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Protection Scheme 1
As described earlier, the EFT and ESD transient have similar energy
levels, while the surge waveform has energy levels three to four
magnitudes greater. Protecting against ESD and EFT is accomplished in
a similar manner, but protecting against high levels or surge requires
more complex solutions. The first solution described here protects up
to Level 4 ESD and EFT and Level 2 surge. The 1.2/50 µs waveform is
used in all surge testing described in this article.
This solution uses the Bourns CDSOT23-SM712 transient voltage
suppressor (TVS) array, which consists of two bidirectional TVS diodes
optimized to protect RS-485 systems with minimal overstress while
allowing the full range of RS-485 signal and common-mode excursions
(–7 V to +12 V) on the RS-485 transceiver. Table 1 shows the voltage
levels protected against for ESD, EFT, and surge transients.
Table 1. Solution 1 Protection Levels
ESD (-4-2) EFT (-4-4) Surge (-4-5) Level Voltage
(Contact/Air) Level Voltage Level Voltage 4 8 kV/15 kV 4 2 kV 2 1 kV
A TVS is a silicon based device. Under normal operating conditions,
the TVS has high impedance to ground; ideally, it is an open circuit.
The protection is accomplished by clamping the overvoltage from a
transient to a voltage limit. This is done by the low impedance
avalanche breakdown of a PN junction. When a transient voltage larger
than the breakdown voltage of the TVS is generated, the TVS clamps the
transient to a predetermined level that is less than the breakdown
voltage of the devices that it is protecting. The transients are
clamped instantaneously (<1 ns), and the transient current is diverted
away from the protected device to ground.
It is important to ensure that the breakdown voltage of the TVS is
outside the normal operating range of the pins protected. The unique
feature of the CDSOT23-SM712 is that it has asymmetrical breakdown
voltages of +13.3 V and –7.5 V to match the transceiver common-mode
range of +12 V to –7 V, therefore providing optimum protection while
minimizing overvoltage stresses on the ADM3485E RS-485 transceiver.
This is just for RS485.