TVs tube parameters and selection

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Preface

Source of task ： One case, Currently used in circuits TVS The tube is out of stock , It is necessary to evaluate whether the tubes of the same type from other manufacturers can replace the current devices , Give reasons for evaluation
TVS Tube protection circuit ： Board level circuits are supported USB3.0 Of type-C Output interface , For external connection USB The storage device .VBUS The signal is from CC Pin detection chip output , At the board level edge VBUS The power line passes TVS The pipe bypass is connected in parallel to the ground .
TVS Tube protection principle ：TVS The tube and the parallel circuit board are connected in parallel with the protected circuit . When the instantaneous voltage exceeds the normal working voltage of the circuit ,TVS Avalanche breakdown will occur （ Avalanche breakdown and zener breakdown belong to diode breakdown , can PN Knot recoverable ）, Thus, an ultra-low impedance path is provided for the instantaneous current , The result is an instantaneous current passing through TVS The circuit flows to the ground , So as to avoid the protected device , And keep the protected circuit in the cut-off voltage state until the voltage returns to the normal value . After that , When the instantaneous pulse ends ,TVS The diode will automatically return to the high resistance state , The whole circuit enters the normal voltage state .
TVS The fast response time of the tube is ps Level .
![ Insert picture description here ](https://img-blog.csdnimg.cn/13c9e22f578d4205affc75da6e7c8614.png

TVS Evaluation and selection of

One 、 The key parameters

TVS The key parameters of the tube are shown in the figure below

VRM： Cut off voltage , VBR： Breakdown voltage , VCL Maximum hoop voltage
VRM, Refers to TVS Cut off voltage of diode （ Maximum operating voltage ）, Can be applied continuously without causing TVS Maximum DC voltage or AC peak voltage for diode deterioration or damage . At cut-off voltage ,TVS The diode is not working , That is, non-conductive , therefore TVS The cut-off voltage of the diode shall be greater than the maximum operating voltage of the protected circuit . In this way, we can guarantee TVS The diode will not affect the operation of the circuit under the normal operation of the circuit .
however TVS The working voltage also determines TVS Clamp voltage level , When the cut-off voltage is greater than the normal working voltage of the line ,TVS The working voltage should not be too high , If it's too high , The clamp voltage will also be higher , So in choosing VRWM when , The working voltage of the protected circuit and the bearing capacity of the later stage circuit should be considered comprehensively .

VBR, Breakdown voltage , Referring to V-I On the characteristic curve , At the specified pulse DC current IT Or near the current condition of avalanche TVS The voltage across the diode . From this point , Mark the TVS The tube enters the avalanche breakdown state .
low pressure TVS diode , Due to the large leakage current , Therefore, the test current is selected IT It's bigger .TVS diode SMAJ5.0A, Test current IT selection 10mA. Use a pulse constant current source to TVS Diode applied IT The magnitude of the current , read out TVS The voltage across the diode is the breakdown voltage . The current application time shall not exceed 400ms, So as not to cause TVS Heat damage .VBR MIN and VBR MAX yes TVS A deviation of the diode breakdown voltage , commonly TVS by ±5% The deviation of . When measuring , Breakdown voltage falls at VBR MIN and VBR MAX Between , It is regarded as qualified product

Vcl Evaluate anti surge (surge) characteristic . Peak pulse current Ipp And clamping voltage Vcl It's a measure TVS Parameters of diode's ability to resist surge pulse current and limit voltage in circuit protection , These two parameters are interrelated . about TVS Clamping characteristics of diode in lightning protection circuit , Refer to the clamping voltage parameter . For the same model TVS diode , Under the same peak pulse current , The lower the clamping voltage , explain TVS The better the clamping characteristics of the diode .TVS The ability of diode to withstand impulse current can be referred to as peak impulse current , Of the same model TVS diode , The higher the peak pulse current , The stronger the ability to withstand impulse current .（10/1000us and 8/20us Test waveforms for two different surges , The energy of the two waveforms is different .8/20us Pulse refers to 8us achieve 100%Ipp,20us achieve 50%Ipp）

ESD and Surge Protection level ,
General TVS The pipes will be datasheet The front page indicates . As shown in the figure below , This type TVS The tube meets the following test criteria ：
IEC61000-4-2 Level 4 ESD Protection—— Electrostatic test standard
IEC61000-4-5（8/20us）—— Surge test standard

Two 、TVS Tube use and Selection Guide

1,TVS Tubes are usually connected in parallel to the protected circuit . In order to limit the flow TVS The current of the tube shall not exceed the allowable peak current of the tube Ipp, It can be considered to connect current limiting elements in series on this line , Such as resistance , Self recovery fuse , Inductance, etc .
2,VRM Selection of cut-off voltage , Generally, it is slightly higher than the normal working voltage of the protected circuit ,（ In this way, when the protected circuit works normally ,TVS The tube is off , Reduce current consumption and increase service life ）.VBR The breakdown voltage should be higher than the normal working voltage of the protected circuit .Vcl The hoop voltage should be less than the maximum allowable transient input voltage of the next stage circuit , Otherwise, the hoop action has no effect . Be careful Vbr And Vcl There is a linear relationship between the value of and the tube junction temperature

3, Selection of peak pulse power , It should be greater than the maximum transient surge power that may appear in the circuit ,TVS The maximum surge power that the tube can withstand Pppm=Vcl*Ipp, When it is used as the second level protection in low-voltage circuit , commonly 600W（10/1000us） That's enough .
4, Leakage current IRM Influence of junction capacitance , At high speed IO Port protection 、 Analog signal adopts 、 Data interface 、 Low power equipment applications , The influence of leakage current and junction capacitance should be considered .Cd by TVS Parasitic capacitance of the pin of the tube , When the signal transmission circuit is protected , The higher the signal rate ,ESD Protective devices （ Low power TVS Tube array ） The lower the junction capacitance .

3、 ... and 、TVS Tube characteristics

advantage ：
1、 Fast response (ps level )、 High transient power 、 Low leakage current ;
2、 The breakdown voltage varies from 6.8V~550V Series value of , It is convenient for the use of different voltage circuits ;
3、 Clamping voltage is more accurate (±5%);
shortcoming ：
1、 The junction capacitance is relatively large ( A dozen pF~ A few hundred pF);
2、 Compared with discharge tube and varistor, the surge resistance is poor , The flow rate is generally 0.52A~544A Within the scope of ;

Four 、TVS The difference between tube and zener diode

The difference lies in
（1） working principle
• TVS tube （ Transient voltage suppression diode ）： Avalanche effect , In case of high-energy instantaneous overvoltage pulse , Its working impedance can be immediately reduced to a very low conduction value , Allow the maximum current to pass through , And clamp the voltage to a predetermined level , So as to avoid the damage of precision components in the circuit .
• Voltage stabilizing diode （ Zener diode ）： The zener tunnel effect , When the reverse voltage reaches and exceeds the stable voltage , The reverse current suddenly increases , The voltage across the diode is constant .
therefore ,TVS The tube usually does not work and does not conduct （ No breakdown ）, At this time, the corresponding leakage current Ir The relatively small . And the voltage regulator is usually in a breakdown state , At this time, the corresponding reverse breakdown current is relatively large .
（2） nature
• TVS Tubes can be divided into unipolar and bipolar , The regulator diode is only unidirectional
• In terms of flow capacity ,TVS Tubes are usually tens of amperes , It can reach hundreds of AMPS at most , The flow capacity of the regulator is very small
• TVS Tube is used for transient voltage protection , In terms of voltage stabilization accuracy, it is a range , But it has stronger high-voltage bearing capacity , The voltage accuracy of the regulator tube is relatively accurate , But the voltage stabilization range is relatively small
• TVS The tube responds quickly , You can achieve ps level
（3） main parameter
• TVS tube ：VBR、VRWM、VCL、IPP、Pppm.
• Regulator tube ： Steady voltage （VZ）、 Steady current （IE）、 Dynamic resistance （RZ）、 Maximum dissipation power （PZM）、 Maximum / Small stable working current (IZmax/IZmin)、 temperature coefficient （AT）.
（4） Use scenarios
• TVS The tube is used to absorb the instantaneous overvoltage of the input in a short time , And effectively clamp the output voltage , When the instantaneous pulse ends ,TVS The tube returns to the high resistance state , That is, the non conduction state .
• The regulator is used to embed the input voltage into a constant voltage , The thermobaric tube is in a continuous reversed-phase conduction state
（5） Protection purpose
• TVS Clamping of tube ： Protect the subsequent chip from overvoltage damage
• Clamping of pressure stabilizing tube ： Provide a stable voltage to the subsequent chip

5、 ... and 、TVS Management and ESD The difference between electrostatic protective tubes

ESD Electrostatic protection tube and TVS The working principle of the tube is the same ,ESD The tube is made up of several TVS The tubes adopt different layouts and are designed as single or multi-channel protection devices with specific functions .
（1） Application circuit
ESD Diodes are generally used in key data interfaces （HDMI\USB\ETH Bus etc. ） Do electrostatic protection on , The junction capacitance is relatively low ;
TVS The tube has fast response ability and strong surge absorption ability , It is mostly used for surge voltage protection on power lines , The junction capacitance is relatively high .
（2） Number of pins
ESD Tubes usually need three pins （ One GPIO, A power supply +, A power supply -）
TVS Tubes are the same as ordinary diode pins
（3） The key parameters
ESD What matters is ESD Anti static grade of golden egg protective tube LEVEL,TVS What we care about is power and packaging form

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Note

References are from the Internet