JPH078011B2 - Dynamic focus phase controller - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a dynamic focus circuit of a picture tube, that is, a dynamic focus circuit capable of obtaining optimum focus on the entire screen.

[Conventional technology]

The electromagnetic focusing dynamic focus circuit includes a method in which a switching circuit and an LC resonance circuit are combined, an auxiliary coil used for dynamic focus is connected as an inductor of the resonance circuit, and a correction current is supplied. In addition,
A device related to this kind of device is disclosed in, for example, Japanese Patent Laid-Open No.
Publications such as 79573 and JP-A-56-98970 are mentioned.

[Problems to be Solved by the Invention] Generally, in a high-definition display that requires a high-resolution image, it is more advantageous to use a magnetic focusing lens than an electrostatic focusing lens as a means for focusing an electron beam.

FIG. 3 is a CRT cross-sectional view for explaining the outline of the magnetic focusing method. In the figure, 1 is a focusing coil (or permanent magnet), 2 is an auxiliary coil, 3 is a magnetic focusing lens, 4
Is a deflection yoke and 5 is a screen.

As shown in FIG. 3, in order to achieve the magnetic focusing method, a focusing coil (or a permanent magnet may be used) 1 is attached around the neck portion of the cathode ray tube, and a focusing lens system 3 is formed inside the neck. . Generally, the magnetic field for obtaining the optimum focus is not constant at a point on the screen 5, and the magnetic field strength is different between the central part and the peripheral part of the screen. Therefore, in order to obtain the optimum focus over the entire surface of the screen 5, it is necessary to change the magnetic field strength according to the point on the screen. So-called dynamic focus is applied. For this purpose, a parabolic wave (or sinusoidal) correction current is made to flow through the auxiliary coil 2 inside the focusing coil (or permanent magnet) 1 to change the magnetic field strength according to the point (position) on the screen to achieve dynamic focus. To be realized.

Next, the problem will be described using the above-mentioned conventional dynamic focus circuit.

FIG. 4 shows a conventional dynamic focus circuit. In the figure, 6 is a power MOS (switch), 7 is a damper diode, 8 is a capacitor, 9 is a choke coil,
Reference numeral 10 is a capacitor, 11 is a variable inductor, 12 is a resonance capacitor, 2 is an auxiliary coil, and 14 is a resistor.

FIG. 5 is an operation waveform diagram of each part of FIG.

The circuit operation will be described with reference to FIGS. First, a drive pulse a having a horizontal cycle is input to the gate of the switching power MOS 6. At this time, the power MOS 6 is repeatedly turned on and off, and the switching pulse b is generated by the series resonance of the choke coil 9 and the capacitor 8. By this switching pulse b, the capacitor 12, the auxiliary coil 2, the resistor 14
A horizontal cycle sinusoidal waveform is generated in the LCR series-parallel resonance circuit configured by, and a horizontal cycle sinusoidal correction current flows in the auxiliary coil 2. If the resonance frequency of this circuit matches the horizontal scanning frequency, as shown by the solid line in FIG. 5C, the drive pulse a of the horizontal cycle and the phase of the correction current match,
Optimal focus is obtained at all points on the screen. However, since the Q of the resonance circuit is high, even if the horizontal scanning frequency or the element value of the resonance circuit is slightly changed, the phase shifts as shown by the dotted line in FIG. . Normally, this phase shift is adjusted by the variable inductor 11. Although this adjustment can cope with variations in the elements, it is necessary to readjust the phase with respect to changes in temperature and aging or changes in horizontal scanning frequency.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a dynamic focus circuit that can compensate the above-mentioned drawbacks and prevent focus deterioration due to phase shift.

[Means for solving problems]

The dotted line in FIG. 5C shows a state in which the phase of the correction current is advanced with respect to the drive pulse of the horizontal cycle. At this time, in order to correct the phase lead of the correction current, the generation timing of the switching pulse b may be delayed by the phase lead. On the contrary, when the phase of the correction current is delayed, the generation timing of the switching pulse b may be advanced by the phase delay. Therefore, when performing phase compensation, it is sufficient to provide a circuit for controlling the generation timing of the switching pulse according to the phase shift.

[Action]

As a method of changing the generation timing of the switching pulse according to the phase shift, there is a method using a PLL (phase locked loop). The PLL is composed of a phase comparator, a low-pass filter, and a voltage-controlled oscillator.The output voltage of the low-pass filter determines the phase of the output pulse of the voltage-controlled oscillator, and the circuit controlled by this output pulse operates to compare the phase of the desired signal. The phase of the desired signal is controlled by feeding back to the comparator's comparison input.

〔Example〕

An embodiment of the present invention will be described below with reference to the waveform diagram shown in FIG. 2 and its operation will be described.

In FIG. 1, 15 and 19 are monostable multi, 16 is a phase comparator, 17 is a low pass filter (LPF), 18 is a voltage controlled oscillator (VCO), 2 is an auxiliary coil, 22 is a resistor, 20 is a comparator, In addition, the same parts as those in FIG. 4 are designated by the same reference numerals.

In FIG. 2, T ₁ indicates a horizontal blanking period and T ₂ indicates a horizontal scanning period. Now, when the horizontal cycle drive pulse a is input to the monostable multi 15, a reference pulse b is generated from its output. This reference pulse a is so arranged that its falling edge and the falling edge of the horizontal cycle drive pulse b coincide with each other.
Set the monostable multi-15 to have a pal width of. The monostable multi 19 is a circuit for generating a drive pulse d for switching the switching power MOS 6 of the dynamic focus circuit. This monostable multi
19 is set so that the falling edge of the output drive pulse d and the falling edge of the output pulse c of the voltage controlled oscillator 18 coincide with each other. The diode 7 is a damper diode. A series resonance circuit is formed by the capacitor 8 and the choke coil 9, and this resonance circuit and the power MOS 6
A switching pulse e is generated by the switching. The energy generated by this switching pulse e is taken out from the tap of the choke coil 9, and then the series-parallel resonance circuit composed of the capacitors 10 and 12 and the coils 21 and 2 and the resistors 14 and 22 is connected to flow a correction current. A sinusoidal correction current is passed through the coil 2. The resistor 22 is a resistor that converts the current of the auxiliary coil 2 into a voltage and detects the phase of the correction current g as the phase of the voltage f across the auxiliary coil. Further, the sine wave voltage detected by the resistor 22 by the zero-cross comparator 20 is converted into a rectangular wave h and input to the comparison input terminal of the phase comparator 16. Assuming that the phase of the correction current g is advanced with respect to the drive pulse d, the phase of the voltage f across the auxiliary coil is also advanced, and the phase of the output waveform h of the zero cross comparator 20 is also advanced.
Therefore, the output of the phase comparator 16 lowers the output of the LPF 17 and delays the generation timing of the output pulse c of the voltage controlled oscillator 18. This is the same as the drive pulse d of the power MOS6 being delayed, so that the switching pulse d is also delayed, the voltage and current phases of the correction coil 2 are also delayed, and the rising edge of the output waveform h of the zero-cross comparator 20 and the reference pulse b are delayed. The rising edges match. In this state, this phase control circuit converges. That is, the phase of the voltage and current of the correction coil becomes the same as the phase state at the time of optimum focusing as shown in FIG.

〔The invention's effect〕

As described above, according to the present invention, the efficiency of the focus adjustment can be improved by automating the phase adjustment in the dynamic focus circuit, and the correction current phase can always be optimized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is an operation waveform diagram of each part thereof, FIG. 3 is a sectional view of a cathode ray tube for explaining an outline of a magnetic focusing system, and FIG. FIG. 5 is a circuit diagram showing the focus circuit, and FIG. 5 is a waveform diagram of each part for explaining the operation of the circuit. 6 ... Power MOS, 7 ... Damper diode 9 ... Choke coil, 2 ... Auxiliary coil 22 ... Resistor, 15 ... Monostable multi 16 ... Phase comparator, 17 ... Low pass filter 18 ... Voltage controlled oscillator, 19 ... Monostable multi 20 ... comparator

Claims (1)

[Claims] 1. A parapolar wave (sinusoidal) correction current having the same period as a horizontal scanning period is passed through an auxiliary coil provided in association with a magnetic field generating means for focusing an electron beam on a screen. In the dynamic focus circuit of the picture tube that achieves optimum focusing over the entire screen, a circuit that generates a reference pulse of the same phase that is synchronized with the horizontal blanking period of the video signal, and a drive pulse that has the same period as the horizontal scanning period A correction current generating circuit that generates the correction current in the form of the parapolar wave (sine wave) and flows in the auxiliary coil in synchronization with it, and a circuit that generates a phase pulse representing the phase of the correction current flowing in the auxiliary coil; A comparator circuit that detects and outputs the phase difference between the phase pulse and the reference pulse, and the generation position of the drive pulse so that the phase difference becomes zero. A circuit for controlling a phase, and a dynamic focus circuit.