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Date: Sun, 29 Dec 2013 19:39:45 +0100
From: wolf_dl4yhf <dl4yhf@freenet.de>
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Subject: Re: LF: MF PA
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Status: O

Hi Stefan,
> Hi Wolf,
>
> What type of FETs are you using? The standard IRFPs? Many of them have 
> internal Z diodes, so no addional RC network or Z idode is needed. The 
> losses are there anyway, of course.
>
I'm using HUF 75343 P3 : 75 A, 55 V, 9 mOhm RDs on, and a hefty gate 
input capacitance.
Just for historic reasons (on LF), and because I still have a tube with 
over a dozen of them.
But the TC4452 has sufficient driving power to switch them rapidly 
enough, even at 470 kHz (and presumably more).

For the DC modulator (with 0.1 V ... 13 V output, depending on the PWM 
duty cycle), there is another HUF 75343, and one IRF 4905, working 
together as a complementary push-pull stage at 100 kHz with a PWM DC of 
"almost" zero (due to the limited speed of the comparator + driver) to 
100 percent. After a simple choke, and a few microfarads (ceramic), this 
provides the 0... 13 Volts for the MF final stage.
In case of mismatch or excessive drain current (actually measured as 
source current in the PA), the PWM DC is gradually reduced in a control 
loop... and the control loop design turned out quite tricky, because the 
loop gain depends on the load, and the loop has a dead time (due to the 
100 kHz switching-mode modulator) which can cause the loop to oscillate 
under certain conditions. This is actually the reason why the PA is not 
"on air" yet...

Cheers,
   Wolf .