AN1826 Datasheet, PDF (1/5 Pages) STMicroelectronics – TRANSIENT PROTECTION SOLUTIONS

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
(ESD¹), electromagnetic part switching and industrial transients (EOS²), atmospheric effects (lightning).
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_cc
or bi-directional : ±V_cc), 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.
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 TVS³
- and ceramic components - Varistor for instance.
Figure 1 shows the electrical characteristics of a
clamping device, this curve has two areas.
The first one located between 0V axis and V_RM4 is
the normal operating zone; in this area the signal is
not be affected by the transient suppressor (low
leakage current and negligible intrinsic capacitance).
Fig. 1: Electrical characteristics of bi-directional
clamping devices.
I
IPP
The second one is the surge suppression zone in
which the surge is clamped at VCL⁵.
V_CL = V_BR + R_d × I _PP
V_BR6: knee voltage of the I/V curve
R_d: dynamical resistance.
I_PP: current value during surge.
AN1826/0104
IRM
V
VRM VCL
VBR
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