AN1826
®
APPLICATION NOTE
TRANSIENT PROTECTION SOLUTIONS:
Transil™ diode versus Varistor
A. BREMOND / C. KAROUI
Since the seventies, electronic modules are more and more present in our life. This is the case for our
entertainment, automotive, telecommunication, production tools, house access equipment, and it is not
really a secret that these smart functions are very sensitive to transients coming from human body itself
1
), electromagnetic part switching and industrial transients (EOS2), atmospheric effects (lightning).
(ESD
These surges are more often lethal for the electronic functions and the constant decrease of silicon
elementary cell size (transistor) makes them more and more compact but more and more fragile.
To assume a correct definition of a protection stage, we have to ask the two following questions :
- what we have to protect?
- against what we have to be protected?
The answer to the first question depends on both the technology used to realize the part to be protected
and normal operating waveform of the signal on the line to be protected. Needed parameters are the
maximum voltage that technology can withstand, the type of normal operating signal (unidirectional : 0-V
or bi-directional : ±Vcc), the maximum and minimum voltages of this signal and its frequency. The answer to
the second one is closely linked to the environment where modules are operating. In each application we
have to focus on one type of disturbances, i.e. in telecom we are faced to atmospheric effects, in domestic
and industrial worlds, disturbances are EOS while in computer or mobile phones, the main surge cause is
due to ESD.
cc
To avoid dramatic consequences on systems submitted to over voltage transients, the best way is to use
protection devices based on clamping action.
In this kind of products we have the choice between two technologies :
- silicon devices named Transil™ - so called TVS
3
- and ceramic components - Varistor for instance.
Figure 1
clamping device, this curve has two areas.
The first one located between 0V axis and V
the normal operating zone; in this area the signal is
shows the electrical characteristics of a
4
RM
Fig. 1: Electrical characteristics of bi-directional
clamping devices.
I
I
PP
is
not be affected by the transient suppressor (low
leakage current and negligible intrinsic capacitance).
The second one is the surge suppression zone in
which the surge is clamped at VCL
VV RI
=+×
CL BR d PP
6
V
: knee voltage of the I/V curve
BR
: dynamical resistance.
R
d
: current value during surge.
I
PP
AN1826/0104
5
.
I
RM
RM
V
CL
V
BR
1/5
V
V
AN1826 - APPLICATION NOTE
In this zone, the main property of the protection device is to have a clamping voltage as close as possible to
the operating voltage. This property is defined by the clamping factor C
V
C
CL
=
F
V
BR
.
F
Fig. 2: Electrical characteristics of unidirectional
clamping devices.
When the signal moving through the line to be
protected is unidirectional (0 to +V
the surge suppressor has to be unidirectional,
I
I
PP
otherwise the protection function is not optimized.
Figure 2
unidirectional clamping device. In this case the
positive behavior of the device is still the same
while the negative one is totally different.
Negative over voltage occurring on the protected
I
RM
I
R
V
RM
V
V
BR
V
CL
line is clamped to a typical forward drop voltage of
a diode.
Figure 3
Transil™ diode and Varistor measured on curve
tracer. Both tested components have been chosen
in the same power and voltage (14V) ranges in
order to make an accurate comparison.
V
R
Fig. 3: Electrical characteristics measured on curve tracer.
200
150
100
50
cc
shows the electrical characteristics of a
gives the electrical characteristics of both
200
150
100
50
or -Vccto 0)
0
-50
Current (µA)
-100
-150
-200
-20 -10 0
Varistor
10 20
Voltage (V)
0
-50
Current (µA)
-100
-150
-200
-20 -10 0
Transil
10 20
Voltage (V)
The first remark we can do, regarding these curves, is that the Varistor operates uniquely in bi-directional
mode while the Transil™ diode can be both uni or bi-directional (here this is a unidirectional type). The
second point is the leakage current which is higher for the Varistor particularly close to the knee of the
curve. The third point to focus on is the slope of these characteristics in the clamping zone (vertical part of
the curve after V
And taking into account that the curve slope represents
), here one can easily see that the Transil™ diode slope is tighter than the Varistor one.
BR
1
, so for the same surge current the clamping
R
d
voltage will be lower for the Transil™ and then its clamping factor will be better.
To verify this point we performed measurements on both Transil™ and Varistor during EOS and ESD
surge test conditions. The EOS tests are based on IEC61000-4-5 1.2/50µs (8/20µs current) composed
waveform while ESD surges were performed in accordance with IEC61000-4-2 system standard.
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