JP2000231998A - Power source circuit for lighting discharge tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power source circuit for lighting a discharge tube, in which conversion efficiency can be enhanced and the number of parts and the size can be reduced. SOLUTION: In a power source circuit having a transformer 13 of a leakage magnetic flux type and a capacitor 8 connected in parallel to a discharge tube 5, the power source circuit comprises switching elements Q1, Q2 connected to both ends of a primary coil, respectively, a pulse width modulating circuit 9 for controlling to alternately turn on or off the switching elements Q1, Q2 and a current detecting circuit 6 for detecting a tube current so as to supply it to the pulse width modulating circuit 9. An ON time of a drive pulse is controlled based on the value of the tube current.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit for lighting a discharge tube such as a cold cathode fluorescent tube for illuminating the back of a liquid crystal display device.

[0002]

2. Description of the Related Art As a conventional power supply circuit of this type, for example, a circuit as shown in FIG. 4 is used. This is provided with an inverter circuit 2 composed of a modified Royer circuit at the subsequent stage of the step-down DC / DC converter 1. Self-excited oscillation is performed by the modified Royer circuit, and the resonance voltage is boosted according to the turns ratio of the transformer 3 and appears on the secondary side. Reference numeral 4 denotes a current limiting ballast capacitor. Dimming of the discharge tube 5 is performed by controlling the output voltage of the DC / DC converter 1. The current detection circuit 6 detects the tube current, feeds back a DC voltage value corresponding to the tube current value, and controls the output voltage of the DC / DC converter 1 in the preceding stage so that the tube current becomes constant.

[0003]

The conversion efficiency of the power supply circuit as a whole is obtained by integrating the two stages of the DC / DC converter 1 and the inverter circuit 2, so that there is a disadvantage that the conversion efficiency is significantly reduced. For example, when the conversion efficiencies of the DC / DC converter 1 and the inverter circuit 2 are 80 to 87% and 80 to 85%, respectively, the total conversion efficiency is 64 to 74%. In addition, there is a problem that the number of components is large, the mounting space is large, and miniaturization is difficult. Moreover,
When a plurality of inverter circuits 2 are connected in parallel to a common DC / DC converter 1 to turn on a plurality of discharge tubes 5, a liquid crystal display device is provided due to a variation in oscillation frequency of each self-excited inverter circuit 2. There is also a problem that interference with the horizontal synchronization signal causes noise on the screen.

[0004]

SUMMARY OF THE INVENTION The present invention relates to a leakage flux type transformer having a primary winding and a secondary winding and having a DC voltage applied to a center tap of the primary winding, and a series connected to the secondary winding. In a discharge tube lighting power supply circuit including an inverter circuit having a connected discharge tube and a capacitor connected in parallel to the discharge tube, a first switching element and a second switching element connected to both ends of a primary winding, respectively. A switching element, a pulse width modulation circuit for supplying a driving pulse to the first switching element and the second switching element to perform on / off control alternately, and detecting a current value flowing through the discharge tube and supplying the same to the pulse width modulation circuit And a control circuit for controlling the on-time of the drive pulse with this current value.

[0005]

FIG. 1 shows an embodiment of a power supply circuit for lighting a discharge tube according to the present invention. Parts corresponding to those of the conventional example are denoted by the same reference numerals in the embodiments and will be described. 13
Is a leakage flux step-up transformer having a primary winding L1 and a secondary winding L2. A DC voltage Vin is applied by a power supply 7 to a center tap for dividing the primary winding L1 into a winding a and a winding b.
Is applied. A discharge tube 5 is connected in series to the secondary winding L2, and a resonance capacitor 8 is connected in parallel to the discharge tube 5. The switching element Q is connected to one end of the primary winding L1.
1 and a switching element Q2 at the other end.
In this embodiment, MOSFETs with built-in freewheeling diodes are used as the switching elements Q1 and Q2.
A bipolar transistor may be used instead of ET.

Reference numeral 9 denotes a pulse width modulation circuit for supplying a drive pulse to the gates of the switching elements Q1 and Q2 to perform on / off control alternately. The current detection circuit 6 detects the tube current and feeds back a DC voltage value corresponding to the tube current value to the pulse width modulation circuit 9 to control the on-time of the drive pulse and stabilize the current flowing through the discharge tube 5. The pulse width modulation circuit 9 controls on / off of the switching elements Q1 and Q2 at a high frequency.
The duty dimming circuit 10 supplies a control voltage for turning on and off at a low frequency of 100 Hz to 1 kHz to the pulse width modulation circuit 9. Then, the dimming of the discharge tube 5 is performed by turning on / off the switching elements Q1 and Q2 by the pulse width modulation circuit 9 only during the ON period of the duty dimming circuit 10. That is, the duty dimming circuit 10 is connected to the terminal 11
It has the function of changing the brightness of the discharge tube 5 in accordance with the control voltage input from the controller.

Next, the operation of the power supply circuit shown in FIG. 1 will be described with reference to FIG.
This will be described with reference to the waveform diagram of FIG. Switching element Q1,
When drive pulses having a constant oscillation cycle are alternately applied to Q2 every half cycle, the switching elements Q1 and Q2 are turned on alternately, and voltages of opposite polarities are alternately applied to the windings a and b of the primary winding L1. Applied. That is, at both ends of the winding a, FIG.
A rectangular wave voltage as shown in (a) is applied, and a voltage as in (b) is applied to both ends of the winding b. When the switching element Q1 is turned off, a flyback voltage is regenerated on the route of the return diode and the winding a, and when the switching element Q2 is turned off, a flyback voltage is regenerated on the route of the return diode and the winding b.

When the switching element Q1 is turned on,
The leakage inductance of the transformer 13 and the capacitor 8 resonate, and a resonance current as shown in FIG.
When the switching element Q2 is turned on, similarly, a resonance current as shown in (d) due to resonance of the leakage inductance of the transformer 13 and the capacitor 8 flows through the winding b. As a result, a high voltage is generated in the secondary winding L2 to light the discharge tube 5. When a current flows through the discharge tube 5, the impedance of the discharge tube 5 decreases, and the increase in the current causes a voltage drop of the leakage inductance. And transformer 13
Is reduced until it becomes equal to the voltage across the discharge tube 5, and a constant current flows through the discharge tube 5. A resonance current due to the leakage inductance and the resonance of the capacitor 8 flows through the windings a and b of the primary winding L1. FIG. 2E shows the output current of the transformer 13 flowing through the discharge tube 5.

FIG. 3 shows an example in which a plurality of inverter circuits 20 are connected to the power supply circuit of the present invention so that a plurality of discharge tubes 5 can be turned on. Both ends of the primary winding L1 of the transformer 3 of each inverter circuit 20 are connected to a common switching element Q
1 and Q2. Therefore, these plurality of inverter circuits are driven at the same frequency. The switching elements Q1 and Q2 may be provided in each inverter circuit 20, and the gates of the switching elements Q1 and Q2 may be connected to a common pulse width modulation circuit 8.

[0010]

According to the present invention, conversion efficiency is improved because of one-stage control. In addition, since the oscillation frequency is constant, it does not interfere with other frequencies and the occurrence of screen noise and the like is small. Even when a plurality of inverter circuits are connected to turn on a plurality of discharge tubes, each inverter circuit can be driven at the same frequency, and the problem due to frequency interference does not occur. Further, there is an effect that the size can be reduced because the number of components is small.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a time chart for explaining the operation of the circuit.

FIG. 3 is a circuit diagram showing another embodiment of the present invention.

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

[Explanation of symbols]

 9 Pulse width modulation circuit 13 Transformer Q1 Switching element Q2 Switching element

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference)

Claims (4)

[Claims] 1. A leakage flux type transformer having a primary winding and a secondary winding and having a DC voltage applied to a center tap of the primary winding, a discharge tube connected in series to the secondary winding, and a discharger. In a discharge tube lighting power supply circuit provided with an inverter circuit having a capacitor connected in parallel to a tube, a first switching element and a second switching element respectively connected to both ends of a primary winding; Switching element and second
A pulse width modulation circuit that supplies a drive pulse to the switching element to perform on / off control alternately, and a current detection circuit that detects a current value flowing through the discharge tube and supplies the current value to the pulse width modulation circuit. Is controlled by the current value. 2. A duty dimming circuit for controlling a pulse width modulation circuit is provided, and a low frequency pulse width modulation is performed by the duty dimming circuit to change an average current flowing in a discharge tube to adjust luminance. A power supply circuit for lighting a discharge tube according to claim 1. 3. A discharge tube lighting power supply circuit according to claim 1, wherein a plurality of sets of said inverter circuits are provided, and both ends of a primary winding of each of said inverter circuits are commonly connected to a first switching element and a second switching element, respectively. . 4. A plurality of sets each including a first switching element and a second switching element connected to both ends of a primary winding of the inverter circuit, wherein the plurality of sets of the first switching element and the second switching element are connected to each other. 2. The power supply circuit for lighting a discharge tube according to claim 1, wherein said switching element is connected to said common pulse width modulation circuit.