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ISO3082: Capacitor between GND1 and GND2

Part Number: ISO3082
Other Parts Discussed in Thread: ISO1410, STRIKE

Hi Team,

ISO3082 is used, Vcc1 is always powered, the right side of ISO3082 is exposed to the outside with a connector and is connected with the other side through cable. That is to say, Vcc2 is not powered and GND2 is float when the other side is not connected through cable.

ESD diodes are connected between A/B and GND2 as shown in the picture below,ISO3082 is broken when there is 8kV air gap discharge near pin A and B when the other side is not connected.

However, when the ESD diodes are connected between A/B and GND1 as shown in the picture below,ISO3082 will not be broken when there is 8kV air gap discharge near pin A and B when the other side is not connected.

It's found that if a capacitor is connected between GND1 and GND2, ISO3082 will not be broken with either connection above. So my questions are:

1. Is it safe to connect a capacitor between GND1 and GND2? If not, any possible risks?

2. Any suggestion for the ESD protection when Vcc2 is not powered and GND2 is float?

Thanks and Best Regards!

Hao

  • Hi Hao,

    1. You can connect a capacitor between GND1 and GND2 but the capacitor needs to be rated for the isolation that the customer cares about. An example would be this capacitor with 3kV DC rating

    2. I currently am not aware of a way to keep a device protected when both Vcc2 and GND2 is unconnected. However, the ISO1410 has stronger ESD performance. It is a newer device that is pin-to-pin compatible. Would the customer be willing to try this?

    Respectfully,

    Lucas

  • The ESD diodes can shunt the current to GND2, but if GND2 is open, the energy cannot go anywhere else. So I doubt that the ISO1410 can do better in this situation.

    Is there a decoupling capacitor between GND2 and VCC2, even when the cable is disconnected?

  • Hi Lucas,

    I agree with Clemens, ISO1410B can save the ESD diodes, but may not be able to solve the issue without adding a capacitor between GND1 and GND2. If there is a decoupling capacitor between GND2 and Vcc2, can the problem be solved?

    Thanks and Best Regards!

    Hao

  • If both GND2 and VCC2 are floating, then a capacitor between them will probably not help.

  • Hi Hao, Clemens,

    Please allow me to respond since Lucas is out of the office.

    As mentioned above, Vcc2 and GND2 are floating when the device is unpowered, and there is no path for the ESD discharge to take other than the local circuit, possibly damaging nearby components -- ISO3082 in this case.

    Decoupling capacitors should definitely be used between Vcc2 and GND2 for device operation although they might not affect ESD air discharge performance; adding a capacitor between GND2 and GND1 is helpful because it creates an additional path for the discharge.

    As Lucas said, a capacitor between GND1 and GND2 can be used as long as it does not compromise the isolation barrier, and for best ESD performance during normal operation, ISO1410 is recommended. Please allow us until next week to respond with additional suggestions for ESD protection.


    Thank you,
    Manuel Chavez

  • Hi Hao,

    The ISO14xx does have stronger barrier strength and would be expected to handle this sort of ESD strike better than the ISO3082. We spec 8kV ESD contact discharge across the barrier for the ISO1410  where the ISO3082 isn't rated at all for this. I recommend trying the ISO1410 if possible.

    Additionally, the capacitor placed across GND1 and GND2 would indeed shunt a portion of the energy from the ESD strike from side to the other. This is why the ISO3082 survives your ESD testing when the capacitor is present. This often referred to as a Y capacitor and is utilized to improve EMC performance. 

    Respectfully,

    Lucas