Talk about the circuit use of TVs tube

TVS namely Transient Volatge Suppressor, Chinese name ： Transient voltage suppression diode , Also known as avalanche breakdown diode . But in general , We call it TVS diode . Its role in the circuit is mainly to protect sensitive circuits , Components protected from high voltage transients . among , Most of them are used for port protection , Prevent the damage of the subsequent circuit caused by the instantaneous voltage impact of the port . The main source of high voltage transients is electrostatic discharge , Electrical fast transients and surge events .

TVS Diodes are made of a single semiconductor process PN Knot or multiple PN Junction integrated device . therefore ,TVS There are unidirectional and bidirectional .

among , A one-way TVS Mainly used in DC power supply circuit , two-way TVS Tubes are mainly used in alternating voltage circuits .

working principle

Just take one way TVS Give an example ！, When it is applied to DC circuit , In parallel with the circuit . When the circuit is working properly ,TVS In the status of deadline , It does not affect the operation of the circuit . When abnormal overvoltage occurs in the circuit and reaches TVS At breakdown voltage ,TVS From high resistance state to low resistance state , Discharge the instantaneous over current caused by abnormal overvoltage to ground , At the same time, the abnormal overvoltage is clamped at a low level , And then protect the subsequent circuit from abnormal overvoltage . When the abnormal overvoltage disappears ,TVS The resistance value returns to the high resistance state .

The picture comes from Baidu experience

Xiaobai is writing the first article USB2.0 In circuit design , A few are placed on the final simple schematic diagram TVS diode , Its main purpose is to protect these external interfaces . besides , For example, the earphone hole of the mobile phone ,SIM This kind of... Will be added at the card slot TVS Diode for protection .

However, different interface pairs TVS The requirements of diodes are also different .

TVS Of datasheet Although many parameters are given , But we have to learn to focus , The following parameters are very important for us .

1. Reverse turn-off voltage VRWM
   When the circuit works normally ,TVS It doesn't work ,VRWM Express TVS The highest voltage at which the tube cannot be turned on . therefore TVS The reverse turn-off voltage of is greater than or equal to the maximum working voltage of the protected circuit . Only in this way can we guarantee TVS Under the normal operation of the circuit, it will not affect the circuit ,TVS The absorbed current is also negligible . If TVS The reverse turn-off voltage of is less than the maximum working voltage of the protected circuit ,TVS It will absorb a large amount of leakage current and be in the state of avalanche breakdown .

Usually, the reverse turn-off voltage is about VCC Of 1.2 times （VCC Is the highest working voltage of the circuit ）; Take my first article USB2.0 For example, , The reverse turn-off voltage is the working voltage that we don't want the data or power line to exceed .USB2.0 The data signal voltage range of is 0-5V, So we hope TVS The reverse turn-off voltage of the diode should at least exceed 5V, To ensure that the data can be transmitted correctly .

2. Breakdown voltage VBR
   stay TVS The voltage limit when the diode begins to conduct a large amount of current to the ground and provide transient protection . Usually
   , The breakdown voltage is slightly greater than the reverse turn-off voltage , In order to start transient protection as soon as possible .

3. Clamp voltage
   Low voltage seen by the system during high voltage transients . It shall be less than the maximum bearable transient safety voltage of the subsequent protection circuit, i.e V( Clamp voltage )<V（max）, among Vmax The highest voltage that the circuit can withstand .

4. Pulse peak current I ppp
   When working in reverse , In the specified pulse waveform （ Such as ：10/1000μs Double exponential waveform ） Under the condition of , The maximum surge current allowed by the device . When the current exceeds this value , It will be permanently damaged .

5. Pulse peak power PPP
   Pulse peak power PPP Refers to pulse peak current IPP With the maximum clamping voltage VC The product of the , namely PPP＝IPP×VC. It is TVS The maximum peak power that the tube can withstand . Under the given maximum clamping voltage , Pulse peak power PPP The bigger it is , The greater the bearing capacity of surge current .

6. Junction capacitance
   The junction capacitance is TVS The parasitic capacitance of . Under high-speed signal , Pay special attention to . Excessive junction capacitance will affect the quality of the signal . To ensure correct transmission , We need to choose one with lower capacitance TVS diode . The following can be used as a reference . At the same time, a smaller capacitor can also be connected in series

Now let's talk about the protection function .
Suppose we are concerned about the protected USB2.0 Data line application 8KV Of ESD impact , The instantaneous high voltage exceeds TVS The breakdown voltage of the diode and activate it .TVS The diode will then begin to conduct current to the ground , And make it work USB2.0 The transceiver of , So as to reduce the supply to the more sensitive USB2.0 The voltage of the transceiver . When ESD After the event ,USB2.0 The voltage at the connector will return below the breakdown voltage ,TVS The diode will stop conducting current , And resume normal data transmission .