Method for driving a fluorescent lamp and an inverter circuit for performing such a method

A method for driving a fluorescent lamp and an inverter circuit for performing the same are used to reduce an amount of electromagnetic interference (EMI) generated by a transformer and an instantaneous loading of a DC voltage source. The inverter circuit comprises a DC square wave voltage source, a bridge DC/AC converter, a transformer, a feedback control unit and a voltage control circuit wherein the voltage control circuit is coupled to the DC voltage source, the bridge DC/AC converter and the feedback control unit. The voltage control circuit is used to convert DC voltage provided by the DC voltage source into a two-level DC square wave, which in turn converts the two-level DC square wave into an AC quasi-sine wave to drive the fluorescent lamp through the bridge DC/AC converter and the transformer. The feedback control unit generates signals to control the voltage control circuit and the bridge DC/AC converter.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 94112524, filed on Apr. 20, 2005. All disclosure of the Taiwan application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method for driving a fluorescent lamp and a circuit for performing such a method, and more particularly to a method for driving an inverter circuit for the fluorescent lamp to reduce an instantaneous power source loading and amount of electromagnetic interference (EMI) generated by a transformer.

2. Description of Related Art

FIG. 1is a conventional inverter circuit including a full bridge converter, which comprises a DC (direct current) voltage source110, a bridge DC/AC (alternative current) converter120, a transformer130, a CCFL140, an LCD145, a voltage sensor160, a current feedback150and a feedback control unit170. The bridge DC/AC converter120comprises a switch A, a switch B, a switch C and a switch D, each of which comprises a metal-oxide-semiconductor field effect transistor (MOS FET) and a diode connected in parallel. More, the feedback control unit170comprises an error amplifier & control circuit171, a drive circuit173and a pulse-width modulator175. Furthermore, the CCFL140is disposed in the liquid crystal display145.

One group comprised of the switch A and switch D and the other group comprised of the switch B and the switch C are alternatively turned on in accordance with a pulse signal provided by the drive circuit173, whereby a DC square wave voltage outputted from the DC voltage source110is converted to an AC square wave with a high frequency. There occurs a voltage difference between nodes P1and P2, which is an output of the bridge DC/AC converter120. The DC square wave with a high frequency is then converted to an AC quasi-sine wave signal with a high frequency and a high voltage for driving the CCFL using the transformer130and capacitors C1and C2.

Subsequently, the current feedback150senses a current signal passing through the CCFL140, the voltage sensor160senses a voltage signal inputting to the CCFL140from a secondary winding of the transformer130and eventually the feedback control unit170proceeds with a negative feedback in accordance with the current signal and the voltage signal. Since a brightness of the CCFL140depends on a magnitude of a current passing through it, the error amplifier & control circuit171can compare the current with a predetermined value and output a range of control signals to a pulse-width modulator (PWM)175in accordance with a magnitude of a deviation of the current. An adjusted pulse-width AC square-wave signal can be obtained at a primary side of the transformer130by using the pulse-width modulator175and the drive circuit173to control a pulse-width of the output signal of the bridge DC/AC converter120. The adjusted pulse-width AC square wave signal is then transformed to an AC quasi-sine waveform signal by the transformer130and the second capacitor C2, which in turn is inputted to the CCFL140, thereby achieving a purpose of stabilizing and adjusting the brightness of the CCFL140.

The detail operation of how to obtain the adjusted pulse-width of the AC square-wave output signal of the bridge DC/AC converter120is described as follows. After the AC quasi-sine waveform signal passes through the CCFL140, the current feedback150senses a current signal outputted from the CCFL140, and the voltage sensor160senses the AC quasi-sine waveform signal as well. Then, the error amplifier and control circuit171outputs a feedback control signal to the drive circuit173in accordance with the current signal outputted from the CCFL140and the AC quasi-sine waveform signal. Subsequently, the drive circuit173outputs an adjusted pulse-width driven signal to the bridge DC/AC converter120, which in turn outputs an AC adjusted pulse-width square wave to the primary side of the transformer130, thereby forming a negative feedback loop for driving the CCFL140. Subsequently, the AC adjusted pulse-width square wave is converted to an AC quasi-sine wave for driving the CCFL140, thereby achieving a purpose of stabilizing and adjusting the brightness of the CCFL140.

FIG. 2shows voltage timing charts present at several components in the circuit shown inFIG. 1, from which it can realized that how an AC square wave with an adjusted pulse-width is obtained from the bridge DC/AC converter120. InFIG. 2, WAV_A, WAV_B, WAV_C and WAV_D show turn-on timing charts of the switch A121, the switch B123, the switch C125and the switch D127, respectively, wherein in WAV_B, the term of “B_ON” stands for an on-state of the switch B; likewise, the similar terms apply to WAV_B, WAV_C, WAV_D. More, WAV_E shows a dead-time timing chart generated from the drive circuit173, by which the switch A121is turned off and the switch B123is turned on after a while i.e. the switch A and the switch B are not turned on at the same time due to a transition state period from a low level to a high level or from a high level to a low level. As a result, pulses D3and D4can prevent the switches A and B from being turn on simultaneously. WAV_F chart shows “switching timing” of the bridge DC/AC converter120, wherein the term of “B and C_ON” stands for the switched B and C being turned on simultaneously and the term of “A and D_ON” stands for the switched A and D being turned on simultaneously. As an operation of the full bridge DC/AC converter120, it can convert a DC voltage output from the DC voltage source110to an AC square wave by alternatively turning on one group switches consisted of switches A and D and the other group switches consisted of switches B and C in accordance with pulses provided by the drive circuit173.

In addition, “Primary Driving Voltage” shows an AC square wave with a positive voltage VCC1and a negative voltage −VCC1outputted from the bridge DC/AC converter120to the primary winding of the transformer130. Finally, “Secondary Voltage” shows an AC quasi-sine waveform signal present at a joint node between the second capacitor C2and the CCFL140.

Obviously, from “Primary Driving Voltage” in theFIG. 2, an instantaneous loading of the DC voltage source110is too high because it is used to generate a single-level square wave with a high voltage VCC1. If the instantaneous DC voltage source110is used to generate a two-level or multi-level square wave, its loading can be alleviated due to a smaller voltage variation. Besides, electromagnetic radiating wave generated by the transformer130can interfere other components in a mother board, which results in an electromagnetic interference (EMI) phenomenon. In addition, EMI also affects a read/write malfunction of a CPU. Most importantly, the bridge DC/AC converter120is particularly susceptible to EMI. Once the bridge DC/AC converter120is interfered by EMI, it cannot function normally so that a stabilized operating current for driving the CCFL cannot be obtained, which causes the CCFL140to have an unstable brightness. Also, the CPU interfered by EMI causes a computer, such as a notebook computer and a palm computer, to have a malfunction.

Therefore, it is needed to provide a method for alleviating EMI generated by the transformer in the inverter circuit for driving the CCFL in a field of manufacturing a liquid crystal display. Furthermore, by reducing amount of EMI generated by the transformer, a purpose of maintaining a stable brightness of the CCFL can be achieved.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an inverter circuit for driving a CCFL, which employ a characteristic of a two-level or multi-level DC square wave at a primary winding of a transformer, thereby reducing amount of EMI generated by the transformer.

The present invention is directed to a method for driving a CCFL, which forms a two-level or multi-level DC square wave at a primary winding of a transformer, thereby reducing amount of EMI generated by the transformer.

According to an embodiment of the present invention, an inverter circuit for driving a CCFL is provided. The inverter circuit comprises a DC voltage source, a bridge DC/AC converter, a transformer, a feedback control unit and a voltage control circuit, wherein the voltage control circuit is coupled to the DC voltage source, the bridge DC/AC converter, a feedback control unit and the voltage control circuit, and the DC voltage source is coupled to the voltage control circuit. In addition, the voltage control circuit is coupled to the bridge DC/AC converter, which in turn is coupled to the CCFL through the transformer, and the CCFL is coupled to the feedback control unit. Finally, the feedback control unit generates a feedback signal to the voltage control circuit and the bridge DC/AC converter in accordance with a current passing through the CCFL.

The present invention is characterized in that first, a voltage control circuit generates a two-level DC square wave, which is converted to an AC square wave by through a bridge DC/AC converter. The AC square wave is then input into a primary side of a transformer, which outputs an AC quasi-square wave at its secondary side, and the AC quasi-square wave passes through the CCFL. Since an amount of EMI generated by the transformer is proportional to a magnitude of a voltage variation present at the primary side thereof, the present invention can significantly reduce EMI due to a smaller voltage variation of the two-level or multi-level square wave and thus effectively prevent a bridge DC/AC converter from being damaged by EMI. More, since there is a smaller step-height in the two-level or multi-level square wave than a single-level square wave, an instantaneous loading of the DC voltage source can be considerably reduced.

For the sake of clarified description, the “two-level square wave,” used herein, refers to two square waves with two voltage levels (VCC1and VCC2) and the “two-level square wave” is converted to “four-level square wave” with four voltage levels (VCC1, −VVCC1, VCC2, −VCC2).

In addition, the feedback control unit provides a voltage control signal for controlling the voltage control circuit to adjust a pulse widths of each voltage level (such as VCC1and VCC2, in accordance with the current passing through the CCFL. More, the feedback control unit provides pulse-width modulation (PWM) signals for controlling the bridge DC/AC converter's converting the two-level DC square wave to four-level AC square wave, thereby achieving a purpose of stabilizing a brightness of the CCFL by using a negative feedback mechanism.

According to one embodiment of the present invention, the voltage control circuit comprises a two-level DC voltage generator and an analog device, wherein the two-level DC voltage generator is coupled to a DC voltage source and generates a first DC voltage (such as VCC1) and a second DC voltage with two different voltage levels. In addition, the analog device converts voltage control signals to a two-level DC square wave with the first DC voltage and the second DC voltage in accordance with the first DC voltage and the second DC voltage provided by the two-level DC voltage generator. Therefore, voltage levels of the two-level DC square wave can be adjusted by using the two-level DC voltage generator.

Furthermore, the feedback control unit comprises an error amplifier and control circuit, a pulse-width modulator and a drive circuit. More, the error amplifier and control circuit receives a current passing through the CCFL and then output pulse-width modulating signals for controlling the pulse-width modulator to output feedback PWM signals to the bridge DC/AC converter in accordance with the current. The drive voltage for driving the CCFL can be adjusted by the feedback PWM signals, and then controls the current passing through the CCFL, thereby stabilizing the brightness of the CCFL.

To adjust pulse widths and voltage levels of the two-level DC square wave, the pulse-width modulator at least comprises a triangle-wave generator, a first comparator, a second comparator and an exclusive-OR gate to provide the aforementioned voltage control signals.

In addition, the triangle-wave generator is used to provide a triangle wave. The first comparator compares the triangle wave with a first reference voltage and then outputs a first periodic square wave with a pulse width that is a duration in which the voltage level of the triangle wave is higher than that of the first reference voltage. Likewise, the second comparator compares the triangle wave with a second reference voltage and then outputs a second periodic square wave with a pulse width that is a duration in which the voltage level of the triangle wave is higher than that of the second reference voltage

Subsequently, the outputs of the first comparator and the second comparator are input to the exclusive-OR gate to proceed to an exclusive-OR operation. As a result, the aforementioned voltage control signals are obtained. By adjusting voltage levels of the first reference voltage and the second reference voltage, pulse widths of the first periodic square wave and the second periodic square wave can accordingly be adjusted.

In addition, the voltage control signals output from the exclusive-OR gate can be further designed to first pass an inverter gate for obtaining a better digital wave shape and then output to the voltage control circuit, as can be easily modified by one of ordinary skill in the art. More, the two-level DC square wave can be designed to a multi-level (for example, three-level) by only replacing the two-level DC voltage generator in the voltage control circuit with a multi-level DC voltage generator that provides multi-level voltages. Meanwhile, a generating method for generating multiple voltage control signals is modified, for example, implementing a plurality of comparators and a plurality of reference voltages. It is obviously that the DC square wave with the more level voltage causes the required circuit to be more complicated.

A method for driving a fluorescent lamp of the present invention comprises first, converting a DC voltage source to a two-level (multi-level) DC periodic square, which is then converted to an AC square wave. After that, the AC square wave is converted to an AC quasi-sine wave prior to being input to the CCFL. The method for driving a fluorescent lamp of the present invention is characterized in that a voltage for driving the CCFL is converted from the two-level (multi-level) DC periodic square, which has a smaller voltage variation and accordingly causes EMI generated by the transformer and an instantaneous voltage loading to be reduced

The method for driving a fluorescent lamp of the present invention further comprises detecting the current passing through the CCFL, according to which PWM signals are generated. The PWM signals facilitates the two-level (multi-level) DC periodic square to be converted to the AC square wave, which is a conventional feedback control method for stabilizing the brightness of the CCFL.

The method for driving a fluorescent lamp of the present invention further comprises obtaining the voltage control signals by using the exclusive-OR gate's operating an exclusive-OR with the first and the second periodic square waves and implementing the voltage control signals to control pulse widths of two-level (multi-level) DC periodic square.

In addition, a method for generating the first and the second periodic square waves comprises implementing comparisons between the first and the second reference voltages with different voltage levels, and the triangle waves. For example, a pulse width of the first periodic square wave can be designed to be a duration in which the voltage of the triangle wave is larger than that of the first reference voltage. Likewise, a pulse width of the second periodic square wave can be designed to be a duration in which the voltage of the triangle wave is larger than that of the second reference voltage.

Based on the above description and preferred embodiments of the present invention, problems of EMI generated by the transformer and a high DC voltage source loading can be resolved, Thus, the present invention not only stabilizes a brightness of the CCFL but prevents the inverter from malfunction because of reduced amount of EMI generated by the transformer. Furthermore, a read/write process of a CPU cannot be interfered by electromagnetic radiation generated by the transformer so as to ensure a computer working normally.

The objectives, other features and advantages of the invention will become more apparent and easily understood from the following detailed description of the invention when taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to an inverter circuit of a present preferred embodiment of the invention, examples of which are illustrated in the accompanying drawings.

Referring toFIG. 3, it shows an inverter circuit according to an embodiment of the present invention. The inverter circuit for driving a CCFL340of the present invention is provided. The inverter circuit comprises a DC voltage source310, a bridge DC/AC converter320, a transformer330, a current sensor350, a voltage sensor360, a feedback control unit370and a voltage control circuit380, wherein the CCFL340is arranged into a liquid crystal display panel345.

In addition, inFIG. 3, a negative feedback circuit comprised of a bridge DC/AC converter320, a current sensor350, a voltage sensor360and current feedback unit370is used to stabilize a brightness of the CCFL340, an operation of which is the same as that inFIG. 1and is not described here again. In addition, the bridge DC/AC converter320can be chosen to be the same as the bridge DC/AC converter120shown inFIG. 1.

Compared with the conventional inverter circuit, the present invention features the feedback control unit370and the voltage control circuit380so that the following description describes these two devices accompanied withFIG. 4. More,FIG. 4shows voltage timing charts present at several nodes in the inverter circuit shown inFIG. 3.

The feedback control unit370comprises an error amplifier and control circuit371, a pulse-width modulator375and a drive circuit373. More, the error amplifier and control unit370further comprises a protection device379coupled between the drive circuit373and the voltage sensor360. The protection device379can achieve a purpose of protecting the inverter circuit through the drive circuit373, when a voltage variation at the secondary side of the transformer detected by the voltage sensor360is abnormal.

In the conventional techniques, the pulse-width modulator375receives an output from the error amplifier and control circuit371and thus adjusts pulse widths of output signals from the bridge DC/AC converter320by using the drive circuit373. In addition, the pulse-width modulator375further comprises a triangle-wave generator377, a first comparator A1, a second comparator A2and an exclusive-OR gate to provide voltage control signals to the voltage control circuit380.

Furthermore, the triangle-wave generator377provides a triangle-wave Vtri to the first comparator A1and the second comparator A2. Referring to waveforms in “input terminal of comparator A1” and “V1” shown inFIG. 4, the first comparator A1compares the triangle-wave Vtri with the first reference voltage Vr1, and then outputs a first periodic square wave V1with a pulse width that is a duration in which the voltage level of the triangle-wave Vtri is higher than that of the first reference voltage Vr1.

Likewise, referring to waveforms in “input terminal of comparator A2” and “V2” shown inFIG. 4, the first comparator A2compares the triangle-wave Vtri with the second reference voltage Vr2, and then outputs a second periodic square wave V2with a pulse width that is a duration in which the voltage level of the triangle-wave Vtri is higher than that of the second reference voltage Vr2. Subsequently, the first periodic square wave V1and the second periodic square wave V2are input into the exclusive-OR gate to proceed to an exclusive-OR operation so as to output a signal V3, shown in “V3” waveform inFIG. 4. To obtain a better digital wave shape, the signal V3is inverted by an inverter gate INV to be an inverting waveform of V3, i.e. the voltage control signal, shown in “voltage control signal” waveform inFIG. 4.

By adjusting the magnitudes of the first reference voltage Vr1and the second reference voltage Vr2, the pulse widths of the first periodic square wave V1and the second periodic square wave V2can be accordingly adjusted. As the voltage control signal is obtained from operating the first periodic square wave V1and the second periodic square wave V2with the exclusive-OR operation, in fact, the voltage control signal is determined by the first reference voltage Vr1and the second reference voltage Vr2.

Furthermore, the voltage control circuit380for receiving the voltage control signal comprises a two-level DC voltage generator381and an analog device383, wherein the two-level DC voltage generator381generates a first DC voltage VCC1and a second DC voltage VCC2with different voltage levels in accordance with the DC voltage provided by the DC voltage source310. More, the analog device383amplifies the amplitude of voltage control signal to the aforementioned first DC voltage VCC1and a second DC voltage VCC2with different voltage levels in response to the input DC voltages VCC1and VCC2as shown in “two-level DC square wave” inFIG. 4.

in addition, the analog device383further receives the voltage control signal that controls pulse widths of each voltage level in the two-level DC square wave. After the two-level DC square wave is converted by the bridge DC/AC converter320to an AC square wave, its duty cycle is also determined by the voltage control signal.

From “V1,” “V2,” and “AC square wave” shown inFIG. 4, a duty cycle of the AC square wave is determined by the signal with a larger pulse width of two “V1” and “V2” signals. For example, the larger pulse width of “V1” signal, the larger the duty cycle of the AC square wave, which means that the passing energy is larger. The smaller pulse width of two “V1” and “V2” signals determines the duration ratio between two “V1” and “V2” signals; for example, the smaller pulse width of “V2” signal, the smaller the duration ratio of VCC1to VCC2.

In summary, the two-level DC voltage generator381is used to determine the voltage levels for each level in the two-level DC square wave and the pulse-width modulator375outputs the voltage control signal for adjusting the duration ratio of each level in the two-level DC square wave. Therefore, the voltage variation at the primary side of the transformer330becomes smaller so as to reduce EMI generated by the transformer330and lower instantaneous loading of the DC voltage source.

Furthermore, the pulse widths of two “V1” and “V2” signals can be determined by the first reference voltage Vr1and the second reference voltage Vr2. In addition, the first reference voltage Vr1and the second reference voltage Vr2can be designed to be determined by the error amplifier and control circuit371in accordance with the current passing through the CCFL. Alternatively, the smaller voltage of the first reference voltage Vr1and the second reference voltage Vr2can be defaulted as a DC reference voltage in the feedback control unit.

The duration ratio of each level in the two-level DC square wave is determined by the smaller voltage of two DC reference voltages Vr1and Vr2(in this embodiment, the smaller one is Vr1), and has a little effect on the brightness of the CCFL. Therefore, the value of this duration ratio can be fixed. However, the duty cycle of the AC square wave is able to affect the brightness of the CCFL and the duty cycle is determined by the larger voltage of two DC reference voltages Vr1and Vr2(in this embodiment, the larger one is Vr2). Therefore, the second DC reference voltages Vr2is designed to be determined by the error amplifier and control circuit371in accordance with the current passing through the CCFL.

The two-level DC square wave can be designed to be a multi-level DC (such as three-level or more) square wave by only replacing the two-level DC voltage generator381in the voltage control circuit380with a multi-level DC voltage generator that provides multi-level voltages. Meanwhile, a generating method for generating multiple voltage control signals is modified, for example, implementing a plurality of comparators and a plurality of reference voltages, which can easily modified by one of ordinary skill in the art.

FIG. 5shows a flowchart of a driving method for driving a CCFL of the present invention. Referring toFIGS. 3 and 5concurrently, first, in a step S510, the DC voltage source310provides a DC voltage. Next, in step S520, the two-level DC voltage generator381generates the first DC voltage VCC1and the second DC voltage VCC2with different voltage levels. After that, in step S530, the analog device383generates the two-level DC square wave in accordance with the VCC1and VCC2, as well as the voltage control signal.

In step S540, the bridge DC/AC converter320modulates the two-level DC square wave's pulse width and executes a converting from DC square wave to AC square wave in accordance with the feedback PWM signal. Subsequently, the transformer330converts the AC square wave to a quasi-sine wave in step S550and then the quasi-sine wave drives the CCFL in step S560. In step S570, the feedback control unit370generates a feedback PWM signal provided to be used in step S540in accordance with the current passing through the CCFL so as to stabilize the brightness of the CCFL. Next, in step S580, the feedback control unit370generates a feedback PWM signal in accordance with the current passing through the CCFL provided to be used in step S530for generating the two-level DC square wave.

In summary, a method for driving a fluorescent lamp and an inverter circuit for performing such a method of the present invention not only eliminates problems of EMI generated by the transformer, but reduces the instantaneous loading of the DC voltage source. Thus, the present invention not only stabilizes a brightness of the CCFL but prevents the inverter from malfunction because of reduced amount of EMI generated by the transformer. Furthermore, a read/write process of a CPU can not interfered by electromagnetic radiation generated by the transformer so as to ensure a computer working normally.