JP2002170694A - Control method of fluorescent tube lighting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a fluorescent tube life longer and also to perform a stable control with some allowance by keeping enough control space. SOLUTION: A prescribed variable range Dd<=D<=Du is set for the duty ratio D in advance by lower limit voltage Ved and upper limit voltage Veu to error voltage Ve, where the duty ratio D of a driving signal Sc is controlled variably when the error voltage Ve is not less than the lower limit voltage Ved and not more than the upper limit voltage Veu, and at the same time, load current Ir is controlled to be at a targeted value by variably controlling frequency f of the driving signal Sc when the error voltage Ve is either less than the lower limit voltage Ved or greater than the upper limit voltage Veu.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method of a fluorescent tube lighting device suitable for lighting a backlight or the like of a liquid crystal display.

[0002]

2. Description of the Related Art Generally, a liquid crystal display of a personal computer or the like has a backlight using a fluorescent tube (cold cathode fluorescent tube) and a fluorescent tube lighting device for lighting the fluorescent tube.

FIG. 5 shows a conventional fluorescent tube lighting device 50.
The fluorescent tube lighting device 50 uses a so-called active clamp circuit system.
1, an inverter circuit 52 and a control circuit 53, and the fluorescent tube 70 is connected to the output of the inverter circuit 52.

The inverter circuit 52 includes a transformer 54 having a primary winding 54f and a secondary winding 54r. One end of the primary winding 54f is connected to the positive electrode of the DC power supply 51 and the other end of the primary winding 54f. Is connected to the source of an FET 55 (switching element), and the drain of the FET 55 is grounded. One end of the primary winding 54f is connected to an FET
56 (switching element) and this FE
The drain of T56 is connected via a reset capacitor 57 to the other end of the primary winding 54f. Reference numeral 58 denotes a stabilizing capacitor connected between both electrodes of the DC power supply 51. On the other hand, one end of the secondary winding 54r is connected to one end of the fluorescent tube 70 via the capacitor 59, and the other end of the fluorescent tube 70 is grounded via the resistor 60. In addition, 54R indicates the secondary-side equivalent leakage resistance of the transformer 54, 54L indicates the secondary-side equivalent leakage inductance of the transformer 54, 70R indicates the equivalent resistance of the fluorescent tube 70, and 61 indicates the secondary winding 5
4 shows a waveform shaping capacitor connected between both ends of 4r. In this case, the secondary side equivalent leakage inductance 54L, the capacitor 59, and the waveform shaping capacitor 61 constitute an LC filter.

The control circuit 53 includes an error amplifier 63,
One input of the error amplifier 63 is connected to the other end of the fluorescent tube 70 via the detection circuit 62, and the other input of the error amplifier 63 is connected to the reference voltage circuit 64. The output of the error amplifier 63 is connected to the signal input of a pulse width modulation circuit (PWM circuit) 65,
The modulated wave input section of the PWM circuit 65 has a triangular wave signal circuit 6
6 is connected. Further, the output of the PWM circuit 65 is connected to the input of the driver 67. The output of the driver 67 is connected to the gate of the FET 55 and to the gate of the FET 56 via the inverting circuit 68.

[0006] The fluorescent tube lighting device 50 configured as described above.
In this case, the FETs 55 and 56 are alternately turned on and off by the control circuit 53, and a load voltage having a frequency of about 100 [kHz] and a steady state voltage of about 600 [V] is applied to the fluorescent tube 70. Thereby, the fluorescent tube 70 is turned on. Also,
The control circuit 53 converts the load current Ir flowing through the fluorescent tube 70 into
Feedback control is performed to achieve the target value, and the fluorescent tube 70 is controlled.
Illuminance is kept constant. In this case, the magnitude of the load current Ir is detected by the detection circuit 62.
2 is compared with the reference voltage Vs applied from the reference voltage circuit 64 in the error amplifier 63, and an error voltage Ve is obtained from the difference. Then, the error voltage Ve is applied to the PWM circuit 65. PWM
Since the triangular wave signal from the triangular wave signal circuit 66 is applied to the circuit 65 at the same time, the drive signal Sc is generated based on the triangular wave signal and the error voltage Ve. This drive signal Sc
Corresponds to the frequency of the triangular wave signal, and the duty ratio D (pulse width) changes according to the magnitude of the error voltage Ve. Therefore, for example, when the voltage of the DC power supply 51 decreases and the load current Ir decreases, the error voltage Ve decreases due to the decrease in the detection voltage Vr obtained from the detection circuit 62, and the duty ratio D (pulse width) of the drive signal Sc decreases. ) Becomes larger. As a result, the load current Ir increases and the feedback control is performed so as to match the target value.

[0007]

However, the conventional method of controlling the fluorescent tube lighting device 50 has the following problems.

That is, as shown in the waveform of the load current Ir shown in FIG.
As shown in FIG. 3A, the waveform of the load current Ir has a relatively small distortion and a symmetrical waveform. However, when the duty ratio D changes to 0.75 or 0.25, the waveform of FIG. As shown in (b) and (c), a distorted portion Ire occurs in the waveform of the load current Ir, and the symmetry is lost. Usually, such a distorted portion Ire of the load current Ir does not significantly affect the illuminance, but is a major cause of shortening the life of the fluorescent tube 70. Therefore, in the conventional fluorescent tube lighting device 50, the load current I
The control range of r (variable range of the duty ratio D) has to be set compromised, and as a result, the life of the fluorescent tube 70 has to be sacrificed to some extent, and the control range cannot be set large. .

The present invention solves the above-mentioned problems of the conventional control method. The present invention can extend the life of a fluorescent tube and, at the same time, ensure a sufficient control range by securing a sufficient control range. It is an object of the present invention to provide a control method of a fluorescent tube lighting device capable of performing the above.

[0010]

A control method for a fluorescent tube lighting device 1 according to the present invention is obtained from a deviation between a detected voltage Vr corresponding to a load current Ir flowing through a fluorescent tube 2 and a reference voltage Vs. The error voltage Ve allows the inverter circuit 3
When the duty ratio D of the drive signal Sc generated by the control circuit 4 for controlling the load current Ir is variably controlled to control the load current Ir to the target value, a predetermined variable range Dd ≦ D ≦ Du is set by the lower limit voltage Ved and the upper limit voltage Veu with respect to the error voltage Ve. When the error voltage Vs is equal to or higher than the lower limit voltage Ved and equal to or lower than the upper limit voltage Veu, the duty ratio D of the drive signal Sc is variably controlled. When the voltage is lower than the voltage Ved and / or exceeds the upper limit voltage Veu, the frequency f of the drive signal Sc is variably controlled to control the load current Ir to a target value.

In this case, according to a preferred embodiment, if the error voltage Ve becomes lower than the lower limit voltage Ved, the duty ratio D is changed to the lower limit Dd of the variable range Dd ≦ D ≦ Du (lower limit voltage Vd).
ed), and the frequency f is variably controlled.
If the error voltage Ve exceeds the upper limit voltage Veu, the duty ratio D is changed to the upper limit Du of the variable range Dd ≦ D ≦ Du (the upper limit voltage Veu).
eu), and the frequency f is variably controlled. Further, the error voltage Ve is applied to the pulse width modulation processing circuit 5 to variably control the duty ratio D, and the error voltage Ve is applied to the voltage control oscillation circuit 6, and the output of the voltage control oscillation circuit 6 To the triangular wave signal circuit 7 connected to the pulse width modulation processing circuit 5,
Is variably controlled. Further, the pulse width modulation processing circuit 5
The magnitude of the error voltage Ve is monitored to determine the variable control area of the duty ratio D and the variable control area of the frequency f, and the frequency f is fixed to the reference frequency fo and the frequency f Is performed, the duty ratio D is fixed to the lower limit Dd or the upper limit Du.

Thus, in the variable range Dd ≦ D ≦ Du of the duty ratio D where no distortion occurs in the waveform of the load current Ir,
The variable control of the duty ratio D with respect to the drive signal Sc is performed, and in a region outside the variable range Dd ≦ D ≦ Du, that is, in a region where the waveform of the load current Ir is distorted,
For example, with the duty ratio D fixed, the drive signal Sc
Is variably controlled with respect to.

[0013]

Next, preferred embodiments according to the present invention will be described in detail with reference to the drawings.

First, a configuration of a fluorescent tube lighting device 1 capable of implementing the control method according to the present embodiment will be described with reference to FIG.

The fluorescent tube lighting device 1 of the embodiment uses a so-called active clamp circuit system, and is roughly divided into a DC power supply 11, an inverter circuit 3 and a control circuit 4. Connected to output section.

The inverter circuit 3 includes a transformer T having a primary winding Tf and a secondary winding Tr. One end of the primary winding Tf is connected to the positive electrode of the DC power supply 11 and the other end of the primary winding Tf Is connected to the source of the FET 12 (switching element), and the drain of the FET 12 is grounded. One end of the primary winding Tf is connected to the source of the FET 13 (switching element), and the drain of the FET 13 is connected to the other end of the primary winding Tf via the reset capacitor C1. C2 denotes a stabilizing capacitor connected between both electrodes of the DC power supply 11. On the other hand, the secondary winding T
One end of r is connected to one end of the fluorescent tube 2 via a capacitor C3, and the other end of the fluorescent tube 2 is grounded via a resistor R1. Rs is the secondary-side equivalent leakage resistance of the transformer T, Ls is the secondary-side equivalent leakage inductance of the transformer T, 2r is the equivalent resistance of the fluorescent tube,
C4 indicates a waveform shaping capacitor connected between both ends of the secondary winding Tr. In this case, the secondary side equivalent leakage inductance Ls, the capacitor C3, and the waveform shaping capacitor C4
Constitutes an LC filter.

The control circuit 4 includes an error amplifier 15. One input of the error amplifier 15 is connected to the other end of the fluorescent tube 2 via the detection circuit 14, and the error amplifier 1
5 is connected to the reference voltage circuit 16. The output section of the error amplifier 15 is connected to a signal input section of a pulse width modulation processing circuit (PWM processing circuit) 5, and a triangular wave signal circuit 7 is connected to a modulated wave input section of the PWM processing circuit 5. Further, the output of the PWM processing circuit 5 is connected to the input of the driver 18. The output of the driver 18 is connected to the gate of the FET 11 and to the gate of the FET 12 via the inverting circuit 19.

In this case, the PWM processing circuit 5 performs not only the pulse width modulation processing but also the control processing for executing the control method according to the present embodiment. Further, a voltage controlled oscillation circuit (VCO circuit) 6 is provided. An error voltage Ve is applied from an error amplifier 15 to an input section of the VCO circuit 6, and an output section of the VCO circuit 6 is connected to a triangular wave signal circuit 7
Connect to In addition, the PWM processing circuit 5 sends the error amplifier 1
5 and VCO circuit 6 are provided with regulation signals Sca and Scb, respectively, corresponding to the magnitude of error voltage Ve.

Next, a control method of the fluorescent tube lighting device 1 according to the present embodiment will be described with reference to FIGS.

First, the principle of the control method according to the present invention will be described. When the fluorescent tube lighting device 1 used in the present embodiment is configured, as shown in FIG.
Changes, the load voltage (terminal voltage) Vo of the fluorescent tube 2 also changes, and shows different characteristics depending on the type of the fluorescent tube 2, that is, the magnitude of the load impedance Ro. Therefore, if the load impedance Ro of the fluorescent tube 2 to be used is obtained in advance and an appropriate frequency f is set based on a change characteristic of the load voltage Vo based on the load impedance Ro, the load current Ir is controlled by variably controlling the frequency f. Can be variably controlled.

In this case, as shown in FIG. 3, for example, if a fluorescent tube 2 having a load impedance Ro of 1000 [kΩ] is used and the frequency f is selected to be 100 [kHz], the load becomes higher as the frequency f becomes higher. The characteristic shows that the voltage Vo [kV] increases. On the other hand, if the frequency f is set to 125 [kHz], the load voltage Vo increases as the frequency f increases.
[KV] is lower than the selected frequency f
It is necessary to set a variable control characteristic according to the size of.
That is, when the frequency f is selected to be 100 [kHz], the variable control characteristic shown by the solid line in FIG.
Is set to 125 [kHz], the VCO circuit 6 or the like is set in advance so that the variable control characteristics that are the inverse characteristics indicated by the dotted lines in FIG. 4 can be used.

On the other hand, in this embodiment, the allowable range in which no distortion occurs in the load current Ir with respect to the duty ratio D of the drive signal Sc in the control circuit 4, ie, the variable range Dd ≦
Select D ≦ Du. In the embodiment, as shown in FIG. 4, the variable range of the duty ratio D is 0.4 ≦ D ≦ 0.6.
(Dd = 0.4, Du = 0.6) is shown. In this case, the lower limit Dd and the upper limit Du of the duty ratio D defining the variable range can be obtained experimentally, for example, while actually observing the waveform of the load current Ir. Also,
Since the duty ratio D and the error voltage Ve correspond, the variable range Dd ≦ D ≦ Du is set to the lower limit voltage V
ed is replaced with the upper limit voltage Veu, and the lower limit voltage Ved and the upper limit voltage Veu are set in the PWM processing circuit 5.

Further, the PWM processing circuit 5 has a control processing function for executing the control method according to the present embodiment. Therefore, the PWM processing circuit 5 monitors the predetermined control processing, that is, the magnitude of the error voltage Ve by the computer processing function unit, and determines the variable control area of the duty ratio D and the variable control area of the frequency f, for example. , Duty ratio D
When the variable control is performed, the frequency f is fixed to the reference frequency fo,
When the frequency f is variably controlled, the duty ratio D is set to the lower limit D
d (lower limit voltage Ved) or upper limit Du (upper limit voltage Veu)
Is performed.

Next, the control method according to this embodiment will be specifically described with reference to the flowchart shown in FIG.

First, in the fluorescent tube lighting device 1, the FETs 12 and 13 are alternately turned on and off by the control circuit 4, and the load voltage at which the frequency is about 100 [kHz] and the steady state voltage is about 600 [V] is applied to the fluorescent tube lighting apparatus. 2 is applied. Thereby, the fluorescent tube 2 is turned on. Further, the control circuit 4 performs feedback control on the load current Ir flowing through the fluorescent tube 2 so as to be a target value, and keeps the illuminance of the fluorescent tube 2 constant.

In this case, the magnitude of the load current Ir is detected by the detection circuit 14. The detection voltage Vr obtained from the detection circuit 14 is supplied to the error amplifier 15 by the reference voltage circuit 16.
Is compared with the reference voltage Vs provided from the above, and an error voltage Ve is obtained from the difference (step S1). Then, the error voltage Ve is applied to the PWM processing circuit 5, and the magnitude of the error voltage Ve is monitored (Step S2).

It is assumed that the error voltage Ve is equal to or less than Ved ≦ corresponding to the set duty ratio D variable range 0.4 ≦ D ≦ 0.6.
If Ve ≦ Veu, the PWM processing circuit 5 takes in the error voltage Ve and generates a drive signal Sc having a duty ratio D corresponding to the error voltage Ve from the triangular wave signal given from the triangular wave signal circuit 7. Output (step S
3, S4). At this time, the PWM processing circuit 5 is connected to the VCO circuit 6
, And the frequency of the output signal output from the VCO circuit 6 is fixed to the basic frequency fo. Therefore, for example, if the voltage of the DC power supply 11 decreases and the load current Ir decreases, the error voltage Ve decreases due to a decrease in the detection voltage Vr obtained from the detection circuit 14,
The duty ratio D (pulse width) of the drive signal Sc increases. As a result, the load current Ir increases and this load current I
The feedback control is performed so that r is equal to the target value.

On the other hand, it is assumed that the error voltage Ve enters a region where Ve <Ved (step S5). in this case,
The PWM processing circuit 5 supplies the error amplifier 15 with the regulation signal Sca.
And the voltage applied from the error amplifier 15 to the PWM processing circuit 5 is fixed to the lower limit voltage Ved so that the duty ratio D becomes the lower limit Dd, and the restriction signal Scb applied to the VCO circuit 6 is released. I do. The error voltage Ve is applied to the triangular wave signal circuit 7 from the error amplifier 15 with the same magnitude. As a result, the frequency f of the triangular wave signal is synchronized with the frequency f of the output signal output from the VCO circuit 6, and the frequency of the drive signal Sc is variably controlled according to the magnitude of the error voltage Ve (steps S6 and S7). , S
8).

Similarly, it is assumed that the error voltage Ve enters a region where Veu <Ve (step S9). Also in this case, the PWM processing circuit 5 sends the regulation signal S to the error amplifier 15.
ca, the voltage applied from the error amplifier 15 to the PWM processing circuit 5 is fixed to the upper limit voltage Veu so that the duty ratio D becomes the upper limit Du, and the regulation signal Scb applied to the VCO circuit 6 is To release. The error voltage Ve is applied to the triangular wave signal circuit 7 from the error amplifier 15 with the same magnitude. As a result, the frequency f of the triangular wave signal is synchronized with the frequency f of the output signal output from the VCO circuit 6, and the frequency of the drive signal Sc becomes the error voltage Ve
(Steps S10,
S11, S12).

Therefore, the duty ratio D (pulse width) of the drive signal Sc output from the PWM processing circuit 5 is lower limit Dd
Alternatively, since the frequency f changes in accordance with the magnitude of the error voltage Ve in this state, for example, the voltage of the DC power supply 11 greatly decreases, and the load current Ir excessively decreases. Even in the case of entering the region of Ve <Ved, the duty ratio D (pulse width) is fixed at 0.4 at which no distortion occurs in the load current.
Feedback control for increasing the load current Ir is performed by the variable control of the frequency f. In this case, Ve <V
If the area returns to the variable range of Ved ≦ Ve from the area of ed,
A process for variably controlling the duty ratio D based on steps S2 to S4 is performed. FIG. 4 shows in principle the relationship between the frequency f and the duty ratio D with respect to the error voltage Ve in the above control method.

As described above, according to the control method of the present embodiment, the variable range Dd ≦ D ≦ Du (0.4 ≦ D ≦ 0.6) of the duty ratio D that does not cause distortion in the waveform of the load current Ir. Thus, the duty ratio D is variably controlled with respect to the drive signal Sc, and the duty ratio D is fixed in a region outside the variable range Dd ≦ D ≦ Du, that is, in a region where the waveform of the load current Ir is distorted. In this state, the variable control of the frequency f with respect to the drive signal Sc is performed, so that the life of the fluorescent tube 2 can be extended and, at the same time, a sufficient control range can be ensured to perform a stable control with a margin.

The embodiment has been described in detail above.
The present invention is not limited to such an embodiment,
The detailed circuit configuration, numerical values, and the like can be arbitrarily changed, added, or deleted without departing from the gist of the present invention. For example, the inverter circuit 3, the PWM processing circuit 5,
The VCO circuit 6 and the like can be replaced by another circuit (circuit configuration) having a similar function. Also, the embodiments are
Error voltage Ve is lower than lower limit voltage Ved and upper limit voltage Veu
When the frequency f exceeds the frequency f, the frequency f of the drive signal Sc is variably controlled. However, the frequency f of the drive signal Sc is changed in accordance with one of the conditions that the error voltage Ve is lower than the lower limit voltage Ved or higher than the upper limit voltage Veu. Variable control may be performed.

[0033]

As described above, in the control method of the fluorescent tube lighting device according to the present invention, the predetermined variable range for the duty ratio is set in advance by the lower limit voltage and the upper limit voltage for the error voltage, and the error voltage is higher than the lower limit voltage. And below the upper limit voltage,
If the duty ratio of the drive signal is variably controlled and the error voltage is below the lower limit voltage and / or exceeds the upper limit voltage,
By variably controlling the frequency of the drive signal, the load current is controlled so as to reach the target value, so that the life of the fluorescent tube can be extended, and at the same time, a sufficient control range is ensured by securing a sufficient control range. This has a remarkable effect that control can be performed.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a processing procedure of a control method according to a preferred embodiment of the present invention;

FIG. 2 is an electric circuit diagram of a fluorescent tube lighting device implementing the control method,

FIG. 3 is a frequency-load voltage characteristic diagram for explaining the principle of the control method;

FIG. 4 is an operation explanatory diagram of the control method,

FIG. 5 is an electric circuit diagram of a fluorescent tube lighting device for implementing a control method according to the related art;

FIG. 6 is a waveform diagram of a load current for explaining a problem of the control method;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fluorescent tube lighting device 2 Fluorescent tube 3 Inverter circuit 4 Control circuit 5 Pulse width modulation processing circuit (PWM processing circuit) 6 Voltage control oscillation circuit (VCO circuit) 7 Triangular wave signal circuit Ir Load current Vr Detection voltage Vs Reference voltage Ve Error voltage Ved Lower limit voltage Veu Upper limit voltage Sc Drive signal

Claims (6)

[Claims] A control circuit for controlling an inverter circuit variably controls a duty ratio of a drive signal generated by a control circuit that controls an inverter circuit, based on an error voltage obtained from a deviation between a detection voltage corresponding to a load current flowing through the fluorescent tube and a reference voltage; In a control method of a fluorescent tube lighting device for controlling a load current to be a target value, a predetermined variable range for the duty ratio is set in advance by a lower limit voltage and an upper limit voltage for the error voltage, and the error voltage is set to the lower limit. When the voltage is equal to or higher than the upper limit voltage and the duty ratio of the drive signal is variably controlled, and when the error voltage is lower than the lower limit voltage and / or exceeds the upper limit voltage, the frequency of the drive signal is variably controlled. And controlling the load current to be a target value. 2. The method according to claim 1, wherein when the error voltage becomes lower than the lower limit voltage, the duty ratio is fixed to a lower limit of the variable range (the lower limit voltage) and the frequency is variably controlled. The control method of the fluorescent tube lighting device described in the above. 3. The method according to claim 1, wherein when the error voltage exceeds the upper limit voltage, the duty ratio is fixed to an upper limit of the variable range (the upper limit voltage) and the frequency is variably controlled. Control method of a fluorescent tube lighting device. 4. The method according to claim 1, wherein the duty ratio is variably controlled by applying the error voltage to a pulse width modulation processing circuit. 5. The variably controlling the frequency by applying the error voltage to a voltage controlled oscillation circuit and applying the output of the voltage controlled oscillation circuit to a triangular wave signal circuit connected to the pulse width modulation processing circuit. 5. The control method for a fluorescent tube lighting device according to claim 4, wherein: 6. The pulse width modulation processing circuit monitors a magnitude of the error voltage, determines a variable control area of the duty ratio and a variable control area of the frequency, and performs variable control of the duty ratio. 6. The control method for a fluorescent tube lighting device according to claim 4, wherein a process of fixing the frequency to a reference frequency and fixing the duty ratio to a lower limit or an upper limit during variable control of the frequency is performed.