Gate driving… and shoot through?

May 20, 2014 by crxguy52 2 Comments

I was reviewing the scope trace I posted yesterday, and noticed that the low side gate drive has a spike in it exactly when the high side turns on (red is high gate and yellow is low gate):

 

 

So I adjusted the dead time to see if it was shoot through, or if it followed the high side gate. Turns out it follows the gate, and was not shoot through:

 

 

This concerns me, since a voltage spike at the gate could cause it to turn on (even partially), and cause shoot through. The magnitude of the spike is ~1.75V normally, with occasional spikes up to 2.75V:

 

 

Next I compared the M+ bar (between high and low side FETs) to the low side gate, to see if the spike was causing the low FET to conduct and cause shoot through (thus lowering the voltage available to the motor):

 

 

Low side gate is in yellow, +M is in red. It doesn’t look like it’s dropping the voltage available to the motor, so it must not be conducting much? The real way to tell would be to have a very fast current sensing probe. Looking closer at the motor versus gate voltage, there is some oscillations in the motor voltage but the frequency doesn’t match and the edge is still somewhat sharp:

 

 

Looks like the FETs are switching at 160ns – pretty fast! I went hunting for the source of the spike, so I made my way back from the gate to the driver. The spike was at full magnitude immediately after the gate resistor on the logic board (ruling out the wiring between it and the FET), but dropped to ~0.5V BEFORE the gate resistor… huh? In order for there to be a positive voltage at the gate, across the gate resistor, would mean that current would be flowing OUT of the gate, even after it’s off.

Could the driver ground be going negative relative to the source? I checked the source on the low side FET, and the voltage is stable. I also checked the low side ground on the gate driver, and it was the same story as before the gate resistor – ~.5V variation. In fact, I probed everywhere on the gate drive IC, and the only place I see the spike is after the parallel gate resistor. Maybe the resistor is picking up noise from the power stage? That’s the only explanation I can think of. Either that, or there is some drain-gate leakage voltage (voltage bumps on the gate when drain to source voltage spikes). If you have any insight, let me know.

I also measured the high side switching times. It’s a bit longer at 290ns, which is understandable since it’s bootstrapped and has to reach a higher voltage than the low side. It looks like the high side FET turns on fully when the gate reaches ~10V – perfect, since all of the miller plateauing happens after that:

 

 

 

I’m not sure what to do about the voltage spike on the low side FET gate, so moving forward I’m just going to keep an eye on it. The next step is to finalize all of the mechanical stuff – drill and tap the heatsinks for the FETs and regulators, install final standoffs, and add RF shielding between logic and power stages. I should get a package this week with all of the stuff I’ll need, so hopefully by next Saturday I’ll have it to the point where I can test it on the cart.

In an unrelated note, the current sensor has a fair amount of drift in it – It should read 2.5V at rest, and it fluctuates around 2.87V. I wonder if it has to do with the supply voltage fluctuating – I grabbed a 0-200k pot for the 5V regulator instead of a 0-10k, which means it’s very sensitive to small changes in pot position.