JPH06296370A - Lighting apparatus of discharge lamp - Google Patents

Abstract

PURPOSE:To provide a lighting apparatus of discharge lamp which can almost simultaneously light a couple of discharge lamps even under the low luminance condition and can change luminance in a wider range. CONSTITUTION:An output of an inverter 1 is detected with a resistor R6 of a discharge lamp current detector 3 and is then input to one input of a comparator COM1 of a luminance control part 2 and a luminance adjusting voltage in input to the other input thereof. When a luminance adjusting voltage is high, a transistor Q4 turns on, a switching frequency is increased by C2 and R9 and the inverter 1 operates continuously for obtaining average duty of continuity and breaking in the switching operation in order to adjust luminance. When a luminance adjusting voltage is low, the transistor Q4 turns off, the switching frequency is deceased by C2 and (R9+R13) and the inverter 1 intermittently operate, providing a sufficiently high voltage output when a transistor Q3 is in the on state.

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 having a dimming function for a discharge lamp.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram of a discharge lamp lighting device having a dimming function by current feedback in a conventional example. In the figure, C1 to C6 are capacitors and COM1
Is a comparator, CR1 to CR3 are diodes, E1 to E4 are output terminals, L1 is a reactor, Q1 to Q3 are transistors, R1 to R3 and R6 to R10 are resistors, U1 is a dimming control integrated circuit, and T1 is a transformer. . In the figure, an inverter unit 1 surrounded by a broken line is composed of transistors Q1, Q2,
Capacitors C1, C4, C5, reactor L1, resistor R1
And a push-pull circuit including a transformer T1 and the like. When the input DC voltage is applied to the terminals of + V _IN and GND, an AC voltage is generated on the secondary side of the transformer T1 of the inverter unit 1. This output AC voltage is supplied to two discharge lamps as loads connected to the terminals E1 to E4.

The brightness control section 4 surrounded by a broken line includes a dimming control integrated circuit U1, capacitors C2 and C3, and a diode CR.
1, transistor Q3, resistors R2, R3, R8 to R10
Etc. The discharge lamp current detection unit 3 surrounded by a broken line is composed of diodes CR2 and CR3 and a resistor R6. Comparator COM1 of dimming control integrated circuit U1
A voltage drop amount of the discharge lamp current detection resistor R6 after filtering by the resistor R7 and the capacitor C6, and a luminance adjustment voltage as a reference voltage via the resistor R10 are input to the. That is, with the brightness adjustment voltage as a reference voltage, the discharge lamp current flowing through the discharge lamp, which is a load, is negatively fed back to the voltage level detected by the resistor R6. The duty is changed to average the conduction / interruption duty of the transistor Q3 to make the average current of the reactor L1 constant and to perform dimming. The switching frequency in this case is constant.

[0004]

However, in the conventional lighting device for a discharge lamp, when two discharge lamps are connected to the output terminals E1 to E4, a brightness adjusting voltage for lighting one and a brightness adjusting voltage for lighting two lights. The difference with the voltage is large. Further, when the brightness adjustment voltage is gradually lowered from the state where the two lamps are turned on in the rated operation, one of them is turned off first. This phenomenon is caused by the variation in the discharge voltage of the discharge lamp and the characteristic of the discharge lamp that the impedance is lower after lighting than before lighting. That is, if the brightness adjustment voltage is increased from zero volt,
The discharge lamp with a low starting voltage lights up first. Then, the impedance of the discharge lamp is lowered and the discharge lamp current is about to increase. However, the discharge lamp current is controlled to be constant by the control loop that applies negative feedback, and the output voltage decreases. This makes it difficult for the discharge lamp with the higher starting voltage to light. Further, when the brightness adjustment voltage is lowered from the state where the two lamps are turned on, the output voltage is lowered, and there is a problem that the discharge lamp with the higher discharge voltage is turned off first.

The present invention has been made in view of the above circumstances, and it is possible to light two discharge lamps almost at the same time even when the brightness is low, and the brightness can be varied over a wider range. An object of the present invention is to provide a lighting device.

[0006]

In order to solve the above problems, a discharge lamp lighting device according to the present invention is a discharge lamp lighting device comprising an inverter circuit for supplying an AC output to the discharge lamp. When the value becomes equal to or less than a certain value, a switching frequency conversion means for changing the switching frequency for switching the DC voltage from a high frequency to a low frequency, and a first dimming for averaging the conduction / interruption duty in the high frequency switching. The light control means and the second light control means for dimming by generating a high voltage by the intermittent operation of the inverter circuit by low frequency switching are provided, and the light control means is selected according to the brightness adjustment voltage level to reduce the operation duty. The feature is that it outputs a high voltage sufficient to turn on the discharge lamp even if the brightness decreases.

[0007]

By lowering the switching frequency from several tens of KHz to several hundreds of Hz at low brightness, the current of the reactor, which was used to flow a constant current through the inverter section, becomes discontinuous and the inverter section operates intermittently. As a result, the voltage of the capacitor charging the feedback voltage decreases while the inverter unit is off, the pulse width is fully opened at the next on time, and a sufficiently high output voltage for discharging is supplied to the two discharge lamps. .

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. It is a circuit diagram of a lighting device of a discharge lamp in one example of the present invention. The same reference numerals as those in FIG. 2 indicate the same elements, and the description thereof will be omitted. Basically different from FIG. 2 of the conventional example, a transistor Q4, a diode CR4, and resistors R11 to R13 are added to the brightness control unit 2, and switching is performed at a high frequency determined by the time constants of C2 and R9 depending on the brightness adjustment voltage level. It is configured to select the operation and the switching operation at a low frequency determined by the time constant of C2 and (R9 + R13). This is a switching frequency conversion means that changes the switching frequency according to the brightness adjustment voltage level. However, this embodiment is an example embodying the technical idea of the present invention, and does not specify the circuit configuration.

In this embodiment, when the brightness adjustment voltage level is equal to or higher than the base-emitter voltage V _{BE of the} transistor Q4 and the forward voltage V _F of the diode CR4, the transistor Q4 is turned on and the switching frequency is C2 and R9. It operates at a high frequency determined by a constant, for example, 50 KHz.
The inductance of the reactor L1 is set to such a value that the primary current at the lowest brightness at this high frequency is within the critical current of the reactor L1. Then, the current flowing through the reactor L1 passes through the transistor Q3 while the transistor Q3 is on, and continues through the diode CR1 while the transistor Q3 is off. Therefore, the inverter section operates continuously. This is the first dimming means for averaging the duty of conduction / interruption in high-frequency switching and dimming.

On the other hand, when the brightness adjustment voltage is lowered and the transistor Q4 is turned off, the switching frequencies are C2 and (R9
+ R13) Low frequency determined by the time constant, eg 300
Operates at Hz. The critical current of the reactor L1 is related to the frequency, and the lower the frequency, the higher the critical current.
In other words, at low frequency critical current, the reactor L
The magnetic flux of the iron core of No. 1 becomes discontinuous. Therefore, when the transistor Q3 is turned off, the voltage applied to the transistors Q1 and Q2 becomes an attenuation waveform after a higher voltage is applied than when the magnetic flux was operating continuously. When the switching frequency is extremely lowered, the transistor Q1 is completely attenuated and the transistors Q1 and Q2 are turned off, and then the transistor Q3 is turned on, so that the inverter section operates intermittently. As a result, the voltage drop of the discharge lamp current detection resistor R6 of the discharge lamp current detection section is reduced and the voltage of the capacitor C6 charging the feedback voltage is reduced while the inverter section is off. When turned on, the pulse width is fully opened, and a sufficiently high output voltage for discharging can be obtained. This is a second dimming unit for dimming by generating a high voltage by the intermittent operation of the inverter circuit by low frequency switching. In this way, a high voltage sufficient to turn on the discharge lamp can be output even if the operation duty is reduced and the brightness is reduced.

[0011]

As described above, in the discharge lamp lighting device according to the present invention, the switching frequency conversion for changing the switching frequency for switching the DC voltage from the high frequency to the low frequency when the brightness adjustment voltage level becomes a certain value or less. Means, a first dimming means for averaging the duty of conduction / interruption in high frequency switching, and a second dimming means for generating a high voltage by intermittent operation of an inverter circuit by low frequency switching. And a dimming unit, which is configured to select the dimming unit according to the brightness adjustment voltage level and output a high voltage sufficient to light the discharge lamp even when the operation duty becomes small and the brightness decreases. Since two discharge lamps can be lit at almost the same time even when the brightness is low, by extending the intermittent period, the conventional method You can vary the luminance in a wider range than.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a lighting device for a discharge lamp according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a discharge lamp lighting device in a conventional example.

[Explanation of symbols]

 1 Inverter part 2, 4 Brightness control part 3 Discharge lamp current detection part C1-C6 Capacitor COM1 Comparator CR1-CR4 Diode E1-E4 Terminal L1 Reactor Q1-Q4 Transistor R1-R3, R6-R13 Resistance U1 Dimming control integrated circuit T1 transformer

Claims (1)

[Claims] 1. A lighting device for a discharge lamp comprising an inverter circuit for supplying an AC output to the discharge lamp, wherein the switching frequency for switching the DC voltage is changed from a high frequency to a low frequency when the brightness adjustment voltage level falls below a certain value. Switching frequency conversion means for controlling, a first dimming means for averaging the duty of conduction / interruption in high frequency switching, and dimming by generating a high voltage by intermittent operation of an inverter circuit by low frequency switching. And a second dimming means for selecting the dimming means according to the brightness adjustment voltage level, and outputting a high voltage sufficient to light the discharge lamp even if the operation duty becomes small and the brightness decreases. A lighting device for a discharge lamp.