Return-Path: Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by klubnl.pl (8.14.4/8.14.4/Debian-8+deb8u2) with ESMTP id x52F2PYf016929 for ; Sun, 2 Jun 2019 17:02:26 +0200 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1hXRsR-0000tZ-Qq for rs_out_1@blacksheep.org; Sun, 02 Jun 2019 15:54:27 +0100 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1hXRpG-0000tQ-CK for rsgb_lf_group@blacksheep.org; Sun, 02 Jun 2019 15:51:10 +0100 Received: from mail-vs1-xe2c.google.com ([2607:f8b0:4864:20::e2c]) by relay1.thorcom.net with esmtps (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1hXRp3-0005p2-V7 for rsgb_lf_group@blacksheep.org; Sun, 02 Jun 2019 15:51:01 +0100 Received: by mail-vs1-xe2c.google.com with SMTP id l125so9647260vsl.13 for ; Sun, 02 Jun 2019 07:50:57 -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=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to; bh=zBQ9/nqhs2MNc8mXNWsphwhCbGFpiQQAgTI/VvndHHU=; b=AE4zkpXs34jQmLkUA7OeeKir6Qva7MRxzXtZBJdMXszaMIRzdhTK61JGKyAZDx1uz7 bQc1V/EElZKJ7ezFao0QLkS5KKebPkyicaqDfOvsyBJGuQzZfIJYbIRFiQcHIJH1pIhy VmGFiVfiUB3UB5VrK6tCrBZqXvJdfTKt1PwAq4GEUkWgAlfMI3IjNzL6e2SEIA8K+MAL jGfweGlFm1IxKEbz3pW0lcrfPkEnT6p6//ctlEo2yr4kfLF/O4Uw5dKTs7vQXqNUBzO7 I6zRi7ZuXyXsucnSSNcmTfQ1yhUidOK0/2ieYLfJhgspa0/KuWGDjtL5ZKfzmqJTJN17 dIVw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to; bh=zBQ9/nqhs2MNc8mXNWsphwhCbGFpiQQAgTI/VvndHHU=; b=f24/nfzbGJhhFXUjocmoMIWPo1H1p0e3tJg1eExWuPQPCbpHZIs0k3E8sBC8CIEuqh VVLd+w/5M3U2e7dy+05Sa2MJkER5/i2IF0xj67cHvKyffxQCITGduYg8O52TBeG1AuDj wrE1/+YqTEhA/D+NHzMouseyb5fU/C047ekRYVkp3fzpzm1YPPw2CEqZL2niURemiu8f RsrR5Rm+Q7SVbvRB5qzTt1CbaDULGSFqW0etIkW0VzEyqXJPDFNAaXa7mmskL1aBOA6O Jqgfce24e231NzTBVcXLHcYKvnsJJWNPtlI5BF9p4gKoMaiNdOTGVMfTQnvJmcJKJkVd XoEA== X-Gm-Message-State: APjAAAUZ8icuLdkHEA4SmFe72rBC2968uStBweC5AtBA+lHZ8GIsQEs1 oi/+79eNSZcujWiFDVRc4tr40Q1AqHsi0cgrcXA1/Q== X-Google-Smtp-Source: APXvYqzHFJKO/eeB9KL9+si3HDH8/Ig0P813e48gJGDXM8NLZOn37e51rIVtOaaF5PjwaRgSfFZ9y9eQ/yoakoJMGTs= X-Received: by 2002:a67:d19c:: with SMTP id w28mr11445324vsi.127.1559487055866; Sun, 02 Jun 2019 07:50:55 -0700 (PDT) MIME-Version: 1.0 References: <779573037.20190602143929@gmail.com> In-Reply-To: <779573037.20190602143929@gmail.com> From: Andy Talbot Date: Sun, 2 Jun 2019 15:50:44 +0100 Message-ID: To: "rsgb_lf_group@blacksheep.org" X-Spam-Score: 0.8 (/) X-Spam-Report: Spam detection software, running on the system "relay1.thorcom.net", has NOT identified this incoming email as spam. The original message has been attached to this so you can view it or label similar future email. If you have any questions, see @@CONTACT_ADDRESS@@ for details. Content preview: Teh 4k7 on the gate is not a pull down. I assume it is associated with gate drive via a capacitor and diode to give DC restoration. A safe way of driving FETs to prevent them beoing switched permanant [...] Content analysis details: (0.8 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- -0.0 RCVD_IN_DNSWL_NONE RBL: Sender listed at http://www.dnswl.org/, no trust [2607:f8b0:4864:20:0:0:0:e2c listed in] [list.dnswl.org] -0.0 SPF_PASS SPF: sender matches SPF record 0.0 FREEMAIL_FROM Sender email is commonly abused enduser mail provider (andy.g4jnt[at]gmail.com) 0.0 HTML_MESSAGE BODY: HTML included in message -0.1 DKIM_VALID_EF Message has a valid DKIM or DK signature from envelope-from domain -0.1 DKIM_VALID Message has at least one valid DKIM or DK signature 0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily valid -0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain 1.0 FREEMAIL_REPLY From and body contain different freemails 0.0 T_REMOTE_IMAGE Message contains an external image X-Scan-Signature: b42857a5fad6e32377447282c21f62b1 Subject: Re: LF: MOSFET gate resistors question Content-Type: multipart/alternative; boundary="000000000000ff30c5058a585ff4" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.8 required=5.0 tests=HTML_30_40,HTML_MESSAGE, TO_ADDRESS_EQ_REAL 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 --000000000000ff30c5058a585ff4 Content-Type: text/plain; charset="UTF-8" Teh 4k7 on the gate is not a pull down. I assume it is associated with gate drive via a capacitor and diode to give DC restoration. A safe way of driving FETs to prevent them beoing switched permanantly on if a driver or logic circuit goes haywire. All the resistor does is prevent the mean DC component on the gate from floatiing off to some arbitrarily high value. The gate series resistor is there to prevent spurious ringing and high frequency oscillations. Using a decent driver chip and short leads / tracks it shouldn't be necessary, but doesn't do any harm. Just use a good driver chip capable of driving the gate cap of your FET, If it doesn't, itself, have a Schmitt input, make sufre its drive with a sharp edged logic drive waveform. Perhaps from a Schmitt gate. Andy www.g4jnt.com On Sun, 2 Jun 2019 at 14:46, Chris Wilson wrote: > Hi all, > > My pal G6ECH and myself are battling poor drain waveforms in Class D > 470 kHz push pull amps. We have two main initial questions. Some > circuits show a gate pull down resistor used, some do not. Is one > needed? How do you ascertain a suitable value? Given a 475 kHz amp > would a pull down of say 47k on the gate be a ball park figure? Do > some FET drivers have this internally? > > Similarly the series gate resistor. Current limiting and also a slow > down function? 10 Ohms seems the common value, how do you know if > higher or lower resistance might be better? > > Maybe our issue is noise on the INPUT to the FET driver IC's, so would > a small pF cap to ground be prudent? Say 30pF at these frequencies, or > not required? > > Finally is it worth delving into "exotic" FET's and drivers that > utilize a negative gate turn off voltage and hence need a dual > polarity supply? FET's like the Cree Wolfspeed C3M0065090D > > https://www.wolfspeed.com/media/downloads/176/C3M0065090D.pdf > > > > > > Thanks. > > > > -- > Best regards, > Chris mailto:dead.fets@gmail.com > > > Virus-free. www.avg.com <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> --000000000000ff30c5058a585ff4 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Teh 4k7 on the gate is not a pu= ll down.=C2=A0 I assume it is associated with gate drive via a capacitor an= d diode to give DC restoration.=C2=A0 =C2=A0A safe way of driving FETs to p= revent them beoing switched permanantly on if a driver or logic circuit goe= s haywire.=C2=A0 =C2=A0All the resistor does is prevent the mean DC compone= nt on the gate from floatiing off to some arbitrarily high value.

The gate series resistor is there to prevent spur= ious ringing and high frequency oscillations.=C2=A0 =C2=A0Using a decent dr= iver chip and short leads / tracks it shouldn't be necessary, but doesn= 't do any harm.

Just use a good driv= er chip capable of driving the gate cap of your FET,=C2=A0 =C2=A0If it does= n't, itself, have a Schmitt input, make sufre its drive with a sharp ed= ged logic drive waveform.=C2=A0 Perhaps from a Schmitt gate.=C2=A0 =C2=A0= =C2=A0

Andy<= div>www.g4jnt.com

<= /div>


On Sun, 2 Jun 2019= at 14:46, Chris Wilson <dead.fet= s@gmail.com> wrote:
Hi all,

My pal G6ECH and myself are battling poor drain waveforms in Class D
470 kHz push pull amps. We have two main initial questions. Some
circuits show a gate pull down resistor used, some do not. Is one
needed? How do you ascertain a suitable value? Given a 475 kHz amp
would a pull down of say 47k on the gate be a ball park figure? Do
some FET drivers have this internally?

Similarly the series gate resistor. Current limiting and also a slow
down function? 10 Ohms seems the common value, how do you know if
higher or lower resistance might be better?

Maybe our issue is noise on the INPUT to the FET driver IC's, so would<= br> a small pF cap to ground be prudent? Say 30pF at these frequencies, or
not required?

Finally=C2=A0 is=C2=A0 it=C2=A0 worth=C2=A0 delving=C2=A0 into=C2=A0 "= exotic" FET's and drivers that
utilize=C2=A0 a=C2=A0 negative=C2=A0 gate=C2=A0 turn=C2=A0 off=C2=A0 voltag= e=C2=A0 and=C2=A0 hence need a dual
polarity supply? FET's like the Cree Wolfspeed C3M0065090D

https://www.wolfspeed.com/media/downloa= ds/176/C3M0065090D.pdf





Thanks.



--
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 mailto:dead.fets@gmail.com


<= br>
= Virus-free. www.avg.com
--000000000000ff30c5058a585ff4--