low-capacitance high-voltage diode

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 I'm looking for a good low-capacitance SMT 600V diode.
 For example, Vishay says a 1N4005 is about 5pF at 100V.
 ON Semi says under 3pF.  Diodes, Inc says about 1.5pF.
 What to think?  I suppose new SMTs are similar to oldies.
 An RGL34J MELF shows about 1.5pF.  Small die is better,
 and I don't need a 1A rating.  Why not a 50mA rating?

 A Vishay Ultrafast UF4005 plot says about 6pF.  Ouch.

 Soft-recovery would be nice.  Diodes' CMR1U-06 part
 doesn't give its capacitance, but an app note says
 it's 80pF.   Whoa, that's way too much.

 Suggestions, favorites?


--  
 Thanks,
    - Win

Re: low-capacitance high-voltage diode
On 28 Jun 2016 12:22:38 -0700, Winfield Hill

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I did a 1400 volt flyback switcher and used MMBD5004S's in the C-W
multiplier. That's a dual 400-volt diode in SOT23, 300 mA rating. They
ran cool, unlike some other diodes that had a lot of reverse-recovery
losses.

The Diodes Inc data sheet claims 0.6 pF typ per diode at 0 volts.
That's 800 volts and 0.3 pF for the diodes in series!

https://dl.dropboxusercontent.com/u/53724080/PCBs/T840_E1.jpg



--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: low-capacitance high-voltage diode
On Tue, 28 Jun 2016 13:06:30 -0700, John Larkin

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I have 6000 in stock if you need a few.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: low-capacitance high-voltage diode
John Larkin wrote...
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 Hey, those are awesome!  About 0.4pF each at 100V, and
 1/2 0.52pF at 50V = 0.25pF at 100V, even better yet!  
 What's-more I had saved the datasheet after seeing you
 use them in your T840 (but without useful identifying
 info).  I had thought you were using a '4004 variant.
 They'll be perfect, thanks!


--  
 Thanks,
    - Win

Re: low-capacitance high-voltage diode
On 06/28/2016 03:22 PM, Winfield Hill wrote:
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Isn't "low-capacitance, high-voltage" sort of like a delicious, fat-free  
diet cheeseburger?

Re: low-capacitance high-voltage diode
For me HV means a big gap, depletion region, distance.
which looks like HV ... C = e_0*Area/distance.
I know zilch about HV diodes.  

George H.  

Re: low-capacitance high-voltage diode
On Tuesday, June 28, 2016 at 6:02:18 PM UTC-7, George Herold wrote:
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That's a good start.   The longest depletion regions are in PIN diodes, made
for low-capacitance OFF states, but usually not any appreciable current
in the forward direction.   Rectifiers with HV standoff are usually limited
by their surface leakages, and deep depletion regions don't help against
that.

DigiKey shows under 'PIN' diodes, a 600V unit MA4PH236-1072T.   Maximum Cj (10V) is 0.5 pF,
but it costs several dollars, and doesn't claim to turn OFF particularly fast (carrier lifetime
issue).
<http://cdn.macom.com/datasheets/packagedpindiodes.pdf

Re: low-capacitance high-voltage diode
On 06/28/2016 09:23 PM, whit3rd wrote:
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It sounds reminiscent of the old physics puzzle of why giant insects  
can't exist in the real world: essentially the compression strength of  
the legs is growing as the square of the bug's larger volume, but the  
mass is growing as the cube.

If I'm remembering the equations right the depletion region capacitance  
is inversely proportional to the square of the donor density, which  
means all things being equal a smaller capacitance is associated with a  
larger donor density.

But the minority carrier lifetime is nearly inversely proportional in  
the donor density, and the conductance is directly proportional to the  
donor density, but as you increase the length of the semiconductor  
regions to avoid breakdown the donor density is decreasing as the larger  
volume, so to get the same value of capacitance as the "shorter" diode  
you have to increase the donor density.  But increasing the donor  
density means you decrease the minority carrier lifetime, and hence the  
conductance, proportional to the volume. So you increase the junction  
area to compensate, but you're once again increasing the junction  
volume, and current density is only increasing proportional to the  
square of the junction area.

Something like that? If I'm looking at this graph it seems like all  
things being equal, the depletion capacitance of a regular  
reversed-biased PN diode overall increases with greater path length.

Re: low-capacitance high-voltage diode
On Tue, 28 Jun 2016 16:39:20 -0400, bitrex

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Not really. The conflict would be between current rating and
capacitance, not voltage and capacitance.

Imagine having a 100 volt, 100 pF diode. 10 in series is 1000 volts,
10 pF. In that case, higher voltage is *less* capacitance.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: low-capacitance high-voltage diode

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Use several in series to reduce the capacitance, the increased
conduction losses won't matter much at high voltage.


--  
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
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Re: low-capacitance high-voltage diode
I was gonna tease with "tube" something or other, but 6AL5 is a couple pF,  
even 1X2A (not that you could handle the voltage drop).  2-01C maybe, but  
they're really rare (Eimac microwave diode -- for RF voltmeter probes, it  
goes in the probe end).

In my defense, you can get a very accurate peak reading -- Vf(If = 0) is a  
little negative in fact, so there's no "0.6V" to worry about.

How many can you series up?  That's kind of along JL's idea.

I think HV diodes (>kV -- stacks of dies) are in that territory, but they  
often have crappy recovery, and obviously the voltage drop is high.  They're  
also kind of scarce.

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: low-capacitance high-voltage diode
On Tue, 28 Jun 2016 18:28:09 -0500, "Tim Williams"

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I used to use 1B3 HV rectifier tubes as rectifiers and variable
resistors, to charge an oil cap from a neon sign transformer. I drove
the filament from a D-cell and a wirewound rheostat (with a long
insulated shaft!), turned the knob until I got 7KV DC on the cap. So a
1B3 is a low-bandwidth amplifier with the filament voltage as the
input.

Amazing I'm still alive.

A 1B3 with no filament power makes a nice 30KV capacitive probe.

TEK used some tiny wire-lead pencil tubes as the HV rectifiers in
their old tube scopes.

An MMBD5004S costs 4 cents, 50 u$ per volt. Official HV diode stacks
do tend to be expensive.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: low-capacitance high-voltage diode
On Tuesday, June 28, 2016 at 12:22:47 PM UTC-7, Winfield Hill wrote:
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BAW101V is a dual, presumably matched in leakage, rated for
series connection.   As I read it, it's under 1 pf for series connection.

<http://www.diodes.com/_files/datasheets/ds32178.pdf


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