Update ... 375 Watt Carrier, Current-Mode Class D

[WD5JKO]

What do your waveforms at 13.56 MHz look like? Wonder what FETs you are using up there. My next missions is to move up to 20, 15 and 10 meters with CMCD. Any pointers would be appreciated.        

 Jay,  

I am away on a business trip, but I will get some pictures in a week or two of the gate drive.

These AE amplifiers use RF drive, and two power FET's in parallel to make 3KW out of 200 VDC. They have three power conversions in the box, 3 phase 208VAC to unregulated DC (with small energy storage cap - PFC correction), then DC to DC regulated 0-200VDC, and then DC to RF. Overall they are 55-65% efficient depending on the power level and the individual unit.

  I am new to these technologies and my time with them has been limited. It appears to me though that RF drive has a problem over digital drive. With RF drive in order to get the FET gates over ~ 8 volts peak, the pulse width gets too wide. I definitely see the unit efficiency drop several % as the RF drive amplitude increases. Conversely as the drive amplitude decreases, I see the % efficiency increase. I tune the RF drive (a coil to bend) while monitoring the fet drain saturation voltage and reducing the gate peak level until I see the the fet 'ON' drain voltage start to rise a tiny bit, and then go back until I am 'just' saturated.  There is nothing to tune on the Drain side.

Jim
WD5JKO

[WA1GFZ]

FETs turn off slower than they turn on so it sounds like the increase in drive is making the FET turn on more than 50%. In a push pull circuit this causes phase overlap and loss of efficiency. I have played with drive transformer  turns ratios to reduce voltage and decrease drive Z to force the gate on and off quicker. Sounds like you know how to find the sweet spot but not sure if that spot is stable for you.
My 75 meter final (22 FQA11N90s in push pull parallel) I run 4:1 turns ratio with the secondary grounded center tapped to drive two phases. I run about 88 or 89 % efficiency class e. It takes about 70 watts of RF drive.

[W1VD]

Agree with Frank - sounds like a gate drive duty cycle issue. On 40 meters it was impossible (at least for me) to break into the 90's without it.

Suspect attention to gate drive is even a bigger deal at 13.56 MHz. Your 'sweet spot' may be the best that can be done in your current configuration ... but may not be optimum. Would be interesting to try a duty cycle controlled setup and look for improvement in drain efficiency.     

[W4NEQ]

I'm noticing the IXDD gate drivers have a Schmitt input.  Has anybody tried sine input to the drivers, varying the level to control the ultimate square wave duty cycle?

Chris

 

[W1VD]

Chris

Sounds like 'flirting with disaster' to me. It's pretty easy to have 'positive control' without a lot of circuitry. A schematic of a suitable circuit at:

http://www.w1vd.com/VFOdriver.pdf

[W4NEQ]

That 40 meter board looks really nice.  Perhaps you could sell your idea to some big preamplifier corporation that might produce them commercially ... ?