Silicon Carbide FETs

[VE3ELQ]

In my search for better switching FETs for RF amps I found the specs for the Cree Silicon Carbide series FETs the C2M0280120D in particular, very promising. It exibits a low gate charge and a Ciss of 259 pf all in a 1200V 10 Amp FET. I already have a box full of MOS FETs that I ordered in twos and fours and most turned out to be poor performers at 4 mhz and above.  This one looks like it might work well in a high voltage half bridge.
Anyone tried this series of FETs before I waste any more money.

http://www.cree.com/Power/Landing-pages/MOSFET-products

73s,   VE3ELQ

[WD5JKO]

   I don't know much about the silicon carbide FET's, but look at this one available for $37.50:

http://www.rfmw.com/ProductDetail/DE475102N21A-IXYS-RF/232147/

I attach the data sheet. I can say that two of these at 13 dot 56 Mhz Class (E) make 3 KW CCS with resonated RF drive to the gates. Runs with a B+ of about 185 volts at this power level. You will need to adapt the package or use as is and clamp the bottom down good. It has a 1.8 KW Pd rating.

Jim
Wd5JKO

[PD0RTT]

I have tried these Carbide Mosfets in a experimental class E amp.
The first remarkable issue what I noticed was that the square wave voltage from the (IXDD609) driver was 'more squared' due the lower gate charge capacity of the Mosfet compared with other Mosfets.
The frequency what I used was relativily low, 1,8MHz.
Comparing with other HEXFET Mosfets, I use the IXFH44N50 the types from Cree what I have tested was the C2M0160120D.
In operational mode as a class E amplfier, I have found no difference between the IXFH44N50 and the Cree Mosfet.
Comparing the two types; draws exactly the same current, and gave same power at the output, you can say they give the same results.
The only difference between the two types that the Cree Mosfets require a lower shunt capacity at the drain in a class E amp.
The most remarkable issue is that these Silicon Carbide Mosfets are sensitive for connecting no load at the output filter.
I had four of these Mosfets and I blew all four up.
Then I used the types what I used before the IXFH44N50 and I have no problems anymore.

For high frequency's as 7 and 14MHz these Silicon Carbide Mosfets could be in a advance, comparing to normal HEXFETS.
They can result in faster switching with lower losses and lower gate drive requirements.
But they are more sensitive and less rugged than HEXFETS.

By the way the DE-475 is not an Carbide Mosfet.

73'Martin

[W1TAG]

Sorry for being late to the party. Kept forgetting to post. I played with some CREE C2M0160120D FETs last month in a Class D application. Definitely had lower input and output capacitance than the 11N90s. Efficiency at 7 MHz was significantly better than I had been able to get, about 90% for 100 watts (2 FETs in P-P). DC power consumption by the IXDD614 drivers went from 16 to 5.5 watts.

The catch was that when modulated, the THD readings were significantly higher on 75 and 160 meters than with the 11N90s. I didn't have time to pursue it then, but will be attempting to use the CREE FETs in another rig that I am slowly working on.

Didn't look for no-load blowups, though!

John, W1TAG

[VE3ELQ]

John your info is encouraging.   Puzzling why the THD went up, cant think why. Perhaps on the high mod peaks the FETs develop some parasitic oscillation which messes things up after all they are way faster than the 11n90s. Might be able to see this on a scope with triangle wave modulation at 100%.   
PDORT  Too bad yours blew but the 44n50 are much higher rated FETs current wise and no where near as fast so its really not a fair comparison. 

The C2M0160120D FETs are not that pricey so next time I make a Digikey order I will get 4 and play.

Trying to finish up an H bridge E rig and get it on the air before going too far off track on another project.
73s  VE3ELQ

[Opcom]

Just a guess but the THD increase could be due to the SiC device improved switching characteristics.

[steve_qix]

Sorry for being late to the party. Kept forgetting to post. I played with some CREE C2M0160120D FETs last month in a Class D application. Definitely had lower input and output capacitance than the 11N90s. Efficiency at 7 MHz was significantly better than I had been able to get, about 90% for 100 watts (2 FETs in P-P). DC power consumption by the IXDD614 drivers went from 16 to 5.5 watts.

The catch was that when modulated, the THD readings were significantly higher on 75 and 160 meters than with the 11N90s. I didn't have time to pursue it then, but will be attempting to use the CREE FETs in another rig that I am slowly working on.

Didn't look for no-load blowups, though!

John, W1TAG

That's interesting on the THD differences.  How are you modulating (what sort of modulator / technology, etc.) and at what percentage of modulation were you making your measurements?

Regards,  Steve

[W4AMV]

I have several of the C2M0280120D SiC MOSFETs for which I have measured s parameter data. I started a conventional class C push pull design. Cree offers Spice models. Although they are targeted at switcher applications, the models MAY provide insight into IMD. The FET I selected had a reasonable tradeoff in capacitance and gain for HF applications, 260 pF Ciss, 23 pF Coss, and a Qgs of 5.6 nC. The gm is quite good at nearly 3S. If you desire models for a specific FET, let me know.  

Regards, Alan

[W1TAG]

Steve,

The modulator setup for the 100 watt PA is a commercial audio power amp (Hafler P3000). It feeds single-ended through blocking caps to the top of a 100 mH power choke (Hammond 195T5) used as a modulation reactor. Since the modulated impedance is around 12 Ohms, no transformer was used. Very nice from 30 Hz to 15 kHz.

Measurements were made at 95% modulation. On 75m, I had 0.7% THD with the 11N90's, and 2.9% with the CREE FETs. The audio instrumentation was an Audio Precision System One, fed with audio from your mod mon pickup. I've put off further experiments until I get farther into the new rig under construction. That'll be a few weeks down the road, as I'm playing with audio processing right now.

John, W1TAG

[VE3ELQ]

An interesting article on development of SiC devices.  Looks like this is the future.
http://spectrum.ieee.org/semiconductors/materials/silicon-carbide-smaller-faster-tougher/0

I now have some C2M0280120D FETs and will testing them as RF switching amps shortly.
73s  Nigel

[VE3ELQ]

Just a quick report on the C2M0280120D SiC FETs.  First I tried two in a 200 volt half bridge at 4 mhz and got excellent results. The OP was a beautiful 200V peak to peak square wave which cleaned up nicely to sign wave with a series resonant LC OP network into a Cantenna load. Best efficiency obtained was 93%.  I was impressed.  So I resurrected a 6 FET parallel push pull amp built 4 month ago with 11n90 FETs and four IXDD614 drivers, pretty much as QIX Steve designed it only with toroid OP transformers which run cold. It worked well but driver current was 1.2 amps at 12 V at 4mhz best efficiency obtained was 92%. I exchanged the 11n90 FETs with six C2M0280120D FETs and removed two drivers so the 3 FETs on each side had one driver. Drive current was now 205ma with great looking gate waveforms.  At 60 Volts it was making 300 watts into the load without breaking a sweat. OP tuning was set for Class E best efficiency and was not overly critical, the power stayed up there just the current dropped a little. Now here is where it gets really interesting.  My first efficiency measurement gave 98%, cant be I thought, so checked meter cal against my fluke, bang on, measured the load at 46 ohms with 2 meters and used that in the calcs the scope is right on and the waveform was sign wave.  Ok try again at 200 watts, 97%, 300 watts 98%. So if its that good it must be running cold right, and yes it is.  Barely noticeable with a finger on a FET after 20 minutes at 300 watts no fans. My Cantenna on the other hand was smoking hot bubbling oil out the vent so I terminated the heat test.  My VFO driver is set up for 3.6 to 4 mhz so will rig something up to drive them at higher freqs and see what they will do.  Oh yea, when you stick your finger on a FET while operating at 300 watts watch out for the drain buss, that really hurts.  I love these FETs.  Try some yourself.
73s  Nigel

[PD0RTT]

Thats great! So just to summarize: in half bridge you got 93% efficiency, and in class E topology you have 98%, is that right?
Do you use one or two of these transistors in balanced class E topology, or only a single one - or more of them in parallel?

73'Martin

[W4AMV]

As I said, I studied all the Cree MOSFETs at the time I looked at what they had to offer. I did some simple modeling and s parameter measurements. These were at the top of my list.

Alan  

[VE3ELQ]

Thats great! So just to summarize: in half bridge you got 93% efficiency, and in class E topology you have 98%, is that right?

The Half bridge had a series tuned LC network with parallel C load shunt.  It was adjusted to put the FETs into class E.  Half bridge amps need to have a slightly inductive load to prevent shoot thorough leading to high current draw or worse FET destruction. O/P tuning is critical.

Do you use one or two of these transistors in balanced class E topology, or only a single one - or more of them in parallel?

As my report says 3 FETs in parallel per side (6 total) driven in co-phase with the same class E O/P network as the half bridge. Tuning is not critical for high output but takes some fiddling to get it just right for best efficiency in class E.

73'Martin

[steve_qix]

I'm starting a project with some of these devices.  1kW on 80/160 meters.

There is a larger device they have (60A at 25 degrees C).  In theory, 4 of them will replace the 24 MOSFETs in my current 80/160 meter RF amplifier.  Furthermore, the gate charge is QUITE a bit less, so 1 driver will be sufficient for each device, and the rise/fall times will be faster.

The _calculated_ efficiency is between 97 and 98 % efficient (MOSFET efficiency - does not account for output transformer losses and tank circuit losses).  The calculated efficiency with standard MOSFETs is around 90%.

The cost is the interesting calculation.  To build the 24 MOSFET transmitter with FQA11N90s, the devices cost between $72 and $96 for all. 12 drivers at around $5.00 each are required, along with the support circuitry, extra power supplies, bypass caps, etc. etc. etc. for 12 drivers.  Looking at $150.00 to $175.00 for the MOSFETs, Drivers and power supplies for the drivers.

The silicon carbide FETs are around $32.00 each (need 4).  I need 4 drivers at $5.00 each, and ONE power supply for all of the drivers.  So, it's pretty much a break even, or possibly a cost savings.  If the efficiency is greater, the heat sink can be made smaller - further savings.

I'm big on price/performance, so of course I had to look at that factor before starting anything.

Will report back!

[steve_qix]

The other thing I forgot to mention is this:  if only one device is used per module, the possibility of parasitics is DRASTICALLY reduced because nothing is in parallel.  Won't THAT be nice!

[n1ps]

Wow....4 FETs to do 1KW.   Steve are you building one of these now? That will save a lot of fabrication time.

p

[VE3ELQ]

I'm starting a project with some of these devices.  1kW on 80/160 meters.

Will report back!

Steve that should be one COOL project, (pun intended).

I've had very good results with the IXDD604 DIP 8 drivers, one per FET, which are inexpensive and a little faster than the 614s. They are on a surface mount PCB strip with the chips on the bottom (through the hole) held in contact with the heat sink with three 256 screws.  Can provide photos and PCB layout. If you mount the FETs up on a 3/8 inch alum spacer you can get the drivers up nice and personal to the FETs with very short gate leads. Just an idea.  We look forward to another of your excellent designs.
73s  Nigel

[W4AMV]

Steve, are your referring to the Cree FETs for the 4// units ?

Alan

[PD0RTT]

These are my amplifiers, the only thing what I have to do is connect them into the transmitter cabinet.

[steve_qix]

Steve, are your referring to the Cree FETs for the 4// units ?

Alan

Yes - from Cree, but the next size up.  I don't remember the number.  The .04 ohm device is the one I'm trying first.  They cost a little over $30.00 each (at least at Mouser).

[DMOD]

I have tried these Carbide Mosfets in a experimental class E amp.
The first remarkable issue what I noticed was that the square wave voltage from the (IXDD609) driver was 'more squared' due the lower gate charge capacity of the Mosfet compared with other Mosfets.
The frequency what I used was relativily low, 1,8MHz.
Comparing with other HEXFET Mosfets, I use the IXFH44N50 the types from Cree what I have tested was the C2M0160120D.
In operational mode as a class E amplfier, I have found no difference between the IXFH44N50 and the Cree Mosfet.
Comparing the two types; draws exactly the same current, and gave same power at the output, you can say they give the same results.
The only difference between the two types that the Cree Mosfets require a lower shunt capacity at the drain in a class E amp.
The most remarkable issue is that these Silicon Carbide Mosfets are sensitive for connecting no load at the output filter.
I had four of these Mosfets and I blew all four up.
Then I used the types what I used before the IXFH44N50 and I have no problems anymore.

For high frequency's as 7 and 14MHz these Silicon Carbide Mosfets could be in a advance, comparing to normal HEXFETS.
They can result in faster switching with lower losses and lower gate drive requirements.
But they are more sensitive and less rugged than HEXFETS.

By the way the DE-475 is not an Carbide Mosfet.

73'Martin

Have you had any issues with the - 3V gate bias requirement?

Phil - AC0OB

[n1ps]

Steve as you are working with high current devices, are you using a heat spreader?

p

[N1BCG]

I'm starting a project with some of these devices.  1kW on 80/160 meters.

There is a larger device they have (60A at 25 degrees C).  In theory, 4 of them will replace the 24 MOSFETs in my current 80/160 meter RF amplifier.  Furthermore, the gate charge is QUITE a bit less, so 1 driver will be sufficient for each device, and the rise/fall times will be faster.

If that's successful, then one FET providing a 240 watt carrier can do full legal limit (1500W PEP) when modulated to +150%. That pretty much makes my current 8x11N90 project obsolete.

[steve_qix]

1 FET should be able to do 250 watts or thereabouts, but don't build a 1 FET transmitter.  Use at least 2 modules (even if each has only 1 FET) for SIGNIFICANT harmonic reduction.

Heat spreaders:  So far, I haven't planned on any heat spreaders.  Hopefully, I don't need them.  I'm only looking at about 10 to 15 watts of TOTAL power dissipation per device.  This is about half R D-S on losses and about half driver power, all of which is dissipated as heat.

We'll see how things work once it's actually built !!