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How to design overvoltage protection circuit?

2020-12-06 11:04:49 Procedural ape without hair loss

Catalog

1、 Save money and worry TVS tube

1.1、TVS Tube characteristics

1.2、TVS Tube selection

1.3、 Practical circuit applications

1.4、 summary

2、 Reliable and efficient circuits


The function of overvoltage protection circuit is : If the internal voltage stabilizing loop of switching power supply fails or the output voltage exceeds the design threshold due to improper operation of users , In order to protect the secondary electrical equipment from damage , Limit the output voltage to a safe value .

This blog will save money and worry TVS This paper introduces how to design the over-voltage protection circuit quickly .

1、 Save money and worry TVS tube

TVS(Transient Voltage Suppressors), Transient voltage suppressor , Also known as avalanche breakdown diodes , It is a high efficiency protection device in the form of diodes .

1.1、TVS Tube characteristics

TVS There are one-way and two-way , A one-way TVS Tubes are generally used in DC power supply circuits , two-way TVS The tube is used in the circuit of alternating voltage .

for example , A one-way TVS When the tube is used in DC circuit , As shown in the figure below . When the circuit is working properly ,TVS In a cut-off state ( High resistance state ), Does not affect the normal operation of the circuit . When abnormal overvoltage occurs in the circuit and reaches TVS( An avalanche ) At breakdown voltage ,TVS Rapidly change from high resistance state to low resistance state , Discharge the instantaneous over current caused by abnormal overvoltage to ground , At the same time, abnormal overvoltage is clamped at a lower level , So as to protect the back stage circuit from abnormal overvoltage damage . When the abnormal overvoltage disappears ,TVS The resistance value returns to the high resistance state .

TVS working principle

choose TVS Despite the following important parameters :

choose TVS Key indicators

(1) Vrwm Cut off voltage

TVS Maximum operating voltage of , Can be applied continuously without causing TVS The highest operating peak voltage or DC peak voltage of deterioration or damage . For AC voltage , Expressed in RMS of the highest working voltage , stay Vrwm Next ,TVS Think it's not working , That is, it doesn't work . The maximum working voltage must be less than Vrwm, Otherwise, it will lead to TVS The action causes the circuit to be abnormal .

(2) IR Leakage current

Leakage current , Also called standby current . At specified temperature and maximum operating voltage , Flow through TVS The maximum current of .TVS The leakage current is usually measured at the cut-off voltage , For a certain model TVS, IR It should be within the specified value range . Yes TVS The voltage applied at both ends is Vrwm, The current value read from the ammeter is TVS Leakage current of IR . For the same power and same voltage TVS, stay Vrwm≤10V when , two-way TVS The leakage current is unidirectional TVS Leakage current 2 times . For some analog ports , Leakage current can affect AD Sample value of , therefore TVS The smaller the leakage current, the better .

(3) VBR Breakdown voltage

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 Voltage at both ends . The current tested It Generally choose 10mA about , The applied current should not be applied for more than 400ms, To avoid damaging the device ,VBR MIN and VBR MAX yes TVS A deviation of breakdown voltage , commonly TVS by ±5% The deviation of . When measuring ,VBR Fall in the VBR MIN and VBR MAX As qualified products .

(4) IPP Peak pulse current ,VC Clamp voltage

Peak pulse current , Given the peak value of the pulse current waveform .TVS General selection 10/1000μs The current waveform . Clamp voltage , Apply the peak pulse current of the specified waveform I PP when ,TVS Peak voltage measured at both ends .IPP And VC It's a measure TVS Parameters of the ability to withstand surge pulse current and limit voltage in circuit protection , These two parameters are interrelated . about TVS Clamping characteristics in lightning protection circuits , You can refer to VC This parameter . For the same model TVS,IPP The bigger it is , The stronger the ability to withstand impulse current , If in IPP In the same way VC The smaller it is , explain TVS The better the clamping characteristics of .

(5)  Junction capacitance Cj

The junction capacitance is TVS Parasitic capacitance in , At high speed IO Port protection needs to focus on , Excessive junction capacitance may affect the quality of the signal .

1.2、TVS Tube selection

choose TVS tube , There are three points to note :

  • The circuit can be protected by proper voltage level ;
  • Introduced TVS The junction capacitance cannot affect the circuit ;
  • TVS Enough power margin , Meet the test criteria , And can't hang up before the insurance tube .

The selection process can be carried out according to the following thinking steps :

  • choice TVS Maximum operating voltage Vrmw;
  • choice TVS Clamp voltage VC;
  • choice TVS Power ;
  • Evaluate leakage current Ir Influence ;
  • Evaluate the effect of junction capacitance .

The details are as follows :

(1) choice TVS Maximum operating voltage Vrmw

Under normal operation of the circuit ,TVS It's not supposed to be work , It's in the cut-off state , therefore TVS The cut-off voltage of the circuit should be greater than the maximum working voltage of the protected circuit . In this way, we can guarantee TVS In the normal operation of the circuit will not affect the 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 . requirement Vrwm Greater than the working voltage , Otherwise, the working voltage is greater than Vrwm It can lead to TVS The reverse leakage current increases , Close to conduction , Or avalanche breakdown , Affect the normal operation of the circuit .

Vrwm You can refer to the following formula :

Vrwm≈1.1~1.2*VCC( among VCC Is the highest working voltage of the circuit )

(2) choice TVS Clamp voltage VC

TVS The clamping voltage should be less than the maximum transient safety voltage of the protected circuit ,VC And TVS Avalanche breakdown voltage and IPP It's proportional to . For the same power level TVS, The higher the breakdown voltage VC And the higher , Selected TVS The maximum clamping voltage of VC It should not be greater than the maximum voltage that the protected circuit can withstand . otherwise , When TVS Clamp in VC Will cause damage to the circuit .

Vc You can refer to the following formula :

VC<Vmax( among Vmax The highest voltage that the circuit can withstand )

(3) choice TVS Power Pppm( perhaps Ipp)

TVS The rated transient power of the product shall be greater than the maximum transient surge power that may occur in the circuit , Theoretically ,TVS The higher the power, the better , Able to withstand more impact energy and times , But the higher the power ,TVS The larger the package of , The higher the price , therefore ,TVS Can meet the requirements of . For different power levels TVS, Of the same voltage specification TVS Its V C The value is the same , It's just I PP Different . so Pppm And Ippm In direct proportion to ,Ippm The bigger it is ,Pppm The greater the . For a circuit , There are corresponding test requirements , Let the maximum test current in the actual circuit be Iactual , be Iactual It can be estimated as :

Iactual=Uactual/Ri(Uactual To test the voltage ,Ri To test the internal resistance )

TVS To pass the test , Therefore, it is required in the actual circuit 10/1000μs Under the waveform TVS Minimum power of P actual by :------- among di/dt Is the waveform conversion coefficient , If the actual test waveform is other waveforms , Such as 8/20μs wave form , Suggest di/dt take , If the test waveform is 10/1000μs, take , In actual selection ,TVS There should be a certain margin ,TVS Power PPMP The choice should follow Pppm>Pactual.

(4) According to the selected TVS The junction capacitance and leakage current are evaluated

if TVS For high speed IO Port protection 、 Analog signal sampling 、 Low power equipment applications , We need to consider the effect of junction capacitance and leakage current , The smaller the parameters, the better .

Model selection examples

for example : The normal working voltage of the circuit VCC yes 24V, Maximum operating voltage Vmax yes 26V, The maximum transient voltage that the post stage circuit can withstand is 50V, The test waveform of the experiment is 8/20μs wave form , Test voltage 500V, Test the internal resistance of the power supply and PPTC The total static resistance of is 2Ω. Based on the above information, select the appropriate TVS.

(1) choice TVS Maximum operating voltage

Vrmw≈1.1~1.2*VCC=26~28V

(2) Choose to TVS Clamp voltage

VC<Vmax=50V

(3) Calculate the actual test waveform power :

Pact=50*(500/3)*1/2=4166W

According to the calculation , May choose 5.0SMDJ26A This TVS, Because of this TVS For power port , The junction capacitance and leakage current can be selected according to the actual circuit .

5.0SMDJ26A TVS The key parameters

1.3、 Practical circuit applications

When the working voltage of the device's port exceeds TVS Maximum breakdown voltage of (VBR MAX),TVS It can be seen as a low impedance resistor , The current flowing through it is very large , The resistance keeps heating , If there are no other measures , This TVS I'll be dead soon , Invalid TVS Maybe the rate becomes an open circuit , The circuit of the later stage is still unprotected . So it's better to TVS Put a safety tube in front of it , stay TVS Before hanging up , The fuse failed first and the circuit was broken , You can protect TVS And post stage circuits , If it's a self recovery fuse , After troubleshooting, you can resume normal work by yourself .

TVS Working principle for protection against over voltage

As shown in the figure below is RS485 Over voltage protection circuit of ,RS485 The working voltage of the chip is usually 5V, The limit voltage that can withstand is usually 12V. Usually the working voltage of the equipment is 12V perhaps 24V, If you mistake 24V The power supply voltage is connected to RS485A-B There is no over-voltage protection on line , Large probability RS485 Chips can be physically damaged .TVS The tube is specially designed for instantaneous overvoltage protection , Unable to cope with prolonged overpressure , Less than 0.5STVS It will burn down because of overheating , After the circuit will lose protection .

If in TVS Add self recovery in front of PTC, And PTC The trip time is short enough , also TVS The clamping voltage of Vc< The highest working voltage of the circuit VCC, stay TVS Before it burned down PTC trip , The protection of the later stage circuit can be realized . 

TVS Tube and self recovery PPTC The process of calculation is as follows :

(1)  The precondition of protection realization

When the applied voltage reaches TVS The breakdown voltage of ,TVS Start to turn on , The impedance goes down , The current flowing through it keeps increasing . As the current increases ,PPTC The impedances are increasing , It keeps getting hot , Final PPTC It becomes an open circuit , The whole back stage circuit is protected . So to achieve circuit protection , need 2 Premise :

  • TVS The power is big enough , Big enough to hold on to PPTC Open circuit ;
  • PPTC The action time should be small enough , Small enough to be in TVS Action before failure .

(2) PPTC The selection

For overvoltage protection ,PPTC The following conditions should be met in the selection of the model :

  • Continuous current Ihold> The maximum working current of the circuit Iwork;
  • Maximum action time Trip The shorter the better , Such as SMD1812B020TF, When passed PPTC The current of is 8A when ,PPTC The action time should not be greater than 0.02s;
  • Maximum overload current Imax, Within the operating temperature range PPTC The current value that cannot be exceeded , exceed PPTC There is a high probability of permanent damage ;
  • Maximum operating voltage Vmax, Within the operating temperature range PPTC The maximum operating voltage value that cannot be exceeded , exceed PPTC There is a high probability of permanent damage .

(3) TVS The selection

TVS The selection requirements are as follows :

  • reference : Above 1.2、TVS Tube selection section ;
  • Calculation TVS Can withstand the maximum heat Qtvs=P*t=P/1000( In general, the specification gives 1000uS The power under , Divide 1000 It's conversion to units S).
  • Calculate the selected TVS Actual working heat :
Qact=Vc*Itrip*Tptc(Vc:TVS The clamping voltage of ;Itrip:PTC Trip current of the fuse ;Tptc: Trip time under trip current .)
  • TVS Can withstand the heat of actual selection Qtvs> Theoretical calculation Qact;

Circuit design example

As shown in the figure below ,PTC and TVS Used in conjunction with RS485 Over voltage protection , The supply voltage of the equipment is 24V,RS485 Chip selection MAX488, Normal operating voltage 5V, The highest affordability 12V, Normal working current <1mA, Choose the right one PTC and TVS.

PTC and TVS Used in conjunction with RS485 Protect

(1) PTC The selection

because RS485 The working current is very small ,PTC Choose the smallest current , The key parameter is trip time Trip,Trip The smaller the better. , The smaller the trip time is , Yes TVS The lower the power requirement of , The smaller the package is , The lower the cost . After selection ,SMD1812B020TF The current and voltage meet the requirements , Trip time Trip It's the shortest , by 0.02S.

(2) TVS The selection :

  • TVS Voltage selection

because RS485 Working voltage of VCC by 5V, The limiting voltage is 12V, therefore TVS Working voltage of Vrwm≥5V, Clamp voltage ≤12V; You can pre select SMBJ5.0A.Vrwm=5V,Vc=9.2V.

  • TVS Power selection

a.  Estimate the actual TVS The amount of heat you need to withstand

hypothesis TVS Working at maximum clamp voltage , The current flowing through is the trip current of fuse tube :

Qact=P*t=U*I*t=Vc*I trip*Tptc=9.2V*8*0.02S=1.472J

b.  Estimate the actual TVS The actual power of

Due to the manufacturer's TVS The test power is in 1us Measured at pulse width of , Therefore, it is necessary to convert the above estimated heat into 1uS Time corresponds to TVS Power .

Ptvs>Qact/1us=1.472J/1us=1472W

 The peak current is :Ipp=P/Vc=1472W/9.2V=160A

therefore , Final TVS choice Vc=9.2V,Ipp=163 Of SMCJ5.0A.

Some friends think that the above calculation process has no derating , In fact, the above calculation process is basically in accordance with the extreme situation , Ignored PTC The resistance increases exponentially with increasing temperature ,PTC The resistance on will share a large part of the voltage , To the end PTC On the verge of an open circuit ,TVS The pressure drop is almost zero . So not only is it not necessary to scale down , even to the extent that TVS You can choose to multiply the actual calculated power by 0.5~0.8 coefficient .

1.4、 summary

TVS There are two limitations for overvoltage protection : Small signal and low speed .

(1) Small signal problem

Small signals are easier to understand , We use normal current in our circuit 0.2A Of PTC, But the choice is 160A Of TVS, Scale close 800 times , It's inconceivable , If you use one 2A Of PTC Power port ,TVS Power needs to exceed 15KW, Price close 10 element , The cost is almost unacceptable .

because PTC It's self recovery , After the fault is removed, it can work normally , Avoid the trouble of frequent replacement , however PTC The trip time is longer , The same is 0.2A The glass seal safety tube of , The circuit achieves 8A When , Almost 10ms It can trip within , and PTC The shortest is 200ms, And that leads to this TVS You have to choose more power , To hold on to PTC trip (PTC than TVS Failure is the only way to protect ).

(2) Low speed problem

General TVS The junction capacitance is tens of pF To hundreds pF, Same power level ,TVS The lower the voltage , The greater the junction capacitance , The power used at the small signal port TVS, Unless it's low capacitance TVS, Conventional power TVS The junction capacitance will be in tens of pF, So the rate of small signal should not be too high , Better not to exceed 1Mbps.

(3) Accuracy problem

The calculation process mentioned above is basically an estimation , This is because TVS and PTC It belongs to voltage and current sensitive devices , The failure mode is thermal failure , A careful friend will find it , Manufacturer's TVS The voltage provided in the manual 、 electric current 、 Temperature and so on , They all give a general graph , No precise formula has been given .

Although the exact formula is used in this paper , But it's all extreme , Actually selected TVS You can multiply the calculated value by a coefficient 0.5~0.8 It's not a big problem , The specific physical test shall prevail . For example, the above example calculates the selection 160A Of TVS, The actual measurement uses 100A Can also meet the requirements .

2、 Reliable and efficient circuits

A simple over-voltage protection circuit is usually added TVS Can achieve , When there is an instantaneous high-energy impact from the outside, it can suppress this energy , Although the power is high , over a thousand W Fine , But the duration of inhibition is very short , In case the device is damaged or the working voltage is higher than normal for a long time , I can't do it .

So the best way is to design an intelligent circuit , As shown below :

 Vin At normal input voltage , There is no reverse breakdown of the zener tube ,R3,R4 The current is basically 0.PNP Transistor's Vbe=0, namely PNP The transistor doesn't work .PMOS tube Q4 Of Vgs By resistance R5,R6 Partial pressure determines ,PMOS The pipe is open , That is, the power supply works normally .

When Vin The input voltage is greater than the normal input voltage , here Vin>Vbr, The voltage regulator is broken down , The voltage on it is Vbr.PNP triode Q1 Conduction ,VCE≈0, namely PMOS Tubular Vgs≈0,PMOS The tube doesn't work , The circuit is open , That is to realize the over-voltage protection .

If the accuracy is high , Of course, voltage detection is also available IC, Realize voltage monitoring .


Reference material :

TVS The selection , So that the old and the new can be handy !

Elder brother Yu teaches you to analyze the design of overvoltage protection circuit , you GET To the essence of it ?

【 Dry cargo sharing 】TVS For conventional overvoltage protection

MOS Tube anti reverse connection over-voltage circuit

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