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Old 07-17-2010, 02:22 AM   #18 (permalink)
electrowizard
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Join Date: Jun 2010
Location: Fort Erie, Canada
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Back from the cottage

It was a great trip; I'm sure I gained another 5 pounds over the few days I was there. Went for a 19km bike ride to town though, was impressed with myself.

I came home and guess what? You know the mosfet that died before I left?
Apparently it is not dead

I immediately checked the others and yes they are all dead for sure.


I realized it was not broken with my ohmmeter; I proved it still worked by powering the circuit up.
Thinking back to the grounding issues raised by saand, I disconnected the oscilloscope ground leads from B-. Immediately the motor shot to full speed as if the mosfet died again, but reconnecting ground fixed the problem.

I tried to check whether or not my power supplies were isolated from line ground or not. (driver and microcontroller supplies) I had assumed they were not isolated. Testing continuity with my ohmmeter between the power supply output and the 110 line contacts indicates no connection on either supply. This means they are *both* isolated. This causes me to suspect that my testing method is not valid. Does anyone have a better way or is this correct?

To correct the issue in the mean time, I connected my microcontroller ground to my driver ground, such that they have the same reference. The problem is apparently resolved.


saand:
Thanks for the encouragement
As I said I am not using an iron but rather a propane torch to heat the bars.
I heat away from the devices, and once the solder is hot enough, I stick the SMT devices down and add some fresh solder around the tab while pressing down on the case to ensure a good contact. Cooling down takes at least ten minutes, even with using scrap copper bars to absorb heat and dump it into cooling water.

I am switching at 16kHz though I am free to choose 4 or 8 easily.

The gate oscillations again.. I removed the 10ohm gate resistor. My drive waveform is now at 100ns with only 3% overshoot and minimal ringing. That gate resistor is a menace. However you were right, the supply rails also have this oscillation. See below for more on this.

Thanks for the ESD information, very good to know. 5 years of engineering school and they never taught us that. . .

All components are cool under load at 50% duty cycle for about an hour. Except the motor :P


Weisheimer:
No problem, it happens to the best of us hehe
As you said the gate resistor can be calculated but it is more empirically chosen to reduce ringing right? So if in my application I have no ringing with no gate resistor and a reasonable turn-on time then I should not include it. Do you agree?

What is your take on my supply isolation; how can I test this properly?




Ringing revisited:
My battery voltage (BM+ to B-) rings an extraordinary amount on mosfet turn-on.
Click image for larger version

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Blue = Vdd; Yellow = Gate

I would expect this to occur due to a lack of controller capacitors.
I have verified that I have about 27mF of capacitance due to a 1.1 second time constant with my 42 ohm precharge resistor. I know that does not indicate a low esr or good ripple capability but it does prove the caps are connected.
I also have two of those red metallized polypropylene capacitors (where P+S's much larger controller uses 3 and some other electrolytics) so I would expect my voltage swing to be minimal on a 14A load.

With a nominal 12V battery voltage the buss bar voltage swings from 28V down to 1V. The period of oscillation is close to the risetime of the driver signal.
Remember the mosfet is logic-level and is thus turned on completely, thus unable to oscillate when Vgs varies in the image.

So I am puzzled by the Vdd oscillations. My controller capacitors should prevent it from happening. These caps can even turn the motor ~1/2 turn from a standstill after the battery was disconnected. Are my caps to blame - bad ESR - or is there something I'm not thinking about?

Also, I still do not know why I am blowing transistors with the starter motor. The window motor works excellently. Curious curious..
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