Return-Path: Received: (qmail 99686 invoked from network); 4 Apr 2005 18:35:08 -0000 Received: from unknown (HELO ptb-spamcore02.plus.net) (192.168.71.3) by ptb-mailstore03.plus.net with SMTP; 4 Apr 2005 18:35:08 -0000 Received: from mailnull by ptb-spamcore02.plus.net with spamcore-l-b (Exim 4.32; FreeBSD) id 1DIWQh-0008vO-EW for dave@picks.force9.co.uk; Mon, 04 Apr 2005 19:36:07 +0100 Received: from [192.168.67.1] (helo=ptb-mxcore01.plus.net) by ptb-spamcore02.plus.net with esmtp (Exim 4.32; FreeBSD) id 1DIWQh-0008vJ-4i for dave@picks.force9.co.uk; Mon, 04 Apr 2005 19:36:07 +0100 Received: from post.thorcom.com ([193.82.116.20]) by ptb-mxcore01.plus.net with esmtp (Exim) id 1DIWTV-0004Pm-U1 for dave@picks.force9.co.uk; Mon, 04 Apr 2005 19:39:02 +0100 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1DIWPd-0005a7-Ci for rs_out_1@blacksheep.org; Mon, 04 Apr 2005 19:35:01 +0100 Received: from [193.82.116.30] (helo=relay.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1DIWPc-0005Zy-US for rsgb_lf_group@blacksheep.org; Mon, 04 Apr 2005 19:35:00 +0100 Received: from hestia.herts.ac.uk ([147.197.200.9]) by relay.thorcom.net with esmtp (Exim 4.43) id 1DIWPa-0004Tt-Ed for rsgb_lf_group@blacksheep.org; Mon, 04 Apr 2005 19:35:00 +0100 Received: from [147.197.200.10] (helo=altair2.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 1DIWPI-0007eA-00 for rsgb_lf_group@blacksheep.org; Mon, 04 Apr 2005 19:34:40 +0100 Received: from [147.197.164.230] (helo=RD40002) by altair2.herts.ac.uk with esmtp (Exim 3.36 #1) id 1DIWOc-0003Z2-00 for rsgb_lf_group@blacksheep.org; Mon, 04 Apr 2005 19:33:58 +0100 From: "james moritz" To: rsgb_lf_group@blacksheep.org Date: Mon, 4 Apr 2005 19:33:57 +0100 Message-ID: <000001c53944$dd1f69f0$e6a4c593@RD40002> MIME-Version: 1.0 X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.4510 Importance: Normal In-Reply-To: <326392110@web.de> X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165 X-UH-MailScanner: No Virus detected X-UH-MailScanner-From: j.r.moritz@herts.ac.uk X-SPF-Result: relay.thorcom.net: 147.197.200.9 is neither permitted nor denied by domain of herts.ac.uk X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=failed,none Subject: LF: RE: RE: TX Cores, Gap from DK 8 ND Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Status: No, hits=0.0 required=5.0 tests=none autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-Spam-Filtered: by PlusNet SpamCORE (v3.00) Dear LF Group, Hans-Albrecht makes an interesting point here - certainly, texts on switch-mode power supplies often refer to problems with push-pull converters caused by an imbalance of flux in the core, either due to the core itself, or some asymmetry of the circuit, so it is surprising it does not seem to be an issue with the push-pull class D amplifiers. I think that there is a self-balancing action that results from feeding the DC input through a high-impedance choke - if the core is closer to saturation with the current flowing in one direction in the primary, it will effectively have somewhat lower impedance than when the current is flowing in the other direction. Since the current is nearly fixed over the duration of one cycle, there will be less voltage drop across the winding during the half cycle where the core is closer to saturation and the winding has lower impedance, and so more flux will be generated in the other half cycle, tending to pull the average flux in the core towards zero and restore balance. As an experiment, I applied a DC bias current through a 3mH air cored choke (or loading coil, as some people call it...) into the 50 ohm secondary winding of a class D P-P current switching output stage, and monitored the voltage waveform at the primary centre tap. With zero DC bias, the waveform is approximately a "full-wave rectified sine", but as the DC bias increases, the voltage peaks become asymmetrical, with the peaks of the higher-flux half of the cycle slightly smaller than the lower-flux ones. Once the DC bias is removed, the original waveform is restored. This apparent balancing would seem to be an argument in favour of current-switching rather than voltage-switching in the transformer-coupled push-pull type of circuit. Cheers, Jim Moritz 73 de M0BMU -----Original Message----- From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Hans-Albrecht Haffa Sent: 01 April 2005 17:30 To: rsgb_lf_group@blacksheep.org 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 -----Ursprüngliche Nachricht (HTML) wurde als Anlage beigefügt.----- Von: rsgb_lf_group@blacksheep.org Gesendet: 01.04.05 11:11:24 An: "Hans-Albrecht Haffa" Betreff: Re: LF: RE: TX Cores __________________________________________________________ Mit WEB.DE FreePhone mit hoechster Qualitaet ab 0 Ct./Min. weltweit telefonieren! http://freephone.web.de/?mc=021201