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I apologise if this is a repost but I couldn't find the problem anywhere else.

I am trying to power an ultrasonic transducer using an h-bridge inverter design to convert arduino signal into a higher power ac square wave (shown in the circuit below).Inverter Schematics It was working well, then suddenly I started getting high voltage spikes in the rising and falling edge of the square wave. I am using a high varying driving voltage of 15v-60v (VDC in the schematics) and I have seen that increasing this voltage distorts the output more. I am hoping to drive the transducer from 20kHz to 30kHz.Distorted output waveformThis is a zoomed in picture of it. DIstorted output waveform zoomed inYou can see that the top of the square wave is collapsed slightly as well as the spikes on the bottom left of the picture. I've tried rebuilding the circuit using new components on a different pcb but I'm still encountering the same problem. I am thinking it might have something to do with the mosfets (irf4227), but I have changed them to a different type of mosfets (irf540n) and the problem still continues. I have also changed the bootstraps capacitors to a larger value of 330nF from the 22pF in the schematics but it did not do anything for the distortion.

Thank you very much for any help you can give.

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    \$\begingroup\$ By any chance are you using a standard X10 probe ? And the EARTH clip at some annoying point \$\endgroup\$ – JonRB Feb 15 at 8:50
  • \$\begingroup\$ You'd need ultra fast recovery diodes. Also 1k ohm gate/source resistors are not needed, IMO. From the picture, the scope is kind of DIY \$\endgroup\$ – Marko Buršič Feb 15 at 9:42
  • \$\begingroup\$ If it isn't just scope pickup, then stray parasitics can also be causing spikes, can you show the PCB layout? \$\endgroup\$ – Unimportant Feb 15 at 11:09
  • \$\begingroup\$ Yes I am using a cheap DIY Oscilloscope and no I don't think it's a standard x10 probe as it has alligator clips at the end. I have the ground clipped to the same ground as the 12v input which is common to the 5v. \$\endgroup\$ – HDR Feb 15 at 11:12
  • \$\begingroup\$ @HDR These clips in the picture? If so, that's probably a part of your problems. You need a low inductance probe like shown in this post. Also, trying to view a 30kHz square wave with a 200kHz "scope" won't give you enough harmonics to see the true wave. \$\endgroup\$ – Unimportant Feb 15 at 11:38
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You dont have 22pF bootsteap caps, you have a 10uF plus 22pF bootstraps caps! Remove the 10 uF twice. They probably cause the spikes below 0V, as the cap pushes the voltage downwards when opening one of the lower fets. (depending on your layout)

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  • \$\begingroup\$ Wait, why are the 22pF drawn as resistors? \$\endgroup\$ – Huisman Feb 15 at 11:06
  • \$\begingroup\$ In any case, 10uF is overkill and 22pF is too little. 100nF or so would probably be more in the ballpark. \$\endgroup\$ – Unimportant Feb 15 at 11:13
  • \$\begingroup\$ @Huisman Haha I think it's a mistake but I definitely have capacitors where they are. \$\endgroup\$ – HDR Feb 15 at 12:33
  • \$\begingroup\$ @Huisman I will try to remove the 10uF caps and see. \$\endgroup\$ – HDR Feb 15 at 12:34
  • \$\begingroup\$ @Huisman I will also try around 100nF capacitors for bootstraps. Thank you all! \$\endgroup\$ – HDR Feb 15 at 12:35

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