VIPER17H in flyback converter

Discussion in 'General Electronics Chat' started by firend, Sep 30, 2014.

  1. firend

    Thread Starter Member

    Feb 18, 2012
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    0
    Hi everybody!
    I have a problem. I use IC viper17H in flyback converter. schematic in attached file below. My schematic is same the application note an2753 of STmicrochip. My problem is D103, which is marked red in schematic. if I use diode 1N4148, my circuit will run. If I use another diode as: STTH2R06U, GF1M...I realise that diode 4148 is small signal, switching, high speed, trr =4ns. diode STTH2R6U has trr =50ns. Can anybody explain for this problem?
    Thank in advance.
     
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  2. SgtWookie

    Expert

    Jul 17, 2007
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    Looks like you've answered your own question; the circuit requires an extremely low trr there. You might also consider using a Schottky diode like a 1n5819 or other that has a suitable voltage rating. Schottkys diodes have essentially zero trr.
     
    Last edited: Oct 1, 2014
  3. firend

    Thread Starter Member

    Feb 18, 2012
    44
    0
    Hi SgtWookie.
    in datasheet of Schottkys diodes, I don't find trr parameter, so I don't know trr of Schottkys diode. I wonder the different 4ns and 50ns of STTH2R06 is negligible. Why doesn't my circuit run?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    It's obviously NOT negligible if it causes the circuit to not run! That extra 45nS is long enough for the charge to be bled off through the diode; it's almost like you had a plain piece of wire there. There isn't a Trr spec in Schottky datasheets because there IS no recovery! They simply won't let current pass thru in reverse bias until the Vr is exceeded and breakdown occurs, destroying the diode.
     
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  5. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    A Schottky diode is basically a majority carrier device, so does not have the recombination (recovery) time of minority carriers (as occurs in a junction diode) to delay the turn off time.
     
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