Ringing with high-side switch; high rush-in current - Page 1
Forum section: electronics / projects

author: fitzgerald
author_rank: Contributor
timestamp: July 15, 2016, 07:58:04 am
content: Hi all!I have a setup where I switch a motor controller using an Infineon ITS428L2high-side switch(datasheet).The setup looks like this:~230V ---- [ AC/DC PSU +24V ] ---- [ ITS428L2 ] ---- [ motor controller ]The cabling between the PSU and the ITS428L2 is about 4.5 meter long AWG 18;between the ITS428L2 and the motor controller is about 0.5 meter cable. Duringnormal operation the device consumes about 0.5 A, but I believe there is quitea bit of rush-in current. The circuit of the high side switch is attached tothis post (schematic.png).With some troubleshooting I made the oscilloscope trace 'profet_in.png'. Here,channel 1 (yellow trace) shows the +24V input at the device; channel 2 (cyantrace) shows the voltage at the 'IN' pin of the ITS428L2. There is some 4 Vovershoot (17 %) on the power line. I included the trace of the IN pin of theITS428L2 because it shows that the switch is shortly disabled during itsturn-on.If I include a second ground wire between the motor controller and thehigh-side switch I get the trace 'profet_in_wgnd.png'. The overshoot andringing is now completely gone. There is still a dip in the IN line, but notenough to turn off the ITS428L2.Looks like a bad ground connection, but for all I can test and see the GND lineseems fine to me.To try something else I included a 100 nF cap across the IN input of theITS428L2 and removed the second ground wire. This is shown in the oscilloscopetrace 'profet_in_wgnd.png'. Note the difference in vertical scaling on channel2. It doesn't seem to have any effect.I measured +24V_local and noticed a 4 V dip in the supply voltage (supply.png) Iincluded a 100 uF low ESR electrolytic cap in parallel with C23, but that had littleto no effect.Any hints/tips on what could be going on?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 15, 2016, 11:36:58 am
content: I'm not seeing a problem here. Do you just not like to see it bounce?Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 15, 2016, 02:23:42 pm
content: your probes' ground clip, where are they connected? to the 24V PSU gnd? or near the ITS428L2 and the opto's ground?

author: fitzgerald
author_rank: Contributor
timestamp: July 18, 2016, 05:42:54 am
content: Quote from: T3sl4co1l on July 15, 2016, 11:36:58 amI think the overshoot is related to another issue that prevents the PC to connect to the motion controller. In the beginning I'd like to understand why the secondary GND connection makes the ringing go away.

author: fitzgerald
author_rank: Contributor
timestamp: July 18, 2016, 05:45:10 am
content: Quote from: Mechatrommer on July 15, 2016, 02:23:42 pmThe probes ground clip was connected at the motion controller GND or at the GND of the high-side switch. The trace looks identical; except when connecting both: that would create the secondary ground connection via the scope and the ringing is gone.

author: Andreas
author_rank: Super Contributor
timestamp: July 18, 2016, 06:12:47 am
content: Hello,I think the dip is created when the internal charge pump of the switch is starting to operate.(did not check the data sheet).The input resistance is rather high.I would do a zener stabilisation on the collector of the photocoupler instead of the high ohmic voltage divider.With best regardsAndreas

author: fitzgerald
author_rank: Contributor
timestamp: July 20, 2016, 08:54:13 am
content: Thanks for the help!In the end I found the issue. Apparently there is an inductor in the GND line of the motion controller between the power entry point and its common ground point. Together with the input capacitor it forms an LC filter that causes the ringing. The second ground wire shunts the inductor and solves the ringing.The zener is indeed a better approach for the photocoupler. Implemented that change as well. Thanks.