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What are TVS diodes?


TVS diodes
TVS (transient voltage suppressor) diodes are used to protect traces from high voltage spikes. They work by shunting currents when the voltage exceeds the avalanche breakdown potential. They are basically high-power Zener diodes. They start conducting at a preset voltage and are tailored for low on-times.
Transient Voltage Suppressor or TVS Diodes are an effective solution to protect other semiconductors from any damaging surge levels they may be exposed to. Some confusion can arise as ESD diodes can often be referred to as TVS diodes in literature. However, there is a clear distinction. TVS diodes are particularly efficient in countering medium-voltage high-energy pulses that last milliseconds, in comparison to ESD protection devices which address higher voltage pulses of lower energy and shorter duration.
And while they may seem like Zener diodes, TVS diodes are specifically designed to suppress transients rather than regulate voltage. To effectively protect the circuit, the TVS diode must provide a low impedance path in order to shunt a transient when it occurs, before returning to its original high impedance mode.
Leakage Current
The reverse-leakage of TVS diodes decreases as the stand-off voltage increases. Be warned, the outflow current of TVS diodes that have low voltage stand-offs (e.g. <10V), can have large leakage currents! A 5V stand-off TVS diode typically has a reverse-leakage current of around 500uA, but TVS diodes with a stand-off voltage of 10V or higher have a reverse-leakage of 1uA or less. Note that at low stand-off voltages, the leakage current of a bi-directional diode can be double that of a uni-directional diode for the same stand-off voltage.

Reverse Polarity Protection
Unusually, TVS Diodes. along with a fuse or other current-limiting device, can act as a very good reverse-polarity protection mechanism on inputs to a PCB. They are usually present on a voltage rail input for the primary reason of reducing ESD. However, if the V+ and GND are connected to the PCB the wrong way around, the TVS diode will forward conduct and clamp the voltage to a normally non-destructive 0.7-1.5V. A current-limiting device like a fuse also has to be present to prevent the TVS diode from overheating.
They are especially suited to this role (when considering other diodes) as the are usually built to dissipate large amounts of heat.

Low Capacitance
There ar a family of TVS Diodes referred to as low-capacitance (or ultra-low) TVS diodes. They have much less capacitance than standard TVS diodes (typical capacitances are between 0.4-0.9pF), and are designed for protecting high-speed data lines such as those used in USB, HDMI, DisplayPort, and Ethernet communication protocols and also for RF antennas such as GPS, FM radio and NFC antenna lines.
This low capacitance is achieved by adding a forward-biased general purpose diode in series with the usual reverse-biased TVS (zener-style diode). The forward-biased general purpose diode has a much smaller parasitic capacitance than the zener diode. Because the parasitic capacitances are in series (grey capacitors in diagram), the total capacitance of the component is greatly reduced!

Special-Purpose TVS Diodes

RS-485 TVS Diodes
TVS diodes built specifically for protecting RS-485 communication protocol bus lines are bi-directional and have two different hold-off voltages to meet the RS-485 spec. They usually embody the character sequence “SM712” in their part name (e.g. SM712-02HTG by Littelfuse and SM712-TP by Micro Commerical).





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