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Jan 11 2020

Transient Voltage Suppressor Tutorial and Applications

Ⅰ. Introduction

In electronics, the transient interference of voltage and current source is the main cause of damage to circuits and equipment, and it often causes immeasurable losses to the society. These disturbances usually come from the  the starting and stopping operation of power equipment, instability of the AC grid, lightning interference and electrostatic discharge, etc. They are almost ubiquitous and always present. Therefore, scientists have developed a high-efficiency circuit protection device TVS to effectively suppress transient interference.

TVS (transient voltage suppressor), is a new product developed on the basis of Zener diode technology. Its circuit symbol is the same as that of ordinary Zener diode. As a common circuit protection component, it is widely used in various fields: automotive electronics, consumer electronics, power drives, industrial power distribution, renewable energy, telecommunications, home appliances, measuring instruments, medical electronics, industrial control, lighting, security systems, building control and automation, audio / video equipment, computers, etc.

 tvs diode

Category

Ⅰ. Introduction

Ⅱ. Terminology

Basic Characteristics

   Electrical Characteristics

     a. Unidirectional TVS (V-I characteristic)

     b. Bidirectional TVS (V-I characteristic)

   Main Parameters

     1) Breakdown Voltage V(BR)

     2) maximum Reverse Pulse Peak Current IPP

     3) Maximum Reverse Working Voltage VRWM

     4) Maximum Clamping Current VC(max)

     5) Reverse Pulse Peak Power PPR

     6) Capacitor CPP

     7) Leakage Current IR

Ⅲ. TVS Selection

Ⅳ. TVS vs Varistor, Capacitor

Ⅴ. Application Examples

Microcomputer System Protection

   DC Regulated Power Supply Protection

   Transistor Protection

   Integrated Circuit (IC) Protection

   Silicon Control Protection

   Electric Relay Protection

   Integrated Op Amp Protection

   Suppression of Electromagnetic Pulse Interference

   Lighting Protection



Ⅱ. Terminology

  • Basic Characteristics

TVS diode, under the specified reverse application conditions, when subjected to a high-energy transient overvoltage pulse, due to it has a very fast response time (sub-nanosecond) and a very high  surge absorption ability, its working impedance can be immediately reduced to a very low on value, allowing large currents to pass, and clamping the voltage to a predetermined level, thereby effectively protecting precision components in electronic circuits from damage. TVS can withstand instantaneous pulse power up to kilowatts, and its clamp response time is only 1ps (10-12S). The forward surge current allowed by TVS can reach 50 ~ 200A under the conditions of TA = 250C and T = 10ms.

TVS diodes work similarly to common Zener diodes. If the breakdown voltage is higher than the mark, the TVS diode will conduct. Compared with the Zener diode, the TVS diode has a higher current conduction capability. When the two poles of a TVS diode are subjected to reverse transient high-energy shocks, the high impedance between the two poles of the TVS diode becomes low at a speed of the order of 10 ^ -12S, while absorbing surge power of up to several kilowatts. The clamped voltage between the two poles is at a safe value, which effectively protects precision components in electronic circuits from being damaged by surge pulses.

Working Characteristic Curve of TVS Diode 

Figure 1. Working Characteristic Curve of TVS Diode

When the reverse voltage of the two poles of the TVS is greater than the maximum reverse voltage, it starts to conduct reversely; after the reverse voltage is greater than the breakdown voltage, it begins to be broken down, while the current starts to change suddenly; when the reverse voltage is greater than the maximum clamping voltage, the tube is in an avalanche breakdown state. At this time, the current flowing through the tube increases sharply, and the voltage difference across the tube does not change much (the voltage is clamped).

Under specified reverse application conditions, the TVS diode will provide a low-impedance path, and the instantaneous current flowing to the protected component will be shunted to the TVS diode through a large current method, while the voltage across the protected component will be limited to the clamping voltage of TVS. When the overvoltage condition disappears, the TVS diode returns to a high impedance state.

 15V 24.4A Bidirectional TVS Diode

Note: Unidirectional and Bidirectional TVS Diodes

Unidirectional TVS SymbolUnidirectional TVS SymbolBidirectional TVS SymbolBidirectional TVS Symbol

1) Look at the signs: for unidirectional tvs diode, there is a thin color ring, connected to the positive electrode, and for bidirectional tvs diode, there are two rings in the middle, or there is no sign, no polarity.

2) Look at the specifications: bidirectional tvs is bidirectional conduction, and unidirectional tvs is unidirectional conduction.

3) Look at the model: The model name of the tvs tube is regular, and most of the tvs diode models can see the parameters on the case. For details, it is necessary to consult the manufacturer.

4) Using multimeter tool: the unidirectional has voltage, while avalanche breakdown characteristics are available on the DC side; voltage is on both sides, and the DC side is symmetrical on both sides.

Bidirectional tvs diodes can absorb instantaneous large pulse power in both forward and reverse directions and clamp the voltage to a predetermined level. In addition, bidirectional TVS is suitable for AC circuits, and unidirectional TVS is generally used for DC circuits.

 

  • Electrical Characteristics

a. Unidirectional TVS (V-I characteristic)

Unidirectional TVS Diode 

Figure 2. Unidirectional TVS Diode

The unidirectional tvs diode has the same forward characteristics as ordinary Zener diodes, and the reverse breakdown inflection point is approximately right angle as a hard breakdown and is a typical PN junction avalanche device. The curve segment from the breakdown point to the VC value indicates that when there is a transient overvoltage pulse, the current of the device increases sharply while the reverse voltage rises to the clamped voltage value and remains at this level.

b. Bidirectional TVS (V-I characteristic)

Bidirectional TVS Diode 

Figure 3. Bidirectional TVS Diode

The V-I characteristic curve of the bidirectional tvs diode is similar to the two back-to-back unidirectional tvs diodes combination. Its forward and reverse directions have the same avalanche breakdown characteristics and clamping characteristics. The symmetry relation of the breakdown voltage on both sides of the positive and negative is as follows: 0.9≤ VBR(positive)/(inverse) ≤1.1, once the interference voltage at both ends of it exceeds the clamping voltage will be immediately suppressed, thus the bidirectional tvs are very convenient for ac loop application.

  • Main Parameters

1) breakdown voltage V(BR)

In the region where the device breaks down, the voltage across the device is measured at the specified test current I (BR), which is called the breakdown voltage. In this area, the tvs diode becomes a low impedance path.

2) maximum reverse pulse peak current IPP

In reverse operation, IPP refers to the maximum pulse peak current allowed by the device under specified pulse conditions. The product of IPP and the maximum clamping voltage VC (max) is the maximum value of the transient pulse power.

The TVS should be properly selected during use, so that the rated transient pulse power PPR is greater than the maximum transient surge power that may occur in the protected device or wires.

When the instantaneous pulse peak current appears, the TVS is broken down and its breakdown voltage value rises to the maximum clamping voltage value. As the pulse current decreases exponentially, the clamping voltage also decreases and returns to the original state. Therefore, TVS diode can suppress the impact of possible pulse power to effectively protect the electronic circuits.

The test waveform of the TVS peak current uses a standard wave (exponential waveform), which is determined by TR / TP.

Peak current rise time TR: The time from when the current reaches 0.9 IPP from 0.1 IPP.

Half-peak current time TP: The time after the current passes through the maximum peak from zero and then drops to 0.5 IPP.

The TR / TP values of typical test waveforms are listed below:

A. EMP wave: 10ns / 1000ns

B. Lightning wave: 8μs / 20μs

C. Standard wave: 10μs / 1000μs

3) Maximum reverse working voltage VRWM

When the device operates in reverse, the voltage across the device is called the maximum reverse operating voltage VRWM under the specified IR, usually VRWM = (0.8 ~ 0.9) V(BR). At this voltage, the power consumption of the device is small. When used, VRWM should not be lower than the normal working voltage of the protected device or circuits.

4) Maximum clamping current VC(max)

The maximum voltage value across the device under the peak pulse current IPP is called the maximum clamping voltage. When used, VC (max) should not be higher than the maximum allowable safe voltage of the protected device. And the ratio of the maximum clamping voltage to the breakdown voltage is called the clamping coefficient.

Clamping coefficient = VC(max) / V(BR), the general clamping coefficient is about 1.3.

5) Reverse pulse peak power PPR

The PPR of TVS depends on the pulse peak current IPP and the maximum clamping voltage VC (max). In addition, it is also related to the pulse waveform, pulse time and ambient temperature.

When the pulse time Tp is constant, PPR = K1‧K2‧VC (max) ‧IPP

(K1 is the power coefficient, and K2 is the temperature coefficient of the power) 

The typical pulse duration tp is 1MS. When the pulse time tp applied to the transient voltage suppression diode is shorter than the standard pulse time, its peak pulse power will increase as tp is shortened.

Peak Pluse Power vs Pluse Time

Figure 4. Peak Pluse Power vs Pluse Time

TVS reverse pulse peak power PPR is related to the pulse waveform subjected to surge, expressed by the power coefficient K1: E=∫i(t)‧V(t)dt     

i (t) is the pulse current waveform, and V (t) is the clamping voltage waveform.

This rated energy value is not reproducible to TVS in a very short time. However, in practical applications, surges often occur repeatedly. In this case, even if the single pulse energy is much smaller than the pulse energy that the TVS device can withstand, if repeat, these single pulse energy will accumulated, in some cases, it will exceed the pulse energy that the TVS device can withstand. Therefore, the circuit design must carefully consider and select the TVS device, so that the accumulation of pulse energy repeatedly applied within the specified interval does not exceed the pulse energy rating of the TVS device.

6) Capacitance CPP 

The capacitance of TVS Circuit 

Figure 6. The Capacitance of TVS Circuit

The capacitance of TVS is determined by the area of the silicon sheet and the bias voltage. In the case of zero bias, the capacitance value decreases with the increase of the bias voltage. The value of the capacitance will affect the response time of the TVS device.

7) Leakage current IR

When the maximum reverse working voltage is applied to the TVS, the TVS tube has a leakage current IR. When the TVS is used in a high impedance circuit, the leakage current is an important parameter. In practice, especially in automotive electronics, this parameter affects static current.

 

Ⅲ. TVS Selection

When selecting tvs diode, the specific conditions of the circuit must be considered, and generally the following principles should be followed:

1) The clamping voltage VC (max) is not greater than the maximum allowable safe voltage of the circuit.

2) The maximum reverse working voltage VRWM is not lower than the maximum working voltage of the circuit. Generally, VRWM can be selected to be equal to or slightly higher than the maximum working voltage of the circuit.

3) The rated maximum pulse power must be greater than the maximum transient surge power present in the circuit.

 

Ⅳ. TVS vs Varistor, Capacitor

1) TVS diode and varistor do not have switching characteristics like switching elements, but have voltage regulation characteristics like zener diodes.

2) The varistor can withstand a larger surge current, and the larger the varistor can withstand the larger surge current, which can reach tens of kA to hundreds of kA at the maximum; but the non-linear characteristics of the varistor are poor and the limiting voltage is higher at large current, and the leakage current is larger at low voltage.

3) The non-linear characteristics of TVS diodes are the same as those of Zener tubes. Leakage current before breakdown is very small. After the breakdown, it is in a standard voltage stabilization. Compared with varistors, the maximum clamping voltage of TVS diode is smaller, but its current capability is poor than a varistor. Since the breakdown voltage VBR and the clamping voltage VC of the varistors are relatively high, the current flow capability is relatively strong, and the surge pulse absorption capability is stronger, so it is more suitable for ESD or surge protection of the power interface.

4) For the reaction speed, the response speed of the TVS is fast (ps level), while the varistor’s is slow ( ns level). In addition, the capacitance of both is large (ps: TVS also have low capacitance products).

5) The TVS tube has high reliability, and a long service life, while the varistor has poor reliability, easy aging and short service life.

  • Other View

Compared with ceramic capacitors, TVS diodes can withstand a voltage of 15 kV, but ceramic capacitors have a lower ability to withstand high voltages. A 5 kV shock will cause about 10% of the ceramic capacitor to fail, and by 10 kV, its damage rate will reach 60%.

 

Ⅴ. Application Examples

  • Microcomputer system protection

In a typical microcomputer system, various interference or transient voltages entering through the power line, input line, and output line may cause the microcomputer to malfunction and fail, especially from the switching power supply. The on-off motor near the microcomputer, voltage surges and transients of AC power, electrostatic discharges, etc. may cause the system to fail, and in severe cases may damage the device. Connecting the tvs diode to the input and output lines of the power supply of the microcomputer can prevent the transient voltage from entering the microcomputer bus, strengthen the microcomputer's resistance to external interference, ensure the normal operation, and improve its reliability.

  • DC regulated power supply protection

A DC regulated power supply with a transistor that expands the current output, adding a tvs diode to its regulated output can protect the equipment, and can also absorb peak voltage from the collector to the emitter in the circuit to protect the transistor. In a word, adding a tvs diode at the output end of each voltage stabilization source can greatly improve the reliability of the whole operation.

  • Transistor protection

Various transient voltages can cause damage to the EB junction or CE junction of the transistor. Especially when the collector of the transistor has an inductive load (coil, transformer, motor), a high-voltage back-EMF can be generated, which often causes the transistor to be damaged. It is necessary to use a tvs diode as a protector.

  • Integrated circuit (IC) protection

As integrated circuits become more integrated, their withstand voltages are getting lower and lower, and they are easily damaged by transient voltages. Protective measures must be taken, for example, adding tvs diode in the circuit, the CMOS circuit has a protection network at its input and output ends.

  • Silicon control protection

The thyristor may has wrong trigger and cause malfunction. The control electrode current cannot be too large and the voltage cannot be too high, in order to do it, TVS can be used for protection.

  • Electric relay protection

Relay contacts often use large currents to switch on and off high-current inductive loads such as motors, and the inductor has a high back electromotive force when switching, and has a large amount of energy. Whats more, the contacts are burned or broken to produce an arc, and the surge current generated by the arc is very large. To protect the contacts by suppressing the occurrence of arcs to protect the relays, adding a tvs diode is more effective. In the past, a capacitor or a capacitor series resistor, a diode or a diode series resistor and other suppression methods were used.

  • Integrated op amp protection

Integrated op amps are very sensitive to external electrical stress. In the process of using op amps, if having excessive voltage or current due to operating errors or abnormal working conditions, especially surges and electrostatic pulses, it is easy to damage the op amp. In the integrating circuit, if the capacitor is charged and discharged to a high potential, and then the power supply voltage is cut off, a transient voltage will be generated at the input terminal, and a large discharge current will occur, resulting in damage to the operational amplifier. At this time, tvs protection method adopted at the input terminal of op amp to avoid device damage. If the capacitance value is large (such as greater than 0.1μF), the protecting effect will be very significant.

  • Suppression of electromagnetic pulse interference

A nuclear explosion will cause a strong electromagnetic pulse, which causes induced voltage in the wire. If the induced voltage exceeds the breakdown voltage of the device, it may cause the breakdown of the component, especially for long-term transmission, it is more easily to cause high voltage.

TVS diodes are connected in parallel to the signal and power lines, which can absorb the induced voltage caused by electromagnetic pulses, ensure the reliability of the system, and avoid radiation damage to components.

  • Lighting protection

In thunderstorm-prone areas, lightning-induced voltage often breaks down some of the integrated circuits in a computer network. The reason is that cables are damaged due to transient high voltage caused by lightning induction, by installing tvs diodes in the microcomputer, it is useful to reduce damages and commercial loss. And the result shows that it is very practical, and it can improve the reliability of the whole machine application.

TVS also have many other applications, for example, for VMOS high power transistors, the tvs diodes between the gate and the source and the machine can prevent gate breakdown and improve the reliability of the VMOS power tube application.

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