Among the conventional protection schemes, semiconductor discharge tubes are widely recognized because of their superior cost performance. This type of device is suitable for the first and second level of the communication system to protect against lightning surges, static electricity, and EMI interference. The products are widely used ：Telephone, fax machine, Modem, XDSL terminal, program-controlled exchange central office equipment, T1/E1 interface, instrumentation, and its distribution frame, RS485/232 data line, Ethernet switch, CATV equipment, security products, remote monitoring , Remote meter reading and other products.
Key points for the selection of semiconductor discharge tubes:
1. The maximum instantaneous peak current IPP must be greater than the specified value of the communication equipment standard.
2. The turning voltage VBO must be less than the maximum instantaneous peak voltage allowed by the protected circuit.
3. When the semiconductor discharge tube is in the conducting state, 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. For exponential wave, square wave, sine wave, and triangle wave, the K values are 1.00, 1.4, 2.2, and 2.8 respectively.
4. The reverse breakdown voltage Vp must be greater than the maximum working voltage of the protected circuit. For example, in POTS applications, the sum of the peak voltage (150*1.41=212.2V) of the maximum ringing voltage (150V) and the peak DC bias voltage (56.6V) is 268.8V, so a device with a Vp greater than 268.8V should be selected. For another example, in ISDN applications, the sum of the maximum DC voltage (150V) and the maximum signal voltage (3V) is 153V, so a device with a Vp 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 that the system can provide.
In addition to lightning and surge protection, semiconductor discharge tubes are more voltage-limiting devices that are widely used. There are three package types: mount, in-line, and axial lead. In the circuit protection scheme, the semiconductor discharge tube is generally connected in parallel to the circuit. When the device does not operate, the resistance value is the highest, which can be regarded as an open circuit and has almost no effect on the circuit. When there is an abnormal pulse, the resistance drops instantaneously and the current is released instantaneously. When the abnormal high voltage disappears, it returns to a high resistance state, and the circuit works normally.
