Defining Power Supply Ground: System, Chassis, and Earth in Your PCBBy ZM Peterson • May 7, 2021
Power supplies can be difficult to properly design, and there is an important element of safety that needs to be considered. This is especially important in DC power supplies, which might output high current that can be dangerous to the user. In a DC power supply, the arrangement of power supply ground in the system will be a major determinant of safety, as well as noise, heat dissipation, and ability to galvanically isolate different portions of the system.
If you’re designing a PCB for a power supply, then you’ll have to confront the safety issue from the start of the design. However, there is also the matter of noise and heat, particularly in switching power supplies. How you define your power supply ground, and its relation to chassis and earth ground, will determine many important aspects of electrical behavior in your system. Follow some of these guidelines if you want to ensure a clear definition of ground in your power supply and prevent common noise problems.
Ground Regions in a Power Supply
The idea that every ground region in your power supply has a potential of 0 V is a fallacy. The reality is that there are multiple ground regions in any power system, and these regions all eventually tie back to earth. Each power supply ground region can also have small potential differences between them, creating the potential for large ground loop currents to move between different sections. These ground loops can also manifest as common-mode noise on the supply’s output voltage. Therefore, it’s important to understand how these different ground regions are linked together to prevent noise. However, this can be difficult in an isolated system, especially once we consider chassis ground.
The diagram below shows the typical arrangement and relationships between various grounds in a power supply. Note that this doesn’t just apply to bench-top power supply units. It can also apply to small regulator modules, and regulator circuits embedded on a PCB.
Power supply ground diagram.
These different ground regions have some parasitic capacitance between them, as shown with the red capacitor symbols around the diagram. Eventually, there is a path back to earth ground, either at the input of the power supply or off-board somewhere. Regardless of where this is, we generally have these three regions. The signal ground region, which could be on the power supply or part of the main board that is receiving power, could also be further subdivided, although this is generally a bad idea in modern electronics.
Finally, on the power supply itself, a power supply that outputs reasonably high voltage, there will be a transformer that bridges the system and signal grounds. This is where the high and low voltage sides of the system are bridged, and the output voltage and current are supplied to the rest of the system. On the board, this is where we need to consider the effect of a split power supply ground and the role played by parasitic capacitances in the system.
The Split Ground Problem in Isolated Switching Supplies
Switching power supplies are the most commonly used type of power supply in any application. This is especially true in high voltage/high current power supplies. The idea behind isolation is that there is no direct path for DC currents on the high voltage side to conduct over to the low voltage side of the power supply via a power plane. For this reason, we also split the ground plane, and power is induced on the secondary side of the transformer via the switching action on the primary side. Sounds simple, right?
SI enthusiasts will immediately say that you’ve committed a cardinal sin of splitting the PCB ground plane, and for good reason. The main problem with this whole split planes thing is that everyone thinks "I'll just split my ground into two physically separated planes so that I don't need to think about return paths, and then I'll just route wherever I feel like and not worry about ground". The problem with that mentality is they end up routing over an area with no ground beneath it. You then create a radiated EMI problem that cannot be solved with simple EMI filtering.
Signal routing in each region should not cross the gap between planes. Power is transferred inductively through the transformer. The capacitor ensures consistent ground potential between the two sides.
A power supply design implicitly states that you're not routing anything over the gap between the system ground and the signal ground. In the case of an isolated supply, where the system ground is physically disconnected from the signal ground, you're using a transformer to couple out power from your switching converter or bridge circuit, such as is the case in an resonant LLC converter. Finally, there is normally a capacitor placed across the gap near the transformer to set the various GND potentials equal so that there are no common-mode currents coupling through the chassis ground. This is one grounding strategy used in Ethernet switches (you'll actually find at least 3 different GND recommendations in Ethernet!). The important point here is: Don’t route anything over the gap between the two sides of your power supply. If you must do so, such as for a feedback line, place a capacitor in parallel to bridge the power supply grounds.
Should You Put Ground Below Coils?
Probably every designer will tell you not to put ground below certain components in a power supply, namely below ferrites and coils on a PCB. The argument is that a noise current on the ground plane will be picked up by the coils and will recycle back towards the supply output. This is a somewhat nonsensical argument as it follows the logic that currents "flow" like water in a pipe, which is not the correct physical picture for current. A signal can be induced in a coil via the coil’s winding capacitance, but this is a parasitic coupling effect due to poorly tracked return paths. I discussed this in a recent article on grounding in Ethernet PCB layouts in one of my recent Signal Integrity Journal articles.
I’ve said this many times: keep track of your return paths in your system, and you won’t need to worry about clearing out ground beneath coils, or any of the other tricks people try to use to force isolation between different sections of a PCB. The funny thing about this is, whenever some cuts out ground directly below a coil, they can never seem to explain why the ground a few mils away is not a problem, but ground directly below a coil is a problem. The answer is: neither is a problem if you know how your return current behaves in your system, including in a DC power supply.
Because DC return currents follow the path of least resistance and not the path of least reactance, it’s best to use some basic layout tricks to guide the return current. With that in mind, I’ll use polygons to guide the return current back to where I want it, in addition to using a ground plane. This gives the added benefit of allowing the polygon to be made wider than a typical trace, so it won’t get too hot during operation.
These two polygons are used to guide the return path instead of dumping the return current in to the ground plane on layer 2.
Groudning sounds like it's simple, but it can quickly become a complex topic that stumps many SI/PI "experts" in the field. A proper grounding strategy affects everything from impedance to EMI and even thermal dissipation in your PCB. Since it's so central to your board's performance and reliability, don't rely on 30-year-old design guidelines when planning out your power supply ground strategy.
Experienced designers can help you define your power supply ground in complex PCB layouts and ensure your system will be safe, efficient, and compliant with industry standards. If your company is pushing the limits of telecom, data center, and low power embedded systems, it pays to work with an experienced electronics design firm. NWES helps private companies, aerospace OEMs, and defense primes design modern PCBs and create cutting-edge embedded technology. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.