From: K S <kresho@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Sat, 14 Aug 2021 06:20:00 -0700
Subject: Re: [EE] Tek DSO display issue?
Thread-Topic: [EE] Tek DSO display issue?
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Seems to me logic level FETs are harder to find these days than before?

I searched digikey and mouser and I'm not finding many with
characteristics quoted at Vgs of 5V or less. Of course I need all the
other bells and whistles to - low Q, Rds, decent Vds (80V+) and Id
(30A+).  Maybe I'm not looking in the right place?

On Sat, Aug 14, 2021 at 10:56 PM Jim <jim@jpes.com> wrote:
>
>
> 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>
> >
> >
> > 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, ch=
eck
> > the snubber resistors & capacitors. What FET are you using ?  5V drive =
may
> > be marginal although it seems to be turning on OK, albeit with some rin=
ging.
> > Another place to check is the ground connection(s) & current paths for =
the
> > drive  signal and the switched line & the snubbers.
> > Your second photo may be showing  artifacts of overloading the front en=
d of
> > the scope (in the 2V range).
> > Is there any resistance in the gate drive circuit? Without it there may=
 be
> > more RF oscillation than is apparent from the scope & this can increase=
 FET
> > dissipation significantly. 10 ohms or so is usually enough. (The scope =
lead
> > capacitance may be reducing the RF as well - is it getting as hot with =
the
> > scope connected?).
> > RP
> >
> > On Fri, 13 Aug 2021 at 17:13, K S <kresho@gmail.com> wrote:
> >
> > > 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. Wit=
h
> > > 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.
> > > --
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