Classification and selection of TVS

TVS devices can be divided into two types based on polarity: unipolar and bipolar; It can be divided into universal and specialized types according to its purpose; According to packaging and internal structure, it can be divided into axial lead diodes, dual in-line TVS arrays, surface mount and high-power modules, etc. The peak power of axial lead products can reach 400W, 500W, 600W, 1500W, and 5000W. High power products are mainly used on power feeders, while low-power products are mainly used in high-density installation scenarios. For high-density installation scenarios, dual in-line and surface mount packaging forms can also be chosen.

Selection of TVS

Determine the maximum DC or continuous operating voltage of the protected circuit, the rated standard voltage of the circuit, and the maximum tolerable voltage.

The rated reverse turn off voltage VWM of TVS should be greater than or equal to the maximum operating voltage of the protected circuit. If the VWM selected is too low, the device may enter avalanche or affect the normal operation of the circuit due to excessive reverse leakage current.

The maximum reverse clamping voltage VC of TVS should be less than the damage voltage of the protected circuit.

Within the specified pulse duration, the maximum peak pulse power PM of TVS must be greater than the peak pulse power that may occur in the protected circuit. After determining the maximum clamping voltage, its peak pulse current should be greater than the transient surge current. The maximum peak pulse power of a typical TVS is given in 10/1000ms non repeating pulses, while the actual pulse width is determined by the pulse source, and the peak power varies depending on the pulse width. For a certain 600WTVS, the maximum absorbed power for a 1000ms pulse width is 600W, but for a 50ms pulse width, the absorbed power can reach 2100W, while for a 10ms pulse width, the maximum absorbed power is only 200W. Moreover, the absorbed power is also related to the pulse waveform: if it is a half sine wave pulse, the absorbed power needs to be reduced to 75%, and if it is a square wave pulse, the absorbed power needs to be reduced to 66%.

The matching of average steady-state power is necessary to introduce the concept of average steady-state power for TVS that needs to withstand regular and brief pulse group shocks, such as in applications such as relays, power switches, or motor control. For example, in a power switch circuit, a pulse train with a frequency of 120Hz, a width of 4ms, and a peak current of 25A will be generated. The selected TVS can clamp the voltage of a single pulse to 11.2V. The calculation of the average steady-state power in this case is: the pulse time interval is equal to the reciprocal of the frequency 1/120=0.0083s, the peak absorbed power is the product of the clamping voltage and the pulse current 11.2V × 25A=280W, and the average power is the product of the peak power and the ratio of the pulse width to the pulse interval, which is 280 × (0.000004S/0.0083S)=0.134W. That is to say, the average steady-state power of the selected TVS must be greater than 0.134W.

For the protection of data interface circuits, attention must also be paid to selecting TVS devices with appropriate capacitance C.

Select the polarity and packaging structure of TVS according to its intended use. It is more reasonable to use bipolar TVS for communication circuits; Choosing TVS array for multi line protection is more advantageous.

Considering temperature, transient voltage suppressors can operate between -55 ℃ and+150 ℃. If TVS needs to operate at a changing temperature, its reverse leakage current ID increases with temperature; The power consumption decreases linearly with the increase of TVS junction temperature, from+25 ℃ to+175 ℃, with a decrease of approximately 50%; The breakdown voltage VBR increases by a certain coefficient with the increase of temperature. Therefore, it is necessary to consult relevant product information and consider the impact of temperature changes on its characteristics.