JPH0589989A - Cold cathode tube lighting device - Google Patents

Abstract

PURPOSE:To provide a cold cathode tube lighting device which does not require components for a ballast on the secondary side and which can hold the current of a cold cathode tube constant even if the voltage of the cold cathode tube is changed and which can restrain lowering of the brightness of the cold cathode tube at low temperatures while simply setting the current of the cold cathode tube and which can use an inverter transformer of low output voltage. CONSTITUTION:A cold cathode tube 8 is directly connected to the secondary side of a voltage resonance type Royer circuit to which power is fed from a low voltage constant current source 9. The output voltage of the voltage resonance type Royer circuit is proportional to input voltage and the input voltage is equal to the output voltage. Power consumed at the cold cathode tube is expressed as the product of tube voltage and tube current. Power output from the constant current source 9 is the product of the voltage and the current. Therefore, the tube current is proportional to the input current of the Royer circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-cathode tube lighting device used for a liquid crystal backlight of a display device such as information equipment.

[0002]

2. Description of the Related Art In recent years, in the field of information equipment, a liquid crystal display device with a backlight has been mainly used as a display device in order to downsize the device and improve the visibility.

Most of the light sources of this backlight use a cold cathode tube because of the advantages of brightness and low power consumption. However, this cold cathode tube has a high lighting voltage,
A cold-cathode tube lighting device that generates a high voltage is required for lighting. Further, the cold cathode tube has the following characteristics. Negative resistance characteristics that the discharge start voltage is higher than the lighting voltage, constant voltage characteristics that the cold cathode tube voltage does not change even when the cold cathode tube current is changed, and the cold cathode tube voltage becomes low when the temperature is high, It has characteristics that it becomes high when the temperature is low, that luminous efficiency becomes high when the temperature is high, and that it becomes low when the temperature is low.

A conventional CCFL driver circuit will be described below with reference to the drawings. FIG. 3 shows a circuit configuration of a conventional CCFL driver circuit, which utilizes a voltage resonance type Royer oscillator circuit. In FIG. 3, 1
Is an inverter transformer, 2 and 3 are transistors that are switching elements, 4 and 5 are resistors, 6 is a capacitor for causing voltage resonance, 7 is a choke coil, 8 is a cold cathode tube, and 1 is a cold cathode tube.
Reference numeral 0 is a low voltage constant voltage source, and 11 is a ballast capacitor for limiting the output current.

Inverter transformer 1, transistor 2,
A circuit composed of 3, resistors 4, 5, a capacitor 6, and a choke coil 7 is called a voltage resonance type Royer oscillation circuit, and is a circuit generally used in a cold cathode tube lighting device.

The operation of the cold-cathode tube lighting device configured as described above will be described below. First, the voltage output from the low voltage constant voltage source 10 is applied to the bases of the transistors 2 and 3 through the resistors 4 and 5 and to the B pin of the inverter transformer 1 through the choke coil 7. At this time, either transistor is turned on first due to a slight difference in balance. For example, if the transistor 2 is turned on first, the A pin of the inverter transformer 1 is connected to the negative side of the low voltage constant voltage source 10, and E
A positive voltage is induced on the D pin with respect to the pin. Therefore, the current passing through the resistor 5 becomes E of the inverter transformer 1.
It is drawn into the pin and supplied to the base of the transistor 2, so that the transistor 2 is surely turned on and the transistor 3 is turned off.

Next, when a half cycle of the resonance frequency of the tank circuit composed of the inductor and the capacitor 6 between the AC pin of the inverter transformer 1 elapses, a resonance phenomenon occurs between the AC pin of the inverter transformer 1. The voltage is inverted, and at the same time, the voltage between the D and E pins is also inverted. Therefore, the supply of the base current to the transistors 2 and 3 is reversed, and the on / off states of the transistors 2 and 3 are reversed.
After that, this operation is continued and the direct current is converted into the alternating current. Inverter transformer 1 uses FG between AC pins as input
A voltage corresponding to the winding ratio is output between the pins. That is, it operates as a transformer that inputs DC and outputs AC.

FIG. 4 shows the output characteristics of the inverter transformer shown in FIG. 3, the load curve when a capacitor serving as a ballast is passed, and the voltage-current characteristics of the cold cathode tube.

In FIG. 4, the output voltage of the inverter transformer 1 is determined by the input voltage of the voltage resonance type Royer circuit and has no relation to the output current, so that it is a straight line with a constant voltage.
On the other hand, the cold-cathode tube is lit by utilizing glow discharge, but there is a transition region to glow discharge in a minute current region, and the cold-cathode tube voltage becomes high. Then, in the glow discharge region where the cold-cathode tube is normally lighted, the cold-cathode tube voltage becomes a little lower, and the cold-cathode tube voltage does not change even if the cold-cathode tube current changes.
Further, the cold cathode fluorescent lamp voltage changes depending on the temperature, and the characteristic curves shown in the figure are obtained at low temperature and high temperature, respectively. Further, the life of the cold-cathode tube shortens if too much current is passed, so it is necessary to set an appropriate cold-cathode tube current.

If the output of the inverter transformer is directly connected to the cold cathode tube, an excessive current will flow and destroy the equipment. Therefore, a ballast component for limiting some current is required. This part includes resistors, capacitors,
A coil can be considered, but of these, the resistance has a large loss, and the one that can withstand a high voltage has a drawback that the shape becomes large. Therefore, a capacitor that has a high breakdown voltage and can be made small in size is used. The load curve when the capacitor is used as a ballast is the load curve of the conventional configuration in FIG. 4, and the points that intersect the characteristic curve of the cold cathode tube are the voltage and current of the cold cathode tube.

[0011]

In the above conventional structure, the current of the cold cathode tube must be set by the capacitance of the capacitor, the oscillation frequency and the output voltage of the inverter transformer. However, when the oscillation frequency is increased, the influence of the stray capacitance becomes large, and when the oscillation frequency is decreased, the inverter transformer causes magnetic saturation, so that the adjustment range is narrow. The output voltage of the inverter transformer is limited by its shape, and the lower limit voltage is also determined by the discharge start voltage of the cold cathode tube at low temperature, so the range of adjustment is limited. And
Since the capacity of the capacitor is set in stages, it is not possible to make delicate settings. Because of these problems, it is difficult to set the cold cathode tube current.

When the cold-cathode tube voltage becomes high at low temperature, the operating point moves and the cold-cathode tube current decreases. The brightness of the cold-cathode tube is low because the luminous efficiency of the cold-cathode tube is low even when the same current as that at the time of high-temperature is passed at low temperature. Will be reduced.

Further, the output voltage of the inverter transformer must be increased due to the voltage drop of the ballast capacitor, and the size of the inverter transformer becomes large, which hinders downsizing of the equipment.

The present invention solves the above-mentioned problems of the prior art by using a low-voltage constant current source as a power source and not requiring a ballast component, and keeping the cold-cathode tube current constant even if the cold-cathode tube voltage changes. A cold cathode fluorescent lamp lighting device that can easily set the cold cathode fluorescent lamp current, can suppress the decrease in the brightness of the cold cathode fluorescent lamp at a low temperature, and can reduce the output power of the inverter transformer. With the goal.

[0015]

In order to achieve this object, a cold-cathode tube lighting device of the present invention comprises a low-voltage constant current source and a voltage which receives power from the constant current source and functions as a current transformer. Resonance type Royer circuit and cold cathode tube on the secondary side,
In order to make the voltage resonance type Royer circuit function as a current transformer, the secondary side is characterized by not having a capacitive component in series with the cold cathode tube.

[0016]

Since there is no ballast capacitor, there is no voltage drop, the cold cathode tube voltage and the output voltage of the inverter transformer can be made equal, and the output voltage of the inverter transformer can be low. The load voltage and the input voltage of the voltage resonance type Royer circuit have a linear proportional relationship although there is a difference between direct current and alternating current. Next, since the output power is larger than the loss power of the voltage resonance type Royer circuit, assuming that the input power of the voltage resonance type Royer circuit is equal to the output power, the relation of the current is the power divided by the voltage. From
The cold cathode tube current is proportional to the input current of the voltage resonance type Royer circuit. Therefore, by supplying electric power from the low voltage constant current source, a constant current flows in the cold cathode tube.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, the portion of the voltage resonance type Royer circuit and the cold cathode tube 8 are the same as in the conventional example. The different point is that the low voltage constant voltage source is used to supply power, but the low voltage constant current source 9 supplies power, and the capacitor for limiting the output current is eliminated.

The operation of the cold-cathode tube lighting device configured as described above will be described below. The process in which the DC voltage is input to the voltage resonance type Royer circuit and the AC voltage is output is as described in the related art.

Here, consider the voltage of each part of the cold cathode tube 8 in the lighting state. First, considering the voltage of the cold-cathode tube 8 of the load, it becomes constant when the temperature is determined, and the output voltage of the inverter transformer 1 and the FG pin voltage also become the same voltage. Since the output is a constant voltage, a voltage determined by the winding ratio is applied between the input side of the inverter transformer 1 and the A-C pin. Therefore, the voltage output from the low voltage constant current source 9 is determined. This voltage is proportional to the cold cathode tube voltage.

This relationship is represented by the following equation (1), where Vout is the cold cathode tube voltage, Vin is the voltage output from the low-voltage constant current source 9, and Kv is the proportional constant determined by the winding ratio.

[0022]

## EQU00001 ## Vout = Kv.times.Vin Next, the power output from the low voltage constant current source 9 is Pin,
When the power consumed by the cold cathode fluorescent lamp 8 is Pout and the loss power of the voltage resonance type Royer circuit is Ploss, the relationship between them is expressed by (Equation 2), and the loss power Ploss of the voltage resonance type Royer circuit is Power Pout consumed by the cold cathode tube 8
Since it is smaller than (Equation 3), the power Pin output from the low-voltage constant current source 9 is equal to the power Pout consumed by the cold cathode tube 8 (Equation 4).

[0023]

[Formula 2] Pin = Pout + Ploss

[0024]

[Formula 3] Pout >> Ploss

[0025]

[Equation 4] Pin = Pout The electric power Pin output from the low voltage constant current source 9 is represented by the product of the voltage Vin output from the low voltage constant current source 9 and the current Iin output from the low voltage constant current source 9. (Equation 5).

[0026]

## EQU00005 ## Pin = Vin.times.Iin Then, the power Pout consumed by the cold cathode fluorescent lamp 8 is expressed as follows: cold cathode fluorescent lamp current Iout and cold cathode fluorescent lamp voltage Vout.
Since there is no phase difference between these voltages and currents and there is no distortion, they are expressed by the product of the current Iout and the voltage Vout (Equation 6).

[0027]

## EQU6 ## Therefore, the cold cathode fluorescent lamp current Iout becomes a value obtained by dividing the electric power Pout output from the low-voltage constant current source 9 by the cold cathode fluorescent lamp voltage Vout (Mathematical Expression 7).

[0028]

## EQU00007 ## Iout = Pout.div.Vout Furthermore, (Equation 7) can be expressed as (Equation 8) from (Equation 4).

[0029]

[Expression 8] Iout = Pin ÷ Vout Further, (Expression 8) can be expressed as (Expression 9) from (Expression 5).

[0030]

[Formula 9] Iout = Vin × Iin ÷ Vout Furthermore, (Formula 9) can be expressed as (Formula 10) from (Formula 1).

[0031]

## EQU10 ## Iout = kv * Iin That is, the cold cathode tube current Iout is proportional to the current Iin output from the low-voltage constant current source 9.

Here, the necessary condition is that the power output from the inverter transformer 1 is represented by the product of voltage and current. Therefore, components other than the resistance component should not be connected to the load in series with the cold cathode tubes 8. should not. That is, when a capacitor is connected as a ballast, the proportional relationship between the input current and the output current of the voltage resonance type Royer circuit is broken and the operation becomes unstable.

FIG. 2 shows the output load curve of the inverter transformer 1 of FIG. 1 and the voltage-current characteristics of the cold cathode fluorescent lamp 8. Here, since the current value of the load curve is proportional to the output current of the low voltage constant current source 9, the current of the cold cathode tube 8 can be set very easily. That is, the voltage resonance type Royer circuit functions as a kind of current transformer in which the output current proportional to the input current flows. Therefore, the cold-cathode tube lighting device according to the present embodiment has a load curve that is a straight line with a constant current as shown in the figure, and is excellent in the stability of the cold-cathode tube current with respect to the voltage fluctuation of the cold-cathode tube 8. The effect is obtained.

Further, the output voltage of the inverter transformer 1 should be equal to or higher than the voltage of the cold cathode tube 8 in the minute current region when the temperature is low, and there is no voltage drop of the capacitor due to ballast and equal to the cold cathode tube voltage. Good. The part indicated by the dotted line of the load curve in the figure may not actually be output due to the dielectric strength of the inverter transformer or the problem of core saturation. Therefore, an inverter transformer having a low output voltage can be used, which can be downsized.

In the embodiments of the present invention, the voltage resonance type Royer circuit is not limited to this circuit configuration. For example, the transistors 2 and 3 may be FETs. Also,
Although the resistors 4 and 5 are connected to the positive side of the low voltage constant current source 9, they may be connected to the B pin of the inverter transformer 1.
A separate power supply may be used as long as the switch element can be biased.

The low-voltage constant current source 9 does not have to operate as a constant current source alone, but may operate as a constant current source when a voltage resonance type Royer circuit is connected. Further, the output current of the low-voltage constant current source 9 need not be a constant current in terms of high frequency, but may be a constant current in terms of direct current.

[0037]

As described above, according to the present invention, a low-voltage constant current source, a voltage resonance type Royer circuit which functions as a current transformer when supplied with electric power from the constant current source, and a cold cathode on the secondary side. By having a tube and not having a capacitive component in series with the cold cathode tube, the cold cathode tube current can be made constant even if the voltage of the cold cathode tube changes, and the cold cathode tube current It is possible to realize an excellent cold-cathode tube lighting device that can be easily set, can suppress the decrease in the brightness of the cold-cathode tube at low temperatures, and can reduce the output power of the inverter transformer.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a cold cathode tube lighting device according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of operating points in the embodiment of the present invention.

FIG. 3 is a configuration diagram of a conventional cold cathode tube lighting device.

FIG. 4 is an explanatory diagram of operating points in a conventional example.

[Explanation of symbols]

 1 Inverter transformer 2 Transistor 3 Transistor 4 Resistor 5 Resistor 6 Capacitor 7 Choke coil 8 Cold cathode tube 9 Low voltage constant current source

Claims (1)

[Claims] 1. A low-voltage constant current source, a voltage resonance type Royer circuit which receives power from the constant current source and functions as a current transformer, and a cold cathode tube on a secondary side, the voltage resonance type Royer. A cold-cathode tube lighting device characterized by having no capacitive parts in series with the cold-cathode tube on the secondary side so that the circuit functions as a current transformer.