The following article is from Big Talk Hardware, by Katter.
1. Varistor
Varistor, English name Voltage Dependent Resistor, abbreviated VDR, or Varistor, Variable (will change) + Resistor (resistor). Its volt-ampere characteristic curve is nonlinear. That is, the resistance value of the varistor is not fixed, and there is dynamic change.
The dynamic change of the resistance is precisely because the voltage at both ends of the varistor is different and shows different resistance values. That is, in a certain current and voltage range, the value of the varistor changes with the voltage, or the resistance value is sensitive to the voltage of the device, so this is why it is called varistor.
The working principle of the varistor is: under normal voltage equivalent to a small capacitor (junction capacitor), when the circuit appears overvoltage, the use of the non-linearity of the varistor, its internal resistance drops sharply and quickly on, its working current increases by several orders of magnitude, at this time the voltage of the varistor will be immediately clamped at a fixed value. The varistor is used in parallel with the circuit and is an overvoltage protection element to protect the circuit from damage caused by overvoltage.
There are many types of materials that make up varistors, common ones are zinc oxide varistors, silicon carbide varistors, titanium oxide varistors and barium titanate varistors. The main material of zinc oxide varistor is bivalent element zinc and hexavalent element oxygen. Therefore, varistors are often referred to simply as MOV, metal oxide varistors.
A varistor, also known as a surge absorber, is a voltage resistor with voltage and current symmetry, which is mainly designed to protect electronic products or components from the effects of surges caused by switches or lightning strikes. Varistor response time is ns level, faster than gas discharge tube, but slower than TVS tube, used in general electronic circuits for overvoltage protection, response time can meet the requirements.
The varistor is identified in the circuit as follows:
As can be seen from the logo of the picture, the schematic symbol of the varistor is added with variable meaning on the basis of resistance.
Varistor package types are in-line and chip.
Figure 1 and Figure 3 long lead type is plug-in type, Figure 2 and figure 4 package as patch type. The shorter the lead, the smaller the parasitic inductance and capacitance, when the surge impact, the residual voltage of the varistor is constant, because the influence of parasitic parameters will lead to different peak volttimes. In use, parameter differences can be compared from different packages of the same type of device. Varistors are most commonly placed at the power port for surge protection using the package shown in Figure 1.
The structure of the varistor device is shown in the figure below
As can be seen from the above diagram, the varistor is mainly composed of 5 components: insulation layer, body, conductive silver paste, connection bar, and pin.
2. Voltammetry characteristic curve of varistor
The voltammetry curve of the varistor is shown in the figure below. It can be seen from the figure above that the voltammetry curve of the varistor has symmetry, which is more suitable for the protection of overvoltage in practical AC circuits.
The volt characteristics of the varistor are non-linear, according to the different voltage, the volt curve of the varistor can be divided into three parts, Leakage Region (Leakage Region), "working area (Normal Region)" and "Upturn Region".
Each of the three zones is described below:
(1) Leakage current zone
When the varistor works in the leakage zone, the voltage at both ends of the varistor is low, and the varistor shows a larger impedance externally, usually at the MΩ level, at this time the varistor will only increase the leakage current in the circuit, and the device will not operate.
(2) Work area
The working area is also the nonlinear region of the varistor, at this time the current changes in a large range, but the voltage at both ends of the varistor does not change much, showing better clamping characteristics, which is also the range where the varistor plays a role.
(3) upturn area
The upturn zone is also known as the saturation zone. When the current borne by the varistor in the non-line area continues to increase, the voltage limiting characteristics of the varistor will disappear, the resistance will drop sharply, and the impedance will become very small, at this time, because the varistor will heat up due to large power consumption, and eventually the varistor will burn or even explode. Therefore, when the varistor is used, it cannot enter the saturation area, that is, the up-turn area, which is basically damaged.
3. Key parameters of varistor
The key parameters of the varistor are shown in the figure below, which is derived from the 5D series varistor of Intele Electronics.
Similarly, the varistor data sheet for Junyao Electronics is shown in the figure below.
From the device specifications of the above two manufacturers, the key parameters of the varistor can be extracted as follows:
(1) Varistor Voltage
The varistor voltage refers to the voltage that passes 1mA DC current at both ends of the varistor to indicate whether it is on or not. In some specifications it is referred to as Vb, while varistors are often expressed as V@ 1mA. It can be seen from the two specifications that the pressure-sensitive voltage is usually a range, the same 18V pressure-sensitive voltage, the voltage range measured by the sound electronic is 16~20V, and the Junyao electronic is 15~21.6V. Therefore, according to experience, the usual error range of the pressure-sensitive voltage is about 15%. When doing the experiment, if the varistor voltage is found to deviate from 15% of the normal value, it can be considered that the varistor is in a failed state.
(2) Maximum Allowable Voltage
The maximum allowable voltage contains two voltages, DC voltage DC value and sinusoidal AC voltage RMS AC value. That is, when the varistor can be continuously used in AC circuits and direct current circuits, the voltage at both ends of the varistor should not exceed this recommended value. According to the empirical value, the maximum allowable voltage Uac=0.6U1mA, Udc=0.8U1mA
If the 5D180K varistor is selected in the circuit, the maximum DC voltage should not exceed 14V, and the effective value of AC voltage should not exceed 11V, if the maximum peak value of AC voltage is calculated at 15.5V.
Maximum Limited (Clamping) Voltage Maximum limited (clamping) voltage
Yinte electronic specification is called the maximum limiting voltage, Junyao electronic specification is the maximum clamping voltage, the meaning expressed is the same. When it comes to clamping voltage, it is compared to explain that the protection types of circuit protection devices are clamping type and switching type, and the varistor group is a clamping type device.
That is, after the varistor plays a role, even if the external voltage is high, it will eventually be clamped by the clamp type device at a fixed voltage. The maximum clamping voltage of a varistor is the voltage present on the varistor when a specified 8/20us wave impulse current is applied to both ends of the varistor. The applied current is a specific pulse current wave, which is described in detail in the specification. As shown in the following picture.
The current is also explained separately in the INTC electronic specification, and the current used by other devices is 5A, but the maximum limit voltage is changed to 1A for devices of 180K to 680K models. In the specifications of Junyao Electronics, each maximum clamping voltage will have a corresponding applied current value.
For example, if you choose the 180K model varistor, you can see that the maximum limiting voltage of the device is 40V, that is, the residual voltage is 40V, and the normal working voltage of the post-stage circuit is higher than 40V, otherwise the varistor will clamp the voltage at 40V, and the post-stage will still be damaged.
(4) Through-flow capacity Withstanding Surge Current
The current capacity is also known as the maximum impulse current, which means that the varistor can withstand the peak impulse current of 1.2/50us and 8/20us. Under specified conditions, at specified time intervals and times, a standard impulse current is applied to allow the maximum current to pass through the device.
In the Yinte electronic specification, it is required to withstand two different current waveforms, but in the Junyao electronic specification, only an 8/20us impulse current is written, which only needs to be applied once. The criterion to determine whether the varistor can withstand the impact of this standard current waveform is whether there is a 10% change in the varistor voltage.
As you can see from the specification, this cocurrent capacity contains two values, one is the standard current, which is relatively small, and the other is the higher surge current, which is relatively large.
(5) Static power Rated Wattage
Static power refers to the maximum average power that can be applied to a varistor at a specified ambient temperature. So this power is related to temperature. Working for 1000 hours at a specific ambient temperature of 85 ° C, the pressure-sensitive voltage changes less than 10% of the maximum power.
(6) Energy tolerance
Energy tolerance means that when a 10/100us or 2ms pulse wave is applied at both ends of the varistor, the pressure sensitive voltage change cannot exceed 10% of the energy that can be tolerated. If a change of 10% will damage the varistor, then the varistor cannot fail after the above capability is applied.
(7) Static capacitance Typical capacitance
Static capacitance, will have a @1KHz, that is, the varistor in such a standard waveform presented under the parasitic capacitance value for the corresponding value, generally in several thousand pF.
(8) Accuracy range
Refers to the accuracy of the device's nominal pressure-sensitive voltage fluctuation. It is generally ±10%
(9) Insulation resistance
Working for 1000 hours at a specific ambient temperature of 85 ° C, the pressure-sensitive voltage changes less than 10% of the maximum power.
(10)Current-temperature coefficient
It refers to the relative change of current flowing through the varistor when the voltage at both ends of the varistor remains constant and the temperature changes by 1 ° C.
(11) Voltage nonlinear coefficient
The ratio of static resistance to dynamic resistance of a varistor under a given applied voltage.
(12) Residual pressure ratio
When the current passing through a varistor is of a certain value, the voltage generated at both ends is called the residual voltage of this current value. Residual voltage ratio is the ratio of residual voltage to nominal voltage.
(13) Leakage current
When the varistor is not operating in the circuit, the current flowing through the varistor is the leakage current.
(14) Voltage temperature coefficient
Refers to the rate of change of the nominal voltage of the varistor in the specified temperature range (temperature is 20 ° C ~ 70 ° C), that is, when the current through the varistor remains constant, the relative change of the voltage at both ends of the varistor when the temperature changes by 1 ° C.
(15) Maximum impulse current (8/20us)
A specific pulse current (8/20us waveform) shocks the varistor once or twice (each interval of 5 minutes) so that the change in the varistor voltage is still within 10% of the maximum impact current.
4. Varistor selection
(1) Determine the normal operating voltage and pressure-sensitive voltage of the application circuit.
According to the working voltage of the circuit, the nominal pressure-sensitive voltage of the varistor is determined by the following way. In general, the actual pressure-sensitive voltage is the nominal pressure-sensitive voltage there is an error, which leads to the calculation of the actual pressure-sensitive voltage, need to be multiplied by 1.1~1.2 times.
If it is a direct current circuit, considering that the actual DC voltage fluctuation is common in the range of 1.2 to 1.5, so in the direct current circuit, it is generally 1.8 to 2 times the rated voltage. If it is an AC circuit, there is a 1.4 times relationship between the peak voltage and the effective value, and the voltage in the AC circuit will also have a waveform, and the fluctuation limit is about 1.4 to 1.5 times, so in the AC circuit, it is generally 2.2 to 2.5 (1.2*1.5*1.4) times of the rated voltage.
The specific calculation and reason analysis are introduced in detail in the manual of Junyao Electronics. For the application of overvoltage protection, it is necessary to fully take into account that although the VDR can absorb a large surge energy, it cannot withstand continuous current above the milliampere level. Generally consider the two parameters of V1mA and flow capacity, VDR pressure sensitive voltage value is greater than the actual circuit voltage value, as well as power supply voltage fluctuations, pressure sensitive voltage accuracy, varistor aging and other factors, generally calculated with the following empirical formula:
a is the fluctuation coefficient of the power supply voltage, generally (0.1~0.3)
v is the DC voltage at both ends of the varistor in the circuit (AC takes the peak voltage)
b is the varistor error, generally 0.1~0.2
c is the aging coefficient of the component, generally 0.1
This gives us the following formula:
V1mA = 1.35~1.8Vp = 1.9~2.5VAC, Vp is the peak value of rated voltage, and the effective value of VAC for AC voltage is generally relaxed to V1mA = (1.8~2.0) Vp = (2.2~2.5) Vp
(2) Determine the protection level and flux of the protected circuit.
According to the protection level to determine the varistor flow rate, assuming that the measurement waveform of the protection device is 8/20us, there are mainly several calculation methods.
Power circuit differential mode protection: For example, a power interface differential mode protection test needs to reach 2000V, then according to the loop internal resistance of 2ω, calculate the loop current I as V test /R internal resistance =100A pass flow selection requirements IMAX>I work ×2=2000A.
Power circuit common mode protection: For example, the mode protection level of a power interface needs to reach 4000V, then according to the loop resistance of 12Ω, calculate the loop current of 330A, and the device flow rate selection requirements of 660A.
Signal circuit common mode protection: for example, the common mode protection of a signal interface is 1000V, according to the internal resistance of the loop 40Ω, the calculation loop 25A, and the device flow rate is 50A.
(3) Determine the junction capacitance at both ends of the varistor.
The junction capacitance shall not affect the normal operation of the protected circuit. The capacitance of the varistor is generally large, and there are several thousand pF levels, so it is not suitable for high frequency occasions, otherwise it will affect the normal operation of the circuit.
If the power circuit is added between the lines of the junction capacitor can not be concerned, if added between the lines then need to consider the impact of leakage current on product safety. The junction capacitance of the signal circuit protection device needs to be determined according to the signal rate of the electrical protection circuit, and the device with a small junction capacitance is selected as far as possible on the high-speed signal line, so as not to affect the normal operation of the protection circuit.
5. Varistor Selection Precautions
The protection device requires the maximum flow rate of the device to be more than twice the theoretical calculated flow rate in order to increase the service life of the device, that is, IMAX>I work ×2=1000A.
When the flow rate can not meet the requirements of use, several individual varistors can be used in parallel, the flow rate is the sum of each single varistor, requiring the parallel varistor voltammetry characteristics as much as possible, otherwise easy to cause uneven shunt and damage varistor, this method can be used, but not recommended.
Because the varistor in the suppression of transient overvoltage, the energy exceeds its rated capacity, once the breakdown short circuit is not recoverable, must be replaced, so before the device is applied to the power circuit must be fitted with a fuse.
When the varistor is used for common mode protection against the ground at the power port, a fuse must be added between the lines to prevent short circuit of the power supply to the ground due to device failure or to use the varistor with a gas discharge tube to the ground.
Due to the large capacitance of varistor junction, the attenuation of working signal should be considered in the design of signal interface. Varistors are generally used in power frequency circuits.
When the varistor is used, it is necessary to consider that the residual voltage is acceptable to the later circuit.
6. Varistor Protection Circuit
(1) AC protection
When the varistor is used for AC protection, the common mode protection will be used with the gas discharge tube, and the differential mode protection will use a single varistor.
(2) DC protection
Dc protection If there is no PE line, directly place the varistor between the positive and negative terminals. If there is a PE line, you can refer to the AC protection method. Both common mode and differential mode need to be protected. The following is a diagram of varistor protection shown on the TDK website.
Considering that the resistance drop is relatively large after the failure of the varistor, when used in DC protection, it is generally used with the fuse, when the rear stage has a large current, the fuse of the front stage can be quickly blown.