Video display protection circuit

This application claims the benefit under 35 U.S.C. §365 of International Application PCT/US99/22660, filed Sep. 29, 1999, which was published in accordance with PCT Article 21(2) on Apr. 6, 2000 in English, and which claims the benefit of U.S. Provisional Application Serial No. 60/102,213, filed Sep. 29, 1998.

The present invention relates to a video display protection apparatus and method, and more particularly, to a video display protection circuit for protecting a cathode ray tube from a high beam current condition.

In a video display device comprising a cathode ray tube (CRT), beam current is generated in response to a video signal and accelerated by an accelerating potential to cause the beam to strike a phosphor coated screen. In such a display device, several different voltages are required to generate the beam current, focus the beam and accelerate the beam toward the screen. These voltages include a filament excitation voltage, a modulated cathode voltage, a grid 1 voltage, a screen grid 2 voltage, focus grid voltages, and a final anode voltage.

The filament voltage (nominally 6.3V) excites the filaments to generate free electrons to supply the electron beam in the CRT. A cathode voltage is generated on each of the R, G, and B cathodes and controls the amount of beam current developed in the CRT. The focus grid voltages generate an electrical field that functions in a lens-like manner to focus the beam in the midst of its trajectory. The final anode voltage, nominally 30-32 KV, accelerates the beam and electron beam currents will not flow from the cathode to the anode in the absence of the final anode voltage.

The beam current is a function of the cathode voltage, grid 1 voltage and a cutoff voltage. More specifically, the beam current corresponds to Vc-(Vk-Vg 1 ), wherein Vc is the cutoff voltage, Vk is the cathode voltage and Vg 1 is the grid 1 voltage. The cutoff voltage is set based on the CRT characteristics and in view of the screen grid voltage. Since the beam current is generally controlled by modulating the cathode, instead of modulating the grid 1 voltage, and beam current is an exponential function of the control voltage, this discussion uses notation that is inverse in sign from physical theory. Thus, for purposes of the present discussion, an increase in screen grid voltage results in increased Vc.

If the grid 1 voltage is approximately zero, and Vc is set to be slightly less than the kinescope driver supply of about +215Vdc, the variation in the cathode voltage Vk controls the amount of beam current that flows in the CRT. For cathode voltage Vk close to Vc, beam current is negligible, while for small Vk, substantial beam current may result.

The cathode voltage is typically generated in response to a video signal by a kinescope driver circuit that includes a multi-stage amplification circuit. The multi-stage amplification circuit is usually coupled to at least a low voltage power supply and a high voltage power supply. The multi-stage amplification circuit may be embodied in the form of an integrated circuit, for example TDA6120Q, manufactured by Philips Semiconductors of Sunnyvale, Calif. It is advantageous to use ICs such as TDA6120Q in high performance video monitors because of their relatively high bandwidth and ability to handle relatively high voltages.

However, in some cases, particularly when the kinescope driver circuit is embodied in the form of an IC, such as the TDA6120Q, the loss of either the low voltage or high voltage power supply to a kinescope driver circuit may result in the loss of cathode voltage. The loss of cathode voltage results in a “zero-biasing” of the CRT, which results in an extremely high beam current. The high cathode current is especially problematic in video display monitors that already operate at relatively high beam currents. High beam currents are desirable to increase the brightness of the display and thereby improve the subjective quality of the picture. However, faults in the kinescope driver circuitry power supplies can easily lead to high beam currents that can permanently damage the phosphors on the display screen. In this connection, faults in the kinescope driver circuitry power supplies may occur due to a variety of different reasons including, but not limited to, break in the wire during manufacturing and/or shipment, and oxidation of the various electrical components.

Currently, safety circuits shutdown the anode voltage supply if the anode voltage levels exceed predetermined levels, in order to prevent excessive x-radiation levels. However, such safety circuits respond to the voltage level of the anode voltage supply, but do not respond to a CRT zero-bias condition that results from faults in the power supplies to the kinescope driver circuit.

Therefore, it is desirable to provide an apparatus and a method for protecting a video display device against a zero-bias condition due to the loss of any one of a plurality of power supplies, which condition results in excessive beam current in the video display device.

In particular, it is desirable to provide an apparatus and a method for protecting a video display monitor against a zero-bias condition due to the loss of any one of a plurality of power supplies associated with any one of a plurality of kinescope driver circuits.

It is also desirable to provide an apparatus and a method for protecting a video display monitor against a zero-bias condition due to the loss of power to a kinescope driver circuit, wherein the kinescope driver circuit is embodied in the form of an integrated circuit.

It is also desirable to provide an apparatus and a method for protecting a video display monitor against the loss of a cathode voltage, which apparatus and method is self-latching and does not reset until the fault is removed.

It is also desirable to provide a video display apparatus and method which disables a high voltage power supply when a fault is detected in any one of a plurality of driver circuit power supplies.

“In an exemplary embodiment, the present invention comprises a protection circuit that shuts down the anode power supply in response to the loss of a power supply to a kinescope driver circuit. In particular, the present invention is a protection circuit comprising: a source of video signals; a driver circuit coupled to the source of video signals and the cathode, the driver circuit coupled to a driver circuit power supply, the driver circuit causing a cathode voltage to be generated on the cathode in response to the video signals and the driver circuit power supply; a high voltage power supply coupled to the anode for providing high voltage accelerating potential in the display device; and a shutdown circuit coupled to the driver circuit power supply and the high voltage power supply, the shutdown circuit disabling the high voltage power supply if an output of the driver circuit power supply decreases below a threshold level.”

The invention will be described with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of a video display protection circuit in accordance with the present invention;

FIG. 2 is a schematic diagram including the summing node for the low voltage and high voltage cathode supplies in accordance with the present invention; and

FIG. 3 is a schematic diagram of a high voltage power supply and shutdown circuit in accordance with the present invention.

FIG. 1 illustrates a block diagram of video display apparatus which includes a protection circuit in accordance with the present invention. The apparatus comprises power supply board 22 coupled to Input/Output board 24 and kinescope driver board 20 via connector cables 31 , 32 , 35 and 45 . Power supply board 22 includes circuitry associated with +12V supply 25 , +215V supply 27 and High Voltage supply 26 . +12V supply 25 and +215V supply 27 are used to develop cathode voltage Vk as described further below. High voltage supply 26 is coupled to the anode and provides the accelerating potential.

CRT 100 provides a video display in response to drive signals from kinescope driver board 20 and high voltage from power supply board 22 . Each cathode of CRT 100 is driven by a respective output from kinescope driver board 20 , here represented by connector 34 , although a connector may be necessary for each cathode. High voltage supply 26 is coupled to CRT 100 via connector cable 33 .

I/O board 24 generally includes coupling devices that allow the user to connect various signal sources to the CRT. In the present embodiment, +12V supply 25 is connected to kinescope driver board 20 via I/O board 24 as a matter of convenience. However, it is clear that +12V power supply may be coupled to kinescope driver board 20 in any manner as desired.

Kinescope driver board 20 receives the video signals, and the low and high voltage power supplies, and develops a cathode voltage for each cathode associated with the video signal. +12V supply 25 is coupled to kinescope driver board 20 via connector 31 , I/O board 24 and connector 32 . +215V supply 27 is coupled to kinescope driver board 20 via connector 35 . Within kinescope driver board 20 , the various power supplies are coupled to each other at sensing node 30 , via respective resistors R 1 and R 2 . The voltage at sensing node 30 is coupled to power supply board 22 via diode D 1 and resistor R 4 . The sensing nodes associated with other cathode driver circuits are connected to node 43 via diodes D 2 and D 3 . As described further below, the present protection circuit is arranged so that if a fault is detected in either power supply 25 or power supply 27 , via summing node 30 and connector 45 , high voltage supply 26 is shutdown by shutdown circuit 44 . The fault may result from the failure of the respective power supplies or various connectors becoming disconnected.

FIG. 2 is a schematic diagram that shows portions of kinescope driver board 20 , protection delay circuit 47 and shutdown circuit 44 . Kinescope driver board 20 includes cathode driver ICs 40 , 41 and 42 for each of the red, green and blue video signals. The circuitry for each of the component colors is identical and only the components associated with the red signal will be described. A suitable driver IC includes, but is not limited to, TDA6120Q manufactured by Philips Semiconductors. Driver IC 40 is coupled to source of signals 36 for receiving the video signal used to control the output to cathode 102 . Driver IC 40 includes a multi-stage amplifier disposed therein, and is coupled to a +12V supply at pin 6 and a +215V supply at pin 10 . The output of driver IC 40 is provided at pin 1 2 and controls the cathode voltage of red cathode 102 . As discussed above, the cathode voltage controls the beam current in CRT 100 .

The +12V power supply is also connected to sensing node 30 via resistors R 5 and R 2 . The +215V power supply is connected to sensing node 30 via resistors R 6 and R 1 . Sensing node 30 is coupled to combined sensing node 43 via diode D 1 . The respective sensing nodes for ICs 41 and 42 are coupled to combined sensing node 43 via respective diodes D 2 and D 3 . The voltage at sensing node 43 is indicative of whether a fault exists on any of the power supplies to ICs 40 , 41 and 42 . Sensing node 43 is coupled to the base of Q 2 of shutdown circuit 44 . Diodes D 1 , D 2 or D 3 will conduct if either the +12V supply or +215V supply to any of the ICs 40 , 41 and 42 is below a threshold value. If any one of the diodes D 1 , D 2 or D 3 conduct, the voltage at combined sensing node 43 goes low, causing shutdown circuit 44 to disable the high voltage drive circuit as described further below. In this manner, a fault in either one of the low or high voltage supplies to any of the ICs 40 , 41 , and 42 causes the high voltage supply to the anode to be shutdown. Delay circuit 47 prevents shutdown of the anode high voltage supply during startup conditions. Since the power supplies to ICs 40 , 41 and 42 may not reach their normal levels until a certain finite amount of time during system startup, Q 1 maintains the necessary voltage on sensing node 43 during startup to prevent activation of shutdown circuit 44 .

Also, the resistors in series with the +12V and +215V supplies are fusable and will open if the ICs fail shorted. Such a failure would cause zero biasing of the CRT and excessive beam current. The excessive beam current is prevented by shutting down the high voltage anode in response to the output of shutdown circuit 44 .

37 FIG. 3 illustrates the various elements associated with shutdown circuit 44 and the anode high voltage power supply. Sensing mode 43 is connected to the base of transistor Q 2 of shutdown circuit 44 via connection 45 and node 48 . Shutdown circuit 44 includes a self-biasing latch comprised of transistors Q 3 and Q 4 . During normal operating conditions when the +12V and +215V supplies provide the necessary voltages to driver ICs 40 , 41 and 42 , transistors Q 2 , Q 3 and Q 4 are in the OFF state. When a fault is detected at any one of the sensing nodes, the voltage at combined sensing node 43 drops below a predetermined threshold value, thereby reducing the voltage at node 48 and placing Q 2 in the ON state. When Q 2 turns ON, current is supplied to the base of Q 4 , placing Q 4 in the ON state. When Q 4 turns ON, current is supplied to the base of Q 3 , placing Q 3 in the ON state. This action latches Q 3 and Q 4 into the ON state.

During normal operating conditions, high voltage transformer X 1 provides high voltage to anode 112 . The operation of high voltage transformer X 1 is controlled by phase lock loop 46 , a push-pull driver comprising transistors Q 5 and Q 6 , and MOSFET Q 7 . Transistors Q 5 and Q 6 are driven by phase lock loop 46 to generate a 10V square wave of approximately 50/50 duty cycle. The square wave drives the high voltage generator MOSFET Q 7 , which energizes high voltage transformer X 1 . When PLL 46 drives Q 6 to the ON state, the Vcesat of Q 6 drives the gate of Q 7 below the threshold voltage required for Q 7 to conduct, thereby keeping Q 7 in the OFF state. When PLL 46 drives the base of Q 6 high, R 28 provides base current to Q 5 , supplying MOSFET Q 7 with 12V-Vcesat, saturating MOSFET Q 7 and energizing the primary of high voltage transformer X 1 with current.

When PLL 46 again provides drive to Q 6 and removes drive current from Q 5 , the voltage at the gate of MOSFET Q 7 falls below the threshold voltage thereby placing MOSFET Q 7 in the OFF state. When MOSFET Q 7 turns OFF, the resonant circuit formed by the primary of high voltage transformer X 1 and the retrace capacitor C 10 present a resonant retrace pulse of approximately 1000V across the primary of high voltage transformer X 1 . During retrace, the energy stored in the core of the high voltage transformer X 1 is transferred to the high voltage winding, or “tertiary”, in the form of an impressed voltage of approximately the primary pulse times the turns ratio. This results in a tertiary charging current, which provides charge to anode 112 of CRT 100 as long as the tertiary diodes are forward biased.

The primary supply voltage, B+, is adjusted to maintain a constant anode potential for a variety of frequency and load conditions. For multiple frequency operation, switching of the high voltage generator is synchronized by PLL 46 , synchronizing the first loop, the high voltage generator, with the horizontal deflection, and sampling the high voltage transformer X 1 retrace pulse for establishment of the second loop error signal.

In the event of a fault which results in a loss of kinescope driver board power supplies, which may cause excessive beam current, the sensed voltage at node 48 causes Q 2 to be placed in the ON state. When Q 2 goes to the ON state, the self-biasing latch comprised of Q 3 and Q 4 goes to the ON state thereby drawing current away from the base of Q 5 . This results in the gate voltage of MOSFET Q 7 becoming insufficient to exceed the threshold voltage necessary to turn ON MOSFET Q 7 . As a result, no energy is stored in the core of high voltage transformer X 1 . If there is any beam current, the anode is quickly discharged, in the absence of a tertiary charging current. As the anode is discharged, there is no anode accelerating potential to allow damaging beam current. The self-biasing latch comprised of Q 3 and Q 4 removes the high voltage power supply to the anode until all power supplies are removed, and then restored, saving the CRT from damaging conditions. The anode is quickly discharged due to the use of a resistive element in the negative feedback of the anode voltage to the high voltage power supply. This results in a very fast time constant.

The present protection circuit may be used to shutdown the anode voltage in response to a high voltage level. It is necessary to shutdown the anode voltage if the high voltage exceeds a predetermined threshold level in order to prevent excessive x-radiation from being generated. Here, the output from an X-ray protection sensing circuitry is coupled to the base of Q 4 via the emitter of Q 8 . As such, if the output of the X-ray protection sensing circuitry indicates an excessively high voltage condition, Q 8 turns ON, thereby latching Q 3 and Q 4 into the ON state and activating shutdown circuit 44 as described above.

It will be apparent to those skilled in the art that although the present invention has been described in terms of an exemplary embodiment, modifications and changes may be made to the disclosed embodiment without departing from the essence of the invention. Therefore, it is to be understood that the present invention is intended to cover all modifications as would fall within the true scope and spirit of the present invention.