A transient-voltage-suppression diode or thyrector is an electronic component
used to protect sensitive electronics from voltage spikes induced on connected
wires. Description
The device operates by shunting excess current when the induced voltage exceeds
the avalanche breakdown potential. It is a clamping device, suppressing all
overvoltages above its breakdown voltage. It automatically resets when
the overvoltage goes away, but absorbs much more of the transient energy
internally than a similarly rated crowbar device.
A transient-voltage-suppression diode may be either unidirectional or
bidirectional. A unidirectional device operates as a rectifier in the forward
direction like any other avalanche diode, but is made and tested to handle
very large peak currents. The 1.5KE series allows 1500 W of peak power, for
a short time. A bidirectional
transient-voltage-suppression diode can be represented by two mutually opposing
avalanche diodes in series with one another and connected in parallel with
the circuit to be protected. While this representation is schematically
accurate, physically the devices are now manufactured as a single component.
A transient-voltage-suppression diode can respond to over-voltages faster than
other common over-voltage protection components such as varistors or gas
discharge tubes. The actual clamping occurs in roughly one picosecond, but in
a practical circuit the inductance of the wires leading to the device imposes
a higher limit. This makes transient-voltage-suppression diodes
useful for protection against very fast and often damaging voltage transients.
These fast over-voltage transients are present on all distribution networks and
can be caused by either internal or external events, such as lightning or
motor arcing. Transient voltage suppressors will fail
if they are subjected to voltages or conditions beyond those that the
particular product was designed to accommodate. There are three key modes
in which the TVS will fail: short, open, and degraded device.
TVS diodes are sometimes referred to as transorbs or the Vishay General
Semiconductor registered trademark, TransZorbs.
Characterization A TVS diode is characterised by:
Leakage current: the amount of current conducted when voltage applied is below
the maximum reverse standoff voltage. Maximum reverse standoff voltage: the
voltage below which no significant conduction occurs.
Breakdown voltage: the voltage at which some specified and significant
conduction occurs. Clamping voltage: the voltage at which
the device will conduct its fully rated current.
Parasitic capacitance: The nonconducting diode behaves like a capacitor, which
can have a deleterious effect on high-speed signals. Lower capacitance is
generally preferred. Parasitic inductance: Because the actual
over voltage switching is so fast, the package inductance is the limiting
factor for response speed. Amount of energy it can absorb: Because
the transients are so brief, all of the energy is initially stored internally as
heat; a heat sink only affects the time to cool down afterward. Thus, a
high-energy TVS must be physically large. If this capacity is too small,
the over voltage will possibly destroy the device and leave the circuit
unprotected. See also
Trisil Zener diode
References Further reading
TVS/Zener Theory and Design Considerations; ON Semiconductor; 127
pages; 2005; HBD854/D. External links
"Tailored ESD Protection for various electronic interfaces", Infineon
Application Note AN248 What are TVS diodes, Semtech Application
Note SI96-01 TVS Diodes compared to other transient
voltage suppression technologies, application note
Transient Suppression Devices and Principles, Littelfuse Application Note
AN9768 Transil™ / Trisil™ Comparison, ST
application note AN574 Transient Protection Solutions: Transil™
diode versus Varistor, ST application note AN1826