應(yīng)用介面	傳輸速率	上升時間	建議使用容值
HDMI 1.3 Data Port	10.2G bps	0.02～0.03ns	＜0.15pF
USB 3.0 Data Port	4.80G bps	0.05～0.06ns	＜0.3pF
USB 2.0 Data Port	480M bps	0.5～0.6ns	＜4pF
USB 1.0 Data Port	12M bps	4～20ns	5～10 pF
Wireless Device	1.5M bps	75～300ns	5～10 pF
RS232, IrDA1.0	115.2 K	1us～8us	10～100 pF
Audio (Microphone/Sperker)	20～20K Hz	0.05ms～5ms	10～1000 pF

Below to introduce the following plug in pressure
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Voltage sensitive resistor, VSR, is a kind of voltage sensitive nonlinear overvoltage protection semiconductor device. It is in the circuit with the word symbol "RV" or "R", figure 1-21 is its circuit diagram symbols. (a) the types of voltage - sensitive resistors
Pressure sensitive resistors can be classified according to the structure, manufacturing process, use of materials and volt ampere characteristics.
1 according to the structure of the classification of pressure sensitive resistors by its structure can be divided into junction type pressure sensitive resistor, type of pressure sensitive resistor, single particle layer pressure sensitive resistor and film pressure sensitive resistor, etc..
Because of the special contact between the resistance body and the metal electrode, the junction type voltage sensitive resistor has the non-linear characteristic.
2 according to the use of material classification of varistors according to the use of different materials can be divided into Zinc Oxide, silicon carbide varistor varistor, metal oxide varistor, germanium (silicon) barium titanate varistor varistor, etc..
3 classified according to its volt ampere characteristic pressure sensitive resistor according to its volt ampere characteristic can be divided into symmetrical type pressure sensitive resistor (without polarity) and asymmetric type voltage sensitive resistor (polarity).
(two) the structure and function of the pressure sensitive resistor
1 the structure characteristic of the pressure sensitive resistor is different from that of the common resistor, which is made according to the non-linear characteristic of the semiconductor material.
Figure 1-22 is the appearance of the pressure - sensitive resistor, its internal structure as shown in figure 1-23. Ordinary resistors comply with Ohm‘s law, and the voltage and current of the voltage sensitive resistor is a special nonlinear relationship. When the voltage at both ends of the voltage sensitive resistor is lower than the nominal rated voltage value, the resistance value of the voltage sensitive resistor is close to infinity, and the internal almost no current flows. When the voltage at both ends of the voltage sensitive resistor is slightly higher than the nominal voltage, the voltage sensitive resistor will quickly lead to the breakdown, and by the high resistance state into a low resistance state, the operating current is also sharply increased. When the voltage at both ends of the voltage is lower than the nominal voltage, the voltage sensitive resistor can be restored to the state of high resistance. When the voltage of the two ends of the voltage exceeds the maximum limit voltage, the voltage sensitive resistor will be completely broken down, unable to resume.
Selection of overvoltage protection devices
There are three main types of circuit protection: over voltage protection, over current protection and over temperature protection. Choosing proper circuit protection devices is the first step to achieve efficient and reliable circuit protection design. Different protection devices have different protection principles, the choice should be based on the protection of the principles, working conditions and the use of the environment to consider. In this paper, Li Gong will introduce some of the common type of overvoltage selection techniques to help you to choose the correct circuit protection devices.
An overvoltage protection device (OVP) to protect the circuit from subsequent load rejection or transient voltage failure, overvoltage protection device commonly used are varistor, transient voltage suppressor, electrostatic discharge suppressor and etc.. Overvoltage protection device selection should pay attention to the following four points:
1) the choice of turn off voltage Vrwm. The general turn off voltage is at least 10% higher than the maximum operating voltage of the line.
2) clamp voltage VC selection. VC refers to the ESD shock state through the TVS voltage, it must be less than the maximum transient voltage of the protected circuit can withstand
3) the selection of surge power Pppm. Different power, the time of protection is different, such as 600W (10/1000us); 300W (8/20us)
4) the choice of pole capacitance. The higher the working frequency of the protected components, the smaller the requirement of TVS.
The role and application of 2 varistors varistors are widely used in household appliances and other electronic products, over voltage protection, lightning protection, suppression of inrush current, absorption peak pulse, amplitude limit, high voltage arc, noise elimination, protection of semiconductor components etc..
Figure 1-24 is a typical application circuit of the voltage sensitive resistor. (three) the main parameters of the voltage sensitive resistor
The main parameters of varistor with nominal voltage, voltage ratio, maximum control voltage and residual voltage ratio, flow capacity, leakage current, voltage, temperature coefficient, temperature coefficient, current voltage nonlinear coefficient, insulation resistance, static capacitance etc..
1 pressure sensitive voltage: the so-called voltage, that is, the breakdown voltage or threshold voltage. Refers to the prescribed voltage and current values of conducting voltage varistors when measured by 1mA DC current in most cases, the varistor voltage range of the products can be 109000V ranging from. According to the specific needs of the correct choice. General V1mA=1.5Vp=2.2VAC, type, Vp for the peak voltage of the circuit. VAC effective value for rated AC voltage. The choice of voltage value of ZnO is very important, it is related to the protection effect and service life. As with a rated supply voltage is 220V, electric appliance, varistor voltage value V1mA=1.5Vp=1.5 * 1.414 * 220V=476V, V1mA=2.2VAC=2.2 * 220V=484V, so the varistor breakdown voltage in 470480V between optional. MYG05K specified by the current is MYG07K, 0.1mA, MYG10K, MYG14K, MYG20K nominal voltage is defined by the 1mA DC current, the voltage value of the voltage at both ends of the voltage.
2 of the maximum allowable voltage (maximum voltage): the voltage of AC and DC two kinds of situations, such as communication, refers to the effective value of AC voltage of the varistor is allowed with the ACrms, so the effective value of AC voltage under the action should be chosen with the maximum allowable voltage of the varistor in fact, V1mA and ACrms between each other are interrelated, know that the former will know the latter, but the ACrms of users more directly, the user can according to the circuit voltage, can directly press ACrms to select the appropriate varistor. There should be in AC circuit, min (U1mA) = (2.2 ~ 2.5) Uac, in which Uac effective value of AC operating voltage in the circuit. The selection principle is mainly to ensure the application of varistors in the power supply circuit, a proper safety margin. In DC DC circuit, there should be: min (U1mA) = (1.6 ~ 2) Udc, where Udc is the DC rated voltage in the circuit. There should be in AC circuit, min (U1mA) = (2.2 ~ 2.5) Uac, in which Uac effective value of AC operating voltage in the circuit. The selection principle is mainly to ensure the application of varistors in the power supply circuit, a proper safety margin. In the signal loop, there should be: min (U1mA) = (1.2 ~ 1.5) Umax, where Umax is the peak voltage signal circuit. The current capacity of the pressure sensitive resistor should be determined according to the design index of the lightning protection circuit. Generally speaking, the current capacity of the pressure sensitive resistance is greater than the current capacity of the lightning protection circuit design.
3 flow capacity: the flow capacity, the maximum peak pulse current is 25 C ambient conditions, the number of impulse current provisions for the impact of current waveform and regulations, change of varistor voltage does not exceed a maximum value when the pulse current + 10%. In order to prolong the service life of the device, the amplitude of surge current absorbed by ZnO is less than the maximum flow rate of the product given in the manual. However, from the protection of the effect of starting, the demand for the flow of a large number of good. In many cases, it is difficult to accurately calculate the actual traffic flow. Speak - also known as flow capacity through simple flow, is refers to the prescribed conditions (the impact of current applied standard to the time interval and the number of the provisions), allow the varistor the maximum pulse current value (peak). General overvoltage is one or a series of pulse waves. There are two kinds of shock wave in experimental pressure sensitive resistance, one is 8/20 S wave, that is usually said the wave head is 8 S wave tail time is 20 s of pulse wave, and the other is 2ms of Fang Bo, as shown below: 4 maximum limit voltage: maximum voltage is the maximum voltage at both ends of the varistor can withstand voltage value, it said in the impact of current Ip regulations by varistor time generated by the voltage at both ends is also called the residual voltage, so the voltage level of the residual voltage of Vo varistors used must be less than the protected object otherwise, it is not up to the purpose of reliable protection, usually Ip impulse current value is large, such as 2.5A or 10A, which corresponds to the maximum limit of varistor voltage Vc is quite large, such as the MYG7K471 Vc=775 (Ip=10A).
5 maximum energy (energy tolerance): the energy absorbed by the pressure sensitive resistance is generally pressed down by the calculation of W=kIVT (J)
Where I - the peak of the flow through the pressure sensitive resistance
V - voltage at both ends of the pressure sensitive resistor when the current I flows through a voltage - sensitive resistor
T - current duration
K - current I waveform factor
Yes：
Square wave 2ms of k=1
8/20 S wave k=1.4
10/1000 mu k=1.4 s
The resistance of the 2ms square wave, the absorption of energy up to 330J per square centimeter; on the 8/20 S wave, the current density of up to 2000A per cubic centimeter, which shows that his flow capacity and energy tolerance are very large.
In general the varistor chip size is bigger, its energy tolerance is large, high impact current is larger, the varistor shall also be considered often less energy and over voltage but the frequencies of the high number, such as tens of seconds, one or two minutes once or several times over voltage, then they should consider the average power of varistor can absorb.
6 voltage ratio: the voltage ratio is the ratio of the voltage value generated when the current of the voltage sensitive resistor is 1mA and the voltage value of the voltage value and the current of the voltage sensitive resistor is 0.1mA.
7 rated power: the maximum power that can be consumed in a specified ambient temperature.
8 the maximum peak current: the 8/20 s standard waveform of the current for an impact of the maximum current value, at this time the pressure change rate is still within + 10%. 2 times: 8/20 s standard waveform of the current for the two impact of the maximum current value, the two impact time interval is 5 minutes, at this time the pressure sensitive voltage change rate is still within + 10%.
9 residual pressure ratio: the current flowing through the pressure sensitive resistor is a value, the voltage generated at the end of it is called the current value for the residual voltage. Ratio of residual pressure and nominal voltage.
10 leakage current: the leakage current is also called the waiting current. It is the current of the pressure sensitive resistor which flows through the pressure sensitive resistor at the specified temperature and the maximum DC voltage.
11 voltage temperature coefficient: voltage temperature coefficient refers to the temperature range specified (temperature of 20~70), changes in the nominal voltage of the varistor, is kept constant in the current through the varistor, relative changes in temperature change of 1 degrees the two ends of the varistor.
12 current temperature coefficient: the current temperature coefficient is defined as the voltage at both ends of the pressure sensitive resistor to maintain a constant temperature, the temperature changes 1 degrees Celsius, the current flow through the pressure sensitive resistor. 13 voltage nonlinear coefficient: the voltage nonlinear coefficient is the voltage sensitive resistor in the given external voltage, the value of its static resistance and dynamic resistance ratio.
14 insulation resistance: insulation resistance refers to the extraction line (PIN) of the voltage sensitive resistor and the resistance value between the insulation surface of the resistor.
15 static capacitance: static capacitance is the inherent capacitance of the voltage sensitive resistor.