How Can Effective ESD Charge Routing Be Achieved in PCB Design?

  • #1
marko12345
1
0
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.

Setup
- 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.
 

Attachments

  • Drawing1.png
    Drawing1.png
    5.9 KB · Views: 1,228
Engineering news on Phys.org
  • #3
marko12345 said:
- pcb has connectors at the top and bottom edge
In general, that is something to be avoided. When possible, all connectors should be along one edge or near one corner of the PCB. That let's you use a "Star Ground" configuration, so that any transients that enter on cables can be clamped to ground along that edge or in that corner, and exit the board without flowing through any sensitive circuitry. You may not be able to re-design this PCBA now, but keep this in mind as a design goal for your future designs.
marko12345 said:
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
That let's the transient currents flow through the middle of the board, which can disrupt any sensitive circuitry there.
marko12345 said:
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
That seems like a good plan. The main thing I would add is to have an RC connection from the PCBA ground plane to that guard ring at each of the 4 standoff holes. You can Do-Not-Load the components at 3 of the corners if you get better performance by loading only the corner nearest the power supply. The R and C components should be rated for that 4kV of ESD that you expect -- I use a 1000pF, 2kV capacitor and a 2kV rated 100kOhm resistor. The capacitor is for high-frequency bypassing between the guard ring and the PCBA ground, and the resistor is a bleed resistor that helps to keep charge from building up a voltage between the ring and PCBA ground. I use just a 2kV rated capacitor because the ESD Human Body Model uses something like a 100-200pF output capacitance, so there is a voltage divider between the ESD source and what the PCBA sees. You can also use a lower voltage rating on those caps if your TVS clamp voltage is lower.
marko12345 said:
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?
That is a more problematic option, although it may be better for Radiated EMI compliance. One of the dangers of isolating your PCBA ground from Earth Ground is that the PCBA ground can be driven by high-speed digital circuits on the PCBA, and this can set up resonances (if not clamped to Earth Ground) that result in common-mode RF currents flowing out the external cables. Those cables can act as radiating antennas, depending on the frequencies and cable lengths involved, which can cause you to fail EMI compliance.

Hope that helps.
 
  • Like
Likes anorlunda
Back
Top