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 x6HIubLN011882 for ; Wed, 17 Jul 2019 20:56:38 +0200 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1hnp2E-0002Nj-15 for rs_out_1@blacksheep.org; Wed, 17 Jul 2019 19:52:14 +0100 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1hnp13-0002Na-5b for rsgb_lf_group@blacksheep.org; Wed, 17 Jul 2019 19:51:01 +0100 Received: from mail-ua1-x930.google.com ([2607:f8b0:4864:20::930]) by relay1.thorcom.net with esmtps (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1hnp0w-0002Fe-UM for rsgb_lf_group@blacksheep.org; Wed, 17 Jul 2019 19:50:57 +0100 Received: by mail-ua1-x930.google.com with SMTP id o19so10049895uap.13 for ; Wed, 17 Jul 2019 11:50:54 -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=WqbuBXdcqCalSw/joj6vYX7EXfuWx47SRqJg724KmhA=; b=WeGBvBNz3y/Y8np0jjuT1TyB+siwDZAtc7JMRSsh0HzCdxo/ziDJvXkc1HYBnZTPTE JIrWFdjJxWqHu6/wqE/U2TtVgpmwUPpclXW1Mrs9ahf3YXyII//YT3xJH8LeZli/Q0gj 6To+Uy5kJmzsJwb97Y4taJpLVsnd2yJzxLU40m4gW5aVHdMdcVEznO+qTjt8ZQsAygxq RowLlnOZH9rUCOECKemopwMm7HG8PMtF4O3EqkI458Ykrs5wka7rLMMjqJkAnZZkX4Ix 9j+DqMxIylImJSLPKcO2JnMQv8UTkzAmJWjULqVMsnSU2DDLZ+vkEX3fxshQmw08P+5y 1D0Q== 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=WqbuBXdcqCalSw/joj6vYX7EXfuWx47SRqJg724KmhA=; b=GvpFeZBuI4JMlpQPRuH7TLFpL2WzA3tl602E9g7UB4dmdNhVlnBFYHQETQrYxw33F4 CLgQmkcNA8XFq7o334j4A3nXVSiOIhBd0xboIsmp6kPpPASp1qwRcj7N4CFJqRsmhQwq inibpMGOaAvVMUUo8G4hhKAueUJ2DYE9P3ExwvgQ/Dj6IV6waSNabPUr42IYASHtEUQ7 ljW60K+GcGgPGBQKvLeotFQs1ylgtGfmSS3eJyEKmwZOqnvbf3MiEr3arYtFAiIwFeUO v16A2Yb70V1NL5miqsAdaL/uQifJdW5xSw1pNC6BwoSN52VU68iWNwm+eg6Ad2zFbj2w XxJg== X-Gm-Message-State: APjAAAWKhilIl54jKvRlUNTlpku8z+Zc1nYB2zvqoLCFcw8+snBuXjP1 ljUJWsTQVBC0Q+UGm77sSvVplhmZ1dtbZxtf1KHIuA== X-Google-Smtp-Source: APXvYqyDFVeKcwRJQh0hmWtLReOK9h0MHBWqWaSMGoD2KLRwyTXeoSCGQjnlZBG6TztzV62S0MSOV5zfppYI40hmDrI= X-Received: by 2002:ab0:60d4:: with SMTP id g20mr5586024uam.20.1563389450892; Wed, 17 Jul 2019 11:50:50 -0700 (PDT) MIME-Version: 1.0 References: <993240756.20190717143805@gmail.com> <5D2F677F.1090208@posteo.de> In-Reply-To: <5D2F677F.1090208@posteo.de> From: Andy Talbot Date: Wed, 17 Jul 2019 19:50:39 +0100 Message-ID: To: "rsgb_lf_group@blacksheep.org" 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: I assume we're talking about the series C from driver to gate, then a high value leakage R from gate to ground, and a reverse biassed diode in parallel with this. AC coupling without any DC restoration means the MEAN voltage on the gate is zero. For LF transmitter uses, where the drive has a 50% duty cycle (or very close to) this is not too much of an issue as [...] 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 [2607:f8b0:4864:20:0:0:0:930 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 X-Scan-Signature: 567a11ac6cfe1e88f3054745c06178fe Subject: Re: LF: DC restoration Content-Type: multipart/alternative; boundary="000000000000ddba32058de4f8e8" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.5 required=5.0 tests=HTML_20_30,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 --000000000000ddba32058de4f8e8 Content-Type: text/plain; charset="UTF-8" I assume we're talking about the series C from driver to gate, then a high value leakage R from gate to ground, and a reverse biassed diode in parallel with this. AC coupling without any DC restoration means the MEAN voltage on the gate is zero. For LF transmitter uses, where the drive has a 50% duty cycle (or very close to) this is not too much of an issue as there will still be a significant positive swing. So provided Vdd / 2 of the driver stage is more than sufficient to fully turn-on the FET than AC coupling with just a C and leakage R is OK. Transformer coupling - allowing galvanic isolation - works very well and that givs true AC drive. The problem comes in SMPSUs where the duty cycle is not zero. Assume a 20% duty cycle and a drive waveform of 0/15V. The mean voltage on the gate still has to 0V, so the get this from a 0/15V wavefomr means a positive excursion of 3/4 * 15V = 12V and a negative excursion of 3V. That will still work OK But an 80% duty cycle is +3V and -12V that certainly won't. By adding a DC restoration diode, the negative excursion is clamped to around -0.6V (ish)) and the positive lifted to 14.4V (ish) WHATEVER the duty cycle. SO even at 50% the drive amplitude is doubled. I think earlier IGBJT devices needed a negative voltage to turn off properly - I doubt that is the case with later ones. And IGBJTs don't seem to have been adopted for LF transmitters anyway. I remember learning about DC restoration when I was quite young and being shown how TVs (monochrome at that time) worked. It was a simple way of getting a constant black level on AC coupled video amps, by clamping the recovered sync pulses to 0V, then biassing to get black level, whnere teh video signal have a varying DC component. Later designs clamped the back porch at true black level. Exactly the same circuit arrangement Andy www.g4jnt.com On Wed, 17 Jul 2019 at 19:26, DK7FC wrote: > Hi Chris, > > Maybe it helps to show a concrete schematic together with your question. > I find reverse diodes quite useless, at least when MOSFETs are used. It > may be a different thing with bipolar transistors... > > 73, Stefan > > Am 17.07.2019 15:38, schrieb Chris Wilson: > > > > Hello rsgb, > > > > Some Class D LF / MF amps use a diode to DC restore the waveform > > after the FET driver chip to the gates. Why do they do this, I am > > thinking the negative part of the waveform would turn OFF the gate > > harder, assuming the positive part is adequate to turn the gates ON > > hard enough? Is there any point in using the modern FET's designed > > to have a negative rail to turn the gates off hard? Cree do the > > Wolfspeed ones with this feature. I have also noticed that with amps > > where the input to the driver is capacitively coupled adding a DC > > restoration diode makes the waveform pretty much exactly 50 / 50 > > mark space, whereas without one this is not the case. Good idea to > > use these restoration diodes there? Thanks. > > > > > > --000000000000ddba32058de4f8e8 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
I assume we're talking about the series C fr= om driver to gate, then a high value leakage R from gate to ground, and a r= everse biassed diode in parallel with this.

AC coupling without any DC restoration means the MEAN voltage on the ga= te is zero.=C2=A0 For LF transmitter uses, where the drive has a 50% duty c= ycle (or very close to) this is not too much of an issue as there will stil= l be a significant positive swing.=C2=A0 So provided Vdd=C2=A0 / 2 of the d= river stage is more than sufficient to fully turn-on the FET than AC coupli= ng with just a C and leakage R is OK.=C2=A0 =C2=A0 Transformer coupling - a= llowing galvanic isolation - works very well and that givs true AC drive.

The problem comes in SMPSUs where the dut= y cycle is not zero.=C2=A0 =C2=A0Assume a 20% duty cycle and a drive wavefo= rm of 0/15V.=C2=A0 =C2=A0The mean voltage on the gate still has to=C2=A0 0V= , so the get this from a 0/15V wavefomr means a positive excursion of 3/4 *= 15V =3D 12V and a negative excursion of 3V.=C2=A0 That will still work OK<= /div>
But an 80% duty cycle is=C2=A0+3V and -12V that certainly wo= n't.

By adding a DC restoration diod= e, the negative excursion is clamped to around -0.6V (ish)) and the positiv= e lifted to 14.4V (ish) WHATEVER the duty cycle.=C2=A0 =C2=A0

SO even at 50% the drive amplitude is doubled.
<= div class=3D"gmail_default" style=3D"font-family:tahoma,sans-serif;font-siz= e:small">
I think earlier IGBJT devices needed a negativ= e voltage to turn off properly - I doubt that is the case with later ones.= =C2=A0 =C2=A0And IGBJTs don't seem to have been adopted for LF transmit= ters anyway.

I remember learning about D= C restoration when I was quite young and being shown how TVs (monochrome at= that time) worked.=C2=A0 =C2=A0It was a simple way of getting a constant b= lack level on AC coupled video amps, by clamping the recovered sync pulses = to 0V, then biassing to get black level, whnere teh video signal have a var= ying DC component.=C2=A0 =C2=A0Later designs clamped the back porch at true= black level.=C2=A0 Exactly the same circuit arrangement


On Wed, 17 Jul 2019= at 19:26, DK7FC <selberdenken= @posteo.de> wrote:
Hi Chris,

Maybe it helps to show a concrete schematic together with your question. I find reverse diodes quite useless, at least when MOSFETs are used. It may be a different thing with bipolar transistors...

73, Stefan

Am 17.07.2019 15:38, schrieb Chris Wilson:
>
> Hello=C2=A0 rsgb,
>
>=C2=A0 =C2=A0 Some Class D LF / MF amps use a diode to DC restore the w= aveform
>=C2=A0 =C2=A0 after the FET driver chip to the gates. Why do they do th= is, I am
>=C2=A0 =C2=A0 thinking the negative part of the waveform would turn OFF= the gate
>=C2=A0 =C2=A0 harder, assuming the positive part is adequate to turn th= e gates ON
>=C2=A0 =C2=A0 hard enough? Is there any point in using the modern FET&#= 39;s designed
>=C2=A0 =C2=A0 to have a negative rail to turn the gates off hard? Cree = do the
>=C2=A0 =C2=A0 Wolfspeed ones with this feature. I have also noticed tha= t with amps
>=C2=A0 =C2=A0 where the input to the driver is capacitively coupled add= ing a DC
>=C2=A0 =C2=A0 restoration diode makes the waveform pretty much exactly = 50 / 50
>=C2=A0 =C2=A0 mark=C2=A0 space,=C2=A0 whereas without one this is not t= he case. Good idea to
>=C2=A0 =C2=A0 use these restoration diodes there? Thanks.
>
>=C2=A0 =C2=A0

--000000000000ddba32058de4f8e8--