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'[OT] anything better than Schottky?'
1999\05\11@171626 by Mik Kim

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Hi all,

This is OT, but just barely. In trying to make a switching amplifier
(original design used a PIC for testing), we were using synchronous
rectifier. Instead of its complexity, I thought maybe we can use
something like Schottky, but much better. Is there such a creature?

Basically, we were trying to switch at 80 MHz (!) and still attain
better than 90% efficiency (inductors are the enemy!). An ideal diode
would've saved few percent, so I guess I'm wondering if there is such a
thing as a near-ideal diode (and minimal capacitance, too). We had
semiconductor fab at our disposal, so new process would be Ok. However,
a commercial product would be the best.

Vf = 0 (or less than 0.1 volt) at 1 ampere
C = 1 pf or less at 6 volts

1999\05\11@174315 by Vincent Deno

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> Hi all,
>
> This is OT, but just barely. In trying to make a switching amplifier
> (original design used a PIC for testing), we were using synchronous
> rectifier. Instead of its complexity, I thought maybe we can use
> something like Schottky, but much better. Is there such a creature?

If you find something faster than a Schottky, let me know.... I'll buy the
stock.
Schottky diodes are majority carrier semiconductors with very low
barriers.  They're as close as you're going to get (with current
technology) to an ideal diode for passive components.

> Basically, we were trying to switch at 80 MHz (!) and still attain
> better than 90% efficiency (inductors are the enemy!). An ideal diode
> would've saved few percent, so I guess I'm wondering if there is such a
> thing as a near-ideal diode (and minimal capacitance, too). We had
> semiconductor fab at our disposal, so new process would be Ok. However,
> a commercial product would be the best.
>
> Vf = 0 (or less than 0.1 volt) at 1 ampere
> C = 1 pf or less at 6 volts

I don't necessarily understand what you mean by 90% efficiency (assuming
power efficiency).  You could use an op-amp to model an ideal diode.  But
judging from your power requirements, you may be hard-pressed to find an
op-amp that fast.  Let me know if you still want the schematic.

You COULD put multiple diodes in parallel (to distribute the current draw),
but I wouldn't recommend it because you are still going to stress the
fastest switching diode.... it may work reliably enough for you.

Good Luck,

Vincent Deno

1999\05\11@213602 by Russell McMahon

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I think that synchronous rectification is a s good as you are going
to get in this sort of application. Even a Schottky diode gets up to
many tenths of a volt at moderate currents - its always better than a
standard Silicon junction (Schottky uses Silicon too in normal
implementations - its a technique, not a material) but its not the
ideal zero resistance switch. A switched FET (or even a bipolar
transistor) can give you lower voltage drop. Note also that
Schottky's tend to be fairly low voltage devices (typically <= about
40 volts?).

RM


{Original Message removed}

1999\05\11@215101 by Bob Drzyzgula

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On Tue, May 11, 1999 at 12:43:58PM +1200, Russell McMahon wrote:
> I think that synchronous rectification is a s good as you are going
> to get in this sort of application. Even a Schottky diode gets up to
> many tenths of a volt at moderate currents - its always better than a
> standard Silicon junction (Schottky uses Silicon too in normal
> implementations - its a technique, not a material) but its not the
> ideal zero resistance switch. A switched FET (or even a bipolar
> transistor) can give you lower voltage drop. Note also that
> Schottky's tend to be fairly low voltage devices (typically <= about
> 40 volts?).

The technote at the link I gave earlier today --
http://www.microsemi.com/micnotes/400ser/401.htm -- Has
an I vs V graph for a Schottky compared to a PN junction,
and gives 100 volts for the highest working peak reverse
voltage one is likely to see in Shottky parts; they claim that
this is true "...since devices moderately above this rating
level will result in forward voltages equal to or greater
than equivalent pn junction rectifiers."

--Bob

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