We have a deep understanding of the importance of the stability of electronic products. Of course, we don’t want to buy a smart bracelet that needs to be corrected every day, and we don’t want to buy an electronic product that is easy to crash. Once this happens in the electronic products we buy, the fate of this electronic product may come to an end. The product in your hand may have to eat ashes in the drawer corner or become a toy in the hands of children who have no use.
Electronic products can produce abnormal, which is affected by many factors, such as ESD, EMI or design defects. So how to design a stable performance of electronic products, we need to pay attention to several aspects.
First, eliminate the principle design defects
This part is very important, especially when the external interface circuit needs low-power design, we should pay more attention to it. For example, a key is usually designed to be pulled up to VDD in normal design, but in low-power design, it needs to be connected to GND and pulled up to VDD when there is action.
In addition to designing the schematic diagram according to the product characteristics, it is also helpful to the selection of components. For example, packaging, power consumption and working environment need to be considered.
Second, rational distribution
This part is a technical or artistic problem. Reasonable layout requires reasonable arrangement of the connection, location and routing of each unit in the system equipment. Attention should be paid to the separation of high-speed and vulgar, analog and digital, and the interference source is far away from the core components. Note that the high-frequency connection should be as short as possible, and the grounding resistance should be as small as possible. I believe this is an endless book for electronic engineers. This reminds me of a sentence: a PCB with excellent performance is a good product, and its layout is a work of art.
Third, do a good job of shielding
Ordinary design of vulgar PCB does not need to do any shielding system can also operate normally. But if you take a look at a mobile phone, handset or other communication equipment, you will find that the key RF circuit is often hidden under the Faraday cage. This is a way of shielding.
Shielding, as an effective way to suppress radiation interference, is often used in severe EMI disaster areas, but not as long as the use of shielding can solve the problem of EMI, accompanied by the problem of grounding. Only when shielding and grounding are used together, can shielding play the greatest role. If there is no grounding, the cage may become a flat panel antenna to collect energy and emit more powerful EMI.
There are electric shield, magnetic shield and electromagnetic shield. Different shielding technologies adopt different shielding design methods
Copper aluminum alloy is usually selected as shielding material for electrical shielding, and silver plating is needed on the surface of shielding cover in high frequency shielding. For its shape, generally choose the box shape to obtain good shielding performance. Try not to open holes on the shield to avoid leakage of electric field energy. If the holes have to be opened for some reason (heat dissipation), the number of holes should be as small as possible and the area of holes should be as small as possible.
Those who have studied electromagnetic field course all know that in low frequency magnetic field, the shielding effect of eddy current is very small, and high permeability materials are usually needed for shielding. The thicker the shielding shell is, the higher the permeability is, and the better the magnetic shielding effect is. The magnetic shield is made of steel plate, and double shield can be used when necessary.
The same method can be used for electromagnetic shielding and electrical shielding. According to the actual situation, the appropriate conductive gasket, comb spring, shielding display window and other methods can be selected to achieve electromagnetic sealing.
For shielding, there are usually the following steps:
For the common IO interface, if you need to detect the external voltage or switch status, you can connect it directly. However, in order to prevent the need to protect the IO interface due to over-current or over-voltage, you can use the method of IO port isolation. The use of transistor and optocoupler is a good method. In addition, there are transformer isolation, component isolation and other methods.
Fifth, good grounding
For the grounding technology, please refer to the grounding technology of electronic design
With the development of technology, I believe there are more methods for PCB design, which also records the growth process of every electronic engineer.