Do you understand the grounding of switching power supplies? The initial grounding technology for switching power supplies was a protective measure to prevent electrical or electronic equipment from being struck by lightning. This allows the current generated by lightning to be conducted to the earth through lightning rods, thus protecting the building. At the same time, grounding technology is also an effective means of protecting personal safety. When a phase wire (such as poor wire insulation or aging wiring) comes into contact with the equipment casing for some reason, a dangerous voltage will be generated on the equipment casing. The resulting fault current will flow through the PE conductor to the earth, thus providing protection. After reading this article, you will have a thorough understanding of how switching power supplies are grounded.
Definition of grounding
In modern grounding concepts, for line engineers, the term usually means "reference point for line voltage"; for system designers, it often refers to a cabinet or rack; and for electrical engineers, it means a green, safe ground wire or connection to the earth. A more general definition is "grounding is a low-impedance path for current to return to its source." Note the requirements of "low impedance" and "pathway."
Common grounding symbols
PE, PGND, FG - Protective ground or chassis; BGND or DC-RETURN - DC - 48V (+24V) power supply (battery) return current; GND - Working ground; DGND - Digital ground; AGND - Analog ground; LGND - Lightning protection ground.
Suitable grounding method
There are various grounding methods, including single-point grounding, multi-point grounding, and hybrid grounding. Single-point grounding is further divided into series single-point grounding and parallel single-point grounding. Generally speaking, single-point grounding is used for simple circuits, to distinguish grounding between different functional modules, and multi-point grounding or multi-layer boards (complete ground plane layers) are used for low-frequency (f10MHz) circuits.
Introduction to signal return and cross-segmentation
For an electronic signal, it needs to find a path with the lowest impedance for current to flow back to ground, so how to handle this signal return becomes very critical.
First, according to the formula, the radiation intensity is directly proportional to the loop area. That is to say, the longer the return path, the larger the loop formed, and the greater the interference of external radiation. Therefore, when laying out a PCB, the area of the power supply loop and signal loop should be minimized as much as possible.
Secondly, for a high-speed signal, providing good signal return current ensures its signal quality. This is because the characteristic impedance of transmission lines on a PCB is generally calculated with reference to the ground plane (or power plane). If there is a continuous ground plane near the high-speed line, the impedance of the line will remain continuous. If there is no ground reference near a section of the line, the impedance will change, and the discontinuous impedance will affect the signal integrity. Therefore, when routing, high-speed lines should be assigned to a layer close to the ground plane, or one or two ground lines should be run parallel to the high-speed line to provide shielding and nearby return current.
Third, the reason why it's advisable to avoid crossing power supply splits during wiring is that when a signal crosses different power layers, its return path becomes much longer, making it more susceptible to interference. Of course, it's not strictly forbidden to cross power supply splits. For low-speed signals, it's acceptable because the resulting interference is relatively negligible. For high-speed signals, careful inspection is necessary, and crossing power supply splits should be avoided as much as possible. This can be achieved by adjusting the routing of the power supply section.
Why separate analog and digital geolocation, and how should they be separated?
Both analog and digital signals need to return to ground. Digital signals change rapidly, resulting in significant noise on the digital ground, while analog signals require a clean ground reference. If analog and digital grounds are mixed, noise will affect the analog signal.
Generally, analog ground and digital ground should be handled separately and then connected together using thin traces or a single-point connection. The overall idea is to minimize noise from the digital ground from reaching the analog ground. Of course, this is not a very strict requirement that analog and digital grounds must be separated; if the digital ground near the analog section is still relatively clean, they can be combined.
How are the signals on the circuit board grounded?
For general devices, grounding as close as possible is best. With the adoption of a multi-layer board design with a complete ground plane, grounding general signals becomes very easy. The basic principles are to ensure the continuity of the traces, reduce the number of vias, and keep them close to the ground plane or power plane, etc.
How are the interface devices on a single-board computer grounded?
Some circuit boards have external input/output interfaces, such as serial connectors and RJ45 Ethernet connectors. Poor grounding design for these interfaces can affect normal operation, leading to issues like bit errors and packet loss during Ethernet interconnection, and they can also become sources of electromagnetic interference, transmitting internal noise outwards. Generally, a separate, independent interface ground is allocated, and its connection to the signal ground uses thin traces, which can be connected in series with a 0-ohm or low-value resistor. Thin traces help block noise from the signal ground from reaching the interface ground. Similarly, careful consideration must be given to filtering the interface ground and interface power supply.
How is the shielding layer of a shielded cable grounded?
The shielding layer of a shielded cable should be connected to the interface ground of the circuit board, not the signal ground. This is because there is various noise on the signal ground. If the shielding layer is connected to the signal ground, the noise voltage will drive common-mode current to interfere outward along the shielding layer. Therefore, poorly designed cables are generally the biggest source of electromagnetic interference. Of course, this is on the premise that the interface ground must also be very clean.
