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low Vf diode

Discussion in 'Electronic Design' started by RHRRC, Mar 14, 2006.

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  1. RHRRC

    RHRRC Guest

    I am looking for a very low Vf diode for a low power flyback.
    The switching frequency is low - max 30K dow to 7K, the peak current is
    low (200mA absolute max), the average current only in the order of
    10mA, but the system efficiency is of paramount importance. With a load
    voltage of just 1.1V the diode Vf becomes a very significant item.
    Worst case (fault) diode reverse volts is 11.5V

    Unfortunately a synchronous solution is not an option.


    Any suggestions gratefully appreciated.

    Regards
     
  2. Genome

    Genome Guest

    Consider how you might modify your converter to allow for synchronous
    rectification.

    DNA
     
  3. Tom Bruhns

    Tom Bruhns Guest

    Forward drop isn't the only consideration for peak efficiency. I
    recall doing some careful work to select the best diode for a
    particular converter, and it wasn't clear from the forward drop alone
    which would be the winner.

    That said, have a look at the available Schottky rectifier diodes.
    Larger-area (higher-current) and lower reverse rated diodes stand the
    best chance of having low forward drop, BUT at the expense of higher
    reverse current. Looks like you should be able to get down into the
    0.25-0.3V region without going to synchronous. Maybe heat the diode to
    get the drop lower??

    Cheers,
    Tom
     
  4. Lord, you MUST be an old phart. We used to use that trick with the first
    germanium transistors to get a little more poo out of them. We figured we
    were STILL saving 90% of the filament power {;-)

    Jim




    Maybe heat the diode to
     
  5. Here is one that would drop about .25 volts at the 200 mA peak current.

    http://www.irf.com/product-info/datasheets/data/10bq15.pdf
     
  6. John Popelish wrote...
    That's a nice part. Large-area Schottky diodes have lower forward
    drop as Tom suggested, and the high leakage-current penalty may not
    be so bad, e.g. 40uA at -3V, 25C, and 350uA at 11.5V, 50C for this
    part. But using a large-area diode has another hidden gotcha for
    low-power circuits: high capacitance. This part approaches 500pF
    at 0V bias, creating additional losses on each switching cycle.

    I'm not sure why RHRRC rules out an active diode, it's not a very
    difficult matter to create a small bias supply to run the MOSFET.
     
  7. legg

    legg Guest

    There's more than one kind of synchronous rectifier.

    You might re-examine bipolar options at this voltage and current
    level. As long as base current ends up in the load, it can be quite
    efficient, and simple, at moderate frequencies.

    It must indeed be a seriously energy-starved application if
    rectification efficiency at 11mW of load power is critical. If this
    isn't just an illusion, I'd be tempted to get my energy some other
    way.

    RL
     
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