Message-ID: <15c6ee59c21.marcocadeddu@tin.it>
Date: Sat, 3 Jun 2017 18:59:20 +0200 (CEST)
From: "marcocadeddu@tin.it" <marcocadeddu@tin.it>
To:  <rsgb_lf_group@blacksheep.org>
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Subject: R: Re: LF: For today the FETs survived... - not enough gate
 voltage?
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Thank you Marcus: haahaa also you kept in council :-)

After fighting to place the oscillo probes on the gates which are a 
bit hidden... (this could be an improvement area...) the square wave is 
swinging from -5 to +8V with respect to ground. please don't ask me to 
measure between source and gate! ;-)

So it should be enough to switch the IRF460As

Marco
----Messaggio originale----
Da: andy.g4jnt@gmail.com
Data: 3-giu-2017 18.28
A: <rsgb_lf_group@blacksheep.org>
Ogg: Re: LF: For today the FETs survived... - not enough gate voltage?

Interesting thought.

Checked my own design (the QEX article is the only remaining
documentation!) and checked I  also have a 1:1+1 driver transformer.
But I do use a 15V rail for the driver, so +/- 7.5V which is a bit more
than Marco's

But never considered that ...

Andy  G4JNT


On 3 June 2017 at 17:13, Markus Vester <markusvester@aol.com> wrote:

> Hi Marco,
>
> looking at your schematic, I think the gate drive may be too low. 
Ideally
> the driver will make 12 V peak-peak, or +-6V at the gate (probably 
slightly
> less in reality, e.g. 5 V). This may not be enough to turn the FETs 
fully
> on. So when increasing the drain suppy voltage beyond a certain 
limit, they
> will go out of saturation and limit the available output current. In 
that
> case you won't get more output power but only much more dissipation. 
Also
> the output impedance will go from low (voltage-source) to high
> (current-source), which could explain the changing frequency 
response.
>
> I would recommend to double the turns ratio of the gate transformer 
(e.g.
> 10:20:20).
>
> 73, Markus (DF6NM)
>
> -----UrsprĂźngliche Mitteilung-----
> Von: marcocadeddu <marcocadeddu@tin.it>
> An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
> Verschickt: Sa, 3. Jun 2017 16:46
> Betreff: R: Re: R: Re: LF: Re: I: Fw: For today the FETs survived...
>
> Hi Stefan,
>
> and thank you :-) so you have too something to mumble while sending
> VLF SMS ;-)
> There is nothing really new.. I started from an avalable
> chassis with on board a PS giving 180Vdc (1200W) and assembled the
> Andy's half bridge 700W switching PA and before give it the full 
power
> I'm checking with lower DC supply. Attached you see the schematic 
with
> some change at the moment..
> The output xfmr has 7T/19T, the coil of the guard circuit is not
> connected (now is in serie with L2) and the resonating caps now are
> 5x1000pF in parallel.
> The aim is finally with 180Vdc of supply have an output of 0,5Ăˇ1kW
> but.... as you probably red in the previous messages, from 10 to 
30Vdc
> the PA runs, at low power of course (up to 15W), and complies the
> calculations; with supply between 30 and 50 Vdc the output go down to 
a
> couple of W.
> I made several trials it the last days, changing the core of the 
xfmr,
> the turn ratio, the kind of capacitators but nothing changes (or at
> least the change are within a fraction of dB).
>
> that's the sad history Stefan...
> All thoughts and suggestions are welcome! (including go to fish ;-)) 
)
> Marco
>
> ...
>
>