From: Richard Prosser <rhprosser@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Fri, 13 Aug 2021 04:12:19 -0700
Subject: Re: [EE] Tek DSO display issue?
Thread-Topic: [EE] Tek DSO display issue?
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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, check
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 ringing=
..
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 end 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. 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.
> --
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