> Date: Sun, 29 Dec 2013 19:39:45 +0100
> From:
[email protected]> To:
[email protected]> Subject: Re: LF: MF PA
>
> 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 .
>
