MOSFETs are getting better, year by year

And exotica, phemts and now Sic and GaN, totally blows away mosfets.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Actually, no, they're on my spreadsheet as well. Blow away the old MOSFETs, yes, but not the new super-junction FETs, that represent about half of the new ones in my table. For example, most SiC parts are nested with the best super-junction parts, except for a few that stand out and are 1.5 to 2x better. There's a specific SiC that's spectacular, but it has power-handling issues.

--
 Thanks, 
    - Win

Is any mosfet comparable to the Cree C2M0280120D? I've pushed that one to the limit.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

That's my favorite one, that I was talking about. But its 0.6-ohm Ron (when hot) is rather high, and its weak 62W case rating is rather low. Also, it comes in a wastefully-huge TO-247 package. But, hey, it's not expensive, and makes great pulses.

--
 Thanks, 
    - Win

The chip is tiny for the package.

Its RDS-on tempco is awful, close to thermal runaway turf. There's a similar ST fet that has a much flatter Rds tempco.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

So is a lot of stuff. Hardly a revelation!

--
Jules: fighting igorance on the interweb since 1998

The wide bandgap parts blow aray silicon MOSFETs at HIGH(er) voltages.

At low voltage, say, 100V and below, silicon have much lower RdsOn, bigger die (important for surge current) and low enough voltage that the shitty diode reverse recovery time isn't much of an issue.

And the price is always higher at the moment for the same size die of SiC than it is for Si. Hopefully that will change someday soon.

SiC is awesome stuff though. I use both SiC and Si.

The substrate diode in the Cree is awful, slow turn-on. The Spice model lies about that.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Factors of 10x in a commodity are rare.

--
 Thanks, 
    - Win

Really ? I think I saw you or someone else say that. I have some Cree SiC FETs now that I am going to try in place of the ROHM parts that are already working awesome. They are cheaper than the Rohm's but also have a smaller die I believe.

Silicon, for hard switching HV power is unusable for me.

I have been all hot for GaN parts since about 2005 but still, there aren't any parts I can use for several hundred volts like there are for SiC FETs.

I think that GaN will be great for all sorts of things eventually as they mature. One day soon I may have something I will have a reason to play with. Only thing that I can think of for my local use might be for a Class-D audio amplifier. RF amps would be fun too.

The Cree parts have nice low gate series resistance, so they switch wicked fast... if you can drive the gates hard enough. A bit of series inductance really helps. Some other SiC parts have absurd gate resistances.

We are having good luck with the EPC BGA GaN parts as super fast switches. Pain to assemble, but electrically great. Unlike SiC, they are happy with 4 or 5 volts of gate drive.

(40 volt supply, 50r out, loaded to make 20 volts)

Gotta find a use for that circuit.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

The next generations of Rohm SiC FETs have MUCH lower gate resistance.

10 or 20 times less and lower gate charge as well. Smaller die for the same RdsOn but that's OK because we'd be using a lower RdsOn part anyway, which has a better short current time spec.

The frequencies that I am running these things at, whcih is less than

50 kHz at the moment, is fairly easy. The newer generations require less gate voltage for full turn on as well. Same with the Cree/Wolfspeed parts. 15V recommended rather than 20V. I also use negative drive when possible but SiC parts are kind of picky and don't want to exceed their ratings for long life.

Would be Interested to see the drive circuit for that.

Well, except maybe for big thyristors; a full 8-inch wafer

2kV 4kA switch would be hard to replace in SiC. Still, thyristor turnoffs aren't a pleasant subject, and the old horizontal output transistors were TERRIBLE switches at HV (but they could tolerate the HV after switching OFF at a moderate voltage).

Trying to use bipolar transistors for high power was almost easier with germanium; they wouldn't take heat, but a 90A switch could do everything you'd want (except fit its fat emitter pin in a TO-3 socket).

The c-b junctions of some horizontal output transistors made fabulous Grehkov (drift step-recovery) diodes. I miss them.

Here's a negative 2KV pulse that was used in a tomographic atom probe.

I applied +80 volts to the junction in the forward direction and waited a while for it to conduct, maybe 100 amps. Then reversed biased it 400 volts through an inductor. When it snapped off, it made a big spike.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

That's kinda private. But regular cmos logic chips can drive low-side GaN fets. ACT maybe. All three sections of NL37WZ16US in parallel is awesome.

The high-side driver in a GaN totem pole is harder.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Texas LMG1020 is also fast option.

Uwe

--
Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de 

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt 
--------- Tel. 06151 1623569 ------- Fax. 06151 1623305 ---------

Close enough !

Oh yes, no problem with silicon as long as the switching frequency is low enough and/or there is no reverse conduction if high frequency is required.

I like the big thyristor pucks !

I suspect John makes discrete driving circuits.

His 1kV pulser is slightly faster than mine, and we're both using the same SiC parts. I think he's seen my circuit, RIS-764Gb, but I haven't seen his. I'm using 1.0 ohm from half of a UCC21520, with NPN / PNP SOT-89 emitter-followers. driving +15V through 4.7 ohms and -3.5V through 1.5 ohms, to SiC gate. It's a pretty serious gate drive, 6 watts of power, can go to 5MHz rep rate. Interleave for 10MHz.

--
 Thanks, 
    - Win