Abstract:
A resonant DC/AC inverter includes a DC power source providing a DC voltage, a half-bridge power switch circuit electrically connected to the DC power source being operative to convert the DC voltage to an AC voltage, a resonant tank electrically connected between an output of the half-bridge power switch circuit and an input of a load being operative to boost and filter the AC voltage to generate an AC power voltage supplied to the load, and a controller being operative to detect a magnitude of current in the load and a magnitude of a voltage across the load and to generate pulse waveforms for turning on and off the half-bridge power switch circuit, wherein the controller substantially instantaneously varies a frequency of the pulse waveforms and a duty cycle of the pulse waveforms so as to operate the resonant DC/AC inverter near a neighborhood of a resonant frequency of the resonant tank regardless of a conduction state of the load and improve the efficiency of the inverter regardless of the higher DC voltage applied to the inverter. Particularly, the resonant DC/AC inverter utilizes a piezoelectric transformer to supply power to a fluorescent lamp which is wildly employed in display panels and is extensively used to provide backlighting for liquid crystal displays (LCDs), especially for backlighting LCD monitors and LCD televisions.

Description:
BACKGROUND OF THE PRESENT INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a resonant DC/AC inverter, and more particularly to a resonant half-bridge DC/AC inverter using a piezoelectric transformer to supply power to fluorescent lamps. Usually, the inverter is usually applied to display devices, such as liquid crystal display monitors, liquid crystal display computers or liquid crystal display televisions. 
         [0003]    2. Description of Related Arts 
         [0004]    CCFLs (cold cathode fluorescent lamps) are wildly employed in display panels. CCFL loads are extensively used to provide backlighting for liquid crystal displays (LCDs), particularly for backlighting LCD monitors and LCD televisions. Such conventional applications require direct current/alternative current inverters (DC/AC inverters) to drive CCFL loads. The critical factors in the design of LCD monitors or LCD televisions include efficiency, cost, size. Additionally, due to liquid crystal display&#39;s thin profile, liquid crystal displays can be used in applications where bulkier Cathode Ray Tube (CRT) displays are impractical. 
         [0005]    Recent advances in ceramics technology have yielded a new generation of so-called “piezoelectric transformers (PTs)” that are useful in certain applications. These devices, which are constructed using laminated thin layers of ceramic material, exploit a well-known phenomenon called the “piezoelectric effect” to provide AC voltage gain, in contrast to the magnetic field effects relied upon by conventional wound transformers. In contrast to electromagnetic transformers, piezoelectric transformers have a sharp frequency characteristic of the output voltage to input voltage ratio, which has a peak at the resonant frequency. This resonant frequency depends on the material constants and thickness of materials of construction of the transformer including the piezoelectric ceramics and electrodes. Furthermore piezoelectric transformers have a number of advantages over general electromagnetic transformers. The size of piezoelectric transformers can be made much smaller than electromagnetic transformers of comparable transformation ratio, piezoelectric transformers can be made nonflammable, and produce no electromagnetically induced noise. Like conventional transformers, piezoelectric transformers are fairly rugged and can be used to obtain voltage gain in high-voltage applications. Additionally, due to their thin profile, piezoelectric transformers can be used in applications where bulkier wire-wound transformers are impractical. For example, piezoelectric transformers are used in power supplies that provide high-voltage power to fluorescent lamps used as backlights in portable computers. Due to their thin profiles, piezoelectric transformers used in such applications do not adversely affect the desired sleekness of the portable computer enclosure. 
         [0006]    Piezoelectric transformers operate most efficiently when operated at frequencies at or near a multiple of a fundamental resonant frequency, which is a function of mechanical characteristics of the transformer such as material type, dimensions, etc. However, piezoelectric transformers are high-impedance devices, and therefore their resonance characteristics as well as other characteristics are sensitive to the loading of the transformer output in operational circuits. Resonant frequency, voltage gain at the resonant frequency, and sharpness of the gain-versus-frequency curve all diminish with increased loading. 
         [0007]    The diminishing of resonant frequency and gain with an increase in loading are purposely exploited when a piezoelectric transformer is used to drive a fluorescent lamp. The frequency of the signal applied to the primary inputs of the piezoelectric transformer is slowly swept from a frequency higher than the unloaded resonant frequency toward lower frequencies. As the resonant frequency is approached, the gain increases to the point that the transformer output voltage is sufficiently high to “strike”, or initiate conduction in, the lamp. Once the lamp begins conducting, it presents a much higher load to the transformer, causing the voltage gain and therefore the output voltage of the transformer to drop considerably. The conduction characteristics of the lamp are such that it continues to conduct current at the reduced voltage, so the circuit then enters a stable, lower-voltage operating condition. The intensity of the lamp is regulated by controlling the frequency of the AC drive supplied to the piezoelectric transformer as a function of the lamp current. 
         [0008]    Referring to  FIG. 1  of the drawings,  FIG. 1  shows a conventional resonant half-bridge DC/AC inverter circuit having a piezoelectric ceramic transformer for driving a CCFL load. As shown in  FIG. 1 , the conventional resonant half-bridge DC/AC inverter circuit  100  comprises a half-bridge power switch circuit  110 , a resonant tank  120 , a lamp current sensing circuit  130 , an integrator  140 , a voltage controlled oscillator (VCO)  150 , and a half-bridge drive circuit  160 . The half-bridge power switch circuit  110  comprises two power switches  110 A,  110 B which are in a half-bridge configuration. The resonant tank  120  comprises an inductor  121  and a piezoelectric ceramic transformer  122 . The integrator  140  comprises an error amplifier  141  which integrates the output of the lamp current sensing circuit  130  and this integrated value affects the operating frequency of the VCO  150 . The half-bridge drive circuit  160  provides two driving signals RA and RB. 
         [0009]    The half-bridge power switch circuit  110  is electrically connected to a DC power source  180  and powered by the DC power source  180 . An output terminal of the half-bridge power switch circuit  110  is electrically connected to an input terminal of the resonant tank  120 . An output terminal of the resonant tank  120  is electrically connected to one end of a fluorescent lamp  170 . An input of the lamp current sensing circuit  130  is electrically connected to the other end of the fluorescent lamp  170 . The inverse terminal of error amplifier  141  of the integrator  140  is electrically connected to the output of the lamp current sensing circuit  130  and the error amplifier  141  integrates the output of the lamp current sensing circuit  130 . This integrated value is a voltage-controlled signal RC which affects the operating frequency of the VCO  150 . Hence the voltage-controlled signal RC determines the operating frequency of a pulse signal RD which is generated by the VCO  150 . The output of the VCO  150  is electrically connected to the half-bridge drive circuit  160 . The half-bridge drive circuit  160  generates two sets of fixed duty cycle driving signals RA and RB. The power switches  110 A,  110 B are driven by the driving signals RA and RB respectively. The upper half of the half-bridge power switch circuit  110  is driven out of phase with the lower half of the half-bridge power switch circuit  110  such that when the power switch  110 A is on, the power switch  110 B is off, and conversely, when the power switch  110 A is off, the power switch  110 B is on. Driven in this manner, the output of the half-bridge power switch circuit  110  consists of a square wave voltage. 
         [0010]    The conventional resonant half-bridge DC/AC inverter circuit utilizes the high frequency switching of the power switches  110 A,  110 B to convert a DC voltage powered by the DC power source  180  to a high frequency square wave signal. The high frequency square wave signal is used to drive the resonant tank  120 . The resonant tank  120  is the combination of the inductor  121  and the piezoelectric ceramic transformer  122 . The combination of the inductor  121  and the piezoelectric ceramic transformer  122  forms a resonant circuit. This results in a sine wave at the output of the resonant tank  120 . On the other hand, the resonant tank  120  utilizes the inductor  121  and the piezoelectric ceramic transformer  122  to filter and boost the high frequency square wave signal to a high frequency sine wave signal. The high frequency sine wave signal is used to drive the fluorescent lamp  170 . 
         [0011]    Referring to  FIG. 2  of the drawings,  FIG. 2  schematically shows output voltage characteristics of a conventional resonant tank with respect to various frequencies input signal. Therefore, the lamp current could be adjusted by controlling switching frequency of the half-bridge power switch circuit. In other words, the lamp current could be adjusted by controlling the switching frequency of the power switches  110 A,  110 B. 
         [0012]    A resonant tank has many resonant frequencies, and a different gain-versus-frequency characteristic in the neighborhood of each. Generally speaking, it is desirable to design that the operating frequency of the resonant half-bridge DC/AC inverter circuit  100  is higher than the operating frequency of the resonant tank  120 . The integrator  140  integrates the output of the lamp current sensing circuit  130  and then generates the stable voltage-controlled signal RC which affects the operating frequency of the VCO  150 . Hence the voltage-controlled signal RC can control the VCO  150  to generate different operating frequencies of a pulse signal RD. According negative feedback theory, the voltage-controlled signal RC can raise the operating frequency of a pulse signal RD while the lamp current is increasing and reduce the operating frequency of a pulse signal RD while the lamp current is decreasing. The half-bridge drive circuit  160  utilizes operating frequencies of a pulse signal RD to provides two fixed duty cycle driving signals RA and RB in order to control the power switches  110 A,  110 B. Therefore, the power switches  110 A,  110 B have the same and fixed duty cycle control to provide a stable and symmetric alternating current to the fluorescent lamp  170 . 
         [0013]    Accordingly, the conventional resonant half-bridge DC/AC inverter circuit can provide stable control of lamp current even though the DC power source  180  provides variable DC voltage. However, in practical the drawback of this prior art is that the efficiency of the conventional resonant half-bridge DC/AC inverter circuit is reduced while the DC power source  180  provides higher DC voltage and the operating frequency of the half-bridge power switch circuit  110  operates far away the neighborhood of the resonant frequency. Hence conventional resonant half-bridge DC/AC inverter circuit could not provide good conversion efficiency while the DC power source  180  provides higher DC voltage and the operating frequency of the half-bridge power switch circuit  110  operates far away the neighborhood of the resonant frequency. 
       SUMMARY OF THE PRESENT INVENTION 
       [0014]    A main object of the present invention is to provide a resonant half-bridge DC/AC inverter that simultaneously varies the operating frequency of the power switches and the duty cycle of the power switches to regulate the output current in order to improve the efficiency of the inverter regardless of the higher DC voltage applied to the inverter. 
         [0015]    Another object of the present invention is to provide a resonant half-bridge DC/AC inverter using a piezoelectric transformer to supply power to fluorescent lamps which are wildly employed in display panels and are extensively used to provide backlighting for liquid crystal displays (LCDs), particularly for backlighting LCD monitors, LCD televisions, computer systems and portable DVD, wherein the resonant half-bridge DC/AC inverter simultaneously varies the operating frequency of the power switches and the duty cycle of the power switches to regulate the lamp current in order to improve the efficiency of the inverter regardless of the higher DC voltage applied to the inverter. 
         [0016]    Another object of the present invention is to provide a resonant half-bridge DC/AC inverter that provides a symmetric alternating current to supply to fluorescent lamps and a necessary high voltage to ignite fluorescent lamps. 
         [0017]    Another object of the present invention is to provide a resonant half-bridge DC/AC inverter further comprising a protection circuit and a dimming control circuit to protect the resonant half-bridge DC/AC inverter under abnormal operation and to adjust the brightness of fluorescent lamps. 
         [0018]    Accordingly, in order to accomplish the one or some or all above objects, the present invention provides a resonant half-bridge DC/AC inverter, comprising: 
         [0019]    a DC power source providing a DC voltage; 
         [0020]    a half-bridge power switch circuit electrically connected to the DC power source being operative to convert the DC voltage to a pulse signal; 
         [0021]    a resonant tank electrically connected between an output of the half-bridge power switch circuit and an input of a load being operative to boost and filter the pulse signal to generate an AC power supplied to the load; and 
         [0022]    a controller being operative to detect a magnitude of current in the load and a magnitude of a voltage across the load and to generate pulse waveforms for turning on and off the half-bridge power switch circuit, wherein the controller substantially instantaneously varies a frequency of the pulse waveforms and a duty cycle of the pulse waveforms so as to operate the resonant half-bridge DC/AC inverter near a neighborhood of a resonant frequency of the resonant tank regardless of a conduction state of the load. 
         [0023]    One or part or all of these and other features and advantages of the present invention will become readily apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of one of the modes best suited to carry out the invention. As it will be realized, the invention is capable of different embodiments, and its several details are capable of modifications in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic diagram of a conventional resonant half-bridge DC/AC inverter circuit having a piezoelectric ceramic transformer for driving a CCFL load. 
           [0025]      FIG. 2  is a schematic diagram of output voltage characteristics of a conventional resonant tank with respect to various frequencies input signal. 
           [0026]      FIG. 3  is an exemplary circuit diagram of a resonant half-bridge DC/AC inverter circuit having a piezoelectric ceramic transformer according to a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    Referring to  FIG. 3 , an exemplary circuit diagram of a resonant half-bridge DC/AC inverter circuit having a piezoelectric ceramic transformer according to a preferred embodiment of the present invention is illustrated. The resonant half-bridge DC/AC inverter circuit  300  includes a DC power source  301 , a half-bridge power switch circuit  302 , a resonant tank  303 , a lamp current sensing circuit  305 , a lamp voltage sensing circuit  306 , a pulse width modulator  307 , a triangle wave generator  308 , a half-bridge drive circuit  309 , a protection circuit  310 , a timer  311 , and a dimming control circuit  312 . The half-bridge power switch circuit  302  comprises two power switches  302 A,  302 B which are in a half-bridge configuration. The power switch  302 A could be a p-type MOSFET. The power switch  302 B could be a n-type MOSFET. However, the power switches  302 A,  302 B are not limited to MOSFET and could be any type of transistor switch such as BJT. The resonant tank  303  comprises an inductor  321  and a piezoelectric ceramic transformer  322 . 
         [0028]    The half-bridge power switch circuit  302  is electrically connected to the DC power source  301  and powered by the DC power source  301 . An output terminal of the half-bridge power switch circuit  302  is electrically connected to an input terminal of the resonant tank  303 . An output terminal of the resonant tank  303  is electrically connected to one end of a fluorescent lamp  304 . An input of the lamp current sensing circuit  305  is electrically connected to one end of the fluorescent lamp  304 . An input of the lamp voltage sensing circuit  306  is electrically connected to the other end of the fluorescent lamp  304 . An output of the lamp current sensing circuit  305  and an output of the lamp voltage sensing circuit  306  are electrically connected to the pulse width modulator  307  and feeds back a lamp current sensing signal and a lamp voltage sensing signal to the pulse width modulator  307 . 
         [0029]    The pulse width modulator  307  comprises an error amplifier  361 , a comparator  364 , an integral resistor  365 , an integral capacitor  366 , a current source  367 , and a switch  368 , wherein an inverse integrator consists of an error amplifier  361 , an integral resistor  365 , and an integral capacitor  366 . An inverting terminal of the error amplifier  361  is electrically connected to the current source  367  via the switch  368 . An output terminal S 1  of the error amplifier  361  is electrically connected to the triangle wave generator  308  via a resistor  362 . An output terminal S 16  of the pulse width modulator  307  is electrically connected to the half-bridge drive circuit  309 . The half-bridge drive circuit  309  is electrically connected to the half-bridge power switch circuit  302 . 
         [0030]    According to the preferred embodiment of the present invention, an output terminal S 2  of the triangle wave generator  308  is electrically connected to a grounding resistor  363 . Additionally, the triangle wave generator  308  comprises another terminal electrically connected to a capacitor  364 . The value of a current S 3  passing through the output terminal S 2  of the triangle wave generator  308  and the capacitance of the capacitor  364  determine the operating frequency of the triangle wave generator  308 . The operating frequency of the triangle wave generator  308  increases when the current S 3  increases. The operating frequency of the triangle wave generator  308  is determined by an output signal at the output terminal S 1  of the error amplifier  361  and the current S 3  because the resistor  362  is connected between the output terminal S 1  of the error amplifier  361  and the output terminal S 2  of the triangle wave generator  308 . In this embodiment of the present invention, when the output signal at the output terminal S 1  of the error amplifier  361  is zero voltage, the resistor  362  is in parallel with the grounding resistor  363  with respect to the output terminal S 2 . Hence the equivalent load resistance with respect to the triangle wave generator  308  is smallest and then the current S 3  passing through the output terminal S 2  of the triangle wave generator  308  is highest. In other words, the operating frequency of the triangle wave generator  308  is highest. On the contrary, when the output voltage at the output terminal S 1  of the error amplifier  361  is close to the voltage at the output terminal S 2 , the current passing through the resistor  362  is zero. Hence the equivalent load resistance with respect to the triangle wave generator  308  is just only the grounding resistor  363 . The current S 3  passing through the output terminal S 2  of the triangle wave generator  308  becomes smaller and then the operating frequency of the triangle wave generator  308  also becomes smaller. When the values of the resistor  362 , the grounding resistor  363 , and the capacitor  364  are fixed, the operating frequency of the triangle wave generator  308  is determined by the voltage at the output terminal S 1  of the error amplifier  361 . In other words, when the voltage at the output terminal S 1  of the error amplifier  361  decreases, the operating frequency of the triangle wave generator  308  increases, and vice versa. In this embodiment of the present invention, the triangle wave generator  308  not only generates a triangle wave S 17  but also a pulse signal S 18  having the same frequency with the triangle wave S 17 , wherein the pulse signal S 18  is supplied to the half-bridge drive circuit  309  to generate driving signals. However, it is not intended to o limit the invention to the triangle wave. It should be appreciated that any ramp signals or sawtooth wave signals could be made in the embodiments described by persons skilled in the art. 
         [0031]    The lamp current sensing circuit  305  is in series with the fluorescent lamp  304  and provides a signal S 4  to indicate the conduction state of the fluorescent lamp  304  and a signal S 5  to indicate the current passing through the fluorescent lamp  304 . The lamp voltage sensing circuit  306  is in parallel with the fluorescent lamp  304  and provides a signal S 6  to indicate the voltage at the end of the fluorescent lamp  304 . 
         [0032]    The half-bridge drive circuit  309  generates two driving signals POUT and NOUT. The timer  311  comprises two sets of comparators  381 ,  382 , and a current source  383 . The dimming control circuit  312  comprises a dimming frequency generator  331  generating a triangle wave S 7  and a pulse signal S 15 , a comparator  332 , and an OR gate  333 . The triangle wave S 7  is applied to a non-inverting terminal of the comparator  332  and a dimming control voltage S 8  is applied to an inverting terminal of the comparator  332 . The comparator  332  compares the triangle wave S 7  and the dimming control voltage S 8  to generate a dimming pulse signal S 9 . The OR gate  333  is used to control the timing when the dimming pulse signal S 9  could be applied to the error amplifier  361  of the pulse width modulator  307 . 
         [0033]    In this embodiment of the present invention, the timer  311  utilizes the current source  383  to charge a timer capacitor  384  so that a voltage S 12  across the timer capacitor  384  increases with time. When the voltage S 12  is lower than a reference voltage Vref 1 , the timer  311  utilizes a comparator  381  to output a reset signal S 11 . When the voltage S 12  is larger than a reference voltage Vref 2 , the timer  311  utilizes a comparator  382  to output a time out signal S 10 . The current source  383  is controlled by an indicative signal S 13  outputted from a system voltage source. When a system voltage of the system voltage source is lower than reference voltage Vref 3 , the indicative signal S 13  communicates with the current source  383  to turn off the current source  383  and also grounds the timer capacitor  384 . Therefore, it could be assured that the timer capacitor  384  is charged from zero voltage and the timer  311  should be reset each start of the resonant half-bridge DC/AC inverter circuit. 
         [0034]    In this embodiment of the present invention, the protection circuit  310  comprises a comparator  374  and a logic control circuit  372 . The signal S 4  provided by the lamp current sensing circuit  305  and a reference voltage Vref 4  are applied to the comparator  374  to determine the conduction state of the fluorescent lamp  304 . When the signal S 4  is larger than the reference voltage Vref 4 , the fluorescent lamp  304  is treated as ignition and the comparator  374  outputs a signal S 114  to indicate that the fluorescent lamp  304  is ignited. The protection circuit  310  determines the execution of the protection action or not according to the signal S 14 , the time out signal S 11 , and the pulse signal S 15 . 
         [0035]    Under normal operation, the timer  311  utilizes the current source  383  to charge a timer capacitor  384  so that a voltage S 12  across the timer capacitor  384  increases with time. When the voltage S 12  is lower than a reference voltage Vref 1 , the timer  311  utilizes a comparator  381  to output a reset signal S 11  so that a switch  368  is turned on and the current source  367  is electrically connected to the inverting terminal of the error amplifier  361 . Hence the current source  367  enforces that a voltage at the inverting terminal of the error amplifier  361  is higher than a reference voltage Vref 5  so that the output of the error amplifier  361  is zero. At this time, the output of the pulse width modulator  307  is zero. The operating frequency of the voltage-controlled-frequency triangle wave generator  308  is far way and higher than the resonant frequency of the resonant tank  303 . 
         [0036]    When the voltage S 12  is larger than a reference voltage Vref 1 , the switch  368  is turned off so that the pulse width modulator  307  starts to work. The voltage at the inverting terminal of the error amplifier  361  is lower than the reference voltage Vref 5  plus a conduction voltage of a diode  352 , the output signal of the error amplifier  361  gradually increases because of negative feedback control theory. The comparator  364  compares the output signal of the error amplifier  361  with the triangle wave S 17  to generate a pulse width modulation signal S 16 . The pulse width modulation signal S 16  and the pulse signal S 18  are applied to the half-bridge drive circuit  309  to generate driving signals POUT and NOUT which drive two power switches  302 A,  302 B respectively. The output of the pulse width modulator  307  determines the turned-on duty cycle of the driving signals POUT and NOUT. When the output of the pulse width modulator  307  is higher, it makes larger turned-on duty cycle of the driving signals POUT and NOUT. With such design, the power switches  302 A,  302 B are driven by a higher frequency and less duty cycle signals POUT and NOUT when the supply voltage is higher. When the power switches  302 A,  302 B are driven by less duty cycle signals POUT and NOUT, less power transferred to the load may prevent the operating frequency far away from the resonant frequency of the resonant tank  303  as the prior art. 
         [0037]    Before the ignition of the fluorescent lamp  304 , the voltage at the end of the fluorescent lamp  304  increases because the duty cycle of the pulse width modulation signal S 16  gradually increases and the frequency of the pulse width modulation signal S 16  gradually decreases. The lamp voltage sensing circuit  306  detects the voltage at the end of the fluorescent lamp  304  and provides a signal S 6  to indicate the voltage at the end of the fluorescent lamp  304 . When the voltage of the signal S 6  is greater than the reference voltage Vref 5  plus the conduction voltage of the diode  352 , the output of the error amplifier  361  becomes smaller and then the duty cycle of the pulse width modulation signal S 16  is reduced and the frequency of the pulse width modulation signal S 16  is increased to reduce the power delivery to fluorescent lamp  304 . If this result causes the voltage of the signal S 6  is smaller than the reference voltage Vref 5  plus the conduction voltage of the diode  352 , the output of the error amplifier  361  becomes larger. Therefore, the voltage applied to the fluorescent lamp  304  could be regulated and stabilized because of negative feedback control theory. 
         [0038]    Once the fluorescent lamp  304  is ignited and reaches steady operation, the voltage across the fluorescent lamp  304  will suddenly drop to half of the ignition voltage of the fluorescent lamp  304  so that the lamp voltage sensing circuit  306  does not work because the lamp voltage sensing circuit  306  could not detect an enough high voltage. 
         [0039]    The lamp current sensing circuit  305  provides a signal S 4  to the lamp current sensing circuit  305  and a signal S 5  to the pulse width modulator  307  to stabilize the current passing through the fluorescent lamp  304  at a fixed value via feedback control. 
         [0040]    In this embodiment of the present invention, the function of the diodes  351  and  352  is to utilize the characteristic of the great difference between the ignition voltage and the normal operation voltage of the fluorescent lamp (for example 2˜2.5 times). Before the ignition of the fluorescent lamp  304 , the diode  352  is conductive and the diode  351  is non-conductive so that the signal S 6  provided by the lamp voltage sensing circuit  306  is applied to the pulse width modulator  307 . Once the fluorescent lamp  304  is ignited, the voltage across the fluorescent lamp  304  drops and the lamp current increases so that the diode  352  is non-conductive and the diode  351  is conductive. Hence the signal S 5  provided by the lamp current sensing circuit  305  is applied to the pulse width modulator  307 . As a result, the inverter could provide a stable high voltage to the fluorescent lamp  304  during start operation and a stable current to the fluorescent lamp  304  during normal operation. 
         [0041]    The detail description of the protection circuit in this embodiment of the present invention is described as below: 
         [0042]    Before the fluorescent lamp  304  connected to the inverter, the signal S 14  automatically is delivered to the logic control circuit  372  to indicate that the fluorescent lamp  304  is not ignited. In order to provide enough time to ignite the fluorescent lamp  304 , the time out signal S 10  is delivered to the protection circuit  310  to enforce the logic control circuit  372  to ignore that the signal S 14  indicates the information of the non-ignition of the fluorescent lamp  304 . Once the time reaches the preset value, the inverter utilizes another digital timer to calculate time on the base of the pulse signal S 15 . If the fluorescent lamp  304  still is not ignited after several clock cycles, the logic control circuit  372  outputs a signal S 20  to stop the operation of the half-bridge drive circuit  309  and the conduction of the power switches  302 A,  302 B. In this embodiment of the present invention, once the protection circuit  310  stops the power switches  302 A,  302 B, the inverter  300  must be turned off and restarted to get rid of the protection action. 
         [0043]    When the fluorescent lamp  304  is broken and open during operation, the signal S 14  is delivered to the logic control circuit  372  to indicate the information of the non-ignition of the fluorescent lamp  304 . The logic control circuit  372  receives the time out signal S 10  provided by the timer  311 . The logic control circuit  372  does not work until the time out signal S 10  is delivered to the logic control circuit  372 . Once the time is over the preset value, the inverter utilizes another digital timer to calculate time on the base of the pulse signal S 15 . If the fluorescent lamp  304  still is not ignited after several clock cycles, the logic control circuit  372  outputs a signal S 20  to stop the operation of the half-bridge drive circuit  309  and the conduction of the power switches  302 A,  302 B. In this embodiment of the present invention, once the protection circuit  310  stops the power switches  302 A,  302 B, the inverter  300  must be turned off and restarted to get rid of the protection action. 
         [0044]    The dimming control circuit  312  utilizes a lower frequency than the operating frequency of the fluorescent lamp  304  to stop or recover to deliver power to the fluorescent lamp  304 . The adjustment of the ratio of lightness and darkness is utilized to adjust the brightness of the fluorescent lamp  304 . The dimming frequency control generally is controlled above 200 Hz in order to avoid the user&#39;s feeling of flicker caused by lower dimming frequency. The dimming control circuit  312  is enabled by two signals. One is the signal S 14  which indicates the conduction state of the fluorescent lamp. The other is the time out signal S 10  provided by the timer  311 . When the signal S 14  indicates that the fluorescent lamp is conductive or the time out signal S 10  indicates that time is out, a switch  336  is turned on to control the output of the dimming signal. A dimming voltage S 21  of the dimming control circuit  312  is higher than the reference voltage Vref 5 . When the dimming voltage S 21  is delivered to the pulse width modulator  307  through switched  336 ,  335  and the resistor  334 , the output voltage of the error amplifier  361  of the pulse width modulator  307  becomes smaller and causes the inverter to stop the power delivery to the fluorescent lamp. When the dimming pulse signal S 9  turns off the switch  335 , the dimming voltage S 21  is not delivered to the pulse width modulator  307 . It is an open circuit between the dimming voltage S 21  and the pulse width modulator  307  so that the inverter recovers to deliver the power to the load. 
         [0045]    In this embodiment of the present invention, the dimming frequency generator  331  generates a triangle wave S 7 . The comparator  332  compares the triangle wave S 7  and the dimming control voltage S 8  to generate the dimming pulse signal S 9 . The dimming pulse signal S 9  has different pulse widths. The present invention utilize a low frequency control to control the ratio of the power stop period or the power supply period each cycle in order to achieve the brightness adjustment. However, the conduction state of the fluorescent lamp  304  could determine when starts to proceed the dimming control and ensure the fluorescent lamp  304  has enough time and continuous power to be ignited. 
         [0046]    In this embodiment of the present invention, in order to the interference between the internal clock of LCD and the low frequency dimming control, the dimming control voltage S 8  could be a low frequency pulse generated by related internal clock of LCD. When the amplitude of the dimming control voltage S 8  is greater than the peak value of the triangle wave S 7  and smaller than the valley value of the triangle wave S 7 , the duty cycle and frequency of the dimming pulse signal S 9  is completely determined by the duty cycle and frequency of the dimming control voltage S 8 . Hence it could reduce the difference frequency interference of user&#39;s sense of sight caused by the difference between operating frequency of dimming control and the operating frequency of LCD. 
         [0047]    In order to provide a symmetric alternating current to drive the fluorescent lamp  304 , the upper half of the half-bridge power switch circuit  110  is driven out of phase with the lower half of the half-bridge power switch circuit  110  such that when the power switch  110 A is on, the power switch  110 B is off, and conversely, when the power switch  110 A is off, the power switch  110 B is on. Driven in this manner, the upper half of the half-bridge power switch circuit  110  and the lower half of the half-bridge power switch circuit  110  have the same duty cycle and alternatively turned on and off with 180° phase shift. 
         [0048]    Additionally, while the present invention makes specific reference to CCFLs, the present invention is equally applicable for driving many types of lamps and tubes known in the art, such as: metal halide lamps, sodium vapor lamps, and/or x-ray tubes. 
         [0049]    Furthermore, while the present invention makes specific reference to piezoelectric, the present invention is equally applicable for any types of transformers known in the art, such as: electromagnetic transformers. 
         [0050]    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. 
         [0051]    The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to best explain the principles of the invention and its best mode practical application, thereby to enable persons skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.