JP2005005059A - Separately excited inverter circuit for discharge tube lighting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable to control a tubular current to approximately constant even in the case of a connecting form in which there is no discharge tube return line. <P>SOLUTION: In a separately excited inverter circuit, which is equipped with a pulse width control part 14 to form a timing signal by comparing a triangular wave signal A from a transmitter 12 and two control voltages Va, Vb, two switching elements Q1, Q2 which are ON/OFF controlled by the timing signal, two transformers T1, T2, and resonance capacitors C1, C2 installed on that secondary side, and in which both ends of discharge tubes 10a, 10b connected between the secondary sides of the both transformers are applied of a high voltage having reverse phase alternately and are made to turn on, a transformer voltage detecting part 22 to detect a transformer voltage of the primary side and a tube current stabilizing amplifier 24 to control the control voltage by that detected voltage are installed, and because the gain of the amplifier is set low several times or less, the detected voltage is made to be fluctuated according to a prescribed tilt to the input voltage, thereby the tube current is indirectly controlled approximately constant. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a separately-excited inverter circuit for lighting a discharge tube. More specifically, the present invention relates to a gain of a tube current stabilizing amplifier that receives a detection voltage of a transformer voltage detection unit that detects a transformer voltage on the primary side. The present invention relates to a separately excited inverter circuit for lighting a discharge tube in which the tube current is indirectly controlled almost constant by setting it to be several times or less. This technique is useful, for example, for a lighting device for a cold cathode discharge tube for a backlight of a large liquid crystal panel.
[0002]
[Prior art]
[Patent Document 1]
Japanese Patent No. 2948684 [Patent Document 2]
JP-A-8-78180 [0003]
As a backlight light source in a liquid crystal display device, a cold cathode discharge tube is usually used. In such a discharge tube, when the input voltage fluctuates, the tube current (discharge current flowing through the discharge tube) also fluctuates, so that the brightness as the backlight changes. Therefore, a current transformer is inserted in series with the discharge tube (for example, see Patent Document 1), a resistor is inserted (for example, see Patent Document 2), and the tube current is directly detected to keep the tube current constant. To be controlled.
[0004]
In recent years, along with the increase in size of liquid crystal panels such as for large liquid crystal display devices, long discharge tubes have been used as the backlight light source. In that case, in a general connection configuration in which the low pressure side of the discharge tube is returned by a return line, the wiring length connected to both ends becomes long, so that the light emission efficiency is lowered or a large amount of expensive high voltage wiring material is required. Therefore, problems such as an increase in cost occur.
[0005]
Therefore, a high voltage that is phase-inverted is applied to both ends of one long discharge tube or U-shaped discharge tube, or two discharge tubes are connected on the low-pressure side, and a high voltage in the opposite phase is applied to each discharge tube. An application method has been developed. As a result, the return line of the discharge tube becomes unnecessary, and the above problem can be solved.
[0006]
[Problems to be solved by the invention]
However, when a discharge tube having a connection form without a discharge tube return line is driven by a separately-excited inverter circuit, a resonant capacitor exists in parallel with the secondary side high-voltage winding of the transformer, so the tube current is directly detected and controlled. I can't. This is because when a resistor or a current transformer is inserted in series in the discharge tube, the tube current and the resonance current flow together and the resonance current also fluctuates in proportion to the input voltage, so that the tube current cannot be controlled stably.
[0007]
Therefore, a method of detecting and controlling the current by inserting a resistor or a current transformer on the primary side of the transformer is also conceivable. Again, the tube current cannot be controlled stably.
[0008]
An object of the present invention is to provide a separately-excited inverter circuit capable of controlling the current flowing through the discharge tube to be substantially constant in the case of a connection configuration having no discharge tube return line. Another object of the present invention is to provide a separately-excited inverter device that can be reduced in size and cost by arranging one or more inverter units having the same structure in parallel and controlling them by a common control unit. .
[0009]
[Means for Solving the Problems]
The present invention includes a transmitter that generates a triangular wave signal having a tube driving frequency, a pulse width control unit that generates a timing signal by comparing a triangular wave signal from the transmitter with a control voltage, and a timing from the pulse width control unit. A plurality of switching elements controlled to be turned on / off based on a signal, first and second transformers driven in opposite phases by the switching elements, and a resonant capacitor provided in parallel on the secondary side of both transformers. This is a separately-excited inverter circuit that turns on the discharge tube by alternately applying a high voltage of opposite phase to both end electrodes of one or two series discharge tubes connected between the secondary sides of both transformers. In the present invention, a transformer voltage detector that detects a transformer voltage on the primary side and a tube current stabilization amplifier that controls two control voltages of the pulse width controller according to the detected voltage are provided to reduce fluctuations in the input voltage. On the other hand, the gain of the tube current stabilizing amplifier is set to be several times or less (for example, about 5 to 0.1) so that the detection voltage changes with a certain slope, so that indirect control is performed so that the tube current becomes substantially constant. This point is unique.
[0010]
As a switching method of the separately excited inverter circuit for lighting a discharge tube, which is a premise in the present invention, for example, there is an active clamp method. In that case, the first and second comparators for generating a timing signal by comparing a triangular wave signal from the transmitter with a triangular wave signal from the transmitter and two different control voltages, respectively. And a first and second switching elements that are on / off controlled based on a timing signal from the pulse width controller. The two comparators of the pulse width control unit compare the triangular wave signal from the transmitter with two different control voltages, respectively, and generate a timing signal for controlling on / off of the two switching elements with the comparison signal. Yes, a simultaneous pause period is provided in the two comparator output periods to prevent circuit damage.
[0011]
However, the present invention is not limited to such an active clamp system, but can be applied to other switching systems, that is, a push-pull system, a half-bridge system, and a full-bridge system separately-excited inverter circuit for lighting a discharge tube. Needless to say.
[0012]
Here, another capacitor is connected in series with the resonant capacitor on the secondary side of the transformer, and an open-circuit voltage / spark discharge detection unit that detects an open-circuit voltage and a high-voltage spark discharge according to the voltage at the intermediate point is provided. It is preferable that the primary oscillation is controlled on the basis of this, and the primary oscillation is stopped based on the detected high-voltage spark discharge voltage. If there is a connector contact failure or soldering failure, high-pressure spark discharge may occur, and pulse voltage noise is superimposed on the tube drive voltage. Then, serious problems such as ignition and smoke may be caused. By detecting such an abnormality and stopping the primary oscillation, safety can be ensured.
[0013]
In addition to such a separately excited inverter circuit for lighting a discharge tube, the present invention includes a plurality of switching elements, two transformers driven in opposite phases by the switching elements, and a secondary side of each transformer. A configuration in which one or more inverter units each having a resonance capacitor provided in parallel are arranged in parallel, and each switching element of the inverter unit is simultaneously driven by a timing signal from a pulse width control unit of the separately excited inverter circuit for discharge tube lighting There is also. When such a configuration is adopted, an inexpensive separately-excited inverter device for lighting a large number of discharge tubes can be obtained.
[0014]
【Example】
FIG. 1 is a circuit diagram showing an embodiment of a separately excited inverter circuit for lighting a discharge tube according to the present invention. In this embodiment, two discharge tubes 10a and 10b are interconnected on the low pressure side, and a high voltage having an opposite phase is applied to each discharge tube, thereby eliminating the need for a return line of the discharge tube. . As described above, this configuration eliminates the need for a discharge tube return line, and therefore has the advantage of reducing costs without requiring a large amount of expensive high-voltage wiring material.
[0015]
The separately-excited inverter circuit for lighting the discharge tubes 10a and 10b having such a connection form is driven by the transmitter 12, the pulse width control unit 14, the switching unit 16 controlled by the pulse width, and the switching unit 16. First and second transformers T1, T2 and resonant capacitors C1, C2 provided in parallel on the secondary side of each transformer are provided. Discharge tubes (cold cathode discharge tubes) 10a and 10b are connected to the transformers T1 and T2, respectively, and this circuit alternately reverses the phase of the opposite electrodes of the two series discharge tubes connected between the secondary sides of both transformers. Lights up when high voltage is applied.
[0016]
The transmitter 12 generates a triangular wave signal having a tube driving frequency (for example, about several tens of kHz). The pulse width control unit 14 divides the power supply voltage _{VCC of the} circuit to generate two different control voltages Va and Vb, three resistors R1, R2 and R3 connected in series, and a triangular wave from the transmitter 12 First and second comparators 18a and 18b for comparing the signal and two different control voltages are provided, and two types of rectangular wave-like timing signals are generated according to the comparison result. The switching unit 16 has a configuration in which a first switching element (p-channel MOS) Q1 and a second switching element (n-channel MOS) Q2 are coupled in series, and the timing signal is supplied via the pMOS driver 20. The first and second switching elements Q1, Q2 are alternately turned on / off. The first and second transformers T1 and T2 are driven in opposite phases by the switching elements Q1 and Q2.
[0017]
An example of a timing signal for driving the switching element is shown in FIG. A triangular wave signal from the transmitter 12 is supplied to the negative ends of the two comparators 18a and 18b, while the two different powers obtained by dividing the power supply voltage of the circuit by a series connection of three resistors R1, R2 and R3. Control voltages Va and Vb are applied to the positive ends of the first and second comparators 18a and 18b. The first comparator 18a compares the control voltage Va and the triangular wave signal to generate a first timing signal, which is input to the first switching element Q1, and similarly, the second comparator 18b is connected to the control voltage Vb and the triangular wave signal. The signals are compared to generate a second timing signal, which is input to the second switching element Q2. Since the first timing signal is for driving the first switching element Q1, which is a p-channel MOS, it has a phase opposite to that of the second timing signal.
[0018]
In this way, the circuit operates when the first and second switching elements Q1, Q2 are alternately turned on and off. The second switching element Q2 functions as a main switch for transmitting energy to the transformer secondary side, and the first switching element Q1 functions as an auxiliary switch for resetting the transformer.
[0019]
Here, the first and second comparators 18a and 18b are provided with a simultaneous pause period td in their output periods. In another example inverter circuit including two switching elements Q1, Q2 and two transformers T1, T2, when two switching elements are alternately turned on / off, there is a delay in the current stop time of the switching element with respect to the off signal. If the switching periods are turned on or off at the same time, a simultaneous on-current flows and the circuit may be destroyed. Therefore, as described above, simultaneous break periods are provided in the two comparator output periods to prevent the circuit from being damaged.
[0020]
Basically, the two discharge tubes 10a and 10b interconnected on the low voltage side in this way are lit. The discharge tube can be equivalently expressed by a series connection of a Zener diode and a resistor. Accordingly, when the input voltage V _DD changes, the tube current also changes, and the brightness changes accordingly. Therefore, it is necessary to stabilize the tube current in order to keep the brightness constant. Since there is no discharge tube return line, the problem that the tube current cannot be directly detected and controlled is solved as follows. In addition, the fluctuation factor on the transformer secondary side with respect to the fluctuation of the input voltage includes the resonance current flowing in the resonance capacitor in addition to the tube current flowing in the discharge tube.
[0021]
In the present invention, as shown in FIG. 1, a transformer voltage detector 22 that detects a transformer voltage on the primary side and a tube that controls two control voltages Va and Vb of the pulse width controller 14 according to the detected voltage. A current stabilizing amplifier 24 is provided, and the gain of the amplifier is set low to several times or less. The specific numerical value of the gain to be set varies depending on the circuit configuration, circuit constants, and the like, but is, for example, in the range of about 5 to 0.1, and is typically about 2 to 0.2. Incidentally, the gain of an error amplifier for constant voltage / constant current control in a general DC stabilized power supply device or the like is much larger than 1 (usually about 100 to 200). The gain of the current stabilizing amplifier 24 is extremely small.
[0022]
The transformer voltage detecting unit 22 detects the transformer voltage (switching voltage) on the primary side with the capacitor C3, blocks the direct current component, rectifies it with the diode D1 and the capacitor C4, and detects the terminal voltage (point A) of the resistor R4. The tube current stabilizing amplifier 24 uses the voltage at point A and amplifies the difference from the reference voltage obtained by dividing the reference voltage Vref to control the control voltage. That is, the ON width of the switching element is controlled by shifting the two control voltages and changing the pulse width of the timing signal.
[0023]
First, a state in which control by the tube current stabilization amplifier 24 is not applied (a state in which the connection between the transformer voltage detection unit 22 and the tube current stabilization amplifier 24 is opened) will be considered. In this state, since the power supply voltage _{VCC of the} circuit is constant, both control voltages Va and Vb are also constant, and the on-time of the switching elements Q1 and Q2 is also constant. When the input voltage V _DD is varied in this state and the detection voltage at the point A is measured, as shown in FIG. 3A, the detection voltage at the point A increases as the input voltage increases, and the tube current also increases. It can be seen that it increases. That is, in an uncontrolled state, the tube current increases as the input voltage increases.
[0024]
Next, the gain of the amplifier is set to be much larger than 1 so that the detection voltage at the point A is constant, and the detection voltage at the point A is constant even when the input voltage increases. If (constant voltage control) is applied, the tube current decreases conversely. That is, when constant voltage control is performed on the detection voltage of the transformer voltage detection unit 22, the tube current also varies with the variation of the input voltage.
[0025]
Therefore, as shown in FIG. 3A, when the detected voltage at point A changes with a certain slope with respect to the input voltage, the tube current is maintained almost constant regardless of the fluctuation of the input voltage. I understand that I can do it. In order to realize this state, the gain of the tube current stabilizing amplifier 24 is set to a low value, for example, 5 to 0.1 (specific values are individually determined and adjusted according to circuit constants as described above). ). Then, the detection voltage (the voltage at the point A) is not maintained at a constant voltage but changes monotonously with a certain slope. The fluctuation amount of the detection voltage reflects the fluctuation amount of the resonance current. In the present invention, this is utilized. In other words, the inclination of the detection voltage with respect to the fluctuation of the input voltage is set so that the tube current is constant.
[0026]
The two control voltages Va and Vb change as shown in FIG. 3B with respect to the fluctuation of the detection voltage adjusted in this way. Since the control voltages Va and Vb decrease corresponding to the increase in the detection voltage, the ON width of the second switching element Q2 is narrowed and the tube current is decreased. On the contrary, since the control voltages Va and Vb increase corresponding to the decrease in the detection voltage, the ON width of the second switching element Q2 increases and the tube current increases. In this way, by setting the detection voltage so as to fluctuate with a predetermined slope with respect to the input voltage, the tube current is indirectly controlled so as to be substantially constant regardless of the fluctuation of the input voltage. .
[0027]
In order to adjust the gain of the tube current stabilizing amplifier (OP amplifier) 24, specifically, the value of the feedback resistor R5 may be adjusted. The inclination of the fluctuation of the detection voltage with respect to the input voltage is determined in consideration of the fluctuation amount of the tube current in the uncontrolled state and the fluctuation amount of the tube current in the constant voltage control state. That is, the detected voltage is corrected in a manner that includes the increase / decrease of the resonance current with respect to the input voltage fluctuation. The tube current can be adjusted by changing the value of the resistor R4 of the transformer voltage detector 22.
[0028]
FIG. 4 is a circuit diagram showing another embodiment of a separately excited inverter circuit for lighting a discharge tube according to the present invention. Since the basic configuration of the separately-excited inverter circuit for lighting the discharge tube is the same as that of the embodiment shown in FIG. 1, the same reference numerals are assigned to the corresponding parts and components for the sake of simplicity.
[0029]
The transformer voltage detector 32 cuts a DC component with the capacitor C3, rectifies it with the diode D1 and the capacitor C4, and uses the voltage at one end (point A) of the resistor R4 as the detection voltage. Using the detected voltage at point A, the tube voltage stabilizing amplifier (transistor) 34 controls the control voltage of the pulse width controller 14 using the first Zener diode ZD1 as a reference voltage. Depending on the relationship with the transformer or the like, the gain of the tube current stabilizing amplifier 34 is set to a low value such as 5 to 0.1. Specifically, the gain is determined depending on a h _fe and emitter resistor R _E of the transistors Q3, can be set to a desired gain by adjusting the emitter resistor. When the detected voltage exceeds the reference voltage, the amplifier operates. Thereby, although indirectly, the tube current can be controlled to be substantially constant.
[0030]
Further, in this embodiment, separate capacitors C5 and C6 are connected in series with the resonant capacitors C1 and C2 on the secondary side of the transformers T1 and T2, respectively, and the voltage at the intermediate point is rectified by the diodes D2 and D3 to open the circuit. An open-circuit voltage / spark discharge detection circuit 36a, 36b for detecting voltage and high-voltage spark discharge is provided.
[0031]
The discharge tube deteriorates as the usage time becomes longer, and it becomes difficult to light the lamp accordingly. For this reason, the tube drive voltage is raised when the power is turned on, and after the start of lighting, the tube drive voltage is lowered to a predetermined tube drive voltage to maintain the lighting. An example of the temporal change in voltage at point B in the open-circuit voltage / spark discharge detection circuits 36a, 36b is schematically shown in FIG. An open voltage is indicated from turning on the power to the start of lighting, and a voltage lower than the open voltage (lighting voltage) after the lighting is started. As the discharge tube deteriorates, the lighting start time is delayed (indicated by a white arrow). Therefore, the open-circuit voltage / spark discharge detection circuits 36a and 36b detect the open-circuit voltage, and the pulse width control unit 14 controls the primary-side oscillation (lowers the ON width of the pulse) based on the reference voltage by the second Zener diode ZD2. Control).
[0032]
By the way, if there is a connector contact failure or soldering failure, high-pressure spark discharge may occur, which may cause serious problems such as ignition and smoke generation. At this time, pulsed high voltage noise is superimposed on the tube driving voltage. Therefore, the output voltage (voltage at point B) from the open-circuit voltage / spark discharge detection circuits 36a and 36b is much higher than the open-circuit voltage (voltage during spark discharge), as indicated by a broken line in FIG. By providing the oscillation stop circuit 38 that stops the primary side oscillation when such an abnormality is detected, safety can be ensured.
[0033]
The oscillation stop circuit 38 divides the voltage detected by the open-circuit voltage / spark discharge detection circuits 36a and 36b by two resistors R6 and R7 connected in series, and a capacitor C9 is provided in parallel with the resistor R7 to provide a resistor R6. , R7 is provided with a comparator 39 for comparing the midpoint voltage with the reference voltage Vref, and a latch-up circuit composed of resistors R8, R9 and a transistor Q6. When the voltage at the middle point of the resistors R6 and R7 rises due to spark discharge and exceeds the reference voltage Vref, the comparator 39 is turned on, and this state is held by the latch-up circuit. The output of the comparator 39 is input to the pulse width control unit 14, and when the comparator 39 is turned on, the control voltage of the pulse width control unit 14 becomes 0V, the pulse width is set to zero, and the oscillation of the switching element 16 is forcibly stopped. Put it in a state. In this way, even if a spark discharge occurs due to a poor connector contact or poor soldering, the safety of the device is ensured.
[0034]
In addition, a dimming circuit for dimming by intermittently stopping the switching operation, a tube open detection circuit for notifying whether the discharge tube is in a lighting state or an open state can be added to the circuit. .
[0035]
FIG. 6 shows still another embodiment of the present invention. In addition to the separately excited inverter circuit for lighting a discharge tube having a tube current control function as shown in FIG. 1, two switching elements Q4 and Q5 and two transformers T3 and T4 driven in opposite phases by both switching elements And an inverter unit 40 including resonance capacitors C7 and C8 provided in parallel on the secondary side of each transformer. Then, the switching elements Q4 and Q5 of the inverter unit 40 are simultaneously driven by a timing signal generated by the pulse width control unit 14 of the discharge tube lighting separately excited inverter circuit. As a result, a small and inexpensive separately-excited inverter device for lighting a large number of discharge tubes 10a,..., 10d can be obtained. Although FIG. 6 shows an example in which one inverter unit is provided, it goes without saying that a plurality of similar inverter units can be arranged in parallel.
[0036]
In the above embodiment, the switching method is the active clamp method, but as described above, the present invention can also be applied to a push-pull method, a half-bridge method, and a full-bridge type separately excited inverter circuit for lighting a discharge tube. Needless to say.
[0037]
【The invention's effect】
As described above, the present invention utilizes the detection voltage of the transformer voltage detector that detects the transformer voltage on the primary side, and is configured to set the gain of the tube current stabilizing amplifier to be several times lower. Even when the resonance current and the tube current flow on the secondary side of the transformer in a connection form having no discharge tube return line, the tube current can be controlled to be substantially constant, though indirectly. Since this connection form does not require a discharge tube return line, it has the advantage of requiring a small amount of expensive high voltage wiring material when constructing a backlight for a large liquid crystal panel. A relatively good device is obtained.
[0038]
In addition, by detecting the open-circuit voltage and spark discharge voltage on the secondary side of the transformer, and controlling the tube drive voltage with the detected open-circuit voltage, and stopping the primary-side oscillation with the detected spark voltage, Smoke and fire due to high-pressure spark discharge caused by poor soldering can be prevented, and the safety of the device can be ensured.
[0039]
Furthermore, when it is necessary to arrange a larger number of discharge tubes, by adopting a configuration in which inverter units having the same structure are arranged in parallel and controlled by a single common control unit, the apparatus can be significantly reduced in size and Cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of another type inverter circuit for lighting a discharge tube according to the present invention.
FIG. 2 is an explanatory diagram of the timing signal for driving the switching element.
FIG. 3 is an explanatory diagram showing how the tube current is controlled to be substantially constant.
FIG. 4 is a block diagram showing another embodiment of the present invention.
FIG. 5 is an explanatory diagram of an output voltage from an open-circuit voltage / spark discharge detection circuit.
FIG. 6 is a block diagram showing still another embodiment of the present invention.
[Explanation of symbols]
10a, 10b Discharge tube 12 Transmitter 14 Pulse width control unit 16 Switching unit 22 Transformer voltage detection unit 24 Tube current stabilization amplifier Q1, Q2 Switching element T1, T2 Transformer C1, C2 Resonance capacitor

Claims (3)

Based on the transmitter that generates a triangular wave signal having a tube driving frequency, a pulse width control unit that compares the triangular wave signal from the transmitter with a control voltage and generates a timing signal, and a timing signal from the pulse width control unit A plurality of switching elements controlled to be turned on and off, first and second transformers driven in opposite phases by the switching elements, and a resonant capacitor provided in parallel on the secondary side of both transformers, In a separately-excited inverter circuit for lighting a discharge tube by alternately applying a high voltage of opposite phase to both end electrodes of one or two series discharge tubes connected between the secondary sides of a transformer,
A transformer voltage detector for detecting the transformer voltage on the primary side and a tube current stabilization amplifier for controlling two control voltages of the pulse width controller according to the detected voltage are provided, and the detection voltage is detected against fluctuations in the input voltage. Separately-excited inverter for discharge tube lighting, characterized in that the gain of the tube current stabilization amplifier is set to be several times lower or less so as to change with a certain slope, and thereby the tube current is indirectly controlled to be substantially constant circuit. A secondary capacitor is connected in series with the resonant capacitor on the secondary side of the transformer, and an open-circuit voltage / spark discharge detection unit that detects an open-circuit voltage and a high-voltage spark discharge according to the voltage at the intermediate point is provided. Based on the detected open-circuit voltage The separately excited inverter circuit for lighting a discharge tube according to claim 1, wherein the primary side oscillation is controlled and the primary side oscillation is stopped based on the detected spark discharge voltage. In addition to the separately excited inverter circuit for lighting a discharge tube according to claim 1 or 2, a plurality of switching elements, two transformers driven in opposite phases by the switching elements, and a secondary side of each transformer in parallel Discharge tube lighting in which one or more inverter units including the provided resonance capacitor are arranged in parallel, and each switching element of the inverter unit is simultaneously driven by a timing signal from the pulse width control unit of the separately excited inverter circuit for lighting the discharge tube. Separately-excited inverter device.

Images