Causes and solutions of unqualified electric fast transient burst test

1. Reasons for nonconformity

The electrical fast transient pulse group test is of typical significance for detecting the anti-interference degree of equipment. Because the rising edge of the electric fast pulse test waveform is very steep, it contains a very rich range of high-frequency harmonic components, which can detect the anti-interference degree of the circuit in a wide frequency range. In addition, because the test pulse is a pulse train that lasts for a period of time, it has a cumulative effect on the interference of the circuit. In order to resist transient interference, most circuits have integral circuits installed at the input end, which have a good suppression effect on a single pulse, but cannot effectively suppress a series of pulses.

The figure shows three reasons for the impact of electrical fast pulses on the device.

(1) During power line transmission, interference energy radiates into space. This energy senses adjacent signal cables and interferes with the circuits to which the signal cables are connected (if this happens, it usually directly injects test pulses into the signal cable, causing the test to fail).

(2) The noise voltage on the circuit power line is too high because it is directly transmitted to the device's power supply through the power line. From the interference injection method shown in the figure, it can be seen that when the fire line or the zero line inject interference separately, there is differential mode interference between the fire line and the zero line, and this differential mode voltage will appear at the DC output terminal of the power supply; When interference is injected into both the fire line and the zero line, there is only a common mode voltage. Due to the fact that most power inputs are balanced (either transformer input or rectifier bridge input), actual common mode interference is rarely converted into differential mode voltage, which has little impact on power output.

(3) When broadcasting over cables (including signal cables and power cables), the secondary radiation energy generated by interfering pulse signals is induced into the circuit and interferes with the circuit.

2. Rectification measures

1) Measures for power lines

(1) Metal chassis. The main method to solve the power line interference problem is to install a power line filter at the power line inlet to prevent interference from entering the device. According to the interference injection method shown in the figure, the voltage injected into the power line is a common mode voltage, and the filter must suppress this common mode voltage to enable the test equipment to successfully pass the test. Currently, many finished power filters on the market are mainly designed for electric fast pulse testing, and designers can directly choose based on product characteristics. The following is a method of using filters to suppress electric fast pulses on power lines.

(2) The chassis of the equipment is non-metallic. If the device uses a non-metallic chassis, a metal plate must be added to 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 path is formed through the distributed capacitance between the metal plate and the ground plane. If the size of the device is small, the size of the metal plate is also small. At this time, the capacitance between the metal plate and the ground plane is small, and it cannot play a good bypass role. Therefore, the characteristics of inductors are very important for equipment to successfully pass the test, and various measures need to be taken to improve the high-frequency characteristics of inductors. If necessary, multiple inductors can be used in series.

2) Measures for signal lines

(1) Signal cable shielding. From the electrical fast transient pulse group test method, it can be seen that the mode of coupling interference pulses to the signal cable is capacitive coupling. The method of eliminating capacitive coupling is to shield the cable and ground it. Therefore, the condition for cable shielding is that the cable shielding layer can be reliably connected to the reference ground plane under test. If the equipment shell is a metal and grounded device, it is easy to meet this condition; 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 a non-metallic chassis, the cable shielding method is invalid.

(2) The signal cable of the equipment adopts twisted pair, and a ferrite magnetic ring is added at the signal line interface of the equipment (i.e., the end close to the equipment). The signal line is wound around the magnetic ring for 2-3 turns. This measure has a good effect for equipment with a weak anti-interference ability.

(3) Install a common mode choke on the signal cable. The common mode choke is actually a low-pass filter. According to the suppression effect of low-pass filters on pulse interference, they can only be effective when the inductance is large enough. However, when the inductance of the choke coil is large (usually with more turns), the distributed capacitance is also large, and the high frequency suppression effect of the choke coil is reduced. The electric fast pulse waveform contains a large number of high-frequency components. Therefore, in practical use, attention should be paid to adjusting the number of turns of the choke coil. If necessary, considering the requirements of high and low frequencies, two choke coils with different turns should be used for connection.

(4) Locally shield sensitive circuits. When the chassis of the device is a non-metallic chassis, or cable shielding and filtering measures are not easily implemented, interference will be directly coupled to the circuit. At this time, only sensitive circuits can be partially shielded, and the shield should be a complete hexahedron.

(5) Install a common mode filter capacitor on the signal cable. This filtering method is better than a choke coil, but requires a metal chassis as a filter capacitor. In addition, this method will attenuate the differential mode signal to a certain extent, and attention should be paid when using it.