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Anybody know of a 100mV Schottky diode or similar device?

  1. Jun 19, 2012 #1

    berkeman

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    I need to bias the inputs to a transformer-coupled communications receiver apart by about 100mV. Since the transformer has split windings, one way to do this would be to add a Schottky diode into the center tap, and bias it with a little current (not enough to saturate the transformer). I would use parallel diodes to maintain the offset when the AC comm signal comes through the transformer.

    But so far I haven't found any Schottky diodes with a Vf of less than 220mV, and those were big power diodes. I just need small signal diodes in the 10-20mA range. I was doing some reading that suggests that lower Vf diodes are not generally made because the reverse leakage current increases with decreasing Vf, so they are not very good rectifiers if you try to push Vf too low.

    But I don't really care about reverse leakage current in this application. Does anybody know of custom Schottky diodes where they could maybe be tuned for the lower Vf as a special? Or does anybody know of any other similar electronic device that will drop a constant 100mV for currents in the 1mA-20mA range?
     

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  3. Jun 19, 2012 #2

    Bobbywhy

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    You may have a look at:

    BAT60A - Silicon Schottky Diode (Rectifier Schottky diode with extreme low VF drop for mobile communication For power supply - Siemens Semiconductor Group
     
  4. Jun 20, 2012 #3

    es1

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    What bus is that? It seems like an odd requirement after going to all the trouble to isolate the lines. :)

    Even if they did make a custom one I don't think you're going to find a schottky that can hold 100mV over process and temp, over a decade of If. The curves are going to be too wild.

    The only single device I can think that could hold a 100mV offset would be a pre-charged large cap that got switched in. Not sure if that could work here. Or maybe the really stupid solution of overdriving the common mode with a large current from pin 1 to 6 and then use a resistor. :)

    Is the problem you don't know which common mode current in the 1mA - 20mA range you're going to get and it can be positive or negative (based on back to back diodes)?

    I am thinking the creators of the bus already have a solution and you're looking for a cheaper/simpler way to do it. :) Depending on the com link specifics, I suspect it could be a hard thing to do. I would be really curious what the standard solution is if there is one.
     
  5. Jun 20, 2012 #4

    es1

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    That BAT60A is a really nice part. But if the 1mA If point is a requirement, per Berkeman's post, then the 25C Vf at that load is pretty close to 0V.
     
  6. Jun 20, 2012 #5

    jim hardy

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  7. Jun 20, 2012 #6

    berkeman

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    Thanks for that Bob. I had missed that part. I'll get some samples and try them out in my test fixture.

    LOL. I'm one of the creators of this comm bus. It's sold in the millions in its standard configuration. We just have a large customer with a strange requirement that has me looking at this solution. :smile:

    I'd thought about using Vcesat, but there are some biasing issues that have so far kept me from using it. I think I'll revisit it though.

    Thanks for the thoughts folks!
     
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