Browse Prior Art Database

Electromagnetic Noise Suppression for Electronics Cards And Boards

IP.com Disclosure Number: IPCOM000102632D
Original Publication Date: 1990-Dec-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 4 page(s) / 168K

Publishing Venue

IBM

Related People

Tomlinson, GE: AUTHOR

Abstract

Disclosed is a design for printed circuit cards and second level electronics package power distribution that reduces the electromagnetic noise stored by introducing materials having large dielectric and magnetic losses, and high electrical resistivity. Ferrites are an example of presently available materials providing these properties.

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Electromagnetic Noise Suppression for Electronics Cards And Boards

       Disclosed is a design for printed circuit cards and
second level electronics package power distribution that reduces the
electromagnetic noise stored by introducing materials having large
dielectric and magnetic losses, and high electrical resistivity.
Ferrites are an example of presently available materials providing
these properties.

      The DC power distribution system in a conventional printed
circuit card has the form of a planar circuit.  Such circuits, which
are open circuit around their edges, store high frequency
electromagnetic energy in equilibrium with the source and the circuit
dissipation mechanisms.  The circuit energy storage properties may be
described by its electrical quality factor, Q.  A typical source is
the switching of digital electronics devices powered by the planar
circuit.  This radio frequen- cy energy interferes with the operation
of the electronics and, also, may radiate from the card.  It is
typical to include discrete capacitors connected between the circuit
planes which dominate the low frequency energy storage of a circuit
card.  These capacitors alter the planar circuit resonance
frequencies, and thus also the frequencies of strong radiation.
These discrete capacitors do not behave as pure capacitors for
frequencies above their lowest resonance.

      Dielectric and magnetic energy dissipation rate is proportional
to the energy density within the lossy material and its volume.
Thus, for a dissipative mechanism to be effective in the design of
electronics circuit cards, the dissipative volume must have storage
capacity comparable to the card and the components on it and/or be
effectively coupled to the current distribution associated with the
stored energy. Since the electric and magnetic fields associated with
the stored energy do not have the same spatial dependence, a
preferred dissipative material would possess both electric and
magnetic loss mechanisms.  This property is provided by many ferrite
materials, for example.  It is useful to consider two frequency
bands.  In the low frequency band the storage capacity of the card is
dominated by the discrete capacitors.  This low frequency band is
approximately between 0 and 20 MHz.  The high frequency band above
this is not dominated by the capacitance of the discrete capacitors
because they do not behave as pure capacitors for these frequencies.

      First, consider the low frequency band where the discrete
capacitors behave approximately as such.  In order for a dissipative
energy storage device to be useful, it must have capacitance or
inductance comparable to the rest of the package.  The ferrites can
meet these requirements within a practical volume because their
relative dielectric (magnetic) permeabilities are as high as 5000
(200) in this frequency band.  A preferred design using ferrite
ceramic around the entire periphery is proposed to di...