JPH06325890A - Dimming circuit - Google Patents

Abstract

PURPOSE:To lessen noise generation to a power line and decrease ill influence upon the picture quality and sound quality. CONSTITUTION:A switch circuit S1 which can be turned on and off with a dimming control signal (b), is furnished so that a capacitor C2 can be connected with a capacitor C1 which serves as the load impedance of a DC-AC inverter circuit. The load impedance is varied by changing over the switch S1 with the dimming control signal (b), and the tube current of a fluorescent lamp is adjusted without putting on and off the operation of the inverter circuit itself, and it is practicable to make dimming with no risk of noise generation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dimming circuit for a low-noise fluorescent lamp, and more particularly, to the brightness of the backlight of a device such as a liquid crystal television set in which dimming noise of the backlight adversely affects images and sounds. The present invention relates to a dimming circuit for adjusting.

[0002]

2. Description of the Related Art FIG. 5 is a diagram showing a conventional dimming circuit, and FIGS. 6A to 6E are signal waveform diagrams of respective parts in FIG. In the figure, 11 is a dimming circuit unit, 12 is an oscillation circuit, 13 is a transformer, and 14 is a fluorescent lamp. 6A shows a dimming control signal b, FIG. 6B shows an output pulse c, FIG. 6C shows a collector output d of the transistor Q1, and FIG.
6D shows the tube voltage waveform e, and FIG. 6E shows the noise a to the power supply line. An oscillation circuit 12 to which a dimming control signal b (FIG. 6A) is input is provided in the dimming circuit section 11, and an output pulse c (FIG. 6) of the oscillation circuit 12 is provided.
By (b)), turning on / off the transistor Q3 turns on / off the operation itself of the inverter circuit,
Thereby, the fluorescent lamp 14 is dimmed by controlling the tube current at the secondary side output of the transformer T1.

[0003]

As described above, in the conventional dimming circuit, the operation of the transistors Q1 and Q2 becomes discontinuous due to the on / off of the transistor Q3, and at that time, the inverter circuit itself turns on / off. It will be repeated, due to the inrush current etc. of the operation start of the inverter circuit,
As shown in FIG. 6 (e), a large noise a is generated even in the power line.
Occurred. This noise a is usually 500 Hz to 5 kHz
It was selected about z, which had a great influence on audio and video, and was a major cause of deterioration of sound quality and image quality.

The present invention has been made in view of such circumstances, and a switching means for switching the load impedance of the inverter circuit is provided, and the switching means is switched by a dimming control signal, whereby the circuit operation itself of the inverter is controlled. It is an object of the present invention to provide a dimming circuit in which the tube current of a fluorescent lamp is adjusted without turning it on and off to suppress the generation of noise that adversely affects it.

[0005]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention uses a dimming control signal in a lighting circuit of a fluorescent lamp applied to a backlight of a liquid crystal television or the like to control the load impedance of an inverter circuit. Noise generation that adversely affects the image quality and sound quality of a liquid crystal television, etc., and is composed of a switching means for switching the operation of the inverter circuit and an adjusting means for adjusting the tube current of the fluorescent lamp without turning on / off the operation itself of the inverter circuit. It is characterized by suppressing.

[0006]

The switching means switches the load impedance of the inverter circuit according to the dimming control signal, and the adjusting means adjusts the tube current of the fluorescent lamp without turning on / off the operation itself of the inverter circuit. As described above, the load impedance of the inverter circuit, that is, the capacity of the stabilizing capacitor on the secondary side of the DC-AC transformer is switched by the dimming control signal to limit the tube current flowing through the fluorescent lamp to perform dimming. Be done.

[0007]

Embodiments will be described below with reference to the drawings. FIG. 1 is a configuration diagram for explaining an embodiment of a dimming circuit according to the present invention, and FIGS. 2A to 2C are signal waveforms of respective parts in FIG. In the figure, 1 is a dimming circuit section, 2
Is a transformer and 3 is a fluorescent lamp. In addition, FIG.
Dimming control signal b, FIG. 2B shows a tube voltage waveform e,
FIG. 2C shows noise a to the power supply line. First, when the dimming is not performed, the dimming control signal b shown in FIG. 2A is not input and is at the “L” level. At this time, the switch S1 is closed by the signal of the dimming control signal b, and the load impedance of the inverter circuit is connected in parallel with the capacitors C1 and C2, which is C1 + C2. For this reason, the AC impedance is lowered, and a large amount of the tube current of the fluorescent lamp 3 flows, so that the brightness becomes bright. Next, when dimming is performed, the dimming control signal b is input to the switch S1, the switch S1 is opened, the impedance of the inverter circuit becomes only the capacitor C1, the AC impedance increases, and the fluorescent lamp 3 The brightness decreases because the tube current is reduced.

FIG. 3 is a diagram showing another embodiment of the dimming circuit according to the present invention, in which a transistor is applied instead of the switch S1 in FIG. When the dimming control signal b is "L", the transistor Q4 is turned on, the load impedance of the inverter circuit becomes C1 + C2, the tube current of the fluorescent lamp 3 is increased, and the fluorescent lamp is in the bright mode. Further, when the dimming control signal is "H", the transistor Q4 is turned off, the load impedance of the inverter circuit is only C1, and the dimming mode is set.

FIG. 4 is a diagram showing still another embodiment of the dimming circuit according to the present invention, in which a relay is used instead of the switch S1 in FIG. At this time, the polarity of the dimming control signal is opposite to that in the case of FIG. When the dimming control signal b is “H”, the relay (RLY1) is activated and turned on, so that the fluorescent lamp 3 is in the bright mode with the load impedance of C1 + C2, and conversely the dimming control signal b is “L”. At times, the relay does not operate and is turned off, so the light control mode is set.

[0010]

As is apparent from the above description, the present invention has the following effects. That is, by switching the load impedance of the lighting circuit of the fluorescent lamp, the tube current of the fluorescent lamp is adjusted to perform dimming, so that noise to the power supply line is significantly reduced, and the dimming circuit of the related art is reduced. In contrast, the adverse effects on image quality and sound quality are greatly reduced. Therefore, it becomes easy to design the pattern of the board, and it is possible to greatly reduce noise countermeasure components.

[Brief description of drawings]

FIG. 1 is a configuration diagram for explaining an embodiment of a dimming circuit according to the present invention.

FIG. 2 is a diagram showing a signal waveform of each part of the light control circuit in FIG.

FIG. 3 is a diagram showing another embodiment of the dimming circuit according to the present invention.

FIG. 4 is a diagram showing still another embodiment of the dimming circuit according to the present invention.

FIG. 5 is a diagram showing a conventional dimming circuit.

6 is a diagram showing signal waveforms of respective parts of the light control circuit in FIG.

[Explanation of symbols]

 1 ... dimming circuit, 2 ... transformer, 3 ... fluorescent lamp.

Claims (1)

[Claims] 1. A switching circuit for switching a load impedance of an inverter circuit in response to a dimming control signal in a lighting circuit of a fluorescent lamp applied to a backlight of a liquid crystal television, and on / off of an operation itself of the inverter circuit. The dimming circuit is characterized in that it suppresses the generation of noise that adversely affects the image quality and sound quality of liquid crystal televisions, etc.