EMC is mainly through the comprehensive evaluation of the electromagnetic interference size and anti-interference ability of the test product, which is one of the important indicators of product quality and safety certification. Many products will encounter unsatisfactory product testing during product safety certification, especially in the electromagnetic compatibility test (ie EMC test), the frequency of errors is more common. Once the product fails the test, the EMC rectification notice must be followed.
In the process of EMC rectification, many managers and technicians do not quite understand where to start. Today we will analyze the problems encountered in EMC rectification and some rectification suggestions.
1、EMC rectification opinions
1. After receiving the rectification opinions, you need to locate the EMC rectification plan in advance. Without a good plan, blindly rectify products like a headless flies, which will only increase the cost of rectification.
2. Positioning means, for the editor here, it can be divided into two main points. First: Intuitive judgment, which needs to rely entirely on the intuition and experience of engineers to judge. Second: comparative test, to analyze the problem based on the data provided by the test instrument.
2、EMC rectification process
1、RE excess standard rectification process:
2、Rectification process of wire and cable exceeding the standard:
3、Signal cable rectification process:
4、Rectification process of shield leakage:
Some suggestions for EMC rectification
1、Capacitor filtering Function
That is, the greater the frequency f, the smaller the impedance Z of the capacitor.
When the frequency is low, the capacitor C has a large impedance Z, so the useful signal can pass smoothly;
When the frequency is high, the capacitance C is already small due to the impedance Z, which is equivalent to short-circuiting the high-frequency noise to GND.
2、When will capacitive filtering fail
In the rectification, components such as capacitors are often used for filtering. There is often a saying that “large capacitors filter low frequencies and small capacitors filter high frequencies.”
Taking the common surface-mount MLCC ceramic capacitor as an example, the equivalent model is as follows:
The capacitance of 10nF, the model parameters of the package 0603 X7R ceramics are as follows:
Since there are both capacitance C and inductance L in the equivalent model, a second-order system is formed, and there is instability. For circuit loops, resonance occurs, and the resonance point is at the following frequency:
The following figure is an example of the resonance curve:
It is often said that there is a capacitor before the resonance point, and it is no longer a capacitor after the resonance point.
3、When to use LC filter
If the series inductance L is connected in parallel to form C, an LC filter is formed:
A single capacitor C is a first-order system, and a single inductance L is also a first-order system, and the attenuation slope at the amplitude is -20dB. However, the second-order system composed of LC has an amplitude attenuation slope of -40dB, which is closer to the ideal “steep” cut-off frequency, that is, the filtering effect is better.
4、What is the PWM frequency
PWM is often mentioned, for example, 20kHz PWM is used to drive the motor. But in fact, this 20kHz only represents the pulse period of PWM is 50us:
So where does the frequency point of the so-called 20kHz PWM in the frequency domain fall, as follows:
For a step signal, since the rise time tr is infinitesimally small, the frequency f is infinite. When the frequency is high, the parasitic parameters cannot be ignored, which will cause many resonance problems.
From the signal point of view, it is a very steep step signal that will have overshoot and oscillation problems. In simple terms, the larger the frequency f, the wider the frequency occupied by noise, that is, the worse the EMC characteristics.
5、How to map the schematic and PCB
Due to the problem of subdividing the type of work, the schematic diagram and the PCB were cut apart, and the two groups of people divided the work:
For example, there are the following circuits on the schematic diagram:
The hidden problem is that there is actually a line on the negative pole of V1 and the negative pole of C1 on the PCB (the words used in the PCB layout tool software are more accurate, Trace, trace/trace).
Often in the design stage A-“B-“C will pay attention. If there is a problem with EMC, in addition to finding the circuit parameters on the schematic, you need to pay special attention to C-》D, that is, the return path.
If the return path is not smooth, it will cause signal distortion:
For example, during the EMC test, the signal collected by the ADC of the MCU is disturbed. In addition to analyzing on the schematic diagram, the signal is highlighted on the PCB, and then patiently look for whether the return path of the signal is not smooth. local:
Imagine the return path in the mind of the signal line, a bit of a sense of stream of consciousness.