TVS Diode (Transil / Transient Voltage Suppressor)
A zener-like protection diode designed to clamp very fast rising transient voltage pulses. Its response time is below a nanosecond and its clamping characteristic is far sharper than a varistor's. On 12 V vehicle lines, classes with a standoff voltage of typically 16–20 V are used, and on 24 V lines 33–40 V.
A TVS is high impedance in normal operation and behaves as if it were not in the circuit. When the voltage exceeds the clamping level, it conducts rapidly, takes the pulse current onto itself and holds the line voltage at the device's clamping value. It is connected in reverse between the supply line and the chassis, right after the fuse.
Two values decide the selection:
- Voltage class: the standoff voltage must be above the normal operating voltage and above the rises occurring during charging, while the clamping voltage must be below the withstand limit of the circuit being protected. The narrower this window, the better the protection. In a 12 V system the line can rise to 14.4 V while the alternator is running; a 13 V TVS therefore conducts continuously, heats up and fails.
- Pulse power: it is chosen according to the energy to be absorbed. A TVS is ideal for short, sharp pulses; but load dump events lasting hundreds of milliseconds carry far more energy and cannot be handled by a small TVS on its own. In such environments the TVS forms a multi-layer chain together with a series inductance and a bus capacitor.
Two notes specific to the vehicle environment:
- The long-duration overvoltage an alternator produces when the battery is disconnected is limited in centrally suppressed systems to a residual level of typically 35 V at 12 V and 58 V at 24 V. Choosing a breakdown voltage below these levels means the TVS takes on all the energy and blows up.
- A unidirectional TVS is used on a DC supply line; this device does not act as reverse polarity protection, a separate circuit is needed for that.
When adding external protection, mount the TVS as close to the board input as possible and with short legs; long connection inductance spoils the clamping.
Context in which this term is used: Installation, redundancy and field continuity