JP2006278180A - Cold-cathode tube lighting circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance energy efficiency of a cold-cathode tube lighting circuit. <P>SOLUTION: A primary resonant circuit which consists of primary coils (L1) of a driving transformer 2 and a resonant capacitor 4 (C1) is driven by two switching transistors TR1 and TR2 alternately. Output voltage of a current detection circuit 3 proportional to a current flowing in cold-cathode fluorescent tube 1 is compared with a reference voltage. PWM controlling signals are generated so that the current flowing in a load of the cold-cathode fluorescent tube 1 may be constant. A PWM signal generator 5 provides the two switching transistors TR1 and TR2 with PWM signals of frequencies 3 to 10% deviated from the center frequency of resonance characteristics. At the time of current off, magnetic energy remained in a primary coil CT of a regeneration transformer 7 is fed back to a power source by a secondary coil CTS of the regeneration transformer 7. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a cold cathode tube lighting circuit, and more particularly, to a cold cathode tube lighting circuit for lighting a cold cathode tube for a backlight of a large liquid crystal display panel.

  Cold cathode fluorescent tubes are used as backlights for liquid crystal display panels. As a lighting circuit for the cold cathode fluorescent tube, a sine wave driving circuit or a pulse driving circuit is used. As the sine wave drive circuit, a Royer circuit is generally used. A pulse drive circuit includes a flyback circuit. Some inverters use piezoelectric transformers.

  As shown in FIG. 5A, the Royer circuit of the sine wave drive circuit uses two switching transistors and self-oscillates on the primary side of the output transformer. An arbitrary voltage can be obtained according to the number of turns of the secondary winding of the transformer. Since oscillation is continued by utilizing the phenomenon of magnetic saturation of the transformer, a large collector current flows at the moment when the transistor is turned off, resulting in an increase in loss. In the separately excited lighting circuit, the switching transistor of the resonance circuit is turned on and off by the PWM signal generation circuit. The brightness can be changed by changing the duty ratio of the drive pulse.

  As shown in FIG. 5B, the flyback circuit generates a high-voltage current on the secondary side of the transformer by turning on and off the current on the primary side of the transformer with a transistor switch. A conventional example of a separately excited lighting circuit that drives a resonant circuit with a PWM current is shown below.

  The “externally-excited inverter” disclosed in Patent Document 1 is an inverter that can prevent flickering of a cold-cathode tube and beat of an LCD and can dimm a plurality of cold-cathode tubes independently. As shown in FIG. 6A, a stable voltage is supplied from the DC / DC converter that oscillates in response to the clock signal to the midpoint of the primary winding of the inverter transformer. The secondary winding of the inverter transformer is connected to both electrodes of the cold cathode tube. The clock signal generation circuit output and the dimming PWM signal generation circuit output are connected to the AND gate. The output of the AND gate is connected to the input of the switching element. A switching element switches one end of the primary winding of the inverter transformer. The output of the DC / DC converter is delayed by the delay circuit until the clock signal generation circuit oscillates stably, and the DC / DC converter is started slowly.

  The “backlight unit” disclosed in Patent Document 2 is a backlight of a liquid crystal display device using a plurality of cold cathode tubes. The control circuit includes a primary side power supply, a clock generation circuit, and a dimming PWM signal generation circuit. The separately excited oscillation output circuit is provided corresponding to the cold cathode tube. The control circuit and one separately-excited oscillation output circuit are mounted on the main board. Another separately-excited oscillation output circuit is mounted on the sub-board. The main board and the sub board are connected by a primary power line, a ground line, a clock signal, and a wiring for a dimming PWM signal.

  The “PWM dimming method time difference lighting method” disclosed in Patent Document 3 allows the brightness of a cold cathode tube for backlight of a liquid crystal display panel to be adjusted to a low brightness. A dimming signal generation circuit and a switch circuit are provided exclusively for the number n of cold cathode tubes for each resonance circuit connected to the cold cathode tubes. The timing signal generation circuit receives the PWM timing signal and generates a signal for selecting which of the first to nth circuits of the dimming signal generation circuit is to be driven. Each switch circuit resonates from the 1st to the nth of the resonance circuit and drives the 1st to nth of the cold-cathode tubes sequentially to light up only during a period when the signal is at a high level.

The “discharge tube drive circuit” disclosed in Patent Document 4 is a discharge tube drive circuit that emits light from a cold cathode discharge fluorescent tube used for a backlight of a liquid crystal display. As shown in FIG. 6B, the control characteristics are improved by adding a ballast capacitor to the separately excited drive circuit. The separately excited drive circuit includes a step-up transformer, a semiconductor switch, an LC resonance filter, an open / close time ratio control circuit, and a ballast capacitor. By adjusting the voltage drop characteristic with a ballast capacitor, even one type of transformer can be used for discharge tubes with various load characteristics.
JP 2001-52891 A JP 2001-110582 JP Japanese Patent Laid-Open No. 2002-15895 Japanese Patent Laid-Open No. 2002-359094

  However, the conventional cold cathode tube lighting circuit can only achieve an efficiency of 81 to 85%. In a flyback circuit that does not use a resonant circuit, the circuit is simple, but the efficiency is rather low. A Royer circuit that self-oscillates using a resonant circuit cannot be dimmed and has low efficiency. In the lighting circuit in which the resonance circuit is separately driven by the PWM signal, the efficiency is 85% at the highest.

  An object of the present invention is to solve the above-mentioned conventional problems and increase the operating efficiency of a cold cathode tube lighting circuit.

  In order to solve the above-described problems, in the present invention, a cold cathode tube lighting circuit has a primary side winding serving as an inductance constituting a primary side resonance circuit and a secondary side winding for lighting the cold cathode fluorescent tube. Of the drive transformer, the resonance capacitor that forms the primary side resonance circuit together with the primary side winding of the drive transformer, the switching transistor that turns on and off the current flowing through the primary side winding of the drive transformer, and the resonance characteristic curve of the primary side resonance circuit A PWM signal generator that supplies a PWM signal having a frequency near the peak away from the center to the switching transistor is provided. In addition, a regenerative transformer having a primary winding for supplying current from the power supply to the primary winding of the drive transformer and a secondary winding connected to the power supply via a reverse diode is provided.

  By comprising as mentioned above, the energy efficiency of the lighting circuit of the cold cathode tube used for the backlight of a liquid crystal display panel can be raised to 90 to 95%.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to FIGS.

  The embodiment of the present invention is a cold cathode tube lighting circuit that drives a primary resonance circuit of a drive transformer with a PWM signal having a frequency near a peak away from the center of a resonance characteristic curve, and uses a regenerative transformer instead of a choke coil. .

  FIG. 1 is a circuit diagram of a cold cathode tube lighting circuit in an embodiment of the present invention. FIG. 2 is a circuit diagram for a plurality of tubes of a cold cathode tube lighting circuit. FIG. 3 is a circuit diagram for driving two circuits of the cold cathode tube lighting circuit. FIG. 4 is a graph showing the relationship between drive frequency and efficiency.

  1, 2, and 3, the cold cathode fluorescent tube 1 is an illumination unit for a backlight of a liquid crystal display panel. The drive transformer 2 is a winding transformer that generates a high-voltage current for lighting the cold cathode fluorescent tube. The primary inductance is L1. The secondary winding of the drive transformer 2 has a number of turns that can obtain a voltage sufficient to light the cold cathode fluorescent tube 1. The midpoint of the primary winding of the drive transformer 2 is connected to the power source via the primary winding CT of the regenerative transformer 7. A cold cathode fluorescent tube (CCFL tube) 1 is connected to the secondary winding of the drive transformer 2. The current detection circuit 3 is a circuit that detects a current flowing through the cold cathode fluorescent tube 1. The resonance capacitor 4 is a capacitor that forms a resonance circuit together with the primary winding of the drive transformer 2. The capacity is C1. The primary winding (L1) of the drive transformer 2 and the resonance capacitor 4 (C1) constitute a primary resonance circuit.

  The PWM signal generator 5 is a circuit that generates a PWM signal for driving the switching transistors TR1 and TR2. The error amplifier 6 is a circuit that generates a PWM control signal by comparing the output voltage of the current detection circuit 3 that detects the current flowing through the cold cathode fluorescent tube 1 with a reference voltage. The regenerative transformer 7 is a transformer for regenerating back electromotive force when the current is off to the power source. The primary winding CT is a power supply winding that functions as a choke coil. The secondary winding CTS is a winding for power regeneration. The secondary winding CTS of the regenerative transformer 7 is connected to the power supply via a diode connected in a direction in which a forward current flows into the power supply. The switching transistors TR1, TR2, TR3, and TR4 are switch means that are driven by passing a PWM current through the drive transformer 2. The ballast capacitor 8 is a capacitor for balancing and stabilizing the currents flowing through the two cold cathode fluorescent tubes 1.

  The operation of the cold cathode tube lighting circuit in the embodiment of the present invention configured as described above will be described. First, an outline of the function of the cold cathode tube lighting circuit will be described with reference to FIG. The primary side resonance circuit composed of the primary winding (L1) of the drive transformer 2 and the resonance capacitor 4 (C1) is alternately driven by the two switching transistors TR1 and TR2. The error amplifier 6 compares the output voltage of the current detection circuit 3 proportional to the current flowing through the cold cathode fluorescent tube 1 with a reference voltage. The PWM signal generator 5 generates a PWM control signal so that the current flowing through the cold cathode fluorescent tube 1 of the load becomes constant. The PWM signal generator 5 supplies a PWM signal having a peak frequency 3 to 10% away from the center frequency of the resonance characteristics to the two switching transistors TR1 and TR2. The magnetic energy remaining in the primary winding CT of the regenerative transformer 7 when the switching transistor is turned off is fed back to the power supply by the secondary winding CTS of the regenerative transformer 7.

  Next, the driving method of the cold cathode tube lighting circuit will be described in detail with reference to FIG. 1 again. The primary side resonance circuit composed of the primary winding (L1) of the drive transformer 2 and the resonance capacitor 4 (C1) is alternately driven by the two switching transistors TR1 and TR2. The PWM signal generator 5 supplies a PWM signal having a frequency 3 to 10% away from the center frequency of the resonance characteristic of the primary side resonance circuit to the two switching transistors TR1 and TR2. The resonance characteristic of the primary side resonance circuit is an effective resonance characteristic including the influence of the load on the secondary side. Details will be described later.

  Although the drive current is a pulse, since the resonance circuit functions as a tank circuit, a current having a waveform close to a resonance waveform, that is, a sine wave flows through the primary side resonance circuit. Since the resonance circuit becomes a capacitive load for the harmonic component, it flows through the resonance capacitor 4 (C1) and hardly flows through the primary winding (L1), and therefore does not propagate to the secondary side. The cold cathode fluorescent tube 1 is lit by a current induced in the secondary winding of the drive transformer 2 by this current. When either of the switching transistors TR1 and TR2 is on, the primary winding CT of the regenerative transformer 7 operates as an inductance having a large Q value.

  A current detection circuit 3 detects the current flowing through the cold cathode fluorescent tube 1 of the load. The error amplifier 6 compares the output voltage of the current detection circuit 3 proportional to the current flowing through the cold cathode fluorescent tube 1 with a reference voltage and generates a PWM control signal. These circuits constitute a negative feedback loop. The reference voltage changes according to the brightness control signal. The brightness control signal can be a level signal or a pulse signal, and the circuit system of the error amplifier 6 may be selected according to the purpose. If the output voltage of the current detection circuit 3 is higher than the reference voltage, a PWM control signal instructing to narrow the pulse width is generated. On the contrary, if the output voltage of the current detection circuit 3 is lower than the reference voltage, a PWM control signal for instructing to widen the pulse width is generated.

  The PWM signal generator 5 generates a pulse signal having a pulse width corresponding to the PWM control signal at the same timing as the input synchronization signal. The switching transistors TR1 and TR2 are alternately turned on and off by the output signal of the PWM signal generator 5. Therefore, the primary side resonance circuit is driven with a pulse width such that the current flowing through the cold cathode fluorescent tube 1 of the load is constant.

  The current for operating the drive transformer 2 is supplied from the power source via the primary winding CT of the regenerative transformer 7. At the moment when the switching transistors TR1 and TR2 are turned off, back electromotive force is generated in the primary winding CT of the regenerative transformer 7 and the primary winding of the drive transformer 2. The magnetic energy remaining in the primary winding CT of the regenerative transformer 7 is returned to the power supply by the secondary winding CTS of the regenerative transformer 7. The energy of the primary side resonance circuit of the drive transformer 2 is absorbed by the resonance capacitor 4.

  Next, a method of lighting two cold cathode fluorescent tubes with one drive circuit will be described with reference to FIG. The basic configuration is the same as the circuit shown in FIG. The difference is that the two cold cathode fluorescent tubes 1 are connected to the secondary winding of the drive transformer 2 via the ballast capacitor 8. The ballast capacitor 8 can balance and stabilize the current flowing through the two cold cathode fluorescent tubes 1.

  Next, a method of driving two drive circuits with one PWM signal generator 5 will be described with reference to FIG. The PWM signal generator 5 drives two switching transistor pairs. The switching transistors TR1 and TR3 flow current in synchronization with the synchronization signal at exactly the same timing. The switching transistors TR2 and TR4 also flow current in synchronization with the synchronization signal at exactly the same timing. However, each pulse width is not necessarily the same. The frequency of the synchronization signal is a frequency that is 3 to 10% away from the center frequency of the resonance characteristic of the primary side resonance circuit including the secondary side load. The PWM signal generator 5 has only one system for generating a pulse signal from the synchronization signal and two systems for the others.

  Next, the relationship between the resonance characteristics and the drive frequency will be described with reference to FIG. In the cold-cathode tube lighting circuit having this configuration, when the energy efficiency is measured while changing the drive frequency, the graph shown in FIG. 4 is obtained. This graph is a qualitative characteristic curve in which the horizontal axis represents the drive frequency and the vertical axis represents the energy efficiency of the circuit. Instead of measuring energy efficiency, the same graph can be obtained by measuring the voltage of the primary side resonance circuit. This graph is considered to be because the primary side resonance circuit has a bimodal characteristic. The resonance characteristic of the primary side resonance circuit is a resonance characteristic considering the influence on the load side. The center frequency of the resonance characteristic curve is called the center frequency of the primary side resonance circuit.

  This resonance characteristic graph has peaks at lower and higher frequencies than the center frequency. The frequency of these peaks is 3-10% lower or higher than the center frequency. If any one of these peaks is selected as the operating point and the frequency is set as the driving frequency, the energy efficiency is increased. Therefore, driving with a PWM signal having a frequency 3 to 10% lower (higher) than the center frequency improves energy efficiency.

  The drive frequency is the frequency of the synchronization signal and is supplied from an oscillation circuit (not shown). The oscillation circuit is adjusted so that the frequency of the synchronization signal matches the peak frequency of the resonance characteristic graph. Strictly matching is difficult due to the dispersion of circuit constants and so on, so it may be set to a value near the peak. Since the vicinity of the peak is almost flat, a large error does not occur. When the cold cathode fluorescent tube is replaced, higher energy efficiency can be maintained by adjusting the oscillation frequency again. A negative feedback circuit that automatically corrects the oscillation frequency in accordance with the change in the resonance characteristics may be provided so as to follow the change in the operating state and the secular change.

  As described above, in the embodiment of the present invention, the cold-cathode tube lighting circuit is driven by the primary resonance circuit of the driving transformer with the PWM signal having the peak frequency away from the center of the resonance characteristic curve, and replaced with the choke coil. Therefore, energy efficiency of about 90 to 95% can be realized.

  The cold-cathode tube lighting circuit of the present invention is optimal as a cold-cathode tube lighting circuit for lighting a cold-cathode tube for a backlight of a large liquid crystal display panel.

It is a circuit diagram of the cold cathode tube lighting circuit in the Example of this invention. It is a circuit diagram for multiple tubes of a cold cathode tube lighting circuit in an embodiment of the present invention. It is a circuit diagram for the two-circuit drive of the cold cathode tube lighting circuit in the Example of this invention. It is a figure which shows the relationship between the drive frequency and energy efficiency of the cold cathode tube lighting circuit in the Example of this invention. It is a conceptual diagram of the Royer circuit and flyback circuit used for the conventional cold cathode tube lighting circuit. It is a circuit diagram of the separately excited resonance circuit type inverter used for the conventional cold cathode tube lighting circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cold cathode fluorescent tube 2 Drive transformer 3 Current detection circuit 4 Resonance capacitor 5 PWM signal generator 6 Error amplifier 7 Regenerative transformer 8 Ballast capacitor
TR1, TR2, TR3, TR4 switching transistor

Claims (2)

A drive transformer having a primary winding serving as an inductance constituting a primary resonance circuit and a secondary winding for lighting a cold cathode fluorescent tube, and the primary resonance circuit together with the primary winding of the drive transformer And a switching transistor for turning on and off the current flowing in the primary winding of the drive transformer, and a PWM signal having a frequency near the peak away from the center of the resonance characteristic curve of the primary resonance circuit to the switching transistor. A cold-cathode tube lighting circuit comprising a PWM signal generator to be supplied. 2. A regenerative transformer having a primary winding for supplying current from a power source to a primary side winding of the driving transformer and a secondary winding connected to the power source via a reverse diode. Cold cathode tube lighting circuit.

Images