What you must know about ESD as a PCB LAYOUT engineer. TVS tube is widely used in practical PCB design. Silicon transient voltage absorption diode has extremely fast response time (sub-nanosecond level), quite high surge absorption capacity, and many voltage grades. It can be used to protect equipment or circuits from static electricity, transient voltage caused by inductive load switching, and overvoltage caused by induced lightning.
Silicon transient voltage absorption diode (TVS tube)
Matters needing attention in the use of TVS tube:
1. When there is no TVS tube with appropriate voltage for adoption, it is allowed to use multiple TVS tubes in series. The maximum current of the tubes in series is determined by the one with the smallest current absorption capacity. And the peak absorbed power is equal to the product of this current and the sum of the voltage of the series tube.
2. To suppress the repeated transient voltage, it is particularly noteworthy whether the steady-state average power of TVS tube is within the safe range. As a semiconductor device, TVS tube should pay attention to the derating problem when the ambient temperature rises. Pay special attention to the length of the lead of TVS tube and the relative distance between it and the protected line.
3. The junction capacitance of TVS tube is the key factor affecting its use in high-speed lines. In this case, a TVS tube and a fast recovery diode are generally connected back to back. Because the fast recovery diode has a small junction capacitance, the equivalent capacitance of the two connected in series is also small, which can meet the requirements of high-frequency use.
4. Solid discharge tube solid discharge tube is a new transient interference absorption device, which has the characteristics of fast response speed (10 ~ 20 ns), large absorption current, stable operation voltage and long service life.Both solid discharge tube and gas discharge tube belong to energy transfer type.When the external interference is lower than the trigger voltage, the tube is cut-off.Once the interference exceeds the trigger voltage, the volt ampere characteristic will turn and enter the negative resistance region. At this time, the current is very large, and the on resistance is very small, so that the interference energy can be transferred.When the through current of the discharge tube is lower than the maintenance current, the discharge tube will quickly go out of the low resistance area and return to the high resistance state to complete a discharge process.One of the advantages of solid discharge tube is its short circuit failure mode (when the device fails, there is a short circuit between the two electrodes), which is necessary for many applications and has been widely used at home and abroad.The PCB design training of solid discharge tube has less voltage level, which is more suitable for network, communication equipment and even component level protection.
5. For the protection of small current load, the current limiting resistance can be added consciously in the line. As long as the resistance value of the current limiting resistance is appropriate, the normal operation of the line will not be affected, but the current generated by the current limiting resistance to the interference will be greatly reduced.It is possible to select TVs tube with smaller peak power to protect the line with small current load.
6. The relationship between the absorbed power (peak value) of transient voltage and the pulse width of transient voltage.It is only the absorbed power (peak value) under a specific pulse width, while the pulse width in the actual circuit is unpredictable, so it needs to be estimated in advance.It should be derated for wide pulse.
There are two kinds of TVS tubes: unidirectional (single diode) and bidirectional (two diodes connected back to back). Their main parameters are 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, so as to prevent the leakage current of TVS from affecting the normal operation of the circuit because the working voltage of the line is close to the breakdown voltage of TVS; And avoid that breakdown voltage of TVS tube from falling into the range of the normal work voltage of the line due to the change of ambient temperature. TVS tubes have various packaging forms, such as axial lead products that can be used on power feeders; Dual-in-line and surface-mounted are suitable for protection of logic circuits, I/O buses and data buses on printed boards.
