optocoupler input with a resistor and reverse voltage protection diode and ESD

[quinn]

optocoupler design esd

I have a optocoupler input with a resistor and reverse voltage protection diode.
See basic circuit attached.

The opto is a ACPL-217-500E device from Avago and is rated for 3000Vrms isolation and 10kV/us (CMR).

My questions are as follows:

1: How does the CMR rating actually relate to ESD? (ESD tests are ns while CMR is us)

2: The product is battery powered and in a plastic enlcosure so i dont have a connection to earth for the opto input (normally use a high voltage cap) so i assume a static charge will build up in the ciruit during esd testing?

2: What more can i do to the opto input circuit to ensure it passes +/-4kV contact ESD test?.

Any suggestions/help would be greatly appreciated.

 

[FvM]

Re: optocoupler esd

My first point is to consider, if the optocoupler connected circuit is actually directly exposed to ESD. That's not obvious to my opinion.

If this can't be avoided by design, it would be meaningful to determine the isolation breakdown voltage of the optocoupler with ESD. It's likely to be much higher than specified maximum ratings but possibly not sufficient. If not breakdown is experienced with ESD voltages, I would expect that it can resist ESD without damage. The transient immunity is very likely insufficient, so false signals from ESD must be expected. That doesn't harm, if no permanent operation fault is caused by the event. If your design requires to avoid any false signals with ESD, the respective circuit has to be protected from direct ESD influence.

Finally, if an isolation breakdown must be expected, an external spark gap, e. g. an air gap designed as a milled small PCB slit is probably the most easy solution.

 

[hakeen]

Re: optocoupler esd

Hi Quinn,

Optocouplers are level triggered devices, therefore fast enough but comparatively slow devices What I mean is a transient is very quick for an optocoupler (especially for ACPL-217-500E). It will not be seen by the component.

Just to ensure (in case ESD transeint might be very high) you can also use some TVS in your input.

Cheers