AN1826 - APPLICATION NOTE

2/5

In this zone, the main property of the protection device is to have a clamping voltage as close as possible to

C

V

V

F

CL

BR

=

Fig. 3: Electrical characteristics measured on curve tracer.

V

I

Fig. 2: Electrical characteristics of unidirectional

clamping devices.

When the signal moving through the line to be

the surge suppressor has to be unidirectional,

otherwise the protection function is not optimized.

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

line is clamped to a typical forward drop voltage of

a diode.

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.

200

150

100

50

0

-50

-100

-150

-200

-20

-10

0

Varistor

Voltage (V)

10

20

200

150

100

50

0

-50

-100

-150

-200

-20

-10

0

Transil

Voltage (V)

10

20

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

And taking into account that the curve slope represents

1

R

d

, so for the same surge current the clamping

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.