Grounding strategy for mains powered digital device

[dkonigs]

So I'm working on a project where I have a device in a metal chassis, which is externally powered by mains AC and the chassis itself is earthed.
I have a number of questions about how to handle various aspects of grounding, and have been struggling to get good answers for a long time now.  All the authoritative sources I can find seem to either conflict with each other, ignore some aspect that's relevant to my situation, or simply the suggestion to ignore half the concerns.  And while I have found other posts where people are asking the same sorts of questions I have, they rarely seem to get good answers.

To hopefully start this off, and make things clear, I put together a simplified diagram of the system I'm working with:


Power enters the device through an AC mains cord connected to the "power board".  This board bonds the "earth" pin to the metal chassis, and contains an AC/DC converter.  Out of the other side, there's a connector with DC +12V and GND wires.

On the other half of the device is the board I'm primarily concerned about here.  It receives the DC +12V/GND connections, runs them through regulators to get typical logic voltages, contains the usual smattering of digital circuitry (microcontroller, peripheral ICs, etc) and has various I/O ports and devices.  Within this board, there's a typical ground plane for DC circuit ground.

All of the mounting holes in the device obviously make electrical connections to the chassis, but what those holes connect to on the actual PCB is up to me.

Now as far as my actual questions.  On the main logic board, here's what I want to know:

• Do I connect DC circuit ground to the chassis ground?
• If I should connect DC circuit ground to the chassis ground, then how? Do I connect it to all the mounting holes?  To one mounting hole?  Directly to one mounting hole, then to the others with capacitors?  To all mounting holes with capacitors? At the power input? Near some critical mass of I/O ports? I've seen suggested variations that include all of these, or none of them, depending on where I look.
• All the external connectors have TVS diodes on them. Do I connect the other end of these TVS diodes to the ground plane or to the chassis ground?
• If I am connecting several things directly to chassis ground, then should I have some sort of separate chassis ground trace/plane/polygon between the holes to improve connections to things on the board that should go to chassis ground?
• Sometimes connectors have shields that can touch the chassis. Various USB device datasheets suggest connecting these to circuit ground using a resistor and capacitor in parallel. Is this a good general idea in other similar cases or not?

So what am I trying to accomplish here?  I'd like to keep things safe, obviously have decent EMI shielding, decent transient protection, avoid ground loops, etc.  And really just follow general best practices.

If follow-up questions would help get me better answers, then please ask them.

P.S. I have the Ott book right here sitting on my desk. It only gives me bits and pieces, but doesn't put the whole system picture together in a clear way.  So that's why I'm asking.

[jkostb]

The answer to your question is related to requirements from safety standards and EMC.

1) The metal chassis must be connected to PE because this is required from safety standards e.g 61010, 60601 etc
2) Regarding the ground of your PCB. Depending on type of circuits it needs a connection to the metal chassis. For example if you DC/DC converters or building a motor drive circuit, the fast switching of voltages can couple voltages/currents into chassis through capacitive coupling. This is described in Ott. These currents needs a return path otherwise they will couple into cabling and cause radiated or conducted emission issues. This return path is provided if you connect ground to chassis.
3) For low frequent currents or DC 1 ground connections is sufficient. But for high frequency currents you need multiple ground connections. For example inverter circuits for motor drivers a star connection would not suffice.
4)  TVS diode needs to be connected to your circuit ground because it clamps the voltage relative to your circuit ground.

Besides Ott there is also another excellet reference Grounds for Grounding.

[dkonigs]

1) The metal chassis must be connected to PE because this is required from safety standards e.g 61010, 60601 etc

Already doing this, as mentioned above.

2) Regarding the ground of your PCB. Depending on type of circuits it needs a connection to the metal chassis. For example if you DC/DC converters or building a motor drive circuit, the fast switching of voltages can couple voltages/currents into chassis through capacitive coupling. This is described in Ott. These currents needs a return path otherwise they will couple into cabling and cause radiated or conducted emission issues. This return path is provided if you connect ground to chassis.

So DC ground needs some sort of path back to the chassis, is how I interpret this.  (no motor drive circuits, but there are DC/DC buck regulators)

3) For low frequent currents or DC 1 ground connections is sufficient. But for high frequency currents you need multiple ground connections. For example inverter circuits for motor drivers a star connection would not suffice.

Okay, so you're saying multiple ground connections from the DC ground plane to the chassis are probably a good idea.  But what does that mean in practice?  I've seen the following suggestions floated about:

• Connect all mounting holes to DC ground and the chassis
• Connect one mounting hole to DC ground and the chassis (in the I/O area is often suggested), but connect the rest through high voltage capacitors to keep an AC path but avoid ground loops

4)  TVS diode needs to be connected to your circuit ground because it clamps the voltage relative to your circuit ground.

All the examples in the Ott book actually show TVS diodes connected to chassis ground, and I've seen other scattered guidance that suggests this as well.  However, nearly all general purpose circuit schematic examples just connect it to circuit ground (though none of those examples include a chassis ground to begin with).
So this is why I'm a bit confused as to what to do here.
Of course depending on exactly how chassis and circuit ground are connected, this distinction may or may not actually matter.

Besides Ott there is also another excellet reference Grounds for Grounding.

Ordered.

[jkostb]

>Connect one mounting hole to DC ground and the chassis (in the I/O area is often suggested), but connect the rest through high > voltage capacitors to keep an AC path but avoid ground loops

In many app notes you see suggestions to use capacitors. But  please note that that ading these capacitors introduces inductance and this defeats the purpose of having a low impedance return path. If your application does not require isolation between chassis and ground, it is better to have direct connection. If your application requires a ground isolated from chasis then you have no option then use capacitors.

>All the examples in the Ott book actually show TVS diodes connected to chassis ground, and I've seen other scattered guidance that suggests this as well.  However, nearly all general purpose circuit schematic examples just connect it to circuit ground (though none of those examples include a chassis ground to begin with).
So this is why I'm a bit confused as to what to do here.
Of course depending on exactly how chassis and circuit ground are connected, this distinction may or may not actually matter.

Goal of TVS diode is to clamp and protect your circuit which is referenced to ground. But you also want to divert the surge current to chassis, because if it flows through your ground plane it can create common mode noise . So  a good design requiers chassis connection close to IO region (where you install TVS). TVS is referenced to ground and if there are chassis connections close to TVS you meet all requirements.  In summary
1) Have PE connect to chassis
2) Have multiple direct chassis connections between chassis and PCB. No capaictors unless you need floating ground plane.
3) Have minimum 1 chassis connection in IO region close where you clamp the inputs with TVS.

[T3sl4co1l]

Please describe the IO connections. What are they? What do they connect to? Do they need to be isolated?  Are the things they connect to, themselves isolated, so it doesn't matter?

And when you say "I/O Port", is it a communication port of some standard, or a literal GPIO pin exposed?  If the former, what interface ICs are used; if latter, what protection and filtering is provided?

Just two ports seems strange (where does the data go but through?), but it could be an extender or something, or a logger or converter/bridge, so that's not too unusual.  Importance being: are you showing actually all the outside connections?  I don't have enough reason to doubt, so will assume yes.

Tim

[dkonigs]

In summary
1) Have PE connect to chassis
2) Have multiple direct chassis connections between chassis and PCB. No capaictors unless you need floating ground plane.
3) Have minimum 1 chassis connection in IO region close where you clamp the inputs with TVS.

Gotcha. This sounds like what I'm basically doing right now, so I guess I'll keep doing it that way.  I've just seen so much conflicting information out there, suggesting some of the other things I've mentioned (or people who prefer to flaunt their expertise without actually being helpful) that I've always had doubts as to whether this is the right approach.

I've also sometimes seen cases where filter components are placed between GND and chassis, and wonder how to best handle that.  (The stereotypical example is an AC line filter Y capacitor setup between L<->E<->N, but I have seen DC power filters occasionally do something similar.)  When those cases come up, is it better to simply not do it, or to simply run the "chassis" connection to a chassis bond point before it has a path back to DC ground?

In any case, it'll be interesting to see if that book has any further useful tips.