[SI-LIST] Off-topic? Questions about measuring power supply ripple/noise (PARD)
- From: "Dixon, Adam" <AD123338@xxxxxxxxxxxxxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 15 Aug 2002 00:29:22 -0400
This is my first post and may be slightly off-topic. I am trying to better
understand the hows and whys of measuring power supply ripple/noise. I have
seen this referenced as "Periodic And Random Disturbance" or PARD in several
power supply suppliers' application notes. Any comments or recommended
reading/references are greatly appreciated. I searched the SI-list archives
unsuccessfully. I am working with 90W to 300W multiple-output PC power
supplies (+3.3V, +5V, +12V,...) and several of the industry standard power
supply design guides (ATX, SFX, TFX, WTX form factors) all state to use a
10uF electrolytic and 0.01uF ceramic capacitor on each ouput and to measure
with an oscilloscope set to 20MHz bandwidth. My questions are (comment on
as many as you like, please!):
1) Why 20MHz scope bandwidth? I saw one Intel application note indicating
that bandwidth should be approx. 100X the switching frequency -- is this the
reason? If I use full bandwidth, I see a higher peak-to-peak noise value
but the 20MHz value seems to be an industry standard.
2) The 10uF and 0.01uF "load" capacitor values do not generally correlate
with the actual application loads. Two or three of the outputs have 200uF
or more of bulk/electrolytic capacitance and another few uF's worth of 0.1uF
and/or 0.01uF bypass/decoupling capacitors. Is this a legacy issue to
provide a common measurement configuration? The electronic loads are used
in constant current mode with the ability to dynamically load one or more
outputs simultaneously.
3) What is the best way to capture the maximum peak-to-peak noise? I am
using a TDS7xx scope and don't see a way to create a trigger condition for a
max/min condition. I am wondering about the validity of setting the display
for infinite persistence mode and capturing a large number of samples on a
slow timebase (100uS to 1mS?) to get a good view of how the output varies.
Does this sound valid? I haven't seen anything in print which discusses
this.
4) Probing where the electronic load is connected is difficult due to the
distance between the +/- connections. One application note recommended
removing the plastic insulator of the scope probe to get the shortest
possible ground connection in order to minimize measurement error from noise
coupling. I have hacked up a passive Tek probe ground wire to shorten it to
approx. 2" including clip -- any thoughts about improving the probing?
5) One last scope question: If I am observing a +5VDC output, what is the
difference between using AC couple mode versus DC couple with a +5V offset
applied? Is there a significant difference internal to the scope that
favors either mode? I guess I will go to the lab and try both but any
comments are welcome.
Thanks for reading!
Sincerely,
Adam Dixon
NCR Corporation
adam.dixon@xxxxxxx
(770) 623-7093
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