# Snubbers



## yarross (Jan 7, 2009)

jackbauer said:


> Not sure if i'm asking this in the right part of the forum. I've built an igbt based controller and i'm noticing some fast voltage spikes between collector and emitter at turn off. Magnitude of about 1.5 times vbat.Just wondering if an snubber of some form across the igbt would be good practice or any other ideas? Would these spikes be the result of the freewheel diode not doing its job or being an incorrect recovery type?


If you are using IGT modules, they have good soft recovery diodes.
Transients are inherent to switching. It's a good idea to clamp such spikes using snubbers.
http://www.pwrx.com/pwrx/app/IGBT-Intelligent-PwrMods.pdf and http://www.irf.com/technical-info/designtp/tpap-5.pdf are good starting points.


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## jackbauer (Jan 12, 2008)

I'm using one igbt as the switch and one with its gate emitter shorted as the freewheel diode. thanks for the info.


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## Tesseract (Sep 27, 2008)

jackbauer said:


> Not sure if i'm asking this in the right part of the forum. I've built an igbt based controller and i'm noticing some fast voltage spikes between collector and emitter at turn off. Magnitude of about 1.5 times vbat.Just wondering if an snubber of some form across the igbt would be good practice or any other ideas? Would these spikes be the result of the freewheel diode not doing its job or being an incorrect recovery type?


Ah, Jack... The fun has just begun for you, my friend 

Spikes across the C-E terminals of the drive IGBT would, indeed, point to the freewheeling diode (FWD) not recovering fast enough. The only way to solve this problem - as the FWD is invariably built into the IGBT module - is to slow down the switching speed. If this is a brand new IGBT module then you should consider 8-10kHz as a realistic switching frequency and 300ns as a realistic switching speed. If you are using older IGBTs - bought used off of ebay or pulled from an old VFD - then I would strongly advise lowering the frequency to 1.5kHz and switching speed to 1.5us or even longer. Some of the older IGBTs had really long "tails" during turn off, lasting 3-5us at times, and the FWD can't start conducting until the IGBT has stopped.

Of course, other things that need attention w/r/t spikes are minimizing the stray inductances in the circuit, especially between the input capacitor and the IGBT and between the IGBT and the FWD (nothing you can do about that one if using a module, and it's already as good as it can get if so anyway).

Once you get over 150-200V and 500A parasitics will really start to vex you.


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## jackbauer (Jan 12, 2008)

Its an '05 fuji igbt. I don't really want to drop the switching frequency (currently 16khz) so I may have to end up using a faster freewheel. I'm going to try a few snubber options just to see what if any effects it may have. I've kept the bus caps right across the bat rails and the connection between the switch and the freewheel as short and as beefy as I could. I seem to be getting a good waveform apart from the turn off spike. Its not looking like the switching speed is to high for the device as i would have expected to see the corners of the the waveform "rounding" if that were the case?

Anyway like you say the fun starts here I *really* need a diferential probe.........


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## Anonymous Expert (Oct 25, 2009)

My random thoughts:
1. Inductance between the caps and the modules. What does your laminated bus look like? Post a picture.
2. Decoupling (usually incorrectly called snubber) caps across the module terminals make a huge difference but can be tricky. They need to be big enough and they must be sized so that you don't set up ringing between the bulk caps and the decoupling caps.
3. Slowing down turn off to 1.5uS sounds rediculous (I'm sorry) even if the switching frequency were low. The losses would be significant.
4. Forget about the inductance between the IGBT and FWD. Fuji knows how to make modules. It will be insignificant compared to the inductance between the caps and module.


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## Tesseract (Sep 27, 2008)

Anonymous Expert said:


> My random thoughts:
> ...
> 2. Decoupling (usually incorrectly called snubber) caps across the module terminals make a huge difference but can be tricky. They need to be big enough and they must be sized so that you don't set up ringing between the bulk caps and the decoupling caps.


Very good point... Usually, though, the ESR in the main electrolytic capacitor(s) damps out any ringing from a locally-mounted "snubber" capacitance. Of course, if you are using a laminated bus structure with nothing but high-Q film caps on the input then you have little choice outside of tailoring the switching trajectories to prevent ringing.



Anonymous Expert said:


> 3. Slowing down turn off to 1.5uS sounds rediculous (I'm sorry) even if the switching frequency were low. The losses would be significant.


First off, there's no need to be rude... (what's up with the forum these days, anyway?)

Secondly, yes, back in the bad old days of Darlingtons and SCRs (and even first gen IGBT modules) it was not uncommon to see switching frequencies as low as 300Hz and transition times of 10us or more. As long as the total time spent in transitioning is kept to between 0.5% and 2% of the switching period, though, the losses will be the same no matter frequency is used. Sure the amount of joules lost per transition goes up with a slowing down of the transition speed, but because the transitions come less frequently at slower switching frequencies the total loss is the same.

Furthermore, faster transitions are not always better (assuming the devices can keep up, that is). If you turn on the IGBT faster than the FWD can recover from blocking then all you'll get for your troubles is a short circuit across the DC bus, which can really excite some ringing and causes switching losses to suddenly skyrocket. Turning the IGBT off too fast can result in huge spikes and even Miller-effect induced ringing (i.e. - the rising C-E voltage is capacitively coupled back into the gate turning it back on just a tad (or even fully!) which slows down the rise of the C-E voltage which lets the gate turn back off, etc... usually caused by too much inductance in the gate drive circuit, but exacerbated by switching too fast).




Anonymous Expert said:


> 4. Forget about the inductance between the IGBT and FWD. Fuji knows how to make modules. It will be insignificant compared to the inductance between the caps and module.


Yep - I believe I mentioned that already...


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## Tesseract (Sep 27, 2008)

jackbauer said:


> Its an '05 fuji igbt. I don't really want to drop the switching frequency *(currently 16khz) *so I may have to end up using a faster freewheel.


Jack - check out the switching frequencies used on new, commercially available VFDs... you'll find that if 16khz is mentioned it is always for "quiet" mode. Most modern VFDs (which is where 99.999% of these modules are used) are spec'ed for full amperage at 8khz (sometimes even lower). Even an '05 vintage IGBT will max out around 8khz. There's a very good reason the Soliton1 uses 8khz in "performance" mode and spread-spectrum switching up to a max of 14khz in "quiet" mode.




jackbauer said:


> Its not looking like the switching speed is to high for the device as i would have expected to see the corners of the the waveform "rounding" if that were the case?


Nope - if you have the timebase set so that you can see 1 or more full cycles then at most you will only see the waveform turn slightly trapezoidal. You have to zoom in on the turn-off transition (set timebase to 50 or 100ns per division) to see what's really going on there. Also, you need to make sure you are driving the expected load impedance (composed of both inductance and resistance) for the switching loss testing to be valid.

Suffice it to say, switching too fast is bad (spikes, noise and ringing) and switching too slow is bad (excessive losses). Both conditions can result in surprise destruction of the IGBT, FWD or both. Can't make an omelet without breaking some eggs, though 

Oh, and skip the differential probe - you'll get much better results with a portable scope - even the handheld ones that look like DMMs on steroids.


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## jackbauer (Jan 12, 2008)

Some real food for thought here. Thanks for the clear explanations. Most books i've read on the subject start off well then disappear into third order differential equations that have little relation to an amateaur designer with limited resources. 

So i'll firstly halve the switching frequency then look at some snubbers. I know if i drop to 8khz i'm into an audible whine not a huge issue but was hoping to avoid.

Tesseract from what i understand so far an igbt is not the best at high frequency (mabey due to its bipolar parts?) so i may have been as well to stick to mosfets? doh! Do you have an audible noise from the solition?

edit: ac drives whine at low speed! forgot!


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## Tesseract (Sep 27, 2008)

Yeah, you can hear a faint whine from some motors* when operating at low RPM/current when the Soliton1 is in performance mode. It's nothing at all like the infamous Curtis whine, though. 

You should ask yourself, though, what advantages are there (if any) in going with a higher frequency than is necessary to render the motor inaudible? My take is that there are no advantages besides reduced ripple voltage reflected back onto the battery pack, but if you are using electrolytic capacitors on the bus then you probably already have much more capacitance than is needed from a strictly dV/dt point of view (the excess being needed to get the ESR low enough so they don't explode from the ripple current).



* - ADC ones sing the most (so far), followed by Kostov and then just barely audible are the NetGain WarPs and D&Ds.


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## jackbauer (Jan 12, 2008)

Makes sense. I did a very quick test with a .1uf x2 cap across c-e. Big difference in the spike. Probably halved its peak but added an extra (but much less amplitude) oscillation.
edit:
Went to 8khz switching. Much better. spike is now about 1/3rd magnitude but seems "faster" ie its rising and falling much faster. period of about 70ns now as against 300ns at 16khz. Ran a few tests on the 12v starter and it only whines at low current and speed.


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## bga (May 25, 2009)

Hi All,
I agree with Tessarect, the switching speed should the the lowest to achieve the requirements. The switching speed limits are easiy approached in IGBTS and have to be considered.

It would be worthwhile making the swithing speed a controllable parameter so that you can experiment with it. It is also likely that, due to the minimum on-time requirements of the IGBT (so that it gets into saturation properly), the switching speed may need to vary at low speed (low duty cycle).

I am currently undertaking a similar project using Powerex 300A/1200V IGBTs. Another factor worthy of consideration is the switching speed options available on different IGBTs. Powerex has a slow speed and high speed version of the devices. I opted for the slow speed type (15KHz max) because thay have a lower saturation voltage and will run cooler than the high speed version in the likely switch speeds I will use of between 5 and 15khz.

On the snubber front, It may be desirable to have some local snubbers on the across each IGBT module's +/- legs so that ringing on these is reduced, say a 2uF IGBT snubber which is good for 500A peaks. If you already have these, then I will have the same problem and had better take note since it may be necessary to use a dvdt limiter of the phase output, although this will increase losses and be a bit tricky to design because of the high peak currents.


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## jackbauer (Jan 12, 2008)

I understand switching speed to be the time to go from fully on to fully off and vice versa as distinct from the switching frequency. My understanding is that the fastest possible (just short of ringing) switching speed is best.


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## bga (May 25, 2009)

Hi Jack,

I think that its a compromise between RFI, ringing (overshoot) and efficiency (switching losses)

Many applications use soft switcing to reduce EFI. Most of these are heaters and welders. (I presume that this is because they have exposed electrodes and other components and more readily emit interference than does a well screened motor controller and its cabling).

Because most of our EV applications are about high current and high voltage, so the switching losses are important to stop the transistors from frying.

I spotted a good manual on the subject:
http://w5jgv.com/hv-ps1/pdf/fuji_igbt_application_manual(REH984).pdf
It looks to be good for 'instant experts' with a wealth of worth while info.

Cheers


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## jackbauer (Jan 12, 2008)

Fitted the 1.5uf snubber today. At 8khz the spike has been dramatically reduced with just a little ringing present. at 16khz its ring city so it looks like 8khz it is!


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## bga (May 25, 2009)

Hi Jack,

Sounds good.
What frequency does it ring at?
I am curious as to what's ringing.

Another thought from industrial controllers: they have a minimum inductance on the motor and need series inductors if the motor is too low. I haven't looked at the exact parameters, but it's to do with didt being too fast for the controller such that the PWM can't provide reasonable current regulation, resulting in a big harmonics on the motor and potential overcurrent on the IGBTs. (the problem with big motors on small controllers)

With a real motor and load on the controller you'll know. I've always felt that the low end 'creeping' performance is the most difficult to control and also the easiest to test and observe.

My test plan is to start testing on low voltage, which a home brew controller will permit, so that the energy in the system is low. I was going to test the safety and basic operation of the systems there before moving on to full bus voltage in steps.
I would estimate that at 48V the motor shoudl develop good torque, but have a speed limit of about 300RPM.

One thing that the Red Suzi.s owner (http://www.evalbum.com/1149) has concluded is that good direct speed sensing of 500 pulses per rev or so is essential to low speed performance (torque). The current 8ppr sensor results in jerky low end performance and less than optimal torque since the controller can't determine the engine revs accurately below a rew hundred RPM.


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## SimonRafferty (Apr 13, 2009)

Some good info in this thread!

I've found that 8kHz is barely audible - though that's probably just because I'm getting old! 

When my ancient CRT scope died, I bought a fluke hand held scope from eBay - and it's just brilliant for analysing transients & ringing - as well as loads of other cool stuff! It cost me £200 - but has saved that in fried IGBT's!

Si


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## jackbauer (Jan 12, 2008)

I just want to say thanks to all who replied in this and other threads. I took the car for its first test drive today at just 24v with 8khz switching. Barely any noise at all.


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## Tesseract (Sep 27, 2008)

You're welcome... Now please don't put me out of business, okay?


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## SimonRafferty (Apr 13, 2009)

Well done Jack - that's fantastic! It's a good feeling when something like this comes together without the smoke escaping!

Si


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## jackbauer (Jan 12, 2008)

Tesseract your in no danger! Its working well but its no solition. Simon i'd say the smoke will get out at some point. That said it ate 400 amps today with only 28c on the heatsink.


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## Woodsmith (Jun 5, 2008)

jackbauer said:


> Tesseract your in no danger! Its working well but its no solition.


Maybe if you built one for me????


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