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ISO7721: what's the influence of barrier capacitance and isolation resistance?

Howard Zou
Genius 12365 points
Part Number: ISO7721

Hi,

could you please help me understand what influence these 2 parameters will have for isolation device?

I have searched around the internet but couldn't find explanation.

My customer wonders if Barrier capacitance in 6.6 insulation specification in ISO7721's datasheet will be influenced by the temperature like isolation resistance.

  • 0
    TI__Mastermind 49465 points

    Hi Howard,

    Did you mean to ask if isolation capacitance and isolation resistance are going to change with external factors like temperature?

    Yes, external factors like temperature can marginally influence the isolation capacitance and isolation resistance values. These can also be different from one device to another due to manufacturing but due to the very tight control in factory on the isolation capacitance value, these variations are very well controlled. Please also do note that the values that are quoted in Insulation Specification section 6.6 of ISO7721 datasheet are the worst-case values with very high margin. We do not expect any devices to have any performance lower than section 6.6 under all stated datasheet operating conditions.

    Hope that answers your questions, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Genius 12365 points

    Koteshwar,

    thanks.

    I know we are giving the worst case. 

    For Isolation resistance, from the datasheet we know that it will decrease at high temperature.

    How about the barrier capacitance? Will it decrease or increase along with the temperature increase?

    Besides, I would like to know what can I get from these two parameters. For example, does larger barrier capacitance means larger CMTI? I don't know what these 2 parameters will actually influence the device's behavior in a real system.

  • 0 in reply to Howard Zou
    TI__Mastermind 49465 points

    Hi Howard,

    Thanks for clarifying your question further. I do not have the information on whether the barrier capacitance will increase or decrease with temperature but do note that the dielectric used in isolation capacitor of ISO7721 is SiO2 unlike the commercial capacitors that are available as passive components. Due to high stability of SiO2 with temperature & moisture and since they can be precisely manufactured with high quality, the capacitance variation remains negligible.

    Isolation capacitance and resistance are isolation components and hence they do not directly impact device electrical and switching characteristics. High isolation resistance (RIO) indicates that isolation barrier is strong, it is measure to know if the isolation barrier is intact. RIO is one of the preliminary term used to find out if isolation still exists. One can use high resistance meter to read RIO of ISO7721 and find out if the isolation of device is intact or broken. Apart from this, there shouldn't be any other significance of this parameter.

    In regards to isolation capacitance, the capacitance value by itself shouldn't be of any relevance to high voltage isolation. The thickness of capacitor (referred to as DTI for isolators) and its dielectric strength are important and these parameters define the high voltage performance of isolator. The overall DTI (distance through insulatino) for ISO7721 is >21µm and this enables the device meet all the isolation voltages given in section 6.6 of datasheet.
    The capacitance value is used by internal modulation/demoulation circuits to transfer data through isolation barrier. The exact capacitor value is again not very important as the circuits can be designed to support whatever the capacitor value is.

    If you would like to know more about the isolator circuit architecture, you could refer to the below useful design guide. Thanks.

    Regards,
    Koteshwar Rao