Date: Sat, 3 Jun 2017 12:13:39 -0400
From: Markus Vester <markusvester@aol.com>
To: rsgb_lf_group@blacksheep.org
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In-Reply-To: <15c6e6a8ba3.marcocadeddu@tin.it>
References: <15c6e6a8ba3.marcocadeddu@tin.it>
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Subject: Re: LF: For today the FETs survived... - not enough gate voltage?
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Hi Marco,

looking at your schematic, I think the gate drive may be too low. Ideally t=
he 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 o=
n. 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 c=
ase you won't get more output power but only much more dissipation. Also th=
e output impedance will go from low (voltage-source) to high (current-sourc=
e), which could explain the changing frequency response.=20

I would recommend to double the turns ratio of the gate transformer (e.g. 1=
0:20:20).

73, Markus (DF6NM)

-----Urspr=C3=BCngliche Mitteilung-----=20
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=20
VLF SMS ;-)
There is nothing really new.. I started from an avalable=20
chassis with on board a PS giving 180Vdc (1200W) and assembled the=20
Andy's half bridge 700W switching PA and before give it the full power=20
I'm checking with lower DC supply. Attached you see the schematic with=20
some change at the moment..
The output xfmr has 7T/19T, the coil of the guard circuit is not=20
connected (now is in serie with L2) and the resonating caps now are=20
5x1000pF in parallel.
The aim is finally with 180Vdc of supply have an output of 0,5=C3=B71kW=20
but.... as you probably red in the previous messages, from 10 to 30Vdc=20
the PA runs, at low power of course (up to 15W), and complies the=20
calculations; with supply between 30 and 50 Vdc the output go down to a=20
couple of W.
I made several trials it the last days, changing the core of the xfmr,=20
the turn ratio, the kind of capacitators but nothing changes (or at=20
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

...
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<div style=3D"font-size:10pt;font-family:arial,helvetica,sans-serif;color:b=
lack;"><div style=3D"color: black; font-family: arial,helvetica,sans-serif;=
 font-size: 10pt;">Hi Marco,<br><br>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 no=
t be enough to turn the FETs fully on. So when increasing the drain suppy v=
oltage beyond a certain limit, they will go out of saturation and limit the=
 available output current. In that case you won't get&nbsp;more output powe=
r but only&nbsp;much more&nbsp;dissipation. Also the output impedance will =
go from low (voltage-source)&nbsp;to high (current-source), which could&nbs=
p;explain the changing frequency response. <br><br>I would recommend to&nbs=
p;double the turns ratio of the gate transformer (e.g. 10:20:20).<br><br>73=
, Markus (DF6NM)<br><br>-----Urspr=C3=BCngliche Mitteilung----- <br>Von: ma=
rcocadeddu &lt;marcocadeddu@tin.it&gt;<br>An: rsgb_lf_group &lt;rsgb_lf_gro=
up@blacksheep.org&gt;<br>Verschickt: Sa, 3. Jun 2017 16:46<br>Betreff: R: R=
e: R: Re: LF: Re: I: Fw: For today the FETs survived...<br><br></div>Hi Ste=
fan,<br><br>and thank you :-) so you have too something to mumble while sen=
ding <br>VLF SMS ;-)<br>There is nothing really new.. I started from an ava=
lable <br>chassis with on board a PS giving 180Vdc (1200W) and assembled th=
e <br>Andy's half bridge 700W switching PA and before give it the full powe=
r <br>I'm checking with lower DC supply. Attached you see the schematic wit=
h <br>some change at the moment..<br>The output xfmr has 7T/19T, the coil o=
f the guard circuit is not <br>connected (now is in serie with L2) and the =
resonating caps now are <br>5x1000pF in parallel.<br>The aim is finally wit=
h 180Vdc of supply have an output of 0,5=C3=B71kW <br>but.... as you probab=
ly red in the previous messages, from 10 to 30Vdc <br>the PA runs, at low p=
ower of course (up to 15W), and complies the <br>calculations; with supply =
between 30 and 50 Vdc the output go down to a <br>couple of W.<br>I made se=
veral trials it the last days, changing the core of the xfmr, <br>the turn =
ratio, the kind of capacitators but nothing changes (or at <br>least the ch=
ange are within a fraction of dB).<br><br>that's the sad history Stefan...<=
br>All thoughts and suggestions are welcome! (including go to fish ;-)) )<b=
r>Marco<br><br>...<br><br></div>
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