JPS6365272B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focusing coil current stabilizing device for stably supplying current to the focusing coil in an electromagnetic focusing cathode ray tube.
In particular, electrostatic deflection and electromagnetic focusing imaging type (hereinafter M-
The object of the present invention is to provide a focusing coil current stabilizing device that is effective for S-type image pickup tubes.

Electromagnetic focusing methods have been widely used in the past in order to improve the degree of beam focusing in video tubes and image pickup tubes. Since stability of the focusing state is often required, a constant current circuit is generally connected to the focusing coil to maintain a constant current even when the current voltage or the resistance value of the coil changes.

FIG. 1 shows a conventional focusing coil current stabilizing device for an M-S type image pickup tube. In Figure 1, 1
2 is a horizontal drive pulse input terminal, and 2 is a sawtooth wave generator that generates a sawtooth wave voltage for horizontal deflection in synchronization with this horizontal drive pulse. Since it is necessary to apply sawtooth wave voltages with opposite polarities to the horizontal deflection electrodes, the sawtooth wave generator has two
Two output terminals 3 and 4 are provided. Since the output voltage of the deflection electrode amplitude is generally large, it is amplified by the amplifier 5 and applied to the two deflection electrodes 6 and 7, respectively. Reference numeral 11 is a schematic diagram of an image pickup tube, and the vertical deflection electrode, electron gun, etc. are omitted. Reference numeral 8 denotes an electromagnetic focusing coil for focusing the electron beam, and coil terminals 9 and 10 for flowing current are provided. The focusing coil 8 has power supply voltage fluctuations,
Connect a constant current circuit that does not fluctuate when the resistance value changes due to changes in coil temperature. The control circuit for this purpose, as shown in FIG. A negative feedback control circuit is constructed by an error amplifier 18 which is connected and compares this detected voltage with a reference voltage source 17. The series control transistor 1
3 is a PNP type, an inverting transistor 19 is provided, its emitter is grounded via a resistor 20, and its base is connected to the output of the error amplifier 18.
Capacitor 14 is a smoothing capacitor.

In this conventional device, when the fluctuation rate of the power supply voltage Vin is large and the resistance component of the coil 15 increases with temperature, at the normal operating point, there is a voltage between the collector and emitter of the series control transistor 13. teeth,
It is necessary to share 30 to 40% or more of the power supply voltage. For this reason, there is a drawback that the utilization efficiency of the power consumed by the coil is extremely poor. Especially in today's video cameras that use small image pickup tubes, low power,
This has become an obstacle to miniaturization. That is, as long as the series control method is used, there is a drawback that the efficiency of power usage is very poor as described above, and the size of the transistor becomes large due to the heat generated by the control transistor 13, which limits miniaturization. As described above, the conventional device has a simple configuration, a small number of parts, and is easy to put into practical use, but there are limits to power saving and miniaturization.

Traditionally, in order to reduce power consumption, or in other words improve power usage efficiency, series control circuits have been replaced with switching regulators, but although the losses of series control transistors are reduced,
On the other hand, since it requires an oscillator to form a comparator for pulse width control, a sawtooth wave or triangular wave generation circuit for comparison, and a protection circuit, it has a drawback that it is against miniaturization.

SUMMARY OF THE INVENTION Therefore, the present invention provides an electromagnetic focusing coil current stabilizing device that has a high power utilization rate and has almost no increase in the number of parts.

The basic configuration of the present invention is to control the pulse width duty by using a sawtooth wave voltage for electrostatic deflection, which is shared with the comparison waveform of the chopper circuit for controlling the focusing coil, and to focus the beam burn when the deflection is stopped. It automatically protects you by dissipating the situation.

Since the sawtooth wave for deflection has bipolar slopes, by using either one, the polarity of the error amplifier output can be selected, and therefore it is possible to form a duty control circuit with negative feedback control. It's summery.

An embodiment of the invention is shown in FIG. Figures 1 and 2
Components that have the same function as the circuit shown in the figure are given the same symbols. The emitter of a switching transistor 21 is connected to the power input terminal 12, and its collector is grounded by a damper diode 22. connected to one end of the Focusing coil winding 15
A detection resistor 16 is connected in series to the other end, and the detected voltage is compared with a reference voltage source 17, amplified by an error amplifier 18, and applied to the base of a transistor 27 forming one input terminal of the comparator. Ru. The base of the transistor 26, which forms the other input terminal of the comparator, is connected to a sawtooth wave generator 2 which generates a waveform for the electrostatic deflection voltage as a comparison waveform.
The signal is obtained from output terminal 4 of . A constant current source 28 is connected to the emitters of transistors 26 and 27 to form a comparator. 28
can be realized by replacing it with a resistor. The output of the comparator is applied to the base of the switching transistor 21 via the current limiting resistor 25 to provide a negative feedback effect.

FIG. 4 shows an explanatory diagram of the chopper control operation in the embodiment of FIG. 2. The waveform of FIG. 4a shows the comparison waveform applied to the base of transistor 26, which is the input terminal of the comparator. The switch level of the comparator is controlled by the DC level of the error amplifier 18 applied to the base of the transistor 27, so if the switching transistor 21 of the chopper is turned on at the normal operating point of time axis _t1 , the switch level of the chopper is switching transistor 21
The output waveform of the collector becomes a square wave with an on period of _T1 as shown in FIG. 4b. Now, if the voltage of Vin increases, the coil current tends to increase, so the output DC level is controlled by the negative feedback error amplifier 18 and moves in the downward direction as shown in Figure 4a. , the comparator switch time changes to _t2 . Therefore, the collector output of the switching transistor 21 becomes a square wave with an on period _T2 as shown in FIG. 3c, and changes depending on the input voltage level of Vin. It can be controlled in the same way when Vin drops.

Since the chopper control frequency matches the horizontal frequency, in the case of an NTSC video camera,
It becomes a high frequency of 15.734KHz, and Chiyoke coil 23
is also possible with small inductance.

Next, the protection function when the image pickup tube deflection is stopped will be explained. If the deflection action of the image pickup tube stops, the beam will concentrate on one point on the target film, so some kind of protection measure is necessary, but in the circuit shown in Figure 3, the horizontal drive pulse no longer comes to input terminal 1. In this case, the output of sawtooth generator 2 is DC
Since the waveform becomes a waveform, the square wave action of the comparator is automatically stopped. Since the chopper-controlled transistor is locked in either fully on or completely off mode, the current in the focusing coil is far from the optimal focusing state, causing the beam to diverge and potentially cause burn-in. sex becomes very weak. It is possible to select the polarity of the comparator so that the switching transistor 21 is in the off mode in order to save power during an abnormality.

The above explanation was based on the case of the electrostatic deflection voltage method, but even in the case of an electromagnetic deflection circuit, when a horizontal sawtooth wave current is passed through the deflection coil, the sawtooth wave voltage is
Needless to say, in the case of a method in which the current is generated at any time and then applied to the current amplifier, it can be implemented in the same manner.

As described above, according to the present invention, it is possible to stably supply a constant current to a focusing coil using a chopper control method with high power utilization efficiency without significantly increasing the number of parts, and to prevent horizontal drive pulse accidents. This has the effect of protecting the image pickup tube from beam burnout against accidents of the sawtooth wave generator that creates the deflection waveform.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a part of a conventional focusing coil current stabilizing device, Fig. 2 is a circuit diagram of other parts of the same device, and Fig. 3 is a focusing coil current stabilizing device in an embodiment of the present invention. FIG. 4 is a waveform diagram for explaining the device. DESCRIPTION OF SYMBOLS 1... Input terminal of horizontal drive pulse, 2... Sawtooth wave generator, 6, 7... Electrostatic deflection electrode, 8... Focusing coil, 15... Winding of focusing coil, 16...
Focusing coil current detection resistor, 18... error amplifier,
21...Switching transistor, 26, 27
...Comparator input transistor.

Claims (1)

[Claims] 1 A sawtooth wave generator that generates a bipolar sawtooth wave voltage synchronized with a horizontal drive pulse, a chopper output circuit that supplies power to a series circuit load of a focusing coil and a current detection resistor, and a chopper output circuit that pulses the chopper output circuit. It includes a comparator that controls the width, and an error amplifier that compares the detection voltage of the detection resistor with a reference voltage and controls one input terminal of the comparator. A focusing coil current stabilizing device, characterized in that the current is applied to the other input terminal of the comparator as a comparison waveform.