Author Topic: Probing techniques for MOSFETS with high RF about and heavy current switching?  (Read 878 times)

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Offline Chris WilsonTopic starter

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Lots of interesting posts recently about MOSFET driving and gate and source waveforms, I am not alone in having issues with these devices it seems :)

I have a mid range 2 channel digital USB scope. 150 MHz with the spec here:  http://www.dataman.com/oscilloscopes/dataman-526-150-mhz-usb-oscilloscope.html

I want to look at the the source pins of both MOSFET's used in a push pull Class D amp running at 136kHz. If I set the probes to X10 and read either source pin on either channel on its own the waveform is fine. if i use both probes and both channels to look at both waveforms simultaneously I get a noisy waveform on both channels. Similarly if I look at a gate on one MOSFET and a source pin on the other. Looking at gate and source on one MOSFET at the same time is no problem. This thing runs 50V at 20 Amps.

Is it technique ( I have tried with short ground pins made with stiff wire wrapped around the ground shoulder of both probes, rather than the long standard ground leads, with no change), the probes (Ebay, Chinese, described as 200MHz probes...), or the scope itself? Thanks.
Best regards,

                 Chris Wilson.
 

Offline T3sl4co1l

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Protip: first, check that what you think is ground, is actually ground.

At AC, ground can bounce all over the place.  Probes don't have particularly good common-mode rejection, partly because of the ground clip (in bad cases, yes, even a short spring won't save you), and partly because of the lossy shield on the cable.

Easy way to tell: connect the probe tip to the ground clip, and probe ground with that.  If you see noise, it ain't "ground" (relative to the scope chassis).

You can improve CMRR by wrapping the probe around big ferrite beads, but this can only do so much.

The best approach is to find a point which is "ground", and track backwards from there to the signal you want to probe.

Remember, most of all, that voltage is a difference.  The scope measures the voltage at its connector.  What you do to get that voltage there, is up to you. ;)

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
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