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

To: <[email protected]>
Subject: LF: RE: [rsgb_lf_group] Re: MF: DC-Bias [was: EbNaut]
From: "Clemens Paul" <[email protected]>
Date: Fri, 4 Mar 2016 18:40:07 +0100
Cc: <[email protected]>
In-reply-to: <[email protected]>
References: <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Thread-index: AdF1YzC3jzdM5FFOT62dA7DVqt3aMgA1ZGsQ
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: [email protected]s.co.uk 
>[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]>
>> ------------------------------------
>>
>>
>> ------------------------------------
>>
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