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[82.70.254.222]) by smtp.gmail.com with ESMTPSA id l18sm28009615wrh.54.2019.06.02.06.39.31 (version=TLS1 cipher=AES128-SHA bits=128/128); Sun, 02 Jun 2019 06:39:32 -0700 (PDT) Date: Sun, 2 Jun 2019 14:39:29 +0100 From: Chris Wilson X-Priority: 3 (Normal) Message-ID: <779573037.20190602143929@gmail.com> To: rsgb_lf_group@blacksheep.org MIME-Version: 1.0 X-Spam-Score: -0.2 (/) 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: 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 [...] Content analysis details: (-0.2 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- -0.0 RCVD_IN_DNSWL_NONE RBL: Sender listed at http://www.dnswl.org/, no trust [2a00:1450:4864:20:0:0:0:330 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 (dead.fets[at]gmail.com) -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 X-Scan-Signature: f38d515a9cad1e977fcfe251941a8120 Subject: LF: MOSFET gate resistors question Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit 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=PRIORITY_NO_NAME 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 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