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What is the difference between Schottky and Fast Recovery Diode?

Discussion in 'Electronic Components' started by [email protected], Jun 27, 2007.

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  1. Guest

    Schottky seems to have much lower values of forward voltage drop and
    breakdown voltage then fast recovery diode. What is their difference
    other then this? I'm confused with which one I should choose as
    bootstrap diode for a MOSFET bridge driver. Thanks for helping.
  2. Eeyore

    Eeyore Guest

    You'll discover that schottky diodes have rather limited reverse voltage
    ratings. That should help aid your decision.

    More on the physics here....

  3. Schottky diodes are essentially half junctions, made of
    metal and P type semiconductor, instead of a PN junction.

    As you say, this allows then to conduct forward current with
    about half the normal voltage drop (half the wasted power)
    than full, PN junctions. And also, as you say, they have
    low reverse voltage ratings (and high reverse leakage
    current. And the reverse leakage goes up very fast as
    temperature rises. Though they generally have high reverse
    biased capacitance than similarly current rated PN diodes,
    and it is a very nonlinear capacitance, with respect to
    reverse voltage, there is no real reverse recovery time that
    PN junctions have.

    Reverse recovery refers to the fact that PN junctions, when
    forward biased, fill each side of the junction with minority
    carrier charges (i.e. the N side has holes injected into it,
    and the P side has electrons injected into it). And if the
    forward bias is changed to reverse bias, suddenly, most of
    those minority carriers have to get sucked out of the
    semiconductor, back across the junction, before the junction
    can develop an insulating, charge depleted layer that
    withstands reverse voltage. So, during this clean up
    process (the reverse recovery time), the diode looks pretty
    much like a short across the reverse bias. And the end of
    this process can result in a nasty snap from high reverse
    current to low current, and that snap can generate lots of
    high frequency energy. Schottky diodes are more graceful as
    they swing to reverse blocking, with only the changing
    junction capacitance carrying a little reverse current.

    So, if you want higher efficiency during forward conduction,
    cleaner, faster switching to reverse blocking and can live
    with the low reverse voltage rating, the higher reverse bias
    leakage current (especially at high temperature), and the
    probably higher price for the Schottky device, then the
    Schottky is probably the best choice.
  4. Guest

    So is it correct to say, Schottky diode is always the best choice,
    particularly in low voltage application, and fast recovery type is
    used only when the required breakdown voltage is higher then what
    Schottky can provide? Which of them has higher recovery speed?
  5. Bob

    Bob Guest

    Great explanation, John. Thanks.

  6. Tim Williams

    Tim Williams Guest

    Yep, that's why you find them in switching power supplies (for 5V, but not
    for the 12V supply).

    Right now I'm using two SBL3040gibberish, or something like that -- dual
    schottky in TO-247, anyway -- on a solid aluminum heatsink connected to a
    CT'd winding I put on an MOT. It's making about 6VDC and I'm drawing around
    60A from the thing. I happen to have some FWB's, but they would entail four
    times the voltage drop and dissipation!

  7. Eeyore

    Eeyore Guest

    Have you looked at synchronous rectification btw ?

  8. Fred Bartoli

    Fred Bartoli Guest

    Tim Williams a écrit :
    See onsemi's NIS6111
  9. Ray King

    Ray King Guest

    I have been following the schottky explanation but getting back to your
    application for the diode for the boot strap. The rule of thumb is the high
    side cap is 0.1 times the value of the non floating cap. Depending upon what
    frequency you are running and the size of the mosfet you are trying to
    drive. A 1 uf 20v floating cap and a 10 uf 20v ground cap will do most
    anything. The diode requirments for 100khz and below at 300v buss is a 0.5
    to1 amp diode at 200ns trr. 100khz to 300khz a 100ns is ok. the most
    importent requirment for the diode is the voltage rating of 20% higher than
    the highest buss voltage. At buss voltages above 30 volts a schottky is not
    a good idea.
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