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How is the selection of semiconductor discharge tube?
Semiconductor discharge tube is a common overvoltage maintenance device in modern communication equipment, which can prevent communication equipment from being damaged by lightning and surge. At present, the wires in the wired communication network are metal wires and optical fibers. The wires are installed outdoors and will inevitably be affected and destroyed by nature and man-made. Lightning, power line induction and touch can cause damage to communication equipment or casualties. With the development of modern communication, people pay more and more attention to the maintenance of communication equipment.
At present, the overvoltage maintenance of communication equipment in China basically uses gas discharge tube as the maintenance device, but the gas discharge tube has the defects of slow response speed, short life, large parameter dispersion, high pulse ignition voltage, open circuit after fault, high maintenance cost, unreliable maintenance and so on, which greatly reduces its maintenance function. Corresponding to the gas discharge tube, there are solid discharge tube, commonly known as button solid discharge tube. Compared with the gas discharge tube, its performance has been greatly improved, but the volume is too small, the device is inconvenient, and it is not suitable for various size requirements. Therefore, it is again enclosed in a housing of similar size to the gas discharge tube. However, in this way, the cost of secondary packaging is increased, the rejection rate is low, and the device cost is high.
Semiconductor discharge tube is a kind of overvoltage protection device, which is made of thyristor principle. PN junction breakdown current trigger device conduction discharge, can flow large surge current or pulse current. The range of its breakdown voltage constitutes the range of overvoltage protection. When in use, the semiconductor discharge tube can be directly connected across the two ends of the protected circuit.
When selecting a semiconductor discharge tube, the following points should be noted:
1. The large instantaneous peak current IPP must be greater than the specified value of the communication equipment standard.
2. The breakover voltage VBO must be less than the large instantaneous peak voltage allowed by the protected circuit.
3. When the semiconductor discharge tube is in the on state (conduction), the power loss P should be less than its rated power Pcm,Pcm = KVT * IPP, where K is determined by the waveform of the short-circuit current. The k values are 1.00, 1.4, 2.2, and 2.8 for exponential, square, sine, and triangle waves, respectively.
4. The reverse breakdown voltage VBR must be greater than the large operating voltage of the protected circuit. For example, in POTS applications, the sum of the peak voltage (150*1.41=212.2V) and the peak DC bias voltage (56.6V) of the large ringing voltage (150V) is 268.8V, so a device with a VBR greater than 268.8V should be selected. For example, in ISDN applications, the sum of the large DC voltage (150V) and the large signal voltage (3V) is 153V, so a device with a VBR greater than 153V should be selected.
5. To make the semiconductor discharge tube self-reset after passing a large surge current, the holding current IH of the device must be greater than the current value provided by the system. That is, IH (system voltage/power supply impedance).
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