Transient Voltage Suppression Diodes
Overvoltage Suppression Facts
Transient Threats – What Are Transients?
Voltage Transients are defined as short duration surges of
electrical energy and are the result of the sudden release
of energy previously stored or induced by other means,
such as heavy inductive loads or lightning. In electrical
or electronic circuits, this energy can be released in a
predictable manner via controlled switching actions, or
randomly induced into a circuit from external sources.
Repeatable transients are frequently caused by the
operation of motors, generators, or the switching of
reactive circuit components. Random transients, on the
other hand, are often caused by Lightning and Electrostatic
Discharge (ESD). Lightning and ESD generally occur
unpredictably, and may require elaborate monitoring to be
accurately measured, especially if induced at the circuit
board level. Numerous electronics standards groups have
analyzed transient voltage occurrences using accepted
monitoring or testing methods. The key characteristics of
several transients are shown in the table below.
VOLTAGE
Lighting
Switching
EMP
ESD
25kV
600V
1kV
15kV
CURRENT
20kA
500A
10A
30A
RISE-TIME DURATION
10 μs
50μs
20ns
<1ns
1ms
500ms
1ms
100ns
The exponential rise time of lightning is in the range
1.2μsec to 10μsec (essentially 10% to 90%) and the
duration is in the range of 50μsec to 1000μsec (50% of
peak values). ESD on the other hand, is a much shorter
duration event. The rise time has been characterized at less
than 1.0ns. The overall duration is approximately 100ns.
Why are Transients of Increasing Concern?
Component miniaturization has resulted in increased
sensitivity to electrical stresses. Microprocessors for
example, have structures and conductive paths which
are unable to handle high currents from ESD transients.
Such components operate at very low voltages, so
voltage disturbances must be controlled to prevent device
interruption and latent or catastrophic failures.
Sensitive microprocessors are prevelant today in a wide
range of devices. Everything from home appliances, such
as dishwashers, to industrial controls and even toys use
microprocessors to improve functionality and efficiency.
Most vehicles now also employ multiple electronic
systems to control the engine, climate, braking and, in
some cases, steering, traction and safety systems.
Many of the sub- or supporting components (such as
electric motors or accessories) within appliances and
automobiles present transient threats to the entire system.
Careful circuit design should not only factor environmental
scenarios but also the potential effects of these related
components. Table 2 below shows the vulnerability of
various component technologies.
Device Type
VMOS
MOSFET
GaAsFET
Vulnerability (volts)
30-1800
100-200
100-300
100
140-7000
250-3000
300-2500
380-7000
680-1000
Table 1. Examples of transient sources and magnitude
Characteristics of Transient Voltage Spikes
Transient voltage spikes generally exhibit a “double
exponential” wave, as shown below for lightning and ESD.
Vp
Vp/2
EPROM
JFET
CMOS
t1
t2
t
Schottky Diodes
Bipolar Transistors
SCR
Figure 1. Lightning Transient Waveform
100%
90%
Table 2: Range of device vulnerability.
Current (I) %
I30
I60
10%
30n
60n
tr = 0.7 to 1.0ns
Figure 2. ESD Test Waveform
2
©2008 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to http://www.littelfuse.com for current information.
LITTELFUSE [ LITTELFUSE ]
