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Author Topic: Update: Experimental 4 Silicon-Carbide MOSFET 1kW Transmitter on the air  (Read 34983 times)
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« Reply #25 on: March 26, 2015, 01:07:38 PM »

Just beautiful Al!  So gorgeous.  b.
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« Reply #26 on: March 26, 2015, 02:16:28 PM »

This is my PWM (PDM).  It's for a 400 watt carrier but is very similar to a 1000 watt carrier unit except the power transformer and filtering is built to handle the increase in power.  I believe the same control boards are the same and the heat sink on the back would be bigger

First pic is the whole transmitter - top to bottom = VFO / digital drive, Modulator / PS and RF deck

Second pic is the modular without the shelving section showing the power supply comprising two power transformer which secondaries are wired in series to get the proper PS voltage

Third pic is the shelving section comprising the pulse width modulation circuitry and protection / efficiency circuitry

gl, Al


First, great work as usual Steve on evaluating and getting on the air with the new Cree FETS.  As promised many times before, one of these days I will be building one myself, but the longer I wait the more improvements there are so my procrastination is paying off  Wink 

To you Al, that workmanship is stellar for sure.  I believe that is what Wayne did for you when he took on the project maybe 2 years ago.  You always sound fantastic with it. 

Joe - GMS 
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« Reply #27 on: March 26, 2015, 02:18:47 PM »

Just beautiful Al!  So gorgeous.  b.

I cannot take credit for building it.  My hands shake too much so a friend of mine WA1SSJ built it.  I offer it as an idea how to approach building a PDM / PS unit.  I think your unit might be higher to accommodate the height of the power transformer and perhaps the capacitor bank.  The concept of using a shelf to the construction might be a good design to follow. You might have to do a wrap-around to avoid the total height of the unit being to high

Good luck, al
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« Reply #28 on: March 26, 2015, 08:39:59 PM »

Wayne is one of the better builders around.  Great work.  Al sounds great on air with it with a lot of meter swing. 

Steve....nice buzzardly meters on that mod deck!  BTW...what is the purpose of the 10.9uH coil in your PU-36 space modulator?

p
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« Reply #29 on: March 26, 2015, 08:59:03 PM »

Wayne is one of the better builders around.  Great work.  Al sounds great on air with it with a lot of meter swing. 

Steve....nice buzzardly meters on that mod deck!  BTW...what is the purpose of the 10.9uH coil in your PU-36 space modulator?

p

Hi Peter,

That is the first inductor in the PWM filter.  The other two inductors are wound on X-Flux cores.  There is too much voltage swing at the first inductor to make it using cores and keep the losses to a minimum.  But, once through the first inductor and capacitor, the swing is very small and the inductances can be fabricated using cores - and take up a whole lot less space!
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« Reply #30 on: March 26, 2015, 10:29:39 PM »

Steve,

Cree is one of those names that have become familiar to many of us for different reasons and all within just a few years. For me, I was looking for something to replace my house full of compact fluorescent light bulbs. I was tired (from the start) of the amount of light and lag in ignition mainly. I waited a long time for CFL's to get better but it just never happened.

I was at Home Depot about a year ago looking for an indoor spotlight replacement for the can over the sink in the kitchen. Looking at the lighted bulb display I picked the one that looked the best and was very bright for a change. Neat thing was, it was a Cree led array. I gave it a try and have never looked back. Next was a replacement for a 100 watt incandescent reading light. My last test was a 100 watt equivalent on a dimmer. That was enough proof for me regarding looks but they still needed to pass the noise test.

Most of the LED arrays use a series set of diodes to use up the 1XX volts or so. Running LED's on AC causes them to blink 60 times a second and is annoying to many people. The fix is an AC to DC regulated supply in the base, but that costs money. Most companies making LED household replacements use cheap switching supplies producing square wave DC. These still tend to blink but not as bad as AC. The thing you have to look for is the best regulated DC power supply you can find and an LED with color that is pleasing. The Cree bulbs are quiet as a mouse. Bright as can be. Much better economically than a CFL and every bit as bright as a incandescent. Best part is the 10 year no BS replacement policy. You cant go wrong. Every bulb in the house and garage are now Cree LED's. I still have 8 fluorescent tubes in places that need tubes. The rest are all silicon.

Cree is AWESOME!  My Class E lighting system.

Mike
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« Reply #31 on: March 27, 2015, 02:13:48 PM »

Steve,

Cree is one of those names that have become familiar to many of us for different reasons and all within just a few years. For me, I was looking for something to replace my house full of compact fluorescent light bulbs. I was tired (from the start) of the amount of light and lag in ignition mainly. I waited a long time for CFL's to get better but it just never happened.

I was at Home Depot about a year ago looking for an indoor spotlight replacement for the can over the sink in the kitchen. Looking at the lighted bulb display I picked the one that looked the best and was very bright for a change. Neat thing was, it was a Cree led array. I gave it a try and have never looked back. Next was a replacement for a 100 watt incandescent reading light. My last test was a 100 watt equivalent on a dimmer. That was enough proof for me regarding looks but they still needed to pass the noise test.

Most of the LED arrays use a series set of diodes to use up the 1XX volts or so. Running LED's on AC causes them to blink 60 times a second and is annoying to many people. The fix is an AC to DC regulated supply in the base, but that costs money. Most companies making LED household replacements use cheap switching supplies producing square wave DC. These still tend to blink but not as bad as AC. The thing you have to look for is the best regulated DC power supply you can find and an LED with color that is pleasing. The Cree bulbs are quiet as a mouse. Bright as can be. Much better economically than a CFL and every bit as bright as a incandescent. Best part is the 10 year no BS replacement policy. You cant go wrong. Every bulb in the house and garage are now Cree LED's. I still have 8 fluorescent tubes in places that need tubes. The rest are all silicon.

Cree is AWESOME!  My Class E lighting system.

Mike

Same results here Mike in regard to the Cree LED bulbs.   In addition they are manufactured here in the USA.   I have never had any EMI issue with them except when I was using an indoor RX antenna very close to the bulbs which is the case down in my Lab.  I did have one failure out of maybe 30 or so bulbs.  Home Depot told me that I was supposed to send them back to Cree in order to get a replacement and pay any shipping charges.  In the end, Home Depot gave me an exchange but they said they were doing me a favor. 

Prior to purchasing the Cree LED bulbs, I had no idea of how diversified of a company they are.  I wonder what FET's they are using in the bulbs?  I can see it now, Cree light bulb FET finds a home in a RF amplifier  Wink 

Joe GMS
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« Reply #32 on: March 27, 2015, 10:20:49 PM »

These devices are quite amazing !  I've been running the transmitter for more than a week now, and am quite impressed with the results.

The efficiency is REALLY high - the small heat with the small fan running at a low speed stays COLD, even after long transmissions.

So far, no issues.  Everything appears to be very stable.   The construction was oh so much easier with 4 devices and 4 drivers as opposed to 24 devices and 12 drivers!  What a difference!!
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« Reply #33 on: March 28, 2015, 01:13:53 AM »


So far, no issues.  Everything appears to be very stable.   The construction was oh so much easier with 4 devices and 4 drivers as opposed to 24 devices and 12 drivers!  What a difference!!

That IS amazing.

Now let's see -  as a mental exercise, imagine, just imagine (24)  C2M0040120Ds...  Shocked

I'm thinking the AM disco duck high tech rig of the future may be an SDR Apache including  $20 and a handful of C2M0040120Ds FETS.   Its getting easier and easier to put a hi fi AM KW on the air these days.

T
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« Reply #34 on: March 28, 2015, 05:42:12 PM »

Steve,

Any IMD issues like the other gentleman (don't remember his name or call,  but on the other thread dealing with these types of devices)  reported? Otherwise,  seems the new hot ticket be found!
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« Reply #35 on: March 28, 2015, 10:17:09 PM »

That was John W1TAG.  I don't know yet.  

The modulation on this RF amplifier is slightly different than my 24 FET transmitter using the same modulator, but I did figure that out.  The 24 MOSEFT transmitter has a lot more capacitance to ground than the 4 MOSFET transmitter.  The 4 MOSFET transmitter actually appears to present a better load to the pulse width modulator than the 24 MOSFET transmitter.  Most likely due to a much lower capacitive load presented to the modulator than does the 24 MOSFET RF amp.

When modulating with a square wave with the new RF amplifier connected, there is quite a bit less overshoot than I get with the 24 MOSFET RF amplifier (using the same modulator and the same voltage and current).

Still experimenting, but working very well.
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« Reply #36 on: March 30, 2015, 09:21:10 PM »

Hey Steve did you figure out why one of the FETs was drawing more current?  I noticed that the required gate drive voltage range is considerably more than the Si equivalent.  For example the Sic gates do not fully turn on until they reach 16V typically.  So I am wondering if this may be an issue and that another driver may be needed more suitable for Sic.  Anyway, I am sure you have looked at this, but some thoughts. 

These devices are going to open a whole new world to those who are interested in a class E/D deck but were intimidated by all the heatsink work of the Si fets.   2 fets to emit 250-400W carrier is impressive.  I think I could do a complete heatsink RF amp in an afternoon using 2 fets...versus a week plus for the 11n90s!  Cheesy


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« Reply #37 on: March 31, 2015, 11:11:22 AM »

Hi Pete,

I was able to get the modules to all operate at the same current.  I ended up using a slightly lower shunt capacitor.  It may be the capacitors are not high tolerance (maybe +/- 20%) which would definitely cause anomalies !

The gate drive is the first thing I tested.  The devices appear to be fully saturated at 15V.  I vary the drive from about 10V to about 18V with absolutely not change whatsoever in the drain current.  These are good FETs!!!
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« Reply #38 on: March 31, 2015, 01:36:01 PM »

Steve

Very neat stuff.

Based on the waveforms that you have measured, do you think these devices (including the IXYS drivers) will work well on 7.3 MHz?

Stu
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« Reply #39 on: March 31, 2015, 02:33:04 PM »

Steve

Very neat stuff.

Based on the waveforms that you have measured, do you think these devices (including the IXYS drivers) will work well on 7.3 MHz?

Stu

Hi Stu, glad you chimed in !  Wisdom always welcome  Cheesy   Yes, based on what I see so far, I think these devices will do very well on 40 meters.  The IXDD414 drivers I'm using definitely work up there.
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« Reply #40 on: April 02, 2015, 12:57:14 AM »

Al,

That transmitter of yours is as pretty they come.
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« Reply #41 on: April 02, 2015, 10:57:25 AM »

These devices, and "E RIG" technology in general really intrigues me. I must admit that I'm not that versed in the overall technology. Can this transmitter be used as an amplifier, driven by my Flex 6500? I'd like to use it on AM for 160/75 and possibly 40m. Exactly what would be required?
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« Reply #42 on: April 02, 2015, 11:51:03 AM »

It sounds like these would be less fussy to drive using an IXDD chip rather than the FQA11N90's.  I haven't bought my semiconductors yet... I'm guessing it would be better to just buy two of these devices rather than 8 FQA11N90's.  The cost is a little higher, but if you just have to buy two it's not that bad.

And I guess analog drive it out for these.

I hate to bring this up again, but to simplify things you could run these class C too.  Two devices, two IXYS chips, DDS driver with a JK flip flop in between and done!

Jon
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« Reply #43 on: April 02, 2015, 01:17:25 PM »

With respect to operation as a linear amplifier:

I compared the specification sheet for the C2M0040120D to the specification sheet for an MRF150 (used in various existing. high power linear amplifier designs).

The transconductance of the C2M0040120D is about 3x that of a single MRF150 [I.e. a single C2M0040120D would have the transconductance of three MRF150's, in parallel]. The reverse transfer capacitance of an C2M0040120D is 4x less than that of a single MRF150 [I.e. that's good for stability]. The input capacitance of a single C2M0040120D is about 3x that of a single MRF150 [I.e. comparable ease or difficulty of driving the amplifier at any given RF frequency and RF output power level]. The sensitivity of the threshold voltage to temperature is comparable. The C2M0040120D has a much larger DS breakdown voltage and is rated at about 3x the continuous DS current of a single MRF150. The C2M0040120D is not rated for use at higher frequencies (150MHz for the MRF150).

It would appear (to me) that a reasonably matched pair of C2M0040120D's, operating in push pull, at 7.3MHz, would work using a typical FET linear amplifier architecture.... and would be capable of around 333W PEP output. Since the efficiency in class AB operation is only around 67% (maybe less)... the heat sink would have to remove around 167W of amplifier dissipation, if operated in AM mode. I.e. on modulation peaks, the total electrical input power would be around 500W = 40V x 12.5A, while the RF output power would be around 67% of 500W = 333W.  At carrier, the electrical input power would be around 250W, while the RF output power would be around 83W.  

I'm thinking about trying this... but it would be helpful to get additional input from those who have actually implemented FET linear amplifiers. I may have overlooked some key issues.

Stu
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« Reply #44 on: April 02, 2015, 01:35:14 PM »

Here is a paper that address using the Cree devices in linear AB RF service.  The power level is much lower, but it still was an interesting read. 

http://www.cree.com/~/media/Files/Cree/RF/Papers%20and%20Articles/118A_GaN_HEMT_Class_AB_RF_Power_Amplifier.pdf

Joe-GMS
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« Reply #45 on: April 02, 2015, 08:58:46 PM »

These devices, and "E RIG" technology in general really intrigues me. I must admit that I'm not that versed in the overall technology. Can this transmitter be used as an amplifier, driven by my Flex 6500? I'd like to use it on AM for 160/75 and possibly 40m. Exactly what would be required?

The E/D transmitters will not work as a linear. You can use a Flex as a VFO to drive an E/D RF deck. As Stu noted the components such as what Steve is testing now, may work in linear service (untested though).  I experimented with the IRFP260 series in linear service.  You have to use some bias on these switching fets and that was a trick to keep the fets clean from crud.

Peter
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« Reply #46 on: April 10, 2015, 11:24:43 PM »

I've messed with the duty cycle of the gate drive a bit, and actually kicked the efficiency up even more!  The heat sink now barely breaks room temp after a very long series of old-buzzard transmissions.

The highest efficiency was obtained by leaving just a little "dead time" between the two phases.  The O'scope pictures show the gate waveforms on 75 meters with one IXDD414 driving each of the silicon carbide MOSFET's gate.

The first picture shows the waveforms of the two phases.

The 2nd picture shows the waveforms overlaid on each other which shows the dead time very nicely !




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« Reply #47 on: April 10, 2015, 11:28:16 PM »

I forgot to mention, the voltage is set to 10V per division on the scope.
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« Reply #48 on: April 11, 2015, 09:04:51 AM »

I've messed with the duty cycle of the gate drive a bit, and actually kicked the efficiency up even more!  The heat sink now barely breaks room temp after a very long series of old-buzzard transmissions.

The highest efficiency was obtained by leaving just a little "dead time" between the two phases.  The O'scope pictures show the gate waveforms on 75 meters with one IXDD414 driving each of the silicon carbide MOSFET's gate.

The first picture shows the waveforms of the two phases.

The 2nd picture shows the waveforms overlaid on each other which shows the dead time very nicely !






Steve that's very interesting, thanks for posting the pics.
I'm wondering why so much dead time, perhaps the Td off of those big FETs is hanging them up a litlte.  My results with the 10A FEts is quite different. Gate drive was adjusted for most output power for lowest input current ( best efficiency) and required very little dead time probably because the C2M0280120D switch much faster with Td on and off at 5.2 and 10.8 ns.  What would be nice to see would be a scope trace of gate on ch1 and drain on ch2, could you make those measurements and post it please.

As for efficiency yes these FETs are amazing. I have concluded that when instrumentation error starts to become a significant factor in the measurements then we are doing something right.  The new standard need to be heat sink temp rise as you mention.  Other than for bragging rights at Nearfest who really cares if its 95% or 98% as long as the thing runs cool with a small heat sink.

This raises another question.  Have you tried your amp in class D.  I have on mine and find there is only a small 3 to 4 % drop in efficiency probably because these FETs switch so fast.  The deck runs just as cool but the low pass filter inductors get slightly warm. I know there is a strong class E culture out there but these FETs open new doors to consider elimination of all that OP tuning hardware.

My gate drive waveforms attached using IXDD604 DIP8 drivers one per FET at 12 V.
73s  Nigel


* gate.PNG (61.66 KB, 1366x768 - viewed 471 times.)
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« Reply #49 on: April 11, 2015, 01:15:10 PM »

Hi Nigel,

I will try to post the gate VS drain waveforms.  There is very little time between the gate voltage falling and the drain waveform rising.  The rig gives a perfect class E waveform - the drain voltage has fallen to 0V _before_ the gate voltage goes high and turns the MOSFET on.  So, the switching losses are extremely low.

Haven't tried class D with this setup.  I like class D also, and my first efforts were class D.  May do another in the future.  I like the flexibility of class E, in that you can work into a reasonably large range of load impedances and everything functions properly.  In a setup such as mine, where there is no antenna tuner (coax fed dipoles), this is a real must.

I like class D because you can build a rig that is smaller and less expensive.

One thing I've never tried is combining class D "modules" to get high power, and working into a single output network.  With class E, this is very easy to do because the waveforms are reasonably rounded, and when combined in a single ended push pull arrangement, contain relatively few harmonics.  Due to the nature of the waveform, it makes combining very simple, and the output network is straight-forward to design.

Good stuff all around !

Regards,  Steve
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