What's going on with my high side gate drive?

[H.O]

(EDIT: I'm sorry, this should of course have been posted in the Project section - not in the Test Equipment section. Any moderator, please feel free to move it.)

Hi,
I'm designing a H-bridge for a small PMDC servo drive I'm playing around with. I'm seeing things I don't like on the gates of the high side switches, I think what I'm seeing is induced turn-on but I'm not sure and, part from PCB layout (which I Think I have done a fairly good job of (can post layout later), I have no idea what to do....

This is what one leg of the bridge looks like, the other half identical, electrolytics and current sense circuitry is shared between both legs:



I'm using PSMN012-100YS MOSFETS, L6387E drivers, 15ohm gate resistors with DB2J20900 diodes in parallel. At the beginning I toasted a couple of L6387 from what I think was negative spike on the outpin of the driver (drain/source node of the switch leg) so per a suggestion in an ST app note (AN1299) I added a (currently) 7.5 ohm resistor in series with the outpin.


Here's what the drive signal looks like with 100ohm wirewound resistor as the load, 20V supply voltage. Ch1 and Ch4 (yellow and blue) are the control signals to the L6387E driver, 500ns deadtime on the PWM signals. Ch2 is the low side gate and Ch3 is the high side gate. First low side turning off, high side turning on.



And then low side turning on, high side turning off:



I'd appreciate any suggestions, thanks!

[siggi]

Your high gate voltage is referenced to the output, not ground. Put one channel on the output and display the difference between the high side gate and the output. That'll show you what the gate driver is doing.


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[E.Klaveback]

Your high gate voltage is referenced to the output, not ground. Put one channel on the output and display the difference between the high side gate and the output. That'll show you what the gate driver is doing.

I would expect this to be the thing. I have had the same things going on on the gate of the high side MOSFET. Here are some scopes of my particular situation where the channel "SWITCH" is the node between the highside/lowside MOSFET connected trough an inductor over to a brushed DC motor.

For some reason the output varies from cycle to cycle in time and amplitude with and without a motor connected.