# Freewheeling diode?



## TheSGC (Nov 15, 2007)

I don't have one off hand, but I would recommend a 1200 volt 600 AMP diode, or get a CM600HA IGBT and use it as a diode. 

I too am slowly working on a diy controller, but my first edition exploded when my 150 volt freewheel diode failed running on a 60 volt system. Turns out that the motor voltage spike can be 8x that of the pack voltage, hence the 1200 volt recommendation.


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## Tesseract (Sep 27, 2008)

Greenflight said:


> Embarrassing as it sounds, I had completely forgotten about the need for a freewheeling diode until major reminded me of it in another thread.
> 
> So, the question is: Can anyone recommend a good diode for this application?


Oh boy... 

I wrote a post about this awhile ago. Do a search for the thread I started called something like, "So you want to build a motor controller, eh?"

Here's the short answer: the FWD should be rated for at least 2x the pack voltage; have a reverse recovery time, t_rr_ less than the switch transition time and is "soft"; and have a 100C current rating equal to the maximum continuous _output_ current.




TheSGC said:


> Turns out that the motor voltage spike can be 8x that of the pack voltage, hence the 1200 volt recommendation.


Where in the world did you get that recommendation from?! It is more likely that a 1200V diode was suggested because, well, the best kind of diode typically used as an FWD is a Fast-Recovery Epitaxial Diode, and FREDs are usually rated for that high of a voltage almost as a byproduct of their construction.

Generally, if you are seeing vicious spikes across your FWD you need to slow down the switch transition time (speed) and/or reduce the loop distance between the bulk input capacitor and the switch/fwd (or, even better/more practical, place a high quality, high pulse current, high dV/dt film cap as close as possible to the switch/fwd).

Or, be really sloppy with the layout but fit an RC or RCD snubber across both the switch and the FWD to dampen the inevitable ringing and spikes... 

In any case, if you are seeing spikes of 8x the supply voltage you are definitely doing something (probably lots of somethings) wrong.


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## Qer (May 7, 2008)

http://www.diyelectriccar.com/forum...ant-build-motor-controller-22398.html?t=22398


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## Greenflight (Sep 13, 2007)

Great thread Tesseract. Thanks for sharing your knowledge. I'll be spending some time looking through it.

I'm considering simply using an IGBT with the gate tied low. Possibly the most economical option, since they're a lot easier to buy used. Even new they cost almost the same...

Thanks for the help guys!


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## ga2500ev (Apr 20, 2008)

Greenflight,

Here's a FRED that may suit your needs:

http://theelectrostore.com/shopsite_sc/store/html/diode-meo450-12da-ixys-meo-450-12-da-fred.html

1200V 450A. $40 used.

Hope this helps,

ga2500ev


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## TheSGC (Nov 15, 2007)

Tesseract said:


> Where in the world did you get that recommendation from?! .


That recommendation came from three different electrical engineers and professors who specialize in electric vehicle technology and high power controls. The reasoning is that they are designed for the fast recovery, and the off chance that something get shot to hell, like bad cabling, there is some stupid profo. It's not often that the spikes are 8x, just it can happen and over engineering things it what I do. I have been told that the freewheeling diode is one of the big problems of the 156+ volt controllers because they put one that is usually too small into it and it just that one chance that blows the thing to hell. I got to look at a cooked Curtis 1231C that had been on a Warp 13 and the thing that went was the freewheel diode, then the logic board.


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## Greenflight (Sep 13, 2007)

Interesting actually, sounds similar to the inconsistent failures that used to be going on with the Raptor 600s... I wonder if it was related to the FWD?


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## Tesseract (Sep 27, 2008)

TheSGC said:


> That recommendation came from three different electrical engineers and professors who specialize in electric vehicle technology and high power controls.


Hmmm... So why is that every commercially produced 230VAC VFD, for example, uses semiconductors rated for 600V, not 2.4kV?




TheSGC said:


> The reasoning is that they are designed for the fast recovery, and the off chance that something get shot to hell, like bad cabling, there is some stupid profo.


The only wiring that affects the production of spikes is that between the anode of the FWD and the Collector (or Drain) of the switch (or the Emitter/Source of the switch and the Cathode of the FWD, if arranged inversely). The less inductance there is between those two terminals then the faster the motor current can transfer from the switch as it turns off to the FWD as it turns on.

Additional possibly failure modes attributable to the FWD are:

1. excessive shoot-through current from the FWD taking too long to recover from blocking mode (i.e. - it's slow)
2. excessive ringing from "snappy" recovery characteristics (i.e. - it's fast enough, but recovers abruptly rather than softly).
3. poor current sharing among paralleled rectifiers (everyone worries about sharing among paralleled switches... no one seems to care about the equally important FWD...)

Careful attention to these details, among others, is what allows a design to succeed without excess.


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## Greenflight (Sep 13, 2007)

IMO, most homebuilt controllers aren't going to have the same inductive characteristics as commercial controllers, so it's probably not a bad idea to have a little wiggle room. I'm probably going to go with a 1200v diode to stay on the safe side. Whether it's necessary or not I don't trust myself to build something that won't have the occasional spike.


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## MPaulHolmes (Feb 23, 2008)

Greenflight said:


> I'm working on a diy motor controller design that's intended to take about 500-600A and up to around 150-180v. PWM frequency is 10kHz.
> 
> Embarrassing as it sounds, I had completely forgotten about the need for a freewheeling diode until major reminded me of it in another thread.
> 
> So, the question is: Can anyone recommend a good diode for this application?


Hey, I'm working on a 144v 600amp controller! I'm using 200v STTH6002C freewheel diodes. It might be OK for 150v. I'm using them because the guy who recommended it to me has his own 144v 600 amp controller and is using 12 of them in parallel.

Check out http://www.zeva.com.au/tech/controller_design/

My build thread is at:

http://ecomodder.com/forum/showthread.php/paul-sabrinas-cheap-144v-motor-controller-6404.html


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## Greenflight (Sep 13, 2007)

Never heard of putting diodes in parallel. I'll have to check out the datasheets. Thanks for the input!


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## yarross (Jan 7, 2009)

Tesseract said:


> Additional possibly failure modes attributable to the FWD are:
> 
> 1. excessive shoot-through current from the FWD taking too long to recover from blocking mode (i.e. - it's slow)
> 2. excessive ringing from "snappy" recovery characteristics (i.e. - it's fast enough, but recovers abruptly rather than softly).
> 3. poor current sharing among paralleled rectifiers (everyone worries about sharing among paralleled switches... no one seems to care about the equally important FWD...)


What about using paralleled SiC Schottkies from Cree? They come in 600V and 1200V ratings and current 2x10A.
Or IXYS DSS 2x101-02A, 2x100A.


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## Tesseract (Sep 27, 2008)

Greenflight said:


> IMO, most homebuilt controllers aren't going to have the same inductive characteristics as commercial controllers, so it's probably not a bad idea to have a little wiggle room. I'm probably going to go with a 1200v diode to stay on the safe side. Whether it's necessary or not I don't trust myself to build something that won't have the occasional spike.


Sure, it's good to have a little extra wiggle room... or a lot, even. Just as long as you keep in mind that the forward voltage drop for a 1200V FRED will typically range from 1.8V at 25C and low current to as much as 3.5V at 125C and full rated current. Also remember that the FWD sees the same average current as the switch, so this could result in a tremendous amount of heat to get rid of. Sort of renders moot the ultra low on drop achievable with a bunch of 200V MOSFETs, doesn't it?


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## Tesseract (Sep 27, 2008)

yarross said:


> What about using paralleled SiC Schottkies from Cree? They come in 600V and 1200V ratings and current 2x10A.
> Or IXYS DSS 2x101-02A, 2x100A.


SiC is still relatively new technology, but besides that there is little or no advantage to using them that I can see for a sub-20kHz motor drive. Most FREDs have a t_rr_ of 35-50nS, and that makes them way more faster than any practical switch at these power levels (you really don't *want* to switch hundreds of amps across even nanohenries of inductance in 100nS or less).

Also, those Cree Schottkys have a rather high V_f_ of 1.5-2.4V for the 20A/600V model. That's in the same range as a 600V FRED... but a SiC Schottky is more expensive, has a higher leakage current and much higher junction capacitance (could result in ringing)... Unless you are running at 100kHz+ I don't think the marginally faster speed is worth it.


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## yarross (Jan 7, 2009)

Tesseract said:


> Also, those Cree Schottkys have a rather high V_f_ of 1.5-2.4V for the 20A/600V model. That's in the same range as a 600V FRED... but a SiC Schottky is more expensive, has a higher leakage current and much higher junction capacitance (could result in ringing)... Unless you are running at 100kHz+ I don't think the marginally faster speed is worth it.


I was thinking rather of lower rr current. It could lower turnon losses of switch, when it effectively shorts power supply due to rr current. Also, it would be less snappy because rr current is almost entirely junction capacitance charging current, so I would expect less ringing on turnon slope.


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## Tesseract (Sep 27, 2008)

yarross said:


> I was thinking rather of lower rr current. It could lower turnon losses of switch, when it effectively shorts power supply due to rr current...


Hmmm... good point. Reverse recovery current would effectively be reduced to zero, which is definitely a plus. Careful analysis would be needed to determine if this benefit outweighs the downsides, but it does seem worth investigating.



yarross said:


> Also, it would be less snappy because rr current is almost entirely junction capacitance charging current, so I would expect less ringing on turnon slope.


This I am less sure of, intuitively. The higher junction capacitance inherent in Schottky's is fairly high-Q and can ring strongly in conjunction with the parasitic inductance present... Not saying this isn't without merit, just that I am less sure of it at this moment.


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## jsodemann (Oct 19, 2008)

I don't know if this will be any help, but from my investigations of Curtis controllers I have determined that the 1221 uses 16 diodes in parallel. The numbers on the diodes are CR4180R, In the 1204 there are 8 diodes labelled DR4180R. I haven't found any data on these other than they are made by ON Semiconductor. Digikey sells a diode I have been working with on my controller which combines two Curtis 1204s. It is also made by ON but is a button diode numbered MR2504 (http://www.onsemi.com/pub_link/Collateral/MR2500-D.PDF). The DR4180R diodes are made from button diodes fused between two plates. My only concern is the voltage drop on the MR2504s are 1.18 volts at 25C. The datasheet doesn't list a Trr but does have a graph of Trr vs Ir/If showing the range to be in the 2 to 12 microsecond band at 10A.


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## bipolar (Mar 11, 2009)

Here is some info from a supplier that we've used for years. Some excellent info on switch dynamics.

http://www.semikron.com/internet/index.jsp?sekId=13


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