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Author Topic: Silicon Carbide FETs  (Read 22273 times)
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PD0RTT
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« Reply #25 on: March 18, 2015, 07:56:25 AM »


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

Phil - AC0OB

Hello Phil. No problems here with the drive requirement, the gate votlages are square wave of 12Vpp from the IXDD609 driver ic.

regards 73'Martin
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VE3ELQ
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« Reply #26 on: March 18, 2015, 08:44:39 AM »

Yesterday I was testing my 6 FET amp at 60V with 130V modulation peaks.  It was doing great for about 10 seconds when I heard a SNAP and saw an arc on the side of a FET, the transformer dug in then the fuse blew.  Unlike the 11N90s these FETs have an exposed drain tab on the back requiring insulation pads. I used mica TO-220 pads that I had available but they are about 1mm too narrow to cover the entire drain tab so that left a small gap at each side up from the heat sink by the thickness of the pads.  They are still insulated but at high power the drain voltage pulses are high enough to arc over this gap.  Fortunately nothing was harmed but the fuse and my nerves.  Have ordered some proper sized pads.

So lesson learned.  Fully insulate these things for high voltage.

73s Nigel
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steve_qix
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« Reply #27 on: March 18, 2015, 01:34:49 PM »

Yesterday I was testing my 6 FET amp at 60V with 130V modulation peaks.  It was doing great for about 10 seconds when I heard a SNAP and saw an arc on the side of a FET, the transformer dug in then the fuse blew.  Unlike the 11N90s these FETs have an exposed drain tab on the back requiring insulation pads. I used mica TO-220 pads that I had available but they are about 1mm too narrow to cover the entire drain tab so that left a small gap at each side up from the heat sink by the thickness of the pads.  They are still insulated but at high power the drain voltage pulses are high enough to arc over this gap.  Fortunately nothing was harmed but the fuse and my nerves.  Have ordered some proper sized pads.

So lesson learned.  Fully insulate these things for high voltage.

73s Nigel

Glad nothing was blown up!!  If you're running class E, at 130V, I would expect you'd be generating peaks at the drain of around 400V - maybe even a little higher, depending on the value of your shunt capacitor and the tuning.

Still curious about your output transformer construction.  The output transformers I use do get fairly warm after a while.  If yours don't, I'd like to use that design.

Thanks and Regards,

Steve
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VE3ELQ
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« Reply #28 on: March 18, 2015, 06:31:55 PM »

0

Still curious about your output transformer construction.  The output transformers I use do get fairly warm after a while.  If yours don't, I'd like to use that design.

Thanks and Regards,

Steve


Steve when I first built it with 6 11n90 FETs I used 4 rows of 3 (12 total) mix 43 ferrite sleeves for the OP transformer(s).  The primary was copper tubing secondary #12 wire.  It  worked OK but never got better than 87% efficiency and after about 10 minutes at 300W 4Mhz the cores were too hot to touch.  So I tried toroids wound 8 turns bifilar of #12 speaker wire from the HW store on 2 inch cores. Sorry I don't know what mix they are or even remember where I got them.  The 8 turns was a SWAG but they worked so well I never tried anything different.  They literally run cold at 300W and efficiency went up to 92% with 11N90 FETs and about 98% now.  I think Amidon -1 mix or perhaps -2 mix would work well.  A pic attached before it was modified for SiC FETs.
73s  Nigel


* 20141220_101119.jpg (770.88 KB, 2048x1536 - viewed 576 times.)
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W4AMV
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« Reply #29 on: March 18, 2015, 06:40:38 PM »

I think the key there is try to get by with the lowest perm possible. Get the L by an increase in the physical size and I have seen an approach where the core was epoxied to the heat spreader. The TLT implementations tend to run cooler while the magnetic coupled approaches run warmer as the core loss is an issue.

Alan
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« Reply #30 on: March 20, 2015, 12:04:27 PM »

I have commenced construction of a 4 device, 1kW RF amplifier.

Heat sink drilled and tapped;  all devices (MOSFETs and drivers) mounted;  output transformers mounted and connected.

To be done:  RF bypass caps, shunt caps, driver bypasses, driver connections to the outside world, transzorbs (on MOSFET drains and driver power common point to eliminate any chance of a catastrophic failure), hookup and testing!

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VE3ELQ
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« Reply #31 on: March 21, 2015, 08:37:42 AM »

I have commenced construction of a 4 device, 1kW RF amplifier.



Steve whats it doing?  Did you make some watt things? Did it go bang?  Was on 80m AM with the crowd last night and we are curious. The silence is deafening.

73s  Nigel
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« Reply #32 on: March 21, 2015, 09:11:17 PM »

Still constructing !  Will definitely report back when I know more.

One thing - the gate drive waveform is very good - one driver per MOSFET.  It's almost square and the driver current is LOW LOW LOW (Under 1 Amp at 15V for 4 devices - that's low!).

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John K5PRO
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« Reply #33 on: March 21, 2015, 10:08:42 PM »

You guys are making me salivate about a solid state kW AM radio. The wide bandgap semiconductors using SiC (and Also GaN at higher frequencies) are really moving the technology to where a transistor = a decent tube.

4-5 years back, Microsemi was jumping into SiC static induction FET transistors, through one of their acquisitions in California. They sold me some devices (at work) on an eval board that could produce 1250 watts pulsed at 200 MHz. Single ended!! They were depletion mode so that a negative bias was required on the gate to stop drain current. In other words, with zero bias they would blow up! The drain voltage was 120-150 volts DC.

http://www.compoundsemiconductor.net/article/86791-microsemi-ups-the-power-of-its-sic-transistors.html

These transistors would make a LOT of power, but the bias requirement was fussy, had to have it on negative before the drain (plate!) voltage came on. Similar to GaAsFETs. 

Unfortunately they were arranged so that best topology would be in common gate, which made the power gain < 10 dB. This is typical for Lband radar transistors, being common base for years, and then when FETs came along, common gate. After blowing up the Microsemi part, and looking at commercial LDMOS devices, we decided that we would buy our amplifiers using Freescale parts, and only 8 would be needed for 5.5 kW amplifier, which was our goal. We have been successful with that project, but I have been wondering when and if SiC would find a new base, and it looks like its right here in HF communications.


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« Reply #34 on: March 22, 2015, 01:06:24 AM »

Ok, I've got the RF amplifier constructed and hooked up.

So far, so good.  I am able to run 45V @ 22.5A with 4 devices on 75 meters.  This is a classic "AM Killowatt" like a KW1 or Johnson Desk KW would run.

Efficiency appears to be amazing.  The RF amplifier is constructed on a 6 x 7.5 inch heat sink and the heat sink is just breaking temperature (I mean barely) with a low speed fan blowing on the fins because the fins are pointed down towards the table (pics and more data tomorrow).

Have not applied modulation as of yet - still making measurements.

The shunt capacitors I am using (Russian made micas - 1500pF at 500V) get too warm for comfort.  Am going to order some multi-layer ceramic caps for this rig.

Will probably modulate tomorrow.  I tend to bring things up rather slowly and make a lot of measurements, particularly when it's new design.

Regards,  Steve
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W4AMV
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« Reply #35 on: March 22, 2015, 11:35:36 AM »

You guys are making me salivate about a solid state kW AM radio. The wide bandgap semiconductors using SiC (and Also GaN at higher frequencies) are really moving the technology to where a transistor = a decent tube.

4-5 years back, Microsemi was jumping into SiC static induction FET transistors, through one of their acquisitions in California. They sold me some devices (at work) on an eval board that could produce 1250 watts pulsed at 200 MHz. Single ended!! They were depletion mode so that a negative bias was required on the gate to stop drain current. In other words, with zero bias they would blow up! The drain voltage was 120-150 volts DC.

http://www.compoundsemiconductor.net/article/86791-microsemi-ups-the-power-of-its-sic-transistors.html

Yes John, however, SiC is enhancement mode as well EPC has enhncement mode GaN. GaN as such is depletion mode and you are spot on, the bias up can be painful.

These transistors would make a LOT of power, but the bias requirement was fussy, had to have it on negative before the drain (plate!) voltage came on. Similar to GaAsFETs. 

Unfortunately they were arranged so that best topology would be in common gate, which made the power gain < 10 dB. This is typical for Lband radar transistors, being common base for years, and then when FETs came along, common gate. After blowing up the Microsemi part, and looking at commercial LDMOS devices, we decided that we would buy our amplifiers using Freescale parts, and only 8 would be needed for 5.5 kW amplifier, which was our goal. We have been successful with that project, but I have been wondering when and if SiC would find a new base, and it looks like its right here in HF communications.



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WD5JKO
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« Reply #36 on: March 22, 2015, 12:39:51 PM »

   As a watcher of the class E Ham technology being developed I am getting more and more interested in building one.

That said, an area of concern is keeping the active devices alive in a fault condition where the SOA (safe operating area) is exceeded. I remember most distinctly the DC to AC inverters from the 1970's using paralled darlington transistors. With a massive lead acid battery bank feeding the inverter, a fault often resulted in what I called the "Gatling Gun Syndrome". Imagine a bank of 32 transistors in a TO-3 case where each fuses the PN junctions inside with so much heat that the metal cover over the transistor is blown away and followed by a split second 3 foot flame from all 32 devices! Those things could be used as a weapon.  Tongue

I fondly remember the IRF-100 FET from IR (International Rectumhorse) in 1979-1980 time frame. These as I recall were 80V 0.2 ohm RDS ON devices. These revolutionized the switchmode power supply, and inverter industry. Long gone are Baker Clamps, and IB2 suck out schemes to minimize the delayed turnoff, and then the ordinarily long fall time of a darlington transistor.

Watching and learning from others on this board, many mega-class E rigs have been built. Many have been reliable, and some not. The use of switching devices with low overall dissipation capability are fine so long as we keep things within the SOA of the devices. March outside the SOA too much or too long, and the ""Gatling Gun Syndrome" rears its ugly head. For example, I once took a pair of 8005 triodes in push pull, and played with drive level, bias voltage, and plate voltage. I could literally get over 1000 watts carrier out from a pair with no color on the plates. Those tubes deep into class C were approaching theoretical efficiency. The problem was, an out of tune plate tank allowed for no longer than 1 second to find resonance. I once took longer than that, and one tube instantly got so hot that the glass was sucked in.  Cry

This thread is about SI carbide FET's, so I am sorry about the somewhat off topic post. The IXYS device I mentioned earlier, although not SIC, they have been 100% reliable as a pair running 3KW out at 13.56 Mhz. These have a Pd rating of 1.8KW each, and in a class E circuit they run close to 90% efficiency on the commercial 21 meter band. Since I mentioned those devices, we had a power supply failure feeding the RF stage, and the power supply went open loop....I could hardly believe my eyes when the wattmeter read 4.8KW!! No failure to those FET's!.

In conclusion, how do we prevent catastrophic SOA violations causing melt downs?  It seems from what others have said, the SIC FET's are fragile in this respect.

Jim
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VE3ELQ
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« Reply #37 on: March 22, 2015, 04:13:40 PM »

  As a watcher of the class E Ham technology being developed I am getting more and more interested in building one.

That said, an area of concern is keeping the active devices alive in a fault condition where the SOA (safe operating area) is exceeded. .........

In conclusion, how do we prevent catastrophic SOA violations causing melt downs?  It seems from what others have said, the SIC FET's are fragile in this respect.

Jim
Wd5JKO



Its often been said tubes have "ratings"  transistors have "specs".
Tubes will (usually) withstand short excursions outside their of their ratings. Transistors wont, even for a few milliseconds survive beyond their specs.  So the designs are, and always have been different.  Plenty of design head room is employed along with transorbs and other protection circuitry for over voltage, over current, high reflected power and anything else needed. My idiot proof 10 year old 1KW FET linear does all of this. It uses 4 SD2933 FETs when only two would do the job and still be under rated.  Sometimes its worthwhile to experiment to actually find the limits.  I did this with a pair of $1.50 11N40 400V 10A MOS FETs in a half bridge.  I drove them to destruction at nearly 1KW at 350V. I now know how far they will go and design accordingly.  Although SiC FETs are new to many of us I see no difference from what are the accepted design practices for solid state devices.

I highly recommend you build an E rig.  There is a lot of good info on how to starting with WA1QIX Steve's site. Start small at the 200 to 300 Watt level and go from there as I am doing.  Its a fun journey but it can become obsessive.

73s Nigel
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W1DAN
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« Reply #38 on: March 22, 2015, 07:20:23 PM »

VE6ELQ and all

i have enjoyed reading about these devices and the real-world tests.

thoughts on linear service as well as practical frequency limit?

my next deck will most probably use these.

jim: my class E deck has operated for around 10 years without failure. staying within maximum limits, having good headroom as well as proper overload detection can result in a very reliable transmitter.

thanks,
dan
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VE3ELQ
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« Reply #39 on: March 23, 2015, 07:58:42 AM »

VE6ELQ and all

i have enjoyed reading about these devices and the real-world tests.

thoughts on linear service as well as practical frequency limit?

thanks,
dan

Dan I thought the same, they may work well in linear amps.  Look at the transconductance curve fig.7 for the C2M0280120D FETs, its nearly a straight line once biased on a little. And with a Tr and Toff of 20ns should do 14 mhz OK, and the smaller ones that I am using are much faster.  We need a champion to work up a design so roll up your sleeves.

73s  Nigel

* C2M0280120D.pdf (800.5 KB - downloaded 216 times.)
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