What are TVS diodes?

 TVS diodes or transient voltage suppression diodes are electronic components used to protect electronic devices from voltage spikes induced on connected wires. TVS diodes acts by shunting excess current when the induced voltage exceeds the avalanche breakdown potential. It can be considered as a clamping device suppressing all overvoltages above its breakdown voltage. Electron avalanche occurs when electrons are accelerated by electric energies and collide with atoms to create more mobile electrons and ions which can lead to high currents beyond the breakdown voltage. Incidentally, I came across the use of TVS diodes in a discussion on BITX forum, where it was mentioned that TVS diodes are used for gate protection of final MOSFETs!


TVS diodes can be unidirectional or bidirectional. The first device seen in the image is a unidirectional TVS diode and can be recognized by the band in the region of the cathode, while other devices are bidirectional and do not have the band. Schematically, a bidirectional TVS diode can be represented as two mutually opposing diodes in series to one another. The device is connected in parallel to the circuit to be protected. Though the representation is as two diodes, units are manufactured as a single component as seen in the image. All the devices below the match stick and without the cathodal band, are bidirectional TVS diodes.

TVS diode can respond to over-voltages faster than varistors or gas discharge tubes, which are other types of over voltage protection devices. Though the actual clamping time is roughly one picosecond, the inductance of the wires leading to the device causes a higher time limit. TVS diodes can protect against very fast and often damaging voltage transients which could occur due to events like lightning or motor arcing. As the transients are very brief, the energy due excess current flow is initially stored within the device and heat sinks can only help in cooling down afterward. To accommodate this process, the TVS diode must be physically large. Otherwise they can be destroyed by the transient and the circuit will be unprotected. 

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