Hi Tobias,
I agree in all points with you.
I just referred to the multiturn primary as an example to prove that it is
indeed the the DC bias
which increases losses and I wanted to circumvent the discussion if the author
of
http://ludens.cl/Electron/mosfetamps/amps.html
is right or not with his theory that a center tapped single turn winding
doesn't constitute a complete turn.
There exist different opinions about this point.
73
Clemens
DL4RAJ
>-----Original Message-----
>From: [email protected]
>[mailto:[email protected]] On Behalf Of Tobias DG3LV
>Sent: Sunday, March 06, 2016 9:06 PM
>To: [email protected]
>Subject: Re: LF: RE: [rsgb_lf_group] Re: MF: DC-Bias [was: EbNaut]
>
>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_UnderD
C.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%2
0vs%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: [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.
>>>>
>>>> 73
>>>> Clemens
>>>> DL4RAJ
>>>>
>>>>> -----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]>
>>>> ------------------------------------
>>>>
>>>>
>>>> ------------------------------------
>>>>
>>>> Yahoo Groups Links
>>>>
>>>>
>>>>
>>>>
>>>
>>>
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