How do you utilize earth ground in a fully isolated system?

Community PCB Design
1

Hi,

I’m designing a mixed signal system that is to be powered from an off-the-shelf AC/DC converter. After conversion from the AC/DC, I have two systems that need DC power. System1 PCB is going to be on the same enclosure as the AC/DC power supply; System2 PCB is at the end of a 12 foot long cable. Since I have sensitive analog circuitry on both boards, I’m planning on using flyback regulators on both boards. Also, System1 and System2 will need to talk to each other, so I’m going to use a galvanically isolated datalink to pass digital data between them.

Is my attached diagram a good way to deal set this up? I am going to design the two pcbs have an edge copper area with mounting holes that will be connected to the AC/DC Earth connection, however, there will be no connection to this edge to anything in the PCB itself. The local power and ground will be from the V+ and V- of the flyback converters. My thought is that this way, the absolute difference in potential between the two systems will be small.

Is this a good way to approach grounding in an isolated power system such as this?

Thanks!

2

Form my experience in designing Floating Ground instrumentation this is basically correct. However there are some other considerations. ESD is one as there is not a low impedance path for for ESD event to get to earth ground. And if there are any high speed digital on the floating side then EMI may be harder to control.

I have tied the earth to float ground through low value caps for EMI and a bi-polar TVS in series with low C back to back diodes for ESD. For your first cut on the PCBs allow provisions for various devices to connect the float to earth ground. What actually is required can only be determined by testing.

3

Thanks for the reply.

I’ve drawn out the PCB profile based on your comments.

Please let me know if this is the correct way to look at it and your suggestions on how to improve it.

Also, do you have any advice on where to put the Capacitors and TVSs on the board? Is there a preferable location?

Thanks!

4

Also, do you have any advice on where to put the Capacitors and TVSs on the board? Is there a preferable location?

That seems to be a black art. I suggest laying out the PCB with several optional points of connection but in general try for the shortest, lowest impedance path the earth ground.

You may also consider a optional large value resistor between the float and earth grounds if needed to bleed off ant voltage potential difference.

I like to have the pads for these optional parts on at least the first run of boards. Even when the board goes to production the parts don’t have to be installed if its found they aren’t needed. But they may be difficult or ugly to add if later found they are needed.

You may find that by having the floating circuits fully surrounded by earth ground that none of these extra parts are needed. I don’t know your full experience with designing this sort of circuit and whether this will be a commercial produce that requires full testing and certification, FCC, CE, UL, etc.

5

Thanks Again for your suggestions. This is very valuable information.

This is my first floating circuit although I’ve done a lot of other circuit designs. So this is very valuable information.

Indeed, this is for a commercial product, so it is going through the full battery of testing, CE, etc.

I like the suggestion of putting the weak ties-ins to earth ground and not populate them.

If you have a reference I can read up on this, that would be very helpful.

Thanks!

6

I don’t have any specific reference. The book “Art of Electronics” has some info but not specially of a floating system.

I’d also leave all the parts between the grounds un-installed until you get to do some testing. Then only install what is needed.