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SN6505B: How small can you go with this design?

Peter Berben
Prodigy 120 points
Part Number: SN6505B

Hi,

I'm trying to match the SN6505B push-pull driver with either HALO TGM-050P3RL or PCA EPC3117S-2 isolation transformer.

I understand from the SN6505B datasheet that Vt product is an import factor in the transformer selection.

For the SN6505B I calculated Vtmin = VIN/(2x Fmin) = 5V(2x363KHz) = 6.8VµS

From the the transformer datasheets:

TGM-050P3RL = 5.5Vµs min > is no match because must be greater > but it works at room temperature > but saturates above 45°C

EPC3117S-2 = 11Vµs typical > is a match because is higher then required > but have many issues because transformers shorts between both secondary coils immediately or after some time.

So question is what am I overlooking in this design?

PS. In case you are wondering how secondary side is constructed I can referring to Figure 42 in the datasheet but using Vin = 5V, schottky's PMEG2005 and LDO = LT1761IS5-3.3

  • 0
    TI__Genius 17100 points

    Hi Peter,

    Thank you for including part numbers in your post! The voltage x time product you have calculated is correct for 5V operation of the SN6505B and 0% power supply tolerance. The EPC3117S-2 V-t product is adequate, but the secondary current may be experiencing thermal runaway due to the Schottky diodes being used.

    Some Schottky diodes have high reverse leakage currents at higher temperatures, so this is an important consideration as mentioned in section 9.2.2.3 of the SN6505B datasheet. Please try replacing the diodes with low leakage diodes such as the RB168MM-40.


    Please let me know if this is helpful! I will await your response.


    Respectfully,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 120 points
    Hi Manuel,

    Thank you for the fast reply and probably well spotted mistake.

    Since I'm using a SOD-323 style diode I'm going to order the NSR0340HT1G from OnSemi.
    Could you confirm this is a suitable alternative?

    Will keep you informed,
    Peter Berben
  • 0 in reply to Peter Berben
    TI__Genius 17100 points
    Peter,

    No worries! These diodes are acceptable.
    For how long will this system be exposed to high temperatures?


    Thank you for your time,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    TI__Genius 17100 points

    Peter,

    I also noticed that the transformer is designed to be used with a different component, and although specifications seem compatible there may be nuances that are affecting proper operation. When ordering the new diodes, please also consider a transformer made for the SN6505B such as the EPC3668G-LF, depending on load requirements.


    Respectfully,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 120 points

    Hi Manuel,

    The average environmental temperature is approximately 45°C.  Also up to 16 isolated channels are packed together making it even more challenging.

    As for the alternative transformer. I'm aware, but very much limited by the allowed transformer build height, hence the title.

    You could also say the referenced driver for the low profile transformer has very similar driving specs as SN6505B but missing the spread spectrum feature.

    Load requirement are very modest. The LDO will also limit any overcurrent.

    Any other concerns or things to check related to the targeted transformer?

  • 0 in reply to Peter Berben
    TI__Genius 17100 points
    Peter,

    Thanks for the clarity. I've confirmed with PCA that this transformer is compatible with the SN6505B, so replacing these Schottky diodes should prevent secondary side shorts. How are the the 16 data channels being isolated?


    Respectfully,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    Prodigy 120 points

    Hi Manuel,

    Received the alternative diodes today and already modified one board.

    I will keep it running to verify, but in order to confirm I will have to build many more.

    This will be a task for the coming days and as soon as I have received additional transformers.

    Then if none of them get damaged I will be able to confirm the diode to be the root cause. Fingers crossed.

  • 0 in reply to Peter Berben
    TI__Genius 17100 points
    Peter,

    Thank you for this update; I am glad to hear the parts are coming in and one system is running. Please let me know the results of testing and if new concerns arise.


    Best,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    TI__Genius 17100 points
    Hi Peter,

    Hopefully all is well. Is there any update to the SN6505B testing with new diodes and transformer?

    I will await your response.


    Thank you, and have a great weekend!
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    Prodigy 120 points

    Hi Manuel,

    All the circuits have been modified with the alternative diodes (low reverse current) + broken transformers have been replaced.

    It's too soon to conclude but since the update two days ago I did not have any additional failures. 

    Sounds like the diodes are indeed the root cause.

    Climate room will confirm.

  • 0 in reply to Peter Berben
    TI__Genius 17100 points
    Peter,

    This is good to hear! Please take all the necessary efforts to confirm this solution works 100%. Can you please follow up with me, maybe Wednesday or Friday next week for completeness?


    Thank you,
    Manuel Chavez
  • 0 in reply to Manuel Chavez
    TI__Genius 17100 points
    Hi Peter,

    Hopefully all is well! I am looking forward to an update on the SN6505 testing with new transformers and diodes. Please let me know!


    Thank you for your time,
    Manuel Chavez
  • 0 in reply to Peter Berben
    Prodigy 120 points

    Hi Manuel,

    Climate room confirmed the circuit to be stable.

    Original diodes can be confirmed to be the root cause.

    Diodes with low reverse leakage current are a must have.

  • 0 in reply to Peter Berben
    TI__Genius 17100 points
    Peter,

    I'm glad to hear it! Thank you for working on this solution and for choosing TI parts!

    Best wishes on your future projects.


    Respectfully,
    Manuel Chavez