Date: Tue, 05 Apr 2005 21:32:43 +0200
Message-Id: <328779818@web.de>
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From: "Hans-Albrecht Haffa" <dk8nd@web.de>
To: rsgb_lf_group@blacksheep.org
Organization: http://freemail.web.de/
Subject: LF: Re: TX Cores, Gap from DK 8 ND, to G3KEV
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Dear Mal, dear Group,

transformers for flyback converters need a gap, transformers for forward converters do not have an gap. The gap of an flyback-transformer is about 1% of the magnetic path length, depending of the energy one wants to store in the primary switching impulse. 
In the case of Power-amplifiers  no gap is needed, exept an residuos gap. 
The number of turns for the primary winding depends on how one splits up copper volume and iron-volume. This refers to all electrical machines. So less core area - more turns at the same poer level. For 136 kHz I recommend an magnetic induction of about 0.05...0.1 Tesla for square wave operation.  Temperature rise is ok, if at room temperature (25°C)  it is just possible to touch the warm core, i.e. temperature rise of 40K is a good approach for temperatures in the cabinet of about 55°C at room temperatures of 40°C (dx-expedition...).  Of course all depends on the insulation materials, classes B, F, H (130°C, 155°C, 180°C). 
There is no general rule in power electronics for the calculus of the number of turns of a transformer. Every design engineer has its own rules, he needs the experience proofed by the experiment, depending of copper, insulation material, core geometry.  Tables in SMPS-books just give hints. For instance according to these tables the current density may vary from 12 A/mm2 (EF16) to 1,8A/mm2 at EE55 core. Choosing extremely flat cores needs new winding tests to find out magnetization and current densities at given frequencies. As far as we use magnetic materials with relative permeability figure of urel =500...3000 any material will do the job. Using VLF material with permeability of urel =6000....10 000 we should reduce manetization, as edding current losses in the core may rise. 

The current fed PP-PA with the dc-Choke. 

In switch mode power supplies this construction is used with free running power-oscillators, built with bipolar power transistors. Due to the storage time of the bipolars (0.5 us...3 us) an overlap time where both transistors are conducting , occurs. The inductivity limits the current rise, acting like a constant current source. So the value of inductivity is not much important, as long the bobbin acts as an current source. But this means that the oscillator with the bipolar transistors  MUST act as a sink for the constant current. Therefore always one of the transistors must be ON, or both must be ON, otherwise the voltage at the center tap of the transformer will rise. In a current fed power converter even the two transistors neednt be driven in push-pull mode, if both are ON at the same time, it will rise current in the choke, but this higher current must then be transferred to the secondary in the next swiching pulses...

Try to get information about SMPS basics by Unitrode Integrated Circuits Company, now under Texas Instruments. 

So much at this evening. More details upon request (Book 2004  cba ok). 

55,
Hans-Albrecht
DK 8 ND

 
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Von:      rsgb_lf_group@blacksheep.org
Gesendet: 02.04.05 21:21:17
An:       "Hans-Albrecht Haffa" <dk8nd@web.de>
Betreff:  Re: LF: Re: RE:  TX Cores, Gap from DK 8 ND


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