JPH08288083A - Discharge lamp lighting device - Google Patents

Abstract

PURPOSE: To reduce the cost, and to prevent the influence of the floating capacitance by using a general comparator inexpensively, which can perform the high speed switching operation, for the control of a voltage control means. CONSTITUTION: In a tube current detecting circuit, which is formed of a tube current detecting resistor 10, diodes 11, 12, and a capacitor 13, the voltage generated across the resistor 10 is rectified by the diodes 11, 12, and smoothed by the capacitor 13, and thereafter, input to the non-inverted input of a comparator 17. The comparator 17 compares the stabilized voltage from a direct current power source 14, which is input to the reverse input, with the voltage from the tube current detecting circuit, which is input to the non-inverted input, and amplifies it more, and inputs the amplified voltage to the non-inverted input of a comparator 21. The comparator 21 compares the voltage, which is input from the comparator 17, with the sawtooth wave, which is generated by a comparator 22 and input to the inverted-input, and performs the ON/OFF control of a chopping transistor 2 in response to a result of this comparison. The current is thereby controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a discharge tube lighting device, and more particularly to a discharge tube lighting device suitable for use as a liquid crystal backlight.

[0002]

2. Description of the Related Art As described in Japanese Utility Model Laid-Open No. 5-80191, a conventional device uses a voltage control means provided in a preceding stage of a lighting circuit in which a current flowing through a discharge tube detected by a tube current detection circuit is provided. The voltage is fed back and the voltage is supplied from the voltage control means to the lighting circuit so that the current flowing through the discharge lamp is kept constant. In general, the tube current detection circuit has a configuration in which a tube current is passed through a resistor and a voltage generated across the tube current is output. The discharge tube lighting device having such a configuration is generally called a current feedback type, and is often used for a liquid crystal backlight using a cold cathode discharge tube having a relatively small impedance and a small diameter as a light source. Since the metal reflection film is close to the periphery of the discharge tube used for the liquid crystal backlight, the stray capacitance attached to the discharge tube is large, and the startability is lowered or the characteristics are easily changed due to the influence. However, in the above-mentioned conventional technique, since the current flowing through the discharge tube is directly detected and fed back, the problem due to the stray capacitance is unlikely to occur, and the startability and the characteristics are stable.

[0003]

However, in order to realize the above-mentioned prior art, it is necessary to control the operation of the voltage control means at high speed so as to keep the fed-back tube current value constant. That is, it is generally known that when a discharge tube is lit at a high frequency of several tens of kHz or more, the luminous efficiency of the discharge tube is improved by about 10%, and the discharge tube lighting device used for a liquid crystal backlight is strongly required to be downsized. In order to realize this, the operating frequency of around 30 kHz to around 100 kHz is often selected as the operating frequency for the purpose of operating at a high frequency of several tens of kHz or more to downsize the transformer. As described above, in order to control the operation of the voltage control means that performs a switching operation at a high frequency of several tens of kHz or more, a plurality of high-speed operational amplifiers are required, which has a disadvantage of significantly increasing the cost.
The circuit of the above-mentioned conventional example uses three operational amplifiers.

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and to realize a discharge tube lighting device which is low in cost and less susceptible to stray capacitance.

[0005]

In the above-mentioned conventional example, three operational amplifiers are used for controlling the voltage control means, but as described above, an operational amplifier capable of high-speed switching operation is required. Therefore, by using a general-purpose comparator, which is inexpensive and capable of high-speed switching operation, in this portion, it is possible to minimize the cost increase. Since the output form of a general-purpose comparator is generally an open collector, it is known that it can be used as an amplifier by connecting a pull-up resistor to the output. However, since the comparator is not designed for linear operation like an operational amplifier, the output voltage is not stable just by connecting a pull-up resistor, and therefore the output voltage of the voltage control means is not stable. Light emission becomes unstable. It will appear to flicker visually. Therefore, by connecting a smoothing capacitor to the output of the general-purpose comparator to stabilize the output of the comparator, the output voltage of the voltage control means is stabilized, and stable light emission of the discharge tube is obtained. One of the causes of instability of the comparator output is switching noise of the lighting circuit, and this switching noise is absorbed by the capacitor.

Further, the capacity of the smoothing capacitor is required to have a capacity capable of obtaining a time constant at a frequency lower than several tens of kHz which is the oscillation frequency of the lighting circuit, and a minimum time constant of 100 μs or more is required. Although the output impedance of the comparator depends on the value of the pull-up resistor, it is about 100 kΩ at the maximum, so a minimum capacitance of 0.01 μF or more is required.

[0007]

Further, in the above-mentioned conventional example, the sawtooth wave generating circuit for determining the operating frequency of the control circuit is constructed by using the operational amplifier. In this system, the charge / discharge of the sawtooth wave generating capacitor is switched by an operational amplifier, and the operating frequency is determined by the charge / discharge time constant. Therefore, switching the charging and discharging of the sawtooth wave generating capacitor with a transistor,
A stable operating frequency can be obtained by turning on / off the charge / discharge switching transistor based on the oscillation operation of the lighting circuit. As a result, the number of operational amplifiers used, and in the present invention, the number of comparators, can be reduced, and equivalent performance can be obtained at a lower cost.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of an embodiment of the present invention, in which 1 is a DC power source, 2 is a chopping transistor, 3 is a diode, 4 is a choke coil, 5 is a resistor, 6 and 7 are switching transistors, 8 is a transformer, and 9 is a transformer. Discharge tube, 1
0 is a tube current detection resistor, 11 and 12 are diodes, 1
3 is a capacitor, 14 is a DC power supply, 15 and 16 are resistors, 17 and 21 and 22 are open collector output comparators, 18 is a feedback capacitor, 19 is a pull-up resistor, 20 is a smoothing capacitor, 23 is a time constant resistor, 24 Is a time constant capacitor, 25, 26 and 27 are resistors, 28 is a pull-up resistor, and 29 is a resonance capacitor. The chopping transistor 2 periodically chops the DC voltage of the DC power source 1 and controls the duty of the chopping to control the voltage supplied to the lighting circuits connected thereafter. The collector voltage waveform of the chopping transistor 2 becomes a discontinuous waveform, but due to the inductance of the choke coil 4,
The current waveform of is a continuous waveform, and a continuous tube current flows through the discharge tube 9 which finally becomes a load. The diode 3 is for absorbing the kickback voltage generated in the choke coil 4. The lighting circuit composed of the choke coil 4, the resistor 5, the switching transistors 6 and 7, the transformer 8 and the resonance capacitor 29 is generally called a constant current push-pull type inverter. By feeding back the resonance voltage generated between the windings to the bases of the switching transistors 6 and 7, the switching transistors 6 and 7 are alternately turned on / off to maintain sinusoidal oscillation. The resistor 5 supplies the base current to the switching transistors 6 and 7. On the secondary side of the transformer 8, a sinusoidal voltage boosted by the turns ratio is generated and applied to the discharge tube 9 to light up. The current flowing through the discharge lamp 9 generates a voltage according to the tube current across the tube current detection resistor 10. The tube current detection resistor 10, the diodes 11 and 12, and the capacitor 13 form a tube current detection circuit. Since the voltage generated across the tube current detecting resistor 10 is an alternating current, it is rectified by the diodes 11 and 12, smoothed by the capacitor 13, and input to the non-inverting input of the comparator 17. A voltage obtained by resistance-dividing the DC power supply 14 by the resistors 15 and 16 is input to the inverting input of the comparator 17. If the stability of the DC power supply 14 is high, this voltage also becomes a stable voltage. The stable voltage input to the inverting input of the comparator 17 and the comparator 17
The voltage input to the non-inverting input of the above, that is, the voltage input from the tube current detection circuit is compared, further amplified, and input to the non-inverting input of the comparator 21. Feedback capacitor 1
The frequency characteristic of this amplification factor is defined by 8, and the output of the comparator 17 is stabilized by increasing the amplification factor for DC and decreasing the amplification factor for AC. Further, it is stabilized by the smoothing capacitor 20 and input to the comparator 21. The pull-up resistor 19 is necessary to determine the OFF potential of the output stage transistor of the open collector type comparator 17. Comparator 2
Reference numeral 1 is a comparison between the voltage input from the comparator 17 and the sawtooth wave generated by the comparator 22 and input to the inverting input, and the chopping transistor 2 according to the result.
ON / OFF control of. This will be described in detail with reference to FIG. In FIG. 2, (a) is a sawtooth wave input to the inverting input of the comparator 21, and (b) is the comparator 21.
Indicates the voltage applied to the non-inverting input of. By comparing the two voltages, the waveform of (c) is generated at the output of the comparator 21. In the waveform of (c), the base current of the chopping transistor 2 flows while the voltage is low, and the chopping transistor 2 is turned on. When the tube current increases for some reason, the voltage at the non-inverting input of the comparator 17 becomes high, and the output voltage of the comparator 17 becomes high because it is amplified. Therefore, comparator 2
Among the voltage waveforms output from 1, the period during which the voltage is low, that is, the time during which the chopping transistor 2 is turned on becomes short. Therefore, the voltage input to the lighting circuit is reduced, and control is performed in the direction of reducing the tube current. In this way, the value of the tube current is kept constant. Now, the generation of the sawtooth wave shown by the waveform (a) in FIG. 2 will be described below. The sawtooth wave is generated by the comparator 22, and this circuit is a known circuit. For example, when the sawtooth wave, that is, the voltage across the time constant capacitor 24 shown in FIG. 2A is low and the output of the comparator 22 shown in FIG.
The voltage synthesized from the resistors 25, 26, 27 and 28 and input to the non-inverting input of the comparator 22 has the voltage level shown in (d). Therefore, the time constant capacitor 24 is charged through the time constant resistor 23 and the voltage gradually rises. The voltage across the time constant capacitor 24 is (d)
When the voltage level shown by is reached, the comparator 22 inverts and the output becomes a low voltage level. As a result, resistors 25 and 2
The voltage synthesized from 6, 27 and 28 and input to the non-inverting input of the comparator 22 changes to the voltage level shown in (e). Therefore, this time, the time constant capacitor 24 is discharged through the time constant resistor 23, and the voltage gradually decreases. When the voltage across the time constant capacitor 24 reaches the voltage level indicated by (e), the comparator 22 is inverted again. After that, the above operation is repeated to generate a sawtooth wave.

FIG. 3 is a circuit diagram of another embodiment of the present invention.
Reference numeral 30 is a resistor, 31 is a capacitor, 34 and 35 are transistors, and 32 is a base resistance of the transistor 34. In addition, the same reference numerals as those in FIG. 1 indicate the same or equivalent portions. The circuit of FIG. 3 has the same overall operation as the circuit of FIG. 1, but the circuit for generating the sawtooth wave is simplified and the number of comparators used is reduced to two. FIG.
The operation will be described with reference to the waveform diagram of FIG. FIG. 4A shows a voltage waveform of a portion between the choke coil 4 and the transformer 8, which is shown in FIG.
(B) The transistor 35 is turned on when the voltage is higher than the above.
Therefore, the collector of the transistor 35 is shown in FIG.
The voltage shown in (c) is generated. In the waveform of FIG. 4C, the transistor 34 turns on when the voltage is at a high level. For example, let's follow the waveform from the moment when the transistor 34 changes from ON to OFF. When the transistor 34 is turned off, the capacitor 31 starts charging via the resistor 30, and the transistor 34 is turned on again according to the waveform shown in FIG.
The voltage rises until, and when the transistor 34 is turned on, the capacitor 31 is reset. By inserting a resistor in series with the transistor 34, the slope of the voltage when the capacitor 31 is reset becomes gentle. Thereafter, this operation is repeated to form a sawtooth wave. This sawtooth wave and the comparator 21
Is compared with the voltage of the non-inverting terminal of the above, and the voltage of FIG. 4 (f) is generated at the output of the comparator 21 to control the tube current.

[0010]

As described above, according to the present invention, a current feedback type circuit can be realized at a low cost, and a liquid crystal backlight having stable characteristics even when the discharge tube is used in a state where the stray capacitance is large. Can be provided.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is an operation explanatory diagram of the circuit of FIG.

FIG. 3 is a circuit diagram of another embodiment of the present invention.

FIG. 4 is an explanatory diagram of the operation of the circuit of FIG.

[Explanation of symbols]

2 ... Chopping transistor, 9 ... Discharge tube, 17 ... Comparator, 20 ... Smoothing capacitor, 21 ... Comparator, 22 ... Comparator, 34, 35 ... Transistor.

Claims (3)

[Claims] 1. A DC power supply, a DC-DC converter, and the D.
DC-AC for converting the output voltage of the C-DC converter into alternating current
A converter, a discharge tube connected to the output of the DC-AC converter, and means for detecting a tube current flowing through the discharge tube,
The DC-D is adjusted so that the tube current value becomes a predetermined current value.
In a discharge tube lighting device comprising a control circuit for controlling an output voltage of a C converter, the control circuit has a first comparator to which an output of a means for detecting the tube current is connected, and an output for the first comparator. A discharge tube lighting device, comprising: a second comparator connected to a DC-DC converter, wherein a resistance and a first capacitor are connected in parallel to the output of the first comparator. 2. A second capacitor, a resistor and a transistor are connected to an input terminal of the second comparator according to claim 1, and the transistor is turned on / off according to a resonance operation waveform of the DC-AC converter. A discharge tube lighting device, which is turned off to input a sawtooth wave to another input terminal of the second comparator. 3. The capacitance value of the first capacitor is 0.01
Discharge tube lighting device characterized by being at least μF.