Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-11-08
2001-08-28
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S091100, C361S091200
Reexamination Certificate
active
06282071
ABSTRACT:
BACKGROUND
This invention relates generally to video displays for multiple video modes and, more particularly, to x-ray protection for cathode ray tube displays.
Protection against generation of harmful X-radiation from a cathode ray tube (CRT) includes an X-ray protection (XRP) circuit that compares a sense voltage, representative of an ultor voltage, against a reference voltage. Generation of the ultor voltage is disabled when the sense voltage is greater than the reference voltage. Accuracy of the XRP circuit to disable generation of the ultor voltage at a proper level relies on the sense voltage maintaining a predetermined relationship to the ultor voltage. This relationship is influenced by the relationship between beam current and ultor voltage. As indicated by the high voltage versus beam current curves
15
or
16
in
FIG. 1
, the slope or impedance is steeper at low beam current than at high beam current.
In monitor or CRT display applications the beam current and ultor voltage are maintained below the CRT's isodose curve. The isodose curve defines variations in ultor voltage and corresponding beam current at an anode of the CRT for a relatively constant level of X-radiation by the CRT. The isodose curve is a trip curve in that when beam current and ultor voltage are above the isodose curve the XRP circuit disables generation of the ultor voltage. As observed from
FIG. 1
, isodose curves
11
and
12
define high voltage VHV in kilovolts (kV) versus beam current (Ib) in microamps for X-radiation levels of 0.5 mR/hr (milliroentgen per hour) and 0.1 mR/hr, respectively. The CRT is operated so that its ultor voltage and corresponding beam current coincide below a particular isodose trip curve to avoid a particular level of X-radiation. Although reduced light output has, in the past, been acceptable in computer monitor applications, in television applications maximum light output is the goal and the high voltage is regulated to operate the CRT as close as possible to its isodose curve and improve the focus at high beam currents.
In a television or monitor a secondary winding, conventionally referred to as an X-ray protection winding, on the high voltage transformer develops a voltage VXRP as the primary of the transformer is driven by a pulse voltage waveform at a particular frequency related or synchronized to the video signal's horizontal scan frequency. The voltage VXRP develops with an amplitude that is proportional to the ultor voltage applied to a CRT's anode. The relationship between the ultor voltage and XRP voltage remains relatively constant over a given range of beam current when the transformer is driven by a pulse at a constant frequency.
Various video signal modes have different horizontal frequencies that require different high voltage generator frequencies at which the transformer is energized. High voltage generators incorporating scan-independent high voltage systems can have variable generating frequencies. The standard definition NTSC signal, high definition ATSC signal, and computer generated SVGA signal have the following respective horizontal frequencies, 15.734 kHz (1H), 33.670 kHz (2.14H), and 37.880 kHz (2.4H). Selection to a higher horizontal frequency signal will require driving the high voltage transformer with a pulse voltage waveform at a higher frequency. For example, in the NTSC broadcast signal mode, the high voltage generator is synchronized to the horizontal scan frequency but operated at 2H or 31.468 kHz, and in the SVGA monitor mode the high voltage generator is locked to the 37.880 kHz (2.4H) video signal frequency.
The high voltage transformer which develops the ultor voltage and voltage VXRP operates with a frequency dependent impedance. As frequency of the voltage energizing the transformer increases the inductive coupling to the secondary winding developing the ultor voltage becomes much more lossy than the inductive coupling to the secondary winding developing the voltage VXRP. Known frequency dependent transformer losses in the inductive couplings between the primary winding and secondary windings may include losses due to inter-winding capacitance and eddy current effects. Energy is dissipated during the charge and discharge of inter-winding capacitance between winding layers of the transformer. At a greater energizing frequency the effects of inter-winding capacitance are more pronounced. Also, at higher frequencies known skin effects occur in which conductors appear to have a higher AC resistance from current crowding at the surface of the conductor. With multiple winding conductors skin effects are more pronounced at greater energizing frequencies. Although these and other types of known transformer losses will vary with transformer construction, the losses will be greater with increases in frequency at which the transformer is energized.
To compensate for the increased loss in inductive coupling producing the ultor voltage and maintain a relatively constant ultor voltage, as frequency increases the pulse voltage driving the primary winding of the transformer is boosted to maintain the ultor voltage relatively constant. Since the inductive coupling to the secondary winding developing the voltage VXRP is not as lossy as that for developing the ultor voltage, voltage VXRP increases as the primary voltage energizing the transformer is increased to maintain the ultor voltage level. As a result, voltage VXRP increases relative to the ultor voltage and cannot be used directly to monitor and determine fault levels in ultor voltage over changes in frequency.
SUMMARY
In accordance with an inventive arrangement there is provided a high voltage circuit comprising: a high voltage generator; first means for developing a first signal representative of the high voltage; second means for developing a second signal indicative of a frequency of operation of the high voltage generator; and third means coupled to the first and second means and responsive to the second signal indicative of the frequency of operation for detecting a fault operation of the high voltage generator in accordance with the frequency of operation.
In accordance with a different inventive arrangement there is provided a cathode ray tube display operable under varying transformer energizing frequencies. The display includes a high voltage transformer having a primary winding for being energized by a voltage at the transformer energizing frequency and a secondary winding comprising a tertiary winding for supplying a high voltage to provide an anode accelerating potential to a cathode ray tube and a protection winding for developing a voltage that is in proportion to the high voltage, the proportion to the high voltage changing according to changes in the transformer energizing frequency. The display further includes a protection circuit responsive to changes in the transformer energizing frequency for disabling normal energization of the primary winding when the proportion of high voltage exceeds a reference voltage as the transformer energizing frequency changes.
In accordance with another inventive arrangement there is provided a high voltage power supply circuit for supplying a high voltage to provide anode accelerating potential in a cathode ray tube. The power supply circuit includes a transformer with primary winding and secondary winding including both a tertiary winding and protection winding; a generator circuit for energizing the primary winding with a pulse voltage at a generator frequency to produce both the high voltage across the tertiary winding and a protection voltage across the protection winding in proportion to the high voltage, proportion of the protection voltage to the high voltage varying with changes in the generator frequency; and a protection circuit responsive to changes in the generator frequency for developing a sense voltage from the protection voltage that is representative of the high voltage over variations in the generator frequency.
REFERENCES:
patent: 3660753 (1972-05-01), Judd et al.
patent: 4052676 (1977-10-01), C
Kolodka J. J.
Laks J. J.
Sherry Michael J.
Thomson Licensing S.A.
Tripoli J. S.
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