From: K S <kresho@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Thu, 12 Aug 2021 22:12:00 -0700
Subject: [EE] Tek DSO display issue?
Thread-Topic: [EE] Tek DSO display issue?
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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.
--=20
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