Depletion-Mode FETs

H

Harry Dellamano

Guest
I am searching for SMD Depletion-Mode FETs to construct normally-on, at
zero power, switches. I find Supertex (DN3525) and Clare (CPC3703C) devices
that are 250BV and 6 ohms Rds(on). Are there any other candidates? Greater
than 25V and less than 1.0 ohms Rd(on) in a surface mount package would be
preferable. Normally-on switches, with no external power, is a pain.
TIA
Harry
 
In article <UjpSd.14327$uc.8748@trnddc09>,
Harry Dellamano <harryd@tdsystems.org> wrote:
I am searching for SMD Depletion-Mode FETs to construct normally-on, at
zero power, switches. I find Supertex (DN3525) and Clare (CPC3703C) devices
that are 250BV and 6 ohms Rds(on). Are there any other candidates? Greater
than 25V and less than 1.0 ohms Rd(on) in a surface mount package would be
preferable. Normally-on switches, with no external power, is a pain.
Click to expand...
How about a few J105 like surface mounted JFETs in parallel?

--
--
kensmith@rahul.net forging knowledge
 
Ken Smith wrote...
Harry Dellamano <harryd@tdsystems.org> wrote:
I am searching for SMD Depletion-Mode FETs to construct normally-on, at
zero power, switches. I find Supertex (DN3525) and Clare (CPC3703C) devices
that are 250BV and 6 ohms Rds(on). Are there any other candidates? Greater
than 25V and less than 1.0 ohms Rd(on) in a surface mount package would be
preferable. Normally-on switches, with no external power, is a pain.

How about a few J105 like surface mounted JFETs in parallel?
Click to expand...
Fairchild JFTJ105 ...


--
Thanks,
- Win
 
In article <111njr7petaj4e2@corp.supernews.com>,
Phil Hobbs <pcdh@SpamMeSenseless.us.ibm.com> wrote:
[....]
How about a 3-V enhancement FET with a lithium battery in series with
the gate?
Click to expand...
A normally closed relay?

--
--
kensmith@rahul.net forging knowledge
 
On Tue, 22 Feb 2005 20:50:43 -0700, the renowned Jim Thompson
<thegreatone@example.com> wrote:

On Tue, 22 Feb 2005 11:37:49 -0600, Rob Gaddi
rgaddi@bcm.YUMMYSPAMtmc.edu> wrote:

Ken Smith wrote:
In article <UjpSd.14327$uc.8748@trnddc09>,
Harry Dellamano <harryd@tdsystems.org> wrote:

I am searching for SMD Depletion-Mode FETs to construct normally-on, at
zero power, switches. I find Supertex (DN3525) and Clare (CPC3703C) devices
that are 250BV and 6 ohms Rds(on). Are there any other candidates? Greater
than 25V and less than 1.0 ohms Rd(on) in a surface mount package would be
preferable. Normally-on switches, with no external power, is a pain.


How about a few J105 like surface mounted JFETs in parallel?


Going to have to use JFETs. The first thing I learned upon graduating
college is that MOSFETs exist neither in depletion mode nor in 4
terminal varieties. Much like unicorns, these are entirely fictional
entities that can be found in theory but not catalog. And are really
just used by the sorts of people who write engineering textbooks so that
they can laugh as they watch your dreams of how to build your circuit
shatter on the rocky shores of availability. *glares over at Win*

BS! I know several processes for which depletion mode FETs exist...
like Fairchild, for example... I've used them as start-up mirrors in
several RFID tags.

...Jim Thompson
Click to expand...
Discretes available off the shelf from Supertex. They are often used
in process current transmitters.

http://www.supertex.com/products/selector_guides/102


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
In article <cvgr9m021e8@drn.newsguy.com>,
Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
[...]
In this discussion, we acknowledge that the popular DMOS / VMOS power
mosfets don't have a 4th body connection, and furthermore they have a
"parasitic" substrate diode connected to the drain. But, hey, they're
another (dominant) beast entirely! One would *never* use a huge part
like that in a low-current charge and leakage-sensitive application.
Click to expand...
I have. If you need an analog switch with nearly zero on resistance, the
ability to withstand a couple of 100V and moderately fast switching, a
selected power MOSFET can be just the ticket.

--
--
kensmith@rahul.net forging knowledge
 
On Mon, 21 Feb 2005 17:59:16 GMT, "Harry Dellamano"
<harryd@tdsystems.org> wrote:

I am searching for SMD Depletion-Mode FETs to construct normally-on, at
zero power, switches. I find Supertex (DN3525) and Clare (CPC3703C) devices
that are 250BV and 6 ohms Rds(on). Are there any other candidates? Greater
than 25V and less than 1.0 ohms Rd(on) in a surface mount package would be
preferable. Normally-on switches, with no external power, is a pain.
Click to expand...
Aromat/Nais offers a normally-on SSR aqv414 - 400V withstand, with 22R
impedance.

http://www.relayasia.com/pdf-files/nais/photomos/aqv414.pdf

As with most isolating or PVI optocouplers, body size of 'smd' version
is large.

RL
 
Ken Smith wrote...
Winfield Hill wrote:
[...]
In this discussion, we acknowledge that the popular DMOS / VMOS power
mosfets don't have a 4th body connection, and furthermore they have a
"parasitic" substrate diode connected to the drain. But, hey, they're
another (dominant) beast entirely! One would *never* use a huge part
like that in a low-current charge and leakage-sensitive application.

I have. If you need an analog switch with nearly zero on resistance,
the ability to withstand a couple of 100V and moderately fast switching,
a selected power MOSFET can be just the ticket.
Click to expand...
OK, and come to think of it so have I. Just illustrates why one should
never say never! :>) When using a power mosfet for analog switching,
one usually struggles with finding a small enough FET, to keep down the
capacitances. I have found that small say 1A high-voltage parts, which
used to be manufactured, have been discontinued, so that the smallest
part in a series is no longer available. As this phenomenon was playing
itself out a few years ago I made lifetime buys of some of the small FETs
in question, to have them for high-speed switches and linear amplifiers.


--
Thanks,
- Win
 
"Harry Dellamano" <harryd@tdsystems.org> wrote in message
news:UjpSd.14327$uc.8748@trnddc09...
I am searching for SMD Depletion-Mode FETs to construct normally-on, at
zero power, switches. I find Supertex (DN3525) and Clare (CPC3703C)
devices that are 250BV and 6 ohms Rds(on). Are there any other candidates?
Greater than 25V and less than 1.0 ohms Rd(on) in a surface mount package
would be preferable. Normally-on switches, with no external power, is a
pain.
TIA
Harry

Well it looks like the Clare (CPC3703C), SOT-89, 4.0 ohm, DMOS FET driven
Click to expand...
by a Toshiba (TLP190B) photovoltaic coupler form a nice 0.25A normally
closed, floating relay in SMD packages. Add a 3.3 meg resistor from gate to
source to insure normally closed operation. Supertex has a line of DMOS and
IRF have photovoltaic couplers that could be used.
Thanks for all the input and the info regarding the J105 J-FET. .
Cheers,
Harry
 
Discretes available off the shelf from Supertex. They are often used
in process current transmitters.

http://www.supertex.com/products/selector_guides/102


Best regards,
Spehro Pefhany
Click to expand...
Hmm, and they're even distributed and in stock. Looks like I learn
something new every day. Having now met this day's quota I can go back
to bed.

-- Rob
 
Phil Hobbs wrote:

How about a 3-V enhancement FET with a lithium battery in series with
the gate?

Cheers,
Click to expand...

And _what_ exactly is wrong with using a 12AX7? Parallel the two
sections and there ya go! Need more power (and who doesn't)? There's
always our old favorite, the 6L6.
 
Brad Albing wrote:
Phil Hobbs wrote:

How about a 3-V enhancement FET with a lithium battery in series with
the gate?

Cheers,



And _what_ exactly is wrong with using a 12AX7? Parallel the two
sections and there ya go! Need more power (and who doesn't)? There's
always our old favorite, the 6L6.
alright, a person after my heart!
Click to expand...
 
On Wed, 23 Feb 2005 17:51:13 GMT, Brad Albing wrote:

Phil Hobbs wrote:

How about a 3-V enhancement FET with a lithium battery in series with
the gate?

Cheers,


And _what_ exactly is wrong with using a 12AX7? Parallel the two
sections and there ya go! Need more power (and who doesn't)? There's
always our old favorite, the 6L6.
Click to expand...
I heard that someone in ?Russia? is still making 6L6/6550's ... I shudder
to think what they must get for them.


Bob
 
On Wed, 23 Feb 2005 18:15:30 GMT, the renowned Bob Stephens
<stephensyomamadigital@earthlink.net> wrote:

On Wed, 23 Feb 2005 17:51:13 GMT, Brad Albing wrote:

Phil Hobbs wrote:

How about a 3-V enhancement FET with a lithium battery in series with
the gate?

Cheers,


And _what_ exactly is wrong with using a 12AX7? Parallel the two
sections and there ya go! Need more power (and who doesn't)? There's
always our old favorite, the 6L6.

I heard that someone in ?Russia? is still making 6L6/6550's ... I shudder
to think what they must get for them.


Bob
Click to expand...
Here's one:
http://www.svetlana-tubes.com/pdf/sv6l6gc.pdf

There are others in Russia, the Czech republic, Slovakia, Ukraine and
China, but it's obviously a niche market.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
On Wed, 23 Feb 2005 17:51:13 GMT, Brad Albing
<itza.secret@none-of.yer-bidness> wrote:

Phil Hobbs wrote:

How about a 3-V enhancement FET with a lithium battery in series with
the gate?

Cheers,


And _what_ exactly is wrong with using a 12AX7? Parallel the two
sections and there ya go! Need more power (and who doesn't)? There's
always our old favorite, the 6L6.
Click to expand...
Well, how about fragile, unreliable, noisy, low transconductance, high
power dissipation, high on resistance, needs filament power, not
available in surface mount.

John
 
In article <421cc28d$1@news.cle.ms.philips.com>,
Brad Albing <bradleyDOTalbingATphilipsDOTcom> wrote:
Phil Hobbs wrote:

How about a 3-V enhancement FET with a lithium battery in series with
the gate?

Cheers,


And _what_ exactly is wrong with using a 12AX7? Parallel the two
sections and there ya go! Need more power (and who doesn't)? There's
always our old favorite, the 6L6.
The heater needs 12V. Use a 2B7. If you parallel the two sides, you can
Click to expand...
get 20mA

--
--
kensmith@rahul.net forging knowledge
 
In article <cvhlk201876@drn.newsguy.com>,
Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
Ken Smith wrote...
[....]
I have. If you need an analog switch with nearly zero on resistance,
the ability to withstand a couple of 100V and moderately fast switching,
a selected power MOSFET can be just the ticket.

OK, and come to think of it so have I. Just illustrates why one should
never say never! :>) When using a power mosfet for analog switching,
one usually struggles with finding a small enough FET, to keep down the
capacitances.
Click to expand...
I needed under an Ohm and 200V or so so it wasn't a problem finding a
small enough one. I did have to taylor the rise on the gate so as not to
drive the amplifiers into saturation with the gate capacitance, though.

I have found that small say 1A high-voltage parts, which
used to be manufactured, have been discontinued, so that the smallest
part in a series is no longer available. As this phenomenon was playing
itself out a few years ago I made lifetime buys of some of the small FETs
in question, to have them for high-speed switches and linear amplifiers.
Click to expand...
Supertex makes some quite small ones.

Some of the newer ones have the advanage of a fast recovery on the body
diode.

--
--
kensmith@rahul.net forging knowledge
 
In article <askn11t6o46mnedup5i20k3lk19a2nrr3o@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[...]
BS! I know several processes for which depletion mode FETs exist...
like Fairchild, for example... I've used them as start-up mirrors in
several RFID tags.
Click to expand...
There are also some companies that sell them. I think his problem was
that you can't find them in Digikey.

--
--
kensmith@rahul.net forging knowledge
 
I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
THIStechPLEASEnology.com> wrote (in <b3sp115ffgt2vbc1jot6hrksmlgb47hki9@
4ax.com>) about 'Depletion-Mode FETs', on Wed, 23 Feb 2005:
Well, how about fragile, unreliable, noisy, low transconductance, high
power dissipation, high on resistance, needs filament power, not
available in surface mount.
Click to expand...
If I had time, I'd point you to a source of *surface-mount* octal
holders for the 6L6. B9A for the 12AX7 is admittedly more difficult.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On Thu, 24 Feb 2005 09:34:05 +0000, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
THIStechPLEASEnology.com> wrote (in <b3sp115ffgt2vbc1jot6hrksmlgb47hki9@
4ax.com>) about 'Depletion-Mode FETs', on Wed, 23 Feb 2005:
Well, how about fragile, unreliable, noisy, low transconductance, high
power dissipation, high on resistance, needs filament power, not
available in surface mount.

If I had time, I'd point you to a source of *surface-mount* octal
holders for the 6L6. B9A for the 12AX7 is admittedly more difficult.
Click to expand...
Well, I suppose that a metal plate with a hole punched in it is,
technicaly, a "surface."

John
 
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