JP2001319795A - Inverter circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a beating noise appearing on a display screen of a liquid crystal display unit inconspicuous. SOLUTION: An inverter circuit is equipped with a resonance part LC consisting of a coil L1A and capacitors 16, 17, switching parts 13, 14 to switch the current flowing in a resonance part XC in order to obtain an oscillating voltage, and an output part 11 that transforms the oscillating voltage obtained from the resonance part LC and that outputs into a back light BL of the liquid crystal display unit which modulates the back light beam in accordance with picture signals reversed at every horizontal scanning. Especially, the resonance part LC has the oscillating frequency wherein the oscillating voltage is set assuming an integer fold of 1/4 of a horizontal synchronization frequency of the picture signals as a reference value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter circuit serving as a backlight power supply for a liquid crystal display device which mainly displays an image by modulating backlight light.

[0002]

2. Description of the Related Art In recent years, liquid crystal display devices have become lighter and thinner.
Because of its characteristic of low power consumption, it is widely used as a monitor display for notebook personal computers, portable information devices, small TVs, and the like.

[0003] A typical liquid crystal display device has a liquid crystal display panel in which a liquid crystal layer is sandwiched between two translucent substrates. On these substrates, a plurality of translucent electrodes are formed facing each other to form a display screen together with the liquid crystal layer. The liquid crystal layer is obtained by injecting and sealing a liquid crystal composition into a cell in which the gap between these translucent substrates is surrounded by a sealing material, and according to the pixel voltage applied between the translucent electrodes of each display pixel. Light transmittance is set. As a result, the liquid crystal display panel modulates light from the backlight disposed on the back side and displays it as an image on the display screen. By the way, since a backlight is generally lit using an inverter circuit as a power supply, when an interfering wave is superimposed on a video signal with the oscillation operation of the inverter circuit, horizontal or oblique stripes may appear on a display screen as beat noise. .
Conventionally, the oscillation frequency in the inverter circuit has been set to substantially an integral multiple of the horizontal synchronization frequency of the video signal. By the way,
For example, even when a channel is in an interference state in a television broadcast, beat noise appears on a display screen such as a CRT.
It is known that this beat noise becomes inconspicuous when the beat frequency of the interference wave is an integral multiple of 1/2 of the horizontal synchronization frequency.

[0004]

However, even if the oscillation frequency of the inverter circuit is set to an integral multiple of 1/2 of the horizontal synchronization frequency in the conventional liquid crystal display device, the beat noise appearing on the display screen is made inconspicuous. Could not.

An object of the present invention is to solve the above-mentioned problems, and to provide an inverter circuit capable of making beat noise appearing on a display screen of a liquid crystal display device inconspicuous.

[0006]

According to the present invention, there is provided a resonance section comprising a coil and a capacitor, a switch section for switching a current flowing through the resonance section to obtain an oscillation voltage, and an oscillation section obtained from the resonance section. An output section for transforming the voltage and outputting the same to a liquid crystal display device that performs optical modulation in response to a video signal inverted every horizontal scanning. The resonance section has an oscillation voltage of 1/4 of the horizontal synchronization frequency of the video signal. An inverter circuit configured to have an oscillation frequency set with an integral multiple as a reference value is provided. In this inverter circuit, the resonance unit is configured so that the oscillation voltage has an oscillation frequency set with an integer multiple of 1/4 of the horizontal synchronization frequency of the video signal as a reference value. The frequency spectrum of the video signal inverted every horizontal scanning has a peak at an integral multiple of 1/2 of the horizontal synchronization frequency. If the oscillation voltage of the inverter circuit is superimposed on the video signal as an interference wave, a beat is generated due to the sum and difference between an integral multiple of 1/2 of the horizontal synchronization frequency of the video signal and the oscillation frequency of the inverter circuit. The beat noise of the difference component appears on the display screen. In terms of frequency spectrum, beat noise appears as a sideband centered at an integral multiple of half the horizontal synchronization frequency, and is the difference between the integral multiple of half the horizontal synchronization frequency and the oscillation frequency of the inverter circuit. It peaks at the frequency.
In order to eliminate this peak, the resonance unit is configured so that the oscillation voltage has an oscillation frequency set with an integer multiple of 1/4 of the horizontal synchronization frequency of the video signal as a reference value. Thereby, beat noise appearing on the display screen of the liquid crystal display device can be made inconspicuous.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below.
The inverter circuit will be described with reference to the drawings. this
The inverter circuit mainly reverses the polarity (or
Compatible with NTSC and PAL video signals
LCD backlight device that modulates the backlight light
It is used as a light source power supply. FIG. 1 shows a circuit configuration of the inverter circuit. Invar
The circuit is choke coil 10, transformer 11, resistance
12, switching transistors 13 and 14,
Quantity switching transistor 15, ballast capacitor 16,
Resonance capacitor 17, transistor 18, resistor 19, average
And a diode 20. Coil 10 is a positive power supply terminal
VDD and the intermediate tap of the primary side first winding of the transformer 11
Connected between the This is the transistor 13 and 14
The emitter-path is the first volume of the primary side of the transformer 11
The two ends of the line L1A are connected in series with each other. G
The emitter of the transistor 13 and the emitter of the transistor 13
The contact between the emitters is the collector-emitter of transistor 18.
Connected to the ground terminal GND via the tapas,
Via the biased diode 20, the positive power supply terminal VD
D is connected. The base of the transistor 18 is a resistor 19
And a control input terminal.
Connected to child CT. Switch base of transistor 15
Connected to input terminal SW, the collector of transistor 15
The emitter path is connected between both ends of the primary first winding L1A.
It is connected in series with the capacitor 16. The capacitor 17 is
In parallel with the series circuit of transistor 15 and capacitor 16
Connected. Primary secondary winding L1B of the transformer 11
Are connected to the bases of transistors 13 and 14, respectively.
Connected. The resistor 12 is connected to the coil 10 and the primary first side.
The contact between the winding L1A and the base of the transistor 14 and
It is connected between the contacts between the primary side second windings L1B. liquid
The backlight BL of the crystal display device is connected to the transformer 11
It is connected between both ends of the secondary winding L2. Primary side first winding L
1A constitutes an inductance element portion, and a transistor 1A
5, capacitor 16, and capacitor 17 are capacitors.
The inductance element portion XC,
Section and capacitance element section XC constitute resonance section LC
I do. Winding of primary side first winding L1A and secondary side winding L2
Line ratio and coupling density are required for backlight BL lighting
Oscillation voltage generated in the primary side first winding L1A
Is set to be obtained in the secondary winding L2 by boosting
It is. Transistor 15 is a liquid crystal display that is NTSC
When displaying the image corresponding to the image signal, the capacitance
Set the element part XC to the capacitance of the capacitor 17
The PAL video signal
Capacitance element when displaying the image corresponding to the
Section XC is the total capacitance of capacitors 16 and 17
Turned on to set the This inverter
Circuit oscillation frequency f₀Means that the transistor 15 is off
The horizontal synchronization frequency f of the NTSC video signal_H(= 1
(5.734 KHz) to 66.86 KHz
When the transistor 15 is on,
Signal horizontal synchronization frequency f_H(= 15.625 KHz)
Then, it is set to 66.4 KHz. DC voltage is applied between power supply terminals VDD and GND
And this is the primary first winding L1A and
And the second winding L1B. Transistor 13
And 14 obtain the oscillation voltage from the primary side first winding L1A.
For this purpose, the current flowing through the resonance part LC is switched. Concrete
Typically, the transistors 13 and 14 are connected to the primary second winding.
Conducts alternately according to the direction of the current flowing through L1B,
To the first side winding L1A and the capacitance element portion XC.
The direction of the current flowing is periodically inverted. This allows oscillation
When a voltage is obtained in the primary side first winding L1A, this oscillation
The pressure is increased by the induction action of the transformer 11
Output from the side winding L2 to the backlight BL. The inverter circuit described above reverses the polarity (or
Display the image corresponding to the video signal whose level is inverted)
If the backlight power supply for the LCD is
Is dependent on the oscillation state of the inverter circuit.
May appear on the display screen. For this reason, NTS
The cause of beat noise in the case of C video signal
explain about. Horizontal sync frequency f of NTSC video signal_HIs 15.734
KHz. The polarity of this video signal is inverted every horizontal scan.
The frequency spectrum of this video signal is shown in FIG.
So that the horizontal synchronization frequency f_H8.867K which is 1/2 of
Hz has a peak at an integral multiple of this reference wave.
One. This means that F (t) = a₀+ A_nIt is represented by (here,
n = 1, 2, 3, ..., a₀= τ / TA ・ a, a_n= 2τ / TA
・ {Sin (nω₀τ / 2)} / {nω₀τ / 2}), the result of Fourier expansion is as shown in FIG. I
The interference signal from the inverter circuit causes the video signal
, The beat becomes the horizontal synchronization frequency f_H1/2 of
It occurs between the integral multiple and the oscillation frequency of the inverter circuit. An example
For example, if the oscillation frequency of the inverter circuit is 65 kHz (= F
When set to 2), this oscillation frequency F2 and the horizontal synchronization frequency
Number f_H62.93 KHz (= F1)
= 4f_H) And a beat occurs, as shown in FIG.
By the sum and difference of these frequencies F1 and F2, F1 +
F2, F1-F2, 2 (F1 + F2), 2 (F1-F
2) The harmonic components are as follows. In particular, the F1-F2 component
Appears as beat noise on the display screen. Frequency spectrum
From the viewpoint of the beat noise, the horizontal synchronization frequency f_HOf 1
Appears as a sideband centered at an integral multiple of / 2. This side
The peak frequency of the waveband is f_H/ 2 ± α. (Α ≒ F₀−
4f_H= 2.07 KHz) This sideband component is oscilloscope
Observed during the period, the sideband component shows a sign as shown in Fig. 6.
The wave is superimposed on the baseband of the video signal. Beetno
Are displayed as stripes on the display screen. The bright part of this stripe
And the dark area are determined at the center level of this sine wave.
The number of stripes is obtained by the following equation. Number of stripes = (F2
−F1) / 60 (Hz) If α = F2−F1 is, for example, 600 Hz, the stripes are 10
The book will be displayed. Where α is the inverter circuit
Oscillation frequency f₀It changes with the change of. Figure 7 is actual
The sideband component superimposed on the video signal is
FIG. 8 is an enlarged view of the sideband component shown in FIG.
You. As can be seen from FIGS. 7 and 8, the amplitude of the sideband component
Changes according to the frequency. As described above, the beat noise is １ ／ of the horizontal synchronization frequency.
Appears as a sideband centered on an integer multiple of
Between the integral multiple of half the number and the oscillation frequency of the inverter circuit
It peaks at the difference frequency. Mask this peak
Therefore, the oscillation frequency of the inverter circuit is
The reference value is set to an integral multiple of 1/4 of the synchronization frequency.
You. That is, the oscillation frequency f of the inverter circuit₀= N
f_H/ 2 + f _H/ 4. (Where n = 1,
2, 3, ...) horizontal synchronization frequency f_HIs 15.734 KHz
In the case of the NTSC video signal which is
Oscillation frequency f₀Set 66.86 KHz as reference value
Is done. Also, the horizontal synchronization frequency f_HIs 15.625 KHz
In the case of the PAL video signal which is
Vibration frequency f₀Is set with reference to 66.4 kHz
You. As a result, the beads appearing on the display screen of the liquid crystal display device
Noise can be made inconspicuous. FIG. 9 shows the oscillation frequency f of the inverter circuit.₀Is NTSC video
If 66.86 kHz is set as the reference value for the signal
The results of the evaluation are shown below. This evaluation is given to different engineers
The oscillation frequency f of the inverter circuit
₀From 65.2KHz around 66.86KHz
In the first range of 68 kHz, beat noise is
It was confirmed that it did not appear on the surface. In addition, the inverter
Oscillating frequency f₀Is 64.3 KH excluding the above range
Even in the second range from z to 69 kHz, only thin beats
Since it does not appear, it is substantially acceptable. In the inverter circuit of the present embodiment, the transistor 1
5 is turned on by the switching signal input from the switching input terminal SW.
In this state, the oscillation frequency of the oscillation voltage,
Oscillation frequency f of the bar circuit₀Is the capacitor 16 and
The total capacitance of the capacitor 17 and the primary first winding L
1A inductance enables horizontal synchronization of PAL video signals
Set to 66.4 KHz, which is an integral multiple of 1/4 of the initial frequency
Is done. Also, the transistor 15 is inputted from the switching input terminal SW.
When the switching signal turns off, the inverter circuit
Oscillation frequency f₀Is the capacitance of the capacitor 17
NTS due to the inductance of the primary side first winding L1A
6 which is an integral multiple of 1/4 of the horizontal synchronization frequency of the C video signal
Set to 6.86 KHz. Thus, Transis
Data 15 for the NTSC video signal and the PAL video signal.
Of the capacitance element part XC so as to correspond to the oscillation frequency.
In order to change the capacitance, a capacitor 16 and
Switching the combination of the
Voltage may interfere with NTSC or PAL video signals as interference.
Beat noise is not noticeable on the display screen
It becomes. Also, the oscillation frequency f of the inverter circuit₀Standards of
Because the value has been specified, the frequency
A good margin can be obtained.

[0008]

As described above, according to the present invention, it is possible to provide an inverter circuit capable of making beat noise appearing on a display screen of a liquid crystal display device inconspicuous.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of an inverter circuit according to one embodiment of the present invention.

FIG. 2 is a diagram showing a frequency spectrum of an NTSC video signal input to a liquid crystal display device using the inverter circuit shown in FIG. 1 as a backlight power supply.

FIG. 3 is an enlarged view showing a part of the frequency spectrum shown in FIG. 2;

FIG. 4 is a diagram obtained by Fourier-expanding an expression showing the video signal shown in FIGS. 2 and 3 on a time axis.

FIG. 5 is a diagram showing the frequency of beat noise generated by mixing the oscillation voltage and the video signal of the inverter circuit shown in FIG. 1;

6 is a diagram showing a state in which the beat noise shown in FIG. 5 is superimposed on a baseband of a video signal, by a waveform on an oscilloscope and stripes on a display screen of a liquid crystal display device.

FIG. 7 shows that sideband components actually superimposed on a video signal are 0 to 1;
It is a waveform diagram shown in the range of 00 kHz.

8 is a waveform diagram showing an enlarged sideband component shown in FIG. 7;

FIG. 9 shows that the oscillation frequency of the inverter circuit shown in FIG.
It is a figure which shows the evaluation result when 66.86KHz is set as reference value for SC video signals.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Choke coil 11 ... Transformer transformer 13,14 ... Switching transistor 15 ... Capacitance switching transistor 16,17 ... Capacitor L1A ... Primary first winding L1B ... Primary second winding XC ... Capacitance element part LC ... Resonance Part BL ... Backlight

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/36 G09G 3/36 5H007 (72) Inventor Takashi Saeki 50, Kamiyobe, Yoyo-ku, Himeji City, Hyogo Prefecture Stock Inside the Toshiba Himeji Plant (72) Inventor Takatoshi Kato 7-1 Nisshin-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa F-term (reference) 2H093 NA32 NC01 NC06 ND40 3K072 CA16 GA01 GB14 GC02 HA02 HA06 5C006 AF71 BB11 BF41 BF46 EA01 FA16 FA23 5C058 AA06 AB03 BA29 BA33 BB25 5C080 AA10 DD06 FF03 JJ02 JJ04 5H007 AA01 BB03 CA01 CB03 CB06 CB09 CC12 CC32 DA03 DB01

Claims (3)

[Claims] A resonance section including an inductance element section and a capacitance element section; a switch section for switching a current flowing through the resonance section to obtain an oscillation voltage;
An output unit that transforms an oscillation voltage obtained from the resonance unit and outputs the oscillation voltage to a liquid crystal display device that performs light modulation in response to a video signal that is inverted every horizontal scanning, and the oscillation unit outputs the video signal. An inverter circuit configured to have an oscillation frequency set using an integer multiple of 1/4 of the horizontal synchronization frequency as a reference value. 2. The capacitance element section includes first and second capacitors and the first and second capacitors for changing the capacitance of the capacitance element section so as to correspond to oscillation frequencies for an NTSC video signal and a PAL video signal. 2. The inverter circuit according to claim 1, further comprising a switch element for switching a combination of capacitors. 3. The switch according to claim 2, wherein the switch element is connected in series with the first capacitor, and the second capacitor is connected in parallel with a series circuit of the switch element and the first capacitor. Inverter circuit.