Technical Topics > The RF Workbench

Beyond 45/48m

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Stretchyman:
Are you sure?

The Qg = 68nC :(

The C3M0280090 =9.7nC :)

Somewhat easier to drive!

Are you using 'bolt down' drivers? I'm using SOIC 8's.

I normally use Transphorm GaN for above 5MHz as Qg = 6nC and 6V is plenty to switch them.

That Qg is too high for me!

Str.

Charlie_Dont_Surf:

--- Quote from: Stretchyman on June 12, 2022, 1831 UTC ---Are you sure?

The Qg = 68nC :(

That Qg is too high for me!

Str.

--- End quote ---

It's actually 64 nC, not 68, but yeah, I noted the same thing.

Keep in mind that with higher power, Qg tends to go up. It has to because in order to meet the required current density demand and preserve other relationships, they make these by replicating unit cell transistors in parallel, meaning there are multiple smaller gates in parallel, which means multiple Qgd and Qgs in parallel too. It adds up. The other thing that matters is the breakdown voltage. The higher the BV, typically/often the input capacitance and Qg increases too. The FET channel will be longer, which means larger surface area of the gate capacitance, which likely means larger Qg.

The Cree C3M0280090 is small (900 V, continuous drain current 6.8 A continuous at 100 C) compared to that Microsemi FET (1200 V, 26 A continuous at 100C), so of course the Cree is more "nimble". Side note: the Qg of these transistors are at different conditions so not exactly comparing apples to apples but still...

I'm about ready to populate a board with the Transform TP65H035G4WS: only 650 V but Qg = 22 and Id continuous at 100 C = 29.5 A. Potentially not the best solution for me but at least it is available right now, which is more than I can say for some other choices!

In any case, if he was trying to run that MicroSemi FET at 15 MHz (picking a number out of the air), it might be low(er) efficiency compared to other solutions due to the higher Qg. (Maybe.) On the other hand, I'm only aware of him using 6285 and 4185 KHz, so maybe he doesn't care about anything above 6.3 MHz.

Also, at some point it may make sense to use RF transistors - as opposed to switching transistors - to meet the performance goals you want, particularly at higher frequencies. (That's why they exist.) Personally, I'm nowhere near that point but I have thought about it.

redhat:
Your not reading again stretchy.  I am talking about 1700v devices with 80mOhm RDSon...not 900v devices at 280mOhm.

The current rig has run up to 10 Mhz.  I have little interest in anything much above that as I don't operate during the day.  Bolt on fet drivers are used to deal with the gate capacity.  I will explore GaN when the price performance ratio gets more attractive.  For now Sic makes more sense in my topologies and power levels.  If they come out with comparable high voltage GaN devices at a reasonable price, I'll consider it.

I like my current approach as it affords high power with a simple low cost PA architecture.  There are ways to get similar power with lower voltages, but circuit complexity, the headache of ferrite, high current losses, etc start to make things unnatractive for my needs.

SiC devices have made what I'm doing possible at much lower cost, and simpler design than was possible just a handful of years ago.  GaN for the moment, does not hold the same advantages.

+-RH

Stretchyman:
Ok Both, All understood!

Regards.

Stretchy.

Charlie_Dont_Surf:

--- Quote from: redhat on June 10, 2022, 0339 UTC ---In the modern era of 56V power tools, higher voltage operation is no longer a challenge.  My Ego trimmer has enough battery on it to run a 50 watt transmitter for over two hours, and recharges in 15 minutes!

--- End quote ---

It's funny that you brought this up since that same day that you wrote this my new Ego leaf blower arrived. I bought it without the battery since I already had one with my Ego weed trimmer; you can share batteries among their equipment, saving ~$100 or so.

In any case, I saw the amp-hour capacity of those batteries a few weeks ago and thought, "you could run a transmitter off this bad boy."  :)

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