WO2006025674A1

WO2006025674A1 - Ballast for cold cathode fluorescent lamps

Info

Publication number: WO2006025674A1
Application number: PCT/KR2005/002850
Authority: WO
Inventors: Fu-Wang Wang
Original assignee: Hela Tech Co., Ltd.
Priority date: 2004-08-30
Filing date: 2005-08-29
Publication date: 2006-03-09
Also published as: KR20060019745A

Abstract

A ballast for a CCFL is provided. The ballast includes a power source 100 for receiving an AC voltage, a rectifier circuit 102 for converting an input AC voltage from the power source 100 into a DC voltage, a power factor conditioner 104 for suppressing a reactive power of the rectified input voltage from the rectifier circuit 102 to improve a power factor, an oscillator 106 for converting the rectified DC voltage from the rectified circuit into a high-frequency AC voltage, a lighting unit 108 for supplying power the CCFL by using the high-frequency AC voltage from the oscillator 106, and a protection circuit 110 for protecting the ballast when an overcurrent or an overvoltage is supplied to the ballast.

Description

BALLAST FOR COLD CATHODE FLUORESCENT LAMPS

Technical Field

[1] The present invention relates to a ballast circuit for inducing the turning on of a lamp, and more particularly, to a ballast circuit for inducing the turning on of a cold cathode fluorescent lamp (CCFL).

[2]

Background Art

[3] A lamp has a structure in which a fluorescent material is coated on the inner surface thereof, mercury is contained in the inside thereof, and a filament is disposed in the middle portion. In the conventional lamp, when heat is applied to the filament, a thermal electron is emitted from the filament. When the thermal electron collides with mercury contained in the lamp, an ultraviolet is generated to excite a fluorescent material on the inner wall of the lamp and thus generate a visible ray. This visible ray comes to human eyes.

[4] To light the lamp, discharge should be generated. To generate discharge, i.e., to have a thermal electron emitted, a voltage of hundreds through thousands of volts should be applied. However, when a high voltage is suddenly applied so as to emit thermal electrons, a lamp can be destroyed. To prevent this problem, a lamp is preheated. In the preheat-type lamp, a filament of the lamp is preheated in advance to some extent before a high voltage is applied thereto. Since the lamp is preheated in advance in this type lamp, a high voltage does not need to be suddenly applied so as to actually emit thermal electrons. Therefore, the preheat-type lamp is more stable than the conventional lamp. However, the preheat-type lamp has many problems. That is, when the lamp is not preheated, the lamp is apt to be destroyed and both ends thereof change into black color. Also, since a power factor conditioner consumes much power even in the case where the lamp is preheated, the efficiency of the lamp is degraded. The preheat-type lamp includes a filament and a cathode. Most of lamp failures are generated because the filament is cut and electricity does not flows therethrough.

[5] To solve this problem, a CCFL has been proposed. The CCFL has no filament, and instead, has a cathode at both ends in the inside of the lamp. Unlike the conventional lamp that preheats the filament, the CCFL does not uses the filament but uses the cathodes disposed at both ends. Specifically, when a voltage is applied to the cathodes, electrons are emitted from the cathodes by an electric field. The emitted electrons collide with mercury contained in the lamp to generate an ultraviolet. The ultraviolet changes into a visible ray while passing through a fluorescent material, and finally comes into human eyes.

[6] The conventional ballast is designed for the preheat-type lamp and includes a rectifier circuit, a power factor conditioner, and an oscillator. When the preheat-type lamp is implemented using the conventional ballast, the energy efficiency is high. On the contrary, when the CCFL is used, the energy efficiency is degraded in comparison with that of the preheat-type lamp because the CCFL has a structure different from that of the conventional preheat-type lamp. Particularly, a lighting unit is much different. Since the energy efficiency degrades when the CCFL is implemented using the con¬ ventional ballast, a ballast suitable for the CCFL is highly required.

[7]

Disclosure of Invention Technical Problem

[8] Accordingly, the present invention is directed to a ballast for a cold cathode fluorescent lamp that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

[9] An object of the present invention is to provide a ballast circuit for inducing the stable turning on of a CCFL.

[10] Another object of the present invention is to provide a ballast circuit capable of extending the lifetime of a CCFL.

[11] A further another object of the present invention is to provide a CCFL ballast with an integrated circuit for integratedly fixing a part of a CCFL ballast circuit thereto.

[12]

Technical Solution

[13] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, there is provided a ballast for stably turning on a CCFL, the ballast including: a rectifier circuit for converting an input AC voltage into a DC voltage; a power factor conditioner for suppressing a reactive power of the rectified input voltage from the rectifier circuit to improve a power factor; an oscillator for converting the rectified DC voltage from the rectified circuit into a high-frequency AC voltage; and a lighting unit for turning on the CCFL by using the high-frequency AC voltage from the oscillator, wherein the lighting unit is connected through one output line to each cathode of the CCFL.

[14] The ballast may further include a power source for receiving an AC voltage.

[15] The ballast may further include a protection circuit for detecting a malfunction to control an operation of the ballast.

[16] The power source may include: a power supply for receiving the input AC voltage; and an input filter circuit for filtering the input AC voltage. [17] The rectifier circuit may include: a rectifier for converting the input AC voltage into the DC voltage; and a smoothing circuit for removing a harmonic noise from the rectified DC voltage.

[18] The protection circuit may include: an overvoltage protection circuit for controlling an operation of the oscillator when a voltage exceeding a predetermined voltage is detected; and an overheat protection circuit for controlling the operation of the oscillator when a temperature exceeding a predetermined temperature is detected.

[19] The oscillator may include: an inverter for oscillating a high frequency; and an external interference absorbing circuit for controlling an oscillating status of the oscillator to stably oscillate the high frequency and suppress generation of a flickering phenomenon.

[20] The inverter may include: a transistor unit including at least two transistors; a negative feedback circuit for limiting a current flowing through the transistor unit; a reverse current prevention unit the current of the transistor unit from flowing in a reverse direction; and a vibrator circuit for alternately turning on/off the transistors to control oscillation of the transistor unit. The oscillator may further include an os¬ cillation inducer for inducing the inverter to oscillate the high frequency.

[21] The lighting unit may include: a resonator circuit for converting the high-frequency

AC voltage into a sine wave to supply power to the CCFL; and a lamp unit for receiving the power to turn on the CCFL. The resonator circuit may include a capacitor and an inductor. The capacitor may be connected in parallel to the inductor in the resonator circuit. The lighting unit may be connected in parallel to the capacitor and the inductor of the resonator circuit.

[22] The protection circuit may include a diode, a first resistor serially connected to the diode, a second resistor serially connected to the first resistor, and a capacitor connected in parallel to the second resistor and connected to a switching device. The external interference absorbing circuit may include a resistor for controlling the os¬ cillating status of the oscillator and a capacitor for suppressing the generation of the flickering phenomenon.

[23] The vibrator circuit may include an inductor for alternately turning on/off the transistors of the transistor unit and a resistor for controlling the oscillation of the transistor unit.

[24] The switching device may be an SCR. The vibrator circuit may include a first resistor, an inductor serially connected to the first resistor, and a second resistor serially connected to the inductor. The inductor may include two coils coaxially matched with each other.

[25] The smoothing circuit may include a capacitor connected in parallel to the rectifier.

The oscillation inducer may generate a sawtooth wave to induce oscillation of the transistor unit. [26] The power factor conditioner may include a first diode, a second diode serially connected to the first diode, a third diode serially connected to the second diode, a first capacitor connected in parallel to the first and second diodes, and a second capacitor connected in parallel to the second and third diodes.

[27] The overheat protection circuit may detect overheat by using a PTC resistor.

[28] The resistor of the external interference absorbing circuit may be connected in parallel to a first transistor of the transistor unit and may be serially connected to a second transistor of the transistor unit, and the capacitor of the external interference absorbing circuit may be connected in parallel to the first transistor of the transistor unit and may be serially connected to the second transistor of the transistor unit. [29] The rectifier circuit, the protection circuit, and the power factor conditioner may be integrated in an IC. [30]

Advantageous Effects [31] A CCFL ballast according to the present invention makes it possible to induce emission of electrons from both cathodes of a CCFL such that the CCFL is stably turned on, and makes it possible to supply power such that the CCFL is continuously turned on. Accordingly, it is possible to stably turn on the CCFL. [32] Also, a ballast circuit according to the present invention includes a vibrator circuit

508 having two inductors L2 that are coaxially matched with each other. Accordingly, it is possible to reduce the temperature of a switching device Q3 serving as a protection circuit and to extend the lifetime of the ballast. [33] Also, some parts of the ballast circuit can be integrated in an IC to implement the ballast circuit. Accordingly, it is possible to enhance the vibration and oxidation prevention functions and to stably operate the ballast circuit. [34]

Brief Description of the Drawings [35] FIG. 1 is a block diagram of a CCFL ballast circuit according to a preferred embodiment of the present invention. [36] FIG. 2 is a block diagram of a power source of the CCFL blaster circuit according to a preferred embodiment of the present invention. [37] FIG. 3 is a circuit diagram of an input filter circuit of the CCFL ballast circuit according to a preferred embodiment of the present invention. [38] FIG. 4 is a circuit diagram of an input filter circuit of the CCFL ballast circuit according to another preferred embodiment of the present invention. [39] FIG. 5 is a block diagram of a rectifier circuit of the CCFL ballast circuit according to a preferred embodiment of the present invention. [40] FlG. 6 is a circuit diagram of a rectifier of the CCFL ballast circuit according to a preferred embodiment of the present invention. [41] FlG. 7 is a circuit diagram of a smoothing circuit of the CCFL ballast circuit according to a preferred embodiment of the present invention. [42] FlG. 8 is a circuit diagram of a power factor conditioner of the CCFL ballast circuit according to a preferred embodiment of the present invention. [43] FlG. 9 is a block diagram of a protection circuit of the CCFL ballast circuit according to a preferred embodiment of the present invention. [44] FlG. 10 is a circuit diagram of an overheat protection circuit of the CCFL ballast circuit according to a preferred embodiment of the present invention. [45] FlG. 11 is a block diagram of an oscillator of the CCFL ballast circuit according to a preferred embodiment of the present invention. [46] FlG. 12 is a block diagram of an inverter of the CCFL ballast circuit according to a preferred embodiment of the present invention. [47] FlG. 13 is a circuit diagram of the protection circuit and the oscillator of the CCFL ballast circuit according to a preferred embodiment of the present invention. [48] FlG. 14 is a diagram illustrating a structure of an inductor of a vibrator circuit according to a preferred embodiment of the present invention. [49] FlG. 15 is a flowchart illustrating an operation of an overvoltage protection circuit according to a preferred embodiment of the present invention. [50] FlG. 16 is a flowchart illustrating an operation of an overheat protection circuit according to a preferred embodiment of the present invention. [51] FlG. 17 is a block diagram of the lighting unit according to a preferred embodiment of the present invention. [52] FlG. 18 is a circuit diagram of the lighting unit according to a preferred embodiment of the present invention. [53] FlG. 19 is a circuit diagram of the lighting unit of a conventional ballast for a preheat-type lamp. [54] FlG. 20 is a waveform diagram of a vibrator circuit according to a preferred embodiment of the present invention. [55] FlG. 21 is a diagram illustrating an ideal on-off waveform of a transistor according to a preferred embodiment of the present invention. [56] FlG. 22 is a diagram illustrating an actual on-off waveform of a transistor according to a preferred embodiment of the present invention. [57] FlG. 23 is a diagram illustrating a signal waveform between a base and an emitter of a transistor according to a preferred embodiment of the present invention. [58] FlG. 24 is a diagram illustrating a signal waveform between a collector and an emitter of a transistor according to a preferred embodiment of the present invention.

[59] FlG. 25 is a perspective view of a CCFL ballast implemented using an IC according to a preferred embodiment of the present invention.

[60]

Best Mode for Carrying Out the Invention

[61] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to accompanying drawings.

[62] FlG. 1 is a block diagram of a CCFL ballast circuit according to a preferred embodiment of the present invention.

[63] Referring to FlG. 1, the CCFL ballast circuit includes a power source 100, a rectifier circuit 102, a power factor conditioner (PFC) 104, an oscillator 106, a lighting unit 108, and a protection circuit 110.

[64] The power source 100 receives an alternating current (AC) voltage. Generally, the power source 100 receives an AC voltage of 200V/60Hz. Various voltages other than the 200V/60Hz AC current may be applied to the power source 100.

[65] The rectifier circuit 102 converts the received AC voltage into an direct current

(DC) voltage.

[66] The power factor conditioner 104 suppresses a reactive power to improve a power factor. A discharge of an initial high voltage is necessary for turning on the CCFL ballast circuit. When the initial high voltage is applied, a current of the CCFL increases. A voltage applied to the CCFL after the initial high voltage discharge decreases as the current of the CCFL increases. This is called a negative resistance characteristic.

[67] Due to the negative resistance characteristic, the discharge by the initial high voltage may cause an unstable turning-on operation or the damage of the CCFL. Therefore, the CCFL ballast circuit must be able to initially supply a high voltage to turn on the CCFL, and to control the current of the CCFL after the turning-on operation. The power factor conditioner 104 performs the current control operation. The power factor conditioner 104 improves a power factor to stabilize the CCFL. A low power factor increases a line capacitance and reduces a line capacitance and a transformer capacity. Therefore, the improvement of the power factor may correspond to the reduction of a transmission loss and a signal distortion.

[68] For the improvement of power efficiency, the oscillator 106 converts the DC voltage of the rectifier circuit 102 into a high-frequency AC voltage. The oscillator 106 may use two transistors for oscillating the high-frequency AC voltage. In this case, the two transistors must operate alternately. Conventionally, an inductor intermediates between alternately-operating transistors for the oscillating operation. In a preferred embodiment of the present invention, an inductor is used for the oscillating operation. However, the inventive inductor has a different structure than the conventional inductor. The inventive inductor is constructed to include two coils that are coaxially matched with each other. This inductor increases the lifetime of the CCFL ballast circuit. The detailed structure of the inductor will be described later with reference to the other drawings.

[69] The lighting unit 108 supplies power to the CCFL by using the high-frequency AC voltage from the oscillator 106. In the conventional preheat-type lamp, each cathode must have two output lines for the turning on of the lamp. However, in the present invention, each cathode of the CCFL has only one output line.

[70] The protection circuit 110 serves to protect the CCFL ballast circuit when the CCFL ballast circuit malfunctions due to an overcurrent or an overvoltage supplied thereto. In a preferred embodiment of the present invention, when the CCFL ballast circuit mal¬ functions, the switching device induces the CCFL ballast circuit to be connected to a ground, thereby stopping the operation of the CCFL ballast circuit.

[71] Although not illustrated in FIG. 1, a surge suppressor circuit may be used in a preferred embodiment of the present invention. The surge suppressor protects the CCFL ballast circuit from a surge, such as an instantaneous high voltage (e.g., thun¬ derstroke) and a spark of an input voltage.

[72] As described above, the present invention is different from the conventional art in view of the oscillator 106, the lighting unit 108, and the protection circuit 110 that induce the CCFL to be stably turned on.

[73] FIG. 2 is a block diagram of the power source according to a preferred embodiment of the present invention.

[74] Referring to FIG. 2, the power source 100 includes a power supply 200 and an input filter circuit 202. The power supply 200 receives an AC voltage through its input terminal. Generally, an AC voltage of 220V/60Hz AC is applied to the power supply 200.

[75] The input filter circuit 202 filters an input voltage. The input filter circuit 202 serves to prevent the CCFL ballast circuit from being affected by an electro magnetic in¬ terference (EMI).

[76] FIG. 3 is a circuit diagram of the input filter circuit according to a preferred embodiment of the present invention.

[77] Referring to FIG. 3, the input filter circuit 202 includes a capacitor Cl and an inductor Ll that are connected in parallel.

[78] FIG. 4 is a circuit diagram of the input filter circuit according to another preferred embodiment of the present invention.

[79] Referring to FIG. 4, the input filter circuit 202 includes three capacitors CIl to Cl 3 and three inductors L4 to L6. The input filter circuit 202 in FlG. 4 is superior in a filtering performance to the input filter circuit in FlG. 3. However, the input filter circuit 202 in FlG. 4 is disadvantageous in that it is larger than the input filter circuit in FlG. 3.

[80] FlG. 5 is a block diagram of the rectifier circuit according to a preferred embodiment of the present invention.

[81] Referring to FlG. 5, the rectifier circuit 102 includes a rectifier 300 and a smoothing circuit 302. The rectifier 300 converts an input AC voltage into a DC voltage. The rectifier 300 may perform full-wave rectification or half-wave rectification. The rectified DC voltage generally contains harmonic noise. The harmonic noise causes various problems such as a distribution line loss. These problems are solved by the smoothing circuit 302. That is, the smoothing circuit 302 removes the harmonic noise from the rectified DC voltage. However, if necessary, the rectifier circuit 102 may be implemented using only the rectifier 300 without the use of the smoothing circuit 302.

[82] FlG. 6 is a circuit diagram of the rectifier according to a preferred embodiment of the present invention.

[83] Referring to FlG. 6, the rectifier 300 includes four diodes. Two diodes D4 and D5 are serially connected and another two diodes D6 and D7 are serially connected. Each of the diode pairs is connected in parallel to the power source 100. It is well known to those skilled in the art that the rectifier 300 may be alternatively implemented using a bridge circuit.

[84] FlG. 7 is a circuit diagram of the smoothing circuit of according to a preferred embodiment of the present invention.

[85] Referring to FlG. 7, the smoothing circuit 302 includes a capacitor C 14. The capacitor C14 is connected in parallel to the rectifier 300. The capacitor C14 is connected in parallel to the rectifier 300 to filter a DC voltage from the rectifier 300. This filtering operation removes a harmonic noise from the DC voltage.

[86] FlG. 8 is a circuit diagram of the power factor conditioner 108 according to a preferred embodiment of the present invention.

[87] A power factor is a ratio of actual power to apparent power in a circuit. The power factor can be expressed as Equation 1 below.

[88] MathFigure 1

COSQ(POJVER FACTOR) = PlVA

[89] where P is actual power, VA is apparent power, and θ is a phase difference between an input voltage and an input current [90] [91] As the phase difference θ decreases, the power factor increases. The increase of the power factor means that the actual power P increases while reactive power Q decreases. As the power factor increases, a power loss and a harmonic content decreases.

[92] Referring to FlG. 8, the power factor conditioner 104 includes diodes Dl to D3 and capacitors C2 and C3. The three diodes Dl to D3 are serially connected, the capacitor C2 is connected in parallel to the upper two diodes Dl and D2, and the capacitor C3 is connected in parallel to the lower two diodes D2 and D3. An input voltage is boosted by the diodes Dl to D3 and the capacitors C2 and C3. At this time, the capacitors C2 and C3 are charged with a DC voltage. In this manner, the power factor conditioner 104 compensates for the phase difference θto suppress the reactive power Q.

[93] FlG. 9 is a block diagram of the protection circuit 110 according to a preferred embodiment of the present invention.

[94] Referring to FlG. 9, the protection circuit 110 includes an overvoltage protection circuit 400 and an overheat protection circuit 402. When a voltage applied to the ballast circuit exceeds a predetermined voltage for a stable operation of the ballast, the overvoltage protection circuit 400 controls the oscillator 106 to stop its operation, thereby stopping the operation of the ballast circuit. When a temperature of the ballast circuit exceeds a predetermined temperature for a stable operation of the ballast, the overheat protection circuit 402 controls the oscillator 106 to stop its operation, thereby stopping the operation of the ballast circuit.

[95] FlG. 10 is a circuit diagram of the overheat protection circuit according to a preferred embodiment of the present invention.

[96] Referring to FlG. 10, the overheat protection circuit 402 includes a resistor R12 and another resistor R13 connected in parallel to the resistor R12. The resistors R12 and Rl 3 may be a positive temperature coefficient (PTC) resistor or a negative temperature coefficient (NTC) resistor. As a temperature increases, the resistance of the PTC resistor increases while the resistance of the NTC resistor decreases. The protection circuit 402 detects the temperature of the ballast circuit by the resistors R12 and Rl 3. Accordingly, when the detected temperature of the ballast circuit exceeds the pre¬ determined temperature, the overheat protection circuit 402 controls the oscillator 106 to stop its operation, thereby stopping the operation of the ballast circuit. Even when the resistor Rl 2 is removed from the overheat protection circuit 402, the overheat protection circuit 402 can perform substantially the same function.

[97] FlG. 11 is a block diagram of the oscillator according to a preferred embodiment of the present invention.

[98] Referring to FlG. 11, the oscillator 106 includes an oscillation inducer 500, an inverter 502, and an external interference absorbing circuit 504. The oscillation inducer 500 induces the inverter 502 to oscillate a high frequency. The oscillation inducer 500 generates a sawtooth wave to induce the oscillation of a high frequency by the inverter 502. Alternatively, the oscillation inducer 500 may generate a square wave for the high frequency oscillation. Accordingly, the inverter 502 oscillates a high frequency by the oscillation inducer 500. At least two transistors are used for the high frequency os¬ cillation. The repeated turning on/off operation of the transistors causes the generation of a high frequency. The external interference absorbing circuit 504 provides conditions such that the inverter 502 stably oscillates a high frequency. Specifically, the external interference absorbing circuit 504 stabilizes the high frequency oscillating operation of the inverter 502 and suppresses generation of a flickering phenomenon.

[99] FlG. 12 is a block diagram of the inverter according to a preferred embodiment of the present invention.

[100] Referring to FlG. 12, the inverter 502 includes a transistor unit 506, a vibrator circuit 508, a reverse current prevention unit 510, and a feedback circuit 512. The transistor unit 506 may include two transistors. The repeated turning on/off operations of the two transistors cause generation of a high frequency. The vibrator circuit 508 provides conditions such that the transistor unit 506 enters a good on/off mode. Specifically, the vibrator circuit 508 alternately turns on/off the transistors of the transistor unit 506 such that the transistor unit 506 oscillators a high frequency in a good manner. This reduces the time for convention of the transistor from an on mode into an off mode, and vice versa. This will be described later in detail with reference to the other drawings.

[101] The reverse current prevention unit 510 prevents a current of the transistor unit 506 from flowing in a reverse direction. The feedback circuit 512 limits a current flowing through the transistor unit 506.

[102] FlG. 13 is a circuit diagram of the protection circuit and the oscillator according to a preferred embodiment of the present invention.

[103] Referring to FlG. 13, the protection circuit 110 includes a diode D8, a silicon controlled rectifier (SCR) Q3, a resistor Rl, a capacitor C4, and another resistor R2. In the protection circuit 110, the diode D8, the SCR Q3, the resistor Rl, and the capacitor C4 correspond to the overvoltage protection circuit 400, and the resistor R2 corresponds to the overheat protection circuit 402. When a voltage applied to the ballast circuit exceeds the predetermined reference voltage, the overvoltage protection circuit 400 controls an instantaneous voltage of the oscillation inducer 500 to increase suddenly and controls the connection between the oscillation inducer 500 and the transistor unit 506, thereby stopping the operation of the transistor Q2 of the inverter 504. That is, when the ballast circuit malfunctions, the overvoltage protection circuit 400 induces the diode DlO to be grounded, thereby preventing the operation of the transistor Q2 to stop the operation of the ballast circuit. The detailed operation of the overvoltage protection circuit 400 will be described layer with reference to the other drawings.

[104] When the temperature of the ballast circuit exceeds the predetermined temperature, the overheat protection circuit 402 controls an instantaneous voltage of the oscillation inducer 500 to increase suddenly and controls the connection between the oscillation inducer 500 and the transistor unit 506, thereby stopping the operation of the transistor Q2. That is, when the ballast circuit malfunctions, the overheat protection circuit 402 induces the diode DlO to be grounded, thereby preventing the operation of the transistor Q2 to stop the operation of the ballast circuit. The detailed operation of the overheat protection circuit 4Ow will be described layer with reference to the other drawings.

[105] The oscillator 106 includes an oscillation inducer 500, an external interference absorbing circuit 502, and an inverter 504. The oscillation inducer 500 includes a resistor R3, a DIAC diode DIl, and a capacitor C6. The oscillation inducer 500 generates a sawtooth wave. The generated sawtooth wave induces the transistor Q2 to oscillate a high frequency. When a voltage of 220V is applied to an input terminal, a pulse DC voltage of about 230V can be generated. This voltage is charged through the resistors R3 and R4 in the capacitor C6. At this time, a DC voltage of the capacitor C6 reaches about 34V. This voltage is transferred through the DIAC diode DIl to a second transistor Q2. When the second transistor Q2 is turned on, a current flows through a first transistor Ql via the inductor L2. When the first transistor Ql operates, the second transistor Q2 stops it operation. That is, the two transistors Ql and Q2 al¬ ternately perform an on/off operation. Consequently, the on/off operations of the transistors Ql and Q2 cause the oscillation of a high frequency. When the CCFL is turned on, the oscillation inducer 500 automatically stops its operation. The waveform generated at the oscillation inducer 500 may be any type of waveform including a sawtooth waveform. It is well know to those skilled in the art that the type of the waveform does not affect the scope of the present invention.

[106] The inverter 502 includes a transistor unit 506, a vibrator circuit 508, a reverse current prevention unit 510, and a feedback circuit 512. The transistor unit 506 may include two transistors. When one of the two transistors operates, the other of the two transistors does not operate. This operation is repeated to oscillate a high frequency.

[107] The vibrator circuit 508 includes resistors R8 and RlO and an inductor L2. The vibrator circuit 508 provides conditions such that the transistor 506 is turned on/off in a good manner. The resistors R8 and RlO limit the oscillation degree of the transistor unit 506. The inductor L2 serves as a medium such that the transistors Ql and Q2 operate alternately. The convention ballast circuit performs the same operation as the inventive ballast circuit. However, the inventive inductor has a different structure than the conventional inductor.

[108] FlG. 14 is a diagram illustrating a structure of the inductor according to a preferred embodiment of the present invention.

[109] Referring to FlG. 14, the inductor L2 includes two coils coaxially matched with each other. This inductor structure can extend the lifetime of the ballast circuit.

[110] The reverse current prevention unit 510 includes resistors R9 and Rl 1 and diodes

D 12 and Dl 3. The reverse current prevention unit 510 may be connected in parallel to the transistor unit 506 and the vibrator circuit 508. The resistors R9 and Rl 1 limit a current flowing through the transistor unit 506. The diodes D12 and Dl 3 prevent a current of the transistor unit 506 from flowing in a reverse direction.

[Ill] The feedback circuit 512 limits a current flowing through the transistor unit 506.

The feedback circuit 512 includes resistors R6 and R7 in FlG. 13. The feedback circuit 512 may be a negative feedback resistor. Also, the feedback circuit 512 may be any feedback resistor other than the negative feedback resistor.

[112] The external interference absorbing circuit 504 includes a first transistor Ql and a second transistor Q2. A resistor R5 and a capacitor C7 are connected in parallel to the first transistor Ql, and are serially connected to a collector of the second transistor Q2. Specifically, the resistor R5 stabilizes an oscillating status of the transistor unit 506, and the capacitor C7 suppresses generation of a flickering phenomenon.

[113] FlG. 15 is a flowchart illustrating an operation of the overvoltage protection circuit according to a preferred embodiment of the present invention.

[114] Referring to FlGs. 13 and 15, the overvoltage protection circuit 400 measures a voltage of the ballast circuit (SlOOO). When the measured voltage exceeds a pre¬ determined reference voltage, the overvoltage protection circuit 400 induces an in¬ stantaneous voltage of the oscillation inducer 500 to increase suddenly (S 1004). On the contrary, when the measured voltage does not exceed the predetermined reference voltage, the overvoltage protection circuit 400 continues to measure a voltage of the ballast circuit. When the instantaneous voltage of the oscillation inducer 500 increases, the overvoltage protection circuit 400 controls the connection between the oscillation inducer 500 and the transistor Q2 (S 1006). That is, the overvoltage protection circuit 400 induces the diode DlO to be grounded so that the oscillation inducer 500 cannot be connected to the transistor Q2. A switching device (Q3) may be an SCR performing the above operation. This causes the transistor Q2 to stops its operation (SlOlO). The switching device may also be any device other than the SCR. It is well known to those skilled in the art that the type of the switching device does not affect the scope of the present invention.

[115] FlG. 16 is a flowchart illustrating an operation of the overheat protection circuit according to a preferred embodiment of the present invention. [116] Referring to FlGs. 13 and 16, the overheat protection circuit 402 detects the temperature of the ballast circuit (SHOO). When the detected temperature exceeds a predetermined reference temperature, the overheat protection circuit 402 induces an in¬ stantaneous voltage of the oscillation inducer 500 to increase suddenly (Sl 104). On the contrary, when the detected temperature does not exceed the predetermined reference temperature, the overheat protection circuit 402 continues to detect the temperature of the ballast circuit. When an instantaneous voltage of the oscillation inducer 500 increases suddenly, the overheat protection circuit 402 controls the connection between the oscillation inducer 500 and the transistor Q2 (Sl 106). That is, the overheat protection circuit 402 induces the diode DlO to be grounded so that the oscillation inducer 500 cannot be connected to the transistor Q2. This causes the transistor Q2 to stops its operation (SlOlO). The resistor R2 in HG. 3 is a PTC thermistor. When the temperature of the circuit increases, the resistance of the resistor R2 increases to check the temperature of circuit. When the checked temperature exceeds the predetermined reference temperature, the operation of the ballast circuit is controlled.

[117] As illustrated in FlGs. 15 and 16, when the ballast circuit malfunctions, the overvoltage protection circuit 400 and the overheat protection circuit 402 induce the diode DlO to be grounded, thereby stopping the operation of the transistor Q2 to stop the operation of the circuit. That is, the ballast circuit stops its operation such that it is prevented from being damaged by the abnormal voltage or current.

[118] FlG. 17 is a block diagram of an lighting unit according to a preferred embodiment of the present invention.

[119] Referring to FlG. 17, the lighting unit 108 includes a resonant capacitor 600, a resonant circuit 602, and a lamp 604. The lighting unit 108 converts the high- frequency AC voltage of the oscillator 106 into a sine wave to supply power to the lamp. The supplied power turns on the lamp.

[120] FlG. 18 is a circuit diagram of the lighting unit according to a preferred embodiment of the present invention.

[121] Referring to FlG. 18, the resonant capacitor 600 includes capacitors C8 and C9. The resonant circuit 602 includes a capacitor ClO and an inductor L3. The capacitor ClO, the inductor L3, and the lamp 604 are connected in parallel to one another. The capacitor ClO is connected in parallel to the inductor L3. However, when the lamp does not resonates, the capacitor ClO and the inductor LlO actually serves as a serial resonance circuit. The resonant circuit 602 can generate a voltage of 1200Vp-p, thereby making it possible to rapidly complete the turning on operation. In general, the turning on time of the CCFL does not exceed 100ms. The resonant circuit 602 may also be implemented in a parallel structure.

[122] As illustrate in FlG. 18, the present invention use the lamp having two output lines. That is, the lamp receives power through the two output lines each being connected to each cathode thereof.

[123] FlG. 19 is a circuit diagram of a lighting unit of a conventional ballast for a preheat- type lamp.

[124] Referring to FlG. 19, the conventional ballast uses four output lines for turning on the preheat-type lamp. That is, the lamp receives power through the four output lines each two being connected to each cathode thereof.

[125] The CCFL implemented using the conventional ballast circuit in FlG. 19 is in¬ efficient in energy use. Accordingly, the present invention proposes the structure il¬ lustrated in FlG. 18, so as to stably turn on the CCFL while maintaining the energy efficiency.

[126] FlG. 20 is a waveform diagram of the vibrator circuit according to a preferred embodiment of the present invention.

[127] Referring to FlG. 20, a high-frequency AC voltage from the oscillator 106 is converted into a sine wave by the resonant circuit, thereby supplying power to the lamp 604. However, it is actually difficult to obtain an ideal sine wave. Actually, a quasi-sine wave illustrated in FlG. 20 is obtained.

[128] FlG. 21 is a diagram illustrating an ideal on-off waveform of a transistor according to a preferred embodiment of the present invention.

[129] Referring to FlG. 21, the ideal on-off waveform is a square waveform.

[130] FlG. 22 is a diagram illustrating an actual on-off waveform of a transistor according to a preferred embodiment of the present invention.

[131] Referring to FlG. 22, the actual on-off waveform becomes a trapezoidal waveform due to the gradual damage of the transistor and a limitation in the on-off speed thereof. The damage of the transistor is the main reason for generation of heat by the transistor. The heat generation of the transistor may causes the damage of the transistor or a failed saturation of the transistor. Accordingly, the temperature of the transistor must be lowered to suppress the heat generation of transistor. When the transistor generates heat, the time T required for conversion of the transistor from an on mode into an off mode (and vice versa) becomes longer than that of a case where the transistor operates ideally. Accordingly, it is important to reduce the required delay time T. The vibrator circuit 508 serves to reduce the required delay time T.

[132] FlG. 23 is a diagram illustrating a signal waveform between a base and an emitter of a transistor according to a preferred embodiment of the present invention.

[133] Referring to FlG. 23, the signal waveform between the base and the emitter is a waveform driven between bases of the transistors Ql and Q2 after the vibration of the inductor L2. The waveform of the base and the emitter basically corresponds to a sine waveform and applies an impulse. [134] FlG. 24 is a diagram illustrating a signal waveform between a collector and an emitter of a transistor according to a preferred embodiment of the present invention.

[135] Referring to FlG. 24, the signal waveform between the collector and the emitter is a square waveform that is an on-off waveform. However, the consumption in the transistor results in a trapezoidal waveform. Accordingly, a well-implemented waveform between the collector and the emitter may be a small, vertically symmetrical, frequently-bent trapezoidal waveform.

[136] FlG. 25 is a perspective view of a CCFL ballast implemented using an IC according to a preferred embodiment of the present invention.

[137] Referring to FlG. 25, some parts of the inventive ballast circuit can be integrated in an IC. The rectifier circuit 102, the protection circuit 110, the oscillation inducer 500, the external interference absorbing circuit 504, the vibrator circuit 508, and the reverse current prevention unit can be integrated in one IC. Also, the other circuits may be further integrated in the IC. It is well known to those skilled in the art that this mod¬ ification does not affect the scope of the present invention.

[138] When some parts of the ballast circuits are integrated in an IC, the vibration and oxidation preventing function can be enhanced to stably operate the ballast circuit.

[139] While the present invention has been described and illustrated herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention that come within the scope of the appended claims and their equivalents.

Claims

[1] A ballast for stably turning on a CCFL (cold cathode fluorescent lamp), comprising: a rectifier circuit for converting an input AC (alternating current) voltage into a

DC (direct current) voltage; a power factor conditioner for suppressing a reactive power of the rectified input voltage from the rectifier circuit to improve a power factor; an oscillator for converting the rectified DC voltage from the rectified circuit into a high-frequency AC voltage; and a lighting unit for turning on the CCFL by using the high-frequency AC voltage from the oscillator, wherein the lighting unit is connected through one output line to each cathode of the CCFL.

[2] The ballast of claim 1, further comprising a power source for receiving an AC voltage.

[3] The ballast of claim 1, further comprising a protection circuit for detecting a malfunction to control an operation of the ballast.

[4] The ballast of claim 2, wherein the power source includes: a power supply for receiving the input AC voltage; and an input filter circuit for filtering the input AC voltage.

[5] The ballast of claim 1, wherein the rectifier circuit includes: a rectifier for converting the input AC voltage into the DC voltage; and a smoothing circuit for removing a harmonic noise from the rectified DC voltage.

[6] The ballast of claim 3, wherein the protection circuit includes: an overvoltage protection circuit for controlling an operation of the oscillator when a voltage exceeding a predetermined voltage is detected; and an overheat protection circuit for controlling the operation of the oscillator when a temperature exceeding a predetermined temperature is detected.

[7] The ballast of claim 1, wherein the oscillator includes: an inverter for oscillating a high frequency; and an external interference absorbing circuit for controlling an oscillating status of the oscillator to stably oscillate the high frequency and suppress generation of a flickering phenomenon.

[8] The ballast of claim 7, wherein the inverter includes: a transistor unit including at least two transistors; a negative feedback circuit for limiting a current flowing through the transistor unit; a reverse current prevention unit the current of the transistor unit from flowing in a reverse direction; and a vibrator circuit for alternately turning on/off the transistors to control os¬ cillation of the transistor unit.

[9] The ballast of claim 8, wherein the oscillator further includes an oscillation inducer for inducing the inverter to oscillate the high frequency.

[10] The ballast of claim 1, wherein the lighting unit includes: a resonator circuit for converting the high-frequency AC voltage into a sine wave to supply power to the CCFL; and a lamp unit for receiving the power to turn on the CCFL.

[11] The ballast of claim 10, wherein the resonator circuit includes a capacitor and an inductor.

[12] The ballast of claim 11, wherein the capacitor is connected in parallel to the inductor in the resonator circuit.

[13] The ballast of claim 12, wherein the lighting unit is connected in parallel to the capacitor and the inductor of the resonator circuit.

[14] The ballast of claim 3, wherein the protection circuit includes a diode, a first resistor serially connected to the diode, a second resistor serially connected to the first resistor, and a capacitor connected in parallel to the second resistor and connected to a switching device.

[15] The ballast of claim 7, wherein the external interference absorbing circuit includes a resistor for controlling the oscillating status of the oscillator and a capacitor for suppressing the generation of the flickering phenomenon.

[16] The ballast of claim 8, wherein the vibrator circuit includes an inductor for al¬ ternately turning on/off the transistors of the transistor unit and a resistor for controlling the oscillation of the transistor unit.

[17] The ballast of claim 14, wherein the switching device is an SCR (silicon controller rectifier).

[18] The ballast of claim 8, wherein the vibrator circuit includes a first resistor, an inductor serially connected to the first resistor, and a second resistor serially connected to the inductor.

[19] The ballast of claim 18, wherein the inductor includes two coils coaxially matched with each other.

[20] The ballast of claim 5, wherein the smoothing circuit includes a capacitor connected in parallel to the rectifier.

[21] The ballast of claim 9, wherein the oscillation inducer generates a sawtooth wave to induce oscillation of the transistor unit.

[22] The ballast of claim 5, wherein the rectifier includes four diodes.

[23] The ballast of claim 1, wherein the power factor conditioner includes a first diode, a second diode serially connected to the first diode, a third diode serially connected to the second diode, a first capacitor connected in parallel to the first and second diodes, and a second capacitor connected in parallel to the second and third diodes.

[24] The ballast of claim 6, wherein the overheat protection circuit detects overheat by using a PTC (positive temperature coefficient) resistor.

[25] The ballast of claim 15, wherein the resistor of the external interference absorbing circuit is connected in parallel to a first transistor of the transistor unit and is serially connected to a second transistor of the transistor unit, and the capacitor of the external interference absorbing circuit is connected in parallel to the first transistor of the transistor unit and is serially connected to the second transistor of the transistor unit.

[26] The ballast of claim 3, wherein the rectifier circuit, the protection circuit, and the power factor conditioner are integrated in an IC (integrated circuit).