How to pick the right Schottky for bridge rectifier?

Discussion in 'General Electronics Chat' started by TheLaw, Nov 17, 2011.

  1. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    I'm putting together a lab/bench power supply and I'm trying to optimize my efficiency where I can since I'm using a rather inefficient linear design.

    So I thought I might try to use Schottkys for the bridge rectifier.

    I'm a little confused though. I see some Schottkys touting high efficiency, while their forward voltages are higher than even a 1n4007. But they apparently have a quicker switching speed which is apparently superior.

    So do I go for a low forward voltage or a faster switching speed? I'm willing to spend a little bit of cash on them...but not like $5/rectifier x4

    Also, is total voltage drop equated by the Forward voltage of ONE diode or do I add them or what?

    Thanks.
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    If you are rectifying off the 120VAC line, "fast recovery" switching speed means nothing. Only applicable in high speed switching circuits.
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
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    In a bridge rectifier circuit there are two diode forward voltage drops in series on each half cycle. You have to add them together.
    Schottkys will reduce the voltage drop for a given forward current and bounty is correct, at line frequency speed is not an issue.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    In a linear regulator the few tenths of a volt reduction in forward voltage achieved by using Schottky diodes is normally not a significant factor in efficiency since you are dropping much more voltage than that across the linear regulator control element. It's only a factor if you want to use a low-voltage dropout linear regulator with output voltage operated as close as possible to the raw DC supply voltage.
     
  5. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
    2
    So if I have a bridge rectifier using diodes rated at 0.5VForward, the actual voltage drop will be 1.0V?
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
    1,606

    Generally yes, excepting when you're using a transformer center tap to get plus and minus voltages. But even then the total output voltage is still two drops below the peak voltage.

    Take your schematic and trace out how the current flows. In a single output bridge the current goes out thru 1 diode and back to the transformer thru another diode, thus you get 2 drops.
     
  7. TheLaw

    Thread Starter Member

    Sep 2, 2010
    228
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    Okay thank you. Next question: How are single package GBU bridge rectifiers rated?

    Like GBU6M is rated for like 0.95V forward @ 4A. That's pretty good if that's true. My expensive Schottkys are like 0.6Vforward @ 4A for ONE diode. Aren't shottkys supposed to be lower vDrop?
     
  8. Evil Lurker

    Member

    Aug 25, 2011
    117
    23
    Keep in mind that one has to take into account reverse leakage into the equation as well.
     
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