Expected quite some noise, but found nearly none. - Page 1
Forum section: electronics / projects

author: eTobey
author_rank: Super Contributor
timestamp: January 11, 2025, 11:34:07 pm
content: Hi,i have a coil, that i send current through with a half bridge. After the coil i have a shunt resistor, and there i measure the current (green trace in ampere), that swings (seemingly) about 2.6A pk-pk with quite a steep slope. It is the second ringing on the screenshots, that i refer to.Blue trace: Gate of HSRed trace: Voltage of shunt (0.104Ohm)Yellow trace: Measured noise with a 15mm shorted loop.Green trace: Ampereof coilFull bandwith with peakdetect.The loop was help right on the cable, but there was only 20mVpk noise. I would have expected about 10 times more. Other chips on the PCB generate higher peak noise.Do i measure a signal that is not there with that amplidtude, or do i have to measure this noise differently?

author: temperance
author_rank: Super Contributor
timestamp: January 11, 2025, 11:55:21 pm
content: The induced voltage depends on the rate of change of current.

author: eTobey
author_rank: Super Contributor
timestamp: January 12, 2025, 11:59:13 am
content: Quote from: temperance on January 11, 2025, 11:55:21 pm227A/us would not be quite a rate of change?

author: jonpaul
author_rank: Super Contributor
timestamp: January 12, 2025, 03:02:19 pm
content: Ringing due to poor probe technique and bad breadboard setup.Read about power supply measurements and probe measurements first.j

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 12, 2025, 03:49:04 pm
content: Your descriptions of the scope traces don't make much sense:QuoteI'm guessing we see two transitions because of dead time in the half bridge? Is the y scaling right, drive voltage is only around 5V?QuoteSo it looks like the current shoots up to >30A (3V/0.1ohm), at which point the trace is off the screen... is that right?Quote.... what?QuoteSo you're measuring current with something else besides the shunt resistor (red trace)? What's the sensitivity of this sensor?

author: temperance
author_rank: Super Contributor
timestamp: January 12, 2025, 04:04:25 pm
content: Quote from: jonpaul on January 12, 2025, 03:02:19 pmThat's not what the question is about? The question is about induced voltage in a loop created by shorting the probe to the tip.

author: eTobey
author_rank: Super Contributor
timestamp: January 12, 2025, 05:28:33 pm
content: Quote from: mtwieg on January 12, 2025, 03:49:04 pmIt is the gate on turn on. See the miller plateau. It goes up to 10V after that.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.Yellow trace is measured with a DIY field probe.No, there is no other sensor, it is a math function.

author: eTobey
author_rank: Super Contributor
timestamp: January 12, 2025, 05:31:43 pm
content: Quote from: jonpaul on January 12, 2025, 03:02:19 pmWhat about you reading my post and looking at my pictures first, before you give an answer? Or dont do drugs or alcohol, since you see things, that are just not there! There isno breadboard, and the probing technique i would considernotpoor.It is of no use whatsoever, to give answers just from ones guessings, when there are informations, so that one would not need to guess!

author: temperance
author_rank: Super Contributor
timestamp: January 13, 2025, 02:33:19 am
content: Quote from: eTobey on January 12, 2025, 05:31:43 pmI 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.

author: mikerj
author_rank: Super Contributor
timestamp: January 13, 2025, 08:59:02 am
content: Quote from: eTobey on January 12, 2025, 05:31:43 pmCalm down, this kind of attitude isn't going to want people to help you. Why would you think that a wire wound power resistor would be appropriate for measuring rapidly changing currents?

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 13, 2025, 12:59:42 pm
content: Quote from: eTobey on January 12, 2025, 05:28:33 pmI 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?QuoteBy C2 you're referring to the pink-ish trace in the third capture? Is that the voltage across the shunt resistor?QuoteI 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.QuoteWhat is a math function? Of what?QuoteIt's hard to say for sure without seeing the larger setup.

author: eTobey
author_rank: Super Contributor
timestamp: January 14, 2025, 06:42:01 am
content: Quote from: temperance on January 13, 2025, 02:33:19 amIts 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 amBecause 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 pmThat 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.

author: eTobey
author_rank: Super Contributor
timestamp: January 15, 2025, 11:19:32 am
content: 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.