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SiC FETs latest?




 
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VE3ELQ
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« Reply #25 on: October 12, 2016, 09:30:31 AM »

The SiC fets work well when all is well, so to speak.  They don't have the SOA characteristics of a standard MOSFET, and therefore are much more intolerant of any sort of current overload.  So do be wary of that when designing.

Yes exactly, these SIC FETS are NOT drop in replacements for the older, more current tolerant silicon FETs like the 11N90. They work best, and most efficiently, with high voltage low current to achieve the desired power. My decks employ a powdered iron toroid OP transformer with a 6T to 5T ratio and the output filter is not matched impedance so they can run safely at 70V carrier or even higher. This still leaves plenty of Drain voltage headroom when modulated and keeps the current down.  Of course the power supply and modulator filter need to be designed accordingly.  Great fun.

73s Nigel
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KA6MOK
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« Reply #26 on: October 13, 2016, 03:23:47 PM »

The SiC fets work well when all is well, so to speak.  They don't have the SOA characteristics of a standard MOSFET, and therefore are much more intolerant of any sort of current overload.  So do be wary of that when designing.

I've been able to achieve VERY high efficiency due to the low R D-S on in experimental implementations.

The drive is more critical for sure. The devices don't saturate like a silicon MOSFET, so absolutely as much drive as is possible will be critical.  Without that, you will notice nonlinearity under modulation, and also random failures on peaks due to die heating.

Just my experience..


I need to watch out for sounding more like an expert than I am...  I have an electronics background,  but specc'ing FETs for conventional circuits is one thing, exotic stuff like this is another.  So, I'm still learning what makes a good Class E FET, and definitely listening to the experienced folks like you and Nigel.

I compared the two SOA graphs, they are rather different.  Kind of strange they don't show a DC line on the Cree FET? 

So far, Ive only done "couch testing",  running LTspice simulations...  no real world yet. I am using the Cree model for the FET we're talking about, (which is supposed to be very good) and think I see the unsaturated effect. Gate voltage matters.  Oddly,  even after much tuning around I only get efficency in the low 80's, with a single FET. I easily see mid 90's using other Si FET models.  Of course, simulation is only as good as your model, and more, being a good match to reality....

A question,  do any of you do any drain current waveform measuring?  I've had a little bit of workplace experience in Class E like stuff on a wireless charging system,  and I found looking at both the current and voltage waveform more insightful when tuning. Been researching DIY ways to do this cheaply and noninvasively...
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steve_qix
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« Reply #27 on: October 13, 2016, 09:28:17 PM »

I would definitely measure the current waveform, but I don't possess the proper current probe for my scope.  I did look into getting one, but they are tres expensive  Shocked

It would be good to measure if I could.

I'm using the Cree FETs, and they are very good - high efficiency with enough drive.  I keep the drive in line with the manufacturer's recommendations - which are pretty much right to the edge.

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VE3ELQ
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« Reply #28 on: October 14, 2016, 08:14:04 AM »

Jon, Never tried modelling them, I just "buy em" and "try em".  Have a fair collection of Duds and Slugs.
Have no idea how to measure drain current at RF, just meters on the DC input.  If you have this capability would appreciate some details.

73s  Nigel
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WD5JKO
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« Reply #29 on: October 14, 2016, 01:42:54 PM »

Every time we try to measure something, there are assumptions, and errors in the measurement. One way to measure current through a FET that is switchmode (Saturation/Open) is look at the Vds while in saturation. If you know the gate drive is enough to saturate the FET for a given Drain current, then the Vds enters into the equation I = Vds/RDS_on. That is a bit of a stretch, but a relatively easy measurement to make.

To minimize the error it takes a good probe with an extremely Short ground return. I attach a photo of a commercial 3 KW Class E amplifier which has an approximate Vds of 25 volts when saturated. With ~1/2 ohm RDS on, that would be 50 amps. The DC draw into the amplifier is about 185V @ 20 amps.

Jim
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* ClassE_Vds.jpg (91.41 KB, 718x474 - viewed 68 times.)
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