Surging
Ideas
TVS Diode Application Note
PROTECTION PRODUCTS
What Are TVS Diodes?
TVS diodes are solid state pn junction devices specifi-
cally designed to protect sensitive semiconductors
from the damaging effects of transient voltages. TVS
diode schematic symbols are shown in Figure 1. The
electrical characteristics of the device are determined
by factors such as junction area, doping concentration,
and substrate resistivity. The surge power and surge
current capability of the TVS diode are proportional to
the junction area. TVS diodes are constructed with
large cross sectional area junctions for absorbing high
transient currents. While the VI characteristic curve of
the TVS diode is similar to that of a zener diode, TVS
diodes are specifically designed, characterized, and
tested for transient voltage suppression. By contrast,
zener diodes are designed and specified for voltage
regulation.
TVS diodes serve as parallel protection elements
(Figure 2). Under normal operating conditions, the TVS
diode presents a high impedance to the protected
circuit. Ideally, the device appears as an open circuit,
although a small amount of leakage current is present.
When the normal operating voltage of the protected
circuit is exceeded, the TVS diode junction avalanches
providing a low impedance path for the transient
current. As a result, the transient current is diverted
away from the protected components and shunted
through the TVS diode. The voltage across the pro-
tected circuit is limited to the clamping voltage of the
TVS diode. The device returns to a high impedance
state after the transient threat passes. TVS diodes will
not wear out nor will there be any degradation of the
electrical parameters as long as the device is operated
within specified limits.
A primary attribute of the TVS diode is its reaction
time. Avalanche breakdown theoretically occurs in
picoseconds. This is very difficult to measure however.
Therefore, TVS diodes are often specified as respond-
ing 聯(lián)almost instantaneously聰. The fast response time
of the TVS diode means that any voltage overshoot is
primarily due to lead inductance and PC board traces.
SI96-01
Unidirectional
Bidirectional
Figure 1 - TVS Diode Schematic Symbols
TVS diodes are available in a wide range of operating
voltages. Traditional device voltages range from 5V to
440V for discrete devices. Recent innovations in TVS
technology have yielded devices, such as Semtech聮s
SLV series, which operate at 2.8 and 3.3V.
The TVS diodes fast response time and low clamping
voltages make them ideal for use as board level
protectors for semiconductors and other sensitive
components. Applications include data and signal
lines, microprocessors & MOS memory, AC power lines,
and telecommunication equipment.
Z
in
TRANSIENT
ENVIRONMENT
Transient
Current
TVS
Diode
PROTECTED
CIRCUIT
Figure 2 - TVS Diode Operation
Revision 9/2000
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