Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2000-01-24
2002-06-25
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S397000
Reexamination Certificate
active
06411048
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the control of the vertical deflection of a spot scanning a screen, for example, screens of television sets and/or computer monitors.
BACKGROUND OF THE INVENTION
A spot scans a screen in two orthogonal directions, namely a line scan and a vertical scan. The line scan takes place at a much higher frequency than the vertical scan. For example, for a television, the line scan may take place at 15625 Hz, whereas the vertical scan takes place, for example, at 50 Hz. When the spot has undergone a forward line scan, a line retrace then occurs in order to scan the next line. Simultaneously with the line scan, the vertical scan allows the spot to scan the screen from the top down. At the end of the downward vertical scan, a rapid retrace of the spot to the top and to the left of the screen (frame retrace) then occurs, this being commonly called “Flyback” by those skilled in the art.
The vertical deflection of the spot therefore comprises a downward vertical scan phase followed by a spot flyback phase. The vertical deflection is generated by a vertical deflection circuit, which is conventionally an inductive circuit generating a magnetic field for deflecting the spot. The conventional devices for controlling vertical deflection circuits (or more simply “vertical deflectors”) are formed by linear class AB amplifiers, i.e. an amplifier with the output stage formed by transistors working in their linear range and consequently producing a continuous output voltage. The amplifier amplifies a reference input signal, generally a sawtooth signal, so as to deliver a signal for controlling the vertical scan of the spot. Moreover, in order to ensure flyback of the spot, these control devices are equipped to generate an overvoltage at the terminals of the deflector.
Currently, such control devices have an actual efficiency of about 50% and generally require relatively large heat sinks.
SUMMARY OF THE INVENTION
An object of the invention is to remedy this drawback by increasing the efficiency of such a control circuit, while at the same time reducing the complexity of the heat sinks, so as to reduce the overall cost of the control device.
Yet another object of the invention is to produce such control devices in the form of integrated circuits.
The invention therefore provides a method for controlling a circuit for the vertical deflection of a spot scanning a screen. In the method, the vertical scan of the spot is controlled by an output amplifier stage supplied by a main supply (for example ±15 volts) and the flyback of the spot is initiated by delivering an overvoltage, produced by an auxiliary supply, to the vertical deflection circuit.
The control device includes an output amplifier stage having at least two transistors which are made to operate in Class-D operation (i.e. it is alternately turned on and off) at least for controlling the vertical scan of the spot. The overvoltage is delivered by controlling a first two-way switch connected to the output stage and to the auxiliary supply so as to allow a current to flow through this first switch. In addition, a second two-way switch connected between a first transistor of the output stage and a first terminal (for example +Vcc) of the main supply is controlled so as to prevent a current from flowing through the first switch during the vertical scan of the spot and thus to prevent the overvoltage being delivered during the vertical scan of the spot.
In Class-D operation, the transistors operate in on/off mode, i.e. when one of them is off the other is on, and vice versa. In Class-D operation, when a transistor is off it does not dissipate power, while when it is on it dissipates very little power. The output voltage of the output stage is a “high” or “low” signal. If the duration of the high state is equal to that of the low state, the average value of the output signal is zero. By modulating the duty cycle of the control signal (control pulses of variable width) of the filters and by filtering the high-frequency component, an average voltage is recovered which varies over time. This variation is very slow compared with the chopping frequency (the frequency of the control signal). It is thus possible to achieve an efficiency that may be as high as 90%, as opposed to approximately 45% to 50% in the case of Class-AB operation.
Furthermore, in combination with this Class-D operation, the invention provides for the delivery of the overvoltage by controlling a first two-way switch so as to allow a current to flow through this first switch. Moreover, it is necessary to provide the second two-way switch which is controlled so as to prevent a current from flowing through the first switch during the vertical scan of the spot, and in particular when, in a second phase of the forward scan, the second transistor of the output stage is off.
All the transistors, whether those of the output stage or else those advantageously forming the various two-way switches, may be bipolar transistors. This being the case, especially for the purpose of having an integrated-circuit construction, it is particularly advantageous for these transistors to be isolated-gate field-effect transistors (MOS transistors), for example of the n-channel type.
Although in theory it is not necessary to connect a diode to each transistor in antiparallel to its terminals, this proves to be preferable, especially if bipolar transistors are used, so as to allow good conduction of the transistors in both directions, but also when MOS transistors are used so as to avoid an increase in the threshold voltage of these transistors, which generally operate with large signals. In practice, the antiparallel-connected diode is produced by connecting the source of the transistor to the substrate (or bulk). More generally, it is pointed out here that, in the context of the present invention, a diode is said to be connected in antiparallel with the terminals of a transistor when the anode of the diode is connected to the source of the transistor while the cathode of the diode is connected to the drain of the transistor (when this a MOS transistor).
According to one embodiment of the invention, the first two-way switch comprises a transistor and a diode connected in antiparallel with the terminals of the transistor, the anode of the diode being connected to the first transistor of the output stage. The second two-way switch, which makes it possible to avoid delivery of the overvoltage during control of the vertical scan of the spot, comprises a transistor connected in series between the first transistor of the output stage and the first terminal of the main supply (for example +Vcc). This second two-way switch also comprises a diode connected in antiparallel with the terminals of the transistor, the cathode of this diode being connected to the output stage and the anode being connected to the said first terminal of the main supply.
The vertical scan of the spot comprises a first phase during which the current flows into the vertical deflection circuit and a second phase during which the current flows from the vertical deflection circuit. Moreover, the flyback of the spot comprises a first phase during which the current flows from the vertical deflection circuit and a second phase during which the current flows into the vertical deflection circuit. The transistor of the first two-way switch (i.e. the one which allows the overvoltage to be delivered) is turned off during the vertical scan, while the second two-way switch (i.e. the one which prevents the overvoltage being delivered during the vertical scan) is controlled so as to turn the transistor of this second two-way switch on at least when the second transistor of the output stage is off during the second phase of the vertical scan. Furthermore, the transistor of the second two-way switch is off during the flyback of the spot, while the transistor of the first two-way switch is turned on at least during the second phase of the flyback of the spot.
In practice, for reasons of simplicity of cont
Guedon Yannick
Maige Philippe
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Chen Shih-Chao
Jorgenson Lisa K.
STMicroelectronics S.A.
Wong Don
LandOfFree
Method and device for controlling a circuit for the vertical... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for controlling a circuit for the vertical..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for controlling a circuit for the vertical... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2961064