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Figure 6. Distributed protection

DISTRIBUTED PROTECTION

The printed circuit board shown in Figure 5 represents a general case. In this board the input/output lines

are protected close to the connector and overvoltages are cancelled close to the disturbance sources. The

other lines to be protected are the power supply wires which carry 3 kinds of disturbances:

– The overvoltages resulting from mains perturbations.

– The surges coming from the other boards supplied by these lines.

– The disturbances generated on the board by the normal operation of the resident module, for example

the di/dt due to the fast switching of a buffer.

To suppress these surges we suggest a powerful Transil (1.5 KE for example) close to the power supply

input on the board, and some lower power devices (e.g. BZW04) distributed around the board area.

CONCLUSION

Due to the parasitic inductance of PCB tracks, a protection device chosen purely according to disturbance

standards does not assure immunity from surges. Carefully designed PCB layout plus correct device se-

lection from the STMicroelectronics range is essential to guarantee adequate protection.

DISTURBANCE

SOURCE

PRINTED BOARD

SENSITIVE PARTS