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
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*
OK, glare at me if you will. I'm sure you know MOSFETs in ICs make extensive use of the effect of substrate/body potentials. We say little about this, but enough to rightly-so draw your interest.
Our MOSFET introduction (AoE pages 117-118) explicitly highlights this 4th element or connection. In our defense, we must mention it, because of the important role it plays for example in IC mosfet switch designs (AoE pg 145). I included Motorola's 2n4351 as an example of a MOSFET with 4th connection (AoE pg 224), which may need further examination.
It's true that few discrete MOSFETs make the body connection available, but some important ones do, like the popular SD210 series. And there are a few other examples.
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.
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
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I love to cook with wine. Sometimes I even put it in the food.
Discretes available off the shelf from Supertex. They are often used in process current transmitters.
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Best regards, Spehro Pefhany
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speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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.
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.
It's a shame that all the SSRs (or all the ones I've tried) have a schmitt-like driver in the opto path. If the control were linear, all sorts of interesting things would be possible.
Well, how about fragile, unreliable, noisy, low transconductance, high power dissipation, high on resistance, needs filament power, not available in surface mount.
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
Well it looks like the Clare (CPC3703C), SOT-89, 4.0 ohm, DMOS FET driven 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
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