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ISO7241C: basic electrical insulation?

Marin Babic
Prodigy 60 points
Part Number: ISO7241C
Other Parts Discussed in Thread: ISO7741, ISO7841

ISO7241C datasheet states:

1. "This coupler is suitable for basic electrical insulation only within the maximum operating ratings."

Viorm is 560 Vpk.

If  permanent votage across I/O isolation barrier is much less than 560 Vpk in application, e.g. 50 Vpk, can ISO7241C be declared as reinforced isolation component?

2. "Compliance with the safety ratings shall be ensured by means of suitable protective circuits."

Is there a reference design of such protctive circuit, e.g. to achieve safety limiting values?

  • 0
    TI__Genius 17100 points

    Hi Marin,

    Welcome to the E2E forums! Thank you for posting your questions.

    The ISO7241C can withstand 50Vpk over its lifetime since this is, as you mentioned, within the Viorm rating of 560Vpk.

    Basic and reinforced isolation however are physical capabilities of each device and their certified isolation ratings ( >1kV = operational, >2.5kV = basic, >5kV = reinforced). Since the ISO7241C is certified for 4kV peak, it will only provide basic isolation to the systems it is in.

    I've attached a design guide which can be found in the "Technical Documents" tab of the ISO7241C product folder. Where did you find the quotation "Compliance with the safety ratings shall be ensured by means of suitable protective circuits."? Methods to protect the IC include zener clamps on the supply pins and using a y-capacitor between ground planes, but they are not always necessary.

    TI has compatible isolators which can achieve higher isolation performance, like the ISO7741 and ISO7841. Is reinforced isolation a requirement for your system? Feel free to respond with your desired design specs and we can optimize the isolation subcircuit.

    I hope this response is helpful! Please let me know which additional expertise I can offer.


    Respectfully,
    Manuel Chavez

    Digital Isolator Design Guide - SLLA284b.pdf

  • 0 in reply to Manuel Chavez
    Prodigy 60 points

    Mr. Chavez,

    Thank you for response.

    1.

    Manuel Chavez said:
    Where did you find the quotation "Compliance with the safety ratings shall be ensured by means of suitable protective circuits."?

    is in datasheet rev. T, on page 11, note (2).

    2. Is it applicable to use miniature fuse on power supply and ground pins to ensure "safety input, output, or supply current" from datasheet point 7.8?

    Is current limiting resistor on signal input/output pins needed?

    3. In case both sides of  ISO7421C are floating (no connection to earth potential or other defined potential):

    Y-capacitor between ground planes is to reduce AC and transient stress on the isolation barrier?

    Is resistor (e.g.1 MOhm) between ground planes needed to reduce DC voltage stress on the isolation barrier?

    4. ISO7741 is also a suitable alternative.

    There is a requirement in our application for reinforced isolation for functional safety (not electrical hazad). On both sides of ISO7421C are microcontroller I/O pins.

    Concern is that in a case of single fault of isolation barrier, some other catastrophic single fault on one side of the barrier (e.g. short to higher power supply) will not destroy circuits on the other side of the barrier too.

    Under which condition is low resistance path between ISO7421C I/O pins possible?

    In ISO7241C datasheet there is "isolation capacitor lifetime projection". What is end-of-life criterion for capacitor?

    There are many questions above, answer to at least some would be beneficiary.

    Best regards,

  • 0 in reply to Marin Babic
    TI__Genius 17100 points

    Hi Mr. Babic,

    Thank you as well for the information provided. Below are my answers to your points:

    1. Some isolation certifications and safety ratings apply not only at the isolation device level, but also at a system-level. This is a disclaimer that although an ISO device may be used to achieve isolation specs, if the system system as a whole has shortcomings (ground planes are shorted, PCB is damaged, incorrect device is used considering altitude/packaging, etc.) the device's safety ratings will not apply.

    2. Yes! miniature fuses or self-resetting (PTC) fuses are a known safety mechanism to protect the Vcc input current from reaching damaging levels. They should be placed in series with the power supply pins and may not be needed on both the power and ground pins. Current limiting resistors are not needed on signal I/O pins due to the device's input and output configurations.

    3. Y-capacitors are helpful for preventing damage to devices due to fast transients. This does reduce stress significantly on the isolator device, but I am unsure of the scenario being described. By floating, do you mean there is no connection to the GND potentials at all or to each other? The ISO device may not function if its ground pins are disconnected from local grounds. A 1MOhm resistor is optional, but not necessary.

    4. For basic isolators like the ISO7241C, a single overvoltage event will damage the device if it exceeds the insulation ratings in section 7.6 of the datasheet, and the device could fail short if the surge is high enough. If using a reinforced isolator, like the ISO7741 or ISO7841, damage or shorts caused to one side of the isolation barrier will not affect circuits on the other side - they are protected. This is the exact purpose of our ISO devices! Conditions for which failures result in a short across the barrier for these devices is described in the document attached.

    Conserving the isolation barrier relies on protecting the device from most absolute maximum ratings violations. Within these conditions, the device's isolation barrier life depends on the working voltage (Viorm) it is continuously exposed to on a day-to-day basis. As Viorm decreases, the insulation capacitor's lifetime increases logarithmically.

    I hope to have been helpful with this response! Please let me know if elaboration on any points is desired or there are additional questions.



    Thank you for your time,

    Manuel Chavez

    Understanding Failure Modes in Isolators - slyy081a.pdf

  • 0 in reply to Manuel Chavez
    Prodigy 60 points

    Mr. Chavez,

    Thank you for the replies.

    we will certainly consider reinforced digital isolators in future designs.

    Just for completeness, in question 3 "floating" means that local ground potentials are isolated from earth potential, or any other common potential. ISO device pins are directly connected to local ground.

  • 0 in reply to Marin Babic
    TI__Genius 17100 points
    Marin,

    You're welcome! We are glad to help. For any future issues or questions please feel free to post again on the E2E forums, and that is true for any TI assistance not just with isolators.

    Thank you for also clarifying the floating case!


    Success on your projects,
    Manuel Chavez