How does chassis ground apply in nonmetallic enclosures?

Question

I am working on a PCB that uses a chassis ground, but I am not able to make a metal enclosure for it. I was wondering if you can use enclosures that are not made of metal for chassis ground and how it would affect the PCB? Also, how do you simulate chassis ground on a breadboard? Do you use a wire to connect the ground plane to a piece of metal? I'm confused about the applications of chassis ground.

Image source: Elliott Sound Products - Earthing Your Hi-Fi - Tips, Tricks and Techniques

Edit: Chassis grounds are needed for the TPS63030 buck-boost converter in my project. Here is the schematic (it also works for the TPS63030 in my project):

Image source: Texas Instruments - TPS6303x datasheet

Answer 1

You are interpreting the diagram in the data sheet wrong.

Your buck-boost IC or your PCB do not need a chassis ground, so you should stop thinking about chassis ground as it has nothing to do with the IC.

Please read datasheet page 3 for short description of ground pins, and pages 7 and 17 for longer explanation.

Use the find function in your PDF reader for the word "ground" and read the text.

Answer 2

Also, how do you simulate chassis ground on a breadboard?

Breadboards have bad parasitics, the connections can be flaky with contact resistance and there is capacitance between rows. The rows also have more inductance than most grounding systems so it would be inadvisable to use a breadboard to simulate grounds.

If you were going to simulate a ground on a breadboard, the put 1oz copper sheet underneath it.

Chassis ground is used for faulting and a good way to shunt ESD away from you device. Chassis ground is required in some IEC requirments for products to protect consumers. It can also be used for shielding a design or using it to stop radiation from a design from affecting other devices.

The ground must be able to handle a fault if you connect it to AC mains to shunt dangerous voltages to earth ground through the wiring systems.

Answer 3

As you've stated, your case isn't metal, so there is nothing to 'chassis ground' to.

On the other hand, your board assumes a chassis ground (actually, it doesn't, we'll get into that below.) The assumption for this depends on the designer's intent, but most often it is meant to form a low-impedance common ground for external connections. Why? To help deal with disturbances such as static discharge and common-mode noise invading the I/O connectors.

My suggestion? If your design needs a chassis ground for some other reason, provide an internal metal plate that ties your I/O panel and board ground together. Make the I/O panel such that it picks up any shield grounds on your I/O. This will greatly improve your system EMI and ESD performance without the full metal enclosure.

If you have an internal power supply, this will have a ground that wants to connect to chassis ground:

• DC supply: tie (-) input to the plate
• AC supply (2-wire): tie Y-cap ground point to the plate
• AC supply (3-wire): tie earth to the plate

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Now, a word about the regulator. The schematic is misleading. The TPS regulator doesn't require a chassis ground. It does however require a thick, low-impedance power ground for the high-current AC paths. This is distinguished from signal ground which is for the control and feedback components.

Their schematic used the following symbols:

• 'chassis' (three slanted lines) for power ground. This is misleading.
• 'common' (three horizontal lines) for signal ground.

At any rate, the two grounds should be star-tied near the device. The datasheet has a layout example that makes this clear:

from https://www.ti.com/lit/ds/symlink/tps63030.pdf

That fat 'GND' area is the low-impedance PGND path, drawn misleadingly as 'chassis' in the schematic. It connects to pin 3, PGND, the thermal pad (also PGND) and finally to pin 9, GND, and then on to C3 and R2. The thermal pad is the 'star tie' point that connects PGND and GND.

Notice that the input and output caps tie to PGND. This isolates their dynamic switching currents to a small, low-impedance loop. This isolation is key for switching regulator performance and low-noise operation.

tl; dr: follow their layout recommendation.