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=-2.4 required=5.0 tests=BAYES_00,DNS_FROM_AHBL_RHSBL, HTML_MESSAGE 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 t7AGs9Ai026039 for ; Mon, 10 Aug 2015 18:54:09 +0200 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1ZOqIK-0005Vb-2a for rs_out_1@blacksheep.org; Mon, 10 Aug 2015 17:51:28 +0100 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1ZOqIJ-0005VS-MU for rsgb_lf_group@blacksheep.org; Mon, 10 Aug 2015 17:51:27 +0100 Received: from mail-wi0-f176.google.com ([209.85.212.176]) by relay1.thorcom.net with esmtps (TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128) (Exim 4.86) (envelope-from ) id 1ZOqHJ-0002s0-9y for rsgb_lf_group@blacksheep.org; Mon, 10 Aug 2015 17:51:26 +0100 Received: by wicja10 with SMTP id ja10so33778145wic.1 for ; Mon, 10 Aug 2015 09:50:11 -0700 (PDT) X-DKIM-Result: Domain=gmail.com Result=Good and Known Domain DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; bh=MMZOqz8aetmbCf6+MjX+CvEbTUfcz4f9a+/c+MItG4o=; b=NASEaWD3t0wWktLf2F276lTLD16HwyKYv1Lr33MkxqrLCPhlyF1YJN4zVyrNcMDbS7 +321VsLUx+EMNYTPUm0jEoIaS2iC+wJLIWDd8lATfwiccJCYS6wcoswVb/jZ1WK2zTXt 41qnbO98JSdkn3zSVDiyemwADdpsaOWLMBryT4uARpyPHopNY6M8JgobK6sBJs7SekvN VZXSK/ShlI0XPTowxeOO9I4LwLZGFYsITq95uzmukEec+Wm0QD3K6lQV2gRa+bC42qgw 0ToihONaC+29K7r615ujA/UBc3KBRry11oHaQVmThptbgSqvWQayX8N+JgPwnMfFtZHV N8nA== MIME-Version: 1.0 X-Received: by 10.180.107.70 with SMTP id ha6mr25258094wib.20.1439225411650; Mon, 10 Aug 2015 09:50:11 -0700 (PDT) Received: by 10.28.98.6 with HTTP; Mon, 10 Aug 2015 09:50:11 -0700 (PDT) In-Reply-To: <001301d0d38a$9658e460$c30aad20$@solva.co.uk> References: <1892164761.20150810130954@chriswilson.tv> <7E7DFBB4D102A04DB5ADC88D66628A4A443E0BC1@ICTS-S-MBX1.luna.kuleuven.be> <1407871247.20150810162853@chriswilson.tv> <001301d0d38a$9658e460$c30aad20$@solva.co.uk> Date: Mon, 10 Aug 2015 17:50:11 +0100 Message-ID: From: Andy Talbot To: rsgb_lf_group@blacksheep.org X-Scan-Signature: 3df6620ae1cf83b986cf2fc02407da16 Subject: Re: Re[2]: LF: Protecting a MOSFET driver chip from MOSFET failure? Content-Type: multipart/alternative; boundary=e89a8f234d0969c829051cf7c7c3 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: 3849 --e89a8f234d0969c829051cf7c7c3 Content-Type: text/plain; charset=UTF-8 There is Miller feedback present, where the drain change of voltage is fed back to the gate via internal capacitiance. The effect is to magnify the Cdg value by the voltage gain, so its bound to give something. Also the Cgs - which fo rlot voltage high current FETs is huge, varies with Vds, so all sorts of things ae happening. And bearing in mind a FET under switching in in a very non-linear region, its just not going to be worth trying to understand / explain every blip in the waveform Raab, in his varius discussion od Class-E at HF using low cost mosfets, in fact suggests sinewave drive. the theory being that the removal of hard edges lowers teh drive requirment anyway, and the actual switching of a class-E transmitter, since it occurs at zero drain volts, doesn't have to be hard switched. Not sure how well that argument applies to class D push pull though. I think for any QRO LF transmitter these days I'd always go the Class-E route now. http://www.g4jnt.com/QRO_500kHz_PA_Breadboard.pdf They're more difficult to set up in the first place, and narrow band, but they are simpler and more efficient when finished, and have a substatial part of the low pass filtering already present in the tank circuitry Andy G4JNT On 10 August 2015 at 17:35, Michael Probert wrote: > Hello Andy and Chris, > > Have been following your discussion with interest. I too experienced the > "hand grenade effect" of removing the drive whilst using DC coupling. Dave > YXM put me right some years ago. > > Thanks Andy for the further explanation. > > I have always been perplexed by the distortion of the gate pulse that > occurs when the drain volts are applied. > This sometimes appears to show up as a slight instability on tail of the > drain pulse. Probably of no importance but never heard anyone mention it. > > Would appreciate any comments > > Mike -GW4HXO > ___________________________________________________________________________ > > Subject: Re[2]: LF: Protecting a MOSFET driver chip from MOSFET failure? > > Hello Andy, > > Monday, August 10, 2015 > > > > No, capacitive coupling with diode restorationends up with near-enough > > the same voltage applied to the gate as you had in thr first place. > > > > A 0/12v output from the driver chips ends up at -0.7 /+ 11.3 V > > (assuming 0.7V diode drop) > > > > > > As for the question of the dual drive with resistors. The resistors > > may as well say in, just to smooth off edges and reduce dissipation in > > the driver. i doubt you'll notice any difference. > > > > Andy G4JNT > > OK, thanks and sorry for the naive questions, lots to learn, I was amused > by my friend calling power FET's "Fire Emitting Transistors". I see his > point now, they do go with a good bang and sizzle! The wife smelt them > downstairs and asked wearily if it meant a longer wait before a job she > attached great importance to, but I classed as a boring irrelevance, got > done... > > > > > > > > -- > Best regards, > Chris mailto:chris@chriswilson.tv > > > > --e89a8f234d0969c829051cf7c7c3 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
There is Miller feedback present, where the drain change o= f voltage is fed back to the gate via internal capacitiance.=C2=A0 The effe= ct is to magnify the Cdg value by the voltage gain, so its bound to give so= mething.

Also the Cgs - which fo rlot voltage high curre= nt FETs is huge, varies with Vds, so all sorts of things ae happening.=C2= =A0 And bearing in mind a FET under switching in in a very non-linear regio= n, its just not going to be worth trying to understand / explain every blip= in the waveform

Raab, in his varius discussion od= Class-E at HF using low cost mosfets, in fact suggests sinewave drive. =C2= =A0the theory being that the removal of hard edges lowers teh drive requirm= ent anyway, and the actual switching of a class-E transmitter, since it occ= urs at zero drain volts, doesn't have to be hard switched.
Not sure how well that argument applies to class D push pull t= hough. =C2=A0 I think for any QRO LF transmitter these days I'd always = go the Class-E route now. http://www.g4jnt.com/QRO_500kHz_PA_Breadboard.pdf
=C2=A0They're more difficult to set up in the first place, and narrow= band, but they are simpler and more efficient when finished, and have a su= bstatial part of the low pass filtering already present in the tank circuit= ry

Andy =C2=A0G4JNT


On 10 August 2015 at = 17:35, Michael Probert <mikegw4hxo@solva.co.uk> wrote:<= br>
Hello Andy and Chris,

Have been following your discussion with interest. I too experienced the &q= uot;hand grenade effect" of removing the drive whilst using DC couplin= g. Dave YXM put me right some years ago.

Thanks Andy for the further explanation.

I have always been perplexed by the distortion of the gate pulse that occur= s when the drain volts are applied.
This sometimes appears to show up as a slight instability on tail of the dr= ain pulse. Probably of no importance but never heard anyone mention it.

Would appreciate any comments

Mike -GW4HXO
___________________________________________________________________________=

Subject: Re[2]: LF: Protecting a MOSFET driver chip from MOSFET failure?
Hello Andy,

Monday, August 10, 2015


> No, capacitive coupling with diode restorationends up with near-enough=
> the same voltage applied to the gate as you had in thr first place.

> A 0/12v output from the driver chips ends up at -0.7 /+ 11.3 V
> (assuming 0.7V diode drop)




> As for the question of the dual drive with resistors.=C2=A0 =C2=A0 The= resistors
> may as well say in, just to smooth off edges and reduce dissipation in=
> the driver.=C2=A0 =C2=A0 =C2=A0i doubt you'll notice any differenc= e.


> Andy=C2=A0 G4JNT

OK, thanks and sorry for the naive questions, lots to learn, I was amused b= y my friend calling power FET's "Fire Emitting Transistors". = I see his point now, they do go with a good bang and sizzle! The wife smelt= them downstairs and asked wearily if it meant a longer wait before a job s= he attached great importance to, but I classed as a boring irrelevance, got= done...







--
Best regards,
=C2=A0Chris=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 mailto:chris@chriswilson.tv




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