Summary of basic aspects of electromagnetic compatibility

This article lists several basic aspects of the issues that have been seen in the last few days.

1 A list of EMC design steps?

Record a seemingly plausible electromagnetic compatibility design methodology. The author divides the electromagnetic compatibility design process into six levels. From the first level, the electromagnetic compatibility problem is too stubborn to enable the latter level. In the end, a complete design is formed, which contrasts with our current popular words, which is equivalent to a design list.

——Selection of active devices and printed circuit board design

—— ground wire design

——Shield design

——Filter design

——Inhibition of transient disturbance

Apply "system-level electromagnetic compatibility design, software anti-harassment design and simulation" to each step, called "electromagnetic compatibility layering and comprehensive design method."


2 What is differential mode interference and common mode interference?

There are two forms of "differential mode" and "common mode" when current and voltage are transmitted in the circuit. For a DC loop, at least two wires are required to transmit an electrical signal, and outside the loop, there must be a reference ground for the system. The interference signal transmitted between the line and the line is defined as a "differential mode interference signal", and the interference signal transmitted between the line and the ground is called a "common mode signal". Both differential mode interference and common mode interference can be expressed in the form of current or voltage. When differential mode interference is conducted in the loop, the parameters in the two wires are in opposite directions; common mode interference, when propagating in the loop, the electrical parameters of the two wires are in the same direction. It can also be said that the differential mode interferes with the common mode interference of a phase difference of 180°.

Two ways of forming differential mode interference, one cause is that the spatial magnetic field is induced in the loop, and the other cause is the common mode interference in the loop, which is formed by conduction conversion of the unbalanced circuit. Differential mode interference is essentially another current in the loop that has a different phase. To avoid differential mode interference, you can use twisted pair, shielded lines or add filter circuits in your circuit.

The reason for the formation of common mode interference is mainly the potential difference of the system to the ground, which may be a poor grounding, or the result of superimposing the radiated electromagnetic interference signals in the same direction or the voltage difference formed by the upstream power source. Common mode interference, in essence, is that the system zero-to-ground voltage difference is not zero. Therefore, in order to eliminate common mode interference, on the one hand, improve the grounding system and reduce the voltage difference between the system and the ground; on the other hand, set a filter circuit at the input to filter out the common mode interference from the conduction; strengthen the shielding to avoid the outside world. The radiation of the interfering signal induces common mode interference in the circuit.

In electric vehicles, the main source of interference is the motor controller. Common mode interference is the main way in which the motor controller has an adverse effect. This has been proven by experimentation. How is common mode interference generated on the motor controller?

In the PWM-driven motor controller power module, the common mode interference signal is generated mainly for two reasons. One is that there is stray capacitance between the IGBT and the heat sink, and the common mode voltage is defined as the potential difference between the neutral point of the inverter loop and the reference ground. The common mode voltage fluctuates continuously with the operation of the circuit. The rate of change of voltage with time acts on the equivalent capacitance forming current 1; during the conduction of the common mode voltage, the voltage is propagating. Current 2 is generated on the path. The sum of current 1 and current 2 is the overall common mode interference current generated by the PWM inverter.

3 The basic form of electromagnetic interference propagation?

There are two main forms of electromagnetic interference signal propagation: conductive coupling and radiation coupling.

Conductive coupling is the propagation of a signal along a conductor, regardless of differential mode interference or common mode interference, but the interference signal may be boosted or weakened during conduction. Differential mode interference will increase with the increase of mutual inductance of the propagation line, and common mode interference will decrease as the mutual capacitance of the propagation line decreases. Common paths for conduction coupling include high and low voltage cables, signal communication cables, public grounds, and others. An electrically conductive object connected between the source of interference and the source of sensitivity.

The radiation coupling coupling is that the interference signal propagates to the surrounding space in the form of electromagnetic radiation, and is restored to the circuit by electromagnetic induction in the vicinity of the sensitive source to form an interference signal. Common paths for radiation are antennas, high and low voltage wires, and enclosures. The radiation capability of the interference signal is directly related to the frequency. The higher the frequency, the stronger the radiation capability.

4 What are the broadband and narrowband in the field of electromagnetic compatibility?

In the field of electromagnetic compatibility, broadband and narrowband are not related to the definition of communication and the Internet. It is a contrast relationship between interference signals and detection instruments. A more appropriate record is that "the wideband and narrowband defined in EMC measurements are only relative to the IF bandwidth of a standard-tested measuring instrument. For the 9kHz-150kHz frequency range, the IF bandwidth is 200Hz; for 150kHz-30MHz, the intermediate frequency The bandwidth is 9 kHz; for 30 MHz-1000 MHz, the IF bandwidth is 120 kHz. If the bandwidth of interference in a measurement band is larger than the IF bandwidth of the EMI receiver, it is broadband noise, otherwise it is narrowband noise.

5 What are the transient test items?

Transient testing, a type of test in which the system encounters extreme conditions to investigate system reliability, more similar to environmental testing, is a general-purpose environment and a general-purpose indicator and has general criteria. The test of the emission and anti-interference ability of the electrical system, the parameters required by different systems are different, and there is no uniform and usable criterion.

Specific tests for transient testing include electrical fast transient bursts, lightning surges, and direct and indirect disturbances of electrostatic discharge.

5.1 Electrical Fast Transient Burst Test

The electric fast transient burst test is a simulation of the loop in which the relay is present, and the relay occurs during the process of breaking. When the relay contact is opened, the distance is relatively close, and the voltage between the static and dynamic contacts breaks down to form an arc. After the process, part of the energy oscillates between the contact and the relay coil until the contact distance is large enough, and the circuit is completely Disconnected. The oscillating energy generates a pulse voltage in the circuit other than the relay, which acts on other devices of the circuit. Once the electrical can not withstand such an impact and cause damage, the circuit cannot work normally.

Electrical fast transient burst testing has well-defined parameters, pulse duration, overall waveform period, pulse maximum, and overall test time.

5.2 Lightning surge test

Lightning surge test, the simulated electrical system is in the lightning environment, and the lightning protection capability of the system is investigated. The generation of lightning is divided into direct lightning and inductive lightning. A direct lightning strike is a discharge phenomenon that occurs directly between a charged cloud block and another cloud block or earth or building. Inductive lightning, when lightning occurs, induces a strong electrostatic field and magnetic field to find a suitable metal conductor discharge on the ground, causing sudden spikes in the conductor-related loop. Direct lightning strikes are more destructive, but the probability of occurrence is small; inductive lightning is the most common hazard in life.

Surge test, select different peak voltages according to different experimental grades, divide four levels from 0.5KV to 4KV, and finally reserve a self-selected level. The system wants to withstand such an impact and must have a special anti-surge device in the circuit.

5.3 Electrostatic Discharge Disturbance Test

The electrostatic discharge disturbance test is to simulate the static phenomenon that occurs in daily life and to test the bearing capacity of the system. Static electricity is a discharge phenomenon in which the voltage is extremely high, the duration is extremely short, and the total energy is not large. May cause insulation breakdown in the circuit.

The electrostatic disturbance test is divided into direct discharge and non-contact discharge. Each test is divided into 5 levels, four specified levels and one optional parameter level.

6 Immunity criteria for transient testing?

The transient test has a general test result judgment criterion, and the criterion divides the evaluation of the product test result into four levels, and the original text is reproduced as follows.

Class A, the product works perfectly;

Class B, product function or indicator has an undesired deviation, but when the electromagnetic disturbance is removed, it can recover itself;

Class C, product functions or indicators appear undesired deviations. After electromagnetic disturbances are removed, they cannot be recovered by themselves. They must rely on the intervention of operators to recover. But does not include hardware repair and software reloading;

Class D, product components are damaged, data is lost, software is faulty, etc.

A, B qualified, C, D failed.