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ISOW7842: Insulation Lifetime in Pulsed Applications

GCummings
Prodigy 40 points
Part Number: ISOW7842

Projected insulation lifetime is shown in Figure 42 of the ISOW7842 datasheet but this information pertains to 60 Hz sinusoidal voltage.  I believe the underlying failure mode is charge injection and consequently insulation lifetimes for DC applications may be considerably longer than AC lifetimes.  However, for pulsed applications (i.e. isolation for MOSFET/IGBT gate drivers) I suspect that insulation lifetime may also depend on switching frequency as well as dV/dt.  If this is true it would seem insulation lifetime may be significantly reduced in some applications.  Application note SLYY063A makes reference to the maximum repetitive voltage (VIORM) and gives the example of gate control for a high-side IGBT in a motor drive system but does not mention switching frequency or rise/fall time.  Can you comment on how insulation lifetime can be determined for such applications based on peak voltage, rise/fall time, and frequency?

  • 0
    TI__Mastermind 49495 points

    Hi GCummings,

    Thank you for showing interest in ISOW7842 and for reaching out to us.

    I understand your question and the concern you have.I agree with your viewpoint that the isolators will mostly have higher DC lifetime compared to AC and the lifetime also depends on the switching frequency. We do have limited data related to pulsed applications (like motor drives) to share with you. Please allow me answer your question related to the three parameters separated - peak voltage, rise/fall time & frequency.

    Peak Voltage:
    The peak voltage that a TI isolator supports for its lifetime doesn't change with rise/fall times and frequency. For ISOW7842, this would be VIORM of 1414 Vpeak.

    Rise/Fall Time:
    Fast high voltage rise and fall times are termed common-mode transients which doesn't affect device lifetime but can affect its signal integrity performance provided the peak voltage ratings of device are not violated. The CMTI spec of 100V/µs (minimum) that is stated in ISOW7842 datasheet indicates how fast can the common-mode transient rise/fall without affecting the signal output. Any signal faster than this but within peak voltage limits can data error in output but doesn't affect device lifetime.

    Frequency:
    We have tested our isolators for higher switching frequencies to emulate the situation in motor drive & inverter applications. Our testing indicates that there is no significant impact on TI isolators lifetime when the switching frequency of HV is increased. We did also try testing a few other competition isolation technology and those lifetimes were significantly affected. Please find below the TI isolation technology TDDB lifetime data for frequencies upto 10kHz. Unfortunately, we don't have data for higher frequencies than 10kHz.
    Please do note that the below data is for the TI capacitive isolation technology while the device ISOW7842 also incorporates a power transformer which is not characterized for high frequency.

    I hope this answers your questions, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 40 points

    Hello Koteshwar,

    Thank you for the detailed reply.  Because we do require an isolator with integrated power supply and the ISOW7842 power transformer has not been characterized for high frequency can you provide additional details regarding insulation between the primary and secondary windings? The datasheet indicates that minimum DTI (Distance Through Insulation) for the transformer is 120 um as compared to only 21 um for capacitive signal isolation.  Sections 5 and 9.1 indicate that the transformer insulation barrier is thin film polymer.  Would this be polyimide?  Would you be able to perform supplemental testing for a representative device with integrated power transformer to determine TDDB lifetime over a range of frequencies up to 10 kHz?

    Thanks,

    Gerald

  • 0 in reply to GCummings
    TI__Mastermind 49495 points

    Hi Gerald,

    Apologies for the delayed response.

    The isolation ratings quoted in ISOW7842 datasheet are also applicable to the transformer, the ratings stated in insulation specifications table apply to the overall device.
    Please allow me to come back to you on the question related to thin film polymer of the transformer insulation barrier.
    I am afraid I won't be able to make any commitments on getting this data for ISOW7842, this testing requires assistance from other development teams outside our product group and hence can't commit to you on this. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Mastermind 49495 points

    Hi Gerald,

    Regarding your question on the transformer insulation barrier, the insulation material is not polyimide. This is a custom material that TI uses to achieve the required isolation performance that is comparable to TI capacitive signal isolation levels.

    Let me know if you have any other questions, thank you.

    Regards,
    Koteshwar Rao