Several EMI design guidelines that hardware engineers easily ignore in PCB design

Several EMI design guidelines that hardware engineers easily ignore in PCB design

The following are several EMI design guidelines that hardware engineers tend to ignore in the early stages of PCB design, but are very useful, and these guidelines are often mentioned in some authoritative books.

Design Guide 1: Minimize Current Loop Area for Power Supplies and High-Frequency Signals

In the design phase, first we need to know two main points:

1. The signal current always returns to the source (ie the current path always exists in the form of a loop).

Design Guide 2: Keep the Signal Return Plane Intact

A complete signal return plane can effectively reduce the inductive reactance of the high-frequency signal loop. The smaller the inductive reactance, the smaller the noise voltage value generated. This is one of the important reasons why it is required to set a complete ground plane in the middle layer of the PCB. Of course, in some cases the signal return plane has to be separated due to the traces. However, this situation occurs less frequently on multilayer PCBs. In addition, for the case of single-layer boards, wrapping can be done around the high-speed signal traces to maintain the integrity of the signal return path.

Design Guideline 3: Do not place high-speed circuits near connectors

We often make the following mistake, during the review or evaluation process of the board design, due to lack of consideration, the high-speed circuit is placed near the connector, which causes the engineer to do a lot of extra filtering and shielding, which increases the cost and improves the machine. Correction difficulty.

Why is the location of the connector so important? At frequencies below three hundred megahertz, the wavelength is on the order of a meter or more. The printed circuit board itself and the traces on the board are often electrically small in size, so the radiation efficiency is relatively low. However, the cables connected to the connectors are generally long, so the antenna effect will be obvious, and the noise inside the board is more likely to radiate out through the cable.

In addition, high-speed circuits located between connectors can easily create potential differences of several millivolts or more between the connectors. These voltages can drive current onto the connected cables, causing the product to exceed radiated emission requirements.

Design Guide 4: Control Signal Edge Transition Time (Rising and Falling Time)

In many cases, the clock noise exceeding the punctuation point is not the fundamental frequency, but the higher harmonics derived from the fundamental frequency. By increasing the transition time of the clock edges, the energy of the higher harmonics can be well controlled. Although excessive edge transition times can cause signal integrity and thermal issues, there are many times when a compromise between functionality and EMC effects is required.

There are three common methods for controlling the rise and fall times of digital signals:

1. Change the chip signal output drive capability

2. Signal line series resistor or ferrite

3. Signal line parallel capacitor

Design Guide 5: Clock Spread

As electronic products have more and more functions, the chip clock frequency is also increasing. For high-speed clocks, the risk of controlling clock edge slew rate to suppress EMI is increasing. At this time, spread spectrum technology becomes a good choice for suppressing electromagnetic interference.

While maintaining the integrity of the clock signal waveform without changing the rising edge and falling edge of the clock, the clock jitter is controlled according to a certain rule, and the clock energy is dispersed into a wider frequency band to achieve the suppression of clock noise in the frequency domain.

The spread spectrum technology not only modulates the clock source, but other data, address and control signals synchronized with the clock source are also modulated at the same time as the clock spread, and the overall EMI peak will be reduced accordingly. Therefore, the clock spread is system-level solutions. This is one of the biggest advantages of spread spectrum technology over other EMI suppression measures.

In general, engineers need to keep an alarm bell in their minds during the PCB design process. While considering how to implement circuit functions, they should pay attention to signals that are prone to noise. When encountering such as clock or PWM, they will generate high-order When designing a PCB with harmonic signals, refer to several EMI guidelines, so that it will become easier for the product to pass the EMC certification.