KR100648385B1 - Video display protection circuit - Google Patents

Abstract

A grid kick circuit that applies beam blocking bias to grid G1 of kinescope 10 to provide spot burn protection has an output coupled to first electrode 1 of grid kick capacitor CK. Second, common, having a first, common emitter switching circuit Q1 having C1, and an output C2 coupled to the second electrode 2 of the capacitor CK, also coupled to the grid G1. A base switching circuit Q2. When no beam blocking is desired, the first switching circuit is turned off (non-conducting) to supply charging current to the capacitor through the first current path R1 from the first voltage source (+200 volts), and at the same time, The common base amplifier Q2 adjusts to a fixed value determined by the second electrode of the capacitor and the second voltage source (+12 volts) in the grid G1. During this time, the operating current for the common base amplifier is provided by the second current paths R2, R5 coupled to the first voltage source (+200 volts) and independent of the first path R1. When beam blocking is desired, the common emitter amplifier Q1 is turned on, clamping the first electrode 1 of the capacitor CK to ground, and the common base amplifier Q2 is connected to the capacitor CK from the second voltage source. Since the second electrode of) is disconnected, the blocking voltage appearing across the capacitor is applied to the grid G1 of the kinescope. During this time, the operating current of the common base amplifier Q2 is switched by the third, threshold conduction switching circuit D2 coupled between the common emitter switching circuit Q1 and the second current paths R2, R5. . Advantageously, independent first and second current paths provide improved capacitor charging efficiency, active pull down of the capacitor voltage provides an increased breaking rate, and the grid bias voltage operates normally when no interruption is desired. Is adjusted.

Kinescope

Description

Video display protection circuit             

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1 is a circuit diagram showing, in partial block form, a television device having a grid bias control circuit according to the prior art;

2 is a circuit diagram in partial block form a television device having a grid bias control circuit according to the present invention;

* Brief description of symbols for the main parts of the drawings *

6.Video processing and deflection unit

7.Multiple input terminals

8 ... Kinescope Drive Amplifier

10 ... Kinescope

This application is based on U.S. Provisional Application 093/700, filed July 22, 1998.

FIELD OF THE INVENTION The present invention relates to display systems such as television receivers, computers and other monitors, in particular beam cut bias in the control grid of a kinescope, in particular when phosphor burn protection of the kinescope is desired. By applying a -off bias, it relates to a protection circuit for phosphor burn protection of kinescopes used in systems such as displays.

Conventional direct-view and projection display systems use kinescopes as display systems. During normal operation of the kinescope, the scanning circuit deflects the electron beam to produce a relatively large area raster on the kinescope's face plate, and the video modulation of the beam activates the phosphors deposited on the face plate. Generates a visible picture. During normal operation, the beam energy is distributed through the entire area of the kinescope face plate. If scan loss occurs, the beam energy may be concentrated in a relatively small area of the face plate, and a high concentration of beam energy may create permanent damage called so-called "spot burns" on the phosphor. Can be. Scanning loss can occur when the display device is turned on (ie, when the receiver repeats at high speed between on and off modes) under a "hot start" state, or, for example, component failure Scanning loss can occur when (component failure) occurs.

In addition, the occurrence of the scan loss state is related when the kinescope is turned on. When power to the kinescope is removed, the cathode continues to emit electrons until it cools sufficiently. As a result, the deflection voltages continue to emit an attenuating electron beam for a limited time period after the deflection voltages are interrupted. In order to prevent spot burn on the face plate during this period, it is desirable to maintain a sufficient bias voltage between the cathode and the control grid to prevent the attenuating electron beam from illuminating the face plate without deflection voltage.

During normal operation, a voltage difference is maintained between the cathode and the control grid. On the display screen a large voltage difference results in low illumination levels and a small voltage difference results in high illumination levels. For example, a typical cathode voltage may be about 180 volts, a typical grid voltage may range from about 10 to 12 volts, and the cathode voltage is modulated to reduce the voltage difference to change the brightness level. When the display is turned off, the cathode and grid voltage differences go to zero, and if there is no deflection voltage, the zero cathode bias causes the electron beam to focus on a very small area of the display screen.

During the turn-off of the kinescope, the method for preventing spot burn is called the "grid kick" method. In such a method, the charge storage device is coupled to the control grid of the cathode ray tube and also to the voltage source via the switch device. Such a charge storage device is charged by a voltage supply through the switch device when the control signal coupled to the control input of the switch device indicates the presence of a deflection signal. The switch device isolates the supply voltage from the charge storage device when the control signal indicates the absence of the deflection signal and provides a negative cutoff voltage to the control grid. In this way, a sufficient voltage difference is maintained between the cathode and the control grid when the cathode voltage decreases, and thus the display screen remains blank. Such a method is described, for example, in US Pat. No. 5,089,754, entitled "Protection circuit for a Cathod Ray Tube," issued February 18, 1992 to John B. George. Another example of the form of a kinescope protection circuit, “grid kick”, is described in US Pat. No. 5,043,639, et al., Gurley, entitled “Video Display Apparatus With Kinescope Spot Burn Protection Circuit”, issued August 27, 1991. The circuit of Gurley et al. Is similar to George's, but uses a switching device and passive charging source coupled between ground and the plate of the capacitor.

Although the aforementioned grid kick circuits use only a single switching device, for example, a device using multiple switching devices such as the grid kick circuit used in the model name CTC-195 Color Television Receiver manufactured by Thomson Consumer Electronics, Inc. Compared to these, relatively complex additional bias circuits use grid kick circuits. 1 of the present specification is a schematic diagram of a grid kick circuit in a television receiver.

Advantageously, the use of a pair of switching devices in the grid kick circuit of the CTC-195 receiver provides an active pull-down of the grid kick capacitor which rapidly reduces the grid voltage, and in addition, During normal operation, a actively regulated DC bias voltage is provided to the grid.

Beneficial features of the CTC-195 grid kick circuit for active pull-down of the kick capacitor when beam interruption is required and active adjustment of grid bias when beam interruption is not required can be obtained, requiring no improvement. I believe it is not.

However, in accordance with one aspect of the present invention, it can be seen that for certain grid kick circuit applications, it is desirable to improve the performance of the grid kick circuit. In particular, in many applications, it can be seen when the supply voltage to the grid kick circuit is relatively low and when there is a need to improve the charging efficiency of the grid kick circuit. As described in detail later with respect to the circuit of FIG. 1 of the prior art, the charging efficiency of the prior art circuit is only about 60% indicated by the voltage stored in the capacitor as a percentage of the value of the high voltage supply. This charging efficiency is perfectly sufficient when a supply voltage of around 250 volts is used, but can result in the lowest performance in applications where a high voltage supply is significantly less than 250 volts.

 Therefore, it would be desirable to provide a grid kick circuit with improved charging efficiency for a kick capacitor.

In accordance with an embodiment of the present invention, an apparatus for applying a beam blocking bias to a grid of kinescopes for spot burn protection includes a capacitor having first and second electrodes and a source for providing a beam blocking control signal. When no beam blocking is desired, a charging current is applied from the first voltage source to the first electrode of the capacitor in response to the first state of the control signal, and when the beam blocking is desired, the capacitor of the capacitor in response to the second state of the control signal is desired. A first switching circuit is provided for coupling one electrode to a source of reference potential. A second switching circuit is provided having an output coupled to a second electrode of the capacitor and a grid of kinescopes, the second switching circuit coupling a second voltage source to the output in response to being enabled and being disabled In response, isolate the output from the second voltage source. Circuit means are provided for enabling the second switching circuit from a source of current independent of the charging current if no beam blocking is desired and disabling the second switching circuit if a beam blocking is desired.

In the illustrated application according to the principles of the present invention, the circuit means are provided with a linear circuit path for supplying an operating current directly from the first voltage source to the input of the second switching circuit when no beam interruption is desired, and when beam interruption is desired. And a non-linear circuit path for diverting the operating current from the linear circuit path to the first switching circuit.

The above and other features of the present invention are illustrated in the accompanying drawings, wherein like elements are denoted by like reference numerals.

The display system 5 of FIG. 1 has a plurality of input terminals 7 for receiving the displayed input signals and an output coupled to the cathode K of the kinescope 10 via a kinescope drive amplifier 8. And a video processing and deflection unit 6 in a conventional design. The video processing and deflection unit 6 also supplies deflection signals to the yoke Y of the kinescope 10. Power for the video processing and deflection unit 6 and the drive amplifier 8 is provided by the power supply 12. Control to the power source 12 may also provide control signals to the video processing and deflection unit 6, for example, to control functions such as channel selection, signal source selection, etc., in a television receiver. Provided by The power supply 12 also supplies +250 volts to the first supply terminal 16 of the grid bias control circuit 20, which is in the form of a "grid kick" of the beam blocking circuit. The bias control circuit 20 has a second supply input terminal 18 coupled to receive a grid bias voltage supply of +23 volts from the power supply 12. Scan loss detector 30 is coupled to the output of video processing and deflection unit 6 to provide an output signal indicative of the occurrence of scan loss to input 22 of grid bias control circuit 20. The scan loss detector 30 can be of conventional design to detect the presence or absence of horizontal pulses, for example, as described in the above-mentioned George patent. Another suitable scan loss detector may be a detector for monitoring the supply voltage in the deflection processing section of the video processing and deflection unit 6. For illustration purposes, when a beam block is desired (ie, during a scan failure state), the detector 30 provides a current to the terminal 22, and when no beam block is desired (ie, during a normal scan state), the detector is Assume that no current is provided to terminal 22. In order to supply a regulated amount of grid voltage of about +23 volts during normal operation of the television receiver, and to supply a grid breaking voltage if beam blanking is desired (ie, during a scan loss state), the kinescope 10 The output 24 of the grid kick circuit 20 is coupled to the control grid G1 of.

In the description and examples, typical device circuit values are included in the schematic diagram of the grid bias control circuit 20. The grid bias control circuit 20 has a common emitter amplifier having a base electrode B1 coupled to the input 22 and a collector electrode C1 coupled to the +250 volt supply terminal 16 via a resistor R1. (Q1) is provided. In addition, the collector C1 is coupled to the emitter E2 of the common base amplifier Q2 and also to the first electrode of the grid kick capacitor CK. The second electrode of capacitor CK is coupled to collector C2 of common base amplifier Q2 and, via resistor R3, to grid kick output terminal 24. The base bias of the common base amplifier Q2 is a diode D1 which conducts the base current flowing from the base electrode B2 to the +23 volt supply terminal 18 and a series resistor R4 coupled in series with the diode D1. Is supplied by Diode D1 protects against Vber breakdown (reverse base-emitter voltage breakdown).

In operation, when the scan is normal and no grid blocking bias is desired, the detector 30 applies a zero turn-on current to the base B1 of the common emitter connected to the amplifier transistor Q1. And thereby, the amplifier transistor Q1 is turned off, a charging current flows from the +250 supply terminal 16 to the first electrode of the grid kick capacitor CK through the resistor R1, and furthermore, Current flows through the +23 volt supply terminal 16 through the collector-base junction of transistor Q2 to charge the capacitor. At the same time, a portion of the current flowing through the resistor R1 saturates the transistor amplifier Q2 at the input (emitter electrode E2) of the transistor Q2 through the resistor R2. This saturated state of the amplifier transistor Q2 clamps the second electrode (and grid G1) of the capacitor CK to a voltage of about 23 volts (small saturation and ignore diode D2 voltage drop). do. Therefore, transistor Q2 adjusts grid G1 to 23 volts while scanning is normally performed.

When a scan loss occurs, the detector 30 is grounded to cause the second electrode (and grid G1) of the capacitor CK to a negative potential proportional to the voltage that charged the capacitor CK during normal operation. The first electrode of the low grid kick capacitor CK is clamped to transmit a turn-on current to the base B1 of the transistor Q1. At the same time, the connection of emitter E2 of transistor Q2 to collector C1 of transistor Q1 turns off transistor Q2 by removing all emitter currents from transistor Q2. In this way, transistor Q2 is controlled to be turned on when transistor Q1 is turned off or turned on.

As described so far, the prior art grid bias control circuit 20 provides the benefits of active pull down of the capacitor (CK) voltage for the blocking of high speed beams, if desired. It can be seen that it provides adjustment of the control grid G1 voltage if not desired.

From the description, during normal operation, i.e. when no beam blocking is desired, part of the current flowing through the resistor R1 proceeds to charge the grid kick capacitor CK and flows through the resistor R1. Another portion of the current proceeds to emitter E2 of common base amplifier transistor Q2 to regulate the grid voltage to +23 volts to keep common base amplifier transistor Q2 saturated. Thus, because part of the current in resistor R1 proceeds to control transistor Q2, capacitor CK is charged to a potential lower than the +250 volt supply at terminal 16.

In quantitative terms, the charging efficiency for the illustrated device values and voltage can be calculated as follows. During normal operation, as transistor Q2 becomes saturated, resistors R1, R2, and R3 form a potential divider. Thus, the actual voltage stored in grid kick capacitor CK is equal to the difference in supply voltages of terminals 16 and 18 (250-23) multiplied by the potential divider ratio equal to 430K / (220K + 430K + 3K). . The diode voltage drop and the saturation voltage of transistor Q2 are ignored. This results in a net stored voltage on capacitor CK of about 149.5 volts at 250 volts, resulting in a charging efficiency of about 60% of the available supply voltage.

As already mentioned, the present invention stems in part from the discovery of low charging efficiency of prior art circuits. Since a +250 volt power supply is available, no particular result is obtained when applying the prior art circuit described now. However, for grid kick applications where the voltage available is rather low (i.e., +200 volts), the performance of prior art circuits may not be sufficient to completely block the kinescope beam due to the efficiency issues described above. In such applications, where improved charging efficiency is desired, the circuit 20 of FIG. 1 can be modified as shown in FIG. 2 in accordance with the present invention.

A major change in the grid bias control circuit of FIG. 2 is the addition of a network 200 for controlling the common base amplifier transistor Q2. In addition, the supply voltages were reduced from +250 volts and +23 volts to +200 volts and +12 volts, respectively. Other changes are described later. In the network 200, the collector C1 of the transistor Q1 is directly connected to the high voltage supply terminal 16 through the added resistor 5, while the resistor 2 is directly connected thereto. (D2) is added, the anode of the diode D2 is connected to the common connection 202 of the resistors R2 and R5, and the cathode is connected to the collector C1 of the transistor Q1. In another variation, the Vber protection diode D1 is connected across the base-emitter junction of transistor Q2 instead of being coupled in series with resistor R4. For typical device values for the modified grid bias control circuit, resistors R1 and R5 are 330 KΩ, resistor R2 is 100 KΩ and resistors R3 and R4 are 220 Ω. In addition, the value of the capacitor CK was increased to 10 ms.

In operation, the network 200 biases transistors enabled (i.e., transistors Q2) in response to a current supplied from a high voltage source 16 through a circuit path in which resistors R2 and R5 are coupled in series. by controlling the operation of the common base amplifier transistor Q2, the enabling current is completely independent of the charging current flowing through the circuit path of the resistor R1. Therefore, all the charging current flowing through the resistor R1 is effective for charging the grid kick capacitor CK, whereby the charging efficiency is very high. In particular, ignoring the diode voltage drop, the capacitor is charged to the same voltage as the difference of the supply voltage of 188 volts. Thus, the efficiency is 188/200 or 94%, which is considerably higher than the charging efficiency of about 60% previously described.

However, even with such an improvement in the charging efficiency, problems related to the control of the transistor Q2 still remain. This problem is caused by the addition of a diode D2 which directs all emitter currents of the amplifier Q2 to ground when the transistor Q1 is turned on and the transistor Q2 is turned off during the grid kick state. Is solved. Thus, capacitor CK is charged to a relatively high voltage through a circuit path comprising resistor R1, while transistor Q2 does not want the control signal provided by scan loss detector 30 to block the beam. Is turned on by an independent current flowing through a circuit path comprising resistors R2 and R5, and transistor Q1 is turned off. When the control signal indicates the case where the beam blocking is desired, transistor Q1 is turned on, and diode D2 is turned on in turn, and the emitter of transistor Q2 through resistor R2 and diode D2. Switch current flow from E2. As a result, transistor Q2 is turned off and, during the charging operation, a relatively high negative voltage generated at the second electrode of capacitor CK is applied to grid G1. It should be noted that resistor R2 has an important function in the overall operation of network 200. In particular, resistor 2 prevents short circuiting of the low voltage supply (+12 volts) when transistor Q1 is turned on. Without resistor R2, only a relatively low base resistor R4 of 220 kV is used to limit the current flowing through diodes D1 and D2 and transistor Q2 when transistor Q2 is turned on. do.

Although the invention has been described by way of exemplary embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit of the invention. It will be appreciated that the present invention can be used in direct and projection display television receivers and computers, and in any video display device that transmits energy to the display screen to provide illumination to the display screen, not just to other monitors. . Although the apparatus shown in FIGS. 1 and 2 shows kinescopes having a single cathode as can be used in a projection display system, the present invention is directed to a plurality of (ie three) cathodes associated with respective color channels. It is also apparent that the present invention can be applied to a device having a kinescope having a. Therefore, the present invention is intended to cover various modifications within the spirit and scope of the invention.

Independent first and second current paths provide improved capacitor charging efficiency, active pull down of the capacitor voltage provides increased beam cutoff rate, and the grid bias voltage is adjusted during normal operation if no interruption is desired. .

Claims (5)

Apparatus for applying a beam blocking bias to a grid of kinescopes: A capacitor (CK) having first and second electrodes (1, 2); A source 30 providing a beam block control signal; In response to the first state of the control signal, when no beam blocking is desired, a first charging current is applied from the first voltage source (+ 200V) to the first electrode of the capacitor, and the second of the control signal In response to a condition, a first switching circuit (Q1) for coupling the first electrode of the capacitor to a source of reference potential, when a beam block is desired; And A second voltage source (+ 12V) coupled to the output, if enabled, having an output C2 coupled to the second electrode of the capacitor and the grid G1 of the kinescope 10; A second switching circuit Q2 for separating the output from the second voltage source in the case of an enable, A third current flowing in the second switching circuit to enable the second switching circuit independent of the first charging current when the beam blocking is not desired, and a third switching circuit for disabling the second switching circuit if the beam blocking is desired. And a switching circuit (D2). The method of claim 1, The first charging current is supplied through a first circuit path R1 coupled between the first switching circuit and the first voltage source, The second current is supplied through second circuit paths R2 and R5 coupled between the first voltage source and the second switching circuit, The third switching circuit is coupled between the first switching circuit and the second circuit path to switch the second current from the second circuit path to the first switching circuit if a beam block is desired. A beam interruption bias application device. The device of claim 1, wherein the third switching circuit comprises a threshold conduction device (D2). 3. The second switching circuit of claim 2, wherein the second switching circuit comprises: a base electrode B2 coupled to the second voltage source, a collector electrode C2 coupled to the output, and an emitter electrode coupled to the second circuit path. And a common base amplifier having E2). 5. The first and second resistors R2 of claim 4, wherein the second circuit path is coupled in series between the source of the first voltage source and the emitter electrode E2 of the common base amplifier Q2. , R5), And the third switching circuit (D2) is coupled between the point between the first and second resistors and the first switching circuit.

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