# Buck converter question



## yarross (Jan 7, 2009)

jehan12413 said:


> I'm wondering if I can connect a large freewheel diode across the motor in addition to the ones in the controllers or will this be asking for trouble?


Generally, it's possible. Some issues to solve - protection from environmental factors, cooling, current distribution of parallelized diodes. Having all this stuff enclosed in a controller is an elegant and safe way to do this.


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## jehan12413 (Feb 4, 2010)

The problem is that some of these controllers I am working on have been modified so much that the voltage is near the ratings of the freewheel diodes and their total current rating is nowhere near the amount that the MOSFETS are switching. If I go much higher with the current something's got to give and the diodes are the weak link! As far as putting some on the motor I can handle the heat and weather issues but am wondering if it will cause any oscillation or negative effects due to the cable length from the motor to controller.


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## yarross (Jan 7, 2009)

jehan12413 said:


> am wondering if it will cause any oscillation or negative effects due to the cable length from the motor to controller.


It would eventually make stray inductances lower by closing circulating currents around less area (depends on actual cable length). Controller diodes would take care about remaining inductance.
But personally I would just upgrade controller diodes.


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## Tesseract (Sep 27, 2008)

You want to keep the freewheeling diode(s) as close as possible to the switch(es)!

The loop area of the motor circuit is nearly inconsequential because the current is substantially DC (the loop area of the battery circuit is a whole other story, though!).

Inductance opposes a change in current, so any inductance between the switch and the FWD slows down the handing off of current from one to the other.The more inductance between the two (and it doesn't take much when you are switching 1000A) and the faster the switching time the higher the overvoltage that will result. If the IGBT is turned off faster than the reverse recovery time of the FWD then there will be a point where current is non-zero but nothing is conducting - voltage goes to the proverbial "infinity". Generally the switches and diodes don't much like that and they promptly blow up to save the insulation on the wire and motor windings (how thoughtful of them, no?).

Jehan, I strongly suggest you do a lot more reading on the design of buck converters before fooling around with higher power levels. Or, at least, keep an ABC fire extinguisher around and wear safety glasses at all times. Oh, and put some earplugs in too.


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## yarross (Jan 7, 2009)

Tesseract, I was not talking about removing diodes from controller, I was talking about supplementing them by external package, that would handle motor inductance.


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## Tesseract (Sep 27, 2008)

yarross said:


> Tesseract, I was not talking about removing diodes from controller, I was talking about supplementing them by external package, that would handle motor inductance.


Then the huge amount of inductance between the internal diodes and the external ones virtually guarantees that the internal diodes will have to handle all of the current during the transition. Might as well not even include the external ones at all... which might explain why no one does it or advocates it.


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## yarross (Jan 7, 2009)

Tesseract said:


> Then the huge amount of inductance between the internal diodes and the external ones virtually guarantees that the internal diodes will have to handle all of the current during the transition. Might as well not even include the external ones at all... which might explain why no one does it or advocates it.


Pulse rating is (2+)x continuous rating  and motor inductance is several times greater than stray inductance wich means kickback voltage will surely be exposed mainly at motor terminals.
Anyway, I vote for the controller diodes upgrade.


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## jehan12413 (Feb 4, 2010)

Tesseract said:


> Then the huge amount of inductance between the internal diodes and the external ones virtually guarantees that the internal diodes will have to handle all of the current during the transition. Might as well not even include the external ones at all... which might explain why no one does it or advocates it.


That answers my question right there! I guess I'll keep searching for a way to upgrade the internal ones. Thanks


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## Tesseract (Sep 27, 2008)

yarross said:


> ...motor inductance is several times greater than stray inductance wich means kickback voltage will surely be exposed mainly at motor terminals.


Yeah, lots of people think that - it's an easy mistake to make - but the motor inductance isn't really the problem here. Take a closer look at a buck converter - think of the motor as the output inductor and load - and see if you can figure out the consequences of inductance in between the switch and freewheeling diode.


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## yarross (Jan 7, 2009)

Tesseract said:


> Yeah, lots of people think that - it's an easy mistake to make - but the motor inductance isn't really the problem here. Take a closer look at a buck converter - think of the motor as the output inductor and load - and see if you can figure out the consequences of inductance in between the switch and freewheeling diode.


Hmm seems I must repeat - I'm not suggesting REMOVING internal FWDs.
Also, motor inductance matters. It's at least 10-50x greater and stores most of the energy of magnetic field. Stray inductance my cause high kickback voltages as well, but pulse energy will be limited. So my conclusion - external FWDs (without removing internal ones) should improve RMS current capability (but may fool current measurement ad confuse controller), thats all.


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## Tesseract (Sep 27, 2008)

yarross said:


> Hmm seems I must repeat - I'm not suggesting REMOVING internal FWDs.


Yaross - the problems here do not occur during the static portions of the operating cycle - ie, switch is on, FWD is off; switch is off, FWD is on. At those points in the cycle you very well may get the external and internal FWDs to share current well, even equally. It is during the dynamic portions of the cycle - when the switch is turning off and the diode is turning on, or vice versa - that the strays rear their ugly heads.



yarross said:


> Also, motor inductance matters.


Not when the current pathway is changing over from switch to diode or vice versa. The motor inductance only serves to force the current to remain constant while the changeover occurs. The only inductance that matters during the changeover is that between the switch and the diode. This is why I said go back and look at the buck converter schematic and imagine extra inductance in between those two devices. The more inductance there is the higher the voltage must rise to force the current to change pathways (specifically, from switch to output terminal over to diode to output terminal). When you put an external FWD in what you have done is add a really long path from switch to that external FWD in parallel with the much shorter path to the internal FWD. In other words, two rectifiers in parallel, but the second has an inductor in series with it. When you look at it that way it becomes clear why this will cause problems during the switching transitions.


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