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
-----Ursprüngliche Nachricht (HTML) wurde als Anlage beigefügt.-----
Von: [email protected]
Gesendet: 02.04.05 21:21:17
An: "Hans-Albrecht Haffa" <[email protected]>
Betreff: Re: LF: Re: RE: TX Cores, Gap from DK 8 ND
__________________________________________________________
Mit WEB.DE FreePhone mit hoechster Qualitaet ab 0 Ct./Min.
weltweit telefonieren! http://freephone.web.de/?mc=021201
-----Ursprüngliche Nachricht wurde als Attachment angehängt.-----
----- Original Message ----- Sent: Friday, April 01, 2005 5:29 PM Subject: LF: RE: TX Cores, Gap from DK 8 ND
Dear Lowfers,
any ferrite core should have a gap. This is necessary to reduce the remanent flux from about 180 mT to as low as 30 mT. It may be just the residuous gap of lapped cores, preferrable is to put a layer of capton tape on one outer leg of an EE-Core (EDT59 and the like). Lowering remanent flux will rise flux swing at operation. Apparently it is not a problem at 136kHz, as core saturation is unlikely at this frequency. Saturation will occure at frequencies below 50 kHz during startup of push-pull switching circuits (symmetrical half bridge or full bridge), designed by beginners. A gap as low as 0.05 mm (one layer self adhesive film tape or a bit of varnish) will hardly lower permeability, so from urel=2700 down to urel=1500. If You can, use this effect. A toroid core is often second choice in power electronics. Two paralleled ETD cores have greater surface than two stacked toroids. With the ETDs copper volume is doubled too. not so with the stacked toroids. So if You can buy ETD cores , do it. They are available at Conrad.com or conrad.de.
55, Hans-Albrecht
DK 8 ND
Hi Hans Are we all talking about the same cores and applications for transformers suitable for matching FETS to a 50 ohm load in PA design for 137 khz. If the core has two halves some say leave a GAP and others say NO. Some cores are circular and therefore there is no choice about GAPS. Can someone please clarify the position, also some designers show a variety of turns needed for the core varying from about 4 turns to 16 for the Prim winding with the Sec up to 30 turns. Core type refers normally to 3C85 type or equiv. 73 de Mal/G3KEV
|
