TVS tube function and its application circuit design
The selection of the TVS tube should be noted when selecting the following:
1 TVS rated reverse shutdown voltage VWM should be greater than or equal to the maximum operating voltage of the circuit being protected.
2 Minimum breakdown voltage VBR=VWM/KBR (where KBR=0.8 to 0.9).
3 The maximum clamp voltage VC of TVS should be less than the damage voltage of the protected circuit, ie VC=KC×VBR (where KC=1.3).
4 The maximum peak pulse power consumption PM of the TVS must be greater than the peak pulse power that may be present in the protected circuit for the specified pulse duration. After the maximum clamp voltage is determined, its peak pulse current should be greater than the transient surge current.
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The role of the TVS tube The TVS tube is a high-performance protection device in the form of a diode. When the two poles of a TVS diode are subjected to a reverse transient high-energy shock, it can change the high impedance between the two poles to a low impedance at a speed of the order of 10-12 seconds, absorbing surge power of up to several kilowatts, making the poles The voltage clamp is placed at a predetermined value to effectively protect the precision components in the electronic circuit from various surge pulses. Because it has the advantages of fast response time, large transient power, low leakage current, small breakdown voltage, easy control of clamping voltage, no damage limit, small size, etc., it has been widely used in computer systems, communication equipment, and /DC power supply, automotive, electronic ballast, household appliances, instrumentation, RS232/422/423/485, I/O, LAN, ISDN, ADSL, USB, MP3, PDAS, GPS, CDMA, GSM, digital camera Protection, common mode / differential mode protection, RF coupling / IC drive receiving protection, motor electromagnetic interference suppression, audio / video input, sensors / transmissions, industrial control loops, relays, contactor noise suppression and other fields.
Comparison of TVS tube and varistor At present, many varistor devices are used on equipment that needs surge protection in China. TVS tubes are generally used for the protection of electrical fast transient bursts, and their characteristics are much better than those of varistor. The comparison of specific characteristic parameters is listed in Table 5.
Table 5 Comparison of TVS tube and varistor 
TVS tube application example AC circuit Figure 4 is a schematic diagram of the microcomputer power supply using TVS tube for line protection. 
Figure 4 Schematic diagram of the microcomputer power supply section The following describes the line protection in Figure 4.
1 Add a bidirectional TVS tube D1 at the incoming AC 220 V to suppress spikes in the 220 V AC grid. Two-way TVS tube D1 
When D1 is selected, it can be obtained by looking up the table according to the above parameters.
2 Add an anti-jamming power line filter to the transformer incoming line to eliminate small spikes.
3 Add a bidirectional TVS tube D2 at the AC output of the transformer at 20 V to suppress interference again. Two-way TVS tube D2 
When D2 is selected, it can be obtained by looking up the table according to the above parameters.
4 When the rectified and filtered output is 10 V DC, the unidirectional TVS tube D3 is added to suppress interference. One-way TVS tube D3 
When D3 is selected, it can be obtained by looking up the table according to the above parameters.
By the above four suppressions, a so-called "purified power source" is obtained. In order to prevent surge voltages such as lightning strikes, a varistor can be added to the AC 220 V input terminal to prevent interference into the CPU and memory of the computer more effectively, thereby further improving the reliability of the system.
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