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Low-Leakage Linearity Coil

IP.com Disclosure Number: IPCOM000116548D
Original Publication Date: 1995-Oct-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 6 page(s) / 162K

Publishing Venue

IBM

Related People

Beeteson, JS: AUTHOR [+3]

Abstract

A new construction of linearity coil for CRT displays is described that has the following major advantages over conventional components: 1. Lower leakage fields, so virtually eliminating any convergence and purity interactions. 2. Lower resistance, leading to lower power losses. 3. A simpler, and potentially lower cost construction.

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Low-Leakage Linearity Coil

      A new construction of linearity coil for CRT displays is
described that has the following major advantages over conventional
components:
  1.  Lower leakage fields, so virtually eliminating any convergence
       and purity interactions.
  2.  Lower resistance, leading to lower power losses.
  3.  A simpler, and potentially lower cost construction.

The new construction uses a toroid topology to give lower leakage and
higher sensitivity (or this may be build up from double U cores or
similar), with sections of the toroid modified to give a region of
magnetic path that can be saturated by a permanent magnet built into
cut outs (or moulded in).

      Conventional linearity coils typically look like Fig. 1.  A
permanent magnet is used to bias and saturate a ferrite core, so that
at zero or negative current in the coil the inductance is low.  As DC
current increases in the coil, then the permanent magnet flux is
gradually cancelled so that the core eventually comes out of
saturation and the inductance increases as shown in Fig. 3.

      A problem with such coils is that the magnet gives a very high
stray field which has historically been very troublesome in causing
convergence and purity errors on CRTs.  A solution to this problem
was found in (*), where a second magnet (also shown in Fig. 1) was
glued to the top of the structure to cancel stray fields, and this
has been successfully used in many programmes.  A second solution has
been to use two coils and place them side by side so that the two
stray fields cancel.

      Both these solutions have been successful in the past, but
modern monitor designs are becoming smaller causing the linearity
coil to be placed closer to the CRT.  As a consequence the smaller,
but still present, leakage fields are causing problems again.

      To find a linearity coil that had fundamentally low leakage
fields we decided to investigate a toroidal topology.  Eventually,
the prototype (Fig. 2) settled on was the one which produced the
results of Fig. 3 (which also gives the inductance/current plot of a
conventional linearity coil for comparison).

      Theory of Operation - The flux from the magnet mainly flows in
the ferrite immediately surrounding it and will saturate the core if
the magnet is strong enough.  The AC flux from the coil wound on the
core would, if no magnet were present, link efficiently round the
whole toroid giving a large inductance value.  However, with the
magnet present the core is locally saturated near the magnet and,
hence, the reluctance of the complete magnetic circuit increases
substantially.  After local area saturation, the magnet flux links
back via the rest of the core, and with core material and dimensions
and the magnet strength correctly chosen, there will be no saturation
of this part of the core and, thus, no substantial leakage of flux.

      Because of the local saturation, the AC flux fro...