Si8652 Supply pins getting shorted in field use

[enggmayank]

Hi Guys,

I am using Si8652/Si8452 digital signal isolaters for isolating RS485 communication pins from MCU. MCU is driven by 3.3V logic whereas MAX485 is driven by 5V. Si8652/Si8452 is supplied respective power supplies for each side.

The device acts as a MODBUS Master, so I have provided fail safe biasing (560E - 120E - 560E).

In field use I have observed that Isolation side Supply and Ground pins on Si8652/Si8452 get damaged (low resistance or short), causing the communication to fail.
 
- The RS485 side of circuit is protected against transients using 2 x Littlefuse SMAJ12CA on A and B lines.
- Si8652 is powered via a regulated 5V source (LM7805) isolated via  MORNSUN B0512S Isolated DC/DC converter.
 
I are trying to replicate the scenarios:
1. DC Test: Directly connected 24VDC to RS485 inputs.
Result: TVS, PPTC protection kicks in and no damage is done to components.
 
2. ESD/EFT/Surge test: To be conducted at a test lab as i don't have required test equipment.
 
Appreciate if you can share your advice/knowledge on the same.
 
Regards