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

To: [email protected]
Subject: Re: LF: RE: [rsgb_lf_group] Re: MF: DC-Bias [was: EbNaut]
From: Tobias DG3LV <[email protected]>
Date: Sun, 6 Mar 2016 21:06:11 +0100
In-reply-to: <[email protected]>
References: <[email protected]> <[email protected]> <[email protected]>
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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.

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.

On the left side you find the circuit version with DC feeding via extra chokes 
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.


-----Original Message-----
From: [email protected]
[mailto:[email protected]]
Sent: Thursday, March 03, 2016 4:42 PM
To: [email protected]
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

[email protected] 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.


-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
Markus Vester
Sent: Tuesday, March 01, 2016 12:03 AM
To: [email protected]
Subject: Re: LF: MF: EbNaut

Posted by: "Clemens Paul" <[email protected]>


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