Do you know the EMC testing work of microcontrollers?

Do you know how to detect EMC of a microcontroller? How to define EMC? In simple terms, it is the ability of a device or system to work normally in its electromagnetic environment without causing unacceptable electromagnetic disturbance to anything in the environment. EMC testing includes two major aspects: testing the intensity of electromagnetic disturbance it sends to the outside world to confirm whether it meets the limit value requirements stipulated in relevant standards; conducting sensitivity testing under electromagnetic environment conditions that specify the intensity of electromagnetic disturbance to confirm whether it meets the limit value requirements stipulated in relevant standards. Confirm whether it meets the immunity requirements specified in relevant standards.

For engineering and technical personnel engaged in the design of microcontroller application systems, it is very necessary to master certain EMC testing techniques. EMC is the abbreviation of Electromagnetic Compatibility, which includes electromagnetic interference (EMI) and electromagnetic susceptibility (EMS). Since electrical products cause electromagnetic interference to other electrical appliances when in use, or are subject to electromagnetic interference from other electrical appliances, it not only affects the reliability and safety of the product, but may also affect the normal operation of other electrical appliances and even cause safety hazards.

Microcontroller system EMC testing

(1)Test environment

In order to ensure the accuracy and reliability of test results, electromagnetic compatibility measurement has high requirements on the test environment. The measurement sites include outdoor open fields, shielded rooms or anechoic chambers, etc.

(2)Testing equipment

Electromagnetic compatibility measurement equipment is divided into two categories: one is electromagnetic interference measurement equipment, which can measure electromagnetic interference when connected to an appropriate sensor; the other is electromagnetic sensitivity measurement, where the equipment simulates different interference sources and passes appropriate Coupling/decoupling networks, sensors or antennas are applied to various types of equipment under test for sensitivity or interference measurements.

(3)Measurement method

There are many measurement methods for electromagnetic compatibility testing based on different standards, but they can be summarized into 4 categories: conducted emission testing, radiated emission testing, conducted sensitivity (immunity) testing and radiated sensitivity (immunity) test.

(4)Test and diagnosis steps

Figure 1 shows the electromagnetic interference emission and failure analysis steps of a device or system. Following this step can improve the efficiency of testing and diagnosis.

(5)Test preparation

① Test site conditions: The EMC test laboratory is a radio wave semi-dark room and a shielded room. The former is used for radiated emission and radiation sensitivity testing, and the latter is used for conducted emission and conducted sensitivity testing.

②Environmental level requirements: The electromagnetic environment level of conduction and radiation should preferably be far lower than the limit value specified by the standard. Generally, the environmental level should be at least 6dB lower than the limit value.

③Test table.

④Isolation of measuring equipment and equipment under test.

⑤ Sensitivity judgment criteria: Generally provided by the party under test, and monitored and judged honestly, the degree of performance degradation is determined by measurement and observation.

⑥Placement of the equipment under test: In order to ensure the repeatability of the experiment, there are usually specific regulations on the placement of the equipment under test.

(6) Test types

Conducted emission test, radiated emission test, conducted immunity test, radiated immunity test.

(7)Commonly used measuring instruments

Electromagnetic interference (EMI) and electromagnetic susceptibility (EMS) testing requires the use of many electronic instruments, such as spectrum analyzers, electromagnetic field interference measuring instruments, signal sources, functional amplifiers, oscilloscopes, etc. Since the EMC test frequency is very wide (20Hz ~ 40GHz), the amplitude is very large (μV level to kW level), there are many modes (FM, AM, etc.), and there are many postures (flat, tilted, etc.), it is very important to use electronic instruments correctly. important. A suitable instrument for measuring electromagnetic interference is a spectrum analyzer. A spectrum analyzer is an instrument that displays the variation of voltage amplitude with frequency. The waveform it displays is called a spectrum. The spectrum analyzer overcomes the shortcomings of the oscilloscope in measuring electromagnetic interference and can accurately measure the interference intensity at each frequency. The spectrum analyzer can directly display each spectrum component of the signal.

When solving electromagnetic interference problems, one of the most important issues is to determine the source of the interference. Only after the source of interference is accurately located can measures to resolve the interference be proposed. Determining the source of interference based on the frequency of the signal is the simplest method, because among all the characteristics of the signal, the frequency characteristics are the most stable, and circuit designers often have a very clear understanding of the signal frequencies at various parts of the circuit. Therefore, as long as the frequency of the interference signal is known, we can deduce where the interference is generated. For electromagnetic interference signals, since their amplitude is often much smaller than the normal operating signal, it is very simple to use a spectrum analyzer to make this measurement. Since the spectrum analyzer has a narrow IF bandwidth, it can filter out signals with a different frequency than the interference signal, accurately measure the frequency of the interference signal, and thereby determine the circuit that generates the interference signal.

Electromagnetic compatibility troubleshooting technology

(1)Solution to conductive problems

① Reduce EMI current by connecting a high impedance in series.

② Short-circuit the EMI current to the ground or lead it to other return conductors by connecting a low impedance in parallel.

③Cut off EMI current through galvanic isolation device.

④Suppress EMI current through its own action.

(2) Capacitive solutions for electromagnetic compatibility

A common phenomenon is to view one side of the filter capacitor not as directly connected to a separate impedance, but as connected to the transmission line. Typically, when the length of an input/output line reaches or exceeds 1/4 wavelength, the transmission line becomes "long". In fact, this change can be approximately expressed by the following formula:

l≥55/f

In the formula: l unit is m, f unit is MHz. This formula takes into account the average speed of propagation, which is 0.75 times that of free space theory.

a. Dielectric materials and tolerances

Most capacitors used in electromagnetic interference filtering are non-polar capacitors.

b. Differential mode (line-to-line) filter capacitance.

c. Common mode (line to ground/chassis) filter capacitor

Common mode (CM) decoupling usually uses small capacitors (10 to 100nF). Small capacitors can short-circuit undesired high-frequency currents to the chassis before they enter sensitive circuits or when they are far away from noisy circuits. In order to obtain a good high-frequency attenuation circuit, reducing or eliminating parasitic inductance is the key. Therefore, it is necessary to use ultra-short wires, and it is especially desirable to use leadless components.

(3) Inductive and series loss electromagnetic compatibility solutions

In the case of capacitors, Zs and Z1, if they are not pure resistors, use their actual values ​​when calculating frequency. When capacitors are connected in series in power or signal circuits, they must meet the following requirements:

① The flowing working current should not cause the inductor to overheat or become too large or worse;

② The flowing current cannot cause magnetic saturation of the inductor, especially for high magnetic permeability materials.

The solutions are as follows:

*Core material;

*Ferrites and ferrite-loaded cables;

*Inductance, differential mode and common mode;

*Ground choke;

*Combined inductor and capacitor components.

(4)Solution to radiation problem

In many cases, radiated electromagnetic interference problems may arise in the conduction stage and be eliminated. Some solutions are to suppress interference devices in the radiated transmission channels, working like field shields. According to shielding theory, the effectiveness of this shielding mainly depends on the frequency of the electromagnetic interference source, the distance from the shielding device and the characteristics of the electromagnetic interference field - electric field, magnetic field or plane wave.

①Conductor tape.

Use copper or aluminum tape to quickly and easily create a direct, shielded and low-resistance connection or bus. They are convenient for both temporary solutions and relatively permanent solutions. The thickness is between 0.035~0.1mm, and there is conductive adhesive on the back for easy installation. If copper conductive tape is used, its passing resistance is about 20mΩ/cm2. Applications: electrical shielding cover; locating leakage points in the event of a failure; as an emergency solution, turning plastic connectors into metal, shielded ordinary flat cables, etc.

②Mesh shielding tape and zipper jacket.

Tin-coated steel mesh tape: mainly used to install on an already assembled electricity bill sheath as an easy-to-install bandage-type shielding cover. To reduce magnetic field radiation or sensitivity issues in electricity bills, steel mesh belts are an effective solution. Zippered Shield Jacket: Used when there are clear signs that electricity is a major cause of EMI coupling.

③EMI gasket.

Applications: When the following conditions exist and true SE is required, EMI gaskets are the most commonly used method to solve radiation problems, sensitivity problems, ESD, electromagnetic pulse and TEMPEST problems.

*Chassis leakage has been identified as the primary radiation path.

*The mating surface is not smooth, flat or hard enough to provide good connection contact by itself.

④ EMI shielding of windows and ventilation panels:

Suitable for shielding apertures.

A rough model for a plane wave is:

SE≈104(-20-lgl)-20lgf

In the formula, the unit of SE is dB; l is the size of the grid or mesh, the unit is mm; the unit of f is MHz. Of course, as the frequency decreases, the upper limit of the mesh's shielding efficiency SE is limited by the metal itself. In the near field, the shielding power SHE for H field shielding is not affected by frequency and can be approximated by the following formula:

SEH≈10lg(πr/l)

Among them, r is the distance between the source and the shielding cover, l is the mesh size, and both units are mm.

⑤ Conductive coating: It is used to build an EMI shielding cover on the plastic shell of the system, increase the shielding effectiveness SE of the existing ordinary or deteriorated conductive surface, prevent ESD or static electricity accumulation, and increase the contact area of ​​the joint surface or sealing gasket.

⑥Conductive foil: Aluminum is a good conductor with no absorption loss below 10MHz, but it has good reflection loss for any frequency of the electric field. Please refer to relevant information for application scenarios.

⑦Conductive cloth: It can be used in any three-dimensional shielding situation where the frequency range from 100kHz to GHz needs to achieve 30~30dB attenuation.

Conclusion

In actual EMC testing applications, in addition to passing the qualification test of the standard qualification laboratory, there are two feasible methods that are also recognized by the industry: TCF (Technical Construction File) and Self Certification (self-test certificate). The anti-interference ability test is a very practical test item. The best way to achieve electromagnetic compatibility is to treat all digital and analog circuits as circuits that respond to high-frequency signals, and use high-frequency design methods to handle electricity shielding, PCB wiring, and common-mode filtering.

It is also important to use a single ground plane and power plane, also for analog circuits, to limit high-frequency common-mode loops. Most transient interferences are of high frequency and produce strong radiated energy. The above is the relevant analysis of microcontroller EMC detection, I hope it can help you.