JPH09330796A - Cold cathode ray tube driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably drive light emission of a cold cathode ray pole tube without flickering in a non-linear load region in which a lamp current is steeply changed. SOLUTION: This circuit is provided with a power converting unit 12 for converting power from a power supply 11 in response to input of a detection signal S1 and for outputting a power supply current I1 corresponding to that conversion, an inverter 13 for converting the power supply current from the power converting unit to a lamp current and outputting this lamp current I2 to a cooling cathode ray tube 14, and a power supply current detecting circuit 16 for detecting a change of a power supply current I1 in comparison with a reference value and outputting a signal associated with this detection as the detection signal S1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold cathode fluorescent lamp which changes the lamp current for driving the cold cathode fluorescent lamp to change the brightness of the backlight when the cold cathode fluorescent lamp is used for a liquid crystal backlight. It is applied to the cathode tube drive circuit, and more specifically, it is suitable for a demand for expanding the control range of the lamp current, that is, a dimming width, a demand for low power consumption by narrowing the lamp current, or a demand for stable lighting in a low temperature environment. The present invention relates to a cathode tube drive circuit.

[0002]

2. Description of the Related Art With reference to FIG. 3, a conventional cold-cathode tube drive circuit used in a lighting device such as a liquid crystal backlight will be described. The drive circuit 10 includes an input voltage source 11 such as a battery and a series regulator, a step-down chopper, a step-up converter, etc., and converts the input voltage source 11 into electric power corresponding to the detection signal S1. A power converter 12 for outputting a DC-AC inverter 13 for generating and outputting an AC lamp current I2 from a DC power supply current I1 from the power converter 12, and a lamp current I2 for emitting light. A fluorescent lamp 14 as an operating cold cathode tube,
The lamp current I2 given to the fluorescent lamp 14 from the inverter 13 is detected, and the detection signal S1 is detected in response to the detection.
And a lamp current detection circuit 15 for outputting to the power conversion unit 12.

The operation of the conventional drive circuit 10 having the above configuration will be described. When a power supply current I1 is applied from the power conversion unit 12 into the inverter 13 through its constant current inductor L1, the transistors Q2 and Q3 push-pull connected to each other are biased in the forward direction by the starting resistors R1 and R2. Start conduction. In this case, one of the transistors Q2 and Q3 conducts deeper than the other due to a slight difference in current amplification factor. For example, in this case, if the transistor Q2 is deeply conducted, the transformer T1
The positive feedback action of the base feedback winding NB of the transistor Q3 reversely biases the transistor Q3 to completely turn on the transistor Q2, and the primary winding NP of the transformer T1.
And the resonance capacitor C1 resonate in parallel. This resonance voltage is fed back to the base feedback winding NB and the transistors Q2, Q
3 will be alternately turned on and off repeatedly. as a result,
A sinusoidal AC voltage is generated across the primary winding NP.
This AC voltage is boosted by the transformation ratio between the primary winding NP and the secondary winding NS of the transformer T1, and the secondary winding NS
A high AC voltage is generated at both ends, and the lamp current I2 flows into the fluorescent lamp 14 through the lamp current limiting capacitor CB. As a result, the fluorescent lamp 14
Emits light.

The lamp current I2 is detected by the lamp current detection resistor RS1 in the lamp current detection circuit 15, and is smoothed by a smoothing circuit composed of a diode D1 and a capacitor C2 to be converted into a direct current. The voltage across the capacitor C2 due to this direct current conversion is the lamp current comparison circuit OP1.
It is applied as a lamp current detection voltage to one of the input terminals +. The lamp current comparison circuit OP1 compares this lamp current detection voltage with the reference voltage Vref of the other input terminal and outputs a voltage corresponding to the comparison to the power conversion unit 12 as a detection signal S1 as an error of the lamp current. To do. The power conversion unit 12 controls the power supply current I1 to the inverter 13 in response to the detection signal S1. In this way, the fluorescent lamp 14 emits light with a constant lamp current I2. The increase / decrease in the lamp current I2 is based on the reference voltage Vr.
This is performed by changing the value of ef by increasing or decreasing, or by changing the resistance value of the resistor R2 in parallel with the capacitor C2.

Now, the load characteristics of the fluorescent lamp 14 will be described with reference to FIG. The vertical axis of FIG. 4 indicates the lamp current I.
2, the horizontal axis is the voltage VS across the secondary winding NS of the transformer T1, and CB = large, CB = medium, and CB = small are the capacities of the capacitors CB for limiting the lamp current, respectively. This is the case when a certain impedance is high, medium, and low.
As shown in FIG. 4, in the case where the capacity of the capacitor CB is large, the fluorescent lamp 14 does not emit fluorescent light until the voltage VS across the secondary winding NS of the transformer T1 reaches a certain area AB. The lamp 14 does not discharge, and its impedance is infinite, but when it reaches between the regions A and B and discharges, its impedance decreases at a stroke, and as a result, the lamp current I2 is limited until it is limited by the impedance of the capacitor CB. Will flow at a sudden rise, and will increase at a value limited by the impedance of the capacitor CB in the area between the areas A and B. As described above, the fluorescent lamp 14 exhibits a typical non-linear load characteristic in which the lamp current I2 rapidly increases between the regions A and B. It is known that such a load characteristic becomes more remarkable when the lamp temperature is low. In the load characteristics of the fluorescent lamp 14 in FIG. 4, the lamp current I2 is small between the regions A and B, but the lamp current I2 changes rapidly, so the lamp current I2 is narrowed down and dimming at low emission brightness and low. It is convenient for demands such as power consumption, and the fluorescent lamp 14 does not discharge in a region where the lamp current I2 below the point A is smaller and does not emit light, and the lamp current I2 is larger in a region above the point B. The fluorescent lamp 14 can be made to emit light with higher brightness.

[0006]

In the above-mentioned conventional drive circuit 10, since the fluorescent lamp 14 has the load characteristics as shown in FIG. There is a problem that the light emission flickers when it is driven to emit light in a low lamp current region between −B and B.
The flicker occurs in the light emission of the fluorescent lamp 14 because the lamp current detection circuit 15 detects the lamp current detection resistor R.
When the lamp current I2 detected in S1 is smoothed by the diode D1 and the capacitor C2 and converted into direct current,
Due to the time constant of the diode D1 and the capacitor C2, a response delay occurs in the detection of the lamp current I2. Then, in this case, in a rapidly completed region such as between regions AB, the lamp current I2 is large and rapidly increases or decreases even if there is a slight response delay with respect to the detection of the lamp current I2. This is because the light emission of 14 causes flicker.

[0007]

In the drive circuit of the present invention according to claim 1, a power conversion for converting electric power from a power supply in response to an input of a detection signal and outputting a power supply current corresponding to the conversion. Section, while converting the DC power supply current from the power conversion unit into an AC lamp current, and an inverter that outputs the converted lamp current to a cold cathode tube, comparing the change in the power supply current with a reference value. Along with detecting
The above-described problem is solved by a configuration including a power supply current detection circuit that outputs a signal related to this detection as the detection signal to the power conversion unit.

In the drive circuit of the present invention according to claim 2, the power supply current detection circuit has a detection resistor connected between the power converter and the inverter, and a voltage appearing across the detection resistor. Is used as a change in the power supply current to solve the above-mentioned problems.

In the drive circuit of the present invention according to claim 3, the lamp current further detects the lamp current in the inverter and compares the detected lamp current with a reference value to detect an error in the lamp current. The above-mentioned problems are solved by a configuration including a detection circuit and using an output of the lamp current detection circuit as a reference value of the power supply current detection circuit.

In the drive circuit of the present invention according to claim 4, the lamp current detecting circuit smooths a lamp current detecting resistor connected to a lamp current path in the inverter and a voltage across the lamp current detecting resistor. And a comparator circuit for comparing the smoothing circuit output with a reference value, wherein the power supply current detection circuit uses the output of the lamp current detection circuit as its reference value. The above-mentioned problem is solved by the configuration.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram of a cold-cathode tube drive circuit according to an embodiment of the present invention. The parts corresponding to those in FIG. 3 are designated by the same reference numerals, and the parts associated with the same reference numerals are designated. Detailed explanation is omitted. Cold cathode tube drive circuit 10 of the present invention
A has a configuration in which a power supply current detection circuit 16 is further provided in addition to the conventional configuration shown in FIG.

The power supply current detection circuit 16 is composed of a power supply current detection resistor RS2 connected in the power supply path between the power conversion unit 12 and the inverter 13, and a power supply current comparison circuit OP2. The power supply current comparison circuit OP2 has one input terminal + connected to one end of the power supply current detection resistor RS2, and the other input terminal − connected to the output terminal of the lamp current comparison circuit OP1 of the lamp current detection circuit 15.

According to the drive circuit 10A of the present invention, since the power supply current I1 is proportional to the lamp current I2, the change in the lamp current I2 can be detected by detecting the change in the power supply current I1 and the lamp current can be detected. Since there is no smoothing circuit such as the circuit 15 which causes the response delay in the detection of the change in the lamp current I2 described above, there is no such a circuit in FIG.
In the load characteristics of the above fluorescent lamp, the lamp current I2 is changed even in the rapidly changing region of the lamp current I2 between the regions A and B.
As a result of being able to detect the change in the temperature without a response delay, the fluorescent lamp 14
Can be controlled with high accuracy and without causing flickering.

At that time, the lamp current I2 can be controlled to be constant only by the power supply current detection circuit 16, but in the present invention, the output of the lamp current detection circuit 15 is set to the output of the power supply current detection circuit 16. Lamp current comparison circuit OP
The other input terminal of 2 is input as a reference value.
This is because the lamp current I
This is to prevent the change corresponding to the change in ambient temperature of No. 2 from becoming undetectable, and the change in the lamp current I2 due to the change in ambient temperature is detected.
The response delay of the detection of the lamp current I2 by the above-mentioned smoothing circuit in FIG.

FIG. 2 shows a circuit for controlling the lamp current I2 to be constant only by the power supply current detection circuit 16 described above. In the circuit of FIG. 2, the lamp current detection circuit 15 is omitted, and the lamp current comparison circuit OP of the power supply current detection circuit 16 is omitted.
A reference power source is connected to the other input terminal-of 2, and a reference voltage Vref is applied from this reference power source. The other configuration is the same as that of FIG.

[0017]

As described above, according to the present invention, the following effects can be obtained.

According to a first aspect of the present invention, the cold cathode tube drive circuit converts a power from a power source in response to an input of a detection signal and outputs a power source current corresponding to the conversion, and the power converter. While converting the DC power supply current from the to the AC lamp current, the inverter that outputs the converted lamp current to the cold cathode tube, while detecting the change in the power supply current by comparing with the reference value,
Since a signal related to this detection is configured as a power supply current detection circuit that outputs the signal to the power converter as the detection signal, when the lamp current is narrowed down in a low lamp current region which is a non-linear load region of the cold cathode tube, The current can be stably controlled even at a low temperature, and the cold cathode tube can be driven to emit light without flicker. Therefore, it becomes possible to expand the dimming range of the cold cathode tube in the non-linear load region, and at that time, it is possible to meet the demand for reducing the power consumption by driving light emission with a small lamp current. This is because when the cold cathode tube drive circuit has a liquid crystal display section as a display section in an electronic device such as a battery-driven small portable terminal or a notebook computer, and is applied to a liquid crystal backlight of the liquid crystal display section, It became possible to apply the battery to the low power consumption mode.

According to a second aspect of the present invention, in the cold-cathode tube drive circuit, the power supply current detection circuit has a detection resistor connected between the power conversion section and the inverter, and the voltage across the detection resistor is the Since it is used as a change in power supply current, a change in power supply current can be accurately detected with a simple configuration.

According to a third aspect of the present invention, the cold-cathode tube drive circuit further detects the lamp current in the inverter and compares the detected lamp current with a reference value to detect an error in the lamp current. Since a circuit is provided and the output of the lamp current detection circuit is used as the reference value of the power supply current detection circuit, the lamp can be operated in response to this temperature change even if the temperature around the cold cathode tube changes. The current can be accurately controlled to stabilize the light emission of the cold cathode tube.

According to a fourth aspect of the present invention, in the cold-cathode tube drive circuit, the lamp current detection circuit smooths a lamp current detection resistor connected to a lamp current path in the inverter and a voltage across the lamp current detection resistor. And a comparison circuit for comparing the output of the smoothing circuit with a reference value, the power supply current detection circuit,
Since the output of the lamp current detection circuit is used as its reference value, it is possible to stabilize the light emission of the cold cathode tube regardless of the temperature change around the cold cathode tube as in the third aspect.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram of a cold cathode tube drive circuit according to another embodiment of the present invention.

FIG. 3 is a circuit diagram of a conventional cold cathode tube drive circuit.

FIG. 4 is a diagram showing load characteristics of a cold cathode fluorescent lamp.

[Explanation of symbols]

 10A Cold-cathode tube drive circuit 11 Input voltage source 12 Power converter 13 Inverter 14 Cold-cathode tube 15 Lamp current detection circuit 16 Power supply current detection circuit

Claims (4)

[Claims] 1. A power conversion unit that converts power from a power supply in response to an input of a detection signal and outputs a power supply current corresponding to the conversion, and a DC power supply current from the power conversion unit that is an AC lamp. An inverter that converts the current into a current and outputs the converted lamp current to a cold cathode tube and detects a change in the power supply current by comparing it with a reference value, and a signal related to this detection is used as the detection signal in the power conversion. A cold-cathode tube drive circuit, comprising: a power supply current detection circuit that outputs the current to the unit. 2. The power supply current detection circuit has a detection resistor connected between the power conversion unit and an inverter, and the voltage across the detection resistor is used as a change in the power supply current. The cold cathode tube drive circuit according to claim 1. 3. A lamp current detection circuit for detecting a lamp current in the inverter and comparing the detected lamp current with a reference value to detect an error component of the lamp current. 3. The cold cathode tube drive circuit according to claim 1, wherein an output of the detection circuit is used as a reference value of the power supply current detection circuit. 4. The lamp current detection circuit includes a lamp current detection resistor connected to a lamp current path in the inverter, a smoothing circuit that smoothes a voltage across the lamp current detection resistor, and the smoothing circuit. 4. A cold-cathode tube drive circuit according to claim 3, further comprising a comparison circuit for comparing the output with a reference value, wherein the power supply current detection circuit uses the output of the lamp current detection circuit as its reference value. .