Silicon transient absorption diode

The operation of silicon transient absorption diodes is somewhat similar to that of ordinary voltage regulators, and they are clamp type interference absorbing devices; Its application is to be used in parallel with the protected device. Silicon transient voltage absorbing diodes have extremely fast response time (sub nanosecond level) and considerable surge absorption capability, as well as a wide range of voltage levels. Can be used to protect equipment or circuits from transient voltages generated during static electricity, inductive load switching, and overvoltage caused by induced lightning.

TVS tubes come in two types: unidirectional (single diode) and bidirectional (two back-to-back connected diodes), with their main parameters being breakdown voltage, leakage current, and capacitance. The breakdown voltage of the TVS tube in use should be about 10% higher than the working voltage of the protected circuit to prevent the leakage current of the TVS from affecting the normal operation of the circuit due to the line working voltage approaching the TVS breakdown voltage; It also avoids the breakdown voltage of TVS tubes falling within the normal operating voltage range of the circuit due to changes in environmental temperature.

TVS tubes come in various packaging forms, such as axial lead products that can be used on power feeders; Dual in-line and surface mount are suitable for protecting logic circuits, I/O buses, and data buses on printed boards.

Characteristics of TVS

The circuit symbol of TVS is the same as that of ordinary voltage regulators. The voltage current characteristic curve is shown in Figure 1. Its forward characteristics are the same as ordinary diodes, and its reverse characteristics are typical PN junction avalanche devices. Figure 2 shows the current time and voltage time curves of TVS. Under the action of surge voltage, the voltage between the two poles of TVS rises from the rated reverse turn off voltage VWM to the breakdown voltage VBR and is broken down. With the occurrence of breakdown current, the current flowing through the TVS will reach the peak pulse current IPP, and the voltage at both ends will be clamped below the predetermined maximum clamping voltage VC. Afterwards, as the pulse current decays exponentially, the voltage between the two poles of the TVS continuously decreases and eventually returns to its initial state. This is the process by which the TVS suppresses potential surge pulse power and protects electronic components. When the two poles of TVS are subjected to reverse high-energy impact, it can reduce the impedance between the two poles at a speed of 10-12 seconds, absorb surge power of up to several kilowatts, and clamp the potential between the two poles at a predetermined value, effectively protecting electronic components from damage caused by surge pulses. TVS has the advantages of fast response time, high transient power, low leakage current, small breakdown voltage deviation, easy control of clamping voltage, and small size. It has been widely used in various fields such as household appliances, electronic instruments, communication equipment, power supplies, computer systems, etc.

Main parameters of TVS

Maximum reverse leakage current ID and rated reverse turn off voltage VWM. VWM is the maximum continuous operating DC or pulse voltage of TVS. When this reverse voltage is applied between the two poles of TVS, it is in a reverse off state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID. Minimum breakdown voltage VBR and breakdown current IR. VBR is the minimum breakdown voltage of TVS. At 25 ℃, TVS below this voltage will not experience avalanche. When a specified 1mA current (IR) flows through the TVS, the voltage applied to the two poles of the TVS is its minimum breakdown voltage VBR. According to the degree of dispersion between the VBR of TVS and the standard value, VBR can be divided into two types: 5% and 10%. For a 5% VBR, VWM=0.85VBR； For a 10% VBR, VWM=0.81VBR。

Maximum clamping voltage VC and maximum peak pulse current IPP. When a pulse peak current IPP with a duration of 20mS flows through the TVS, the maximum peak voltage appearing at both ends is VC. VC and IPP reflect the surge suppression capability of TVS. The ratio of VC to VBR is called the clamping factor, which is generally between 1.2 and 1.4.

Capacity C. The capacitance C is determined by the TVS avalanche junction cross-section and is measured at a specific frequency of 1MHz. The size of C is directly proportional to the current carrying capacity of TVS. If C is too large, it will cause signal attenuation. Therefore, C is an important parameter for selecting TVS in data interface circuits.

Maximum peak pulse power consumption PM. PM is the maximum peak pulse power dissipation value that TVS can withstand. At a given maximum clamping voltage, the greater the power consumption PM, the greater its ability to withstand surge currents; At a given power consumption PM, the lower the clamping voltage VC, the greater its ability to withstand surge currents. 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 non repetitive, and the device's specified pulse repetition frequency (the ratio of duration to interval time) is 0.01%. If there are repetitive pulses in the circuit, the accumulation of pulse power should be considered, which may damage the TVS.

Clamping time TC. TC is the time from zero to the minimum breakdown voltage VBR. For unipolar TVS less than 1 × 10-12s; For bipolar TVS less than 10 × 10-12s.