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Phase shift full bridge SMPS problems?

Discussion in 'General Electronics Discussion' started by eem2am, Feb 15, 2014.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    Hello
    Please can you confirm that the Phase shift full bridge SMPS has many characteristics that have little to be desired? (when compared to a "plain" full bridge converter)

    Our SMPS spec is...
    Vin = 18 to 32V (28V nominal)
    Vout = 34v
    Iout = 25A maximum


    I declare (in the way of a question) that the Phase shift full bridge smps has worse secondary sync fet snubber losses than a "plain" full bridge, because of the enhanced leakage inductance, which rings with the output capacitance of the secondary sync fets which needs snubbing....this is a serious disadvantage of the phase shift full bridge, since the amount of snubbing required will vary with the leakage inductance, which is generally a loosely toleranced thing....so one tends to have to use overly big snubbers at the secondary side.....surely its best to just do a "Plain" full bridge, and design it to get the minimum amount of leakage inductance in the transformer?

    Page 20 of the LTC1922-1 phase shift full bridge controller datasheet confirms the high secondary snubber losses of the phase shift full bridge topology.

    Also, it's known that even small amounts of leakage inductance in a "Plain" full bridge transformer mean that the actual duty cycle tends to be far higher than calculated. The phase shift full bridge converter does nothing to alleviate this problem.

    Also, the phase shift full bridge converter, if designed with enough leakage inductance, will get Zero voltage switch_on of the primary fets...however, it does nothing to reduce fet switch-off losses....and switch-off switching losses are far worse than switch-on switching losses.

    So really, especially with the increased secondary snubber losses that the phase shift full bridge converter brings, why would anyone want to do a phase shift full bridge converter rather than a "plain" full bridge converter?

    Also, do you agree with my above points?
    (the datasheets and research notes on this say nothing of the disadvantages of the phase shift full bridge topology)


    LTC1922-1 phase shift full bridge pwm controller....
    http://cds.linear.com/docs/en/datasheet/1922f.pdf
     
    Last edited: Feb 15, 2014
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Where did you copy that from wiki?
     
  3. eem2am

    eem2am

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    Aug 3, 2009
    Not even wiki tells of this.....it is not possible to find a single article or book telling of any of the drawbacks of the phase shift full bridge smps. And every app note one reads tells that the phase shift full bridge smps is the answer to "all of the worlds problems"......however, as we suspect, the phase shift full bridge smps is a bogus topology....promising all, and delivering very very little...do you agree?
     
  4. davenn

    davenn Moderator

    13,866
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    Sep 5, 2009
    Who is WE, did you mean I ?

    so why do YOU that it's bogus ?
    You are the one making these claims, not us
    what is your proof, what are your reasons ?

    Dave
     
  5. eem2am

    eem2am

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    Aug 3, 2009
    The proof is that the phase shift full bridge has a bigger transformer leakage term...this means more ringing, and more need for snubbing so that the secondary side sync fets and diodes don't get overvoltaged...thus more snubber losses.

    Also the higher leakage term means even more turn on delay to the power current, and thus bigger than expected duty cycle...never a desired thing.
     
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Look up EMC issues and noise. I think you will find they are better.
     
  7. eem2am

    eem2am

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    Aug 3, 2009
    ..I agree with this...however, would you agree that the secondary snubber losses are greater than for a simple full bridge, and also that the duty cycle needs to be greater (than a simple full bridge) due to the "extra" leakage term slowing up the rise of the primary current?
     
    Last edited: Feb 22, 2014
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yes and no
     
  9. eem2am

    eem2am

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    Aug 3, 2009
    Yes that the secondary snubber losses are greater in PSFB and no that the duty cycle doesn't have to be greater than a plain full bridge?

    This centres on the bogus use of resonant converters in applications that don't warrant it....the said resonant converters having bogus "advantages".
    Are resonant converters the subject of "design for CV" rather than "design for the application"?...I often think so
     
    Last edited: Feb 22, 2014
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