Return-Path: X-Spam-DCC: paranoid 1290; Body=2 Fuz1=2 Fuz2=2 X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on lipkowski.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=5.0 tests=BAYES_00,DNS_FROM_AHBL_RHSBL, FORGED_RCVD_HELO,SUBJ_RE_NUM autolearn=no version=3.1.3 Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by paranoid.lipkowski.org (8.13.7/8.13.7) with ESMTP id t7AETA18024673 for ; Mon, 10 Aug 2015 16:29:11 +0200 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1ZOo1R-0004gs-FF for rs_out_1@blacksheep.org; Mon, 10 Aug 2015 15:25:53 +0100 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1ZOo1R-0004gj-6e for rsgb_lf_group@blacksheep.org; Mon, 10 Aug 2015 15:25:53 +0100 Received: from smarthost01b.mail.zen.net.uk ([212.23.1.3]) by relay1.thorcom.net with esmtp (Exim 4.86) (envelope-from ) id 1ZOo0b-0002Ld-TX for rsgb_lf_group@blacksheep.org; Mon, 10 Aug 2015 15:25:52 +0100 Received: from [82.70.254.222] (helo=OfficeWin7.lan) by smarthost01b.mail.zen.net.uk with esmtp (Exim 4.80) (envelope-from ) id 1ZOo0M-0001Kk-GT for rsgb_lf_group@blacksheep.org; Mon, 10 Aug 2015 14:24:46 +0000 Date: Mon, 10 Aug 2015 15:24:46 +0100 From: Chris Wilson Organization: Gatesgarth Developments X-Priority: 3 (Normal) Message-ID: <177655808.20150810152446@chriswilson.tv> To: Andy Talbot In-Reply-To: References: <1892164761.20150810130954@chriswilson.tv> MIME-Version: 1.0 X-Originating-smarthost01b-IP: [82.70.254.222] X-Scan-Signature: 3f8d74a8c70dd416d0a3a1dbcb9bb89f Subject: Re[2]: LF: Protecting a MOSFET driver chip from MOSFET failure? Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit 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-Scanned-By: MIMEDefang 2.56 on 10.1.3.10 Status: O X-Status: X-Keywords: X-UID: 3843 Hello Andy, Monday, August 10, 2015 > Forgot to mention, although I hope it is more than self-evident to > readers here, the diode is orientated so it points 'upwards'; > allowing a positive drive voltage to appear on the gate and clamping the negative  to ground. > Andy  G4JNT > On 10 August 2015 at 13:31, Andy Talbot wrote: > Yes there is.  Or , are: several. > Don't drive the MOSFET directly from the IC.  Instead use a > capacitor and DC restorer circuit.  That guards against both  the > driver staying high permanently if drive is removed, and also > against IC damage if the FET fails > Capacitor (typically 100nF for 137/475kHz) from IC output to gate > Diode like IN914 , 1N4148 etc from gate to ground.  Shunt the diode > with a resistor of a few kohms to stop it floating > The capacitor decouples the chip output at DC.  The diode clamps > the waveform at the cap output / gate so it's most negative > excursion is forced to sit near enough at ground. > Or use transformer coupling, as in my QRO 137kHz transmitter. http://www.g4jnt.com/137tx.pdf > Andy  G4JNT Thanks for the replies gentlemen! Would I need to make any other circuit changes to capacitively couple the gates? With the gates individually driven from separate 6R8 resistors in the G3YXM circuit, do I leave those resistors in? Or replace them with the caps and the diode / resistor shunts? Thanks Andy. -- Best regards, Chris mailto:chris@chriswilson.tv