Components in protective circuit

With the continuous progress of society , The development of the Internet of things , Outdoor application scenarios of electronic products , Sustained high growth , Electronic products have been widely used , Whether it's public utilities , Or commercial or civil , It has gone deep into all fields , This also leads to the diversification of product functions 、 The complexity of the application environment . With more and more product functions , Its functional interfaces are becoming more and more abundant , such as ： Network interface （ belt POE function ）、 Analog video interface 、 Audio interface 、 Alarm interface 、RS485 Interface 、RS232 Interface, etc. . Functions are constantly increasing , But the volume requirements for products are getting smaller and smaller , While increasing the difficulty of design, it will also make the product face more threats , For example, with the increase of thunder and lightning in rainy season , Damage of product batch ; During equipment installation and commissioning in winter , The function of the equipment is abnormal due to static electricity . This paper mainly introduces the basic application of common protective devices in products , Through the protective circuit to improve the product antistatic 、 Ability to resist surge interference , So as to improve the stability of the product .

      Communication products are in the process of application , Overvoltage and overcurrent caused by lightning strike and other reasons will cause damage to the equipment port , Therefore, corresponding protective circuits should be designed , Each port depends on its product family 、 Network status 、 Target market 、 Application environment 、 Different signal types, implementation costs and other factors correspond to different protection circuits .

1、 Gas discharge tube

chart 1   Schematic symbols of gas discharge tubes

 

 

Gas discharge tube is a switch type protective device , The working principle is gas discharge . When the voltage between the two poles is large enough , The gap between poles will be electrically punctured , From the original insulation state to conductive state , Similar to short circuit . The voltage maintained between the two poles in the conductive state is very low , Generally in 20～50V, Therefore, it can protect the subsequent circuit . The main indicators of the gas discharge tube are ： response time 、 DC breakdown voltage 、 Impulse breakdown voltage 、 Flow capacity 、 Insulation resistance 、 Interelectrode capacitance 、 Freewheeling trip time .

The response time of gas discharge tube can reach hundreds ns Even number ms, It is the slowest among protective devices . When the Lightning Overvoltage on the cable causes the gas discharge tube in the lightning arrester to puncture and short circuit , The initial breakdown voltage is basically the impulse breakdown voltage of the gas discharge tube , The maintaining voltage between the two poles drops to 20～50V; On the other hand , The flow rate of the gas discharge tube is higher than that of the varistor and TVS Be big , Gas discharge tube and TVS When the protective devices are used, most of the overcurrent should be discharged through the gas discharge tube , Therefore, the gas discharge tube is generally used in the front stage of the protective circuit , The protection circuit of the subsequent stage is composed of varistor or TVS Tube composition , The response time of these two devices is very fast , The protection effect of the later stage circuit is better . The insulation resistance of the gas discharge tube is very high , It can reach the order of gigaohms . The value of interelectrode capacitance is very small , Generally in 5pF following , The inter pole leakage current is very small , by nA level . Therefore, the parallel connection of gas discharge tubes on the line will not have any impact on the line .

The freewheeling trip of gas discharge tube is a key problem to be considered in the design of circuit . As mentioned earlier , When the gas discharge tube is in conductive state, the freewheeling maintenance voltage is generally 20～50V, When applied in DC power circuit , If the voltage between the two lines exceeds 15V, Do not apply discharge tube directly between two wires . stay 50Hz When used in AC power circuit , Although the AC voltage has zero crossing , It can realize the continuous flow interruption of the gas discharge tube , But after many times of conductive breakdown, devices such as gas discharge tubes , Its freewheeling interrupting capacity will be greatly reduced , After long-term use, the zero crossing point of the AC circuit can not realize the freewheeling interruption ; There is also a case that if the current and voltage phases are inconsistent , It may also cause the follow-up flow cannot be blocked . Therefore, in the phase line of the AC power circuit to protect the ground wire 、 It is not suitable to use gas discharge tube alone between phase line and neutral line , When the electric equipment adopts single-phase power supply and cannot guarantee that there is no possibility of reverse connection between phase line and neutral line in practical application , It is also inappropriate to use a gas discharge tube alone for the neutral line to protect the ground wire , At this time, the gas discharge tube needs to be connected in series with the varistor . Gas discharge tubes are basically not used in the protection of phase line to neutral line of AC power circuit .

In the design of lightning protection circuit , Attention should be paid to the DC breakdown voltage of the gas discharge tube 、 Impulse breakdown voltage 、 Selection of isoparametric value of flow capacity . Discharge tube set on ordinary AC line , It is required that it cannot operate within the normal operating voltage of the line and its allowable fluctuation range , Then its DC discharge voltage should meet ：min(ufdc)≥1.8UP. In style ufdc DC breakdown voltage ,min(ufdc) Represents the minimum value of DC breakdown voltage .UP It is the peak value of the normal operating voltage of the line .

The gas discharge tube can be mainly used in the phase line of AC power port 、 Ground protection of the central line ; Direct RTN And the protection between the protected areas ; Protection of signal port line to ground ; Protection of the shielding layer by the feeder core of the antenna feeder .

The failure mode of gas discharge tube is open circuit in most cases , When the discharge tube is burned out due to short circuit for a long time due to circuit design or other factors , Failure modes that can also cause short circuits . The service life of gas discharge tube is relatively short , The performance will decline after multiple impacts , At the same time, other discharge tubes will have natural failure due to air leakage after long-term use , Therefore, the lightning arrester composed of gas discharge tube has the problem of maintenance and replacement after long-term use .

2、   Varistor

  

chart 2   Schematic symbols of varistors

Varistor is a kind of voltage limiting protective device . Using the nonlinear characteristics of varistors , When the overvoltage occurs between the two poles of the varistor , The varistor can clamp the voltage to a relatively fixed voltage value , So as to realize the protection of the subsequent circuit . The main parameters of varistor are ： Voltage sensitive voltage 、 Flow capacity 、 Junction capacitance 、 Response time, etc .

The response time of the varistor is ns level , Faster than air discharge tube , Than TVS Tube a little slower , In general, the response speed of overvoltage protection for electronic circuits can meet the requirements . The junction capacitance of varistor is generally hundreds to thousands pF Order of magnitude range , In many cases, it is not suitable to be directly applied to the protection of high-frequency signal lines , When applied to the protection of AC circuit , Because its junction capacitance will increase the leakage current , It is necessary to fully consider when designing the protective circuit . The through current capacity of the varistor is large , But smaller than gas discharge tube .

The voltage of the varistor （min(U1mA））、 Current carrying capacity is a key consideration in circuit design . In the DC circuit , Ought to have ：min(U1mA) ≥(1.8～2)Udc, In style Udc Is the DC rated working voltage in the circuit . In the AC circuit , Ought to have ：min(U1mA) ≥(2.2～2.5)Uac, In style Uac Is the effective value of AC working voltage in the circuit . The above value principle is mainly to ensure that when the varistor is applied in the power supply circuit , Have appropriate safety margin . When in the signal circuit , Ought to have ：min(U1mA)≥(1.2～1.5)Umax, In style Umax Is the peak voltage of the signal circuit . The current capacity of varistor shall be determined according to the design index of lightning protection circuit . generally speaking , The current capacity of varistor shall be greater than or equal to that of lightning protection circuit design .

Varistors are mainly used for DC power supply 、 The ac power 、 Low frequency signal line 、 Antenna feeder with feed .

The main failure mode of varistor is short circuit , When the overcurrent passing through is too large , It may also cause the valve disc to be cracked and open . The service life of varistor is short , The performance will decline after multiple impacts . Therefore, the lightning arrester composed of varistors has the problem of maintenance and replacement after long-term use .

3   Voltage clamped transient suppression diode （TVS）

 

chart 3  TVS Schematic diagram of pipe  

TVS（Transient Voltage Suppression） It is a voltage limiting protection device , Its function is very similar to that of varistor . It also uses the nonlinear characteristics of the device to clamp the overvoltage to a lower voltage value to realize the protection of the subsequent circuit .TVS The main parameters of the tube are ： Reverse breakdown voltage 、 Maximum clamping voltage 、 Instantaneous power 、 Junction capacitance 、 Response time, etc .

TVS The response time can reach ps level , It's the fastest among the voltage limiting surge protection devices . When it is used for over-voltage protection of electronic circuits, its response speed can meet the requirements .TVS The junction capacitance of the tube depends on the manufacturing process , It can be roughly divided into two types , High junction capacitance type TVS It's usually in the hundreds ～ Thousands of pF The order of magnitude , Low junction capacitance type TVS The junction capacitance of is generally in a few pF～ Dozens of pF The order of magnitude . It's usually split TVS The junction capacitance is higher , Watch and stick TVS Both types of tubes have . In the protection of high frequency signal line , We should mainly choose low junction capacitance TVS tube .

TVS The nonlinear characteristic of the tube is better than that of the varistor , When passed TVS When the over-current of the tube increases ,TVS The rising speed of the clamping voltage of the tube is slower than that of the varistor , Therefore, a more ideal residual voltage output can be obtained than that of varistors . In many electronic circuits that need fine protection , application TVS Tube is a better choice .TVS The current capacity of the tube is the smallest among the voltage limiting surge protective devices , It is generally used for the last level of fine protection , Because of its small flow , Generally not used for the protection of AC power lines , Use of lightning protection circuit of DC power supply TVS Tube time , Generally, it also needs to be used with varistors and other devices with large current capacity .TVS The tube is easy to integrate , Very suitable for use on single board .

TVS Another advantage is that unidirectional or bidirectional protection devices can be flexibly selected , In unipolar signal circuit and DC power supply circuit , Choose one-way TVS tube , A relatively low residual pressure can be obtained .

TVS Reverse breakdown voltage 、 Current carrying capacity is a key consideration in circuit design . In the DC circuit , Ought to have ：min(UBR)≥(1.3～1.6)Umax, In style UBR DC TVS Reverse breakdown voltage ,Umax Is the voltage peak in the DC circuit .

TVS Tubes are mainly used for DC power supply 、 Signal line 、 Lightning protection of antenna feeder line .

TVS The main failure mode of the tube is short circuit . But when the overcurrent is too large , It can also cause TVS The tube was blown open .TVS The service life of the tube is relatively long .

4   Voltage switching transient suppression diode （TSS）

chart 4   TSS Schematic symbols of tubes

Voltage switching transient suppression diode （TSS,Thyristor SurgeSuppressor） And TVS It's the same , It is also a voltage limiting protection device made of semiconductor process , But its working principle is similar to that of a gas discharge tube , And with varistors and TVS Different tubes . When TSS The overvoltage at both ends of the tube exceeds TSS When the breakdown voltage of the tube ,TSS The tube will clamp the overvoltage to a lower level than the breakdown voltage 0V At the level of , after TSS The tube continues this short circuit , Until it flows TSS After the overcurrent of the tube drops below the critical value ,TSS Restore the open circuit .

TSS It is a voltage switching transient suppression diode , Surge suppression transistor , Or conductor discharge tube , Solid discharge tube and so on .LangTuo Waiting brand .TSS Tube is a protective device made of semiconductor process , It is mainly used for lightning protection of signal circuit . It cannot be used in the power port .TSS The through current capacity of devices can generally reach 150A（8/20uS）.

TSS Tube and TVS Tubes are voltage limiting protective devices made of semiconductor technology ,TSS The tube is a voltage switching type .TVS It is voltage clamped .TSS Tube in response time , The junction capacitance is similar to TVS Tubes have the same characteristics , Easy to make surface mount devices , Very suitable for use on single board .TSS The transistor is suitable for signal circuit protection with high signal level .

TSS Tube in response time 、 The junction capacitance is similar to TVS Tube has the same characteristics . Easy to make surface mount devices , Very suitable for use on single board ,TSS After the tube moves , Pull down the overvoltage from near the breakdown voltage value to close 0V The level of , At this time, the junction voltage drop of the diode is small , Therefore, it is used for lines with high signal level （ for example ： Analog subscriber line 、ADSL etc. ） Flux ratio during protection TVS Guanda , The protection effect is also better than TVS keep tabs on .TSS It is suitable for the protection of signal lines with high signal level .

In the use of TSS One thing to pay attention to when managing is ：TSS After the tube breaks down under the action of overvoltage , When flowing through TSS After the current value of the tube drops below the critical value ,TSS The tube returns to the open circuit state , therefore TSS When tubes are used in signal lines , The normal current of the signal line should be less than TSS Critical recovery current of the tube . The critical recovery current varies with TSS The model and design application of the pipe are different , When using, pay attention to find out the exact value of the specific model used in the device manual .

TSS Breakdown voltage of the tube （min(UBR））、 Current carrying capacity is a key consideration in circuit design . When in the signal circuit , Ought to have ：min(UBR)≥(1.2～1.5)Umax, In style Umax Is the peak voltage of the signal circuit .

TSS Many tubes are applied to lightning protection of signal lines .

TSS The main failure mode of the tube is short circuit . But when the overcurrent is too large , It can also cause TSS The tube was blown open .TSS The service life of the tube is relatively long .

5   Positive temperature coefficient thermistor (PTC）

PTC It is a current limiting protection device , It has an action temperature value TS, When its body temperature is lower than TS when , Its resistance value remains basically constant , The resistance at this time is called cold resistance . When the temperature of the resistance body with positive temperature coefficient is higher than TS when , Its resistance value increases rapidly , The maximum resistance value that can be reached can be higher than the cold resistance value 104 About times . Because its resistance value can increase rapidly with the increase of temperature , Therefore, it is generally used in series on the line as overcurrent protection for transient large current .PTC It is used in signal lines and power lines .

 

PTC The reaction speed is slow , Generally, it is above the millisecond level , Therefore, its nonlinear resistance characteristics can not play a role when lightning overcurrent passes , Only according to its normal resistance （ Cold resistance ） To estimate its current limiting effect . The role of thermistor is more reflected in occasions where long-term overcurrent protection occurs, such as power line touch , It is often used in the protection of user lines .

at present PTC There are mainly polymer materials PTC And ceramics PTC Two kinds of , Among them, ceramics PTC The over-voltage withstand capacity of is better than that of polymer materials PTC good , But polymer materials PTC The response speed is faster than that of ceramics PTC fast . Usually ceramics PTC Low resistance cannot be achieved , Low resistance PTC Are made of polymer materials .

 

6   Insurance management 、 Fuse 、 air switch

Insurance management 、 Fuse 、 Air switches are protective devices , For short circuit inside the equipment 、 Under fault conditions such as overcurrent , It can disconnect the short-circuit load or overcurrent load on the line , Prevent electrical fire and ensure the safety characteristics of the equipment .

 

The fuse tube is generally used for the protection on the veneer , Fuse 、 The air switch can generally be used for the protection of the whole machine . The following is a brief introduction to the use of safety tubes .

For the power circuit, the air discharge tube 、 Varistor 、TVS Protective circuit composed of tubes , It must be equipped with a safety tube for protection , In order to avoid safety problems after the protective circuit in the equipment is damaged . chart 4-5 Two examples of insurance application are given , among a In the circuit, the protective circuit and the main circuit share a fuse , When the protective circuit is short circuited and fails, the power supply of the main circuit will be disconnected at the same time ,b The main circuit and protective circuit in the circuit have their own fuses , When the protective circuit fails, the fuse of the protective circuit is disconnected , The main circuit can still work normally , But when overvoltage occurs at the port again , The port may cause damage to the internal circuit due to loss of protection . Both circuits have advantages and disadvantages , In the design process, you can choose according to your needs . Signal line without feed 、 It is not necessary to adopt fuse tube for the protection of antenna feeder line .

chart 5 Two examples of insurance applications

The characteristics of the safety tube mainly include ： Rated current 、 Rated voltage, etc . The rated voltage can be divided into DC and AC .

The voltage rating marked on the fuse indicates that the fuse can safely and reliably interrupt its rated short-circuit current in a circuit with a voltage equal to or less than its rated voltage . The voltage rating series is included in N．E．C In the regulations , And it is also a requirement of Underwriters' Laboratory , As a protective measure against fire hazards . For most small size fuses and micro fuses , The standard rated voltage adopted by fuse manufacturers is 32、63、125、250、600V.

In summary , The fuse can be used at any voltage less than its rated voltage without damaging its fusing characteristics . Safety tube in protective circuit , Explosion proof slow fuse should be selected .

7   inductance 、 resistance 、 conductor

inductance 、 resistance 、 The wire itself is not a protective device , However, in the protective circuit composed of multiple different protective devices , Can play a coordinating role .

In protective devices , The gas discharge tube is characterized by large flow 、 But the response time is slow 、 High impulse breakdown voltage ;TVS The through flow of the pipe is small , Fastest response time , The voltage clamping characteristic is the best ; The characteristics of varistors lie between the two , When a protective circuit requires large overall flux , When fine protection can be achieved , Protective circuits often need these protective devices to work together to achieve ideal protective characteristics . But these protective devices cannot be simply used in parallel , for example ： Connect the varistor with large flux and the varistor with small flux TVS The tubes are directly connected in parallel , Under the action of overcurrent ,TVS The tube will be damaged first , It can't give full play to the advantage of high flux of varistor . Therefore, when several protective devices are used together , Inductance is often needed 、 resistance 、 Wires, etc. are matched between different protective elements . These components are introduced below ：

inductance ： In series DC power supply protection circuit , There should be no large voltage drop on the feeder , Therefore, hollow inductors can be used for the coordination of inter pole circuits , Here's the picture ：

 

chart 6 Use inductance to realize the cooperation of two-stage protective devices

The role of inductance ： When the protective circuit reaches the designed flux ,TVS The overcurrent on the should not reach TVS The maximum flow rate of the pipe , Therefore, the inductance needs to provide sufficient current limiting capacity for lightning overcurrent .

In the power circuit , Several problems that should be paid attention to in the design of inductance ：

1、 The inductance coil shall be able to work normally without overheating when flowing through the full allocated working current of the equipment ;

2、 Try to use hollow inductors , The inductance with magnetic core will be magnetically saturated under the action of overcurrent , The inductance in the circuit can only be calculated by the inductance without magnetic core ;

3、 The coil shall be wound as single layer as possible , This can reduce the parasitic capacitance of the coil , At the same time, it can enhance the resistance of the coil to transient overvoltage ;

4、 The insulation layer on the wire of wound inductance coil shall have sufficient thickness , To ensure that there is no breakdown and short circuit between turns of the coil under the action of transient overvoltage .

In the design of protective circuit of power port , Inductance is usually taken as 7～15uH.

resistance ： In the signal line , The components connected in series on the line should suppress the high-frequency signal as little as possible , Therefore, resistance can be used for pole to pole coordination , Here's the picture ：

chart 7 Use resistance to realize the coordination of two-stage protective devices  

The resistance should play the same role as the inductance mentioned above . Pictured above is an example , The value calculation method of resistance is ： Measure the impulse breakdown voltage of the air discharge tube U1, check TVS The device manual gets TVS tube 8/20us Maximum flux under impulse current I1、 as well as TVS Maximum clamping voltage of the tube U2, Then the minimum value of resistance is ：R≥（U1-U2）/I1.

In the signal line , Several problems that should be paid attention to in the use of resistors ：

1、 The power of the resistor should be large enough , Avoid the resistance damage under the action of overcurrent ;

2、 Try to use linear resistors , Make the influence of resistance on normal signal transmission as small as possible .

conductor ： Some friends / The full working current of DC equipment is very large , exceed 30A, In this case, the use of inductance in the pole to pole coordination of the protection circuit will cause the problem of excessive volume , To solve this problem , The protective circuit can be divided into two parts , The front protection and the rear protection are not designed on the same circuit board , At the same time, the feeder with specified length can be used to cooperate between the two-stage circuits .

chart 8 Use wires to realize the coordination of two-stage anti-corrosion devices  

In the protective circuit formed by this combination , The function of the specified length feeder , It has the same function as inductance , because 1 The inductance of meter long wire is 1～1.6uH Between , The feeder has reached a certain length , Can play a good role in cooperation , The wire diameter of the feeder can be flexibly selected according to the full working current , It overcomes the defect that the inductor cannot flow a large working current when using the inductor as the pole to pole coordination .

8   transformer 、 Optocoupler 、 Relay

transformer 、 Optocouplers and relays themselves are not protective devices , However, in the design of port circuit, the isolation characteristics of these devices can be used to improve the anti overvoltage ability of port circuit .

There are two ways to design common mode protection for port lightning ：

1、 Install voltage limiting protector on the line to the ground , When lightning overvoltage is introduced into the line , The voltage limiting protector becomes a short-circuit state and discharges the overcurrent to the earth ;

2、 Isolation elements are designed on the line , The circuits on both sides of the isolating element are not grounded , When lightning overvoltage is introduced into the line , This instantaneous overvoltage is applied to both sides of the isolating element . As long as the overvoltage acts on the isolating element , The isolation element itself will not be broken down by insulation , And the high-voltage signal line in front of the isolation element will not puncture other low-voltage parts , Lightning Overvoltage on the line cannot be converted into overcurrent and enter the equipment , The internal circuit of the device is also protected . At this time, only differential mode protection needs to be designed on the line , The protective circuit can be greatly simplified . For example, the protection of Ethernet port can adopt this idea . The main components that can realize this isolation are ： transformer 、 Optocoupler and relay .

The transformer here mainly refers to various signal transmission transformers used for signal ports . Transformers generally have primary / Index of insulation and withstand voltage between secondary , Impulse withstand voltage value of transformer （ Applicable to lightning ） It can be converted according to DC withstand voltage value or AC withstand voltage value . The approximate estimation formula is ： Impulse withstand voltage =2× DC withstand voltage =3× AC withstand voltage .

chart 9 Use transformer to realize isolation

The diagram above shows a design of signal port protection circuit combining transformer . When lightning strikes , The inductive common mode overvoltage to ground on the cable outside the equipment acts between the primary and secondary of the transformer , Pictured 9. As long as the beginning / No insulation breakdown occurs in the secondary , The Overvoltage on the cable outside the equipment will not be converted into overcurrent and enter the equipment . At this time, the port only needs differential mode protection , Using the isolation characteristics of transformers and other devices , It is conducive to simplifying the lightning protection circuit of the port .

Using this method of design needs to pay attention to ： transformer 、 Components such as optocouplers and relays should have high insulation and withstand voltage （ For example, the impact withstand voltage is greater than 4kV）, Otherwise, insulation breakdown will easily occur under the action of overvoltage , It cannot improve the port withstand voltage . in addition , When using the isolation characteristics of the transformer , Attention should be paid to the initial of the transformer / There is distributed capacitance between the secondary , In some cases, common mode Overvoltage on external cables can be coupled from primary to secondary through distributed capacitance , So as to enter the internal circuit , This destroys the isolation effect of the transformer , Therefore, transformer with primary inter pole shielding layer should be selected as far as possible , And the outer lead of the transformer shielding layer is grounded in the single board , Pictured 9 Shown . At this time, the effective insulation withstand voltage of the transformer becomes the insulation withstand voltage value between the primary and the shielding grounding terminal . Another problem that needs attention when using common mode isolation design is the primary circuit and other circuits on the board 、 The printed wire of the ground should be separated on the single board , And have enough insulation distance . commonly , The distance between the upper edges of the printed board 1mm Two printed wiring , Intolerance 1.2/50us Impulse voltage 4kV about .

In the process of device selection of protective circuit, the gas discharge tube 、 Varistor 、 Thermistor 、 Insurance management 、 Fuse 、 Air switches, etc. should choose devices with safety certification .