Abstract:
An over-current protection circuit is used to prevent damage to a television receiver due to excessive beam current. The television receiver includes a high voltage generating circuit for supplying a beam current to a picture tube. The over-current protection circuit a measurement circuit for directly detecting the beam current, a comparison circuit for comparing the detected beam current with a predefined threshold level, and an output circuit coupled to a control input of the high voltage generating circuit, for generating a control signal for turning off the high voltage generating circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     THIS APPLN CLAIMS BENEFIT OF 60/275,393, filed Mar. 13, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates to television receivers, and more particularly, to preventing the beam current in television receivers from exceeding a predefined maximum amount. 
     2. Description of the Related Art 
       FIG. 1  shows the high voltage circuitry in a known projection television receiver. The high voltage output/driver circuit  10 , under the control of a high voltage drive pulse, applies a B+ voltage of 130 V. to a primary winding L 1  of a high voltage transformer  20 , the primary winding L 1  being connected to ground via a capacitor C 1 . The high voltage transformer  20  includes a plurality of secondary windings L 2 -L 5  serially connected by diodes D 1 -D 3 . A diode D 4  connects the free end of secondary winding L 5  to the input of a high voltage splitter  30  which applies anode voltages to the red, green and blue projection tubes (not shown). Diode D 2 , in addition to being connected to the secondary winding L 4 , is also connected to a main focus output  22  of the high voltage transformer  20 . A power supply voltage Vb is connected to the free end of secondary winding L 5  via a series resistor R 1  and across a capacitor C 2  connected to ground, and supplies the beam current Ib. This beam current Ib is also supplied to an automatic beam current limiting (ABL) circuit (not shown) 
     Virtually every television receiver has an automatic beam current limiting, ABL, circuit. Its function is to limit the maximum beam current that is necessary to produce a bright picture on the screen when the brightness and the picture (contrast) controls are at maximum. In general, the object of an ABL circuit is to keep the maximum beam current below, for example, 1.8 ma. U.S. Pat. No. 4,017,681 shows a typical beam current limiting circuit. 
     When the brightness or contrast increases, more beam current, Ib, is drawn from the power supply, Vb, through the resistor R 1  into the secondary winding L 5  of the high voltage transformer  20 , the high voltage splitter  30  and the anodes of projection tubes. Any amount of beam current in excess of, for example, 1.8 ma should be kept to a minimum so that the high voltage transformer  20  and the projection tubes will operate reliably. Otherwise, the high voltage transformer  20  and/or the projection tubes may fail leading to the costs for replacing these components which are expensive. In addition, too much beam current may cause the projection tubes to emit too much X-ray radiation which is harmful to health. To this end, the ABL circuit detects the power supply voltage Vb and correspondingly limits the luminance and chrominance drive circuits which, in turn, limits the brightness and contrast of the television receiver. 
     When the ABL circuit fails due to, for example, a failed component therein, or when the picture tube is zero biased, or both, a large amount of beam current over 1.8 ma will result. The high voltage output circuit C should then be disabled. As such, there will be no beam current. An over-current protection circuit is used to disable the high voltage output circuit when the beam current exceeds the maximum beam current by a certain percentage. 
       FIG. 1  shows an over-current protection circuit which uses a switching circuit  40  (e.g., IC SC78130P1 by Motorola) to disables the high voltage output circuit. In particular, the high voltage transformer  20  includes an auxiliary winding L 6  which applies a flyback pulse, through a series arrangement of a diode D 5  and a resistor R 2 , to input  3  of the switching circuit  40 . Input  3  is connected to ground via a resistor R 3  and a capacitor C 3 , and is also connected to an input  1  via a resistor R 4 . Input  4  is connected to ground and inputs  2  and  8  are connected to ground via capacitors C 4  and C 5 , respectively. A supply voltage of +15 V. is applied to input  8  via a resistor R 5 . In addition, an on/off signal is applied to input  7  via a resistor R 6 . Finally, an output  8  of the switching circuit  40  is connected to a control input of the high voltage output/drive circuit  10 . 
     The over-current protection circuit of  FIG. 1  relies on the change of the voltage amplitude of the flyback pulse on the auxiliary winding L 6  of the high voltage transformer  20 . When the beam current increases, the amplitude of the flyback pulse decreases. When there is excessive beam current over 1.8 ma, the voltage amplitude of the flyback pulse decreases to such a low amplitude that, after it is rectified by diode D 5  and the capacitor C 3 , the d.c. voltage at input  1  is less than the reference voltage at input  2 . This actives the switching circuit  40  and causes the high voltage output/drive circuit  10  to shut down and to be latched. 
     Applicant notes, however, that this system does not monitor the beam current directly because the change of voltage amplitude of the flyback pulse at the auxiliary winding L 6  of the high voltage transformer  20  does not directly track the exact change of the beam current. Furthermore, the amplitude of the flyback pulse will change due to the changing of the flyback time or due to the tolerances between transformers. Therefore, the exact amount of beam current over 1.8 ma cannot be tightly controlled and predicted precisely. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an over-current protection circuit that directly tracks the beam current in a television receiver. 
     This object is achieved in an over-current protection circuit for preventing damage to a television receiver due to excessive beam current, said television receiver including a high voltage generating circuit for supplying a beam current to a picture tube, said high voltage generating circuit having a control input, wherein said over-current protection circuit comprises means for directly detecting the beam current; means for comparing the detected beam current with a predefined threshold level; and means, coupled to the control input of said high voltage generating circuit, for generating a control signal for turning off said high voltage generating circuit. 
     It is a further object of the invention to provide an over-current protection circuit that is immune to tolerance changes in the high voltage transformer, and is capable of accurately controlling and precisely predicting the amount by which the actual beam current exceeds a predetermined maximum amount. 
     Applicant has found that the beam current power supply voltage is directly proportional to the beam current. Hence, by monitoring the change of this voltage, the amount of change of the beam current can be directly determined. 
     In a further embodiment of the invention, the generating means latches said high voltage generating circuit in an off state. Arranged as such, the over-current protection circuit prevents the high voltage generating circuit from being turned on again when the beam current drops, at least until power has been removed from the television receiver. As such, re-starting of the television receiver must be a conscious act by the consumer. 
     In a further embodiment of the invention, the over-current protection circuit further comprises means for generating a reference signal equivalent to said threshold level; and temperature compensating means for making said reference signal temperature independent. Arranged as such, the over-current protection circuit is able to precisely predict the beam current without regard to temperature. 
     In a further embodiment of the invention, the over-current protection circuit further comprises means for preventing said generating means from erroneously generating said control signal due to picture tube arcing and/or random noise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With the above and additional objects and advantages in mind as will hereinafter appear, the invention will be described with reference to the accompanying drawings, in which: 
         FIG. 1  shows schematic block diagram of a prior art over-current protection circuit for a projection television receiver; and 
         FIG. 2  shows a schematic block diagram of an over-current protection circuit in accordance with the subject invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIG. 2 , a projection television receiver includes a high voltage generator  50  which may comprise the high voltage output/drive circuit  10  and the high voltage transformer  20  of FIG.  1 . An input of the high voltage generator  50  receives the B+ voltage of 130 V. which is connected to ground via a capacitor C 11 . One output of the high voltage generator  50  constitutes the main focus output, while three other outputs carry the anode voltages for the red, green and blue projection tubes. The beam current Ib is applied at input  2 , and results from the application of the beam supply voltage Vb to the series arrangement of resistors R 11 , R 12  and capacitor C 12 , the junction between resistor R 12  and capacitor C 12  being connected to the input  2 . This input  2  is also connected to an automatic beam current limiting (ABL) circuit (not shown). For a fixed value of R 1 , the amount of change of the voltage B (at the junction between resistors R 11  and R 12 ) is directly proportional to the amount of change of the beam current. Therefore, by monitoring the change of the voltage B, the amount of change of the beam current can be directly determined. For example, if R 11  is 3300 ohms, the beam supply voltage Vb is 28 V. and the beam current is 1.8 ma, the voltage B is equal to:
 
(28−3.3×1.8)=22.06  V   1
 
If the beam current increases to 2.3 ma, the voltage B will drop to:
 
(28−3.3×2.3)=20.41  V   2
 
     The over-current protection circuit is divided into four sections each performing a particular function. The first section provides a reference voltage A. The beam supply voltage Vb is applied to the series arrangement of resistors R 13 , R 14  and diodes D 11  and D 12 , diode D 12  being connected to ground. The junction between resistors R 13  and R 14  is connected to ground via a capacitor C 13  and to the base of an NPN transistor Q 1 . The collector of transistor Q 1  receives the beam supply voltage Vb, while the emitter is connected to ground via a resistor R 15 . The reference voltage A appears at the emitter of transistor Q 1 . The diodes D 11  and D 12  are used for temperature compensation so that the reference voltage A will not change with temperature. 
     The second section of the over-current protection circuit compares the voltage B with the reference voltage A and performs the switching function. In particular, the voltage B is applied to the base of a PNP transistor Q 2  via a diode D 13 , and the reference voltage A is applied to the emitter of the transistor Q 2 , a capacitor C 14  interconnecting the base and the emitter. If the voltage B is lower than the reference voltage A by the sum of the forward voltage drop of diode D 13  and the base-emitter voltage drop of transistor Q 2 , transistor Q 2  will turn on and apply a potential to the series arrangement of resistors R 16  and R 17  connected to ground. 
     The third section of the over-current protection circuit includes a diode D 14  connected to the junction between resistors R 16  and R 17 , and the parallel arrangement of a capacitor C 15 , a diode D 15  and a resistor R 18  coupled across the combination of diode D 14  and resistor R 17 . This third section rectifies the voltage at the junction of resistors R 16  and R 17 , and serves to block interference due to picture tube arcing and random noises. 
     The fourth section of the over-current protection circuit is the output stage. The diode D 14  is connected, on the one hand, via a resistor R 19  to the collector of a PNP transistor Q 3  and, on the other hand, to the base of an NPN transistor Q 4 . A resistor R 20  connects the emitter of transistor Q 4  to the grounded end of resistor R 17 , while a resistor R 21  connects the collector of transistor Q 4  to the base of transistor Q 3 . The parallel arrangement of a resistor R 22  and a capacitor C 16  connects the base of transistor Q 3  to its emitter, which is connected, on the one hand, via a resistor R 23  to a power supply source Vc, and, on the other hand, via diode D 16  to the ON/OFF input of the high voltage generator  50 , which also receives an ON/OFF voltage via a resistor R 24 . The rectified voltage at diode D 14  turns on transistors Q 3  and Q 4 , while the diode D 15  protects transistor Q 4  from reversed base-emitter voltage breakdown. Transistors Q 3  and Q 4  are arranged in such a configuration that the voltage at the ON/OFF input of the high voltage generator  50  will be clamped to approximately 0 volts by the diode D 16  when transistors Q 3  and Q 4  are turned on. This clamping action is continued even when the voltage at the junction between resistors R 16  and R 17  drops to zero. The clamping action by the diode D 16  is only released when the power supply voltage Vc is interrupted for a few seconds. When the voltage at the ON/OFF input of the high voltage generator  50  is at zero voltage potential, the high voltage generator  50  is disabled, and as such, there is no beam current. 
     The following equation may be used to calculate the total amount of beam current I before the over-current protection circuit acts and shuts down the high voltage generator  50 :
 
( Vb×R   13 + Vd ( R   13 + R   14 ))/ R   11 ( R   13 + R   14 )  3
 
Eq. 3 assumes that the base-emitter voltages of transistors Q 1  and Q 2  are the same as the forward diode voltage, Vd, of the diodes D 11 , D 12  and D 13 . As such, Vd is approximately 0.6 V.
 
     In addition, once the value of resistor R 11  is chosen, and the ABL voltage V 1  is known for the maximum current, Ib (e.g., 1.8 ma), the value of resistor R 12  can be determined using the equation:
 
 Ib =( Vb−Vl )/ R   11 + R   12 )  4
 
     Numerous alterations and modifications of the structure herein disclosed will present themselves to those skilled in the art. However, it is to be understood that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.