In product design, the grounding is considered more from the safety or function, and less from the perspective of suppressing disturbance
When choosing grounding mode, grounding point and grounding wire, some avoidable mistakes will appear. In addition, a good grounding design must have good reliability
Good assembly process as a guarantee, in order to achieve the desired purpose.
1. In the grounding design, the grounding mode and grounding point should be selected according to the actual situation
For example, the frequency of computer radiated disturbance exceeding the limit value is concentrated in the range of 30 ~ 200MHz, so the internal units and shielded cables of the microcomputer are in phase
The enclosure shall be grounded at multiple points nearby. Using single point grounding will increase the length of grounding wire, if the length of grounding wire is close to or equal to
When the wavelength of the disturbance signal is 1 / 4, the radiation ability will be greatly increased, and the ground wire will become the antenna. Generally speaking, the length of grounding wire should be less than
2.5cm。 The grounding of shielded cable is shown in Figure 1.
2. Selection of grounding wire
It is often seen that the internal ground wire of such a product is a very thin single strand wire. When the high-frequency current passes through the product, the high-frequency current will cause the high-frequency current
The impedance is very large, and the grounding effect can be imagined. Therefore, considering the skin effect, the ribbon braided wire should be selected for the grounding wire. If grounding is required
This is mainly to reduce the surface resistivity of the conductor, so as to achieve the purpose of reducing the high frequency impedance of the grounding wire.
3. The grounding wire should be well lapped with the ground
It is generally stipulated in the standard that the DC lap impedance between the grounding wire and the grounding ground should be less than 2.5MW. In order to achieve high quality grounding, the grounding ground should pass through the surface
Avoid oxidation and corrosion. There shall be no lock washer or gasket between the grounding wire and the grounding plane, and no gasket, bolt or gasket shall be used
The nut is part of the ground loop.
4. Three grounding modes: floating grounding, single point grounding and multi-point grounding
The purpose of floating ground is to isolate the circuit or equipment from the common ground or the common line that may cause circulating current. Disadvantages: the equipment is not connected with the earth
Direct connection, easy to accumulate static electricity, to a certain extent, will produce breakdown, which is a destructive source of disturbance. Compromise treatment
In order to eliminate the influence of static electricity accumulation, a discharge resistor with large resistance is connected between the floating ground and the earth. The way to achieve floating: Change
Voltage isolation, charging isolation. Floating ground not only makes the ground wire "float", but also solves the problem of potential inconsistency in single ground system.
Single point grounding means that only one physical point of grounding is defined as the grounding reference point, and other points that need to be grounded are directly connected to this point. If
The working frequency of the system is very high. When the length of the grounding wire can be compared with the working frequency (the wavelength of the signal), the single point grounding can not be used
The way of grounding (the grounding effect is not ideal), but to use the concept of multi-point grounding.
Multipoint grounding refers to that each grounding point in a system is directly connected to the nearest grounding plane, so that the length of the grounding wire is the shortest. Grounding
The point can be the bottom plate of the equipment, the ground wire through the whole system, and the structural frame of the equipment. Advantages of multipoint grounding
The circuit structure is simpler than single point grounding. Since multi-point grounding is adopted, many grounding circuits are formed, so the quality of grounding system is improved
It becomes very important to maintain the good conductivity.
Mixed grounding: multi point grounding is used only for the places requiring high frequency grounding, and single point grounding is used for the rest. The grounding length is 0.05 λ~ zero point one five λ To measure,
Those beyond this value shall be grounded at multiple points.
In addition, the relay and other occasions with large current mutation should be grounded separately to reduce the transient coupling to other circuits.
Direct grounding of the load is not appropriate. Excellent shielding performance can also be obtained with tightly wound twisted pair.
When the shielded cable transmits high frequency signal, the outer shield of the cable should be grounded at multiple points. The typical cut-off point is 100kHz. If the cut-off point is higher than this value, it should be grounded at multiple points
Grounding, below this value with a single point grounding, multi-point grounding should be done every 0.05 λ~ zero point one λ There's a ground point.
The shielding layer should not be grounded by braid, but should be wrapped around the core wire and then grounded 360 degrees.
5. Ground design criteria
Circuit size less than 0.05 λ It can be grounded at a single point, greater than 0.15 λ Multi point grounding can be used in case of emergency.
For the system with wide working frequency, hybrid grounding shall be used.
When the problem of ground loop occurs, floating ground isolation can be used (such as transformer, photoelectric).
All ground wires shall be short.
The grounding wire shall have good conductivity and avoid high resistance.
The signal line, the signal return line, the power system return line and the bottom plate or shell shall have a separate grounding system, and then these return lines can be connected
To a reference point.
For those circuits with large current mutation, there should be a separate grounding system or a separate grounding loop to reduce the impact on other circuits
Transient coupling of the circuit.
The place where the grounding wires of low-level circuit must cross shall make the wires perpendicular to each other.
Use balanced differential circuit to minimize the disturbance effect of grounding circuit.
For the maximum size far less than λ/ 4 circuit, using single point grounding tightly twisted wire (whether shielding depends on the actual situation), to make the equipment sensitive
The sensitivity is the best.
The AC and DC flow lines cannot be bound together. The AC line itself has to be twisted.
When terminating the cable shield, avoid using the braided lead out of the shield.
When coaxial cable is needed to transmit signal, signal circuit shall be provided through shielding layer. The low frequency circuit can be grounded at a single point at the signal source end; High frequency electricity
The multi-point grounding is adopted for the circuit.
High frequency and low-level transmission lines shall be shielded by multiple layers, and each shielding layer shall be grounded by single point.
For safety, the ground wire of the test equipment is directly connected with the ground wire of the tested equipment; Or from the safety point of view, to ensure that the grounding connection device can
It can cope with the unexpected fault current, and can cope with the impact of lightning current when the outdoor terminal is grounded.
