From: Jim <jim@jpes.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
Sender: "piclist-bounces@mit.edu" <piclist-bounces@mit.edu>
Date: Fri, 13 Aug 2021 07:36:28 -0700
Subject: RE: [EE] Tek DSO display issue?
Thread-Topic: [EE] Tek DSO display issue?
Thread-Index: AdeRDP+6x28zteNLTxiY0CjaA5NOvw==
Message-ID:
 <20210813073628.06a56fd6300ec2bfca6379d4e10ab8d2.b90cbbeb8f.wbe@email27.godaddy.com>
Reply-To: Microcontroller discussion list - Public. <piclist@mit.edu>
Accept-Language: en-US
X-MS-Exchange-Organization-SenderIdResult: Pass
X-MS-Exchange-Organization-PRD: mit.edu
X-MS-TNEF-Correlator:
received-spf: None (protection.outlook.com: jpes.com does not designate
	permitted sender hosts)
dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
	d=mitprod.onmicrosoft.com; s=selector2-mitprod-onmicrosoft-com;
	h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
	bh=ThFNtQLxIsCC6DZgqbeP3IJTz8rW0mp92sRhhB8wBbw=;
	b=cRsl0jF5DUHt9E2zzGmqWTb/cc1OH7L0Nn6foni0C1PO+U9wpx7T8Gw5X+jUhqFKLyU5ssOsuGsFZALvIVJQnJd+Fbw7tZPyRxnTJpdRz6mOYvvj+JPvhTEYCE/cY29mHglAaSTSyKlqqRzGlZDtdCuE8Z36YsclrxCvHgNO4ns=
authentication-results: spf=none (sender IP is 216.69.139.48)
	smtp.mailfrom=jpes.com; mit.edu; dkim=none (message not signed)
	header.d=none;mit.edu; dmarc=none action=none header.from=jpes.com;
x-originating-ip: 98.200.152.44
user-agent: Workspace Webmail 6.12.9
x-cmae-analysis: v=2.4 cv=I5ag+Psg c=1 sm=1 tr=0 ts=6116836e
	a=UEXjU7X+PLwIesFmDkxagw==:117 a=y2T9wV5VyygA:10 a=RqfB-CqXghgA:10
	a=IkcTkHD0fZMA:10 a=MhDmnRu9jo8A:10 a=5KLPUuaC_9wA:10 a=pGLkceISAAAA:8
	a=Eq9pAxoCAAAA:8 a=F07sB3KiAAAA:8 a=Gz7s5_CCAAAA:8	a=PEWgjIede9_7Y3ladD4A:9
	a=QEXdDO2ut3YA:10 a=xeDuJAulGugA:10 a=pX47PpmJ-_ski9dZ2BE6:22
	a=8rYrM0obaTRgCqsvCuYq:22
errors-to: piclist-bounces@mit.edu
list-id: "Microcontroller discussion list - Public." <piclist.mit.edu>
list-post: <mailto:piclist@mit.edu>
x-beenthere: piclist@mit.edu
x-mailman-version: 2.1.6
x-sid: EYIPmD6buogSI
x-topics: [EE]
x-mime-autoconverted: from quoted-printable to 8bit by PCH.mit.edu id
	17DEaq2Z030671
x-mailman-approved-at: Sat, 14 Aug 2021 08:56:20 -0400
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0


I can't see the picture, but if you have a large vds, that usually means
the FET is in the linear region, and not going into saturation.   Try
using a pullup on the gate to a voltage higher than 5v so the FET goes
into saturation.  There should be less than 1v vds when operating this
way.  And your current gate connection can then pull the gate to g nd to
turn it off.  If this gives operation that you're looking for, then you
either need to figure a way of getting a higher gate voltage, ior get a
different FET with lower gate threshold voltage to turn it fully on.
Something like the logic level FETs.

Jim



Regards,

Jim

> -------- Original Message --------
> Subject: Re: [EE] Tek DSO display issue?
> From: Richard Prosser <rhprosser@gmail.com>
> Date: Fri, August 13, 2021 6:12 am
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
>=20
>=20
> Have you checked the decoupling caps on the MP24830 driver chip & its
> supply? The falling edge looks to have a bit of a slope to it. Also, chec=
k
> the snubber resistors & capacitors. What FET are you using ?  5V drive ma=
y
> be marginal although it seems to be turning on OK, albeit with some ringi=
ng.
> Another place to check is the ground connection(s) & current paths for th=
e
> drive  signal and the switched line & the snubbers.
> Your second photo may be showing  artifacts of overloading the front end =
of
> the scope (in the 2V range).
> Is there any resistance in the gate drive circuit? Without it there may b=
e
> more RF oscillation than is apparent from the scope & this can increase F=
ET
> dissipation significantly. 10 ohms or so is usually enough. (The scope le=
ad
> capacitance may be reducing the RF as well - is it getting as hot with th=
e
> scope connected?).
> RP
>=20
> On Fri, 13 Aug 2021 at 17:13, K S <kresho@gmail.com> wrote:
>=20
> > Help me PICLIST!
> >
> > I'm working on a buck converter and the main switching FET is heating
> > up plenty. To see what's going on I am measuring Vgs (purple) and Vds
> > (blue) with my trusty 20year old Tek TPS2014 - 4 isolated channels,
> > nice!
> >
> > See pic here: https://we.tl/t-t4t2USmkIU
> >
> > Vgs reaches just over 5V. I'm not too happy about that but it's what
> > the chip (MP24830) gives me and it should be just enough. To see what
> > I'm losing across in the FET I'm measuring Vds and hoping it's low
> > when the FET is on. So there are 2 shots in the pic, one with the
> > vertical scale set to 5V and the other at 2V/div. The 5V trace looks
> > beautiful, as expected. The 2V trace looks hmmm. Vds is shown clipped
> > in both cases; it reaches 48V.
> >
> > I'm noticing lots of funnies when I switch from 5V to 2V. I thought
> > I'd try something else: rather than measure volts directly across the
> > drain-source I thought I'd measure both the drain and source volts
> > w.r.t. GND and use the scope math function to get the difference. With
> > scales set to 5V/div I get what I'm expecting, with 2V the trace
> > disappears all together! I then tried this last experiment on a mate's
> > scope, also a Tek, TDS2022B, and got similar results. (I did try
> > across the FET itself with his scope but was getting FET misfirings -
> > I suspect that some small capacitive loading from this non-isolated
> > scope's inputs are causing some issues - I have many more pics if
> > anyone is interested).
> >
> > Anyone have any idea what might be happening here?
> >
> > TIA,
> > Kris.
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
> --=20
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.