What is the purpose of EMC design grounding?
Filtering, shielding, grounding; is known as the three major methods of EMC design; grounding design is an important issue in electronic product design! The purpose of grounding is as follows:
A. Grounding allows all of the unit circuits in our circuit system to have a common reference 0 potential, that is, there is no potential difference between the circuits to ensure stable operation of the circuit system;
B. Prevent external electromagnetic interference. For example, the chassis is grounded; it provides a bleed channel for transient interference (ESD); it can also discharge a large amount of charge accumulated on the chassis due to static induction through the earth; if the circuit has a shield using a shield or circuit, select With proper grounding, you can get better shielding!
C. Guarantee safe work. When the electromagnetic induction of the Surge occurs, the damage of the electronic equipment can be avoided;
Therefore, grounding is the main method for suppressing noise and preventing interference for EMS. Grounding can be understood as an equipotential point or equipotential surface is the reference potential of an electronic circuit system, but not necessarily earth potential; to prevent damage from lightning strikes and to protect the safety of products & equipment operators, the housing and power supply system of electronic equipment The metal components must be connected to the earth! Moreover, the grounding resistance is required to be small and cannot exceed the specified value.
Most products require grounding; although the grounding can be a true grounding, isolated or floating but grounded structure must exist. Grounding Sometimes we are confused with the loop that supplies current to the signal. Most of the problems in practice are related to PCB design; problems with complex systems can sometimes be attributed to the design of connecting connections between analog and digital circuits and providing high-frequency connections between the PCB's formation and the metal casing!
Although the grounding design is the most important aspect of EMC design, the problem usually is not easy to understand. In fact, each circuit must have a reference grounding source. The circuit design first considers the grounding design. Grounding is not desirable. An important method for minimizing noise & interference and isolating the circuit. The grounding of the PCB and the design of the grounding copper reduce many noise problems. A well-designed grounding system prevents undesired interference and emissions at a low cost. For example, if the ground plane in the high-speed digital circuit is incomplete, after the cable is connected, it will be driven by the noise voltage on the ground plane, which will cause EMI!
First share the idea of ground current:
A. Once a current flows through a certain impedance, a certain voltage drop will occur; using Ohm's law on the electronic circuit board, there will be no OV potential voltage or current unit may be in the microvolt or microampere range, there is a comparison Small finite value.
B. The current always returns to its source; the loop may have many different paths, and the magnitude of the current on each path is different, which is related to the impedance of the path. It is not desirable to have some current flowing on one of these paths, so you need to take a suppression on that path!
Therefore, when designing the product, it is the most economical way to consider grounding first. Failure to consider during the design phase means that the system may fail in terms of EMC. A well-designed grounding system; including PCB design from the electronics, the ability to prevent EMI and EMS from a system perspective is the key design.
Share the key design with the following PCB grounding and clock grounding design case:
The design of PCB layout in electronic products is the method adopted by many design engineers for the current low-cost IoT and smart products; due to the need for crystal-driven MCU/CPU high-frequency operation in the product system; PCB paving Ground design can bring advantages to system EMC design!
Theory and analysis of double-panel PCB system paving; what effect does copper paving have on EMI?
Good aspects of paving:
Note that the trace of the edge of the PCB of the signal line is paved; the external signal coupling of this signal line is reduced; the copper is placed on the edge of the PCB; the copper is laid between the signal lines. It can reduce crosstalk between signals, and the EMI radiation in crosstalk is indirectly related to it! If both can achieve or achieve EMI effect will be good! The paving is not the number of places to be counted by area. It is necessary to have a target to lay the ground to have a good effect!
My point of view: PCB has copper plating conditions, and this copper plating is useful, if it can improve the performance of EMI; it is recommended to use the rule of PCB copper plating!
In this design, it is necessary to ensure that the ground plane of the signal and the impedance of the reference ground potential of the system are small enough; otherwise, the paving will have an impact on radiation interference or radiation interference!
For high-frequency application electronic product lines; due to the common mode current interference EMI problem, we need to increase the common mode inductance and its combination of LCM*C at the DC-DC DC current output terminal and the interface connector end of the functional circuit. The design of the EMI filter; the PCB layout copper layout at this time will have an impact on our high frequency EMI; the analysis is as follows:
When there is a filter design of the common mode inductor in the electronic circuit, the PCB of the TOP layer and the PCB floor of the BOTTOM bottom layer will have a coupling capacitor Cp when the front and rear stages are designed for PCB copper plating; the high frequency disturbance signal will be coupled. The capacitance affects the noise impedance performance of the common mode inductor; the equivalent circuit is as follows:
For example, the design of the system LCM device has a stray capacitance of 2pF; its resonant frequency is around 4MHZ; the design of the ground copper of the PCB is 6pF due to the wiring of the PCB, the input trace and the copper of the PCB. Coupling capacitance parameters; analysis is as follows:
After the resonance point of its LCM, its impedance value is reduced - the frequency & impedance characteristic curve reference data shown above; when performing EMI test, it will bring high frequency EMI with high frequency > 4MHZ!
In the design of PCB double-panel wiring copper; in some circuit design to improve PCB layout and routing can reduce high-frequency EMI electromagnetic interference; simple optimized PCB design reference is as follows:
Note: If there is a noise coupling source in the ground plane, the ground plane should not be close to the sensitive input circuit.
It is common for a two-panel system to have a clock signal; the system crystal oscillator has a high clock frequency and the interference harmonic energy is strong; the interference harmonics are radiated from the space in addition to the two layout wires of the input and output, if the layout The wiring is unreasonable, it is easy to generate strong spike noise radiation problem. Therefore, it is very important to layout the crystal oscillator and CLK signal line when the PCB board is laid out. The high-frequency clock signal needs to be able to mirror the reflow. of.
The layout of the clock source is briefly described as follows:
A. For active crystal oscillators: The coupling capacitors should be placed as close as possible to the power supply pins of the crystal oscillator.
According to the inflow direction of the power supply, the values of the capacitors are placed from the largest to the smallest, and the capacitor with the smallest capacitance is closest to the pin.
B. The outer shell of the crystal must be grounded to design the crystal signal source; prevent external radiation, and shield the external signal from the crystal.
C. Do not route other traces under the crystal to ensure complete grounding. Do not route other traces within the 300 mil range of the crystal to prevent the crystal noise source from coupling to other wiring and devices.
D. The trace of the clock signal should be as short as possible, and the crystal oscillator should not be placed on the edge of the PCB board!
If the edge of the board has a reset and data & address signal lines, etc., the edge of the board of the PCB needs to be designed with copper plating; the mirror symmetry design of the ground return path of the critical signal line is ensured.
Therefore, the copper paving of the double-panel of the PCB is not laid by the area according to the area. The paving should have a good purpose to lay the ground to have a good effect. The incorrect paving design will deteriorate the EMC performance of the system.