by **ke1gf** » Fri Aug 05, 2005 2:42 pm

Yeah stu, so driving the gates a bit harder helped eh? From what you said you weren't even driving them out of the triode region or just barely.

I get my efficiency and stability from drive, many a time I have thumped the "switchmode drive" bible to Tom Vu and Tina while they were building their big rigs. I like to have a stable rig that fires up each time I key the thing no matter what, it gets imbarrasing when you have a crap out.

It's very important!

And I don't use transzorbs, they can create more problems than they solve most of the time.

I use this formula from IXYS's MOSFET and IGBT driver bible to compute how much drive power will be needed

Work = (Gate Capacitance)x[((P-t-P Voltage) x .707)^2]x(frequency in hz)

or you can do it this way, it's the same thing

Work = (Total Gate Charge)x(Vcc)x(frequency in hz)

sorry but you can't lick the laws of physics, at least not yet.

Uhh, so you probably say what advantage does 30V p-t-p have over 24V p-t-p, other than making more heat?

it's the duty cycle, it goes a little something like this

lets say that the FET acts as a switch at +10V G-S

so if I run 30V p-t-p which is 15V pk = A (sinewave)

10V = (15V)sin(2xPIxfxt)

so at 4 Mhz, we know that t = 250ns they are hand in hand

but, we want to know how much of the period is the FET acting like a switch...

make sure your calculator is in radians, hehe!!!

arcsin( 10V / 15V ) / ( 2 x PI x 4000000 ) ~= 29ns after zero crossing

this is where the miller plateu occurs, the little notch in your sinewave.

and we can subtract back from 125ns at the zero crossing and get

125ns - 29ns - 29ns = 67ns of ON time per period

so the %DC is 67ns / 250ns ~= %27, you can recompute it for different amplitudes...

this is at 4 Mhz, at 8 Mhz though the %DC increases, and as the frequency gets higher the %DC gets closer and closer to %50 on...

No magic here, but if you go back to my equations the other day I was using %50 DC to do my figuring.

I'm to the point now that I design drivers for the E rig and I use sinewave drive at the final transistors because of the timing aspects that you get for free, no more rice box.

I'm really not interested in fiddling for a week with a tapped tank so my rice box is happy, and then when I "YAAAAAAY" into the mic watch my SWR foldback kick in... SNAP BANG POOF!

Cheers

-Bill 'GF