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System grounding and ESD

  1. Nov 5, 2012 #1
    There are many documents available about PCB/component level ESD protection, but I have found only few about how to get the charge out of the PCB and into earth.

    - plastic enclosure with metal base plate
    - enclosure contains 2 devices: plastic power supply (smps) and controller pcb
    - at least 3 unshielded cables goes into enclosure: 1 input power cable for power supply (230VAC/50Hz) and rest are for controller io
    - metal base plate is grounded to PE-wire of the power input cable
    - DC output 0V of the smps is internally connected to PE
    - pcb has connectors at the top and bottom edge
    - PCB's ground plane is uninterrupted with no traces in that layer
    - external controller cable wires vulnerable to ESD 4kV

    Unmodified design
    - PCB is fully isolated from base plate and is connected to earth only through the DC 0V wire and smps
    - all TVS's are connected to PCB's ground plane
    - in case of ESD hit any connector PIN, TVS directs the charge into ground plane -> DC power connector -> 0V wire, power supply -> PE wire

    I am thinking about changing strategy so that after TVS, the ESD charge would have more direct route to PE. I can think of two slightly different ways to do this.

    Design A
    - external ground pour (red) is added to pcb, that goes around PCB edges and all TVS directs charge to that
    - pour is connected to base plate through 4 mounting holes at pcb corners/connector edges

    Design B
    - PCB's ground plane (green) is extended to PCB edges and connected to base plate through mounting holes at pcb corners/connector edges
    - is there any chance that DC return current goes to power supply's 0V through base plate instead of pcb power connector wire?

    Considerations about designs and conditions that must be met setup to be effective?

    I would appreciate any details, how this is done in commercial products that passes emc tests.

    Attached Files:

  2. jcsd
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