Re: LF: RE: [rsgb_lf_group] Re: MF: DC-Bias [was: EbNaut]

[Tobias DG3LV <DG3LV@gmx.net>]

Hi Clemens !

> > I was referring to a *multiturn* center tapped winding.
A multi-turn (e.g. 2-Turn) primary of an PA output transformer has 
several disadvantages in real life:
1. The ohmic resistance of the primary is doubled (vs. a one-turn 
non-tapped primary ). At some 10 amperes -as typical for PAs- this is 
negative for efficiency and heat-production rises (see Pauls EMail).
2. the secondary has to have double as much turns as for a single turn 
primary. Ohmic resistance rises as well.
3. At a given cylindrical ferrite bead output transformer you get into 
trouble for space of all those turns in a given inner diameter. You 
cannot use thicker wires then to compensate for all the (ohmic) losses 
you introduced.
4. as you pointed out, a DC bias at an output transformer may lead to 
losses. It is better to separate supply- and transforming- funktion to 
separate ferrites (DB1NV).
Those bifilar supply chokes are no witchcraft and they are easy to build.

All this leads me to my opinion, that the typical one-turn primary 
output transformer in conjunction with a bifilar supply choke is a good 
choice for a homebrew push-pull PA. The power-efficiency and 
power-output is better (than those center-fed ones) and therefore the 
heat-production is smaller, the heat-spreader (and/or fan) need not be 
that big or stays cooler. It's a win-win situation.
73 de dg3lv Tobias

Am 04.03.2016 um 18:40 schrieb Clemens Paul:
> Hi Bodin & Tobias,
>
> see the article on ferrite losses with DC bias.
> https://www.dropbox.com/s/4zu1apz99mopmj6/03_Core_Losses_UnderDC.pdf?dl=0
>
> @Tobias
> I know the article you've pointed to.
> It addresses only the special situation with a *single turn* center tapped 
> primary winding of an output transformer
> which - according to the author's opinion - doesn't represent a real single 
> turn due to the center feed.
> I was referring to a *multiturn* center tapped winding.
> See my spice simulation for a center tapped output tranformer as opposed a DC 
> feed via chokes.
>
> https://www.dropbox.com/s/u252hmj1p9610bu/Center%20tap%20feed%20vs%20separate%20choke%20feed.png?dl=0
>
> On the left side you find the circuit version with DC feeding via extra chokes 
> (V.1).
> On the right there is the center tap version (V.2).
> As you can see the center tap is grounded for RF by 200nF and the DC feed has a 
> choke for
> additional choking besides the transformer primary windings (in sum 10µH like 
> in V.1).
> The current (yellow trace ) in the 50 Ohm load  resistor R1 is identical in 
> both versions
> hence you have the same output power in both circuits.
> But compare the currents in the transformer primaries of both versions.
> In V.1 this current (green trace) swings approx. between +/- 6A
> while in V.2 (with center tap) in the upper half of the winding the current 
> swings between zero and -12A (green trace)
> and in the lower half between zero and +12A respectively (white trace)
> i.e. you have a current swing of +/-12A per cycle as opposed to V.1 with +/-6A 
> per cycle.
> I conclude therefore that the flux  in the core is doubled in V.2 (center tap).
> I also created a center tapped model with 2x10µH prim. so that the resistance 
> to ground was the same
> for each FET  as opposed to the floating primary with coupled 2x2.5µH = 10µH 
> between the FETs.
> The result was just the same.
> So it seems that only the the DC current fed via the center tap can be the 
> reason for the doubled
> current / flux in the transformer core in the center tap circuit.
>
> 73
> Clemens
> DL4RAJ
>
>
>
> > -----Original Message-----
> > From: rsgb_lf_group@yahoogroups.co.uk
> > [mailto:rsgb_lf_group@yahoogroups.co.uk]
> > Sent: Thursday, March 03, 2016 4:42 PM
> > To: rsgb_lf_group@yahoogroups.co.uk
> > Subject: Re: [rsgb_lf_group] Re: MF: DC-Bias [was: EbNaut]
> >
> > But if the winding is bifilar, usually a twisted pair, then, from the
> > core's point of view, it will look almost like a single wire carrying
> > current in alternating directions. The core has no way of knowing that
> > there are two wires instead of one so it will "see" a pure AC
> > excitation. Am I missing something obvious?
> >
> > 73 de
> > Johan SM6LKM
> >
> > cpaul@gmx.net wrote:
> > > Hi Markus,
> > >
> > > > Clemens, I think that saturation from DC-biasing would only be
> > > > a problem if the two transformer primaries (LP1 and LP2) were
> > > > wound on separate cores. On a single toroid, the flux from the
> > > > two DC currents should cancel.
> > > It would appear that the flux from the two DC currents will
> > not cancel because they don't flow at one time.
> > > The current into the center tap alternates from travelling
> > through one half of the winding then the
> > > other due to the push pull action of the FETs.
> > >
> > > 73
> > > Clemens
> > > DL4RAJ
> > >
> > > > -----Original Message-----
> > > > From: owner-rsgb_lf_group@blacksheep.org
> > > > [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of
> > Markus Vester
> > > > Sent: Tuesday, March 01, 2016 12:03 AM
> > > > To: rsgb_lf_group@blacksheep.org
> > > > Subject: Re: LF: MF: EbNaut
> > >
> > > ------------------------------------
> > > Posted by: "Clemens Paul" <cpaul@gmx.net>
> > > ------------------------------------
> > >
> > >
> > > ------------------------------------
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