Abstract:
A DC/AC inverter provided for transforming a DC voltage source into an AC power source, wherein the AC signal is used for driving a fluorescent lamp. The inverter comprises a half-bridge switch circuitry electrically connected to the DC voltage source, for outputting an AC signal, a resonant tank electrically connected between the half-bridge switch circuitry and the fluorescent lamp, for stepping up and filtering the AC signal into a high-voltage AC power source which is supplied to a load, and a controller feeding back an output of the fluorescent lamp, providing a pulse width modulation signal for turning on and off the half-bridge switch circuitry, such that the fluorescent lamp can operate consistently and provide a consistent brightness.

Description:
BACKGROUND OF THE PRESENT INVENTION 
     1. Field of Invention 
     The present invention relates to a DC/AC inverter, and more particularly to a DC/AC inverter for driving a background fluorescent lamp of an LCD, especially for a plurality of power operated half-bridge DC/AC inverter designed for driving a plurality of fluorescent lamps. 
     2. Description of Related Arts 
     As liquid crystal displays (LCD) thinner than conventional cathode ray tube (CRT) monitors, they are being used in more and more homes and public display. However, since LCD is operated by its optical rotary power and optical characteristic to display image and text information, hence not illuminable, it requires an additional backlight source. An example of backlight source for LCD is fluorescent lamps. 
     A typical DC/AC inverter utilizes a full-bridge inverter circuit, along with a resonant tank and a step-up transformer, a DC power input can be filtered and converted to a high AC voltage by the resonant tank and the step-up transformer, so as to drive the fluorescent lamp. 
     In order to stabilize the light emitted by the fluorescent lamp, and prevent a change in light intensity while there is a change in the power input voltage, most inverters are incorporated with negative feedback circuits for stabilizing the current in the fluorescent lamp. As the life-span of the fluorescent lamp is affected by the symmetry of the waveform of the current, it is most popular to use full-bridge inverter to drive fluorescent lamps. 
     Referring to  FIG. 1  of the drawing, a conventional full-bridge inverter is illustrated. As shown in  FIG. 1 , a full-bridge inverter  100  comprises a DC voltage source  101 , a full-bridge switch circuitry  102 , a resonant tank  103 , a fluorescent lamp  104 , a current sensing circuit  105 , a pulse width modulator  106 , a frequency generator  107 , and a full-bridge switch driver circuit  108 , wherein the full-bridge switch circuitry  102  comprises four power switches  101 A,  101 B,  101 C and  101 D. The resonant tank  103  comprises a step-up transformer  120  and two resonant capacitors  121  and  122 . The frequency generator  107  comprises a triangular wave generator  110  and a pulse generator  109 . The full-bridge switch driver circuit  108  provides four sets of driving signal R 1 , R 2 , R 3  and R 4 . 
     The DC voltage source  101  is electrically connected to the full-bridge switch circuitry  102 , wherein the output of the full-bridge switch circuitry  102  is electrically connected to an input of the resonant tank  103 . An output of the resonant tank  103  is electrically connected to a terminal of the fluorescent lamp  104 . The series connection between the full-bridge switch circuitry  102 , the resonant tank  103  and the fluorescent lamp  104  is a typical example of a power transfer connection. 
     The current sensing circuit  105  is electrically connected to the fluorescent lamp  104  and the pulse width modulator  106 . The pulse width modulator  106  is then electrically connected to the frequency generator  107  and the full-bridge switch driver circuit  108 , which is electrically connected to the gate terminals of the full-bridge switch circuitry  102 , forming a control loop connection. 
     Conventional full-bridge inverter is operated based on the a fixed high frequency conduction between the four power switches of the full-bridge switch circuitry  102 , such that the DC voltage output by the DC voltage source  101  is transformed to and outputted as a fixed high-frequency AC square wave, which is provided for being inputted to the resonant tank  103 . The resonant tank  103  utilizes the step-up characteristic and the filter function of the step-up transformer  120  to transform the fixed high-frequency AC square wave to a fixed high frequency AC sine wave, which is provided to the fluorescent lamp  104 . 
     The control loop utilizes the current sensing circuit  105  to produce a feedback signal R 5 , which corresponds to a fluorescent lamp current, which is then transferred to the pulse width modulator  106 . The pulse width modulator  106 , together with the a triangular wave output R 6  by the triangular wave generator  110  of the frequency generator  107 , utilizing the theory of negative feedback, produces a pulse width modulation signal R 7  for inputting to the full-bridge switch driver circuit  108 , wherein the full-bridge switch driver circuit  108  utilizes the pulse width modulation signal R 7  and the frequency generator  107  to produce the four sets of driving signals R 1 , R 2 , R 3  and R 4  so as to drive the four power switches  101 A,  101 B,  101 C and  101 D. 
     By controlling the conduction period between the two power switches  101 A and  101 D, and the conduction period between the two power switches  101 C and  101 B, the alternating conduction between  101 A and  101 D, and  101 C and  101 B provides a stable fluorescent lamp current which is an AC current having a symmetrical waveform. 
     This conventional type of full-bridge inverter circuits can stably control the current of a fluorescent lamp, however, has the draw back of having a great number of switches, pushing the production cost of such circuits higher. 
     As a result, the present invention is to provide a cheaper and more reliable DC/AC inverter. 
     SUMMARY OF THE PRESENT INVENTION 
     A main object of the present invention is to provide a circuitry of a DC/AC inverter for driving a fluorescent lamp circuit, wherein the DC/AC inverter uses less power switches to control the operation of the fluorescent lamp, producing lower DC voltage ripple, which in turn lowers noise caused by system ripples. 
     Another object of the present invention is to provide the circuitry of a half-bridge DC/AC inverter, having an advantage of using less power switches and lower production cost, together with an alternating operation to achieve lower DC voltage ripple, which in turn lowers noise caused by system ripples when multi fluorescent lamps are operated. 
     Another object of the present invention is to provide two sets of power switch driving signals of the DC/AC inverter, such that the duty cycle of each of the two sets of power switch driving signals alters symmetrically with respect to that of the other power switch driving signals. Since the power switches do not conduct simultaneously upon receiving the DC voltage, the noise of the DC voltage source is minimized. 
     Another object of the present invention is to provide a plurality of sets of power switch driving signals of the DC/AC inverter, which is applied to a plurality of fluorescent lamps, such that the fluorescent lamps can utilize frequency generators to generate a plurality of signals with identical frequency and different phases as frequency sources. 
     Accordingly, in order to accomplish the above objects, the present invention provides a DC/AC inverter for transforming a DC power source to an AC power source, an AC signal of which is used to drive a fluorescent lamp, wherein the DC/AC inverter comprises: 
     a half-bridge switch circuitry electrically connected to the DC power source; 
     a resonant tank electrically connected between the half-bridge switch circuitry and the fluorescent lamp, wherein the resonant tank steps-up and filters the AC signal from the half-bridge switch circuitry, such that the AC signal is transformed to the AC power source; and 
     a controller feeding back an output of the fluorescent lamp, providing a pulse width modulation signal to control, the conduction of the half-bridge switch circuitry so as to drive the fluorescent lamp being operated around the resonant frequency of the fluorescent lamp according to the conduction condition of the half-bridge switch circuitry. 
     These and other objectives, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art of a full-bridge fluorescent lamp inverter circuitry. 
         FIG. 2  illustrates a circuitry of a DC/AC inverter according to a preferred embodiment of the present invention. 
         FIG. 3  illustrates some waveforms of the circuitry of the DC/AC inverter according to the above preferred embodiment of the present invention. 
         FIG. 4  illustrates a circuitry of two DC/AC inverters according to an alternative embodiment of the present invention. 
         FIG. 5  illustrates a sequential marked graph according to the above alternative embodiment of the present invention. 
         FIG. 6  illustrates a circuitry of a plurality of DC/AC inverters according to yet another alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 2  of the drawings, a circuitry of a DC/AC inverter according to the preferred embodiment of the present invention is illustrated, wherein the DC/AC inverter is a half-bridge DC/AC inverter. 
     As shown in the drawing, an inverter controller  200  comprises a DC voltage source  201 , a half-bridge switch circuitry  202 , a resonant tank  203 , a fluorescent lamp  204 , a current sensing circuit  205  for sensing a lamp current of the fluorescent lamp  204 , a voltage sensing circuit  206  for sensing a terminal voltage of the fluorescent lamp  204 , a pulse width modulator  207 , a frequency generator  208 , a driver circuit  209 , which is a half-bridge switch driver circuit, a protection circuit  210 , a timer  211 , and a dimming control circuit  212 . 
     The DC voltage source  201  is electrically connected to the half-bridge switch circuitry  202 , one of the switch  202 A is connected to a DC voltage line and the other one  202 B is connected to the ground, wherein through an output of the half-bridge switch circuitry  202 , the half-bridge switch circuitry  202  is electrically connected to the resonant tank  203  though an input of the resonant tank  203 . Through an output of the resonant tank, the resonant tank  203  is also electrically connected to the fluorescent lamp  204 . The resonant tank  203  further comprises a step-up transformer  221  and resonant capacitors  222 ,  223 , and  224 . 
     The electrical connection method between the elements of the DC/AC inverter is conventionally known as power transfer connection. According to the preferred embodiment of the present invention, a Low Q resonant tank is used so as to provide easy designing of the circuitry. 
     Under such a circuitry design, the waves that drive the step-up transformer  221 , and the fluorescent lamp  204  is quasi sine waves or quasi square waves, as oppose to pure square waves or pure sinusoidal waves.  FIG. 3  of the drawings illustrates the voltage wave form at different positions under the power transmission route design according to the preferred embodiment. 
     In  FIG. 3  of the drawings, S 51  is the voltage waveform of the output end of the half-bridge switch circuitry  202  and S 16  is the voltage waveform of the driving signal output by the step-up transformer  221 . 
     The current sensing circuit  205  and the voltage sensing circuit  206  are electrically connected to the ends of the fluorescent lamp  204  respectively. The current sensing circuit  205  is also electrically connected to the pulse width modulator  207 , wherein the pulse width modulator  207  is also electrically connected to the frequency generator  208  and the driver circuit  209 . The driver circuit  209  is in turn electrically connected to the half-bridge switch circuitry  202 , forming a control loop connection. 
     The half-bridge switch circuitry  202  comprises two power switches  202 A and  202 B. According to the preferred embodiment of the present invention, the power switch  202 A connected to the voltage line is a P-type MOSFET, while the power switch  202 B connected to ground line is an N-type MOSFET. However, the power switches are not limited to MOSFET, and can also be another semiconductor switches, such as BJT switches. 
     The frequency generator  208  generates a triangular wave signal S 1  and a pulse signal S 2 , wherein both signals have the same frequency. However, the present invention is not limited to the use of triangular wave signals, where all ramp signals and sawtooth wave signals are applicable. 
     The current sensing circuit  205  and the fluorescent lamp  204  are in series to provide a signal S 3  for indicating the conduction of the fluorescent lamp  204 , and utilize a second signal S 4  to show the current value of the current flowing through the fluorescent lamp  204 . The voltage sensing circuit  206  utilizes the resonant capacitors  223  and  224  of the resonant tank  203  to obtain a third signal S 5  for indicating a terminal voltage of the fluorescent lamp  204 . 
     The pulse width modulator  207  comprises an error amplifier  261 , a resistor  262 , and a capacitor  263 , and a comparator  264 . The pulse width modulator  207  also comprises a controlled current source  265 , which is electrically connected to an inverse input of the error amplifier  261  through a switch  266 . 
     The driver circuit  209  comprises two driving signals POUT and NOUT. The protection circuit  210  comprises a logic control circuit  272 . The protection circuit  210  receives the signal S 3  capable of indicating the conduction state of the fluorescent lamp, the third signal S 5  capable of indicating the terminal voltage of the fluorescent lamp, and an output signal S 6  of the error amplifier  261  in the pulse width modulator  207 . 
     The timer  211  comprises two comparators  281  and  282 , and a current source  283 . The dimming control circuit  212  comprises a dimming control frequency generator  291 , wherein a triangular signal S 7  and a dimming control voltage S 8  are generated by the dimming control frequency generator  291 . The triangular signal S 7  is delivered to a non-inverting input of a comparator  293 , and the dimming control voltage S 8  is delivered to an inverting input of the comparator  293 . After comparison, a dimming control pulse signal S 9  is generated, the dimming control circuit  212  further comprises an OR gate  296  for controlling the timing of the outputting of the dimming control pulse signal S 9  to the pulse width modulator  207 . 
     According to the preferred embodiment of the present invention, the timer  211  functions in a manner such that a timer capacitor  284  is being charged by the current source  283 , such that a voltage S 12  of the timer capacitor  284  increases as time increases. Before the voltage S 12  reaches a first reference voltage Vref 1 , a reset signal S 11  is being sent out. When the voltage S 12  reaches a second reference voltage Vref 2 , a time out signal S 10  will be sent out. 
     The current source  283  is being controlled by a signal S 13  capable of indicating the system voltage, such that when the system voltage is lower than a third reference voltage Vref 3 , the current source  283  will be cut off, and the voltage S 12  of the timer capacitor  284  earthed. Through such design, every time when the system starts up the DC voltage source  201  from a zero voltage, the timer capacitor  284  of the timer  211  is charged started from zero voltage. 
     According to the preferred embodiment of the present invention, the frequency generator  208  is also controlled by a fourth signal S 14 , indicating whether or not the fluorescent lamp is conducted. When the fluorescent lamp is conducted, an operation frequency is sent out. When the fluorescent lamp is not conducted, a start-up frequency is sent out. 
     An advantage of such a design is that the resonant frequency of the resonant tank  203  can operated by the different operation frequencies according to the conduction state of the fluorescent lamp, such that the system can be operated around the resonant frequency, whether or not the fluorescent lamp is conducted, such that the system is operated efficiently. 
     The fourth signal S 14  is determined by the signal S 3  provided by the current sensing circuit  205 , and a comparator  274  of the protection circuit  210 , such that when the signal S 3  exceeds a fourth reference voltage of Vref 4 , the fluorescent lamp  204  is considered as being conducted. 
     Under normal circumstances, the detailed operation procedures according to the preferred embodiment of the present invention are as follows: 
     After staring up to supply voltage to the system, the timer  211  initializes the charging of the timer capacitor  284 , such that before the voltage of the timer capacitor  284  reaches the first reference voltage Vref 1 , the reset signal S 11  being sent out by the timer  211  passes through an OR gate  267 , turning on a switch  266 , such that the current source  265  is connected to the inverting input of the error amplifier  261 , forcing an input voltage of the inverting input of the error amplifier  261  to be higher than a fifth reference voltage Vref 5 , which in turn forces an error output S 6  to be zero. 
     When the timer capacitor  284  continue to charge until the capacitor voltage is greater than the first reference voltage Vref 1 , the switch  266  will be turned off, such that the pulse width modulator  207  is initialized, and because of the fluorescent lamp  204  is not conducted, the inverting input voltage of the error amplifier  261  becomes lower than the fifth reference voltage Vref 5 , causing the output signal S 6  outputted by the error amplifier  261  to increase under the effect of negative feedback. 
     After comparing with the triangular wave signal S 1 , the comparator  264  of the pulse width modulator  207  outputs a pulse width modulation signal S 15 . The driver circuit  209  receives the pulse width modulation signal S 15  and the pulse signal S 2  so as to produce the driving signals POUT and NOUT to drive the power switch  202 A and  202 B respectively. 
     Before the conduction of the fluorescent lamp  204 , the voltage S 16  of the fluorescent lamp  204  will increase due to a widening of duty cycle of the pulse width modulation signal S 15 . Upon sensing the third signal S 5  exceeding a sixth reference voltage Vref 6 , the voltage sensing circuit  206  sends out an over voltage signal S 17 , which passes through the OR gate  267  and turn on the switch  266  of the current source  265 , to the inverting input of the error amplifier  261 , so as to reduce the output signal S 6  of the error amplifier  261 , whereby decreasing the duty cycle of the pulse width modulation signal S 15  and decreasing the amount of electrical power input to the fluorescent lamp. 
     If the effect of this decreasing in amount of electrical power input to the fluorescent lamp is a sensing of the third signal S 5  to be less than the sixth reference voltage Vref 6 , the switch  266  will be turned off, increasing the output signal S 6  of the error amplifier  261 . As a result, the voltage S 16  of the fluorescent lamp  204  is stably adjusted under such a negative feedback control. 
     As soon as the fluorescent lamp is conducted by a sufficient voltage S 16  of the fluorescent lamp  204  and for a substantial period of time, according to the characteristic of the fluorescent lamp, the voltage S 16  of the fluorescent lamp  204  will drop to less than half the voltage required for conducted operation, such that the voltage sensing circuit  206  loses its function due to a non-detection of a higher voltage. 
     At the same time, the current sensing circuit  205  sends out the signal S 3  to the protection circuit  210 , producing the fourth signal S 14  to alter the output frequency of the frequency generator  208 , and outputting the second signal S 4  to the pulse width modulator  207 , such that the current flowing through the fluorescent lamp is stabilized on a pre-determined value through the negative feedback control. 
     According to the preferred embodiment of the present invention, the protection circuit is operated as follows: 
     When the fluorescent lamp is not connected, the third signal S 5  will continuously send out a signal indicating that the terminal voltage of the fluorescent lamp exceeds the sixth reference voltage Vref 6  to the logic control circuit  272 , which receives the time out signal S 10  of the timer  211 . 
     The logic control circuit  272  will take no action until the time out signal S 10  is inputted. Once the time out signal S 10  reaches the logic control circuit  272 , and in the condition of the terminal voltage of the fluorescent lamp exceeds the sixth reference voltage Vref 6 , it times with another digital timer (not shown in the diagram), by the pulse signal S 2  produced by the frequency generator  208  to time. 
     If the terminal voltage of the fluorescent lamp still exceeds the sixth reference voltage Vref 6  after a predetermined period of time, a terminating signal S 18  will be outputted by the logic control circuit  272  to the driver circuit  209 , so as to cut off the conduction of the power switches  202 A and  202 B. 
     If the fluorescent lamp is damaged during operation, the fourth signal S 14  will be sent out, indicating that the fluorescent lamp is not conducted, to the logic control circuit  272 , receiving the time out signal S 10  from the timer  211 . The logic control circuit  272  will not take any action until the receiving the time out signal S 10 . 
     When time is up, the logic control circuit  272 , under the condition of the fourth signal S 14  indicating that the fluorescent lamp is not conducted, will time with a second digital timer, through a second pulse signal S 21  produced by the low frequency dimming control circuit  212 . 
     If the lamp still is not conducted after a predetermined period of time, the logic control circuit  272  will output the terminating signal S 18  to the driver circuit  209 , so as to cut off the conduction of the power switches  202 A and  202 B. 
     Also, when, the step-up transformer  221  encounters serious damage, such as power leakage, creating an overloading effect, the entire system will be overloaded. Under such conditions, the error amplifier  261  will continue to increase its outputting of the output signal S 6 , so as to provide sufficient power to stabilize the current of the fluorescent lamp. If the leakage is greater than the maximum power provided by the system, the error amplifier  261  will definitely exceed the peak value of the triangular wave signal S 1 . 
     The protection circuit  210  compares the output signal S 6  of the error amplifier  261  with a seventh reference voltage Vref 7 , the value of which is slightly higher than the peak value of the triangular wave signal, to obtain an overloading signal S 19 , indicating whether or not the system is overloaded. 
     Similarly, if the overloading signal S 19  indicates that the system is overloaded when the timer  211  initializes the protection circuit  210 , and, if the timing using the pulse signal S 2 , which passes through the logic control circuit  272  and is produced by the frequency generator  208 , also exceeds the predetermined period of time, the logic control circuit  272  then outputs the terminating signal S 18  to the driver circuit  209 , cutting off the conduction of the power switches  202 A and  202 B. 
     According to the preferred embodiment of the present invention, the inverter further has the dimming control circuit  212  provided for controlling the termination and restarting of the power provided to the fluorescent lamp. It makes use of the adjusting of brightness ratio to adjust the brightness of the fluorescent lamp. In order to avoid the creating of the flashing feeling created by a low frequency, the brightness frequency is normally controlled to be above 200 Hz. 
     The dimming control circuit is controlled by two signals, the first one being the fourth signal S 14  indicating whether or not the fluorescent lamp is conducted, and the second one being the time out signal S 10  of the timer  211 . Only when the fourth signal S 14  indicates that the fluorescent lamp is conducted or the timer  211  receives the time out signal S 10 , a switch  236  controlling the output of the dimming control signal will be turned on. 
     A dimming control voltage S 20  of the dimming control circuit is higher than the fifth reference voltage Vref 5 . When the dimming control voltage S 20  passes through the switches  235  and  236  to be connected to a second resistor  234  and the pulse width modulator  209 , the output signal S 6  of the error amplifier  261  of the pulse width modulator  297  is decreased, cutting off electricity transferring of the system to avoid overloading. 
     When the switch  235  is turned off by the dimming control pulse signal S 9 , the pulse width modulator  207  is reopened and restarting to provide electrical power to the system. 
     Dimming control effect can be achieved by a low frequency to control the ratio between the stopping and the restarting of providing electrical power of each cycle. In order to ensure that the fluorescent lamp has sufficient and continuous electrical power so as to be ignited in a predefined period of time, the time when brightness can be adjusted is determined by whether or not the fluorescent lamp is conducted. 
     In order to provide an AC current with good symmetry to drive the fluorescent lamp  204 , according to the preferred embodiment of the present invention, when the system is operated steadily, the half-bridge switch circuitry  202  is alternately conducted with the same duty cycle, but shifted by 180 degrees. 
     Referring to  FIG. 4  of the drawings, an alternative embodiment of the present invention is illustrated, wherein two sets of DC/AC inverters are operated and applied to two fluorescent lamps simultaneously. The elements in each of a first inverter set  301  and a second inverter set  302  is substantially the same as that of the DC/AC inverter as shown in  FIG. 2  of the drawings. 
     It should be noted that a timer  303  is shared by the first and the second inverter set  301  and  302 , and, in order to be applicable to the two sets of inverters according to this embodiment of the present invention, a frequency generator  304  and a brightness adjusting circuit  305  must be appropriately altered. 
     A frequency control signal T 3  of the frequency generator  304  determines when a change in frequency is required, according to a first conduction confirmation signal T 1  (similar to S 14  of  FIG. 2 ) and a second conduction confirmation signal T 2  of the first and the second inverter set  301  and  302  respectively, and a timer signal T 4  of the timer  303 . 
     After passing the conduction confirmation signals T 1  and T 2  through an AND gate  311 , a third conduction confirmation signal T 5  is obtained. After passing the third conduction confirmation signal T 5  and the timer signal T 4  though an OR gate  312 , the frequency control signal T 3  is obtained. 
     An operation frequency of the frequency generator  304  will be changed after the fluorescent lamps are all conducted or the timer signal T 4  outputted by the timer  303 . As a result, the frequency of the system can still be altered even when one of the fluorescent lamps is damaged. 
     The frequency control signal T 3  is also used for controlling the dimming control circuit  305 . The outputting moment of the dimming control circuit  305  for adjusting the brightness is also after the conduction of the fluorescent lamps, or after the timer  303  outputted the timer signal T 4 . As a result, not only can it be ensured that both lamps are successfully lit up, brightness adjustment can still be achieved even when one of the lamps is damaged. 
     A second pulse signal T 7  is produced when a first pulse signal T 6 , which is outputted by the frequency generator  304  to the DC/AC inverter, passes through an inverter  313 . Utilizing the first pulse signal T 6  and the second pulse signal T 7  having the same frequency as but out of phase of the first pulse signal T 6 , such that the first DC/AC inverter outputs a first set of driving signals POUT 1  and NOUT 1 , for driving a first power switch P 1  and N 1 . 
     The second DC/AC inverter outputs a second set of driver out signals POUT 2  and NOUT 2  for driving a second power switch P 2  and N 2 , wherein the second set of driving signals has the same frequency as but out of phase of the first set of driving signals. 
     Referring to  FIG. 5  of the drawings, a sequential marked graph of the driving signals according to this alternative embodiment of the present invention is illustrated. The dashed portion of  FIG. 5  shows the change in duty cycle of the driving signals POUT 1 , NOUT 1  and POUT 2 , NOUT 2 . 
     In order to keep the symmetry of the lamp driving current, the change between the duty cycle between the driving signals POUT 1 , NOUT 1  and POUT 2 , NOUT 2  is symmetrical. Since the driving signals POUT 1  and POUT 2  will not be conducted simultaneously, a voltage noise of the power source will be reduced. 
     Referring to  FIG. 5  of the drawings, because POUT 1  and POUT 2  is out of phase by 180 degrees, the current flowing into the fluorescent lamps  318  and  319  will be reversed. Also, by adjusting the polarity of the transformers  321  and  322 , the current flowing into the fluorescent lamp  318  and  319  can be altered to be in phase. 
     When more than two fluorescent lamps are in use, a plurality of frequency generators, each having the same frequency but out of phase with each other, is used as a frequency source to drive the fluorescent lamp. 
     Referring to  FIG. 6  of the drawings, a plurality of frequency generators provided for driving N number of DC/AC inverters according to this alternative embodiment of the present invention is illustrated. The input of the plurality of frequency generators  501  can be an external clock pulse  502 , which can be any one frequency related to the control signal of the LCD display The other input is a frequency control signal  503 . 
     The frequency control signal  503  utilizes an AND gate  504  and an OR gate  505  to control the changing of the operation frequency of the fluorescent lamp, according to the conduction confirmation signals  506 ,  507 ,  508  . . . N, confirming whether or not all the N number of fluorescent lamps are conducted, or upon the outputting of a timer signal  509  by the timer. 
     Each of the triangular wave signal  510  output by the plurality of frequency generators  501  to each of the pulse width modulator has the same frequency but out of phase. Each of the pulse signal  511  output to each of the switch driver circuit has the same frequency as and in phase with the triangular wave signal  510 . 
     After the dimming control signal  531  entering the plurality of frequency generators  501 , the plurality of frequency generator  501  generates a dimming control pulse signal  532  that is produced with a frequency that is relative to the LCD control frequencies, such that the frequency control signal  503  controls the switch  533 , so as to control when the dimming control pulse signal  532  is outputted to each of the DC/AC inverters. 
     Such a plurality of frequency generator  501  can be achieved by the use of a conventional micro control unit (MCU)  521 , together with a direct digital synthesizer (DDS)  522 . 
     Due to the fact that not the power switch of the DC voltage source will not be conducted all at the same time, as oppose to conventional circuitry, noises related to the power source is minimized. And since the operation frequency is synchronized with the LCD controller, visual disturbance due to interference caused by frequency difference can be minimized too. 
     The usage of the outputting method of the frequency generator is not limited to half-bridge DC/AC inverters. When there are more than two sets of fluorescent lamp, this outputting method can also be applied full-bridge or other control systems that use the same frequency, so as to minimize noises related to the power source and visual disturbance. Also, the present invention utilizes fluorescent lamp to illustrate the preferred embodiment, but its application should not be limited to fluorescent lamp. The present invention as disclosed above can be applied to any lighting element. 
     One skilled in the art will understand that the embodiment of the present invention as shown in the drawings and described above is exemplary only and not intended to be limiting. 
     It will thus be seen that the objects of the present invention have been fully and effectively accomplished. It embodiments have been shown and described for the purposes of illustrating the functional and structural principles of the present invention and is subject to change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.