JPH0973989A - Cold cathode tube lighting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce dielectric loss, transformer loss, and the like and downsize a shape by directly coupling the output portion of an inverter circuit and the input portion of a cold cathode tube. SOLUTION: The output voltage VS of an inverter circuit 2 is directly applied to a cold cathode tube FL, and the output voltage VS depends on tube current- tube voltage characteristics held by the tube FL. The output voltage VS is proportional to the output voltage Vout of a PWM inverter circuit 1a, a tube current is changed by changing the output voltage Vout . Thereby, the brightness control of the tube FL can be conducted. A lighting maintaining voltage becomes equal to the output voltage VS, and is reduced by the voltage portion applied to a ballast capacitor in a conventional device.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-cathode tube lighting device used as a backlight for a liquid crystal display or the like.

[0002]

2. Description of the Related Art The structure of a conventional CCFL driver circuit will be described with reference to FIG. DC is a DC power supply, and DC power supply DC
Is input to the DC / DC converter circuit 1b, and the negative electrode of the DC power supply DC is grounded.

In the circuit configuration of the DC / DC converter circuit 1b, the emitter of the PNP type transistor 11 is a DC power source D
It is connected to the positive electrode of C and one end of the capacitor 14, the base of the transistor 11 is connected to the output end of the PWM 12, the collector of the transistor 11 is connected to the cathode of the diode 13 and the one end of the coil 16, and the The other end is connected to one end of the capacitor 17 and serves as an output end of the DC / DC converter circuit 1b, and the other ends of the capacitors 14 and 17 and the diode 1 are connected.
The anode of 3 is grounded.

Reference numeral 2 denotes an inverter circuit. The circuit configuration of the inverter circuit 2 is such that one end of the choke coil 20 is DC /
The output end of the DC converter circuit 1b, that is, the other end of the coil 16, is connected, and the other end of the choke coil 20 is connected to the resistor 2
One end of 1, 22 and low voltage side windings 23, 2 of the transformer T
4 is connected to one end. NPN type transistor 25
The collector of is the one end of the capacitor 27 and the low voltage side winding 2
3, the base of the transistor 25 is connected to the other end of the resistor 21 and one end of the base winding 28, and the emitter of the transistor 25 is grounded. The collector of the NPN transistor 26 is connected to the other end of the capacitor 27 and the other end of the low voltage side winding 24, and the transistor 2
The base of 6 is connected to the other end of the resistor 22 and the other end of the base winding 28, and the emitter of the transistor 26 is grounded. Then, one end of the high voltage side winding 29 is grounded and the other end is an output end of the inverter circuit 2.

The output end of the inverter circuit 2, that is, the other end of the high-voltage side winding 29 is connected to one end of a ballast capacitor CB, and the other end of the ballast capacitor CB is connected to one end of a cold cathode tube FL. The other end of FL is grounded.

Reference numeral 3a is a voltage detection circuit. The circuit configuration of the voltage detection circuit 3a is such that the output terminal of the DC / DC converter circuit 1b is connected to one end of the resistor 34, and the other end of the resistor 34 is connected to one end of the variable resistor 35. The other end of the variable resistor 35 is connected to one end of the resistor 36 and serves as an output end of the voltage detection circuit 3a, and the other end of the resistor 36 is grounded.

Reference numeral 4 is an error amplification circuit, and the error amplification circuit 4
In the circuit configuration, the output end of the voltage detection circuit 3a, that is, the other end of the variable resistor 35 is connected to the inverting input end (-) of the differential amplifier 41 and one end of the impedance Z, and the other end of the impedance Z is connected to the differential amplifier 41. Of the differential amplifier 41, the positive terminal of the DC power supply 42 serving as a reference voltage is connected to the non-inverting input terminal (+) of the differential amplifier 41, and the negative terminal of the DC power supply 42 is grounded. There is.

The output end of the error amplification circuit 4 is DC
By being connected to the input terminal of the PWM 12 in the / DC converter circuit 1b, a cold cathode tube lighting device including a negative feedback control circuit is configured.

Next, the operation of the conventional CCFL driver circuit will be described with reference to FIG. Input voltage V of DC power supply DC
When in is applied to the DC / DC converter circuit 1b, P
The transistor 11 is turned on by the controlled pulse voltage Vpwm from the WM 12, a square wave is output to the collector of the transistor 11, and the DC output voltage Vout is output by the diode 13, the coil 16 and the capacitor 17.

The output voltage Vout of the DC / DC converter circuit 1b is smoothed by the choke coil 20 of the inverter circuit 2, is applied to one end of the low voltage side windings 23 and 24 of the transformer T, and the transistors 25 and 2 are also provided.
It is applied to the base of 6 through resistors 21 and 22, respectively. At this time, one of the transistors 25 and 26 is turned on due to the difference in the internal constants or the constants of the peripheral circuits, and thereafter, this ON operation is caused by the inversion of the induced electromotive force generated in the base winding 28. , Alternating with each transistor. Transistor 2
When either one of 5 and 26 is turned on, resonance occurs due to L and C in the low voltage side windings 23 and 24 in the inverter circuit 2 and the circuit such as the capacitor 27, and a sinusoidal voltage is generated between A and B in FIG. Occurs. At this time, the oscillation operation is continued at the resonant frequency of the sine wave determined by L and C in the circuit such as the inductance of the low voltage side windings 23 and 24 and the capacitance of the capacitor 27. Then, in the high voltage side winding 29, a high voltage proportional to the voltage between A and B (voltage of the low voltage side winding) is generated in proportion to the number of turns of the high voltage side winding 29, and becomes the output voltage Vs of the inverter circuit 2. . The output voltage Vs of the inverter circuit 2 is the voltage Vcb via the ballast capacitor CB.
Is applied to the cold cathode fluorescent lamp FL. Here, the ballast capacitor CB serves to stabilize the current (hereinafter referred to as tube current) I flowing through the cold cathode tube FL and protect the cold cathode tube FL.

On the other hand, the DC / DC converter circuit 1
The output voltage Vout of b is also input to the voltage detection circuit 3a and divided by the resistors 34 and 36 and the variable resistor 35, and the divided voltage of the resistor 36 becomes the output voltage V ₁ of the voltage detection circuit 3a. The output voltage V ₁ of the voltage detection circuit 3a is input to the inverting input terminal (−) of the differential amplifier 41, and the differential amplifier 4
Reference voltage Vre input to the non-inverting input terminal (+) of 1
Compared with f, the output voltage V ₂ of the error amplification circuit 4 is output from the output terminal of the differential amplifier 41, and is input to the input terminal of the PWM 12 in the DC / DC converter circuit 1b. The output voltage V ₂ of the error amplification circuit 4 is pulse width modulated in the PWM 12, and the modulated pulse voltage Vpwm
This controls the conduction of the transistor 11. As described above, the series of operations of the cold cathode tube lighting device are controlled.

In the cold-cathode tube lighting device shown in FIG. 3, the brightness of the cold-cathode tube FL can be adjusted by controlling the tube current I, and the voltage Vcb applied to the cold-cathode tube FL is used to control the tube current I. Change. To change the voltage Vcb, the output voltage Vs of the inverter circuit 2 may be changed. Since the output voltage Vs is proportional to the output voltage Vout of the DC / DC converter circuit 1b, the cold cathode tube can be controlled by controlling Vout. The voltage Vcb applied to FL can be controlled.

The output voltage Vout of the DC / DC converter circuit 1b is divided by the resistors 34, 35 and 36 in the voltage detection circuit 3a and outputs the voltage V ₁ . V ₁ can be expressed by an equation.

V ₁ = (36 / (34 + 35 + 36)) × Vout ... V ₁ is input to the inverting input terminal (−) of the differential amplifier 41,
Invert and amplify the difference from the reference voltage Vref to output DC voltage V
₂ is input to PWM12. The PWM 12 generates a pulse according to the output voltage V ₂ of the error amplification circuit 4,
Since the transistor 11 is driven and Vout is controlled so that V ₁ and Vref are equal to each other, Vout can be expressed by the following equation.

Vout = ((34 + 35 + 36) / 34) × Vref From the formula, Vout is adjusted by the variable resistance 35, that is, the tube current I is adjusted, and when the variable resistance 35 is increased, the tube current I becomes When the variable resistance 35 is decreased, the tube current I is decreased, that is, the brightness increases.
The brightness decreases.

Another example of the conventional CCFL driver circuit is composed of a negative feedback control circuit as shown in FIG. As shown in FIG. 4, the cold-cathode tube lighting device includes a DC power supply DC, a PWM inverter circuit 1a, an inverter circuit 2, a ballast capacitor CB, and a cold-cathode tube F.
L, a tube current detection circuit 3b, and an error amplification circuit 4. 4, those having the same numbers as those in the circuit shown in FIG. 3 have the same configuration and operation as those in FIG.
Is the same as Regarding the PWM inverter circuit 1a,
A square wave output voltage Vou is obtained by removing the coil 16 and the capacitor 17 from the DC / DC converter circuit 1b of FIG.
The tube current detection circuit 3b is configured by adding a resistor 31, a diode 32, and a capacitor 33 that perform voltage conversion, rectification, and smoothing to the voltage detection circuit 3a of FIG. 3, respectively. ing.

A DC power supply DC is used to connect each circuit and part.
Is input to the PWM inverter circuit 1a, the output end of the PWM inverter circuit 1a is connected to the input end of the inverter circuit 2, the output end of the inverter circuit 2 is connected to one end of the ballast capacitor CB, and the other end of the ballast capacitor CB is connected to It is connected to one end of the cold cathode tube FL, the other end of the cold cathode tube FL is connected to the input end of the tube current detection circuit 3b, and the output end of the tube current detection circuit 3b is an inverting amplifier 4 in the error amplification circuit 4.
1 is connected to the inverting input terminal (-) of the error amplifier circuit 4 and the output terminal of the error amplifier circuit 4 is connected to the input terminal of the PWM 12 in the PWM inverter circuit 1a to form a cold cathode tube lighting device that serves as a negative feedback control circuit. .

With the circuit configuration of FIG. 4, the differences from the operation of the circuit of FIG. 3 are as follows. The output voltage Vout of the PWM inverter circuit 1a is input only to the inverter circuit 2, and the output voltage Vs of the inverter circuit 2 becomes the voltage Vcb via the ballast capacitor CB and becomes the cold cathode tube F.
Applied to L. The tube current I flowing through the cold cathode tube FL is input to the tube current detection circuit 3b, converted into a voltage by the resistor 31, rectified by the diode 32, and the capacitor 3
It joins 3 and is smoothed. Charging voltage V of capacitor 33
₃ is divided by the resistors 34 and 36 and the variable resistor 35, and the divided voltage of the resistor 36 becomes the output voltage V ₁ of the tube current detection circuit 3b and is input to the error amplification circuit 4. Other operations are the same as those of the conventional example shown in FIG.

By constructing the negative feedback control circuit as shown in FIG. 4, when the tube current I increases and the brightness increases, the charging voltage V ₃ of the capacitor 33 also increases and the voltage division of the resistor 36 is increased. Also increases, and the output voltage V ₁ of the tube current detection circuit 3b also increases. Then, the output voltage V ₂ of the differential amplifier 41 decreases, and the time width of Vpwm = 0 of the pulse voltage Vpwm output from the PWM 12 decreases, so that the ON duty of the transistor 11 (the time of one cycle from ON to OFF). The ratio of the ON time in) decreases and the output voltage Vout also decreases. As a result, the tube current I decreases and converges to the set value, and the predetermined brightness is maintained. Further, when the tube current I becomes small, the control reverse to the above is performed and the predetermined brightness is maintained. Further, when it is desired to arbitrarily set the brightness of the cold cathode tube, the resistance value of the variable resistor 35 may be changed. In this case, if the resistance value of the variable resistor 35 is increased, the tube current I increases and the brightness rises.
Further, when the resistance value of the variable resistor 35 is reduced, the tube current I is reduced and the brightness is lowered.

The negative feedback control circuit shown in FIG.
Is controlled to be constant, and when the tube current I is constant, the cold cathode tube FL has a characteristic that the voltage Vcb input to the cold cathode tube FL becomes high when the ambient temperature decreases. The output voltage Vs of 2 increases, and the luminance change of the cold-cathode tube FL only decreases the luminous efficiency due to the temperature change, and the luminance change due to the ambient temperature change becomes smaller than that of the circuit of FIG.

[0021]

However, the above-mentioned conventional CCFL driver circuit has the following problems. The voltage required to start lighting of the cold cathode fluorescent lamps (hereinafter referred to as lighting starting voltage) is higher than the voltage required to maintain lighting after lighting of the cold cathode fluorescent lamps (hereinafter referred to as lighting maintaining voltage),
Further, since the lighting start voltage increases with a decrease in ambient temperature, the output voltage Vs of the inverter circuit 2 needs to be equal to or higher than the voltage at which lighting can be started even at a low temperature. Further, once the cold cathode fluorescent lamp FL is turned on, the voltage applied to the cold cathode fluorescent lamp FL needs to be lowered to the lighting maintaining voltage as described above. That is, during lighting of the cold cathode tube, the output voltage Vs of the inverter circuit 12 is reduced by the voltage Vs-Vcb by the ballast capacitor CB and applied to the cold cathode tube FL. As a result, an extra voltage is applied by the amount of Vs-Vcb. Therefore, it becomes necessary to provide an insulating portion for withstanding voltage around the transformer T,
The shape is larger and the cost is higher.

In consideration of power loss, there is a dielectric loss due to the ballast capacitor CB, and it is necessary to secure the withstand voltage of the transformer T.
The degree of coupling between the high-voltage side winding 29 and the high-voltage side winding 29 is reduced, and the winding ratio of the low-voltage side windings 23 and 24 and the high-voltage side winding 29 is increased to secure a high output voltage. In addition, the leakage current was increased due to the influence of the stray capacitance between the transformer output section and the conductive section around the transformer T, and the loss was increased.

Therefore, a main object of the present invention is to provide a cold-cathode tube lighting device in which loss such as dielectric loss and transformer loss is reduced, the shape is further reduced, and the cost is reduced.

[0024]

In order to achieve the above object, in a cold cathode tube lighting device of the present invention, a DC power source, a stabilizing circuit for stabilizing an effective value of an input voltage, and the stabilizing circuit An inverter circuit that converts the output voltage into a sine wave, a cold cathode tube that is lit by the output voltage of the inverter circuit, and a detection circuit that converts the current flowing through the cold cathode tube into a voltage according to the amount of current and detects the voltage. In a cold-cathode tube lighting device including a negative feedback control circuit configured by an error amplification circuit that amplifies a difference voltage between a detection voltage detected by the detection circuit and a reference voltage, the output section of the inverter circuit and the cold-cathode tube The feature is that the input part of is directly connected.

Further, the circuit for stabilizing the effective value of the input voltage is characterized by comprising a PWM inverter circuit for converting an input DC voltage into a square wave.

Furthermore, the circuit for stabilizing the effective value of the input voltage is characterized by comprising a DC / DC converter circuit for converting an input DC voltage into a DC voltage.

As a result, it is not necessary to use a ballast capacitor, the output voltage of the inverter circuit required to maintain the lighting of the cold cathode tube is lowered, and the size of the transformer is reduced. Also, since the power loss due to the ballast capacitor is eliminated, the withstand voltage of the transformer can be lowered, the degree of coupling between the low voltage side winding and the high voltage side winding is increased, and the turns ratio of the low voltage side winding and the high voltage side winding is small. Therefore, the transformer loss is reduced. Moreover, the leakage current due to the influence of the stray capacitance between the transformer output section and the conductive section around the transformer is reduced, the loss is reduced, and the cost is reduced.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION One of the embodiments of the present invention will be described below with reference to FIG. As shown in FIG. 1, the cold-cathode tube lighting device includes a PWM inverter circuit 1a, an inverter circuit 2, a tube current detection circuit 3b, and a differential amplifier circuit 4.
And a cold cathode tube FL. Since the configuration of each circuit is the same as that of the conventional example, the same numbers are assigned and the description thereof is omitted. In the cold-cathode tube lighting device of the present invention, the output end of the inverter circuit 2 and the cold-cathode tube FL are directly connected, and the output end of the cold-cathode tube is input to the tube current detection circuit 3b. With this configuration, the changes in the circuit operation are as follows.

The output voltage Vs of the inverter circuit 2 is directly
The output voltage Vs applied to the cold cathode fluorescent lamp FL is equal to that of the cold cathode fluorescent lamp FL.
Depends on the tube current-tube voltage characteristic of the. Since the output voltage Vs of the inverter circuit 2 is proportional to the output voltage Vout of the PWM inverter circuit 1a, the PWM inverter circuit 1a
The brightness of the cold cathode fluorescent lamp FL can be adjusted by changing the output voltage Vout of the above and changing the tube current I.

With the above circuit configuration, the lighting sustaining voltage becomes equal to the output voltage Vs of the inverter circuit 2, so that the lighting sustaining voltage is compared to the lighting sustaining voltage of the conventional device, and the ballast capacitor in the conventional device is reduced. It becomes smaller by the applied voltage. For example, assuming that the lighting sustaining voltage of the cold cathode fluorescent lamp FL is 500 Vrms, the output voltage V of the inverter circuit 2 is conventionally due to the existence of the ballast capacitor.
s was required to be 700 to 800 Vrms, but in the present embodiment, Vs is equal to 500 Vrms, that is, the lighting maintaining voltage of the cold cathode fluorescent lamp FL.

From the above, the following effects can be obtained. By eliminating the ballast capacitor, downsizing and cost reduction can be realized. Moreover, since the output voltage of the inverter circuit is greatly reduced, the measure for preventing electric shock can be simplified. In terms of power loss, by eliminating the ballast capacitor, the loss due to the ballast capacitor is eliminated, and the withstand voltage of the transformer can be designed to be low.
The degree of coupling between the low-voltage side winding and the high-voltage side winding is high, and the turn ratio of the low-voltage side winding and the high-voltage side winding is low, so that the transformer loss (iron loss / copper loss) is reduced. Further, since the output voltage of the inverter circuit during lighting is greatly reduced, the leak current due to the influence of the stray capacitance between the transformer output section and the surroundings is reduced, and the loss is also reduced.

The PWM inverter circuit 1a in the cold-cathode tube lighting device has a capacitor 15 as shown in FIG.
The DC / DC converter circuit 1b including the coil 16 and the coil 16 may be used. As a result, the DC voltage is input to the inverter circuit 2 and can be converted into a clearer sine wave in the inverter circuit 2.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a cold cathode tube lighting device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing another example of the stabilizing circuit according to the embodiment of the present invention.

FIG. 3 is a circuit diagram showing an example of a conventional CCFL driver circuit.

FIG. 4 is a circuit diagram showing another example of a conventional CCFL driver circuit.

[Explanation of symbols]

 1 Stabilization circuit 1a PWM inverter circuit 1b DC / DC converter circuit 2 Inverter circuit 3 Detection circuit 4 Error amplification circuit DC DC power supply FL Cold cathode tube

Claims (3)

[Claims] 1. A DC power supply, a stabilizing circuit for stabilizing an effective value of an input voltage, an inverter circuit for converting an output voltage of the stabilizing circuit into a sine wave, and a cold cathode which is turned on by an output voltage of the inverter circuit. From a tube, a detection circuit for converting the current flowing in the cold cathode tube into a voltage according to the amount of current and detecting the voltage, and an error amplification circuit for amplifying the difference voltage between the detection voltage detected by the detection circuit and the reference voltage. A cold-cathode tube lighting device comprising a configured negative feedback control circuit, wherein an output part of the inverter circuit and an input part of the cold-cathode tube are directly connected. 2. The cold-cathode tube lighting device according to claim 1, wherein the stabilizing circuit comprises a PWM inverter circuit that converts an input DC voltage into a square wave. 3. The cold cathode tube lighting device according to claim 1, wherein the stabilizing circuit comprises a DC / DC converter circuit for converting an input DC voltage into a DC voltage.