How to select TVS diodes

When selecting TVS, the following main factors should be considered:

(1) If the TVS is likely to withstand spikes (surge voltages) from both directions, it should be bipolar, otherwise it can be unipolar.

(2) The Vc value of the selected TVS should be lower than the maximum voltage of the protected element. "Vc is the voltage of the diode in the cutoff state, that is, the voltage that passes through the TVS during the ESD surge state. It cannot be greater than the allowable limit voltage of the protected circuit, otherwise the device may be in danger of being damaged.".

(3) TVS should not be in a breakdown state under normal working conditions, preferably below VR. The appropriate TVS should be selected based on comprehensive consideration of both VR and VC requirements.

Stabilizing tube

(4) If an accurate surge current IPP is known, VCIpp can be used to determine power; If the approximate range of IPP cannot be determined, it is better to choose a higher power TVS. PM is the maximum peak pulse power dissipation value that TVS can withstand. Under a given maximum clamping voltage, the greater the power consumption PM, the greater the withstand capacity of its surge current; Under a given power consumption PM, the lower the clamping voltage VC, the greater the withstand capacity of the surge current. In addition, peak pulse power consumption is also related to pulse waveform, duration, and ambient temperature. (5) The transient pulses that TVS can withstand are non repetitive, and the specified pulse repetition frequency (ratio of duration to intermittent time) of the device is 0.01%. If repetitive pulses occur in the circuit, the accumulation of pulse power should be considered, otherwise it may damage the TVS.

(6) For the protection of small current loads, it is possible to consciously add a current limiting resistor to the line. As long as the resistance value of the current limiting resistor is appropriate, it generally does not affect the normal operation of the line, but the current generated by the interference caused by the current limiting resistor will be greatly reduced. However, it is possible to select TVS tubes with low peak power to protect small current load lines.

(7) The capacitance C is determined by the cross-section of the TVS avalanche junction, which is measured at a specific frequency of 1 MHz. The magnitude of C is proportional to the current withstand capacity of the TVS, and too large a C will attenuate the signal. Therefore, C is an important parameter for selecting TVS for data interface circuits. For circuits with higher data/signal frequencies, the greater the interference of diode capacitance on the circuit, and the greater the intensity of noise or attenuation signals. Therefore, it is necessary to determine the capacitance range of the selected device based on the characteristics of the circuit. Generally, the capacitance of high-frequency circuits should be selected as small as possible (such as LCTVS and low capacitance TVS, with a capacitance not greater than 3 pF), while for circuits with low capacitance requirements, the capacitance can be selected above 40 pF.

(8) In order to meet the IEC61000-4-2 international standard, TVS diodes must be able to handle a minimum of 8 kV (MB, contact) and 15 kV (BM, air) ESD impacts. Some semiconductor manufacturers have used higher impact resistance standards on their products. For some portable device applications with special requirements, designers can select devices as needed.