Characteristics and application of TVs

Characteristics and application of TVs
The transient voltage suppressors (Transient Voltage suppressor) are referred to as TVs and are a high-performance protection device in the form of diodes. When the two poles of a TVs diode are subjected to a reverse transient high energy shock, it can turn the high impedance of the two poles into a low impedance at a speed of 10-12 seconds, absorb up to thousands of watts of surge power, and clamp the voltage between the poles to a predetermined value, effectively protecting the precision components in the electronic circuit from the damage of a variety of surge pulses. Due to its fast response time, large transient power, low leakage current, small breakdown voltage deviation, clamping voltage is easy to control, no damage limit, small size and other advantages.

has been widely used in computer systems, communications equipment, AC/DC power supply, automobiles, household appliances, instrumentation, rs232/422/423/485, I/O, Ethernet. LAN, ISDN, ADSL, USB, MP3, TABLET, GPS, CDMA, GSM, WCDMA/TD, digital camera protection, common mode/differential mode protection, RF coupling/IC drive receive protection, motor electromagnetic interference suppression, audio/video input, sensor/transmission, Industrial circuit, relay, contactor noise suppression and other fields. The following three major characteristics:

1, the TVs diode is added to the signal and the power line, can prevent microprocessor or microcontroller due to instantaneous pulse, such as electrostatic discharge effect, the surge of AC power supply and the noise of the switching power supply caused by the failure.

2, electrostatic discharge effect can release more than 10000V, more than 60A pulse, and can continue 10ms, and the general TTL device, when encountering more than 30ms 10V Pulse, it will lead to damage. TVs diodes are used to effectively absorb pulses that cause damage to the device and to eliminate interference caused by switching between buses (Crosstalk).

3, the TVs diode placed in the signal line and ground between, to avoid data and control the bus is not affected by unnecessary noise.

First, the characteristics and main parameters of TVs

1, the characteristic curve of TVs

TVS have the same circuit symbols as regular zener diodes. Its forward characteristic is the same as the ordinary diode, and the reverse characteristic is typical PN junction avalanche device.

Under the action of the transient peak current pulse, the current flowing through the TVs, from the original reverse leakage current ID up to IR, its bipolar voltage from the rated reverse shutdown voltage VWM rise to the breakdown voltage Vbr,tvs is penetrated. With the advent of peak pulse currents, the current flowing through TVs reaches the peak current-pulse IPP. The voltage at its poles is clamped below the predetermined maximum clamping voltage. Thereafter, as the pulse current is attenuated exponentially, the voltage on the TVs poles is also declining and finally returning to the starting state. This is the whole process of TVs suppressing the potential surge pulse power to protect electronic components.

2, the characteristics of TVs parameters

① Maximum reverse leakage current IR and rated reverse shutdown voltage VWM.

The VWM is the maximum continuous DC or pulse voltage of the TVs, when the reverse voltage is added to the TVs ' poles, it is in reverse shutdown state, and the current flowing through it should be less than or equal to its maximum reverse leakage current IR.

② minimum breakdown voltage VBR and Breakdown current IR

VBR is the smallest avalanche voltage of TVs. At 25 ℃, TVs are not conductive until this voltage is applied. When TVs flows through the specified 1mA current (IR), the voltage between the TVs poles is added to its minimum breakdown voltage VBR. According to the discrete degree of VBR and standard value of TVs, TVs can be divided into two kinds of ±10% VBR ±5%VBR coexist. For ±5%VBR, VWM=0.85VBR; for ±10% VBR, vwm=0.81 VBR.

③ Maximum clamping voltage VC and Maximum peak pulse current IPP

The maximum peak voltage that occurs between the poles of a pulsed peak current with a duration of 20 microseconds flowing through TVs is VC. It is a combination of a series resistor and a voltage rise due to a temperature coefficient. VC and IPP reflect the surge suppression capability of TVs devices. The ratio of VC to VBR is called clamping factor, usually between 1.2~1.4.

④ capacitance C

The capacitance C is determined by the TVs Avalanche Junction section and measured at a specific 1MHZ frequency. The size of C is proportional to the current withstand capacity of TVs, and C is too large to attenuate the signal. Therefore, C is an important parameter of selecting TVs for data interface circuit.

⑤ maximum peak pulse power consumption pm

PM is the maximum peak pulse dissipation power that TVs can withstand. For the specified test pulse waveform and the various TVs PM values, please refer to the relevant product manuals. At a given maximum clamping voltage, the larger the power consumption pm, the greater the withstand capacity of the inrush current, and the lower the clamp voltage VC under a given power consumption pm, the greater the withstand capacity of its inrush current. In addition, peak pulse power consumption is also related to pulse waveform, duration and ambient temperature. And TVs can withstand the transient pulse is not repeated, the device set pulse repetition frequency (duration and interval time ratio) is 0.01%, if there is a repetitive pulse in the circuit, should consider the "accumulation" of pulse power, it is possible to damage the TVs.

⑥ Clamp-Time TC

The TC is the time from zero to the minimum breakdown voltage VBR. To TVs less than 1x10-12 seconds.

Ii. Classification of TVs diodes

TVS devices can be divided into unipolar and bipolar by polarity, as well as single-and multi-line distinctions. According to the use can be divided into a variety of circuits are applicable to general-purpose devices and special circuits suitable for (such as high-speed circuit) special-purpose devices.

Third, the selection of TVs skills

1. Determine the maximum DC or continuous operating voltage of the protected circuit, the rated standard voltage of the circuit and the "high-end" tolerance.

2, TVs rated reverse shutdown VWM should be greater than or equal to the maximum operating voltage of the protected circuit. If the selected vwm is too low, the device may enter an avalanche or cause a reverse leakage current too large to affect the normal operation of the circuit. Serial connection sub-voltage, parallel connection sub-current.

3, TVs maximum clamping voltage VC should be less than the damaged voltage of the protected circuit.

4. For the specified pulse duration, the maximum peak pulse power consumption PM of TVs must be greater than the peak pulse power that may occur in the protected circuit. When the maximum clamping voltage is determined, the peak pulse current should be greater than the transient surge currents.

5, for the protection of the data interface circuit, it is also necessary to pay attention to selecting a TVs device with appropriate capacitance C.

6. Choose the polarity and package structure of TVs according to the use. It is more reasonable to choose bipolar TVs in AC circuit, and it is more advantageous to choose TVs Array for multi-line protection.

7, temperature considerations. Transient voltage suppressors can operate between -55~+150℃. If a TVs is required to operate at a varying temperature, the inverse leakage current of IR is increased with the increase of the output, and the power dissipation decreases with the increase of the TVs junction temperature, from +25℃ to +175℃, approximately linearly 50% rain breakdown voltage VBR increases with temperature by a certain coefficient. Therefore, it is necessary to consult the relevant product data to consider the effect of temperature change on its characteristics.
Iv. comparison of TVs and Varistors
At present, many of the domestic equipment required for surge protection is the use of varistors. Varistors are a metal varistor. TVs are much more characteristic than varistors, and the comparison of specific characteristic parameters is shown in the table below.

Comparison table of attribute parameters
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