I'm studying the ISO1500 for an RS485 application. In particular, I'm looking for a TVS diode for bus protection. I've found that a common TVS diode used for RS-485 applications is the SM712 (like Bourns or Littlefuse) because it has asymmetrical reverse stand-off voltages to match the common-mode operating range of RS485 (-7V to 12V).
According to IEC 61000-4-5, for Class 2, a 1kV surge voltage will discharge through a resistor of 42\$\Omega\$, producing a current of 24A (as specified here).
I have a doubt regarding how the SM712 can provide surge protection. In SM712 datasheet it seems that the max clamping current is lower than 24A. Moreover, It seems that the clamping voltage is higher than the absolute maximum voltage on the bus pins (±18V from ISO1500 datasheet).
In this case the device will be unprotected by the surge pulse?
Best Answer
Who says it's a class 2 device? Who says class 2 is 2 kV?
EN 61000-4-5 says this about the voltage impulse: -
So a level 1 device would survive a 500 volt surge and this translates to a surge current of about 11.9 amps. A level 2 device would survive a surge current of 23.8 amps.
The Bourns CDSOT23-SM712 suggests that the limiting voltage will be 14 volts with a surge of 17 amps. You might say that this will still destroy the ISO1500 and so it might but, the ISO1500 has internal protection that can survive an ESD of 4 kV so, it is up to the designer to merge the indirect lightning protection offered by the TVS diode to the levels that are compatible with the ISO1500.
Nobody says it's easy to do.