“Effects of Plastic Shell Connectors and Metal Shell Connectors ON ESD
1. Phenomenon description
During the ESD test of a multimedia product with a metal casing, when the audio interface is subjected to a 2 kV electrostatic test, it is easy to cause mosaic and image freezing on the monitor, and the test fails.
2. Reason analysis
After observation, it is found that the outer shell of the audio interface is a plastic shell, and the connector of the audio signal line is outside, so the electrostatic interference signal can be directly coupled to the PCB through the audio signal line, thereby causing the device to operate abnormally. Static electricity is a discharge of high-voltage energy. During the electrostatic discharge test, the electrostatic interference signal has the characteristics of discharge from the nearby relatively low-potential conductor. When using the audio head of the plastic shell, the audio signal line is the conductor closest to the electrostatic discharge gun head. Electrostatic interference signals are only discharged to the audio signal line nearby. Such high-Voltage electrostatic signals are transmitted to the inside of the device through the signal line, which will inevitably cause abnormal operation or damage to the device, as shown in Figure 1.
Figure 1 Principle of electrostatic failure
It would be much better if the ESD point didn’t happen on the signal line. In order to make the device pass the ESD test, the best way is to let the ESD energy go away from a good grounding path, so that any circuits, devices and signals inside the device are not directly disturbed by the electrostatic energy.
For this device, to achieve this goal, it is necessary to change the discharge path of electrostatic discharge energy when discharging at the audio interface. Using an audio interface connector with a metal shell and grounding the connector shell can change the ESD path well, as shown in Figure 2.
Figure 2 The metal shell connector is used as a good design against electrostatic discharge interference, and it is necessary to make some supplementary explanations.
In the circuit shown in Figure 3, when the electrostatic discharge point is on the metal casing of the device, because the metal casing itself has poor laps and holes, when the electrostatic discharge current flows through the bad laps and holes, voltage will inevitably be generated. Drop ΔU, this voltage drop has a direct impact on the grounded circuit, even the ungrounded internal circuit (that is, there is no thick line in Figure 3), it will also have an impact on the circuit due to capacitive coupling. At the same time, if the size of the aperture can be compared with the wavelength of the frequency of the electrostatic discharge signal, it will also become a slot antenna and emit the electromagnetic energy of the electrostatic discharge current.
Figure 3 Radiation due to electrostatic current
Therefore, as a discharge path for electrostatic discharge, it is necessary to maintain a low impedance. Otherwise, there may be a lower impedance path created by the arc through the Electronic circuit. Impedance increases at high frequencies due to the skin effect, which can be alleviated by increasing the surface area. What is low impedance in EMC? Practice has proved that a complete (no gap, no opening) metal plane with an aspect ratio of less than 3 can well meet the discharge of electrostatic discharge.
3. Treatment measures Use electrostatic discharge to interfere with signals
The characteristics of nearby discharge, change the static discharge path, change the audio interface connector of the original plastic shell to an audio connector with a metal shell, and keep the metal shell and the chassis in good electrical continuity, so that electrostatic discharge interference Flow from the audio connector housing – the chassis to ground, protecting the signal in the audio interface. After testing, the anti-static discharge interference capability of the multimedia device using the metal shell audio connector is up to ± 8 kV for air discharge and ± 6 kV for contact discharge, and the problems described in this case have been solved.
Fourth, thinking and inspiration
(1) For the signal connector interface of the device, the selection of the connecting device and the design of the structure should avoid the electrostatic discharge interference signal being directly coupled to the signal line.
(2) In the selection of connectors, if the plastic shell cannot meet the required air discharge insulation distance, a connector with a metal shell must be used, and in the structural design, the connector shell has a good quality. grounding characteristics.