AN9768 Datasheet, PDF (7/8 Pages) Littelfuse – Transient Suppression Devices and Principles

Application Note 9768
The zener diode leakage is about 100 times higher at 5V
than the varistor, 200µA vs less than 2µA, in this example.
100µA PER VERTICAL DIV.
1V PER HORIZONTAL DIV.
P6KE 6.8
V8ZA2
FIGURE 8. CHARACTERISTIC OF ZENER P6KE 6.8 vs
LITTELFUSE VARISTOR V8ZA2
The leakage current of a zener can be reduced by specifying
a higher voltage device.
“Aging”
It has been stated that a varistor's V-I characteristic changes
every time high surge current or energy is subjected to it.
That is not the case.
As illustrated in Figure 9, the V-I characteristic initially
changed on some of the devices, but returned to within a few
percent of its original value after applying a second or third
pulse. To be conservative, peak pulse limits have been
established on data sheets. In many cases, these limits have
been exceeded many fold without harm to the device. This
does not mean that established limits should be exceeded,
but rather, viewed in perspective of the definition of a failed
device. A “failed” varistor device shows a ±10% change of
the V-I characteristic at the 1mA point.
Failure Mode
Varistors subjected to energy levels beyond specified ratings
may be damaged. Varistors fail in the short circuit mode.
Subjected to high enough energy, however, they may
physically rupture or explode, resulting in an open circuit
condition. These types of failures are quite rare for properly
selected devices because of the large peak pulse
capabilities inherent in varistors.
Zeners can fail either short or open. If the die is connected
by a wire, it can act as a fuse, disconnecting the device and
resulting in an Open circuit. Designers must analyze which
failure mode, open or short, is preferred for their circuits.
When a device fails during a transient, a short is preferred,
as it will provide a current path bypassing and will continue
to protect the sensitive components. On the other hand, if a
device fails open during a transient, the remaining energy
ends up in the sensitive components that were supposed to
be protected.
Another consideration is a hybrid approach, making use of
the best features of both types of transient suppressors (See
Figure 10).
INPUT
VARISTOR
R
ZENER
INPUT
VARISTOR
L
ZENER
FIGURE 10. HYBRID PROTECTION USING VARISTORS,
ZENERS, R AND L
8 x 20µs WAVE V31CP20
45
44
43
42
41
40
39
38
37
36
35
34
33
32
31
30
01
2
3
45
6
7
8
9 10
NUMBER OF PULSES
FIGURE 9. 250A PULSE WITHSTAND CAPABILITIES
Capacitance
Depending on the application, transient suppressor
capacitance can be a very desirable or undesirable feature.
Varistors in comparison to zener diodes have a higher
capacitance. In DC circuits capacitance is desirable, the
larger the better. Decoupling capacitors are used on IC
supply voltage pins and can in many cases be replaced by
varistors, providing both the decoupling and transient
voltage clamping functions.
The same is true for filter connectors where the varistor can
perform the dual functions of providing both filtering and
transient suppression.
There are circuits however, where capacitance is less
desirable, such as high frequency digital or some analog
circuits.
10-108