JP2002359094A - Discharge tube driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve control characteristics by adding a ballast capacitor to a separately excited driving circuit of a cold cathode discharge tube. SOLUTION: A discharge voltage is transferred to the ballast capacitor and is supplied to the discharge tube, in this driving circuit composed of a step-up transformer, plural semiconductor switches, an LC resonance filter including C of the distributed capacitance of the transformer and L of leakage inductance, and a control circuit for controlling the make-to-break ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a discharge tube driving circuit for emitting a cold-cathode discharge fluorescent tube used for a backlight of a liquid crystal display such as a notebook computer or a digital camera.

[0002]

2. Description of the Related Art A conventional driving circuit for a cold-cathode discharge tube uses a self-excited oscillation circuit having a boosting function and a separately excited oscillation circuit separately provided with an oscillator. In each case, the DC voltage is several tens k
Hz, and then several hundred V
After the pressure was increased to, the voltage was applied to a discharge tube to emit light. In the self-excited oscillation circuit, a capacitor called a ballast capacitor is connected between a high voltage output terminal of a transformer and a high voltage terminal of a cold cathode discharge tube. This ballast capacitor is provided for the purpose of smoothly continuing self-sustained pulsation even if the impedance of the cold cathode discharge tube of the load is greatly changed after the start. On the other hand, the separately excited oscillation circuit does not include a ballast capacitor because the oscillator is separately provided. When this ballast capacitor is provided, a voltage drop occurs there, and the output voltage of the transformer is increased, thereby deteriorating the conversion efficiency of the drive circuit. Therefore, there is no circuit having a ballast capacitor in the separately-excited oscillation circuit.

FIGS. 3 and 4 show the configuration of a conventional self-excited oscillation circuit and voltage waveforms at main parts thereof. 3, the DC / DC converter includes a semiconductor switch 55, an inductor 32, a capacitor 73, and a diode 63, and obtains an arbitrary variable DC voltage from both ends of the capacitor 73 according to a set value of a variable resistor 81 of the control circuit 42. is there. Here, a terminal 421 is a power supply terminal of the control circuit 42, a terminal 422 is a common ground terminal of the control circuit 42, a terminal 423 is a drive terminal of the semiconductor switch 55, and a terminal 424 is a current detection terminal of the discharge tube 2. Terminal. The voltage V71 of the DC power supply 1 is converted into a pulsed voltage V63 according to the opening and closing operation times of the semiconductor switch 55,
The DC voltage becomes V73 by the smoothing circuit. FIG. 4 shows the voltage waveforms described above. The DC voltage V73 can be arbitrarily selected according to the ratio of the closing operation time (TON) to the opening operation time (TOFF) of the semiconductor switch 55. Therefore, the variable DC voltage is calculated by V73 = V71 × TON / (TON + TOFF).

Next, the voltage-controlled DC voltage V 73 is applied to the intermediate tap 342 of the transformer 34 via the inductor 33. The semiconductor switch 56 is connected to the terminal 341, the semiconductor switch 57 is connected to the terminal 343, and a capacitor 74 is connected in parallel. The resonance circuit includes an inductance and a capacitor 74 as viewed from the primary side of the transformer 34. Semiconductor switches 56 and 57
The opening and closing operation of the frequency timing of the resonance circuit is performed by the operation of the drive windings of the terminals 346 and 347. Sine wave voltage V between the secondary terminals 344 and 345 of the transformer 34
72 can be obtained. The current of the discharge tube 2 is detected by the resistor 85, and the control circuit 42 controls the voltage V of the capacitor 73.
By controlling 73, the current of the discharge tube 2 is stabilized.

This self-excited oscillation circuit resonates at a resonance frequency determined by the inductance between the terminals 341 and 343 of the transformer 34 and the capacitor 72, and a sine wave voltage V72 is applied between the terminals 341-343, 344-345 or 346 of the transformer 34. -347. The voltage waveform V56 shown in FIG. 4 is the voltage between the collector and the emitter of the semiconductor switch 56. When the terminal 347 connected to the base is at a negative voltage, the semiconductor switch 56 is in the open operation period. Therefore, a sine wave voltage is applied to the voltage of the terminal 341 connected to the collector of the semiconductor switch 56. , Terminal 347
Changes to a positive voltage, a closing operation period occurs, and the voltage at the terminal 341 has no difference. On the other hand, as shown by V57, the voltage waveform of the semiconductor switch 57 has an antiphase relationship with V56. The winding voltage waveform of the transformer 34 is a composite voltage of V56 and V57,
The voltage waveform is a positive / negative symmetric voltage waveform as shown by V72 in FIG.

V 342 is a voltage waveform at the terminal 342 of the transformer 34. DC voltage V73 of capacitor 73 and terminal 34
2 will appear between the terminals of the inductor 33. A voltage pulsation equal to the voltage-time products S331 and S332 is included between the terminals of the inductor 33. For this reason, the average voltage value of the terminal 342 of the transformer 34 and the voltage value of the capacitor 73 are equal.

FIGS. 5 and 2 show the configuration of a conventional separately-excited oscillation circuit and voltage waveforms at main parts thereof. In FIG. 5, a power supply 1 is connected to a smoothing capacitor 71, a control circuit 41, and a DC terminal of a bridge circuit of semiconductor switches 51 to 54. Gates of the semiconductor switches 51 to 54 are connected to a control circuit. The AC terminals of the bridge circuits of the semiconductor switches 51 to 54 are connected to the input terminals 311, 312 of the transformer 31 via the capacitor 72. The output terminal 313 of the transformer 31 is connected to the high voltage terminal of the discharge tube 2 of the load. The low voltage terminal of the discharge tube 2 of the load is connected to a resistor 8
5 and to the terminal 314 of the transformer 31 and to the negative terminal of the power supply 1. The terminal 417 of the control circuit 41 is connected to a connection point between the resistor 85 and the discharge tube 2 of the load to detect the terminal voltage of the resistor 85. The brightness setting variable resistor 81 is connected to the control circuit. The switching frequency of the semiconductor switches 51 to 54 is determined based on the triangular wave signal of the oscillator 43. It is well known that the open / close time ratio of the semiconductor switches 51 to 54 can be easily selected by converting the triangular signal voltage of the oscillator 43 and the set voltage signal into a pulse signal by a comparison circuit.

In FIG. 2, semiconductor switches 52, 53
While the switch is closed, the semiconductor switches 51 and 54 are open. At this time, a positive voltage appears as the AC terminal voltage V50 of the bridge circuit of the semiconductor switches 51 to 54. While the semiconductor switches 51 and 54 are closed, the semiconductor switches 52 and 53 are open. Therefore, a negative voltage appears as the voltage V50 opposite to the above. The voltage V50 is supplied to the input terminals 311, 3
12 has been added. When the transformer 31 is represented by an equivalent circuit, as shown in FIG.
5, internal resistance 317, output inductance 316, and distributed capacitance 318. The capacitor 72 and the leakage inductance 315 are connected in series, and the output inductance 316 and the distributed capacitance 318 are connected in parallel. The distributed capacitance of the load is also connected in parallel with the distributed capacitance 318. At the resonance frequency including the output inductance 316, the distributed capacitance 318, and the load capacitance of the parallel connection, the combined impedance has the maximum value. At the resonance frequency of the series-connected capacitor 72 and leakage inductance 315, the combined impedance becomes the minimum value. Both of these resonance frequencies
When the frequency is close to the frequency of the voltage V50, components other than this frequency extremely decrease. As a result, the output terminal 3 of the transformer 31
A sine wave voltage V31 is generated at 13, 13 and 314.

When the output voltage V31 of the transformer 31 is applied to the discharge tube 2 of the load, a current flows through the discharge tube 2. By detecting the voltage 85 with the resistor 85 and controlling the switching time ratio of the semiconductor switches 51 to 54 based on the value, the current of the discharge tube 2 is stabilized. The voltage-current characteristic of the cold cathode discharge tube is as shown by the curve of the characteristic 23 in FIG. 6, is not a straight line, and the current is not proportional to the voltage. This characteristic is shown as a value when a temperature rise of the cold cathode discharge tube 2 of the load is stabilized by applying a sine wave voltage having a constant frequency. The practical range is in the range of 1 mA to 6 mA, which is the reverse characteristic range in which the voltage decreases as the current increases. On the other hand, the voltage-current characteristics when the frequency of the discharge tube driving circuit and the switching time ratio of the semiconductor switches 51 to 54 are fixed show the voltage drop characteristics of characteristics 24 to 26. The vertical movement of the characteristics 24 to 26 can be achieved by changing the open / close time ratio of the semiconductor switches 51 to 54. However, the slopes of the characteristics 24 to 26 depend on the leakage inductance 315 of the transformer 31 and other values.

[0010]

Since the discharge tube driving circuit having good conversion efficiency exhibits the voltage-current characteristic 24, when the load of the cold-cathode discharge tube 2 having the voltage-current characteristic 23 is connected, the switching of the semiconductor switches 51 to 54 is performed. Depending on the setting of the time ratio, FIG.
As described above, since there are two or more intersections, the operation becomes unstable. In addition, the discharge tube driving circuit having the voltage-current characteristic 26 has a large voltage drop and needs to increase the step-up ratio of the transformer 31, so that the conversion efficiency is poor. The optimum discharge tube drive circuit has a voltage-current characteristic 25 that matches the voltage-current characteristic of the cold cathode discharge tube 2 as a load. In recent years, in order to meet demands for miniaturization and high efficiency, the transformer 31 has been designed each time in accordance with the voltage-current characteristics of the discharge tube 2 of the load. For this reason, if the discharge tube 2 is changed when the model is changed, the transformer 31 before the change cannot be used, and the defective stock is disposed. Even for similar discharge tubes, only the discharge tube manufacturer is different and the voltage-current characteristics are not the same. Therefore, transformers having the same shape and different turn ratios have been manufactured.

[0011]

SUMMARY OF THE INVENTION The present invention comprises a plurality of semiconductor switches and a control circuit connected to the gates of the semiconductor switches, wherein the semiconductor switches are bridge circuits,
An AC terminal of the bridge circuit is connected to an input terminal of a transformer via a first capacitor, and an output terminal of the transformer is connected to a high voltage terminal of a discharge tube. And a discharge tube driving circuit for connecting a second capacitor between the high voltage terminal of the discharge tube and the high voltage terminal of the discharge tube. Since the second capacitor is connected in series to the discharge tube, the voltage-current characteristic of the apparent load is determined by adding the impedance of the second capacitor (the value at which the voltage increases in proportion to the current value) to the voltage-current characteristic of the discharge tube. , Changes to the added characteristics. By changing the value of the second capacitor, the voltage-current characteristics of the apparent load can be adjusted, so that the transformer can be shared by a plurality of discharge tubes. Thus, the number of types of transformers can be smaller than the number of types of discharge tubes. In the present invention, the control circuit includes a current detector for the discharge tube, a reference voltage generation circuit, an error amplifier, and an open / close time ratio conversion circuit. Preferably, a resistor is connected between the low voltage terminal of the discharge tube and the output terminal of the transformer, and a terminal of a current detector of the control circuit is connected to a connection point between the resistor and the discharge tube. Further, the discharge tube driving circuit of the present invention includes a series connection circuit of a leakage inductance of the transformer and a first capacitor, and a parallel connection circuit of a capacitor including an inductance of an output winding of the transformer and a distributed capacitance of the discharge tube. And a series resonance / parallel resonance filter. Since the capacitor 76 is connected in series to the discharge tube 2 of the load, the voltage-current characteristic of the apparent load is determined by adding the impedance of the capacitor 76 (the value at which the voltage increases in proportion to the current value) to the voltage-current characteristic of the discharge tube 2. , Changes to the added characteristics. By changing the value of the capacitor 76, the voltage-current characteristics of the apparent load can be adjusted. Since the value can be handled by the value of the capacitor 76, the transformer 31 can be shared. Thus, the number of types of the transformer 31 can be smaller than the number of types of the discharge tubes 2 of the load.

[0012]

FIG. 1 shows an embodiment according to the present invention. FIG. 1 shows a conventional example, in which a capacitor 76 is added to FIG. The power supply 1 includes a smoothing capacitor 71
And the control circuit 41 and the DC terminals of the bridge circuits of the semiconductor switches 51 to 54. Semiconductor switches 51-
The gate of 54 is connected to the control circuit. The AC terminals of the bridge circuits of the semiconductor switches 51 to 54 are connected to a capacitor 7.
2 (first capacitor) and connected to the input terminals 311 and 312 of the transformer 31. The output terminal 313 of the transformer 31 is connected to the high voltage terminal of the discharge tube 2 of the load via the capacitor 76 (second capacitor). The low-voltage terminal of the discharge tube 2 of the load is connected to the terminal 314 of the transformer 31 and the negative terminal of the power supply 1 via the resistor 85. The terminal 417 of the control circuit 41 is connected to a connection point between the resistor 85 and the discharge tube 2 of the load to detect the terminal voltage of the resistor 85. The brightness setting variable resistor 81 is connected to the control circuit.

Therefore, the operation of FIG. 1 is the same as that of FIG.
In the example, the semiconductor switches 51 and 54 are open while the semiconductor switches 52 and 53 are closed. At this time, a positive voltage appears as the AC terminal voltage V50 of the bridge circuit of the semiconductor switches 51 to 54. Semiconductor switch 5
1 and 54 are closed while the semiconductor switches 52 and 5 are closed.
3 is open. Therefore, the reverse voltage V50 is
A negative voltage appears. The voltage V50 is applied to input terminals 311 and 312 of the transformer 31 via the capacitor 72. As shown in FIG. 7, an equivalent circuit of the transformer 31 includes a leakage inductance 315, an internal resistance 317, an output inductance 316, and a distributed capacitance 318. The capacitor 72 and the leakage inductance 315 are connected in series, and the output inductance 316 and the distributed capacitance 318 are connected in parallel. The distributed capacitance of the load is also connected in parallel with the distributed capacitance 318. The transformer 3 includes a parallel resonance circuit including an output inductance 316, a distribution capacitance 318, and a load capacitance, and a series resonance circuit including a capacitor 72 and a leakage inductance 315.
A sine wave voltage V31 is generated at one output terminal 313, 314.

When the output voltage V31 of the transformer 31 is applied to the discharge tube 2 of the load via the ballast capacitor 76, a current flows through the discharge tube 2. By detecting the voltage 85 with the resistor 85 and controlling the switching time ratio of the semiconductor switches 51 to 54 based on the value, the current of the discharge tube 2 is stabilized. The terminal voltage of the ballast capacitor 76 is V =
It becomes the value of the formula of I / (2πfC). C is the value of the capacitor 76, f is the frequency of V31, and I is the load current. From this equation, if f and C are constant, the terminal voltage of the capacitor 76 is proportional to the load current. The value of C is
Is inversely proportional to the terminal voltage. As shown by the characteristics 26 to 28 in FIG. 6, the slope of the voltage drop can be selected by the value of the capacitor 76. For example, when the f frequency is 50 kHz, the C capacitor 76 is 470 pF, and the load current is 5 mA, the voltage drop is about 34 V, which can increase the voltage drop by about 7% with respect to the load voltage of 500 V.

[0015]

As is apparent from the above description, the discharge tube driving circuit according to the present invention can adjust the voltage drop characteristics by adding the ballast capacitor 76 and using the value. By combining several types of ballast capacitors with one type of transformer, it is possible to cope with the discharge tube characteristics of various types of loads.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a separately excited oscillation circuit according to an embodiment of the present invention.

FIG. 2 is a voltage waveform diagram during operation of a separately excited oscillation circuit.

FIG. 3 is a circuit diagram showing a conventional self-excited oscillation circuit.

FIG. 4 is an operating voltage waveform diagram of the self-excited oscillation circuit.

FIG. 5 is a circuit diagram of a separately excited oscillation circuit of a conventional example.

FIG. 6 is a voltage-current characteristic diagram of a discharge tube driving circuit.

FIG. 7 is a configuration diagram of a filter circuit used in one embodiment of the present invention.

[Explanation of symbols]

 1: DC power supply 2: Discharge tube 21: Load resistance 22: Load capacitance 23 to 28: Voltage and current characteristics 31 and 34: Transformers 311 to 314 and 341 to 347: Terminals 32 and 33: Inductor 315: Leakage inductance 316: Output inductance 317: Internal resistance 318: Distributed capacitance 41 and 42: Control circuit 411 and 421: Power supply terminal 412 and 422: Common terminal 413, 414, 415 and 416: Driving terminal 417 and 424: Detection terminal 51 to 57: Semiconductor switch 63: diode 72: first capacitor 76: second capacitor 81: variable resistor 83 to 85: resistor

Claims (4)

[Claims] 1. A semiconductor device comprising: a plurality of semiconductor switches; and a control circuit to which a gate of the semiconductor switch is connected. The semiconductor switch is a bridge circuit. A discharge capacitor driving circuit connected to a terminal, and an output terminal of the transformer connected to a high voltage terminal of the discharge tube, wherein a second capacitor is connected between the output terminal of the transformer and the high voltage terminal of the discharge tube. Discharge tube drive circuit. 2. The discharge tube drive according to claim 1, wherein the control circuit includes a discharge tube current detector, a reference voltage generation circuit, an error amplifier, and a switching time ratio conversion circuit. circuit. 3. A resistor is connected between a low voltage terminal of the discharge tube and an output terminal of the transformer, and a terminal of a current detector of the control circuit is connected to a connection point between the resistor and the discharge tube. The discharge tube driving circuit according to claim 2. 4. A series circuit comprising a series connection circuit of a leakage inductance of the transformer and firstly a capacitor, and a parallel connection circuit of a capacitor including an inductance of an output winding of the transformer and a distributed capacitance of a discharge tube. 2. A resonance / parallel resonance filter is provided.
4. The discharge tube driving circuit according to any one of claims 1 to 3.