1. Guide to selection

1, first determine the maximum DC or continuous working voltage of the protected circuit, the rated standard voltage and the "high end" tolerance of the circuit.

2, the rated reverse turn off voltage (VWM) of TVS should be greater than or equal to the maximum working voltage of the protected circuit. If the selected VWM is too low, the device may enter the avalanche state or affect the normal operation of the circuit because the reverse leakage current is too large.

3, the maximum clamping voltage VC of TVS should be less than the damage voltage of the protected circuit.

4, the maximum peak power PW of TVS must be larger than that of the peak power in the protected circuit.

5. After determining the maximum clamping voltage of TVS, the peak impulse current should be larger than that of transient surge current.

6, for the protection of the data interface circuit, we must pay attention to selecting the TVS device with the capacitance value of C as small as possible.

7, the TVS diode with A is better than the TVS diode without A. The C type in front of the TVS diode A shows the bidirectional TVS diode.

8, the DC protection generally selects one way TVS diode, the AC protection generally selects the bidirectional TVS diode, the multiple protection selects the TVS array device, and the high power protection selects the TVS special protection module. Special cases, such as RS-485 and RS-232 protection, can choose bidirectional TVS diode or TVS array.

9, TVS diode can work between -55 and +150. If TVS is required to work at a changing temperature, because its reverse leakage current ID increases with the increase of temperature, the power consumption decreases with the increase of the temperature of TVS junction, so the effect of temperature change on its specificity should be considered when selecting TVS.

10, TVS diode can be used in series / serial connection, with serial connection voltage sharing and parallel connection current sharing. But taking into account the discreteness of TVS, we should reduce the number of Strings / clusters as much as possible.

11, unipolarity or bipolar - there is often a misunderstanding that bidirectional TVS is used to suppress reverse surge pulses, but this is not the case. Two way TVS is used for alternating current or positive and negative bidirectional pulses. TVS is sometimes used to reduce capacitance. If the circuit has only a positive level signal, then a one-way TVS is enough.

The TVS operation is as follows: when the forward surge, the TVS is in the reverse avalanche breakdown state; when the reverse surge, the TVS is similar to the forward bias diode and absorbs the surge energy. This is not the case in a low capacitance circuit. Bidirectional TVS should be used to protect the low capacitance components in the circuit from the damage of reverse surge.

Two. Annotation

1, VWM - the DC or pulse voltage of the maximum continuous operation of TVS. When this reverse voltage is added to the TVS pole, it is in the reverse turn off state, and the current flowing through it is less than or equal to its maximum reverse leakage current ID.

2, VBR - the smallest avalanche voltage of TVS. At 25 C, the TVS is not protected before this voltage. When TVS flows through the specified 1mA current IR, the voltage between the TVS poles is the minimum breakdown voltage VBR.

3, IT - - test current.

4, ID - - reverse leakage current.

5, VC - when the peak current IPP of 20us duration flows through TVS, the maximum peak voltage between the two poles is VC. It is a combination of voltage rise between series resistance and thermal temperature rise.

6, IPP - maximum peak pulse current.

7, C---- capacitance value (pF). In the receiving / sending bus interface circuit, it is particularly advantageous to select TVS devices with small capacitance.

Three. An example of power protection in AC circuit

Fig. 1 is a schematic diagram of TVS power line protection for microcomputer power supply.

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Figure 1 Schematic diagram of the part of a microcomputer power supply

The line protection in Figure 1 is described below.

(1) the bidirectional TVS tube D1 is added to the 220 V of the incoming line to suppress the spike interference in the 220 V AC network. Bi-directional TVS tube D1

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D1 is selected according to the parameters mentioned above.

(2) add an anti-interference power line filter to the transformer inlet line to eliminate small spike interference.

3. At the 20 V of the transformer output end, the bidirectional TVS tube D2 is added to suppress the interference once again. Bi-directional TVS tube D2

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D2 is selected according to the parameters mentioned above.

3. When rectifier and filter output DC 10 V, plus one-way TVS tube D3 suppresses interference. One way TVS tube D3

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D3 is selected according to the parameters mentioned above.

Through the above 4 suppression, the so-called "purified power supply" has been obtained. In order to prevent surge voltage such as lightning, the voltage sensitive resistor can be added at the end of AC 220 V to prevent interference into the CPU and memory of the computer more effectively, so as to further improve the reliability of the system.