JPH10340788A - Discharge lamp lighting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable stable lighting of a plurality of discharge lamps at low cost by detecting a current flowing through the discharge lamp, arranging a current detecting means even in a discharge lamp to which current is not fed back, judging the lighting of the discharge lamp based on a detected current value, and stopping the supply of electricity to the discharge lamp when it is not lit. SOLUTION: In the case that a discharge lamp 14 is not lit for some reason, since voltage is not generate at both ends of a resistance 32, a transistor 45 is not turned on. Therefore, a capacitor 41 is charged via the resistance 40 with the voltage of a DC power source 1, the base voltage of a transistor 44 reaches on-voltage after fixed time has elapsed. As a result, a transistor 26 and a switching transistor 2 are turned off. Voltage does not generate on the secondary side of a transformer 7. The tube current of the discharge lamp 14 to which the tube current is not fed back is detected with a resistance 28, the operation of the switching transistor 2 is stopped, and high degree of safety can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art A conventional power supply for lighting a discharge lamp is disclosed in Japanese Utility Model Laid-Open No. 5-801.
As described in JP-A-91, the tube current value detected by the tube current detection circuit is converted into a voltage and fed back to the voltage control means provided in the preceding stage of the lighting circuit so that the tube current flowing through the discharge lamp is constant. Operation of the voltage control means so that
M control was performed.

In the prior art, even if the power supply voltage fluctuates, the voltage supplied to the lighting circuit is controlled to be constant by controlling the open / close duty of a switch element provided in the voltage control means. In this configuration, a constant tube current always flows.

[0004]

In the above-mentioned prior art, for example, when two discharge lamps are turned on, the following method is conceivable.

(1) A method in which the tube current is detected separately for each of the two discharge lamps, and the voltage control means corresponding to each of the discharge lamps is separately controlled to control the current flowing through the discharge lamps.

[0006] (2) Only the tube current of one of the two discharge lamps is detected, and power is supplied to one of the two discharge lamps from one voltage control means. One voltage control means is controlled so as to have a value. By branching the output of one transformer with a ballast capacitor having the same capacity and supplying the tube current to the two discharge lamps, the same current is supplied to the discharge lamp that does not detect the tube current.

Although the method (1) is complicated in terms of circuit configuration, even if there is a large variation in the characteristics of the discharge lamp and a large variation in the stray capacitance around the discharge lamp, feedback control is performed by detecting the tube current separately. Therefore, there is a feature that variation in tube current can be reduced. In addition, when one of the discharge lamps did not light, the tube current detection circuit did not detect the tube current, so that the discharge lamp was not lit. When the state where the discharge lamp is not turned on for a certain time or more is continued, it is easy to prevent the danger of continuing to output a high voltage by stopping the operation of the circuit.

However, since each discharge lamp must have a tube current detection circuit and a voltage control means, the cost is high. In particular, the cost increases as the number of lit heads increases.

On the other hand, in the method (2), a single tube current detection circuit and voltage control means can turn on multiple discharge lamps, so that the cost can be reduced. In addition, since the current flowing through one of the plurality of discharge lamps is fed back, the device operates stably against disturbances such as a change in discharge lamp characteristics due to a change in ambient temperature and a change in power supply voltage. The discharge lamps other than the discharge lamp detecting the tube current are branched by the ballast capacitor, and the current substantially equal to the current of the discharge lamp detecting the tube current flows. Compared with the method (1), the variation in the tube current tends to increase due to the variation in the discharge lamp characteristics and the variation in the stray capacitance around the discharge lamp.
Therefore, the method (2) is an excellent method capable of stably lighting a plurality of discharge lamps at low cost.

However, in the case of the method (2), if the discharge lamp which does not detect the tube current does not light, it can be easily detected that the lamp is not lit as in the method (1). Therefore, a high voltage is continuously output to both ends of the discharge lamp. For example, since the lamp is not lit, there is a risk of electric shock when the user attempts to replace the discharge lamp.

When the discharge lamp on the side where the tube current is detected does not turn on, the control circuit moves in a direction to increase the output, but at that time the discharge lamp on the side where the tube current is not detected is turned on. In this case, the operation is performed with a current value larger than a predetermined current value. As a result, abnormal heat generation occurs, and in extreme cases, there is a danger of smoke or ignition. A similar danger exists when returning the sum of the two tube currents.

[0012]

In order to solve the above problems, the present invention has the following configuration.

That is, in the above method (2), a current detecting means is provided also for the discharge lamp which does not detect and feed back the current flowing through the discharge lamp, and the discharge lamp is determined by the current value detected by the current detecting means. It can be realized by determining whether or not the lamp is lit, and when not lit, stopping the power supply to the discharge lamp.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of an embodiment of the present invention.
Is a DC power supply, 2 is a switching transistor, 3 is a choke coil, 4 is a resistor, 5 is a base resistance of the switching transistor 2, 6 is a diode, 7 is a transformer, 8 is a resonance capacitor, 9 and 10 are transistors, 11 and 13 are Ballast capacitors, 12 and 14 are discharge lamps, 15 and 28 are tube current detection resistors, 16 and 29
Is a rectifier diode, 17 and 30 are smoothing capacitors,
18, 19, 31 and 32 are resistors, 20 is an error amplifier, 21 is a reference voltage source, 22 is a feedback capacitor, 23 is a sawtooth wave generating circuit, 24 is a voltage comparator, 25 is a base resistor of a transistor 26, 33, 33 35, 36, 39, 4
0, 42 and 43 are resistors, 34 and 41 are capacitors, and 37, 38, 44 and 45 are transistors.

The DC voltage output from the DC power supply 1 is chopped by the switching transistor 2, further smoothed by the choke coil 3, and input to the transformer 7 as a DC voltage according to the duty of the switching transistor 2. A circuit composed of the transistors 9 and 10, the resonance capacitor 8, the resistor 27, and the transformer 7 is a so-called push-pull voltage resonance circuit, and a sine wave oscillation operation is performed at a frequency substantially determined by the inductance of the transformer 7 and the capacitance value of the resonance capacitor 8. I do. The DC voltage input to the transformer 7 is converted into an AC voltage according to the voltage value by the push-pull voltage resonance circuit. Further, this AC voltage is boosted to a secondary voltage corresponding to the turns ratio of the transformer 7 and applied to both ends of the discharge lamps 12 and 14 via the ballast capacitors 11 and 13, so that the discharge lamps 12 and 1
4 lights up. When the discharge lamps 12 and 14 are turned on, a tube current determined by the impedance of the ballast capacitors 11 and 13 and the secondary voltage of the transformer 7 flows through the discharge lamps 12 and 14.

The tube current flowing through the discharge lamp 12 is converted to a voltage value proportional to the current value by a tube current detection resistor 15, rectified and smoothed by a diode 16 and a capacitor 17, and
And 19 to the error amplifier 20. The error amplifier 20 outputs to the voltage comparator 24 a voltage obtained by amplifying the difference between the voltage value corresponding to the input tube current and the voltage value of the reference voltage source 21. The voltage comparator 24 compares this voltage with the output voltage of the sawtooth wave generator 23, and outputs the result of the comparison.

The triangular waveform shown in FIG. 2A is the output voltage waveform of the sawtooth waveform generator 23, and the solid straight line is the output voltage waveform of the error amplifier 20. FIG. 2B shows an output voltage waveform of the voltage comparator 24. The switching transistor 2 is turned on because the transistor 26 is turned on while the voltage is high, and the switching transistor 2 is turned off while the voltage is low. If the voltage of the DC power supply 1 increases, the voltage input to the transformer 7 also increases, so that the tube current flowing through the discharge lamp 12 increases. For this reason, FIG.
As indicated by the broken straight line, the output voltage of the error amplifier 20 increases. Therefore, the high voltage period of the output voltage waveform of the voltage comparator 24 changes in the direction of shortening, the duty of the switching transistor 2 decreases, the voltage input to the transformer 7 is suppressed, and the current flowing through the discharge lamp 12 is reduced. Be kept constant. If the ballast capacitors 11 and 13 have the same capacity and the discharge lamps 12 and 14 have substantially the same characteristics, almost the same current as the current flowing through the discharge lamp 12 flows through the discharge lamp 14. Therefore, even if there is disturbance such as a change in the voltage of the DC power supply 1, the current flowing through the discharge lamps 12 and 14 is kept substantially constant.

When the discharge lamp 12 is not lit for some reason, for example, when the discharge lamp is damaged, no voltage is generated across the resistor 19, so that the transistor 38 does not turn on. Therefore, the capacitor 34 is charged with the voltage of the DC power supply 1 via the resistor 33. Resistance 3
The voltage across the capacitor 34 gradually increases with the time constant of the resistance of the resistor 3 and the capacitance of the capacitor 34, and after a lapse of a certain time, the base voltage of the transistor 37 reaches the ON voltage. Therefore, the transistor 37 is turned on, and the transistor 26 and the chopping transistor 2 are turned off. When the chopping transistor 2 is turned off, no voltage is generated on the secondary side of the transformer 7. For this reason, if the discharge lamp 12 is not turned on, the secondary voltage of the transformer 7 is not generated after a certain period of time, so that it is safe to accidentally touch the secondary side.

As described above, since the tube current on the side where the tube current is detected and fed back is constantly monitored, it can be reliably detected that the discharge lamp has not been turned on. However, if the discharge lamp on the side that does not feed back the tube current is not lit by this alone, this cannot be detected, and there is a risk of accidentally touching the secondary side and causing an electric shock.

Therefore, the tube current is detected and fed back, the tube current of the discharge lamp 14 is also detected by the resistor 28 and the like, and the operation of the chopping transistor 2 is stopped by the same operation as when the discharge lamp 12 is not turned on. High security can be obtained.

That is, when the discharge lamp 14 does not light for some reason, no voltage is generated across the resistor 32, so that the transistor 45 does not turn on. Therefore, the capacitor 41 is charged with the voltage of the DC power supply 1 via the resistor 40. The voltage between both ends of the capacitor 41 gradually increases with the time constant of the resistance of the resistor 40 and the capacitance of the capacitor 41, and after a certain period of time, the base voltage of the transistor 44 reaches the ON voltage. Therefore, the transistor 44 is turned on, and the transistor 26 and the chopping transistor 2 are turned off.
Becomes When the chopping transistor 2 is turned off, no voltage is generated on the secondary side of the transformer 7. In this way, a safe discharge lamp lighting device can be realized by detecting the tube current including the discharge lamp that does not feed back the tube current.

FIG. 3 is a circuit diagram of a second embodiment of the present invention.
In FIG. 3, reference numeral 46 denotes a second transformer. Other Figure 1
The same reference numerals indicate the same or equivalent parts. In the embodiment of FIG. 1, two discharge lamps are connected in parallel to the transformer 7, but in the embodiment of FIG. 3, two discharge lamps are connected in series to the transformer. Transformer 7 and transformer 46
Are connected as shown in FIG. 3, and at the moment when a positive polarity voltage is generated at the winding start terminal of the transformer 7 (indicated by a black circle), a negative polarity voltage is symmetrically generated at the winding start terminal of the transformer 46. ing. For this reason, the connection point between the secondary side of the transformer 7 and the secondary side of the transformer 46 always shows a value close to the GND potential. Other operations are the same as those in the embodiment of FIG.

In FIG. 3, since the two discharge lamps are connected in series, the secondary voltage also needs to be twice as high, so that the necessary output voltage is output using two transformers. If a transformer with a large output voltage is used, it can be realized with one transformer. In any case, if one of the two discharge lamps does not light for some reason, the operation of the circuit stops, so that a safe discharge lamp lighting device can be realized.

[0024]

According to the present invention as described above,
Even if one of the multiple discharge lamps does not light for some reason, it can be detected reliably, the operation of the device can be stopped, and a high voltage does not continue to be generated, so a safe lighting device Can be realized.

[Brief description of the drawings]

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

FIG. 2 is an operation waveform diagram of one embodiment of the present invention.

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

[Explanation of symbols]

REFERENCE SIGNS LIST 1 DC power supply 2 Switching transistor 3 Choke coil 4 Resistance 5 Base resistance of switching transistor 6 Diode 7 Transformer 8
Resonant capacitor, 9 transistor, 10 transistor, 11 ballast capacitor, 13 ballast capacitor, 12 discharge lamp, 14 discharge lamp, 15 tube current detection resistor, 28 tube current detection resistor, 16 rectifier diode
29: rectifier diode, 17: smoothing capacitor, 30 ...
Smoothing capacitor, 18 resistor, 19 resistor, 31 resistor, 32 resistor, 20 error amplifier, 21 reference voltage source, 22 feedback capacitor, 23 sawtooth wave generation circuit,
24: voltage comparator, 25: base resistance of transistor 26, 33, 35, 36, 39, 40, 42, 43 ... resistance, 34, 41 ... capacitors, 37, 38, 44, 45
... transistors.

Claims (2)

[Claims] 1. A discharge lamp lighting device for lighting a plurality of discharge lamps using a DC power supply as a power supply, comprising: means for detecting all current values flowing through the plurality of discharge lamps for each discharge lamp; A part of the current value detected by the means for detecting the current flowing through the lamp is fed back to the control circuit to control the switching duty of one switch element so that the current flowing through the discharge lamp becomes a predetermined value. Having a function of controlling, if all the current values flowing through the plurality of discharge lamps are detected by the means for detecting each discharge lamp individually, the current value flowing through at least one discharge lamp is smaller than a predetermined value; Alternatively, a discharge lamp lighting device comprising means for stopping power supply to some discharge lamps. 2. The discharge lamp lighting device according to claim 1, wherein all of the current values flowing through the plurality of discharge lamps flow through at least one of the current values detected by the means for detecting each discharge lamp. Means for stopping power supply to all or some of the discharge lamps when the state in which the current value is smaller than the predetermined value has continued for a predetermined time or more, or interrupted.