Do AC input optocouplers require an ESD protection?

Question

I intend on using a circuit I designed for detecting an AC input line for control purposes. One of the inputs will be accessed by third party users and a cable might be connected with a decent length(>3m). The optocoupler I intend on using(VOL628A) has a diode max reverse voltage of 6V and the intended circuit is shown in the following image:

Since the diodes are connected in antiparallel, I was wondering if I should protect them from ESD. If an ESD voltage level should occur, each diode should clamp and protect the other. I know that the diodes react pretty slowly when it comes to ESD and there is a possibility that they get destroyed, but I was also struggling to find a TVS diode with a clamping voltage bellow 6V.

Therefore my question on whether or not should I use a TVS diode. Maybe you could recommend an alternative solution if TVS wouldn't help in this case.

Answer 1

The forward current for the diodes in the opto is +/- 60 mA maximum. The series impedance is 4x 27 kΩ = 108 kΩ. With half the maximum rated current flowing from a threatening surge, the voltage across 108 kΩ is 3,240 volts. This is a scene setter.

For ESD, the event will be over so quickly that I doubt there would be any problem but, with an indirect lightning surge (as per EN61000-4-5) then you might find that a 4 kV surge (sourced from a 2 Ω source impedance) is long enough in duration to harm the resistors. This in turn might cause one to fail short circuit and then damage the opto.

In short, you need to pick the resistor types carefully and ensure they can survive the peak voltages associated with the surge. You won't really find that using a TVS across the opto input is going to help much. You might decide to absorb any residual energy that might get to the opto with a 10 nF capacitor across the input terminals. Do the math to see how big this value can be without upsetting 60 Hz operation.

Answer 2

Your D1 and D2 are slow diodes and will not react sufficiently fast for ESD speeds. You need TVS diodes for that.

But honestly, you probably don't need to take any special measures with such a ridiculously high series resistance, the system parasitic capacitance can probably absorb it/

You could add a small parallel capacitance to absorb it better if you want. At 60Hz your system is is so slow that you could calculate an effective capacitance for ESD that is still small enough not to affect your phase response.

The human body model uses 100pF for the capacitance of a human body. With your current resistors you could have 1nF capacitance, which is ten times that and still have a cutoff frequency of 1592Hz. Far higher than your line's 60Hz which minimizes phase delay for your zero-cross detection. That alone will cut the ESD voltage by 1/10th and that's not even considering the energy that your resistors will dissipate yet.