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Original question:

ISOW7842: VISO outputs a square wave at 250kHz, gets very hot, VISO is 1.965V

ISOW7841: High rate of part failures

Derek
Mastermind 9772 points
Part Number: ISOW7841

Hello, 

   Originally we saw one bad part but now it appears that we are finding bad parts right and left. Original thread is included, but that was for just one part. When we replace the part, the issue appears to go away.

Update:

We tested five boards. Each has two channels (named DP1 and DP2) of ISOW7841. The problem is that sometimes when we turn these on, we are not seeing 5V at the output. Test consisted of turning on the 5V to the isolator for one second, and then verifying that the voltage coming out was good. Then it turns off power for one second.

Unit 0: Original batch of boards, nothing changed on it at all. Ran 172k tests and passed. This is the only one of the original parts that worked completely.

Unit 1: This is the unit that had the square wave output, so I removed the ISOW7841, then measured current on the VISO load. It was 18mA, so no short there. Then (with the chip removed) I powered the isolated side from a bench power supply; everything worked ok. I put a new ISOW7841 part in, and ran the test. Ran 63k times and still running.

Unit 2: Failed after 30 tests on channel 1. VISO is 0.78V continuous. Removed isolator, verified that the isolated section current is normal and everything there works, and installed a new ISOW7841. The test stopped after 9400 tests because the second channel (with original chip) failed. So I restarted the test with only the updated chip. Still running, 8400 tests and counting.

Unit 3: Upgraded input capacitor from 47uF to 100uF. Failed test even once.

Unit 4: Upgraded input capacitor from 47uF to 100uF. Failed after 87 tests. Output is only 3.53V and the ISOW7841 is quite hot.

Units 5 & 6: ISOW7841 working but the TI GPIO expander is not responding over I2C. Other I2C peripherals on isolated side work fine.

See linked thread for additional information on other troubleshooting steps. We verified VCC rise time into the ISOW7841, tested varying levels of input capacitance, etc.

Attached image: top IC is the one I pulled from the board that did not work. (Lot code 93APZXT, CM purchased from Digikey a couple months ago) Legs are a little bent because I first tried lifting the legs while troubleshooting.

Bottom IC is brand new purchased from Digikey last week before I installed it. Lot code 88AGN5T.

This is now causing production and deliveries to stop so we really are looking forward to working on a solution.

  • 0
    TI__Genius 17100 points

    Hi Derek,

    Thank you for posting to E2E. For all of these tested units, were they all tested on the same PCB or were they each tested on their own board?

    Additionally, if the startup conditions are different than the ones described on the original post, can you describe the test? Is this an on/off test or is a specific startup waveform applied to Vcc?

    Lastly, what is the source powering the ISOW7841 devices?


    I understand some of this might already be answered in the original thread; please feel free to repeat information since this is a separate thread.


    Best,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Mastermind 9772 points

    Hello,

    1. Each is on its own different PCB, two to a PCB.

    2. Startup is the same as the original post. For this test we turn on power to the ISOW7841, wait 500mSec, test I2C, wait 500mSec, then turn off power for 1 sec. I can see on the scope that VISO completely drains between cycles. This cycle is repeated until we get to 100,000 cycles or there's a failure. But the original units didn't even get to 20 cycles.

    3. Input to the ISOW is a LMS3635 (5.0V @ 3.0A) and then goes through a AP22802 load switch (2A nominal). I've verified that rise time at input is about 5mSec.

    Today I swapped out the second channel's ISOW7841 and so far these boards with the new parts have been passing the test just fine. Again, the parts that we replaced them were with the 8xxxx date code. The parts that were not working were the 9xxxx date code.

    I have all parts if you wish to inspect them or put them on an evaluation board.

    Thanks, I know this is a tough one.

    Regards,

    Derek

  • 0 in reply to Derek
    Mastermind 9772 points

    Update: overnight we tested two boards that have both ISOW7841 isolators swapped out for ICs with the 8xxxx date code, and they are working great, having passed 60,000 cycles of testing.

  • 0 in reply to Derek
    Mastermind 9772 points

    Hello, is there any update on this? We've tested more units and we're seeing about 30% failure rate.

  • 0 in reply to Derek
    TI__Guru 55575 points

    Hi Derek,

    Sorry to hear that you are seeing even higher failure rate. Like I have mentioned earlier, the issue you are observing is very new for us. We haven't seen anybody or any of our tests show such an issue. This could be related to your system as a whole but to confirm that I think we need good discussion. Hence, can I reach out to you on your email which you have used to create your TI account?

    The reason why I do not want to suspect the device alone at first is because all the units that ship out of the factory get tested for operation and performance. This applies every individual unit that gets shipped. I of course do not want to ignore an issue at device level, but before I do that I would like to look at your system deeper to try understand the possible cause for the issue. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Mastermind 9772 points

    Yes, I look forward to your email.

    We thought that it would be a system level problem, yet replacing devices solves the problem.

    Thanks,

    Derek

  • 0 in reply to Derek
    TI__Guru 55575 points

    Hi,

    This discussion has moved to private e-mails to look deeper into system and debug the issue. Once we reach a resolution, will come back and post an update with a summary. Thanks.

    Regards,
    Koteshwar Rao