JPH11238594A - Discharge tube drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adjust emission luminance by installing a bidirectional flyback transformer, having a semiconductor switch which has a reverse directional blocking characteristic and an inductor function, and generating an oscillating voltage satisfying a resonance condition at the time of flyback operation by controlling the exciting current energy of the transformer. SOLUTION: The threshold level of the terminal voltage of a resistor 82 is set by a variable resistor 81, the reference voltage value set by the variable resistor 81 is compared with the terminal voltage value of the resistor 82 in a control circuit 4, and the closing time of a semiconductor switch 51 or 52 is controlled by the magnitude of a value obtained by amplifying the difference voltage of the comparison. When the voltage between both the terminals of the resistor 82 is lower than the reference voltage set by the variable resistor 81, the excitation current of a transformer 31 is increased by making the closing time of the semiconductor switch 51 or 52 long and the current value flowing through a discharge tube 2 is increased by making the voltage after the opening operation period of the semiconductor switch 51 or 52 high, and thereby emission luminance is increased. Thus, the emission luminance is controlled by means of the variable resistor 81.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a booster circuit for driving a discharge tube for emitting a cold-cathode discharge fluorescent tube used for a backlight of a liquid crystal display such as a notebook personal 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 resonance type oscillation circuit having a boosting function. After converting a DC voltage to an AC voltage of several tens of kHz, the voltage was raised to several hundred volts by a transformer and applied between terminals of a discharge tube to emit light. However, since it is of the resonance type, it is practically difficult to operate the circuit outside the resonance frequency in principle, and the output voltage is substantially constant and the voltage is varied within a very limited range. In such a circuit configuration, the voltage applied to the terminals of the discharge tube was fixed, and the function of adjusting the brightness was unavoidably omitted or an incomplete voltage control circuit was provided. On the other hand, the circuit becomes complicated in order to provide sufficient luminance adjustment characteristics, but by inserting a DC / DC converter in the front stage, first, the DC voltage on the input side is changed to a variable voltage according to the luminance of the discharge tube. , An AC conversion circuit.

FIGS. 4 and 5 show a circuit configuration of this conventional system and voltage waveforms at main parts thereof. In FIG. 4, the DC / DC converter is a semiconductor switch 53,
Inductor 32, capacitor 73 and diode 63
And smoothed by a capacitor 73 to obtain a variable DC voltage. The voltage V71 of the DC power supply 1 is converted into a pulsed voltage V63 according to the opening and closing operation time of the semiconductor switch 53, and the DC voltage V63 is converted by the smoothing circuit.
It becomes 73. FIG. 5 shows the voltage waveforms described above. The DC voltage V73 can be arbitrarily selected according to the ratio between the closing operation time (TON) and the opening operation time (TOFF) of the semiconductor switch 53. Therefore, the variable DC voltage is V73 = V71 × TON / (TON
+ TOFF).

Next, the voltage-controlled DC voltage V73 is applied to the intermediate tap 312 of the transformer 31. Terminal 311
Is connected to the semiconductor switch 51, the terminal 313 is connected to the semiconductor switch 52, and the capacitor 72 is connected in parallel. The resonance circuit is constituted by the leakage inductance viewed from the primary side of the transformer 31 and the capacitor 72. The sine wave voltage V72 can be obtained between the secondary terminals 314 and 315 of the transformer 31 by performing the opening and closing operations of the semiconductor switches 51 and 52 at appropriate timings according to commands from the control circuit 4. The voltage applied between the terminals of the discharge tube 2 is about 800 V in effective value. DC power supply 1
Is about 10 V, the primary to secondary turns ratio of the transformer 31 is selected to be around 1: 200, which is a transformer having a high turns ratio far out of the normal range of the turns ratio.

The self-excited resonance type oscillation circuit includes a transformer 31
Resonates at a resonance frequency determined by the inductance between the terminals 311 and 313 and the capacitor 72, and a sine wave voltage V72 is obtained from the terminals 311-313, 314-315 or 316-317 of the transformer 31. The voltage waveform V51 shown in FIG. 5 is the voltage between the collector and the emitter of the semiconductor switch 51. When the terminal 317 connected to the base is at a negative voltage, the semiconductor switch 51 is in the open operation period. Therefore, a sine wave voltage is applied to the voltage of the terminal 311 connected to the collector of the semiconductor switch 51. , Terminal 3
When the voltage of the terminal 17 changes to a positive voltage, a closing operation period is started, and the terminal 311
Voltage difference disappears. On the other hand, as shown by V52, the voltage waveform of the semiconductor switch 52 has an antiphase relationship with V51.

V312 is a voltage waveform at the terminal 312 of the transformer 31. DC voltage V73 of capacitor 73 and terminal 31
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 312 of the transformer 31 and the voltage value of the capacitor 73 are equal. Further, the conventional push-pull circuit or bridge circuit does not utilize the flyback voltage. For this reason, a transformer having a characteristic with as low a flyback voltage as possible has been used. Furthermore, since a transistor that conducts in the reverse direction is generally used as a constituent semiconductor switch, the flyback voltage due to the back electromotive force generated during the period when the switch is opened is reduced by the effect of the reverse conduction characteristic. A voltage higher than the voltage could not be generated.

[0007]

The cold cathode fluorescent lamp requires an overvoltage of 150% to 200% of the lighting voltage at the start of discharge. Furthermore, if the lighting voltage waveform does not have symmetry, the life will be significantly shortened. Therefore, the conventional self-excited resonance type oscillation circuit is a drive circuit which satisfies the condition. However, in the case of adjusting the light emission luminance in the conventional driving circuit, a DC / DC converter circuit for varying the supply power supply voltage is required separately. For this reason, there is a technical problem that the loss of the voltage conversion unit in the DC / DC converter is added and the conversion efficiency is low. Further, since two inductors and a transformer are required as winding components, there is a problem that it is expensive and a mounting space is required for them, and it is difficult to reduce the size of the drive circuit.
In addition, there is a great demand for a backlight drive circuit for a liquid crystal display to be inexpensive and small.

[0008]

SUMMARY OF THE INVENTION The present invention provides a semiconductor switch having a reverse blocking characteristic and employs a bidirectional flyback transformer having an inductor function to reduce the excitation energy of the transformer during flyback operation. The present invention has conceived a circuit system for generating an oscillating voltage satisfying a resonance condition and supplying the oscillating voltage to a discharge tube. Further, by controlling the exciting current energy of the transformer, the light emission luminance can be adjusted. As a result, the DC / DC converter circuit used in the prior art can be omitted, and the inductor as a winding component can be eliminated.

[0009]

FIG. 1 shows an embodiment according to the present invention. FIG. 1 shows a case where the semiconductor switch has a push-pull configuration. The primary winding terminal 312 of the transformer 31 is connected to the positive terminal of the DC power supply 1, a capacitor 72 is inserted in parallel between the terminals 311 and 313, and a diode 6.
Semiconductor switches 51 and 52 are connected to the ground side of DC power supply 1 via 1 and 62. Further, a common resistor 82 for current detection is provided on the ground side of the semiconductor switches 51 and 52.
To the negative terminal of the DC power supply 1. A capacitor 71 for ripple current smoothing is inserted in parallel with the DC power supply 1. The gates of the semiconductor switches 51 and 52 are respectively connected to the control circuit 4, and the periods of the opening and closing operations are controlled. Further, the control circuit 4 has a function of detecting the voltage across the resistor 82 and monitoring the overcurrent, and also has a constant current function. The overcurrent value setting is obtained by adjusting the variable resistor 81.

FIG. 3 shows an example of a voltage waveform of each part. Q in the figure
Reference numerals 51 and Q52 denote operating states of the semiconductor switches 51 and 52, respectively, and indicate periods and timings of the opening operation and the closing operation. V82 is a voltage waveform between the terminals of the resistor 82, and indicates a current passing through the semiconductor switches 51 and 52.
V72a and V72b are voltage waveforms of the capacitor 72. V72a is a case where the capacitance of the capacitor 72 is reduced to increase its resonance frequency, and V72b is a capacitor voltage at a lower frequency.

The operation of FIG. 1 will be described in detail. When the semiconductor switch 52 is closed, the terminal 312 of the transformer 31 is closed.
From the terminal 313 to the DC power supply 1 through the diode 62 and the semiconductor switch 52. As shown in the figure, the exciting current increases with time, and when the semiconductor switch 52 shifts to the opening operation, the magnetic energy due to the exciting current stored in the transformer 31 is charged to the capacitor 72 as electric energy. After that, the voltage charged in the capacitor 72 causes a current to flow from the terminal 313 to the terminal 311 through the terminal 312, so that the voltage of the capacitor 72 decreases. This series of operations is a damped oscillation determined by the inductance between the terminal 311 and the terminal 313, the capacitor 72, and the load as shown by the voltage waveform of V72a.

Next, when the semiconductor switch 51 is closed, the terminal 312 is connected to the terminal 311 through the diode 61.
Excitation current flows in the direction, and rises over time. Further, when the semiconductor switch 51 is opened, the magnetic energy generated by the exciting current stored in the transformer 31 continues to flow in the direction from the terminal 312 to the terminal 311. Therefore, the terminal 31 is connected to the terminal 31 through the capacitor 72.
A current flows in the direction 3 to charge the capacitor 72 and increase the voltage. When the current decreases from the terminal 313 to the terminal 311 through the terminal 312 and stops, the voltage of the capacitor 72 becomes the maximum value. Thereafter, a current flows from the terminal 311 to the terminal 313 through the terminal 312 due to the voltage charged in the capacitor 72, and the voltage of the capacitor 72 decreases. This operation is exactly the same as the operation of the semiconductor switch 52, and attenuates while vibrating.

The role of the diode 61 will be described. Immediately after the semiconductor switch 52 shifts to the open operation, the voltage at the terminal 313 increases to twice or more the voltage of the DC power supply 1. In this case, between the terminals 311 and 312 of the transformer 31 and the terminal 31
Since the coils having the same number of turns are wound between 2 and 313, the potential of the terminal 311 is lower than the negative potential of the DC power supply 1. Therefore, if the diode 61 is not provided, a current flows from the negative terminal of the DC power supply 1 to the drain from the source due to the reverse conduction characteristics of the resistor 82 and the semiconductor switch 51, and the current flows from the terminal 311 to the terminal 312 via the terminal 312. Regenerative current flows to the positive terminal. On the other hand, the voltage of the terminal 313 reaches about twice the voltage of the DC power supply 1 and is clamped. Since the diode 61 is for preventing the reverse conduction characteristic of the semiconductor switch 51, it is unnecessary when the semiconductor switch 51 does not exhibit the reverse conduction characteristic. In addition, after the semiconductor switch 52 is opened, if the voltage rise at the terminal 313 is less than twice the voltage of the DC power supply 1, the diode 61 is unnecessary. Similarly, the diode 62 performs the above-described operation to prevent the reverse conduction characteristic of the semiconductor switch 52 after the semiconductor switch 51 is opened. The semiconductor switch 51 and 5
2, the diodes 61 and 62 are unnecessary.

The variable resistor 81 sets a threshold value of the terminal voltage of the resistor 82. In the control circuit 4, the reference voltage value set by the variable resistor 81 is compared with the terminal voltage value of the resistor 82,
The semiconductor switch 51 has the magnitude of the amplified value of the difference voltage.
Alternatively, the closing time of 52 is controlled. Resistor 82
Is compared with the reference voltage value set by the variable resistor 81. If the voltage is low, the closing time of the semiconductor switch 51 or 52 is lengthened to increase the exciting current of the transformer 31. When the exciting current of the transformer 31 increases, the semiconductor switch 51
Alternatively, the voltage after the open operation period of 52 becomes high, and the discharge tube 2
The light emission luminance increases due to an increase in the value of the current flowing through the device. In this way, the light emission luminance can be adjusted by the variable resistor 81.

FIG. 2 shows a half-bridge circuit configuration as another embodiment of the present invention. The positive terminal of the DC power supply 1 is connected to the terminal 313 of the primary winding of the transformer 31 through the semiconductor switch 52 and the diode 62. Further, the terminal 313
Is connected to the negative terminal of the DC power supply 1 through the diode 61 and the semiconductor switch 51. On the other hand, the positive terminal of the DC power supply 1 is connected to the terminal 311 of the primary winding of the transformer 31 through the capacitor 74. The terminal 311 is connected to the negative terminal of the DC power supply 1 through the capacitor 75. The capacitor 72 is connected in parallel between the terminals 311 and 313 of the primary winding of the transformer 31. The capacitors 74 and 75 have a sufficiently large capacity with respect to the capacitor 72.
The ripple current flowing through the DC power supply 1 is smoothed by the capacitor 71. Terminal 314 of secondary winding of transformer 31
And the discharge tube 2 to the terminal 315. Semiconductor switch 5
The gates of 1 and 52 are connected to the control circuit 4 respectively.
The closing operation time of the semiconductor switches 51 and 52 is performed by a variable resistor 81 for setting.

The circuit thus constructed is the same as the circuit shown in FIG.
The same operation as in the case of is performed. The circuit operation of FIG. 2 will be described. When the semiconductor switch 52 is closed, current flows from the terminal 313 through the diode 62, and an exciting current flows through the capacitor 75. Next, when the semiconductor switch 52 shifts to the opening operation, the magnetic energy stored in the transformer 31 is replaced by the capacitor 72 as electric energy. When the charging current from the terminal 313 to the terminal 311 decreases to zero, the voltage of the capacitor 72 reaches the maximum value. Thereafter, a current flows from the terminal 311 to the terminal 313 in the primary winding of the transformer 31 by the voltage charged in the capacitor 72, so that the voltage of the capacitor 72 decreases. This operation is performed by the V72a described in the case of FIG.
Is exactly the same as

Next, when the semiconductor switch 51 is closed, the potential of the terminal 313 becomes zero.
, A current flows from the terminal 311 to the terminal 313 through the diode 61 and the semiconductor switch 51. Further, when the semiconductor switch 51 is controlled from the closed state to the open state, the magnetic energy stored in the transformer 31 causes the current to continue flowing from the terminal 311 to the terminal 313 in the primary winding of the transformer 31. 313 to capacitor 7
A current flows in the direction of the terminal 311 through 2 to charge the capacitor 72 and increase the voltage. When the charging current decreases and stops, the voltage of the capacitor 72 becomes the maximum value. Thereafter, a current flows from the terminal 313 to the terminal 311 in the primary winding of the transformer 31 due to the voltage charged in the capacitor 72, so that the voltage of the capacitor 72 decreases. This operation is a voltage of damped oscillation similar to the above. The role of the diodes 61 and 62 is the same as in the embodiment of FIG. 1, and is unnecessary if the semiconductor switch uses a switch that does not exhibit reverse conduction characteristics.

Although FIG. 2 shows a half-bridge circuit configuration, the same effects as those of the present invention shown in FIG. 1 can be obtained by using the reverse blocking semiconductor switches composed of the above-described semiconductor switches and diodes for the capacitors 74 and 75. Also,
The closing time of the semiconductor switch 51 or 52 is set by the variable resistor 81. The resistor 82 of FIG. 1 does not control the current value, but uses the semiconductor switch 51 or 52.
Since the exciting current value of the transformer 31 is controlled by setting the closing time of, the emission luminance can be adjusted. In this case, the resistor 81 and the error amplifier circuit are unnecessary, but there is a disadvantage that the voltage fluctuation of the DC power supply 1 appears as the fluctuation of the exciting current value of the transformer 31. 1 and 2, the capacitor 72 is connected. However, if the distributed capacitance formed between the windings of the transformer 31 is used, the capacitor 72 can be omitted or replaced with a capacitor having a reduced capacitance. .

In the experimental prototype, a flyback voltage higher than the excitation voltage could be used, so the flyback voltage was designed to be six times the excitation applied voltage. Therefore, the number of windings of the conventional transformer is as follows: primary winding: 10 turns * 2 winding, secondary winding:
In contrast to 1800 turns, according to the present invention, the primary winding is designed to be 10 turns * 2 windings, the secondary winding is set to 450 turns, and the number of turns is designed to be 1/4. In terms of external dimensions, conventionally, for a 20.3cc volume of 145mm * 20mm * 7mm,
The size was 8.0cc, 95mm * 14mm * 6mm, and the volume ratio was 39%.

[0020]

As is apparent from the above description, the booster circuit according to the present invention does not require a winding component such as an inductor. The winding component is a transformer only, and the light emission luminance of the discharge tube can be adjusted. According to the present invention, the number of components is small, the reliability is improved, and the device can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is an embodiment according to the present invention.

FIG. 2 is another embodiment according to the present invention.

FIG. 3 is an example of a voltage waveform when the present invention is implemented.

FIG. 4 is a circuit diagram showing a conventional example.

FIG. 5 is a voltage waveform according to the related art.

[Explanation of symbols]

1 DC power supply, 2 discharge tubes, 31 transformers, 311
317 terminals, 33 inductors, 4 control circuits, 51
~ 53 Semiconductor switch, 61-63 Diode, 7
1-75 Capacitor, 81 Variable resistor, 82-84
Resistor

Claims (6)

[Claims] 1. A discharge tube driving circuit that includes a DC power supply, a plurality of semiconductor switches, a control circuit, and the like on a primary side of a transformer and connects a discharge tube to a secondary side, wherein a capacitor is connected in parallel to a primary winding. A discharge tube driving circuit, wherein the semiconductor switch has a reverse non-conducting characteristic, and the semiconductor switch and the transformer have a push-pull configuration. 2. A discharge tube driving circuit comprising a DC power supply, a plurality of semiconductor switches, a control circuit, and the like on a primary side of a transformer, and a discharge tube on a secondary side, wherein a capacitor is connected in parallel to a primary winding. The semiconductor switch has a reverse non-conducting characteristic, and the semiconductor switch and the transformer are configured as a half bridge. 3. A discharge tube driving circuit that includes a DC power supply, a plurality of semiconductor switches, a control circuit, and the like on a primary side of a transformer, and connects a discharge tube to a secondary side. A discharge tube driving circuit, wherein the semiconductor switch has reverse non-conductive characteristics, and the semiconductor switch and the transformer are configured in a full bridge. 4. The discharge tube driving circuit according to claim 1, wherein the voltage waveform on the secondary side does not substantially include even-order harmonics. 5. The discharge tube driving circuit according to claim 1, wherein the primary to secondary turns ratio of the transformer is 1: 4 or less. 6. The transformer according to claim 1, wherein the stray capacitance between the primary and secondary of the transformer is used to omit the capacitor, or a low-capacitance capacitor is connected in parallel. Discharge tube drive circuit.