Communications dropping on with sbRIO with converter switching

[Spex]

Adding link to complementary discussion forum post.

[BMac]

Status update:

• NI R&D has engaged with the design team to review/revise the wiring block diagram and identify possible conducted emissions paths, and is providing troubleshooting guidance to help identify the source of the problem. A next step is to review the mechanical design and identify possible radiated emissions paths.

Knowns:

• The problem occurs only when the power converter current exceeds certain levels.
• The heatsink is grounded to chassis and Earth ground as required for safety, but the DC link is floating per application requirements. The gate driver is powered from the same supply as the sbRIO GPIC (not recommended: see GPIC user guide system diagrams section.)
• IGBT switching action is generating voltages/currents with respect to Earth ground. Somewhere in the design (either in wiring or mechanical location of components) there is a path or loop being created from the high voltage side to the low voltage side. That emissions path is either conducted and/or radiated. The team is working to identify the path through various troubleshooting techniques, starting with the least effort tests.
  • For example:
    • One test will be to take measurements on unused wires of the Ethernet port itself to identify the nature of the common mode voltages appearing on the cable.
    • Using a magnetic field sniffer probe attached to a scope, need to find the most likely source of radiated emissions (i.e. power supply cables from gate driver, heat sink, etc.) that correlates with the common mode voltages seen on the Ethernet cable.
    • Need to determine whether it is the laptop Ethernet port or the sbRIO Ethernet port that's losing connection.
    • Need to analyze the mechanical layout with respect to radiated emissions and determine whether key circuit boards, cables, etc. are too close to electrical or magnetic field generator sources.
• Once the source(s) and nature (conducted or radiated) of the problematic emissions are identified, the strategy is to break the path. The nature of the solution will depend on the nature of the problem. Typical solutions: Shielding, ferrite beads, copper tapes, RF gaskets/seals, separate isolated power supplies, etc.

Note: NI offers free design reviews for OEM design teams developing GPIC interface boards on request. The ideal time for the design review is before the mating PCBs design and cable harness drawings have been finalized. Contact brian.maccleery@ni.com to request/schedule a design review.

[LabviewingToday]

A quick update on current status of the troubleshoot. The converter in question just stopped having communication issues without any changes to the setup. This was a bit disappointing since we were not able to run all the tests that we wanted to before shipping the system. On the other hand, the converter is working great.

We have had this issue on other topologies where we have seen better results by grounding the logic power supply and using ferrite beads on the power supply inputs. We will continue to run the recommended tests as we observe this problem in future units. I will update the forum if I have any new findings.              

Thanks to the NI team for their time and help,