High performance protection device - characteristics and application of TVS

Transient Voltage Suppressor (TVS) is a high-performance protection device in the form of a diode. Some documents are also TVP, AJTVS, SAJTVS, etc. When the two poles of a TVS diode are subjected to a reverse transient high-energy shock, it can change the high impedance between the two poles to a low impedance at a speed of the order of 10-12 seconds, absorbing surge power of up to several kilowatts, making the poles The voltage clamp is placed at a predetermined value to effectively protect the precision components in the electronic circuit from various surge pulses. Because it has the advantages of fast response time, large transient power, low leakage current, small breakdown voltage, easy control of clamping voltage, no damage limit, small size, etc., it has been widely used in computer systems, communication equipment, and power supplies. , household appliances and other fields.

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Specifically, it has the following three characteristics: 1. Adding a TVS diode to the signal and power line can prevent the microprocessor or the MCU from being damaged by an instant, such as electrostatic discharge, AC power surge and switching power supply noise. Failure.

2, the electrostatic discharge effect can release more than 10000V, 60A or more of the pulse, and can last 10ms; and the general TTL device, when encountered more than 30ms 10V pulse, it will lead to damage. With the TVS diode, it can effectively absorb the pulse that will cause damage to the device and eliminate the interference caused by the switch between the buses (Crosstalk).

3. Place the TVS diode between the signal line and the ground to avoid unnecessary noise from the data and control bus.

First, the characteristics and main parameters of TVS

1. Characteristic curve of TVS

The circuit symbol of TVS is the same as that of a normal Zener diode. Its forward characteristics are the same as for a normal diode; the reverse characteristic is a typical PN junction avalanche device.

Under the action of the transient peak pulse current, the current flowing through the TVS rises from the original reverse leakage current ID to IR, and the voltage presented by the two poles rises from the rated reverse shutdown voltage VWM to the breakdown voltage VBR, and the TVS is breakdown. As the peak pulse current occurs, the current flowing through the TVS reaches the peak pulse current IPP. The voltage at its two poles is clamped below a predetermined maximum clamp voltage. Then, as the pulse current decays exponentially, the voltage at the two poles of the TVS also drops, and finally returns to the initial state. This is the entire process of TVS suppressing the possible surge pulse power and protecting electronic components.

2. Characteristic parameters of TVS

1 Maximum reverse leakage current ID and rated reverse shutdown voltage VWM. VWM is the maximum continuous DC or pulse voltage of TVS. When this reverse voltage is added between the two poles of TVS, it is in the reverse-off state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID.

2 minimum breakdown voltage VBR and breakdown current IR

VBR is the smallest avalanche voltage of TVS. At 25 ° C, the TVS is non-conducting before this voltage. When the TVS flows through the specified 1 mA current (IR), the voltage between the two poles of the TVS is added to its minimum breakdown voltage VBR. According to the dispersion degree of TVS VBR and standard value, TVS can be divided into ±5% VBR and coexistence ±10% VBR. VWM = 0.85 VBR for ± 5% VBR and VWM = 0.81 VBR for ± 10% VBR.

3 maximum clamp voltage VC and maximum peak pulse current IPP

When the pulse peak current IPP with a duration of 20 microseconds flows through the TVS, the maximum peak voltage appearing between its two poles is VC. It is a combination of a series resistance and a voltage rise due to both temperature coefficients. VC and IPP reflect the surge suppression capability of TVS devices. The ratio of VC to VBR is called the clamping factor and is generally between 1.2 and 1.4.

4 capacitance C

Capacitance C is determined at a specific 1 MHz frequency as determined by the TVS avalanche junction profile. The size of C is proportional to the current carrying capacity of the TVS, and C is too large to attenuate the signal. Therefore, C is an important parameter for the TVS of the data interface circuit.

5 maximum peak pulse power consumption PM

PM is the maximum peak pulse dissipation power that TVS can withstand. For the test pulse waveforms specified and the PM values ​​of various TVS, please refer to the relevant product manual. At a given maximum clamping voltage, the greater the power consumption PM, the greater the inrush current capability; at a given power consumption PM, the lower the clamping voltage VC, the more the inrush current withstand capability Big. In addition, peak pulse power consumption is also related to pulse waveform, duration, and ambient temperature. Moreover, the transient pulses that TVS can withstand are not repeated. The pulse repetition frequency (the ratio of duration to pause time) specified by the device is 0.01%. If there is a repetitive pulse in the circuit, the "accumulation" of the pulse power should be considered. It is possible to damage the TVS.

6 clamp time TC

TC is the time from zero to the minimum breakdown voltage VBR. It is less than 1×10-12 seconds for unipolar TVS and less than 1×10-11 seconds for bipolar TVS.

Second, the classification of TVS diodes

TVS devices can be divided into unipolar and bipolar according to their polarities. According to their applications, they can be divided into general-purpose devices and special-purpose devices suitable for various circuits. Such as: a variety of AC voltage protectors, 4 ~ 200mA current environmental protection, data line protectors, coaxial cable protectors, telephone protectors. According to the package and internal structure can be divided into: axial lead diode, dual in-line TVS array (for multi-line protection), patch, component and high-power modular.

Third, TVS selection 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. The TVS rated reverse turn-off 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 the reverse leakage current will affect the normal operation of the circuit. The serial connection is divided into voltages, and the divided currents are connected in parallel.

3. The maximum clamp voltage VC of TVS should be less than the damage voltage of the protected circuit.

4. The maximum peak pulse power consumption of the TVS must be greater than the peak pulse power that may be present in the protected circuit for the specified pulse duration. After the maximum clamp voltage is determined, its peak pulse current should be greater than the transient surge current.

5. For the protection of the data interface circuit, care must also be taken to select a TVS device with a suitable capacitor C.

6. Select the polarity and package structure of TVS according to the application. It is more reasonable to use bipolar TVS for AC circuit; it is more advantageous to use TVS array for multi-line protection.

7, temperature considerations. The transient voltage suppressor can operate between -55 and +150 °C. If TVS is required to operate at a varying temperature, its reverse leakage current ID increases with increasing; power consumption decreases with increasing TVS junction temperature, from +25°C to +175°C, approximately linearly drops by 50%. The penetration voltage VBR increases with a certain factor with increasing temperature. Therefore, it is necessary to consult the relevant product data to consider the effect of temperature changes on its characteristics.

8. TVS diodes supplied by ProTek USA have the following different power options:

~500W: SA series

~600W: P6KE, SMBJ series

~1500W: 1N5629~1N6389, 1.5KE, LC, LCE series

~5000W: 5KP series

~15000W: 15KAP, 15KP series

Fourth, the comparison of TVS and varistor

At present, many varistor devices are used on equipment that requires surge protection in China. A varistor is a metallization varistor. TVS is much superior to varistor characteristics, and the comparison of specific characteristic parameters is shown in the table below.
-------------------------------------------------- ----------------
Key parameters or limit values ​​| TVS | Varistor
--------------------|-------------------------|--- ----------------
Reaction speed | 10-12s | 5×10-8
--------------------|-------------------------|--- ----------------
Is there any aging phenomenon? No | Yes
--------------------|-------------------------|--- ----------------
Maximum operating temperature | 175°C | 115°C
--------------------|-------------------------|--- ----------------
Component Polarity | Unipolar and Bipolar | Unipolar
-------------------|-------------------------|---- ----------------
Reverse leakage current typical | 5μA | 200μA
--------------------|-------------------------|--- -------------------
Clamp factor (VC/VBR) | ≤1.5 | ≥7~8
-------------------|-------------------------|---- ------------------
Sealing properties | Sealing is not breathable | Breathable
--------------------|-------------------------|--- ------------------
Price | expensive | cheap
---------------------------------------------|---- ------------

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