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Expected quite some noise, but found nearly none.

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mtwieg:

--- Quote from: eTobey on January 12, 2025, 05:28:33 pm ---It is the gate on turn on. See the miller plateau. It goes up to 10V after that.
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I highly doubt that whole thing is one miller plateau, unless you intentionally added a ton of Cgd for some reason. Can you show what Vds looks like at the same time?


--- Quote ---The red trace is a bit missleading, as i have F3 activated (which is of no interest). C2 ( the other red) is not leaving the screen.
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By C2 you're referring to the pink-ish trace in the third capture? Is that the voltage across the shunt resistor?


--- Quote ---Yellow trace is measured with a DIY field probe.
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I understand using sniffer coils/pickup loops for detecting HF ringing, but it's impossible to make heads or tails of its output without knowing more about it (like what is its self resonant frequency) and exactly how it's positioned.


--- Quote ---No, there is no other sensor, it is a math function.
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What is a math function? Of what?


--- Quote ---the probing technique i would consider not poor
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It's hard to say for sure without seeing the larger setup.

eTobey:

--- Quote from: temperance on January 13, 2025, 02:33:19 am ---I think you have to admit that what you wrote isn't very clear and the picture doesn't show much either. It shows a probe attached to a power resistor and some wires. Where is the DIY field probe? Because that's what you're question is all about.

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Its a wireloop on the tip of a probe, square about 15-20mm. Not worth a picture IMHO.


--- Quote from: mikerj on January 13, 2025, 08:59:02 am ---Why would you think that a wire wound power resistor would be appropriate for measuring rapidly changing currents?

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Because it does conduct current, and it creates a voltage drop without going up in flames. Why would it not be appropriate?


--- Quote from: mtwieg on January 13, 2025, 12:59:42 pm ---....

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That Millerplateau looks distorted, since the gate current is controlled by the driver (6EDL7141).


Yes its the pinkish trace. Its a mess with these colors...
Yes its the voltage accross the shunt.


Would the selfresonant frequency be so much of a factor, when there are only a few swings, or even on edge?
The wireloop was positioned on the cable in the best way (tested on multiple places) to pick up the most noise/signal.


Its the math function of the scope.

eTobey:
I conducted another test, as the method of current measurement was questioned:

What i found is, that the magnetic field, that is produced by the changing current, induces a bigger voltage on the signal, than the actual voltage that is produced by the current in the shunt itself.

I did it like this:
Used a resistor wire, and measured with a short standart ground spring (pink trace), and did another measure with a longer ground wire (red trace), that had about 2-3 times as much loop area. Those two measures were taken on the same point (pretty close at least) of the resistor wire The second measure had much bigger Vpp values.

Therefore i did not measure to little noise, i just measured too much current.

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