----- 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
