JP2000133484A - Discharge tube driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remarkably reduce the circuit components and to adjust the luminance of a discharge tube by converting the excitation energy of a bidirectional flyback transformer into the self-excited energy satisfying the resonance condition in the flyback operation, to be fed and applied to the discharge tube. SOLUTION: A driving circuit comprises a power source part, a control circuit part, a converter circuit part of push-pull connection structure, and a set-up circuit part. The power source voltage is stabilized by a ripple current smoothening capacitor 71 connected to a DC power source 1 in parallel. A control circuit 41 monitors the overcurrent and comprises the constant current function by detecting the both end voltage of a resistor 85. The primary current of a transformer 31 is detected by a terminal 415, and a terminal 416 monitors the current of a discharge tube 2. A diode 61 controls the potential of the terminals 311, 312 to prevent the clamping condition of the voltage, and the potential of a terminal 311 is risen to be at least 2 times voltage of the DC power source 1. A variable resistor 81 determines a threshold of the terminal voltage of the resistor 85, whereby the close time of the semiconductor switches 51, 52 is controlled to adjust the impressed voltage and to adjust the luminescent brightness.

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 suitable for 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 Conventionally, a self-excited resonance type driving circuit having a boosting function has been used to drive a cold cathode discharge tube. This type of circuit was applied because of its stable operation and its output being a sine wave. In this self-excited resonance type, DC voltage is several tens of kilohertz by a self-excited resonance circuit.
After generating an AC voltage of z, the voltage is raised to several hundred volts by a transformer and applied to a discharge tube to emit light. But,
Since the primary side of the transformer constitutes a resonance circuit, circuit operation outside the resonance frequency is impossible in principle, and the output voltage or frequency during operation is substantially fixed. The brightness of the cold cathode discharge tube is adjusted by the magnitude of the voltage applied between the electrodes. However, since the self-excited resonance type oscillation circuit described above cannot control the output voltage, the brightness is designed to be constant, and the brightness is usually adjusted. Function was omitted. For this reason, an imperfect voltage control circuit is added only when it is unavoidable. However, the demand for providing high-performance luminance adjustment characteristics has increased as the demand for higher functionality has increased, and a method of inserting a DC / DC converter before the booster circuit has been adopted. However, the addition of the booster circuit not only complicates the circuit configuration but also poses a major obstacle to miniaturization and low cost.

FIGS. 4 and 5 show a circuit configuration of a conventional system and voltage waveforms at main parts thereof. The illustrated circuit includes a DC power supply, a DC / DC converter, a self-excited oscillation circuit, a booster circuit, and a control circuit. The DC / DC converter includes a semiconductor switch 53, an inductor 32, a capacitor 73, and a diode 63. This DC / DC
In the converter, the voltage V71 of the DC power supply 1 is temporarily changed to a pulsating flow by the chopping operation of the semiconductor switch 53, and a target DC voltage V73 is obtained from both ends of the capacitor 73. When the set value of the variable resistor 81 provided in the control circuit 42 is changed,
Since the duty of the drive pulse supplied from the terminal 423 to the semiconductor switch 53 changes, the set value of V73 can be changed. Since the constant power supply voltage V71 is converted into an appropriate DC voltage V73 and supplied to the self-excited oscillation circuit and the booster circuit, a variable DC power supply is connected to the primary side, and as a result, The discharge tube applied voltage is controlled. As the control circuit 42, a chopper IC is generally used. The terminal 424 is a current detection terminal of the discharge tube 2 and is used for overcurrent protection or control at the time of discharge current.

Next, the self-excited resonance operation will be described. The self-excited oscillation circuit includes the primary winding of the transformer 34, the semiconductor switches 54 and 55, the capacitor 74, and the like. The input voltage V73 is supplied to the intermediate tap terminal 342 of the transformer 34 through the inductor 33. For example, when the semiconductor switch 54 starts conducting, positive feedback is applied to the base side, so that the semiconductor switch 54 suddenly shifts to the closed state, and the other semiconductor switch 55 is opened because the base side is negatively biased. . At this time, the electric charge of the capacitor 74 is discharged through the primary winding of the transformer 34. After a predetermined time, when the discharge current changes from increasing to decreasing, the phase of the induced voltage between the terminals 346 and 347 of the transformer 34 is inverted, so that the base of the semiconductor switch 54 becomes negative and the base of the semiconductor switch 55 becomes positive. Be biased. For this reason, the state of the semiconductor switch 54 is rapidly changed from closed to open, and the state of the semiconductor switch 55 which has been open is simultaneously inverted to closed.

Further, after a predetermined time, each semiconductor switch is inverted again to return to the initial state. Self-excited oscillation with such a state change as one cycle continues. With this operation, the transformer 34
A sine wave voltage V72 is induced between the secondary terminals 344 and 345. As is clear from the above description, the predetermined time is a resonance cycle determined by the inductance of the capacitor 74 and the transformer 34. The voltage applied to the discharge tube 2 is 8 as an effective value.
Since the voltage of the DC power supply 1 is
Since it is about V, the primary and secondary turns ratio of the transformer 34 uses a transformer having a high turns ratio of about 1: 200. Further, the output voltage V72 is stabilized by detecting the current of the discharge tube 2 with the resistor 85 and inputting it to the control circuit 42.

FIG. 5 shows waveforms of main parts in the operation described above. First, in the DC / DC converter, 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 53, and is converted to an average DC voltage V73 by the smoothing circuit. V73 can be appropriately selected by the ratio (TON / (TON + TOFF)) of the closing operation time (TON) and the opening operation time (TOFF) of the semiconductor switch 53. This is set by the variable resistor 81 of the control circuit 42 or the like. In the self-excited resonance circuit, since a resonance circuit is formed by the inductance between the terminals 341 and 343 of the transformer 34 and the capacitor 72, a sine wave voltage V72 is obtained from the secondary winding of the transformer 34. The voltage waveform of V54 is the collector-emitter voltage of the semiconductor switch 54. When the terminal 347 connected to the base is at a lower potential than the emitter, the semiconductor switch 54 is in the open operation period, and a half-wave of a sine wave as shown in the figure appears between the collector and the emitter. However, the terminal 347 shifts to a positive potential with respect to the emitter to enter a closing operation period, and the forward saturation voltage appears at V54, and the voltage drop is substantially zero. On the other hand, since the semiconductor switch 55 performs an operation completely opposite to that of the semiconductor switch 54, V55 has an anti-phase relationship with V54 as shown in the figure.

V 342 is a terminal 342 of the transformer 34.
FIG. Since a difference voltage between the DC voltage V73 of the capacitor 73 and the voltage V342 of the terminal 342 is generated between the terminals of the inductor 33, the voltage-time product values S331 and S332 (corresponding to the hatched portions in the figure) become equal, and the voltage pulsation of the inductor 33 is obtained. Observed as minutes. Therefore, the terminal 3 of the transformer 34
The average voltage at 42 is equal to V73. Since the conventional push-pull circuit does not utilize the flyback voltage, a transformer in which the flyback voltage is suppressed as much as possible has been designed and used. Since a semiconductor switch generally uses a transistor or the like that does not have a blocking function in the reverse direction, it cannot block a flyback voltage generated during a switch operation period in a closed state, and the primary voltage of the transformer exceeds the power supply voltage. The voltage could not be increased. For this reason, a transformer having a high turns ratio is used, which is a design problem. In addition, 2
Since the secondary side voltage cannot be controlled, the capacitor 75 is set to a value equal to or higher than the discharge starting voltage of the discharge tube 2, and
(Ballast capacitor) to share the overvoltage. As a result, an increase in the voltage of the discharge tube 2 has been suppressed.

[0008]

In the prior art, the output voltage could not be adjusted and controlled, so that an overvoltage of 150% to 200% of the lighting voltage was set at the start of discharge, and the overvoltage was distributed to the ballast capacitor 75 after the start of discharge. I was pressured. In addition, if the lighting voltage waveform does not have symmetry, the life will be significantly shortened.Therefore, the conventional self-excited resonance type circuit requires a large number of circuit components, particularly winding components, to satisfy the condition. Used a lot. As a result, the drive circuit configuration becomes expensive and unsuitable for miniaturization. Further, in the prior art, a DC / DC converter circuit is inserted before the booster circuit to adjust the light emission luminance.
In addition to the loss of circuit elements in the DC / DC converter,
This has created a cause for further lowering the efficiency of the drive circuit. However, at least an inductor and a transformer are required to boost the voltage, which is a major bottleneck in weight reduction and space saving. In general, LCD backlights are light and thin.
There is a great potential need to reduce the cost and size of the drive circuit due to the variety of applications.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and can be solved at once by applying a bidirectional flyback transformer to the conventional problems. The present invention has been made under the heading. In the prior art, the application of a flyback transformer was not considered from the beginning, because a sine wave was applied to the discharge tube as an output voltage due to the demand for symmetry of the output waveform. However, if a bidirectional flyback transformer is applied, the output waveform can be made a symmetrical waveform within a practically acceptable range, and the output voltage can be controlled. More specifically, the circuit is of a circuit type capable of converting the excitation energy of a transformer into a self-excitation voltage satisfying a resonance condition during a flyback operation and supplying the voltage to a discharge tube. The present invention focuses on the fact that it is easy to control the magnetic energy by the excitation current of the transformer, and adds a light emission luminance adjustment characteristic suitable for a discharge tube driving circuit. Furthermore, when applying a bidirectional flyback transformer, processing of reverse voltage during flyback was a problem, but as a countermeasure, by connecting circuit elements such as diodes in series to a DC power supply, inconvenient flyback Could be solved. As a result, it is possible to omit the DC / DC converter circuit used in the prior art, and it is possible to greatly reduce circuit components such as inductors or semiconductor switches as winding components. Further, it is not necessary to use a transformer having a high turns ratio, and the circuit design is simplified. The technical idea of the present invention is
As is apparent from the above description, it is a method of simplifying the discharge tube driving circuit, and is a method of obtaining performance equal to or higher than that of the related art with a minimum necessary circuit element configuration.

[0010]

FIG. 1 shows an embodiment according to the present invention. As shown, the drive circuit of the present invention includes a power supply unit, a control circuit unit, a converter circuit unit having a push-pull connection configuration, and a booster circuit unit. The primary side of the transformer 31 is connected to the terminal 31
1 and a terminal 313, a capacitor 72 is connected in parallel, and semiconductor switches 51 and 52 are connected to respective terminals.
A common resistor 82 is connected in series to form a converter circuit. An intermediate tap is provided in the primary winding of the transformer 31, and a diode 6 is provided between the terminal 312 and the DC power supply 1.
1 is inserted at the polarity shown. Semiconductor switches 51 and 5
The gates 2 receive a control signal from a control circuit 41 using a push-pull IC or the like, and the periods of the opening and closing operations are controlled. This open or closed period is limited to a resonance period determined by the leakage inductance of the capacitor 72 and the transformer 31. If the opening or closing cycle of the semiconductor switches 51 and 52 is longer than the resonance cycle, the operation becomes unstable, so that a stable voltage cannot be applied to the discharge tube 2. Therefore, it is necessary to satisfy this restriction condition when setting the operation of the semiconductor switches 51 and 52. A detailed description of the operation will be described later.

As compared with the conventional circuit shown in FIG. 4, the DC / DC converter is omitted in the present invention, and a simplified circuit configuration in which the transformer 31 does not have a feedback winding can be provided. Also, a capacitor 71 for ripple current smoothing is connected in parallel with the DC power supply 1 to take account of stabilization of the power supply voltage. Further, the control circuit 41 has overcurrent monitoring and constant current functions by detecting the voltage across the resistor 85. The constant current is obtained by changing the set value of the variable resistor 81. On the other hand, terminals 411 and 412 are power terminals of the control circuit 41, and terminals 413 and 41
4 is a base drive terminal of the semiconductor switches 52 and 51. Further, at the terminal 415, the primary current of the transformer 31
(V82) Detect and monitor the current of the discharge tube 2 via the terminal 416.

FIG. 2 shows examples of voltage waveforms at various parts when the present invention is implemented. A time chart of the opening and closing operations of the semiconductor switches 51 and 52 is shown in the figure. V82 is a voltage waveform between terminals of the resistor 82,
And 52, that is, the primary current of the transformer. On the other hand, V72a and V72b indicate the voltage waveform of the capacitor 72, V72a when the capacitance of the capacitor 72 is reduced to increase the resonance frequency, and V72b when the capacitance is increased and the resonance frequency is low. The circuit operation will be described in detail with reference to FIGS. When the base potential of the semiconductor switch 52 is made positive by the control circuit 41, the current flows in from the terminal 312, exits from the terminal 313, passes through the semiconductor switch 51, and flows back to the DC power supply 1. This current is an exciting current for the transformer 31 and increases linearly with time as the semiconductor switch 52 closes. When the semiconductor switch 52 shifts to the opening operation after a predetermined time, the magnetic energy stored in the transformer 31 is stored as electric energy in the capacitor 72, and the capacitor terminal voltage V72
Rises rapidly. Since V72 forms a closed circuit from the terminal 313 to the terminal 311 through the terminal 312, it once rises to the maximum value and then falls. This series of operations results in damped oscillation determined by the inductance between the terminals 311 and 313, the capacitor 72 and the equivalent load resistance of the discharge tube 2 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. After a while, the semiconductor switch 51
Is opened, the exciting current of the transformer 31 is kept at the terminal 3
A current for charging the capacitor 72 from the terminal 12 to the terminal 311 continues to flow. Therefore, V72 falls once and then rises. At the moment when the charging current decreases to zero, the voltage of the capacitor 72 reaches the maximum value. Thereafter, a current corresponding to the vibration flows from the terminal 311 to the terminal 313 through the terminal 312, and the capacitor 72
Voltage decreases. This operation is exactly the same as the operation of the semiconductor switch 52, but has an opposite phase relationship.

Next, the role of the diode 61 will be described.
Immediately after the semiconductor switch 52 shifts to the open operation, the terminal 313
Is more than twice as high as the DC power supply 1. Transformer 3
1 between terminals 311 and 312 or terminals 312 and 313
Since the number of turns of the winding between them is the same, the potential of the terminal 311 becomes lower than the negative potential of the DC power supply 1. For this reason,
When the diode 61 is not inserted, a current flows from the source to the drain due to the reverse conduction characteristic of the semiconductor switch 51 in the DC power supply 1, and a regenerative current flows from the terminal 311 to the positive terminal of the DC power supply 1 via the terminal 312. . For this reason, the voltage of the semiconductor switch 52 cannot be increased to more than twice the voltage, and the voltage is clamped. However, if the diode 61 is inserted, the potential of the terminal 311 stops at the negative voltage value of the DC power supply 1. By raising the potential of the terminal 312 from the positive potential of the DC power supply 1, the potential of the terminal 311 can be raised to twice or more the voltage of the DC power supply 1. on the other hand,
Immediately after the semiconductor switch 51 shifts to the opening operation, the voltage of the terminal 311 rises to twice or more the voltage of the DC power supply 1. When the voltage rise is less than twice the voltage of the DC power supply 1, the diode 61 is unnecessary. In addition, it is more convenient for the diode 61 to be inserted on the load side than the control circuit 41 against overvoltage.

The variable resistor 81 sets the threshold value of the terminal voltage of the resistor 82. In the control circuit 41, the reference voltage value set by the variable resistor 81 is compared with the average value (or effective value) of the terminal voltage of the resistor 85, and the semiconductor switch 51 or 52 is amplified by the magnitude of the amplified value of the difference voltage. Is to control the closing time. The voltage across the resistor 85 is the variable resistor 81
Is compared with the reference voltage value set in (1), and when it is lower than the reference value, the closing time of the semiconductor switch 51 or 52 is lengthened to increase the exciting current of the transformer 31. On the other hand, transformer 3
1 increases, the semiconductor switch 51 or 5
2, the voltage after the open operation period increases, and the light emission luminance increases due to an increase in the value of the current flowing through the discharge tube 2. In this state, the luminance can be adjusted by lowering the setting of the variable resistor 81 and suppressing the imprint voltage on the discharge tube.

Another embodiment according to the present invention is shown in FIG. The operation principle is the same as that of FIG. 1, but the configuration of the converter is different. FIG. 3 shows a push-pull configuration and FIG. 3 shows a bridge configuration. The semiconductor switches 56 and 52 and the semiconductor switches 57 and 51 respectively open and close at the same time, but the opening and closing operations are in opposite phase relation. That is, the semiconductor switch 5
While the groups 6 and 52 are open, the semiconductor switches 57
And 51 are in the closed period. The semiconductor switch 57 → terminal 3 from the DC power supply 1 via the diode 61
12 and 313 → the semiconductor switch 51 → the exciting current flows through the resistor. Conversely, semiconductor switches 51 and 57
Is in the open period, the capacitor 72 is charged with the back electromotive force of the transformer 31 and the voltage rises. At this time, unless the diode 61 is inserted, the potential of the terminal 313 does not become higher than the voltage of the DC power supply 1 due to the reverse conduction characteristics of the semiconductor switches 52 and 56, and the magnetic energy stored in the transformer 31 is regenerated to the DC power supply 1. You.

Since a high flyback voltage due to the exciting current can be used, the flyback voltage was designed to be about six times the applied voltage in the confirmation experiment. The discharge tube with a discharge starting voltage Vs = 1200 V was turned on using a 10 V DC power supply. A primary winding: 14 turns × 2 windings, a secondary winding: a transformer having a turn ratio of 40 times that of 560 turns was prototyped. When the voltage generated by the transformer is calculated based on the DC power supply voltage, the secondary voltage is 10 V × 40 = 400 V, and does not reach the discharge start voltage Vs = 1200 V. In the conventional method, the discharge tube could not be lit at this turn ratio. However, in the present invention, since the flyback voltage works, before the start of discharge, 2400 V which is six times or more than that can be applied, so that a transformer having a small number of secondary windings can be used.

On the other hand, in the conventional system, the number of turns of the transformer required a turn ratio of 1200 V = 10 V × 120 or more in order to generate a discharge starting voltage Vs = 1200 V or more. In the conventional method, the turns ratio of the primary winding is 10 turns × 2 turns, and the secondary winding is 1800 turns, and the turns ratio of the transformer according to the present invention is 40. Comparing only the next number of turns, it could be reduced to 1/3 or less. It can be easily understood that the transformer can be reduced in size according to the present invention even if only the turns ratio is compared. Furthermore, comparing the outer dimensions of the core of the transformer, the size is 29.5 cc, which is 145 mm x 20 mm x 7 mm, and is 8.0 cc, which is 95 mm x 14 mm x 6 mm. .

[0019]

As described above, the present invention provides a discharge tube driving circuit capable of changing the secondary voltage of the transformer even if the DC / DC converter is omitted, that is, adjusting the luminance, as is apparent from the detailed description. it can. This method can be realized by applying a bidirectional flyback transformer, which has not been neglected in the past, and inserting a diode into a DC power supply, and does not require winding components such as an inductor. D
Since the circuit configuration is simplified by eliminating the C / DC converter and the inductor, space saving in circuit mounting can be obtained. At the same time, it can be configured with inexpensive parts, and enables more efficient operation.

[Brief description of the drawings]

FIG. 1 is a circuit configuration showing an embodiment according to the present invention.

FIG. 2 is an example of a voltage waveform of an embodiment according to the present invention.

FIG. 3 is a circuit configuration showing another embodiment according to the present invention.

FIG. 4 is a circuit configuration showing a conventional technique.

FIG. 5 is a waveform example in the related art.

[Explanation of symbols]

1 DC power supply, 2 discharge tubes, 31, 34 transformer, 3
11 to 315, 341 to 347 terminals, 32, 33 inductor, 41, 42 control circuit, 51 to 57: semiconductor switch, 61, 63 diode, 71 to 75 capacitor, 81 variable resistor, 82 to 85 resistor

Claims (6)

[Claims] A DC power supply, a converter circuit, and a control circuit are connected to a primary winding of a transformer, and an AC voltage having a required frequency is induced in a secondary winding of the transformer and applied to a cold cathode fluorescent lamp. In the discharge tube driving circuit, the transformer is configured as a bidirectional flyback transformer, a capacitor is connected in parallel to the primary winding, and a diode is inserted in a forward direction with respect to the DC power supply. Tube drive circuit. 2. The discharge tube driving circuit according to claim 1, wherein said converter circuit is configured as a push-pull. 3. The discharge tube driving circuit according to claim 1, wherein the converter circuit is configured as a bridge. 4. The discharge tube driving circuit according to claim 1, wherein the induced voltage of the secondary winding has a symmetric waveform substantially not including even-order harmonics. 5. The discharge tube driving circuit according to claim 1, wherein the primary to secondary turns ratio of the transformer is 1:50 or less. 6. The discharge tube driving circuit according to claim 1, wherein a secondary winding induced voltage is controlled by detecting a current flowing through the cold cathode discharge fluorescent tube.