ground design
Grounding is one of the important means to suppress electromagnetic interference and improve the EMC performance of electronic equipment. Correct grounding can not only improve the ability of the product to suppress electromagnetic interference, but also reduce the external EMI emission of the product.
1. The meaning of grounding
The "ground" of electronic equipment usually has two meanings: one is "earth" (safety ground), and the other is "system reference ground" (signal ground). Grounding refers to the establishment of a low-resistance conductive path between the system and a certain potential reference plane. "Connecting to the earth" is to use the earth's potential as the reference and the earth as the zero potential to connect the metal casing of the electronic equipment and the reference point of the circuit to the earth.
Connecting the ground plane to the earth is often due to the following considerations:
A, to improve the stability of the equipment circuit system;
B, to discharge static electricity;
C, to provide safety protection for operators.
In products such as switching and access networks, the handle bars of the general boards are connected to the protective ground through the positioning holes of the handle bars to discharge static electricity. I am doing the ESD experiment of PON16. Because the handle strip of the DMU is not connected to PGND (the positioning hole that should have been metallized is misdesigned as a non-metallized hole, so the handle strip of the board is not connected to the protection ground). Therefore, when the electrostatic test (contact discharge and air discharge) is performed on the rear panel of the chassis (local end or remote end), it is easy to cause reset. After changing the pad design and connecting the handle bar to PGND, the reset problem is solved and the ESD test is passed.
2. Grounding purpose
A, safety considerations, that is, protective grounding:
B, providing a stable zero-potential reference point (signal ground or system ground) for the signal voltage;
C, shielding grounding.
3. Basic grounding methods
There are three basic grounding methods in electronic equipment: single-point grounding, multi-point grounding, and floating grounding.
3.1 Single point grounding
Single-point grounding means that in the entire system, only one physical point is defined as the ground reference point, and all other points that need to be grounded are connected to this point.
Single-point grounding is suitable for circuits with lower frequencies (below 1MHZ). If the operating frequency of the system is so high that the operating wavelength is comparable to the length of the system grounding lead, there is a problem with the single-point grounding method. When the length of the ground wire is close to 1/4 wavelength, it is like a transmission line with a short-circuit terminal. The current and voltage of the ground wire are distributed in standing waves, and the ground wire becomes a radiation antenna instead of "ground". . In order to reduce the ground impedance and avoid radiation, the length of the ground wire should be less than 1/20 wavelength. In the processing of power circuits, single-point grounding can generally be considered. For the digital circuits widely used by our company, because they contain rich high-order harmonics, it is generally not recommended to use single-point grounding.
3.2 Multi-point grounding
Multi-point grounding means that each grounding point in the equipment is directly connected to the ground plane closest to it, so that the length of the grounding lead is the shortest.
The structure of the multi-point grounding circuit is simple, and the phenomenon of high-frequency standing waves that may appear on the grounding line is significantly reduced, and it is suitable for occasions with high operating frequency (>10MHZ). However, multi-point grounding may cause many ground loops inside the device, thereby reducing the device's ability to resist external electromagnetic fields. In the case of multi-point grounding, attention should be paid to the ground loop problem, especially when networking between different modules and devices.
Electromagnetic interference caused by ground loops:
The ideal ground wire should be a physical entity with zero resistance and zero resistance. However, the actual ground wire itself has both resistance and reactance components, and when a current passes through the ground wire, a voltage drop will occur. The ground wire will form a loop with other connections (signals, power lines, etc.). When the time-varying electromagnetic field is coupled to the loop, an induced electromotive force will be generated in the ground loop, and the ground loop will be coupled to the load, posing a potential EMI threat.
Take the following picture as an example:
Due to the effect of distributed capacitance, there is a loop B-C-D-E-F-A between the transmission line and the ground. Due to the existence of ground impedance, when the local current Ig flows through the ground plane, a voltage drop Vi will be generated in Zg, that is, a voltage drop Vi appears between the two points B and E of the circuit. This voltage Vi is common to the two signal connections, causing currents I1, I2 to flow in the two wires. Since the impedances of the paths through which I1 and I2 flow are different, the impedance imbalance generates a differential mode voltage VO at both ends of the load, which is one of the sources of ground loop EMI. The external electromagnetic field is linked with the ground loop,
Inductive voltage Vi is generated in the loop, there is
This voltage acts as a potential source of electromagnetic interference for the common mode currents I1 and I2 around the two loop areas comprising the two connecting wires.
3.3 floating ground
Floating ground refers to a grounding method in which the equipment grounding system is electrically insulated from the earth. Due to some weaknesses of the floating ground itself, it is not suitable for our general large-scale system, and its grounding method is rarely used, so I will not introduce it in detail here.
3.4 Mixed grounding method composed of the above various methods
4. General selection principles for grounding methods
For a given device or system, at the highest frequency concerned (corresponding to wavelength ) , when the length of the transmission line is L) , it is regarded as a high-frequency circuit, otherwise, it is regarded as a low-frequency circuit. According to the rule of thumb, for circuits below 1MHZ, it is better to use single-point grounding; for circuits above 10MHZ, it is better to use multi-point grounding. For frequencies in between, a single-point ground can be used to avoid common impedance coupling as long as the length L of the longest transmission line is less than /20 Ω.
The general selection principles for grounding are as follows:
(1) For low-frequency circuits (<1MHZ), it is recommended to use single-point grounding; (2) For high -frequency
circuits (>10MHZ), it is recommended to use multi-point grounding;
grounded.
4.1 Board grounding method
It is inappropriate to specify specific requirements for grounding at the circuit level. For a specific board, we generally perform necessary division processing according to the device manual. For the connection between different types of grounds on the board, it is recommended to complete it through magnetic beads, or directly through a single point on the PCB. For a power supply that is directly connected to a single point without a magnetic bead, it is recommended to place a 0.1u (or 0.01u) capacitor at the single point junction for filtering (one end is connected to the power supply, and the other end is connected to the corresponding ground).
In terms of power supply and ground division, attention should be paid to cutting off the path of EMI from the primary to the secondary through the reference plane, especially in the division of filters, common mode coils, magnetic beads and other devices.
The following is the ground division diagram of its isolation transformer provided by Pulse:
For the above picture, we noticed that in the separation of the isolation transformer:
A, the location of the division: the primary and secondary junctions:
B, the width of the division line: not less than 100MIL
. The reason for the above division is that In order to achieve primary and secondary isolation, the interference at the control source is coupled to the secondary through the reference plane. From the following segmentation map recommended by INTEL, we can also find that the position of the dividing line is very important. In addition to achieving isolation, we also need to consider the wiring of adjacent layers to avoid important signal lines from crossing partitions.
For most of the ICs we use now, suppliers generally provide detailed PCB design requirements (including power supply and ground division), and hardware personnel should work with CAD engineers to carefully analyze these requirements. Where practical, try to follow the supplier's requirements.