Paul & Sabrina's cheap DIY 144v motor controller - Page 124 - Fuel Economy, Hypermiling, EcoModding News and Forum

MazdaMatt · 05-14-2009, 11:02 AM

Oh right, the ringing

That's right on the FM radio band! You could tune in to your controller's ringing!

I have noticed that IGBT's are very often used with a hardcore low-value capacitor across the emitter and collector to prevent ringing. Have you looked into that on this controller? Is it a good or bad idea? I don't know, i'm no power electronics guru. Maybe you can run it by the power expert.

MPaulHolmes · 05-14-2009, 11:10 AM

The ringing doesn't go very high or low, so I don't think there is any danger of exceeding the voltage ratings of anything, but it does make the noise. I think a capacitor would have a bad effect on those mosfets keeping them in their transition region, so the heat loss may be bad.

MazdaMatt · 05-14-2009, 11:19 AM

we're not capacitorizing the gate, so i don't know if it will slow down the transition. We are talking about a capacitor that is just big enough to filter a 50Mhz ring to some reasonable degree. Since your transition times are between 1 and 2us, this should have little/no effect on switch times. I think it is worth looking at. I may be crazy, they could just be for IGBT's. Again... i'm no power engineer.

MPaulHolmes · 05-14-2009, 11:33 AM

Oops! Collector to emitter. I should read your messages before responding to them! haha! Interesting! Maybe some little ceramic 1000v thing. RdsOn is 0.008 ohms.

R*C = 1/100,000,000

C = 1.25 uF

spendy! 200v 1.25uF low ESR MLCC. Through hole versions are $10 each. surface mount has a minimum of 1000 order. It would be perfect for surface mount, though. Right from drain to source.

I'm confused! This isn't an RC circuit is it?

MazdaMatt · 05-14-2009, 11:42 AM

Careful, your R may be .008/10, unless you want to use 10 capacitors. Probably the closer to the leads the better, so you may be right, but you're talking about 100 dollars in caps, then. However, capping here may greatly help the other capacitors and batteries... the whole works, really. Again, i'm talking about a method usually used on IGBT's and I'm no power engineer ( did i mention that?)

MPaulHolmes · 05-14-2009, 11:52 AM

I found some surface mount 250v 1.2uF caps for $1.50 each or so on Mouser. Is this the RC snubber I've heard about? I think they do do this on mosfet based controllers, but I don't know. I'm confused by how it works. Nevermind. I drew a picture and I think I get it now. the capacitor discharges while the mosfet is ON, and acts like a shock absorber when the mosfet suddenly gets turned off.

I think I'm wrong about the capacitor value. Choose a convenient resistance. Let's say 1 Ohm. Then choose a 10 pF cap. Put them in series, and connect it from drain to source. I think that's how it should work. I don't know, though.

MazdaMatt · 05-14-2009, 11:55 AM

Google knows everything. I found this:
Snubber Circuits Suppress Voltage Transient Spikes in Multiple Output DC-DC Flyback Converter Power Supplies - Maxim
it is an app note from maxim regarding snubber RCD and RC circuits for use in mosfet driven flyback transformers (similar stuff to what we're doing here). I gave it a skim and it seems simple enough... I'd run it by our lab tester, though.

jyanof · 05-14-2009, 01:02 PM

I believe I used the same maxim app note when playing around with snubbers on my charger power section. This was before I realized that the ringing was due to the parasitic inductance of the input wiring and that it needed to be minimized to reduce the ringing.

Anyway, I hacked together an RCD snubber from spare parts and it eliminated the ~3 MHz ringing on the Drain-Source voltage and reduced the initial voltage spike. Once I redid the wiring and reduced the parasitic inductance, there was virtually no ringing and only a very little voltage spike so I saw no need to add the RCD snubber back in.

Paul, I believe you've already minimized the parasitic inductance with the power board layout... are you having problems or being proactive about EMI?

Oh, I also came across 'snubber' capacitors when I was researching snubber circuits. I believe are designed as a very low ESR low value cap that can handle the rapid full charge/discharge it'd see.

Hondo · 05-14-2009, 01:12 PM

Hey Paul,

I think I can finally help on this project. I made this up quick in my company's metal shop. It is made from 14 guage sheet metal, but I could make it out of .080 aluminum just as easily, ( except we don't have any .080 in stock right now, it's on order and will be here in a week or so). Do you think it will work? We would just need to bolt the bottom onto the flanges and make some kind of end cap. If it will work, I can make some up and send them to you for free.

Hondo