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UC2844: UC2844 functional safety related information

Yunjing Wang1
Expert 4300 points
Part Number: UC2844
Other Parts Discussed in Thread: UC2842

Tool/software:

Hi team,

I am posting a public post related to below E2E, because customer need a public post to catch up this case.

https://e2e.ti.com/support/power-management-group/power-management---internal/f/power-management---internal-forum/1394681/uc2844-uc2844-fmeda-and-behavior-when-opto-coupler-broken-cause-open-loop/5335887#5335887 

Again, my customer is using UC2844 in their product which need to get SIL2 functional safety certification. (PPM ID:488187) Need some related information of UC2844 device to help customer pass the functional safety certification.

  1. Could you pls help generate a FEMA analysis file for UC2844?
  2. UC2844 datasheet is mentioned this device max duty cycle is 50% due to internal T-flip flop design, but do you have any test data that UC2844 Dmax is safety clamped at <50%?  Such as what kind of Vout, Iout, FB condition will make the device work with Dmax, and what is our Dmax test data/report at such conditions?
  3. Also about UC2842, what kind of internal design make UC2842 able to achieve 100% duty cycle? 
  4. If we use UC2842 in flyback, is UC2842 100% duty cycle able to mantain volt-sec balance in flyback?
  5. Base on UC2844 flyback design, when we work at full load condition and the opto-coupler is broken(open at primary side), what will UC2844 behave and how could we calculate Flyback Vout at this condition?

Thank you!

Yunjing

 

  • 0
    TI__Guru 74115 points

    Hi,

    1.

    Quality, reliability, and packaging data download | TI.com

    The above link is what we have at now. 

    2.

    Its max 50% duty cycle is from its operation mechanism. Yes, the devices are test at production to check their max duty cycle within the datasheet specs.

    The below block diagram (and also Fig 8-1) can help to know when Dmax is made. If you have problem to understand, please let me know - it looks to me the diagrams provide clear idea when the Damx can be made.

    3. 

    See Fig 8-1, again, let me know if you have problem to understand.

    4.

    Volt-second balance is achieved by the flyback snubber circuit not by the IC. Please explain what your understanding is to the volt-seconds balance in a flyback - your question seems to me you do not understand how a flyback volt-second is achieved. I think you need to review the flyback operation and particularly how the transformer is reset in a flyback.

    5.

    You need to provide your circuit to show how your opto-coupler is connected to help answer your questions.

  • 0 in reply to Hong Huang
    TI__Expert 4300 points

    Hi Hong,

    May I know if the internal T-flip flop is failure, how does UC2844 clamp Dmax to 50% at this condition? Could we help analysis this failure rate and failure mode of UC2844?

    Thanks!

    Yunjing

  • 0 in reply to Yunjing Wang1
    TI__Guru 74115 points

    Hi,

    If the internal T-flip flop fails then the ic fails. Just like other circuit, say if VREF fails the IC fails, so do other fictions.

    The reliability data is on ti.com where you can review. The reliability is for whole ic not individual functions.

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    Hi,

    ti.com has something like this below for reliability. This is for the IC all functions.