When I was design switcher power supply boards we did one with a 100 Hz
switching frequency and you could hear it buzz when it was running... mainly
because the 100 Hz is in the audio range of frequencies between 20 and 20K
Hertz. We named it the 'Hummer' because of that 'feature'... You will also
notice the hum you hear in the florescent lighting fixtures that are
vibrating at 60Hz which is the frequency of the line voltage in your home...
It can be rather annoying...
Best regards,
Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
Datron World Communications, Inc.
_______________________________________
San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com
-----Original Message-----
From: JaMi Smith [mailto:[log in to unmask]]
Sent: Wednesday, June 08, 2005 4:32 PM
To: [log in to unmask]
Subject: Re: [DC] The Buck Switch-mode Power supply buzzs
Sk,
Sometimes we tend to forget that an inductor (sometimes also called a choke)
is a
magnetic animal, in that with or without an iron core, it is actually an
electromagnet.
This electromagnet in your circuit appears to possibly be interacting with
some
other ferrous object in your design, or depending on the actual type of
"inductor"
you are using, possibly the "winding" is loose, and vibrating on or within
it's own
core.
Firstly, it appears that it is mechanically "vibrating" at low enough
frequency to
be heard, or at least interacting with something else at a low frequency.
What type and value of inductor are you actually using? Voltage, Current,
etc. (all
of which determine the size and strength of the magnetic field).
Can you describe it's physical construction, or possibly give an link to a
data
sheet?
How well is it attached to the board?
Are there any components nearby with steel components or pieces, such as a
spring
clip on a TO-220 regulator or transistor in a nearby circuit, or a large
device with
steel rather than copper leads?
Another question to ask is are you possibly using two inductors in close
proximity
to each other? If you are, it is possible that they are magnetically
"coupling" to
each other.
Whenever you use an inductor in a design, you have to take the footprint of
it's
actual magnetic field into account, to make sure it is not either
interfering with
something else in the design, or that it itself is not being interfered
with,
magnetically speaking.
You also must remember that a magnetic field can easily penetrate your PCB,
such
that an inductor on one side of the board could actually interfere with (or
be
interfered with by) an inductor on the other side of the board, in the same
vicinity.
If you ever have to place inductors near each other in a circuit, the first
thing
you want to do is to get as much physical separation as possible, and then
try to
place the inductors such that their "cores" are at right angles to each
other, which
will provide the least amount of magnetic coupling.
While many engineers and techs may already be very familiar with this
problem, I
would recommend that if a designer gets a chance, he or she should ask a
engineer or
tech to give them a demonstration of this phenomena, which can be easily
done by
simply hooking up another inductor to a scope probe, and moving the "probe /
inductor" (insulated of course) around in an active circuit, such as the one
that
you are describing.
Actually, in your type of circuit, which usually calls for a rather large
inductor
with a ferrite core or bobbin, I would vote for the winding being loose on
the core.
JaMi
----- Original Message -----
From: "Liew, Sen Joon" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, June 08, 2005 3:14 AM
Subject: [DC] The Buck Switch-mode Power supply buzzs
Hi, Everyone,
I have one issue to ask for assistance. I am designing a simple Switch-mode
Buck
power
supply. I noticed that when I changed to certain inductor value. The whole
board
seems to
buzz loudly. Does it mean I used the wrong inductor value or otherwise.
Has everyone encountered the same phenomenon before, and what is the
possible cause.
Your response is very much appreciated. Thanks!
Best regards,
Sk Liew
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