SemiSouth SiC Power JFETs

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Eli Duttman

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Joined 2004
Guys,

I'm definitely on unfamiliar ground here. So, please take it easy on me. Whatever expertise I have lies in the "hollow state" arena.

Perhaps these SiC power JFETs could be mated to Plitron's PAT 4152-00 (sic) SS O/P trafo. Source follower topology and no NFB, at all, would be used around the O/P stage.

Miles Prower has already pointed out the need to use a tube like rail voltage with these "puppies". Tubed small signal amplification can be very linear, without the application of loop NFB. Hybrid amp, anybody?
 
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Miles Prower

Miles Prower

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Joined 2005
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Eli Duttman said:
Hybrid amp, anybody?
Click to expand...

Definitely a possibility. I considered the alternative of using an LTP implemented with cascoded 6BQ7s. I've already used this topology for the front end of another, hollow state, project. Works great.
 

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wrenchone

wrenchone

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Joined 2004
To add some fuel to the fire, I just read an article about Microsemi's new series of SiC SITs. They are rated for 1200V and touted as the next best thing for radar applications. I haven't had the time to seek out any more information, but the more avid types here might feel compelled to give it a go....
 
wrenchone

wrenchone

Member
Joined 2004
Assuming they're even available to the general public, I would expect the Microsemi parts to be as expensive as those from Semisouth. RF is a pricey niche business, especially high power. To add more confusion, a Scandinavian firm is offering SiC bipolar transistors that are rated for high voltage and high temp, and have much smaller storage times than their silicon little brothers. Wait 'till the GaN parts hit the street....
 
StevenOH

StevenOH

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wrenchone

wrenchone

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Joined 2004
It's nice to know that Kneeeejerk/Farnell have the Semisouth parts, though the price is dear. Has anyone tried the equally expensive Cree SiC parts (stocked by Digi-Key)? I haven't looked too closely, as the price tag is somewhat scary, but I think these are enhancement-mode mosfets of the more usual sort, though maybe posessed of some magic for those brave enough to bias them into linear operation (?).
 
jan.didden

jan.didden

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A question to the gathered intellects here.

I am working on a research program (ahem) for which I need a fast, rugged switch to disconnect or connect amplifier loads to ground. FETs suggest themselves but they normally have an anti-parallel build-in diode, so for an AC switch the usual way is to use two FETs in series, in opposite direction.

These new SITs apparently don't have the anti-parallel diode, are fast and rugged so seem a perfect fit for me. Only, I am not sure if I can use them as AC switches - bidirectionally. I think I can, because off, they're off, and when on, they are a very low R. But I would like it confirmed before raising my mortgage again ;) . Any issues I missed?

jan didden
 
jan.didden

jan.didden

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DuncanV said:
Hi Jan,

That's an interesting application. I've used small signal JFETs in the past for solid state 50 Ohms input termination switching. Don't forget a JFET has an intrinsic gate to channel diode which a MOSFET is missing fortunately. ;)

Duncan.
Click to expand...

Duncan,

Does that account for the very low impedance specified as the gate input impedance? So, are these really FETs with an essential capacitive input Z, or more like a B-E junction. I'm confused now.

BTW, if the Vds is inverted while they are in the off state, is there any leakage current you think?

jan didden
 
D

DuncanV

Member
Joined 2009
Hello Jan,

janneman said:
Duncan,
So, are these really FETs with an essential capacitive input Z, or more like a B-E junction. I'm confused now.
Click to expand...

I'm affraid both... I took a brief look on this application note:
http://www.semisouth.com/application/AN-SS1rev1.pdf

Please have a look on the equivalent circuit shown on page 2.
What confused me too is the normally-off variant of this JFET. This requires forward biasing of the gate to channel junction, represented as two diodes in this diagram, to obtain a conducting channel.

janneman said:
BTW, if the Vds is inverted while they are in the off state, is there any leakage current you think?

jan didden
Click to expand...

Do you mean channel leakage or gate leakage?


Duncan.
 
jan.didden

jan.didden

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DuncanV said:
Hello Jan,



I'm affraid both... I took a brief look on this application note:
http://www.semisouth.com/application/AN-SS1rev1.pdf

Please have a look on the equivalent circuit shown on page 2.
What confused me too is the normally-off variant of this JFET. This requires forward biasing of the gate to channel junction, represented as two diodes in this diagram, to obtain a conducting channel.

Do you mean channel leakage or gate leakage?

Duncan.
Click to expand...

Hi Duncan,

Yes, thanks for that link. Gate drive is clear to me now.

As for the use as an AC switch, I was wondering what the channel leakage would be when reverse-biased? Can these devices withstand reverse bias? A 'normal' FET with an antiparallel diode can never be reverse biased since the diode would conduct, but that's not the case here.

jan didden
 
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