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Author Topic: Simple SiC TX  (Read 1504 times)

Offline OgreVorbis

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Re: Simple SiC TX
« Reply #15 on: June 14, 2019, 0028 UTC »

Sometime in the future a new PA layout is in order...2 RF fets for 1.5KW, 4 for 2.5KW...carrier that is.  5KW in 5 rack units, for the power modules anyway....wow.

+-RH

What FETs are you talking about? What frequency? That's an insane amount of power. Is this some design you are currently working on?
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Offline redhat

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Re: Simple SiC TX
« Reply #16 on: June 14, 2019, 0715 UTC »
Same ones I'm currently using, C2M0080120D's and C2M0045170D's.  Of course its all theory until you build one... 

A big part of the problem in getting above 1KW is the thermal management.  Large bonded fin heatsinks and fan capacities over 200CFM become mandatory, especially for gear meant to survive operation in outdoor summer conditions.

It will be quite the feat when it is running.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline OgreVorbis

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Re: Simple SiC TX
« Reply #17 on: June 15, 2019, 0524 UTC »
Same ones I'm currently using, C2M0080120D's and C2M0045170D's.  Of course its all theory until you build one... 

A big part of the problem in getting above 1KW is the thermal management.  Large bonded fin heatsinks and fan capacities over 200CFM become mandatory, especially for gear meant to survive operation in outdoor summer conditions.

It will be quite the feat when it is running.

+-RH

https://www.fischerelektronik.de/web_fischer/en_US/K%C3%BChlk%C3%B6rper/D/L%C3%BCfteraggregate/index.xhtml

These are a bit pricey, but it's a good company and they have everything.
Radio and Programming Blog: http://dosaidsoft.com/wp/

Offline redhat

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Re: Simple SiC TX
« Reply #18 on: June 15, 2019, 2106 UTC »
I found a local company that does custom bonded fin stuff for reasonable prices.  I'll see how that works out.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Josh

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Re: Simple SiC TX
« Reply #19 on: June 15, 2019, 2137 UTC »
No pelts or is that too much current?
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Offline redhat

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Re: Simple SiC TX
« Reply #20 on: June 16, 2019, 0524 UTC »
You mean peltier junction modules?  I haven't looked into it, but I would guess too much power consumption for the drop in temperature you get.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Josh

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Re: Simple SiC TX
« Reply #21 on: June 16, 2019, 1808 UTC »
Shame there isn't something that can absorb all that heat and turn it to lectricity.
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Offline ChrisSmolinski

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Re: Simple SiC TX
« Reply #22 on: June 16, 2019, 1849 UTC »
Shame there isn't something that can absorb all that heat and turn it to lectricity.

Curse you, Laws of Thermodynamics!
Chris Smolinski
Westminster, MD
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Online JimIO

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Offline Josh

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Re: Simple SiC TX
« Reply #24 on: June 19, 2019, 1854 UTC »

   https://www.autozone.com/cooling-heating-and-climate-control/heater-core/toyota/corolla

While I'm sure our brother Jimmy is being droll, water cooling might be an option, taking expense in funds and current as well as complexity/reliability issues into consideration.
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Offline ChrisSmolinski

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Re: Simple SiC TX
« Reply #25 on: June 19, 2019, 1913 UTC »

   https://www.autozone.com/cooling-heating-and-climate-control/heater-core/toyota/corolla

While I'm sure our brother Jimmy is being droll, water cooling might be an option, taking expense in funds and current as well as complexity/reliability issues into consideration.

Years ago I worked with an engineer originally from Egypt, he worked at their broadcasting department, I think mostly with the MW transmitters. He mentioned how they had some Soviet supplied transmitters that used steam cooling for the tubes, which was actually an excellent way to do it, initial thoughts aside.
Chris Smolinski
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Offline redhat

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Re: Simple SiC TX
« Reply #26 on: June 20, 2019, 0537 UTC »
Water cooling is a good option for stationary service but for field use is totally impractical.  If you have a good source of cold process water from a well or something much less than ambient it would be worth doing In some situations.  It may in Some cases allow you to use fewer power devices for a given output power.  I considered this for 1kw on up as it would allow two devices to reach 3kw carrier, but only if you could keep the chill block temperature under 80 degrees F.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Josh

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Re: Simple SiC TX
« Reply #27 on: June 20, 2019, 2032 UTC »
Perhaps hsf units with heatpipes as commonly used in the pc world could be a compromise?
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Offline Stretchyman

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Re: Simple SiC TX
« Reply #28 on: June 22, 2019, 1124 UTC »
A decent design should generate very little heat.

As you seemed determined to use outdated drivers which wil generate 5W of heat each trying to drive 4nF of gate C you will need an enormouse heatsink.

Sticking with the C2M0080120D should help as they have less than 1nF of gate C to drive.

Just to explain whats going on here.......

To maintain high efficiencies with class D or E the switching devices must be turned on and off very quickly.

Any time spent in switching between on or off leaves the switching devices in a resistive region and will generate heat.

So if it's off it's open circuit (no current drawn) and if it's on the only heat genearated is Current squared X RDS(on).

So with the C2M0080120D (RDS(on) is only 80mR! super low, so with lets (say) 10A flowing thru' it will only generate 8W of heat.

So not much!

The trick is to use high voltages and as these devices have a VDS of 1200V, you can use (say) 150V with no issues.

So we know we can push 10A and use 150V, so that's 1500W of potential power.

However unless the design is over 95% efficient, your going to generate a fair amount of heat.

Even 5% is 75W!

I dont think any design is going to work with just 2 devices.

Looking at the current 'Trend' maybe 8 fror a 1KW carrier, there's certainly no need to use 24 11N90's, that's a bit silly now....

Anyway you seem to have hijacked my thread as I was trying to promote simplicity, not high power, I'll start another thread...

Str.

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Offline redhat

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Re: Simple SiC TX
« Reply #29 on: June 22, 2019, 2146 UTC »
My current test bed has logged may broadcast hours using just two RF devices running at 600-700W carrier and about 165VDC of B+ before the modulator.  The problem with adding more devices is you multiply the number and quantity of circuit strays, and this in turn winds up limiting your operating frequency.  This is the primary reason those frankenstein rigs with dozens of fets barely work on 80 meters, and not at all on 40 meters.

If you want big power, you need big fets, which have higher input capacity.  What I'm doing now was not practical prior to the introduction of SiC devices due mainly to limited Vds ratings and input capacity fo conventional fets.  The thermal management problem is the last hurdle.  The current PA runs about 93% efficient at 6.9 MHz.  Add to that the losses in the modulator, around 4-5%, total DC to RF efficiency is on the order of 88%.  At carrier that is approximately 30W per device, and it scales from there.  At 1500W and average modulation, its around 77W per device and the thermal headroom starts to get thin.  Maximum case temperature at 77W for the C2M0080120D is about 100 degrees C.  The aluminum oxide insulators we use have a thermal resistance of about 0.315 degrees C per watt, so expect a 25 degree C rise right there.  A reasonable heatsink temperature is about 50-60C, so we would need a heatsink capable of dissipating 260W of heat and exhibiting a rise of only 10 degrees C.  This means we need a heatsink with a rating of 0.038 degrees C per watt, which is tough, but attainable under forced air with a bonded fin heatsink.

I'll try some of your exalted fet drivers this week and see how they do.  If history is any indication, I expect them to slide off the board when trying to drive one of my fets.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com