It must be one of the difficulties of switching power supply to deploy a sophisticated PCB board (a poor PCB design may lead to the situation that no matter how the parameters are debugged, it is not alarmist). The reason is that there are still many factors considered when PCB layout, such as: electrical performance, process routing, safety requirements, EMC influence, etc. Among the factors considered, electrical is the most basic, but EMC is the most difficult to understand. , The bottleneck of the progress of many projects lies in the EMC problem; let’s share with you the PCB layout and EMC from 22 directions.

What EMC issues should be considered when PCB layout？

1. The EMI circuit of PCB design can be carried out calmly after being familiar with the circuit.

The impact of the above circuit on EMC can be imagined. The filter at the input end is here; the pressure sensitive for lightning protection; the resistance R102 to prevent inrush current (cooperate with the relay to reduce the loss); the key consideration is the differential mode X capacitor and the The inductance is matched with the Y capacitor for filtering; there are also fuses that affect the safety board layout; each device here is very important, and you must carefully savor the function and role of each device. The EMC severity level that must be considered when designing the circuit is calmly designed, such as setting several levels of filtering, the number of Y capacitors, and the location. The choice of varistor size and quantity is closely related to our demand for EMC. Welcome everyone to discuss the seemingly simple EMI circuit, but each component contains profound truth.

2. Circuit and EMC: (The most familiar flyback main topology, see which key places in the circuit contain the EMC mechanism).

There are several parts in the circuit in the above figure: the impact on EMC is very important (note that the green part is not), such as radiation, everyone knows that electromagnetic field radiation is spatial, but the basic principle is the change of magnetic flux, which relates to the effective cross-sectional area of ​​the magnetic field. , Which is the corresponding loop in the circuit. Electric current can produce a magnetic field, it produces a stable magnetic field, which cannot be transformed into an electric field; but a changing current produces a changing magnetic field, and a changing magnetic field can produce an electric field (in fact, this is the famous Maxwell equation, I use plain language), change In the same way, the electric field can generate a magnetic field. So be sure to pay attention to those places with switch states, that is one of the EMC sources, here is one of the EMC sources (here, of course, I will talk about other aspects later); for example, the dotted loop in the circuit is the switch tube opening. And the closed loop, not only the switching speed can be adjusted to affect EMC when designing the circuit, but also the loop area of ​​the board layout has an important impact! The other two loops are the absorption loop and the rectification loop. Learn about it in advance and talk about it later!

3. The association between PCB design and EMC.

1). The impact of the PCB loop on EMC is very important, such as the flyback main power loop. If it is too large, the radiation will be poor.

2). The wiring effect of the filter. The filter is used to filter out the interference, but if the PCB wiring is not good, the filter may lose the effect it should have.

3). In the structural part, poor grounding of the radiator design will affect the grounding of the shielded version, etc.

4). Sensitive parts are too close to the source of interference, such as the EMI circuit and the switch tube are very close, it will inevitably lead to poor EMC, and a clear isolation area is required.

5). RC absorption circuit routing.

6). The Y capacitor is grounded and routed, and the location of the Y capacitor is also critical, and so on.