JPH0314273B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a television receiver that performs brightness control and contrast control using a microprocessor such as a microcomputer and is equipped with a switching regulator power source.

Conventional configurations and their problems In recent years, television receivers have come to have control functions such as brightness control and contrast control using microcomputers, and have come to use switching regulator power supplies.

An example of a conventional television receiver will be described below with reference to the drawings. FIG. 1 shows a block diagram of a conventional television receiver.

In the figure, 1 to 4 are rectifier diodes, and 5 is a smoothing capacitor. 6 is a switching transformer, 7 is a switching control circuit, 8 is an overcurrent protection circuit, 9 is a switching transistor,
10 is an overcurrent detection resistor. These 1 to 10 constitute a switching power supply. 11 is a rectifier diode, and 12 is a smoothing capacitor. A predetermined +B power source is obtained by these. 13 is a synchronization, deflection, and video circuit block. 14 is a cathode ray tube (CRT). 15 is a tuning control circuit; 16 is a control microcomputer; 1
7 is a brightness control circuit, and 18 is a contrast control circuit. 19 is a television receiver including 1 to 18;

FIG. 2 shows the ABL circuit extracted from the synchronization, deflection, and video circuit block 13 of FIG. 1. 21, 22, 24 are resistors, 23 is a diode, 25 is a capacitor, 26 is a neon tube, 2
7 is a flyback transformer, and 28 is a horizontal output transistor.

The operation of the television receiver configured as described above will be described below. First, in Figure 1,
The diodes 1 to 4 and the capacitor 5 rectify and smooth the AC power to generate a primary voltage. next,
+B power is supplied to the secondary side by a switching regulator power supply composed of a transformer 6 and a capacitor 12. The +B power supply operates the synchronous deflection and video circuit 13, and images are displayed on the CRT 14. The video amplification circuit is controlled by a brightness control circuit 17 and a contrast control circuit 18 in the channel selection control circuit 15.

When the video load increases, the collector current of the switching transistor 9 increases, and a voltage above a certain level is generated across the emitter of the resistor 10 for overcurrent detection, thereby providing overcurrent protection 8.
and stop the switching operation. Second
In the figure, a high voltage is generated in a flyback transformer (FBT) 27 by a horizontal output transistor 28.

In this circuit, the average brightness level (APL)
When receiving a low signal, the beam current is small, so the voltage drop across the resistor 24 causes a
The potential at the point is set by resistors 21 and 22 b
Since the potential is higher than that at the point, the diode 23 becomes conductive. Further, the capacitor 25 connected to point a is charged with positive polarity, and the ground side is charged with negative polarity. However, when a high APL signal is received, the beam current is large and the potential at point a due to the voltage drop across resistor 24 becomes lower than the potential at point b set by resistors 21 and 22, so diode 23 is cut off. . When the diode 23 is cut off, the potential at point a, which was almost constant when the APL was low, drops as the APL becomes high. Finally, it drops to negative polarity. As the potential at point a falls, the brightness control circuit 17 and the contrast control circuit 18 are controlled to lower the brightness and contrast and reduce the beam current. The capacitor 25 starts discharging at the same time as the potential at point a falls, and at the same time the potential at point a becomes negative, the ground side of the capacitor 5 becomes positive.

However, in the above configuration,
When switching channels from receiving a low APL signal to a high APL signal,
The ABL circuit does not operate during the discharge time constant of the capacitor 25, and an excessive beam current flows.
When the television receiver is used in a high ambient temperature and reduced voltage state, the core of the switching transformer 6 becomes saturated due to this excessive beam current, and the overcurrent detection circuit 8 operates, causing the switching regulator to operate. The problem was that it would stop.

Purpose of the Invention The present invention is intended to solve the above-mentioned problems of the conventional art.It is an object of the present invention to prevent the switch from switching from a low APL signal to a high APL signal by channel switching in a high ambient temperature and reduced voltage condition. An object of the present invention is to provide a television receiver in which the core of a switching transformer does not become saturated and, as a result, an overcurrent protection circuit does not operate.

Structure of the Invention The television receiver according to the present invention generates a mute pulse having a time width greater than the time constant of the ABL circuit in synchronization with the timing of channel switching, and applies it to the brightness control circuit and the contrast control circuit. This reduces the brightness and contrast when switching channels to reduce the video load, thereby preventing the core of the switching transformer from becoming saturated and, as a result, preventing the overcurrent protection circuit from malfunctioning. be.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. 3 of the drawings.

FIG. 3 shows a circuit diagram of a contrast control circuit and a brightness control circuit in a television receiver according to an embodiment of the present invention. The circuit shown in FIG. 3 is applied to a part of the channel selection control circuit 15 of the television receiver shown in FIG.

In the figure, 31 is a microcomputer for channel selection control, 32 is an output terminal for a mute pulse that is synchronized with channel switching and has a time width greater than the time constant of the ABL circuit, and 33 is an output terminal for a PWM signal for brightness control. A terminal 34 is an output terminal for a PWM signal for contrast control. The brightness control signal output from the terminal 33 is inverted and amplified by resistors 35, 37, and 38 and a transistor 39, smoothed by a resistor 41, a diode 40, and a capacitor 42, and made into a brightness control voltage, which is added to the brightness control circuit 17. It will be done. On the other hand, the contrast control signal output from the terminal 34 is transmitted to the resistor 43,
45, 47 and a transistor 46, and is inverted and amplified by a resistor 49, a diode 48, and a capacitor 5.
0 and is made into a contrast control voltage and then applied to the contrast control circuit 1
Added to 8.

Further, in this circuit, a mute pulse is transmitted from the terminal 32 through the diode 36 to the transistor 3.
A mute pulse is applied to the base of transistor 4 from terminal 32 through diode 44.
Applied to the base of 6.

With this configuration, when switching channels, the microcomputer 31 for channel selection control is activated.
ABL synchronized with channel switching from terminal 32 of
A mute pulse of positive polarity with a width equal to or greater than the time constant of the circuit is generated and passes through diodes 36 and 44.
It is inverted and amplified by transistors 39 and 46, and reduces the brightness control voltage and contrast control voltage, respectively, to lower the image load. As a result, even when switching from a low APL signal to a high APL signal under conditions of high ambient temperature and reduced voltage, the core of the switching transistor 6 will not saturate during the time constant when the ABL circuit does not operate. , the overcurrent protection circuit 8 also has the effect of not malfunctioning.

Effects of the Invention As described above, according to the present invention, when the ambient temperature is high and the core tolerance of the switching transformer is small, it is possible to
Even when a signal with a low APL is switched to a signal with a high APL, the core of the switching transformer does not saturate during the time constant when the ABL circuit does not operate, and the overcurrent protection circuit does not malfunction. It is possible to obtain the following effects.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional television receiver, FIG. 2 is a circuit diagram of the ABL circuit diagram of the receiver in FIG. 1, and FIG. 3 is a brightness diagram of a television receiver according to an embodiment of the present invention. FIG. 3 is a circuit diagram of a control section and a contrast control section. 6...Converter transformer, 7...Switching control circuit, 8...Overcurrent protection circuit, 9
...Switching transistor, 13...Synchronization,
Deflection, video circuit, 14... cathode ray tube, 17... brightness control circuit, 18... contrast control circuit, 31... microcomputer,
32...Mute pulse output terminal, 33...Brightness control signal output terminal, 34...Contrast control signal output terminal, 36, 44
...Diode, 39,46...Transistor,
40, 48...diode, 42, 50... capacitor.

Claims (1)

[Claims] 1. A switching regulator power supply that supplies DC voltage to the secondary side of a switching transformer, and a flyback transformer that drives a CRT with the power from the switching regulator power supply. The ABL has a charging/discharging section consisting of a resistor, a capacitor, and a unidirectional element that are charged and discharged by the current flowing through this flyback transformer, and the beam current is controlled by the control voltage held by this charging/discharging section. a switching transistor that is coupled to the switching transformer and controls a voltage supplied to the secondary side of the switching transformer; and an overcurrent protection device that detects a current flowing through the switching transistor and limits overcurrent. and an arithmetic circuit that generates a mute pulse that is synchronized with channel switching and has a time width greater than the time constant of the charging/discharging section of the ABL circuit, and an arithmetic circuit that generates the mute pulse when switching channels. and means for controlling the control voltage so as to reduce the video load by reducing the control voltage.