GOODBYE CLASS "E": WELCOME CLASS D REVERSE "1/D"

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WD5JKO:
Lots of South Americans on 7160 AM...

Looks like some really good circuit development, both tube type, and solid state.

Quite a bit of stuff on Class D Reverse, getting 400Watts out on 40m at somewhere around 95% efficiency, and using RF drive instead of digital drive.

https://lu5hah.blogspot.com/

http://lu6dcs.blogspot.com/

http://lu1agp.blogspot.com/

Amplificador RF Clase D Inversa 40m 450W (CMCD - Current Mode Class D Amplifier 40m):
https://youtu.be/Qs-iScjrtaA

Looks like 40M AM in Argentina is alive and prospering.

Added a spreadsheet, rename with .xlxs at the end instead of pdf.

Use Google Translate to switch to your preferred language..

Jim
Wd5JKO

K9MB:
Hi Jim,
Thanks for sharing the information on South American Class D operation, etc.

I have a couple of questions.

1. This all looks like the transmitters that W1VD developed several years ago called CMCD (Current Mode Class D) as opposed to VMCD (Voltage mode Class D?
It looks like the same design.

CMCD is fascinating in a lot of ways and designs have also managed to cover two bands by switching inductors and capacitors.

The only thing that troubles me about Class D is the necessity for the load to be close to 50 ohms or the design output impedance of choice.
Most just feed the transmitters into a tuner to effect a match to their real world antennas.

The 92+% efficiency of the amplifier is impressive, but the effects of the tuner are not stated.
Cheap tuners may lose between 10 and 20%  and very good ones seldom are better than 90% efficient.
If a 93% efficient transmitter with 400 watts input has an output of 372 watts into a 50 ohm dummy load and is then put through a tuner to match a 100ohm antenna at  a great 90% efficiency, the power into the antenna is 372 x .9 =335watts
335/400 = 84% efficiency for the current mode Class D amplifier.

Are my calculations valid?

If so, then a Class E transmitter which inherently is 90% efficient (360 watts output) would be considerably more efficient than a system using Current Mode Class D.
I now that Class E is more difficult to adjust and requires better FETs with higher voltage ratings, but the idea that CMCD is clearly a much better system is not as convincing for me in a real world application after looking at the full implementation of the design.

Please discuss. I do not know enough to do more than ask questions here.
73, Mike K9MB

M0VRF:
Your right, it's nowt new.

I'm sticking with class E, works for me and covers a good portion of the band without tuning.

Plenty of high voltage FETs around both GaN and SiC.

However good info!

JB.

K9MB:
Quote from: M0VRF on April 23, 2022, 03:59:00 PM

Your right, it's nowt new.

I'm sticking with class E, works for me and covers a good portion of the band without tuning.

Plenty of high voltage FETs around both GaN and SiC.

However good info!

JB.




Thanks for the comments.
I have a question on the SIC and GaN devices.

The SIC devices seem to have very low input and output capacitance and low reverse transfer capacitance, so they look impressive there, but the heat transfer from junction to case is several times higher on the cheaper versions I have looked at.
The C3M0280090D is a commonly used SIC device by WolfSpeed that is a good case in point.
The gate resistance (Rgs) on this device is 22 ohms, while older silicon devices like the FQA11C90 have Rgs values less than 4 ohms.
The input capacitance is only 220pF, which is 9 times smaller than the 11c90, but the increased Rgs cancels a lot of that advantage-right? It might still be faster to load and unload the gate, however in spite of the much higher series resistance of the gate.

The transfer rate of heat from junction to case is 22 degrees C/watt compared to 0.47 degrees/watt on the old 11c90 FET.
It appears that any heat generated inside the device faces a much higher transfer gradient in this C3M0280090D SIC device compared to the venerable FQA11C90 device.

I found a very nice SIC at Mouser made by LittleFuse that has very low Rgs and good heat transfer while also having low capacitance and high rise times, etc, the
LSIC1MO120E0160

The only downside to this device is the cost is more than $12 each. 11c90 can still be purchased for less than $4 each.

The GaN devices are very impressive, though voltages seem low and also the cost is pretty high on them.

Choosing the best devices for a particular design seem less cut and dried for me, therefore.
I would appreciate your ideas on these tradeoffs and why you advocate the SIC Nd GaN devices, given their own inherent limitations and higher cost?
73, Mike

M0VRF:
I've never bothered to use the Fairchild FETs that most folk seem hung up on. Way too hard to drive on 40M as I use SOIC drivers and need a low Qg. The GaNs from Transphorm are even better and have a pair running on 20M using the same drivers.

I've published my designs on here for quite sometime with next to zero interest as I'm only interested in 100W carrier designs at 48V and this lot want multi KW.

The Cree C3Ms are fine, use the lowest Qg (280090) ones for 40m and 80m + 65090s for 160m.

Stick to the ClassD design that's floating around with tuned O/P on the drivers, I think it's 2 half bridges in series? Or the W1VD design as it's fine.

You won't go far wrong using either.

JB

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