Note: Descriptions are shown in the official language in which they were submitted.
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-1- RCA 82,664
ISOLATIN~ HIGH VOLTAGE TRANSFORMER FOR
VIDEO APPARATUS
This invention relates to transformers for video
apparatus and, in particular, to high voltage transformers
providing electrical isolation between the primary and high
voltage windings.
A video apparatus, such as a television receiver
or a computer monitor, may incorporate user accessible
terminals or jacks ~o facilitate input or o~tput of video
or audio signals. These user accessi~le terminals or jacks
must be electrically isolated ~rom the AC line supply in
order to protect the user from shock hazard. Electrical
isolation may be provided by isolation transformers
associated with the input and output circuits themselves,
but this technigue may increase the cost and complexity of
video apparatus having many input or output terminals.
Electrical isolation may also be provided in the power
supply circuitry, Such as via a chopper transformer in a
switched mode power supply, for example.
In a video apparatus having a power supply
utilizing an SCR regulator, electrical isolation may be
provided ~ia the high voltage transformer. The high
voltage transformer typically incorporates a primary
winding to which a regulated B+ voltage is applied. One or
more secondary or load circuit windings are provided.
Voltages developed across the secondary windings are used
to power various load circuits of the video apparatus. A
high voltage winding develops a high voltage or ultor
potential for the cathode ray tube of the video apparatus.
The voltage levels present within the transformer require
that care be taken in the design and manufacture of the
high voltage transformer in order to reliably maintain the
electrical isolation barrier during the lifetime of the
video apparatus.
In accordance with an aspect of the present
invention, a high voltage transformer providing electrical
isolation for use in a video apparatus comprises a
magnetically permeable core. A first bobbin encircles the
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core and has a first winding for being energized from a
first voltage. A second bobbin surrounds the first bob~in
and has a second winding coupled to a load circuit that is
electrically isolated from the first voltage. A third
bobbin surrounds the second bobbin and has a high voltage
winding, electrically lsolated from the first voltage, for
generating an ultor potential in response to the
energization of the first winding.
In the accompanying drawing:
FIGURE 1 is a block and schematic diagram of a
portion of a video apparatus in accordance with an aspect
of the present invention;
FIGURE 2 is an exploded isometric view of a
portion of a high voltage transformer in accordance with an
aspect of the present invention;
FIGURE 3 is a bottom plan view of a part of the
transformer shown in FIGURE 2; and
FIGURE 4 is a side elevational cross sectional
view o~ a transformer similar to that shown in FIGURE 2.
Referring to FIGURE 1, a power source 10, such as
an AC line supply, is coupled to a rectifying circuit 11,
the output of which is filtered by a capacitor 12 to
provide a source of unregulated DC voltage At a texminal
13. The unresulated DC voltage is applied to one terminal
of a winding 14 of a novel high voltage transformer 15, the
detailed construction of which will be explained later.
The other terminal of winding 14 is coupled to the anode of
an SCR 16 via an inductor 17. The conduction of SCR 16 is
controlled in a manner that will be described later to
produce a regulated DC voltage acros~ capacitor 19 at a
terminal 20, located at the cathode of SCR 16. The
regulated DC voltage is applied via a primary winding 21 of
transformer 15 to the collector of a horizontal deflectian
output transistor 22, which forms part of a horizontal
deflection output circuit 23.
The video apparatus shown in FIGURE 1, such as a
television receiver or computer monitox, for example,
illustratively receives an input signal from an antenna 24,
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in the case of a television receiver, or via an input
terminal block 25 from an external source o~ signals, in
the case of a computer monitor. The radio frequency signal
from antenna 24 is applied to tuner and inter~ediate
frequency (IF) circuitry 26, the output of which is applied
to signal processing circuitry 27 and to synchronizing
~sync) pulse separator circuit 28. Signal processing
circuitry 27 may, for example, include the functions of
video detection, chrominance processing and luminance
1~ processing. Signal processing circuitry provides the drive
signals to the electron gun assembl~ 30 of a cathode ray
tube 31 via a conductor 32. Sync separator 28 provides the
individual horizontal, or line rate, and ver~ical, or field
rate, pulses from the composite video signal output of
signal processing circuitry 27. The signal from terminal
block 25 illustratively provides direct red, green and blue
video signals designated R, G and B to signal processing
circuitry 27, as well as a composite synchronizing signal,
designated CS, to sync pulse separator circuit 28.
The vertical, or field-rate, synchronizing signal
is applied via a conductor designated VS to a vertical
def~ection circuit 34, which produces vertical deflection
current via terminals V and V' in a vertical deflection
winding 45, located on the neck of CRT 31. Deflection
2S current in winding 45 causes the vertical deflection or
scanning of a representative electron beam 43, produced by
electron sun assembly 30, at a field rate across the
phosphor display screen 44 of CRT 31.
The horizontal, or line-rate, synchronizing
signal is applied via a conductor designated HS, to
horizontal deflection and regulator control circuitry 33,
which provides a horizontal rate switching signal to a
driver transistor 35. Switching of transistor 35, in turn,
causes switching pulses to be applied to the base of
horizontal deflection output transistor 22 via driver
transformer 36. Horizontal deflection output circuit 23
illustratively comprises a conventional resonant retrace
circuit including a damper diode 37, a retrace capacitor
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40, a horizontal deflection winding 41, located on the neck
of CRT 31, and an S-shaping capac.itor 42. The operation of
horizontal deflection output circuit 23 causes de~lection
current to flow in deflection winding 41 via terminal~ H
and H', thereby generating electromagnetic deflection
fields that horizontally deflect or scan electron beam 43
at a line rate on the phosphor display screen 44 of CRT 31.
Horizontal deflection and regulator control
circuitry 33 also produces horizontal deflection rate
gating pulses to the gate terminal of SCR 16 via a
trànsformer 38 in order to switch SCR 16 into conduction.
The time of occurrence of a gating pulse within each
horizontal deflection interval is controlled in accordance
with a feedback signal in order to maintain a constant
regulated voltage level at terminal 20. SCR 16 is
commutated off in a conventional manner by retrace related
pulses appearing across winding 14. The hori~ontal retrace
pulses appearing across primary winding 21, produced by
horizontal output circuit 23 in response to the switching
of horizontal output transistor 22, cause voltage pulses to
be developed across the other windings of transformer 15,
including the previously described SCR commutating pul~es
produced across winding 14. The voltage developed across
high voltage or tertiary wlnding 47 is rectified to
provide, at a terminal designated HV, a high voltage or
ultor potential of the order of 28 KV, that is applied to
ultor terminal 46 of CRT 31 in order to provide the
accelerating potential for electron beam 43. The voltage
developed across secondary winding 50 is rectified by diode
51 and filtered by capacitor 52 to provide a regulated DC
voltage source at a terminal 53 that may illustratively be
used to power various load circuits of the video apparatus,
for example, horizontal deflection and regulator control
circuitry 33. The voltage developed across winding 39 is
rectified by diode 48 and filtered by capacitor 49 to
illustratively provide the feedback signal to horizontal
deflection and regulator control circuitry 33 via a
terminal 58.
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-5- RCA 82,664
The input terminals and/or jacks of terminal
block 25 represent user accessible terminals that must be
electrically isolated from the power source 10 in order to
reduce user shock hazard. In accordance with an aspect of
the present invention, high voltage tra~sformer 15 provides
electrical isolation, which reduces the maximum current
that can flow between two isola-ted circuit points, between
the AC line supply 10 and the user accessible terminals,
including ~erminal block 25, for example, of the video
apparatus.
Referring to FIGURES 2, 3 and 4, the construction
details o~ transformer 15, illustrating the novel aspects
of the present invention, will now be described.
Transformer 15 comprises a first bobbin 55, upon which are
illustratively wound winding 14 and primary winding 21.
Windings 14 and 21 are electrically nonisolated from the AC
line supply 10, and are referenced ~o a point of reference
potential referred to in FIGURE 1 as "hot ground" and
designated with a particular ground symbol. Dashed lines
155 in FIGURE 1 schematically represent bobbin 55. Bobbin
55 camprises a cylindrical portion 56 about which the
windings are wound. As shown in FIGURE 4, the windings
wound on bobbin 55 illustratively traverse substantially
the entire winding region defined by winding stops 57 and
59. Bobbin 55 also includes a radially extending foot or
base portion 60, about which are distributed terminal pins
61. Terminal pins 61 are selectively connected to the
windings wound on first bobbin 55 for making contact with
the appropriate video apparatus circuit elements via a
printed circuit board (not shown).
A second bobbin 62 is dimensioned to fit around
first bobbin 55 such that first bobbin 55 nests within
second bobbin 62. Bobbin 62 comprises a cylindrical
portion 63 about which the secondary windings,
illustratively including winding 39 and 50, for example,
are wound. Windings 39 and 50 are illustratively wound to
cover substantially the full traverse of the winding region
defined by winding stops 64 and 66. Winding 39 and 50, and
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the associated video apparatus load circuits connected
thereto or powered therefrom, are electrically isolated
from windings 14 and 21, and consequently are lsolated from
the AC line supply lO. Windings 39 and 50, aDd associated
load circuitry, are referenced to a point of reference
potential referred to in FIGURE 1 as "cold ground" and
designated by a multiple horizontal line ground symbol.
Dashed line 162 in FIGURE 1 schematically illustrates
bobbin 62. Terminal pins 65 are distributed about the
perimeter of the base of the cylindrical portion 63 of
bobbin 62 in order to provide electrical contact via a
printed circuit board between windings 39 and 50 and
various circuit components of the video apparatus. Second
bobbin 62 also incorporates a base portion which defines an
area by way of wall struc~ure 67. When first bobbin 55
becomes nested within second bobbin 62 during assembly of
transformer 15, the area defined by wall structure 67 acts
to enclose terminal pins 61 of first bobbin 55, as can be
seen in FIGURE 3, thereby providing a physical separation
~0 barrier between the electrically isolated terminal pins 61
and 65. The use of the two bobbins 55 and 62 result in the
physical presence of the cylindrical portion 63 of second
bobbin 62 being located between the "cold" windings 39 and
50 and the "hot" windings 14 and 21, thereby providing an
~5 effective and reliable electrical isolation barrier.
A high voltage bobbin 70, around which is wound
high voltage or tertiary winding 47, surrounds second
bobbin 62. Dashed line 170 in FIGURE 1 represents high
voltage bobbin 70. High voltage winding 47 is also
referenced to the "cold" ground reference potential. The
"cold" low voltage windings 39 and 50 are therefore
physically located between the "cold" high voltage winding
47 and the "hot" windings 14 and 21, thereby providing a
high voltage discharge path to "cold ground" in the event
arcing of the high voltage winding should occur. The
bobbin structure formed by bobbins 55, 62 and the high
voltage bobbin 70, are located within a transformer housing
71. Housing structure 71 may be filled with epoxy in a
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conventional manner in order to encapsulate or pot the
wound bobbin structure. A magnetically permeable core 72
is inserted within the interior of cylindrical portion 56
of bobbin 55 to provide proper inductance for the tuning of
transformer 15 in a conventional resonant r~trace circuit
of the video apparatus.
FIGURE 4 illustrates a cross sectional view of
assembled transformer 15, showning the positioning of the
structural elements shown in FIGURE 2. The winding stops
57 and 59 on first bobbin 55 and winding stops 64 and 66 on
second bobbin 62 also act to stabilize the relative
positioning of bobbins 55, 62 and 70 of the assembled
bobbin structure. Second bobbin 62 also includes an
inwardly extending lip 73 that provides an additional
physical barrier, thereby increasing the length of a
possible arc path between windings 14 and 21, and windings
39 and 50. As can be seen in FIGURE 1, core 72 is also
referenced to "hot ground" with the result that core 72 and
the windings on first bobbin 55 are referenced to "hot
ground", while the windings on second bobbin 62 and high
voltage winding 47 are referenced to "cold ground", thereby
necessitating only a single electrical isolation barrier in
high voltage transformer 15.
