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From: Andrew Kohlsmith
Subject: Philips MKT cap failures - LV pulse application
Date: Tue, 24 Sep 2002 22:03:25 -0400
NNTP-Posting-Date: Tue, 24 Sep 2002 22:06:11 EDT
Organization: WorldCom Canada Ltd. News Reader Service
I've been using Philips MKT .012uF caps in a 5V sawtooth generator circuit
for many years. Just recently I've been noticing parts failures both in
test and in the field - the capacitance dropped to roughly 10nF or so,
really screwing up the slope of the waveform :-)
From looking at old boards I've noticed that the original parts were marked
MKT-HQ (or HO, hard to tell)
About a year ago these parts read
All the parts that are failing are marked
The datasheet for the MKT 370 series doesn't tell me anything about these
two-letter markings but my gut instinct is telling me that it must have
something to do with either dv/dt or clearing ("healing") energy
Has anyone run across this? Does anyone know what this HQ/PH/BC suffixes
are about? These are all 100V rated parts and their specified dv/dt
characteristics are well within what I'm doing.
My parts vendor claims there is no difference in these parts but my gut
instinct is telling me that these two-letter codes are indicating
*something*; I just can't tell what.
Basic circuit is an LM339 arranged as an integrator -- the + pin at +4V and
the - at 5V through 10k. The output is connected to - through this cap,
and there is an everyday generic LM3904 transistor across the cap which I
activate to short it out and start the cycle over again.
With this transistor and the driver (output from a 74HC XOR gate) I can't
believe I'm exceeding the 110V/us dv/dt rating. Just to be sure, I've
sub'ed in a specific pulse-rated self-healing cap (Wima FKP series) --
Panasonic's ECQ series (ECQP1H123GZ to be exact) seems to be another
substitute. Both have far greater dv/dt ratings but I'm wondering if
anyone out there has any experience with either of these (or any other
370-footprint part) in low-voltage, pulse applications.
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