POWER SUPPLY CIRCUIT

Abstract: A power supply circuit is provided with a voltage control circuit (1) with negative feedback for applying a rapidly changing high voltage to the anode of a penetron-type cathode ray tube (3) for a colour display system and with a deflection voltage correcting unit (5) for obtaining colour-independent pictures on the screen of the cathode ray tube. The deflection voltage correcting unit is controlled by at least a correction signal derived from an input voltage of the voltage control circuit.

Description: Power supply circuit. The invention relates to a power supply circuit, provided with a voltage control circuit with negative feedback for applying a rapidly changing high voltage to the anode of a penetron type cathode ray tube for a colour display system, and with a deflection voltage correcting unit for obtaining colour-independent pictures on the screen of the cathode ray tube. To increase the legibility of a display, a multicolour tube is employed. Owing to the short working distance, it is unde- sirable to use a shadow-mask tube for this purpose; a penetron tube gives considerably better results. The penetron tube has two different phosphor layers. With a certain anode voltage the electrons excite mainly the first phosphor layer, while with a certain higher anode voltage the electrons are sufficiently accelerated to penetrate the barrier layer between the two phosphor layers and excite the second phosphor layer. The two anode voltages light up the screen in a certain colour; with intermediate anode voltages mixed colours are obtainable. Four-colour displays are in common use. In such colour displays the electron beam will hit the screen at a different place for each colour with a specific deflection voltage; as the electrons continually acquire a different acceleration by the anode voltages corresponding with the various colours. The deflection voltages should therefore be corrected by a fixed factor for each colour. To switch the anode voltage from a first to a second value, a certain time will elapse before the anode voltage settles to its second value and stabilises. In this switching period the beam electrons are subjected to a change in acceleration, causing positional errors to be displayed, even if fixed correction factors are applied to the deflection voltages for the individual colours, unless the display is blanked during the switching. A power supply circuit as set forth in the opening paragraph is known from the German Offenlegungsschrift 26 19 520, in which the deflection voltages are permanently corrected by a fraction of the anode voltage. To improve the long-term stability of the circuit, it is preferable to avoid a direct coupling between the deflection channels and the anode voltage. According to the invention the deflection voltage correcting unit is thereto controlled by at least a correction signal derived from an input voltage of the voltage control circuit. This correction signal is therefore directly related to the control voltage of the voltage control circuit with negative feed-back. With an input voltage typical of a certain colour the voltage control circuit can be set in such a way that the control voltage and hence the correction signal are adjusted to a value corresponding with this colour. This however requires an exceptionally broad range of the control voltage. It is therefore preferable to compose the deflection voltage correcting unit of a coarse and a fine correcting circuit, where the coarse correcting circuit is controlled by a signal typical of a particular display colour and the fine correcting circuit by the correction signal derived from an input voltage of the voltage control circuit. The control voltage and hence the correction signal for the fine correcting circuit can then be continually adjusted to zero. In the coarse correcting circuit the deflection voltages are corrected by a fixed factor required for each colour, while in the fine correcting circuit errors due to . imperfections in the switching of the anode voltage are compensated. If now the voltage control circuit is divided into a coarse and a fine control and the correction signal for the fine correction circuit is derived from the control voltage of the fine control of the voltage control circuit, the errors which would arise through variations in the anode voltage could be compensated in the deflec¬ tion voltage very accurately. In particular the compensation of the errors, which would arise during the settling time while switching the anode voltage, give the voltage control circuit the effect of one with very short switching times. The invention will now be described with reference to the accompanying figures, of which Fig. 1 is a circuit diagram of a power supply circuit according to the invention, and Fig. 2 a block diagram of a preferred embodiment of such a circuit. The power supply circuit of Fig. 1 comprises a voltage control circuit 1, of which the output voltage V,, is fed back via a control amplifier 2, to the input voltage V of voltage control circuit 1. The power supply circuit produces the anode voltage for a penetron type cathode ray tube 3. Input voltage V is supplied by a digital-analogue converter - . A computer delivers a digital code of the colour displayed to converter , converting this code in a corresponding voltage V_. From voltage V\. and a fraction of the anode voltage V,. fed back a control voltage -Vp + H is produced for voltage control circuit 1. The power supply circuit of Fig. 1 further comprises a deflection voltage correcting unit 5 to adapt the reflection voltages for cathode ray tube 3 for each of the colours displayed in such a way that the displayed pictures are independent of colour. Correcting unit 5 is provided with a coarse correcting circuit 6 , which is controlled by the computer-supplied digital colour code, represented by signal G, and with a fine correcting circuit 7, which is controlled by a signal F derived from the control voltage -Vp + αV,.. Signal G, corresponding with a certain display colour, is used to correct the deflection voltage, applied via line 8, by a fixed factor determined by the colour displayed, while signal F is used to correct the deflection voltage in accordance with imperfections in the switching of the anode voltage. It should be noted that for the sake of simplicity only one of the two orthogonally acting deflection voltages is considered. In a preferred embodiment of the power supply circuit in Fig. 2 voltage control circuit 1 comprises a coarse and a fine control. Control circuit 1 incorporates an input circuit 9 for both the coarse and the fine controls. Input circuit 9 comprises two combination circuits 10 and 11 and a divider 12 for delivering the voltages -a.V p (l - the supply of voltages V„ and αV,, from combination circuit 10 is the control voltage for the coarse control in the power supply circuit; this control voltage is representative of the absolute error in the anode voltage. If the anode voltage corresponds ___ OMPI _ with the applied input voltage Vp typical of a display colour, d, a different controls the anode voltage in such a way that this control voltage is returned αV to zero. The voltage -b(l - k-τ j —) supplied by divider 12 is the V C control voltage for the fine control in the power supply circuit; this control voltage is representative of the relative error in the anode voltage if this voltage has already approximated the value corresponding with that of Vp last applied. During the settling period after an anode voltage change initiated by a change of Vp the relative error retains its maximum value, that is with the switching during the "slew" decreases from maximum to zero, its value; the latter voltage decreases from maximum to zero during the settling time. Voltage control circuit 1 further comprises four compara¬ tors 13-16, a transformer 17, two current sources 18 and 19, a high voltage supply unit 20 and a d.c.-level adjusting unit 21, which are used to obtain a coarse and a fine control in the voltage control circuit. Comparators 13 - 16 supply the control signal for the switching and adjusting currents of the current sources connected thereto via transformer 17. With small deviations in the anode voltage, only one of the comparators 15 and 16 (the socalled settling comparators), depending on the sign of these deviations, is activated by the control voltage from divider 12 to switch on the adjusting current of the particular current source; with large deviations in the anode voltage the control voltage from combination circuit 10 is sufficient to activate one of the comparators 13 and 1 - (the socalled slew comparators) to switch on the switching current of the particular current source. The d.c.-level adjusting unit 21 can be applied to reduce the switching level to about zero volt. The two current sources 18 and 19 and d.c.-level adjusting unit 21 are fed by the high voltage supply unit 20. If current source 18 is activated, there will be a current flow from the anode of tube 3 through adjusting unit 21 and current source 18; if current source 19 is activated, there will be a current flow through current source 19 and adjusting unit 21 to the anode of tube 3. These currents remain as long as the control voltages applied to the comparators are not zero. Since the embodiment of voltage control circuit 1, apart from input circuit 9, is no part of the invention and many other embodiments are.feasible, only a brief description of a suitable embodiment is given. Signal F from divider 12 is of concern to the control of the fine correcting circuit 7 in deflection voltage correcting unit 5. Although the output signal of combination circuit 11 could also have been used as control voltage for the fine control in voltage control circuit 1, signal F has been selected as control voltage for the fine control in control circuit 1 as well, owing to the fact that signal F was required for the fine correcting circuit 7. The fine correcting circuit comprises a multiplier 22 and a combination circuit 23. Representing the coarsely corrected deflection voltage from coarse correcting circuit 6 by V ., the output voltage of the fine correcting circuit 7, that is the fully corrected deflection voltage V. , can be represented by: , then The dimensions of input circuit 9 with respect to fine correcting circuit 7 are such that 3b =1. Moreover, by dimensioning input circuit 9 in such a way that, after stabilisation of a high voltage V H required for a certain display colour, with respect to the reference voltage V corresponding with this colour, 0, during the adjustment, V, can be expressed by V, =A As already stated, a fixed factor is applied for the coarse correction of the deflection voltage by means of signal G, while the fine correction is effected through a multiplication factor obtained from the square root of the ratio between anode voltage V,, and reference voltage V p determining the display colour. When the anode voltage assumes the value it requires in accordance with the reference voltage, the multiplication factor is equal to 1 and signal F is equal to 0. Therefore, fine correction occurs only during switching and settling to the new anode voltage level. fδ OM Λ. WIP

Claims: • Claims: 1. Power supply circuit, provided with a voltage control circuit with negative feedback for applying a rapidly changing high voltage to the anode of a penetron-type cathode ray tube for a colour display system, and with a deflection voltage correcting unit for obtaining colour-independent pictures on the screen of the cathode ray tube, characterised in that the deflection voltage correcting unit (5) is controlled by at least a correction signal derived from an input voltage of the voltage control circuit (1). 2. Power supply circuit, as claimed in claim 1, characterised in that the deflection voltage correcting unit (5) comprises a coarse and a fine correcting circuit (6 and 7 respectively), where said coarse correcting circuit (6) is controlled by a signal G typical of a certain display colour and said fine correction circuit (7) by a correction signal F derived from an input voltage of the voltage control circuit (1). 3. Power supply circuit, as claimed in claim 2, characterised in that the voltage control circuit (1) comprises a coarse and a fine control, and the input voltage supplying the correction signal F is the input of said fine control. 4. Power supply circuit as claimed in claim 3, characterised in that the fine correcting circuit (7), in response to a coarsely corrected deflection voltage V , and to the correction signal F, produces a corrected deflection voltage AV,(l + βF), where A and 3 are constants, and that the voltage control circuit (1) comprises an input circuit (9) that, in response to a reference voltage Vp typical of a certain display colour and to a fraction α of the high voltage V., fed back, produces the correction signal F, expressed where k is a constant, for application to the fine control in the voltage control circuit. ■ & 5. Power supply circuit, as claimed in claim 3, characterised in that the input circuit (9) is so dimensioned that when a high voltage V,,, required for a certain display colour, is stabilised with respect to the reference voltage Vp corresponding with said display colour. OM Λ- WIP

Inventors: PLOUMEN H
Applicants: HOLLANDSE SIGNAALAPPARATEN BV, PLOUMEN H
Application year: 1980
Main classification: H04N9
IPC codes: H04N3, H04N9