Magnetic shielding on inductors is great, but it is still necessary to
exercise caution. The shielding is generally not complete, and where the
gaps exist, expect significant fringing. As has been said, distance is your
friend; but where fringing is between two closely spaced pieces of ferrite
you are very unlikely to get equal coupling in differential traces because
the distances between traces is no-longer small in relation to the fringe
width. This leads to what then appears as an imbalanced induction as it
primarily affects one of the two signals in the pair and may ruin your
differential signal. As it is not common induction, it cannot be filtered
out easily.
While a little bit off-subject, it is interesting to note that the routing
of the PSUs feedback signal starts at or near the inductor's output side
and frequently needs to come back to the controller by coming round (or
under) the inductor. If that feedback path gets too close to the fringing
location, the output voltage of the supply is likely to carry a lot more
noise than the data sheet and simulations might suggest, and in more
extreme cases be unstable.
Regards
Jon
On 15 March 2017 at 15:26, Curt McNamara <CurtM@xxxxxxxxxxx> wrote:
Inductors are used in switching power supplies to store energy in the form
of a magnetic field. This field increases and decreases over every
switching cycle. The resulting magnetic field external to the (magnetically
shielded) inductor can be minimized by careful layout.
However, copper is ineffective in reducing magnetic fields. Distance is
your best strategy.
Linear Tech has some good app notes on the topic.
http://cds.linear.com/docs/en/application-note/an139f.pdf
http://cds.linear.com/docs/en/application-note/an25fa.pdf
Curt
Curt McNamara, P.E.
Engineering Consultant
612.305.0440 x248
www.npe-inc.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of John Lin
Sent: Wednesday, March 15, 2017 1:01 AM
To: si-list <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Ground planes beneath power inductor/choke to isolate
high speed digital signal.
Dear SI gurus,
Could someone share experience on routing high speed digital signal
10GHz(layer 3), beneath inductors ( on the top layer)in switching power
circuit area with ground plane(s) (layer 2)for isolation?
What tools should be used to simulate induced noise coupling?
How many ground planes should be used to isolate noise induced by the
inductors?
Thank you for helps in advance.
Regards,
John Lin
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