PCB News - Selection of components in PCB copy board

PCB is one of the important parts of the electronics industry. Almost every kind of electronic equipment needs PCB, as long as there are electronic software such as integrated circuits, PCB is also used for the electrical interconnection between them.
So today I will introduce things related to PCB.
Starting from the requirements of safety design, first is the selection of safety-critical parts carrying dangerous voltages. For example: 220V power sockets, fuses, power modules, etc. must use components or parts that have passed safety certification or 3C certification (China Compulsory Certification Committee). The general selection method of IC circuit selection for other safety extra-low circuits: In the case of suitable price and function, surface-mounted SMT copy board devices are preferred to TTL dual in-line devices, and TTL devices are preferred to separate components.
As long as the power of the IC circuit and the speed of IC operation (switching circuit rise and fall time) can meet reliability, it is opposed to the wrong view that the greater the IC power, the faster the switching speed, the better. Because everything is multi-faceted, a certain characteristic goes to an extreme, and other problems will follow. For example, sensitivity and anti-interference are a pair of contradictions. All design indicators must be compatible to properly solve the problem.
Resistance, capacitance, and inductance, generally SMT capacitors with large capacity can also be used, and other forms of devices can be considered as appropriate.
The selection of components should be based on the prerequisite of satisfying the function; 3 reductions and reductions have been proposed.
1, reduce the switching speed of the IC circuit and reduce the harmonic components.
2, reduce the working current and reduce the power.
3, reduce the circulation area.
SMT devices have the smallest circulating current area and are the most suitable, with high integration and good reliability, so they are the first choice. Figure 7 uses three different devices assembled on the same PCB board test results.

The third type of SMT has the lowest radiation.
Summary: The choice of devices does not advocate that the greater the power, the faster the better, but it is recommended that as long as the design function requirements are met, the design compatible with various indicators can be used, and the cost can be reduced.
can achieve the design goal perfectly. This design combination is considered to be the best combination. Of course, there are different best combinations for different types and grades of machines.
Printed board level design and wiring
It is very important to correctly understand the power supply, ground wire interference and radiation conditions before the printed board wiring. When the power supply and ground wire increase or decrease the current in a transient state, due to the effect of inductance and capacitance, the power supply and ground wire See Figure 8 for the noise voltage and current waveforms of the power line (VCC, ICC) and ground line (Ig, Vg) when interference occurs.
This is the situation when an IC circuit is working. When many circuits are working, the interference and radiation on the power and ground wires are very serious. Therefore, it is recommended to use a four-layer copy board for more complicated PCB boards. Its advantage is that signal lines can be routed on the top and back sides, which increases the wiring space. The more important thing is that there is a low impedance ground plane and power plane. Especially the ground plane greatly reduces the circulation area and ground impedance of all IC circuits.
In principle, the top layer is the signal line layer, the second layer is the DC ground line layer, the third layer is the DC power layer, and the fourth layer is the signal line layer. When the printed board IC circuits are all switching circuits or all analog circuits, their ground wires do not need to be isolated. Sometimes, there are often several power sources in the DC power layer, which are usually separated and solved by gap isolation methods. When there are logic circuits and analog circuits in the printed circuit board copy board, through analysis, the logic circuit ground wire and the analog circuit ground wire can be partitioned (isolation band width> 3mm) single short circuit or use magnetic beads, etc. Method connection to obtain the same potential reference ground.
When there are dozens of connections between the logic circuit and the analog circuit in the printed board, the situation is very complicated. It is necessary to grasp that they must have independent power supply and ground areas, and consider the IC loops that have a connection relationship. The principle of minimum circulation area goes
design, and ensure that there is a very low impedance ground wire.
The double-layer board ground wire is designed to be formed by a grid-like enclosure, that is, a more parallel ground wire is laid on one side of the printed board, and the other side is a vertical ground wire, and then they are connected by metalized vias at the place where they cross ( The via resistance should be small).
In order to consider that there should be a ground wire near each IC chip, one ground wire is often laid out every 1 ~ 115cm, so that the dense ground wire makes the area of the signal loop smaller, which is beneficial to reduce the radiation. The ground network design method should be before the signal line, otherwise it will be difficult to implement.
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