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Aug 26 2019

Basic Informantion of A Varistor | Definition,Types,Parameters,Function

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Article Core

Varistor

Purpose

Introduce what is the Varistor and its parameters are

Application

Semiconductor industry

Keywords

Resistor


Catalog

Ⅰ Definition

Ⅱ Types of Varistor

III  Introduction of varistor parameters

IV Basic performance of Varistor

V Function of Varistor


Definition

Varistor is the abbreviation of voltage sensitive resistor, and it is a kind of nonlinear resistance element. The varistor resistance is related to the voltage applied at both ends. When the voltage added to the varistor is within its nominal value the resistance of the resistor is in an infinite state with almost no current passing through. When the voltage at both ends of the varistor is slightly larger than the nominal voltage, the varistor breaks through the conduction quickly, and its resistance value decreases quickly, which makes the resistor in the conduction state. When the voltage is reduced below the nominal voltage, the resistance value of the voltage-sensitive resistor is increased, and the voltage-sensitive resistor is again in a high-resistance state. When the voltage at both ends of the varistor exceeds its maximum limit voltage, it will break through the damage completely and will not be able to recover on its own.

Varistors

Varistor (VSR,) is a kind of nonlinear overvoltage protection semiconductor element which is sensitive to voltage. It is represented by the text symbol "RV" or "R" in the circuit, and figure 1 is its circuit graphic symbol. Varistors are different from ordinary varistors in that they are made according to the nonlinear characteristics of semiconductor materials.

Circuit Graphic Symbol of Varistor 

Figure 1 Circuit Graphic Symbol of Varistor

Varistor is an ideal protective element with the characteristics of high price, small volume, wide working voltage range, fast response to overvoltage pulse, strong resistance to impulse current, low leakage current (less than a few microamperes to tens of microamperes), small resistance temperature coefficient and so on. It is widely used in household appliances and other electronic products, and is often used to form the overvoltage protection circuit, the de-noising circuit, the anti-spark circuit, the lightning protection circuit, the surge voltage absorption circuit and the protective semiconductor component.

The Shape of Varistor 

Figure 2 The Shape of Varistor

Figure 3 Internal Structure of Varistor


 Types of varistors

Varistors can be classified by structure, manufacturing process, use material and volt-ampere characteristics:

(1) According to the structure of varistors, varistors can be divided into junction varistors, single particle varistors and thin film varistors.

The junction varistor has nonlinear characteristics because of the special contact between the resistor and the metal electrode, and the nonlinear of the varistor is determined by the semiconductor properties of the resistor itself.

(2) Varistors can be divided into zinc oxide varistors, silicon carbide varistors, metal oxide varistors, germanium (silicon) varistors, barium titanate varistors and so on.

(3) According to its volt-ampere characteristics, varistors can be divided into symmetrical varistors (non-polar) and asymmetric varistors (polar).


  Introduction of varistor parameters

(1) Varistor voltage: Breakdown voltage or threshold voltage, refers to the voltage value at a specified current. In most cases, the voltage values measured by 1mA DC current when passing through the varistor can range from 10 to 9000V. It can be selected correctly according to the specific needs.

The varistor has an inflection point in the pre-breakdown area of its V ≤ I characteristic curve, which corresponds to a specific inflection point voltage and a specific inflection point voltage. When the applied voltage is higher than this inflection point voltage, the varistor enters the "on" state (the resistance value becomes smaller); When the applied voltage is lower than this inflection point voltage, the varistor enters the "cut-off" state (the resistance value becomes large). The most important characteristic of varistor is that the resistance value varies with the applied voltage. The inflection point voltage in the V ≥ I characteristic curve can best reflect this important characteristic of varistor, so we can understand the inflection point voltage as the varistor voltage UN of the varistor (the threshold voltage between the on and off states). Because varistor is a kind of ceramic element with completely uniform interior, even if it is a varistor of the same specification, the inflection point of each element is different. In order to standardize the need, the International Electrotechnical Commission(IEC)  artificially specified two DC reference currents I0-1mA and 0.1mA for measuring the inflection point of the varistor. Almost all of the varistor manufacturers currently use U1mA or U0.1 mA to represent the pressure-sensitive voltage.

Under normal circumstances,  there should be:

V (1 mA) = 1.5Vp = 2.2 VAC

 where Vp is the peak of the circuit's rated voltage. VAC is the effective value of the rated AC voltage.

For DC in the DC loop, there should be:

min (U1mA) ≥ (1.6-2) Udc

Udc is the DC rated operating voltage in the loop.

The selection of voltage value of ZnO varistor is very important, which is related to the protection effect and service life.

(2) Maximum continuous operating voltage(MCOV)

Because the varistor has positive and negative symmetry volt-ampere characteristics, it can be applied to both DC circuit and AC circuit. MCOV refers to the maximum DC voltage (UDC )or the maximum AC voltage effective value (URMS) that the varistor can withstand for a long time.

Varistors have a very special characteristic: long-term static power is very small, and instantaneous dynamic power is very large. Because the static power of varistor is very small, the long-term working voltage applied to both ends of varistor is absolutely smaller than its varistor voltage UN, otherwise the varistor will burn out due to overburden.

If the varistor is used in a AC circuit, the principle for determining the URMS is: the peak value (1.41URMS) of the maximum continuous alternating voltage is greater than the lower limit of tolerance (±10%) of the varistor voltage UN, and the formula is expressed as follows:

 Relation between MCOV and Input Voltage under AC Power Supply of Varistor

Figure 4 Relation between MCOV and Input Voltage under AC Power Supply of Varistor

If the varistor is used in a DC circuit, the principle for determining the UDC is: The power consumption of the varistor under the action of UDC is roughly equal to or slightly smaller than the power consumption under the action of URMS and its power consumption under the action of URMS, and the empirical formula obtained by the invention is as follows:

Relation between MCOV and Input Voltage under DC Power Supply of Varistor 

Figure 5 Relation between MCOV and Input Voltage under DC Power Supply of Varistor

(3) Maximum peal current

It is the maximum pulse current value when the varistor voltage does not exceed ±10% for the specified impulse current waveform and the specified impulse current number at the ambient temperature of 25 ℃.

In order to extend the service life of the device, the amplitude of the surge current absorbed by the ZnO varistors shall be less than the maximum flow rate of the product given in the manual. However, from the point of view of protection effect, the selected flow rate is required to be larger.

In many cases, the actual flow is difficult to calculate accurately. Actually, the flow capacity, also known as the flow rate, refers to the maximum pulse (peak) current value allowed to pass through the varistor under specified conditions (applying standard impulse current at a specified time interval and number of times). The general over-voltage is one or a series of pulse waves. There are two kinds of shock waves used in experimental varistors, one is 8 / 20 μ s wave, that is, a pulse wave with a wave head of 8 μ s and a tail time of 20 μ s, the other is 2ms square wave, as shown in the figure below.

Impulse Current Waveform Used in Test Varistor 

Figure 6 Impulse Current Waveform Used in Test Varistor

Flow rate correspondence table of commonly used varistor 

 Flow rate correspondence table of commonly used varistor

(4) Residual Voltage(UR)

Residual voltage UR refers to the peak voltage at both ends of a particular waveform when the surge (surge) current flows into the varistor. In general, the peak value of surge current flowing into varistors is above 1mA. For general varistors and protective varistors, the specific waveform refers to the 8 / 20 μ s standard lightning current waveform specified in IEC 60060 ≤ 2 ≤ 1973 standard, which is shown in the figure.

01 represents the apparent origin, the Ts is called the apparent front time, the Tr is called the apparent half peak time, and the Im is called the electric peak value. Because it is difficult to find the origin 01 accurately on the oscilloscope, the approximate method will be used to measure the origin at the front time Ts, and the half peak time Tr. The specific method is as follows: the value of T1 is measured on the oscilloscope first, and then the value of Ts is obtained by using the formula Ts=1.25 × T1 approximating, and the starting point of the actual measurement of Tr at half peak time is changed from 01 to 0. In addition, the IEC allows for a small amplitude of reverse polarity oscillations in the inrush current waveform used for measurement.

Schematic Diagram of Surge Waveform 

Figure 7 Schematic Diagram of Surge Waveform

(5) Residual Voltage ratio(Kp)

When the current flowing through the varistor is a certain value, the voltage generated at both ends of the varistor is called the residual voltage of this current value. The ratio of residual voltage to nominal voltage is the ratio of residual voltage to nominal voltage: KR=UR/UN

The residual voltage ratio reaction varistor limits the energy of overvoltage, which has been widely used in the research of varistor materials. It has become the standard electrical performance parameter in lightning protection varistor, arrester valve plate and high energy varistor valve plate.

(6) Maximum limit voltage(Up)

Limiting voltage Up is a special form of residual voltage UR, and it is also a characteristic index to evaluate the ability of varistors of specific specifications to suppress transient overvoltage. Firstly, a basic equivalent assessment current IP shall be specified for the varistor of different chip diameters, and the limiting voltage Up of the varistor of each chip diameter shall correspond to the specified good assessment current(as shown in Table). Secondly, the limited voltage Up is not the residual voltage measured by IP, but the upper limit value of the residual voltage stipulated by each manufacturer. Therefore, the limited voltage Up is actually the protection voltage level of each specification that manufacturers promised to the users. In the IEC standard, the limited voltage is also called the voltage under the grade current.

 Protection Voltage Level of Varistor

Figure 8 Protection Voltage Level of Varistor

(7) Leakage current

Leakage current, also known as waiting current, refers to the current that the varistor flows through the varistor at the specified temperature and maximum DC voltage.

(8) Ratio-voltage

The voltage ratio is the ratio of the voltage generated when the varistor current is 1 mA to the voltage generated when the varistor current is 0.1 mA.

(9) Ceiling capacity(Em)

The maximum energy, Em, is the maximum energy of a surge current or a pulse current that can be dissipated by the varistor. The meaning of bearing is that the varistor voltage UN after shock is less than ±10% compared with that before shock, and visual and visible mechanical damage can occur at the same time.

The energy absorbed by the varistors is generally calculated as follows: W=kIVT(J)

I: the peak flow through the varistor

V: The voltage across the varistor when the current I flows through the varistor

T: Current duration

Square waves of 2ms: K=1

8 / 20 μ s wave: K=1.4

10 / 1000 μ s: K=1.4

Em is closely related to current waveform. The energy test waveform specified by IEC is 2ms standard square wave, as shown in the figure.

 Waveform Parameters of 2ms Standard Square Wave

Figure 9 Waveform Parameters of 2ms Standard Square Wave

TD is called effective square wave duration (also called T0.9), TT is called effective square wave total time (also written as T0.1), and I2ms is called square wave (average) current. IEC60060-2:1973 stipulates that the tolerances of TD are + 20% and-0%, TT ≤ 1.5 TD, and I’/ I and I "/ I do not exceed 10%. When the 2ms standard square wave current flows over the varistor, the residual voltage waveform of the varistor is 2ms voltage wave. Moreover, it is more regular than 2ms electric wave. After the average residual voltage U2ms of varistor in 2ms range can be measured by a method similar to I2ms measurement, the actual dissipative energy of varistor can be calculated by using the following formula:

E2ms=U2msI2ms×2×10-3(J)

For 2ms square wave, the absorption energy of varistor can reach 330 J per square centimeter. For 8 / 20 μ s wave, the current density can reach 2000 A per cubic centimeter, which indicates that its current flow ability and energy tolerance are very large. Generally speaking, the larger the chip diameter of varistor, the greater its energy tolerance and shock current resistance.

(10) Power rating(Po)

Rated power Po refers to the maximum average power that varistors can withstand and maintain thermal stability and no structural failure under the action of current pulse group. The maximum number of shocks per second N is calculated according to the lower formula:

图片17.png 

(11) Temperature Coefficient(Tc)

The voltage temperature coefficient refers to the change rate of the nominal voltage of the varistor in the specified temperature range (the temperature is 20 ≤ 70 ℃). That is, when the current through the varistor is kept constant, the relative change in voltage across the varistor is changed by 1 °C as the temperature changes.

Voltage and Temperature Coefficient of Varistor 

Figure 10 Voltage and Temperature Coefficient of Varistor

Tupper is the upper class temperature of the varistor (°C), the maximum allowable operating temperature. The definition formula of voltage temperature coefficient Tc actually only indicates the average voltage temperature coefficient in the range from room temperature to its upper limit category temperature, which is generally greater than-0.05% ℃. Strictly speaking, the voltage temperature coefficient is not a constant, and the TC value is different at different temperatures, but it is usually not necessary to give the relationship curve between Tc and temperature.

(12) Current temperature coefficient

The current temperature coefficient refers to the relative change in current across the varistor is changed by 1 °C as the temperature changes when the voltage through the varistor is kept constant.

(13) Insulation resistance

Insulation resistance refers to the resistance value between the lead line (pin) of the varistor and the insulation surface of the resistor.

(14) Voltage non-linear coefficient(α)

Nonlinear index α is a sign of whether the resistance value of a element varies with voltage or electric flux and whether the change is sensitive or not. The general resistor (linear resistor) is a voltage-sensitive resistor with a value of 1. The geometric meaning is the reciprocal of the slope of the V ≥ I characteristic curve drawn by the double logarithmic coordinate method.

IEC provides that:

 

图片19.png 

(15) Static capacitance

Static capacitance refers to the inherent capacitance capacity of varistor itself.

(16) Response time(T)

In IEEE C62.33 ≤ 1982 standard, the response time tau of varistor is defined as shown in the figure. The voltage Vc in the figure refers to the residual voltage of 8 / 20 μ s standard lightning current wave by varistor. When the peak value of surge current is equal, but at the front time TS is shorter than 8 μ s, the residual voltage V1 is higher than that of Vc, (V1-Vc) called voltage overshoot VOS. The time widthτ (t2-t1) between the peak time T1 of V1 and the overshoot time T2 of 50%VOS is called the "response time" of varistor, and its measured value is generally within 25ns.

Varistor Response Time 

Figure 11 Varistor Response Time

(17) Pulse current stability(10,000 impact life)

The 8 / 20 μ s standard lightning current wave of Ia applies the peak value to the varistor, which strikes 104 times in one direction and has an interval of 10 s. The specified value of Ia is shown in Table, followed by recovery at room temperature for 1-2 hours. After recovery, the varistor shall meet the following requirements:

Visual inspection: No visible damage, and the sign is clear.

Varistor voltage (voltage under specified current): The change rate shall no more than±10%.

he Correspondence of Ia Value Applied in Pulse Current Stability Test 

Figure 12 The Correspondence of Ia Value Applied in Pulse Current Stability Test


 Basic performance of varistor

(1) Protection characteristics: When the impact strength of the impact source (or impulse current Isp=Usp/Zs) does not exceed the specified value, the limited voltage of the varistor is not allowed to exceed the impulse withstand voltage (Urp). That the protected object can bear.

(2) Impact resistance: The varistor itself should be able to withstand the specified impulse current, shock energy, and the average power when multiple shocks occur one after another.

(3) Life characteristics: First, the continuous working voltage life, that is, the varistor should be able to work reliably at the specified ambient temperature and system voltage conditions (hours). The second is the impact life, that is, the number of times that can reliably withstand the prescribed impact.

(4) Quadratic effect: After the voltage-sensitive resistor is involved in the system,  it not only has the protection effect of the "safety valve", but also brings some additional influence, which is the "secondary effect", and it should not lower the normal working performance of the system. The factors to be considered are three, one is the capacitance of the varistor itself (several tens to tens of thousands of PF), the second is the leakage current under the system voltage, and the non-linear current of the voltage-sensitive resistor is affected by the coupling of the source impedance to the other circuits.


 The function of varistor

The main function of varistor is to protect the transient voltage in the circuit. Because of the working principle mentioned above, the varistor is equivalent to a switch, only when the voltage is higher than the threshold, the resistance value is infinitesimal and the switch is closed, so that the current flowing through it surges and the influence on other circuits does not change much, thus reducing the influence of overvoltage on the subsequent sensitive circuit. This protection function of varistor can be used repeatedly many times, and it can also be made into a disposable protective device similar to current fuse.

Varistors are mainly used for transient overvoltage protection in circuits, but because of their volt-ampere characteristics similar to those of semiconductor voltage regulators, varistors also have a variety of circuit element functions. For example, a voltage-sensitive resistor is a direct-current high-voltage small-current voltage-stabilizing element; Varistor can be used as voltage fluctuation detection element, DC level shift element, fluorescence starting element, voltage equalizing element and so on. Varistors are widely used in household appliances and other electronic products, such as overvoltage protection, lightning protection, suppression of surge current, absorption of peak pulse, limiting, high voltage arc extinguishing, denoising, protection of semiconductor components, etc.

87.png 

Figure 13  Typical Application Circuit of Varistor

The protection function of varistor has been widely used. For example, the power supply circuit of home color TV is to use varistor to complete the overvoltage protection function. When the voltage exceeds the threshold, the varistor reflects its clamp characteristics, lowers the excessive voltage, and makes the rear circuit work in the safe voltage range.


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