Electrical fast transient bursts are typical for the immunity of test equipment

One、 Reasons for disqualification of electrical fast transient burst (EFT)

For the EMC immunity of testing equipment, the electrical fast transient burst test is of typical significance. Because the rising edge of the electric fast transient pulse group test waveform is very steep, it contains very rich high-frequency harmonic components, which can detect the circuit immunity in a wide frequency range. In addition, because the test pulse is a pulse train, it has a cumulative effect on the interference of the circuit. In order to resist transient interference, most circuits are installed with integral circuits at the input end, which can suppress a single pulse well, but can not effectively suppress a series of pulses.

The figure shows three reasons that affect the equipment:

The noise voltage on the circuit power line is too high because it is directly transmitted to the equipment power supply through the power line. It can be seen from the interference injection method shown in the figure that when the live line or zero line is injected separately, there is differential mode interference between the live line and the zero line, which occurs at the DC output end of the power supply; When the live line and the zero line are injected at the same time, there is only common mode voltage. Because most of the power input is balanced (transformer input or rectifier bridge input), the actual common mode interference is rarely converted into differential mode voltage, which has little impact on the power output.

In the process of power line conduction, interference energy radiates into space. These energies induce the adjacent signal cables and interfere with the circuit connected by the signal cables (if this happens, the test pulse is often directly injected into the signal cables, resulting in test failure). The secondary radiation energy generated when the interference pulse signal is transmitted on the cable (including signal cable and power cable) is inducted into the circuit and interferes with the circuit.

TWO、 Electric fast transient burst (EFT) solution

1. Measures for power line:

1.1. Metal chassis. The main method to solve the problem of power line interference is to install a power line filter at the power line inlet to prevent interference from entering the equipment. From the interference injection method shown in the figure, it can be seen that the voltage injected into the power line is common-mode voltage, and the filter must be able to suppress the common-mode voltage so that the test equipment can pass the test successfully. At present, many finished power filters on the market are mainly designed for fast electric pulse test, and designers can directly select according to product characteristics. The following is a method to suppress the fast electric pulse on the power line with a filter.

1.2. The equipment chassis is non-metallic. If the equipment uses a non-metallic chassis, a metal plate must be added at the bottom of the chassis to ground the common mode filter capacitor in the filter. As shown in the figure, the common mode interference current channel is formed by the distributed capacitance between the metal plate and the ground wire layer. If the size of the equipment is small, it means that the size of the metal plate is small, and the capacitance between the metal plate and the ground wire layer is small, which cannot play a better bypass role. Therefore, the characteristics of inductors are crucial for the equipment to pass the test successfully. Various measures should be taken to improve the high-frequency characteristics of inductors. If necessary, multiple inductors can be used in series.

2. Measures for signal line:

Signal cable shield. It can be seen from the test method that the method of interference pulse coupling into the signal cable is capacitive coupling. The method to eliminate capacitive coupling is to shield the cable and ground it. Therefore, the cable shielding layer can be reliably connected with the reference ground wire layer in the test. If the equipment shell is metal and grounding equipment, it is easy to meet the requirements; When the equipment shell is metal, but not grounded, the shielded cable can only suppress the high-frequency components in the electric fast pulse, and is grounded through the distributed capacitance between the metal shell and the ground; If the chassis is non-metallic, the cable shielding method is invalid.

The common mode choke is installed on the signal cable. The common-mode choke is actually a low-pass filter. According to the suppression effect of low-pass filter on pulse interference, it can only be effective when the inductance is large enough. However, when the inductance of the choke is large (usually more turns), the distributed capacitance is also large, and the high frequency suppression effect of the choke is reduced. The electric fast pulse waveform contains a large number of high-frequency components. Therefore, in actual use, it is necessary to pay attention to adjusting the number of turns of the choke coil, and if necessary, connect two choke coils with different turns, taking into account the requirements of high frequency and low frequency.

The twisted pair is used as the signal cable of the equipment, and a set of ferrite magnetic ring is added at the interface of the equipment signal line (that is, close to the end of the equipment), and the magnetic ring is wound for 2~3 turns. The effect of this measure is still very good for equipment with low immunity.

The common mode filter capacitor is installed on the signal cable. This filtering method has better effect than choke coil, but it needs metal case as the position of filter capacitor. In addition, this method has certain attenuation to the differential mode signal, which should be paid attention to when using.

Locally shield sensitive circuits. When the case of the equipment is non-metallic, or the shielding and filtering measures of the cable are not easy to implement, the interference will be directly coupled into the circuit. At this time, the sensitive circuit can only be partially shielded, and the shielding body should be a complete hexahedron.