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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