JPH0582788B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a beam current detection circuit that detects a beam current by detecting the cathode current of a cathode ray tube.

Background Art and Problems ABL circuits (automatic brightness limiting circuits) have been proposed to protect cathode ray tubes due to heat generation, prevent X-ray emission due to excessive beam current, and prevent overloading of high voltage generation circuits. That is, the beam current is detected by detecting the high voltage current, and the beam current of the cathode ray tube is limited by negatively feeding back this detection signal to the brightness adjustment circuit.

Such an ABL circuit is similarly provided, for example, in a three-tube projector having cathode ray tubes for red, green, and blue. In this case, conventionally, as shown in Fig. 1, a detection circuit 2 detects the total high voltage current flowing through the red, green and blue cathode ray tubes 1R, 1G and 1B, and controls based on this. . In FIG. 1, HV is a high voltage, and 3R, 3G, and 3B are drive circuits, respectively.

However, in this case, it is difficult to maximize the luminous ability of the cathode ray tube. For example, consider a case where only 1 mA of beam current can be passed through each of the cathode ray tubes 1R, 1G, and 1B. In this case, if the brightness level, that is, the beam current is limited by a detection current of 1 mA, the beam current flowing through each cathode ray tube 1R, 1G, and 1B for white, red, green, and blue screens will be as shown in Figure 2A. Since the exposure level is limited to the level shown in , problems such as damage to the cathode ray tube will not occur. However, in the case of a white screen, each cathode ray tube 1
The beam currents flowing through R, 1G, and 1B are significantly less than 1 mA, and the light emitting capabilities of the cathode ray tubes 1R, 1G, and 1B are not fully exhibited. On the other hand, if the brightness level, that is, the beam current, is limited by a detection current of 2.2 mA, for example, white, red,
For green and blue screens, each cathode ray tube 1R, 1
Since the beam current flowing to G and 1B is limited to the level shown in Figure 2B, the brightness in the case of a white screen is 2.2 times that of 1 mA, and the luminance ability can be fully demonstrated. - In green and blue screens, a beam current of up to 2.2 mA flows through each of the cathode ray tubes 1R, 1G and 1B, causing problems such as damage to the cathode ray tubes.

Therefore, in order to ensure the safety of the cathode ray tubes 1R, 1G, and 1B while maximizing their luminous ability, the beam current is detected by detecting the cathode current of each tube, and the beam current is determined based on the detected current. It is conceivable to limit the brightness level.

FIG. 3 shows the principle diagram. In the same figure, 4 is a terminal to which the color video signal S _V is supplied, and the video signal S _V is then supplied to the signal processing circuit 5, from which the red, green, and blue primary color signals R, G, and B are output. is obtained. The color signals R, G, and B are supplied to the cathodes of cathode ray tubes 1R, 1G, and 1B via drive circuits 3R, 3G, and 3B, respectively. Each cathode ray tube 1R, 1G
And a flyback transformer 6 is installed on the anode of 1B.
Higher voltage HV is supplied. In addition, cathode ray tube 1
Current detection circuits 7R, 7G and 7B are connected to the cathodes of R, 1G and 1B, respectively. The respective cathode current detection signals S _R , S _G and S _B obtained from these detection circuits 7R, 7G and 7B are as follows:
They are supplied to one input of comparator 9 via diodes 8R, 8G and 8B, respectively. That is, the detection signal
The largest of S _R , S _G and S _B is supplied. The other input of this comparator 9 is supplied with a set level V _REF . Then, the comparison error signal S _C is supplied to the signal processing circuit 5, and based on this signal S _C , the brightness level, that is, the level of each color signal R, G, and B, is limited, and the beam current is limited. In this example, consider the case where only 1 mA of beam current can be passed through each of the cathode ray tubes 1R, 1G, and 1B, as described above. In this case, if the brightness level, that is, the beam current is limited by a detection current of 1 mA, the current flowing through each cathode ray tube 1R, 1G, and 1B for white, red, green, and blue screens will be as shown in Figure 4. limited by level. Therefore, there is no inconvenience such as destruction of the cathode ray tube, and for example, when a white screen is displayed, the cathode ray tubes 1R, 1R,
The luminance of 1G and 1B is 2.2 times higher. In this way, by doing as shown in the example in Fig. 3, while ensuring the safety of the cathode ray tubes 1R, 1G, and 1B,
Its luminous ability can be maximized.

However, in systems that detect the beam current by detecting the cathode current, parasitic capacitance (e.g. 8pF
~15pF), the charging current that charges this parasitic capacitance is included in the cathode current, and the detection current becomes greater than the beam current, resulting in the inconvenience that the beam current cannot be detected correctly and the ABL operation cannot be performed correctly. be. That is, in FIG. 5, when 10 is a parasitic capacitance, this parasitic capacitance 1
0 is charged with a current Ic, and the cathode detection current is the sum of this current Ic and the beam current _Ib . In FIG. 5, 1 is a cathode ray tube, 3 is a drive circuit, and 7 is a current detection circuit.

Purpose of the Invention In view of the above, the present invention has been developed to
This allows the beam current to be detected accurately.

Summary of the Invention The present invention provides a beam current detection circuit in which a drive circuit 3 is connected to the cathode of a cathode ray tube 1 and the beam current flowing through the cathode is detected.
A current supply circuit 3a is constructed by dividing the drive circuit 3 into two so that when a high frequency signal Sv is supplied to the drive circuit 3, a charging current Ic is applied to the parasitic capacitance 10 between the cathode of the cathode ray tube 1 and the ground. The beam current Ib flowing to the cathode of the cathode ray tube 1 when the high frequency signal of the circuit 3 is supplied and the parasitic capacitance 10
The drive circuit 3 subtracts the charging current Ic supplied via the current supply circuit 3a from the current Ib + Ic, which is the sum of the discharge current Ic charged by the charging current.
This beam current detection circuit is characterized in that it includes a current drawing circuit 3b configured by dividing the current drawing circuit 3b into two, and uses the detected current from the current drawing circuit 3b as the beam current Ib.

Therefore, the current drawn into the current drawing circuit is the beam current and the charging current to the parasitic capacitance, the current supplied from the current supply circuit is the charging current to the parasitic capacitance, and the difference current is the beam current. Therefore, the beam current can be detected accurately.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to FIG. 6 and subsequent figures.

FIG. 6A illustrates the principle of the invention. That is, the cathode drive circuit of the cathode ray tube 1 is divided into two systems, one of which is a current supply circuit 3a that only supplies current, and the other a current draw circuit 3b that only draws current. When a high frequency signal is input as a signal, charging and discharging occur in the parasitic capacitance 10. In such a configuration, the charging current Ic to the parasitic capacitance 10 is supplied from the current supply circuit 3a as shown in FIG. 6B. In addition, the current drawing circuit 3b
As shown in FIG. 6C, the sum of the beam current Ib and the current Ic from the parasitic capacitance 10 (same as the charging current) is drawn. Therefore, in the present invention, the current drawn into the current drawing circuit 3b (Ib
+Ic), the current supplied from the current supply circuit 3a
Subtract Ic and detect beam current Ib.

FIG. 7 shows an embodiment of the present invention,
Portions corresponding to those in FIG. 6A are designated by the same reference numerals.

In the figure, 11 is an npn type transistor forming the current supply circuit 3a, 12 is a pnp type transistor forming the current drawing circuit 3b,
Transistors 11 and 12 are complementary connected. A signal is commonly supplied to the bases of the transistors 11 and 12, and the connection point of their emitters is connected to the cathode of the cathode ray tube 1.

In addition, the collector of the transistor 11 is the collector/emitter of the pnp transistor 13, and the resistor 1
4 to a power supply terminal 15 to which a positive DC voltage +B is supplied. Further, the power supply terminal 15 is connected to the collector of an NPN transistor 19 via a resistor 16, the emitter/collector of a PNP transistor 17, and the emitter/collector of a PNP transistor 18. In addition, transistors 13 and 1
The bases of transistors 7 and 7 are connected to each other, and the base of transistor 18 is connected to the collector of transistor 13. In this case, transistors 13, 17 and 18
A current mirror circuit is constructed, and equal current flows through the transistor 13 and the transistors 17 and 18.

In addition, the collector of the transistor 12 is the collector/emitter of the npn transistor 20, and the resistor 2
1 to ground. In addition, the transistor 19
The emitter of is grounded via a resistor 22. Further, the base of the transistor 19 is connected to its collector to form a diode connection. Furthermore, the bases of transistors 19 and 20 are connected to each other. In this case, the transistors 19 and 20 constitute a current mirror circuit, and the same current flows through the transistors 19 and 20.

Further, the collector of the transistor 12 is grounded via a resistor 23 for beam current detection.

This example is configured as described above and operates as follows.

That is, the current Ic that charges the parasitic capacitance 10 flows from the transistor 11, and the current Ic flows from the transistor 13. Here, the transistor 13,
Since 17 and 18 constitute a current mirror circuit, a current Ic flows through the transistors 13, 17, and 18, and this current Ic flows into the transistor 19. Also here, transistors 19 and 2
0 constitutes a current mirror circuit, so
A current Ic is caused to flow through the transistor 20. On the other hand, the transistor 12 has a beam current Ib
A current (Ib+Ic), which is the sum of the current Ic from the parasitic capacitance 10, flows into. And this current (Ib+
Of the current Ic), current Ic flows through the transistor 20. Therefore, only the beam current Ib flows through the resistor 23. Therefore, by detecting the voltage across the resistor 23, only the beam current Ib is detected.

In this example, the beam current Ib can be detected accurately without being affected by the parasitic capacitance 10.

Effects of the Invention As is clear from the embodiments described above, according to the present invention, the current supplied from the current supply circuit (parasitic Since this method subtracts the current (charging current to the capacitor) and detects the difference current (beam current) as the beam current, the beam current can be accurately detected without the influence of parasitic capacitance. Therefore, by using the beam current detection circuit according to the present invention, for example, an ABL circuit can be operated correctly. Incidentally, since the parasitic capacitance is not considered as a fixed value, it is possible to cope with variations in capacitance due to differences in wire materials and component placement without any problems.

[Brief explanation of drawings]

FIGS. 1 to 5 are diagrams for explaining conventional beam current detection circuits, FIG. 6 is a configuration diagram showing the principle of the present invention, and FIG. 7 is a configuration diagram showing an embodiment of the present invention. be. 1 is a cathode ray tube, 3a is a current supply circuit, 3b is a current draw circuit, 10 is a parasitic capacitance, and 23 is a detection resistor.

Claims (1)

[Scope of Claims] 1. In a beam current detection circuit configured to connect a drive circuit to the cathode of a cathode ray tube and detect the beam current flowing through the cathode, parasitics between the cathode of the cathode ray tube and the ground The drive circuit is connected to the capacitor in such a way that charging current flows when a high frequency signal is supplied to the drive circuit.
a beam current flowing to the cathode of the cathode ray tube when the high frequency signal of the drive circuit is supplied;
A current drawing circuit configured by dividing the drive circuit into two so as to subtract the charging current supplied via the current supply circuit from the current obtained by adding the discharge current charged by the charging current to the parasitic capacitance. A beam current detection circuit comprising: The detection current from the current drawing circuit is used as a beam current. 2. Beam current detection according to claim 1, wherein the current supply circuit and current draw circuit configured by dividing the drive circuit into two are configured of a complementary circuit and first and second current mirror circuits. circuit.