AN1702

Transient voltage suppression

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Transient voltage suppression

Transient voltage surges are generated generally by inductive load switching, for example,

the equipment starts and stops, lightning strikes, etc. The standard IEC 61000-4-5 Immunity

Test describes a method to test circuits with a waveform simulating these conditions (see

Figure 6).

Figure 6.

IEC 61000-4-5 transient voltage test V and I waveforms

Surge voltages inducted on both differential lines do not affect the entire system operation, if

the bus is isolated, and the ground voltage shift generated does not exceed the maximum

ratings. However, the use of additional transient voltage suppressors (TVS) is advisable, in

order to protect against surges in case of miswiring, incorrect grounding and other fault

conditions. ST offers a wide range of TVS. Visit the site:

http://eu.st.com/stonline/discretes/index.shtml. The selection criteria are the stand-off

voltage (VRM), the clamping voltage (VCL) and the peak power dissipation (PD). The stand-

off voltage must not exceed the operating common mode voltage range (-7 to +12 V), in

order to avoid clamping during the normal operation, while the clamping voltage must be low

enough to hold the line voltage with no damage to the protected circuits. The peak power

dissipation is the product of the clamping voltage and the peak current that can be managed

by the TVS without being harmed. One TVS suitable for a RS-485 network is the ST’s

SMA12CA. It has:

Equation 2

Figure 7.

Usage of the transient voltage suppressors in the RS-485 network

V

V

CL

19.9 V

=

20A;

=

P

D

400 W

=

B