The design of the TVS tube mentioned in the lightning protection design of airborne electronic equipment based on TVS tube is aimed at the lightning protection of aviation airborne electronic products. Here I borrow the TVS
tube selection method and lightning protection design ideas described in it to do the electrostatic protection (ESD) design of the circuit board.
Article Directory
1. ESD protection design
The electrostatic protection design is to allow all kinds of metal button switches connected to the circuit board to discharge in contact with the outside air or contact discharge . After the large energy that appears at this moment is injected into the circuit board, it can be released to the In the earth, so as to protect the back-end circuit from the influence of static electricity.
2. Basic characteristics of TVS tube
The protection principle of the TVS tube is determined according to the characteristics of the PN structure. When the voltage at both ends of the TVS tube is greater than a fixed value, the TVS tube will be reversely broken down, and a conduction path will be formed in an instant, leading out the large current at both ends of the tube, and at the same time Clamp the voltage at both ends of the tube to a fixed voltage value. The use and working characteristics of the TVS tube are shown in the figure:
combined with the TVS operating characteristic curve in the above figure, when the reverse voltage at both ends of the tube is greater than VRWM , the tube starts to reverse direction When the reverse voltage is greater than VBR , the tube begins to be reversely broken down , and the current begins to change suddenly. Finally, when the reverse voltage is greater than VC , the tube is in an avalanche breakdown state , and the current flowing through the tube increases sharply. And the voltage value at both ends of the tube does not change much .
3. Selection method of TVS tube
In the TVS tube selection process, according to the actual protection signal amplitude characteristics, three parameters need to be considered, reverse working voltage VRWM, breakdown voltage VBR, and clamping voltage Vc. Firstly, the working voltage of the circuit to be protected must be specified. The maximum working voltage should be less than VRWM to ensure that the signal characteristics remain basically unchanged. Secondly, VBR should be
determined according to VRWM. VBR = VRWM/KBR (KBR=0.8~0.9), and finally according to the back-end interface chip Or the maximum transient withstand voltage value of the processor chip determines Vc, ensuring that the voltage value of Vc is less than the maximum withstand voltage value of the back-end chip pin.
For high-frequency data lines, the parasitic capacitance of the TVS tube is also a consideration. If the parasitic capacitance is too large, it will affect the transmission of high-frequency data lines, thereby affecting data communication. The selection of parasitic capacitance of the TVS tube is shown in the table:
4. TVS tube layout details
When doing the layout of the TVS tube on the circuit board, it is necessary to focus on the layout position of the TVS tube. Place it at the input end of the protected circuit. best working results.
5. References
Yang Qifan, Zhang Daozze, Zhu Enliang, Ma Xiaochen. Lightning Protection Design of Airborne Electronic Equipment Based on TVS Tube[J]. Aviation Computing Technology, 2020,50(06):96-100. ↩︎