Message-ID: <002901c53513$3d541ae0$6507a8c0@Main>
From: "Alan Melia" <Alan.Melia@btinternet.com>
To: "LF-Group" <rsgb_lf_group@blacksheep.org>
Date: Wed, 30 Mar 2005 11:27:57 +0100
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Subject: Fw: Re: LF: More on cores...]
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Posted for Andre
Alan

----- Original Message -----
From: "Andre Kesteloot" <andre.kesteloot@verizon.net>
To: "Alan Melia" <Alan.Melia@btinternet.com>
Sent: 30 March 2005 04:55
Subject: [Fwd: Re: LF: More on cores...]


> Hello Alan,
> I have recently (this week) posted two e-mails on blacksheep, but
> neither has appeared back in my mail box.
> Is the RSGB reflector down, or is this a simple case of discrimination
> against us "Murkans ?   :-)
> Best regards
> André  N4ICK
> ****************************
> -------- Original Message --------
> Subject: Re: LF: More on cores...
> Date: Tue, 29 Mar 2005 15:00:31 -0500
> From: Andre Kesteloot <andre.kesteloot@verizon.net>
> Reply-To: andre.kesteloot@ieee.org
> To: rsgb_lf_group@blacksheep.org, sq5bpm@wp.pl
> References: <001301c533a6$e3f37c60$2102000a@ibm>
> <004701c533c7$5f9b2a00$546ca450@preinstapdq0tf>
> <000801c533ce$756d3ce0$2102000a@ibm> <42494D98.8060506@wp.pl>
>
>
>
> Marek SQ5BPM wrote:
>
> > Hello all,
> >
> > I noticed that people are using much more turns than me in the output
> > transformer, so I would appreciate any help in calculating the correct
> > number of turns for the cores I am using here. Messing around the
> > trafo would probably allow me to achieve more power than now (450W).
> > Trying to go higher causes heavy ringing + FET heating. I've tried
> > almost everything to reduce this (different combinations of Zobel
> > network, decoupling, inductances in series with gates, screening,
> > grounding etc.). In every case around 400W things start to go bad,
> > 450W is a border of manageable heat and ringing. I am thinking about
> > making a new FET driver in a way that allows regulation of the time of
> > the FET on-state, leaving some rest period in-between.
>
> In  a single-ended Class C stage, we think of the load as the resistance
> that will allow the collector to swing from the supply to
> ground, hence                                     R = V2/P and P =
> (Vrms)2 / R  (1)
> This is a rms voltage, and since  Vrms = (Vpeak / 1.414) we can replace
> the value of Vrms in (1) hence
>                                                             P = (Vpeak)2
> / (1.414)2 * R = (Vds)2 / 2R
>
> When we have a push pull, we can consider it as being made up of two
> halves, each supplying P/2.
> Hence                                             Zdd =
> (Vds)2/2*(Pout/2)+(Vds)2/2*(Pout/2)=2(Vds)2/Pout (2)
> So far, so good, for _sinusoidal_  signals.
>
> Now it would seem to me that when we deal with class D circuitry, we
> are dealing with (almost perfect) square waves, where average power,
> r.m.s. power and peak power are all the same.
>
> When a Power MosFet conducts, its internal resistance is so low that it
> effectively brings the corresponding output transformer side  to
> ground.  Because of the transformer action, the other drain is now at
> 2*Vdd, as can be seen clearly on a scope.
> We can consider that each MosFet works 1/2 of the time from a drain
> voltage that is actually (2*Vdd).
> Hence the power switched into the core by both devices is
>                                                         Pout = (2 *
> Vdd)2 / Zdd .
>
> When calculating the number of turns required for good transformer
> action, is not unusual to consider each individual winding inductance as
> having to be larger than 10 x  the load and source impedance presented
> to it.
>
> Dye and Grangberg "Radio Frequency Transistors" is of course excellent,
> but tends to focus more on wide-band RF amplifiers. For a very good
> treatment of transformer-coupled class D power amplifier, you may want
> to reference "Solid State Radio Engineering", by Krauss, Bostian and
> Raab, pp.438-448.
>
> Hope this helps
>
> 73
> André N4ICK
>