JPH08111289A - Discharge lamp lighting device - Google Patents

Abstract

PURPOSE: To ensure the safety of a circuit by sensing over-voltage, and controlling the pause period in oscillation of the output voltage from a DC/DC converter provided in an inverter. CONSTITUTION: When a voltage 41 between the primary side of a transformer 9 and a choke coil 4 or a voltage 23 sensed by diodes 14, 15 in a tube current sensor is sensed as an over-voltage, a transistor 22 is turned on to change the resistance value for controlling the voltage between the dead time control terminal of IC 19 and GND. Thereby oscillation is continued in the condition that the IC is stopped oscillating or the duty cycle is low, and either the circuit operation should be ceased or the voltages at the over-voltage sensing points 41 and 23 are sunk to the safe state. Thus the safety of elements in the circuit is ensured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lighting device for a discharge lamp.

[0002]

2. Description of the Related Art A conventional discharge lamp lighting device is an actual Kaihei 5-80.
As described in Japanese Patent No. 1911, the tube current detected by the tube current detection circuit is fed back to the voltage control means provided in the preceding stage of the lighting circuit to control the voltage so that the tube current flowing through the discharge tube becomes constant. The means is operated and the lighting voltage is supplied to the lighting circuit. The tube current detection circuit is usually configured to flow a tube current through a resistor and output a voltage generated across the resistor. The discharge lamp lighting device as described above is generally called a current feedback type lighting circuit, and like a liquid crystal backlight using a cold cathode discharge tube of a small diameter with a relatively high impedance as a light source, a metal is provided around the discharge tube. It has the feature that it is possible to obtain a lighting state with stable startability and characteristics, which is less likely to cause a problem that the startability is lowered or the characteristics are likely to change due to the influence of stray capacitance caused by the close proximity of the reflective film There is.

[0003]

According to the above method, the discharge tube has a lamp starting voltage which is higher than the state when the discharge tube is lit, as a lamp starting voltage from the start of the discharge lamp lighting device to the start of lighting of the discharge tube. It is necessary to apply a high voltage. At this time, in the transformer of the inverter, when the power supply voltage input to the primary side is higher than the expected voltage, the voltage output to the secondary side becomes higher than a predetermined voltage, and corona discharge may occur. At this time, there is a risk that the transformer will emit smoke or fire. Further, when the circuit element in the tube current detector included in the inverter is opened or short-circuited, the current flowing in the discharge tube becomes an overcurrent than in a steady state, and the circuit element of the DC / DC converter included in the inverter or the discharge tube. There is a risk that the temperature of the product itself will rise and cause smoke or ignition.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a highly safe discharge lamp lighting device having a protection circuit for suppressing the corona discharge of an inverter transformer and the current flowing in a discharge tube below a predetermined current.

[0005]

In order to achieve the above object, a circuit for detecting a voltage on the primary side of a transformer and an output voltage of a tube current detection circuit in a discharge lamp lighting device is provided, and one of them is equal to or higher than a predetermined voltage. Then, the output voltage of the primary side of the transformer or the tube current detection circuit is made to be equal to or lower than a predetermined voltage by prolonging the oscillation idle period at the output voltage of the DC / DC converter of the discharge lamp lighting device. Can achieve the purpose.

[0006]

That is, the voltage on the primary side of the transformer in the discharge lamp lighting device is detected. When the detected voltage exceeds a predetermined voltage, the duty of the switching circuit of the DC / DC converter is limited to a predetermined value or less. Limiting the duty can be achieved by changing the input voltage to the idle period limiting circuit. When the input voltage is low, the idle period limit circuit reduces the maximum duty that the switching circuit can operate, and when the input voltage is high, the maximum duty that the switching circuit can operate is large. To work.

When a component of the voltage control means for keeping the tube current constant fails, the tube current increases and the transformer and the choke coil generate heat. However, it is impossible to detect the voltage on the primary side of the transformer alone because the voltage change in this case is minute.

Therefore, the abnormal voltage is determined by also including the output voltage of the tube current detection circuit. Originally, this part has a function of being kept at a constant voltage, so that even a slight change can be detected.

[0009]

EXAMPLE An example of the present invention will be described with reference to FIG. In the figure, 1 is a DC power supply, 2 is a chopping transistor, and 3
Is a diode, 4 is a choke coil, 5 and 6 are switching transistors, 7 is a base resistance of the transistors 5 and 6, 8 is a resonance capacitor, 9 is a transformer, 10 is a ballast capacitor, 11 is a discharge lamp, and 12 is a tube current detection resistor. A tube current detector composed of 13, a diode 14 and 15, a capacitor 16, a tube current detection resistor 17 and 18, a control IC 19 for controlling the duty of the chopping transistor 2, and 20 and 36 a dead time control voltage value. A resistor for determining, 21 is a timing resistor for determining a reference triangular wave, 24 resistors and 25 and 26 capacitors are for setting the gain of the IC, 27 is a base resistor of the chopping transistor 2, 28 is a variable for changing the current flowing in the discharge lamp 11. It is resistance.

The DC voltage of the DC power supply 1 is chopped by the chopping transistor 2 and reduced to a voltage according to the chopping duty. The reduced DC voltage is input to the transformer 9 via the choke coil 4. The diode 3 is for absorbing a surge voltage generated in the choke coil 4 by chopping. Transformer 9, switching transistors 5 and 6,
The base resistor 7 and the resonance capacitor 8 are so-called push-pull type voltage resonance type inverter circuits, so that the switching transistors 5 and 6 are alternately turned ON / OFF to cause a resonance current to flow between the transformer 9 and the resonance capacitor 8 and a DC voltage. To AC voltage. A voltage according to the winding ratio of the transformer 9 is generated on the secondary side of the transformer 9, and is applied to the discharge lamp 11 via the ballast capacitor 10 to light the discharge lamp 11.

When the discharge lamp 11 is turned on, the tube current detector 1
A voltage corresponding to the value of the tube current flowing through the discharge lamp 11 is generated at the output of 2. The AC tube current is detected as a DC voltage by the diodes 14 and 15. From the tube current detector 12, when the transistor 19 is ON, the resistors 13 and 17 are connected.
A voltage value obtained by subtracting the forward voltage of the diode 15 from the value obtained by multiplying the combined resistance value of 18 and 18 by the tube current value is output. Then, a voltage corresponding to the difference between the output voltage of the tube current detector 12 and the reference voltage of the reference voltage source 32 inside the IC is output from the error amplifier 29. The solid line in FIG. 2A is the error amplifier 2
9 is an output waveform of No. 9. Next, the output voltage of the error amplifier 29 and the output voltage of the reference triangular wave generator 33 as shown by the solid broken line in FIG. The output voltage waveform of the comparator 30 is the solid line waveform of FIG. The output voltage of the comparator 30 is
When the voltage level is ON in FIG. 2, the transistor 31 is ON, and when the output voltage of the comparator 30 is the voltage level OFF, the transistor 31 is OFF. When the transistor 31 is turned on, the chopping transistor 2 is turned on.

When the voltage 41 between the primary side of the transformer and the choke coil in this circuit becomes overvoltage compared to the normal state, that is, when the discharge tube 11 is in a no-load state, the transistor 22 is turned on. At this time, in order to set how much voltage 41 between the primary side of the transformer and the choke coil becomes an overvoltage, the resistance is divided by the resistors 38 and 39 in order to set the switching of the transistor 22. The current flowing between the dead time control terminal of the IC 19 and the GND of the IC 19 is the triangular wave oscillator 3
Since it is the reference wave from 3, it is always constant, and the transistor 22 is normally OFF. Therefore IC1
9 dead time control terminal and IC19 GND
The voltage between them depends on the combined resistance value of the resistors 20 and 36. At this time, the relationship between this voltage and the duty with which the oscillation of the IC can operate becomes 0% when the voltage is 0.7 V or less and 100% when the voltage is 1.3 V or more. At the voltage 41 between the temporary side of the transformer and the choke coil, an overvoltage is detected and the transistor 22 is turned on.
When it becomes N, the voltage of the dead time control of the IC 19 depends only on the value of the resistor 20. For this reason, the voltage of the dead time control is lower than that in the normal state because the current is constant and the impedance is reduced. Therefore, the output of IC19 is ON
The duty cycle will be lower. Resistance 2
Depending on the setting of 0 and 36, the IC 19 then stops oscillating or continues oscillating in a low duty cycle state. As a result, the circuit stops operating, or the voltage 41 between the primary side of the transformer and the choke coil drops to a voltage at which the detection circuit does not work, that is, a voltage at which the transistor 22 is turned off. For this reason, the voltage applied to the primary side of the transformer 9 does not exceed a certain voltage value, so that abnormal heat generation, smoke emission, or ignition of the transformer 9 can be prevented. At the same time, since the output voltage of the secondary side of the transformer 9 is also limited, it is possible to take measures for preventing the corona discharge of the transformer.

However, with the above circuit method alone, when the resistor 24 or the capacitors 25 and 26 are short-circuited, a larger amount of current flows through the discharge lamp 11 when the discharge lamp 11 is lit than in the steady state. At this time, the voltage 41 between the primary side of the transformer and the choke coil also becomes an overvoltage as compared with the steady lighting, but if the detection is set to work at this voltage, the discharge lamp cannot be lighted. This is because the voltage at the start of the discharge lamp in the normal state is higher than this voltage.

Therefore, it is necessary to detect the voltage 23 detected by the diodes 14 and 15 in the tube current detector 12. When the voltage 23 detected by the diodes 14 and 15 becomes an overvoltage, the above-mentioned function similarly operates and the voltage is controlled similarly. This makes it possible to prevent the tube current of the discharge lamp 11 from exceeding a certain level.

The diodes 39 and 40 are either the voltage 41 between the primary side of the transformer and the choke coil to the voltage 23 detected by the diodes 14 and 15 in the tube current detector or the diode 1 in the tube current detector.
This is to prevent current from flowing from the voltage 23 detected by 4 and 15 to the voltage 41 between the primary side of the transformer and the choke coil.

[0016]

As described above, according to the present invention, the circuit parts of the inverter are at the end of their life, the foreign matters such as metal pieces enter the lighting device, and the parts of the lighting device are short-circuited with a simple circuit configuration. Due to the open state, even if an overvoltage occurs on the secondary side of the normal transformer and smoke and ignition occur, these abnormalities can be detected in advance and the danger of smoke and ignition can be avoided.

[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 embodiment.

[Explanation of Codes] 1 ... DC power supply 2 ... Chopping transistor 9 ... Transformer 11 ... Discharge lamp 12 ... Tube current detector 29 ... Error amplifier 30 ... Comparator

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ikuya Nomoto 888 Fujibashi, Ome City, Tokyo Hitachi, Ltd. Living Equipment Division

Claims (2)

[Claims] 1. An inverter that inputs a DC voltage of a DC power supply to a DC / DC converter, converts an output voltage of the DC / DC converter into a high frequency voltage and supplies the high frequency voltage to a discharge tube, and a current flowing through the discharge tube. In a discharge lamp lighting device having a tube current detector that detects a voltage, the primary side of the transformer in the discharge lamp lighting device and the output voltage of the tube current detection circuit are detected, and one of them becomes a predetermined voltage or higher. A discharge lamp lighting device having a protection circuit for stopping the operation or lowering the voltage below the predetermined voltage. 2. The protection circuit detects an output voltage of a primary side of a transformer in the discharge lamp lighting device and an output voltage of the tube current detection circuit, and when either one of the output voltages exceeds a predetermined voltage, the DC By controlling the quiescent period of oscillation at the output voltage of the DC / DC converter and lengthening the quiescent period, DC
The discharge lamp lighting device according to claim 1, wherein the output voltage of the / DC converter is lowered or the operation of the DC / DC converter is stopped.