NOT SQUARE

[PDDD]

Hello,
I have drawn and built an H-Bridge circuit after the design which Ron Simpson posted for me. See PDF attached.
With no power on the Mosfet Drains, the signal to the Gates are square to within 100nS. See BK2.BMP
With 24V on the Mosfet Drains, the signal on the Lower Mosfet Gates remain square, but the Signal to the Upper Gates are no longer square. See BK3.BMP.
Of course, the signal at the gate looks similar, only 12V lower at the peak.
Any advise would be greatly appreciated.
Thank you.
PDDD

 

[PDDD]

Sorry, I meant to say Source on line 4.

 

[Les Jones]

Don't forget that the drivers for the upper mosfets are driving them with respect to their sources so the signal that you are looking at with respect to ground is the 12 volt square wave sitting on top of the H bridge output.

Les.

 

[PDDD]

Thank you, Les!
Wow, if I understand correctly, you're basically saying that my circuit is working perfectly. What still confuses me is that twice I got a square wave that went from 36V high down to 3V, then tapered off. But after a couple of signals it started down to 24V, then tapered off. Other times it drops to 15V, then tapers off.
Can you shed any light on why this might be?
Thanks again Les.

 

[PDDD]

Here's two attachments.
BK000002.BMP is Side B of the H-Bridge.
BK000003.BMP is Side A. Do you know what can account for the difference. I'm using new Mosfets and drivers that are healthy and have not been stressed.
Thanks for any help.

 

[PDDD]

Sorry, Here's BK000003.BMP

 

[Les Jones]

I'm not familiar with the driver chips so I would only be guessing at an explanation. I did wonder about the slow decay on the waveform. Are the two "square wave" drive signals 50% duty cycle and exactly complementary ? If there is a time when both drive signals are low then there would be a time when none of the mosfets were conducting so there would be nothing to pull the output low so it may be just drifting low. I have no idea why both sides of the bridge give different waveform.

Les.