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Application note inquiry

Danilo A.
Mastermind 28660 points

Hi Team,

Page 6 of the application note "Designing an LLC Resonant Half-Bridge Power Converter" states that the "The rectifier diodes are not softly commutated and reverse recovery losses exist". According to our customer, this is the reverse of what is generally accepted. Can you please clarify about this?

Regards,

Danilo

  • 0
    TI__Genius 16155 points

    Hi Danilo,

    When you operate above resonant frequency, secondary side rectifiers wont have zero current switching, so there will be reverse recovery.

    Below and at the resonant frequency, secondary side rectifiers do have ZCS switching.

    However, As move on left side of the gain plot curve (away from resonant frequency and towards left side of gain plot curve) of LLC, conduction losses will increase.

    So ideal point would be to operate the converter at resonant frequency for rated power. So, the design should be made that way to achieve high efficiency.

    Please refer the following screenshots for the current waveforms on both primary and secondary:

    Please let me know if you have any further questions.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    TI__Mastermind 28660 points

    Hi Manikanta, 

    Thank you for your response. Here is the feedback of our customer and his client.

    We believe you have a significant error in this ap note –

    Thanks Danadak, but they are wrong....if you run the attached LTspice sim you can see.

    Also attached is sec diode voltage when above resonance......no reverse recovery......when diode current goes off,

    the voltage across the diode was already zero...the reverse voltage did not build up across the sec diode until the

    diode current had gone to zero.......green is sec diode current...red is sec diode voltage..

    Reverse recovery, AYK, is when a diode that is conducting current, suddenly has a reverse voltage put across it...........as

    can be seen, this does not happen with LLC above resonance.

    Sim attached

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Genius 16155 points

    Hi Danilo,

    During above resonance, the current slew rate is very high (compared to other modes) just before current going to zero. 

    I believe this is what it causes the reverse recovery (reverse recovery is a function of current slew rate).

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    TI__Mastermind 28660 points

    Hi Manikanta,

    According to our customer,

    Sure looks like this still needs to be corrected –

     Sorry but they are wrong again.........i can see what they in fact meant to say...............they in fact are talking about a situation which happens

    when the dual sec windings of a CT output LLC are not well coupled....but they make no reference to that whatsoever....what they have

    actually written is complete and utter nonsense.

    It looks to me that some specialist, who maybe no longer works there, explained it to the writer, and the writer got it all mixed up.

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Genius 16155 points

    Hi Danillo,

    Here is the response from app note writer:

    above resonant frequency (refer fig 4c in the same app note), the current in the secondary side diodes are still zero yet but the diode voltage change the direction so to cause reverse recovery losses then the didoes not softly off but forced off.