Transient Voltage Suppressor Diode TVS

Transient Voltage Suppressor (TVS) is a diode-type high-efficiency protection device. When the two poles of a TVS diode are subjected to a reverse transient high-energy impact, it can change the high impedance between the two poles into a low impedance at a speed of 10 to the negative 12th power of seconds, absorbing surge power of up to several kilowatts, clamping the voltage between the two poles to a predetermined value, and effectively protecting the precision components in the electronic circuit from damage by various surge pulses. It has the advantages of fast response time, large transient power, low leakage current, breakdown voltage deviation, easy control of clamping voltage, no damage limit, and small size.
It has been widely used in computer systems, communication equipment, AC/DC power supplies, automobiles, electronic ballasts, household appliances, instruments (electricity meters), RS232/422/423/485, I/O, LAN, ISDN, ADSL, USB, MP3, PDAS, GPS, CDMA, GSM, digital camera protection, common mode/differential mode protection, RF coupling/IC drive receiving protection, motor electromagnetic interference suppression, audio/video input, sensor/transmission, industrial control circuit, relay, contactor noise suppression and other fields. It has the following three specific features:

1. Adding TVS diodes to signal and power lines can prevent microprocessors or single-chip computers from malfunctioning due to instantaneous surges, such as electrostatic discharge effects, AC power surges, and switching power supply noise.
2. Electrostatic discharge effects can release pulses exceeding 10,000V and 60A, and can last for 10ms; while general TTL devices will be damaged when encountering 10V pulses exceeding 30ms. TVS diodes can effectively absorb pulses that can damage devices and eliminate interference (Crosstalk) caused by switches between buses.
3. Placing TVS diodes between signal lines and ground can prevent data and control buses from being affected by unnecessary noise.
1. Characteristics and main parameters of TVS
1. TVS characteristic curve
The circuit symbol of TVS is the same as that of ordinary voltage regulator diodes. Its forward characteristics are the same as those of ordinary diodes; its reverse characteristics are typical PN junction avalanche devices. Under the action of transient peak pulse current, when the current flowing through the TVS rises from the original reverse leakage current ID to IR, the voltage at its two poles rises from the rated reverse shutdown voltage VWM to the breakdown voltage VBR, and the TVS is broken down. With the emergence of the peak pulse current, the current flowing through the TVS reaches the peak pulse current IPP. The voltage at its two poles is clamped below the predetermined maximum clamping voltage. Afterwards, as the pulse current decays exponentially, the voltage at the two poles of the TVS also continues to decrease, and finally returns to the starting state. This is the whole process of TVS suppressing 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 working DC or pulse voltage of TVS. When this reverse voltage is added between the two poles of TVS, it is in the reverse shutdown state, and the current flowing through it should be equal to or greater than its maximum reverse leakage current ID.

② The maximum breakdown voltage VBR and breakdown current IRVBR are the maximum avalanche voltage of TVS. At 25℃, before this voltage, TVS is not conductive. When the specified 1mA current (IR) flows through TVS, the voltage between the two poles of TVS is its maximum breakdown voltage VBR. According to the degree of dispersion of TVS VBR and standard value, TVS can be divided into two types: ±5% VBR and coexisting ±10% VBR. For ±5% VBR, VWM=0.85VBR; for ±10% VBR, VWM=0.81VBR.

③ Maximum clamping voltage VC and maximum peak pulse current IPP
When a pulse peak current IPP with a duration of 20 microseconds flows through the TVS, the maximum peak voltage between its two poles is VC. It is a combination of the voltage rise on the series resistor and the voltage rise due to the temperature coefficient. VC and IPP reflect the surge suppression capability of the TVS device. The ratio of VC to VBR is called the clamping factor, which is generally between 1.2 and 1.4.

④ Capacitance C
Capacitance C is determined by the cross section of the TVS avalanche junction and is measured at a specific frequency of 1 MHz. The size of C is proportional to the current carrying capacity of the TVS. If C is too large, the signal will be attenuated. Therefore, C is an important parameter for selecting TVS in data interface circuits.

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 PM values ​​of various TVS, please refer to the relevant product manual. Under a given maximum clamping voltage, the greater the power consumption PM, the greater its surge current tolerance; under a given power consumption PM, the lower the clamping voltage VC, the greater its surge current tolerance. In addition, the peak pulse power consumption is also related to the pulse waveform, duration and ambient temperature. Moreover, the transient pulses that TVS can withstand are non-repetitive. The pulse repetition frequency (ratio of duration to intermittent time) specified by the device is 0.01%. If repetitive pulses appear in the circuit, the "accumulation" of pulse power should be considered, which may damage the TVS.

⑥ Clamping time TC
TC is the time from zero to the maximum breakdown voltage VBR. For unipolar TVS, it is 1×10-12 seconds; for bipolar TVS, it is 1×10-11 seconds.

2. Classification of TVS diodes
TVS devices can be divided into unipolar and bipolar types according to polarity, and can be divided into general devices suitable for various circuits and special devices suitable for special circuits according to their uses. Such as: various AC voltage protectors, 4~200mA current environmental devices, data line protectors, coaxial cable protectors, telephone protectors, etc. According to the package and internal structure, they can be divided into: axial lead diodes, dual in-line TVS arrays (suitable for multi-line protection), patch type, component type and high-power module type, etc.

3. TVS selection skills
1. Determine the maximum DC or continuous working voltage of the protected circuit, the rated standard voltage of the circuit and the "high-end" tolerance.
2. The rated reverse shutdown VWM of the TVS should be greater than or equal to the maximum working voltage of the protected circuit. If
the selected VWM is too low, the device may enter avalanche or affect the normal operation of the circuit due to excessive reverse leakage current. Serial connection divides voltage, and parallel connection divides current.
3. The maximum clamping voltage VC of the TVS should be greater than the damage voltage of the protected circuit.
4. Within the specified pulse duration, the maximum peak pulse power consumption PM of the TVS must be greater than the peak pulse power that may appear in the protected circuit. After determining the maximum clamping voltage, its peak pulse current should be greater than the transient surge current.
5. For the protection of data interface circuits, it is also necessary to pay attention to the selection of TVS devices with suitable capacitance C.
6. Select the polarity and packaging structure of the TVS according to the purpose. It is more reasonable to use bipolar TVS for AC circuits; it is more advantageous to use TVS arrays for multi-line protection.
7. Temperature considerations. Transient voltage suppressors can work between -55 and +150°C. If TVS needs to work at a changing temperature, its reverse leakage current ID increases with the increase; power consumption decreases with the increase of TVS junction temperature, from +25°C to +175°C, it decreases linearly by about 50%, and the breakdown voltage VBR increases with the increase of temperature at a certain coefficient. Therefore, it is necessary to consult the relevant product information and consider
the impact of temperature changes on its characteristics.

4. Comparison between TVS and varistor
: Many devices in China that require surge protection use varistor. Varistor is a metallized variable resistor. TVS has much better characteristics than varistor.