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  #21  
Old 07-03-2011, 03:22 PM
valerun valerun is offline
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Default Re: Another DIY DC controller mock

Quote:
Originally Posted by jackbauer View Post
Can you measure Vce sat? Might give you an indication of which one is taking the bulk of the current.
can try. need to read up on that ;-)

Also, after reading through the SEMIKRON doc you posted on the other forum (attached), I realized that my incoming DC connections might not be routed right. Here's what I have:


HTML Code:
DC "-", Motor "-"  ========================

                   ========================  DC "+"
                    Q1     D1     Q2     D2 
                   ========================  Motor "+" (or "AC")
So that might be the reason for uneven sharing - when IGBTs (Q1 and Q2) are on, conduction path is from DC "+" through Q1 & Q2 to Motor "+", then through motor, then to Motor "-"/DC "-". Therefore "ON" path is skewed towards Q2 (which is exactly the side that heats more). When IGBTs are off, conduction path is from DC "-" through D1 & D2 to Motor "+", then through the motor back to DC "-"/Motor "-". Therefore "OFF" path is symmetrical.

So I gather that I need to move DC+ cable to the left side.

Am I on the right track?

V

Last edited by valerun; 07-03-2011 at 03:28 PM.
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  #22  
Old 07-04-2011, 04:11 AM
valerun valerun is offline
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Default Re: Another DIY DC controller mock

110V pack, 900A output (~12kW). Turn-off spikes at ~200V on C-E (attached is a 600A plot showing ~140V overshoot). So total voltage stress of 310V. Seems to be linear with the output current and not very dependent on the pack voltage. Not sure I need to worry about the spikes anymore given that my parts are all 1200V...

Anyway, 2uF snubbers are on order. Will at least use on DC rails - that should shave off ~1/3 of the spike voltage. A bit concerned about using them across the switch, though - 2uF discharged through the switch 5000 times a second is ~500W extra dissipation per IGBT at a hypothetical 300V pack voltage that I might try on this thing at some point (;-)...

Connected current sensor and control board (adopted from the charger). Can see the current / temp / voltage / etc on the screen now. Kind of nice. Unfortunately, will have to give up the screen as every refresh eats up 200-300ms which was fine for the charger but is a deal-breaker in a controller that needs to respond to throttle change in <50ms or so...

Checked thermal balance after rerouting the DC link - seems to be better but not quite equivalent (one side ~5 deg C cooler than the other...). Perhaps that's ok and I just should stop stressing out about that. With 3 92mm PC fans and 7lbs finned heatsink, 250Amps continuous keeps heatsink temp under 55 degrees. As is 600Amps for 30-60 sec. Not bad but expect losses to double-triple when going to higher pack voltage. The last card in the sleeve is automatic PWM frequency reduction with current - at 100uH for something like 9" motor, anything above 400A at 5kHz gives < 40% current ripple. Hence could go down to 2.5kHz for high/low duty cycle and / or high output current. The downside, of course, is the whining becoming more and more noticeable with lowering frequency...

V
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Last edited by valerun; 07-04-2011 at 04:14 AM.
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  #23  
Old 07-04-2011, 08:01 AM
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jackbauer jackbauer is offline
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Default Re: Another DIY DC controller mock

Interesting. Always thought i was solving a problem with the snubber across the switch ....... might just remove it to the dc bus and see what happens.
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Now, Cole, when you shift the gear and that little needle on the ammeter goes into the red and reads 1000 Amps, that's bad.
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  #24  
Old 07-04-2011, 12:46 PM
valerun valerun is offline
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Default Re: Another DIY DC controller mock

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Originally Posted by jackbauer View Post
Interesting. Always thought i was solving a problem with the snubber across the switch ....... might just remove it to the dc bus and see what happens.
cap across the switch does help but the loss is going to be ~U^2 so if your pack is 100-150V, you will have 4-9 times lower dissipation than at 300V. 50-100W per one of these devices is not a problem at all. I'm just a bit worried about 500W ;-)

However, by adding a cap across the switch, I think you are reducing turn-off losses within IGBT as you are slowing down the voltage ramp across the switch and the current has a chance to die down before a significant voltage develops across the switch. As the turn-off loss is the integral of voltage across the switch times current through the switch during the transition period, this reduces the torn-off loss.

So there might be some optimal value of the cap across the switch when you actually reduce total losses... Haven't seen any literature on this, though - everyone is talking either about RC or RCD snubbers...

V
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  #25  
Old 07-04-2011, 12:49 PM
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jackbauer jackbauer is offline
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Default Re: Another DIY DC controller mock

Yeh it hard to find any real info on the subject. I did look back over my videos and the cap on the switch made a big difference. I may change the value a little.

Damn you Val for starting this thread now i have to go build another one of these things
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Now, Cole, when you shift the gear and that little needle on the ammeter goes into the red and reads 1000 Amps, that's bad.
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  #26  
Old 07-04-2011, 03:03 PM
JRoque JRoque is offline
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Default Re: Another DIY DC controller mock

Hi.
Quote:
Originally Posted by valerun View Post
Haven't seen any literature on this, though - everyone is talking either about RC or RCD snubbers...

V
RCD snubbers are critical in this type of application. Here's some reference, if you don't already have it: http://www.daycounter.com/Calculator...lculator.phtml.

Hopefully you'll switch from a mock to actual project once you've pieced more things together. Looks very interesting and educational.

JR
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  #27  
Old 07-04-2011, 04:01 PM
valerun valerun is offline
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Default Re: Another DIY DC controller mock

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Originally Posted by JRoque View Post
Hi.


RCD snubbers are critical in this type of application. Here's some reference, if you don't already have it: http://www.daycounter.com/Calculator...lculator.phtml.

Hopefully you'll switch from a mock to actual project once you've pieced more things together. Looks very interesting and educational.

JR
Thanks JR. I remember seeing this page at some point early in design of my charger but I decided back then that for <100A currents I see in a charger these snubbers were too complex. Now, of course, it's a different story... Still, though - no mentioning of how a simple cap across the switch could work...

Anyway, the 'method 3' from your link peaks my attention as it is lossless. The lossy snubbers all dissipate something like 500W in my calculations (at 1000A 300V hypothetical scenario). I just need to think a bit about how to adopt the Method 3 design to a buck topology.

Re converting this mock to a project - it's already kind of a project but I just don't want to over-promise before going through all the testing.

Thanks for your encouragements!

V
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  #28  
Old 07-04-2011, 04:02 PM
valerun valerun is offline
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Default Re: Another DIY DC controller mock

Quote:
Originally Posted by jackbauer View Post
Yeh it hard to find any real info on the subject. I did look back over my videos and the cap on the switch made a big difference. I may change the value a little.

Damn you Val for starting this thread now i have to go build another one of these things
Oh, Damien - love the pics! Very nice. Liquid cooling is the way to go for high power and even load sharing...

V
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  #29  
Old 07-04-2011, 05:22 PM
JRoque JRoque is offline
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Default Re: Another DIY DC controller mock

Hello V,

This is probably redundant to your own research but here are some links on snubber design:

Videos: http://www.snubberdesign.com/snubber-videos.html
Paper: http://www.newark.com/pdfs/techartic...ell/design.pdf
Book: http://store.kagi.com/cgi-bin/store....SMPSTechnology

JR
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  #30  
Old 07-04-2011, 09:29 PM
valerun valerun is offline
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Default Re: Another DIY DC controller mock

Quote:
Originally Posted by JRoque View Post
This is probably redundant to your own research but here are some links on snubber design:
JR
Thank you JR! Not redundant - haven't seen these before. On my side, the best doc I found so far was http://focus.ti.com/lit/an/slup100/slup100.pdf.

So I decided to try the RCD snubber across the switch - in the clamp configuration (when RC >> switching period). The snubber dissipation is supposed to be much less than in the RC snubbers that recharge on every transition - the only loss in this snubber should be the energy stored in the stray inductances. In my non-ideal snubber design, I have put together a 1200V 60A u-fast diode, 4.7uF metal film cap, and a 5kOhm resistor (the latter could really be much reduced but that was the only >200Ohm resistor I had in a non-inductive package). Below is the screenshot of C-E voltage on turn-off at 800A (110V DC rail). Pretty ugly. My guess is that the snubber diode has too slow forward recovery (indeed, datasheet says 700ns!) and so it doesn't even turn on for the most of the initial spike. Then, when it does turn on, it sets off the ringing. Somewhere around the same time, main diode turns on, as well. The result is a bit of a jagged mess. Not really sure about the origin of the super-high frequency ringing you see on the pick (~30MHz). Perhaps self-resonance of the snubber? Anyway, clearly not the optimal design solution ;-)

I think the key to making it work is very fast forward recovery diode. In fact, JR, in the paper you forwarded, Rudy says that this indeed may be a problem so I suspect it even more now. What do you guys think of using a Silicone Carbide Shottky diode for a snubber? For example, http://search.digikey.com/scripts/Dk...=C2D05120A-ND?

V

PS. Also had a curious inadvertent experiment - while putting things back together after installing the snubber, forgot to plug in the second IGBT driver. Then, after running a few 700A ramps, noticed it! Learnings:
1. There is quite a bit of reserve in a 2-IGBT, 2-diode design - if I can run 700A for 20 sec on just 1 IGBT...
2. It's good to have driver boards bolted to the IGBT. No matter what happens to your upstream circuitry, IGBT won't turn on.
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