT. Most "low voltage IGBTs are 600V rated. And yeah, spikes can be that high. It depends on a lot of things including switching speeds. And that bus design doesn't help. 

I'd suggest you put one of those larger "square" snubbers on each IGBT module. Does it have an electrolytic cap bank or any more film caps? Looks like the + and - go somewhere, but I can't tell from the photo.

You'd think they could at least have plated the copper bars  And do the brass bolts just tighten against some fiberglass board to connect to battery and motor? That's trouble down the road.

Do you mind posting where you bought the thing? I want to be sure to avoid them.

Regards,

major


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## david85 (Nov 12, 2007)

Major, you are right. I just re checked the specs and they are 600V. 400 is the peak amp rating. My head is pretty full at the moment.

Check my other threads for where I got the setup. Poor quality or not, its what I have to work with even if I have to rebuild everything. If it were not for the single loose connection, it would have worked fine. The car did drive well up to that point. I am kicking myself for not inspecting the controller more thoroughly before running it, because as a matter of principle I do that with more auto expensive parts no matter what country the come from.

And yes, there is a bank of 4 caps in addition to the MKP snubbers off to the side.


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## david85 (Nov 12, 2007)

This is the exact model of IGBT that my controller has, FF400R06KE3:

http://www.infineon.com/dgdl/DS_FF4...90004&fileId=db3a304412b407950112b434a7a560dc


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