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I read in an application note or comment on grounding and ground planes for better EMI and EMC that one method is to isolate your board power and ground planes from the chassis ground, creating a barrier on the edge of the board with multiple points where the chassis ground is and then linking the board ground plane and chassis ground with TVS diodes, bypass capacitors or other components.

On the underside of this board there is an array of capacitors that I think are bypass capacitors for discharging high frequency noise onto the chassis ground plane before it can affect inner board circuitry. The inner board circuitry consists of an H-bridge with bootstrap capacitor design that is driving an actuator with PWM. The actuator leads are landed on the terminal block where these capacitors are located under the board. I just thought it was strange that two capacitors were used in series for each input output. Maybe that was for selecting better values or getting the voltage isolation from the chassis ground.

I would like to discuss what these capacitors are doing and reasons they were arranged in the way they were and if there are any application notes or literature on selecting bypass capacitors for "grounding" actuators driven by H-bridges.

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    \$\begingroup\$ What does the ACT+ and ACT- notation mean? \$\endgroup\$
    – SteveSh
    Mar 8, 2023 at 15:41
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    \$\begingroup\$ Two capacitors in series are used in applications where you cannot tolerate a capacitor shorting. Do you know what type of capacitors these are? Ceramic? Tantalum? \$\endgroup\$
    – SteveSh
    Mar 8, 2023 at 15:43
  • \$\begingroup\$ Please provide a schematic. \$\endgroup\$
    – Andy aka
    Mar 8, 2023 at 15:49
  • \$\begingroup\$ @steveSh ACT + = actuator positive lead. ACT - = actuator - lead \$\endgroup\$
    – user298907
    Mar 8, 2023 at 15:51
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    \$\begingroup\$ @SteveSh These are definitely MLCCs. Tantalum caps look very different. \$\endgroup\$
    – Hearth
    Mar 8, 2023 at 15:59

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I agree that these capacitors are probably there to filter noise. The intention could be to filter noise coming out of or coming into the board (or both).

Capacitors connected to chassis often times need quite a bit of voltage rating. Maybe they are connected in series to increase their effective voltage rating. However, increased voltage rating could be achieved with a single (but bigger) capacitor as well.

Another explanation could be functional safety. In safety-relevant applications capacitors are sometimes put in series to prevent the signal from connecting to - in your case - chassis, if one capacitor fails. Ceramic capacitors like this usually fail as a short.

If it's not for functional safety a related explanation could be to just increase the availability of the system in general. Maybe an earlier revision of the board suffered from capacitors failing in this area, as there might be a lot of mechanical stress due to the connector. Two capacitors in series should be more robust to mechanical stress than one big capacitor with the same voltage rating as two smaller capacitors in series.

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