To: rsgb_lf_group@blacksheep.org
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From: Tobias DG3LV <DG3LV@gmx.net>
Message-ID: <56DC8DB3.5020300@gmx.net>
Date: Sun, 6 Mar 2016 21:06:11 +0100
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Subject: Re: LF: RE: [rsgb_lf_group] Re: MF: DC-Bias [was: EbNaut]
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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 10焙 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 2x10焙 prim. so that the resistance to ground was the same
> for each FET  as opposed to the floating primary with coupled 2x2.5焙 = 10焙 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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