Abstract:
A DC/AC cold cathode fluorescent lamp (CCFL) inverter circuit includes a transformer with a primary winding and a secondary winding for providing increased voltage to a CCFL, a first and second MOSFET switches for selectively allowing direct current of a first polarity and a second polarity to flow through the transformer respectively. The primary and secondary windings of the transformer are electrically coupled to ground. A capacitor divider is electrically coupled to the CCFL for providing a first voltage signal representing a voltage across the CCFL. A first feedback signal line receives the first voltage signal. A timer circuit is coupled to the first feedback signal line for providing a time-out sequence of a predetermined duration when the first voltage signal exceeds a predetermined threshold. A protection circuit shuts down the first switch and the second switch when the first voltage signal exceeds the predetermined threshold after the predetermined duration.

Description:
CROSS REFERENCE TO RELATED UNITED STATES PATENT APPLICATIONS 
       [0001]    This application is a Continuation application of the co-pending, commonly-owned U.S. patent application with Attorney Docket No. O2-0011.CON4, Ser. No. 10/935,629, filed Sep. 7, 2004, by Yung-Ling Lin, and entitled “High-Efficiency DC/AC Converter”, which itself is a Continuation application of the commonly-owned U.S. patent application with Attorney Docket No. O2-0011.CON3, Ser. No. 10/776,417, filed Feb. 11, 2004, now U.S. Pat. No. 6,804,129, which itself is a Continuation application of the commonly owned U.S. patent application with Attorney Docket No. O2-0011. CON2, Ser. No. 10/132,016, filed Apr. 24, 2002, which itself is a Continuation application of the commonly-owned U.S. patent application with Attorney Docket No. O2-0011. CON1, Ser. No. 09/850,222, filed May 7, 2001, now U.S. Pat. No. 6,396,722, which itself is a Continuation application of the commonly-owned U.S. patent application with Attorney Docket No. O2-0011, Ser. No. 09/437,081, filed Nov. 9, 1999, now U.S. Pat. No. 6,259,615, all of which claim priority to the Provisional Application Ser. No. 60/145,118, filed Jul. 22, 1999, all of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to a DC to AC power converter circuit. More particularly, the present invention provides a high efficiency controller circuit that regulates power delivered to a load using a zero-voltage-switching technique. General utility for the present invention is found as a circuit for driving one or more Cold Cathode Fluorescent Lamps (CCFLs), however, those skilled in the art will recognize that the present invention can be utilized with any load where high efficiency and precise power control is required. 
       DESCRIPTION OF RELATED ART 
       [0003]      FIG. 1  depicts a conventional CCFL power supply system  10 . The system broadly includes a power supply  12 , a CCFL driving circuit  16 , a controller  14 , a feedback loop  18 , and one or more lamps CCFL associated with an LCD panel  20 . Power supply  12  supplies a DC voltage to circuit  16 , and is controlled by controller  14 , through transistor Q 3 . Circuit  16  is a self-resonating circuit, known as a Royer circuit. Essentially, circuit  16  is a self-oscillating dc to ac converter, whose resonant frequency is set by L 1  and C 1 , and N 1 -N 4  designate transformer windings and number of turns of the windings. In operation, transistors Q 1  and Q 2  alternately conduct and switch the input voltage across windings N 1  and N 2 , respectively. If Q 1  is conducting, the input voltage is placed across winding N 1 . Voltages with corresponding polarity will be placed across the other windings. The induced voltage in N 4  makes the base of Q 2  positive, and Q 1  conducts with very little voltage drop between the collector and emitter. The induced voltage at N 4  also holds Q 2  at cutoff. Q 1  conducts until the flux in the core of TX 1  reaches saturation. 
         [0004]    Upon saturation, the collector of Q 1  rises rapidly (to a value determined by the base circuit), and the induced voltages in the transformer decrease rapidly. Q 1  is pulled further out of saturation, and V CE  rises, causing the voltage across N 1  to further decrease. The loss in base drive causes Q 1  to turn off, which in turn causes the flux in the core to fall back slightly and induces a current in N 4  to turn on Q 2 . The induced voltage in N 4  keeps Q 1  conducting in saturation until the core saturates in the opposite direction, and a similar reversed operation takes place to complete the switching cycle. 
         [0005]    Although the inverter circuit  16  is composed of relatively few components, its proper operation depends on complex interactions of nonlinearities of the transistors and the transformer. In addition, variations in C 1 , Q 1  and Q 2  (typically, 35% tolerance) do not permit the circuit  16  to be adapted for parallel transformer arrangements, since any duplication of the circuit  16  will produce additional, undesirable operating frequencies, which may resonate at certain harmonics. When applied to a CCFL load, this circuit produces a “beat” effect in the CCFLs, which is both noticeable and undesirable. Even if the tolerances are closely matched, because circuit  16  operates in self-resonant mode, the beat effects cannot be removed, as any duplication of the circuit will have its own unique operating frequency. 
         [0006]    Some other driving systems can be found in U.S. Pat. Nos. 5,430,641; 5,619,402; 5,615,093; 5,818,172. Each of these references suffers from low efficiency, two-stage power conversion, variable-frequency operation, and/or load dependence. Additionally, when the load includes CCFL(s) and assemblies, parasitic capacitances are introduced, which affects the impedance of the CCFL itself. In order to effectively design a circuit for proper operation, the circuit must be designed to include consideration of the parasitic impedances for driving the CCFL load. Such efforts are not only time-consuming and expensive, but it is also difficult to yield an optimal converter design when dealing with various loads. Therefore, there is a need to overcome these drawbacks and provide a circuit solution that features high efficiency, reliable ignition of CCFLs, load-independent power regulation and single frequency power conversion. 
       SUMMARY 
       [0007]    Embodiments in accordance with the present invention provide a DC to AC cold cathode fluorescent lamp inverter circuit, which includes a step-up transformer with a primary winding and a secondary winding for providing increased voltage to a cold cathode fluorescent lamp, a first MOSFET switch coupled to the step-up transformer for selectively allowing direct current of a first polarity to flow through the step-up transformer, and a second MOSFET switch coupled to the step-up transformer for selectively allowing direct current of a second polarity to flow through the step-up transformer. The primary winding and the secondary winding of the step-up transformer are electrically coupled to ground. The DC to AC cold cathode fluorescent lamp inverter circuit further includes a capacitor divider electrically coupled to the cold cathode fluorescent lamp for providing a first voltage signal representing a voltage across the cold cathode fluorescent lamp, a first feedback signal line coupled to the capacitor divider for receiving the first voltage signal from the capacitor divider. The DC to AC cold cathode fluorescent lamp inverter circuit further includes a timer circuit coupled to the first feedback signal line for providing a time-out sequence of a predetermined duration when the first voltage signal exceeds a predetermined threshold, and a protection circuit coupled to the timer circuit, the first switch and the second switch for shutting down the first switch and the second switch when the first voltage signal exceeds the predetermined threshold after the predetermined duration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Features and advantages of embodiments of the claimed subject matter will become apparent as the following detailed description proceeds, and upon reference to the drawings, wherein like numerals depict like parts, and in which: 
           [0009]      FIG. 1  is a conventional DC/AC converter circuit; 
           [0010]      FIG. 2  is one embodiment of a DC/AC converter circuit of the present invention; 
           [0011]      FIG. 2   a - FIG. 2   f  are exemplary timing diagrams of the circuit of  FIG. 2 ; 
           [0012]      FIG. 3  is another embodiment of a DC/AC converter circuit of the present invention; 
           [0013]      FIG. 3   a - FIG. 3   f  are exemplary timing diagrams of the circuit of  FIG. 3 ; and 
           [0014]      FIG. 4   a - FIG. 4   f  depict emulation diagrams for the circuits shown in  FIG. 2  and  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    It will be appreciated by those skilled in the art that although the following detailed description will proceed with reference being made to embodiments and methods of use, the present invention is not intended to be limited to these embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited as only set forth in the accompanying claims. While not wishing to be bound by example, the following detailed description will proceed with reference to a CCFL panel as the load for the circuit of the present invention. However, it will be apparent that the present invention is not limited only to driving one or CCFLs, rather, the present invention should be broadly construed as a power converter circuit and methodology independent of the particular load for a particular application. 
         [0016]    Accordingly, embodiments in accordance with the present invention provide a improved system for driving a load and obtain a improved operation for various LCD panel loads, thereby the reliability of the system can be enhanced. 
         [0017]    In one embodiment, a DC/AC converter circuit for controllably delivering power to a load, includes an input voltage source, a first plurality of overlapping switches and a second plurality of overlapping switches selectively coupled to the voltage source. The first plurality of overlapping switches define a first conduction path, and the second plurality of overlapping switches define a second conduction path. A pulse generator is provided to generate a pulse signal. Drive circuitry receives the pulse signal and controls the conduction state of the first and second plurality of switches. The DC/AC converter circuit also includes a transformer includes a transformer having a primary side and a secondary side. The primary side is selectively coupled to the voltage source in an alternating fashion through the first conduction path and, alternately, through the second conduction path. A load is coupled to the secondary side of the transformer. A feedback loop circuit is provided between the load and the drive circuitry and supplies a feedback signal indicative of power being supplied to the load. The drive circuitry alternates the conduction state of the first and second plurality of switches, and controls the overlap time of the switches in the first plurality of switches and the overlap time of the switches in the second plurality of switches, in order to couple the voltage source to the primary side based at least in part on the feedback signal and the pulse signal. 
         [0018]    The drive circuitry is constructed to generate a first complimentary pulse signal from the pulse signal, and a ramp signal from the pulse signal. The pulse signal is supplied to a first one of the first plurality of switches to control the conduction state thereof, and the ramp signal is compared with at least the feedback signal to generate a second pulse signal. A controllable conduction overlap condition exists between the conduction states of the first and second switches of the first plurality of switches. The second pulse signal is supplied to a second one of the first plurality of switches to control the conduction state thereof. The drive circuitry further generates a second complimentary pulse signal based on the second pulse signal. The first and second complimentary pulse signals control the conduction states of a first switch and a second switch of the second of the second plurality of switches, respectively. Likewise, a controllable conduction overlap condition exists between the conduction states of the first and second switches of the second plurality of switches. 
         [0019]    The present invention also provides a method for controlling a zero-voltage switching circuit to deliver power to a load. In one embodiment, the method includes supplying a DC voltage source, coupling a first and a second transistors defining a first conduction path and a third and a fourth transistors defining a second conduction path to the voltage source and a primary side of a transformer, generating a pulse signal having a predetermined pulse width, coupling a load to a secondary side of the transformer, generating a feedback signal from the load, and controlling the feedback signal and the pulse signal to determine the conduction states of the first, second, third and fourth transistors. 
         [0020]    In the first embodiment, the present invention provides a converter circuit for delivering power to a CCFL load, which includes a voltage source, a transformer having a primary side and a secondary side, a first pair of switches and a second pair of switches defining a first and second conduction path, respectively, between the voltage source and the primary side, a CCFL load circuit coupled to the secondary side, a pulse generator generating a pulse signal, a feedback circuit coupled to the load for generating a feedback signal, and drive circuitry for receiving the pulse signal and the feedback signal and coupling the first pair of switches or the second pair of switches to the voltage source and the source and the primary side based on the pulse signal and the feedback signal to deliver power to the CCFL load. 
         [0021]    Additionally, the first embodiment provides a pulse generator that generates a pulse signal having a predetermined frequency. The drive circuitry includes the first, second, third and fourth drive circuits. The first pair of switches includes the first and second transistors, and the second pair of switches includes the third and fourth transistors. The first, second, third and fourth drive circuits are connected to the control lines of the first, second, third and fourth transistors, respectively. The pulse signal is supplied to the first drive circuit so that the first transistor is switched in accordance with the pulse signal. The third drive circuit generates a first complimentary pulse signal and a ramp signal based on the pulse signal, and supplies the first complimentary pulse signal to the third transistor so that the third transistor is switched in accordance with the first complimentary pulse signal. The ramp signal and the feedback signal are compared to generate a second pulse signal. The second pulse signal is supplied to the second drive circuit so that the second transistor is switched in accordance with the second pulse signal. The forth driving circuit generates a second complementary pulse signal based on the second pulse signal and supplies the second complementary pulse signal to the fourth transistor so that the fourth transistor is switched in accordance with the second complimentary pulse signal. In one embodiment, the simultaneous conduction of the first and second transistors, and the simultaneous conduction of the third and fourth transistors, respectively, controls the amount of power delivered to the load. The pulse signal and the second pulse signal are generated to overlap by a controlled amount, thus delivering power to the load along the first conduction path. Since the first and second complementary pulse signals are generated from the pulse signal and second pulse signal respectively, the first and second complementary pulse signals are also generated to overlap by a controlled amount to deliver power to the load along the second conduction path. As such, power is delivered to the load in an alternating fashion between the first and second conduction paths. 
         [0022]    Also, the pulse signal and first complementary pulse signal are generated to be approximately 180° out of phase, and the second pulse signal and the second complementary signal are generated to be approximately 180° out of phase, so that a short circuit condition between the first and second conduction paths can be avoided. 
         [0023]    In addition to the converter circuit provided in the first embodiment, the second embodiment includes a flip-flop circuit coupled to the second pulse signal, which triggers the second pulse signal to the second drive signal only when the third transistor is switched into a conducting state. Additionally, the second embodiment includes a phase-lock loop (PLL) circuit having a first input signal from the primary side and a second input signal using the feedback signal. The PLL circuit compares the phase difference between these two signals and supplies a control signal to the pulse generator to control the pulse width of the pulse signal based on the phase difference between the first and second inputs. 
         [0024]    In both embodiments, the converter circuit includes the feedback control loop having a first comparator for comparing a reference signal with the feedback signal and producing a first output signal. A second comparator is provided for comparing the first output signal with the ramp signal and producing the second pulse signal based on the intersection of the first output signal and the ramp signal. The feedback circuit can also include a current sense circuit receiving the feedback signal and generating a trigger signal, and a switch circuit between the first and second comparator. The switch circuit receives the trigger signal and generates either the first output signal or a predetermined minimum signal, based on the value of the trigger signal. The reference signal can include, for example, a signal that is manually generated to indicate a desires power to be delivered to the load. The predetermined minimum voltage signal can include a programmed minimum voltage supplied to the switches, so that an over-voltage condition does not appear across the load. 
         [0025]    Likewise, in both embodiments described herein, an over-current protection circuit that receives the feedback signal and controls the pulse generator based on the value of said feedback signal can be included. An over-voltage protection can be provided to receive a voltage signal from across the load and the first output signal and compare the voltage signal from across the load and the first output signal, to control the pulse generator based on the value of the voltage signal from across the load. 
         [0026]    As an overview, the present invention provides circuitry to controllably deliver power to a load using feedback signals and pulse signals to adjust the ON time of two pairs of switches. When one pair of switches are controllably turned ON such that their ON times overlap, power is delivered to a load (via a transformer), along a conduction path defined by the pair of switches. Likewise, when the other pair of switches are controllably turned ON such that their ON times overlap, power is delivered to a load (via a transformer), along a conduction path defined by other pair of switches. Thus, by selectively turning ON switches and controlling the overlap between-switches, the present invention can control power delivered to a given load in a relatively precise way. Additionally, the present invention includes over-current and over-voltage protection circuits, which discontinues power to the load in the event of a short circuit or open circuit condition. Moreover, the controlled switching topology described herein enables the circuit to operate irrespective of the load, and with a single operating frequency independent of the resonant effects of the transformer arrangement. These features are discussed below with reference to the drawings. 
         [0027]    The circuit diagram shown in  FIG. 2  illustrates one embodiment of a phase-shift, full-bridge, zero-voltage-switching power converter of the present invention. Essentially, the circuit shown in  FIG. 2  includes a power source  12 , a plurality of switches  80  arranged as diagonal pairs of switches defining alternating conduction paths, drive circuitry  50  for driving each of the switches, a frequency sweeper  22  which generates a square wave pulse to the drive circuitry  50 , a transformer TX 1  (with an associated resonant tank circuit defined by the primary side of TX 1  and C 1 ) and a load. Advantageously, the present invention also includes an overlap feedback control loop  40  which controls the ON time of at least one of each pair of switches, thereby permitting controllable power to be delivered to the load. 
         [0028]    A power source  12  is applied to the system. Initially, a bias/reference signal  30  is generated for the control circuitry (in control loop  40 ) from the supply. For example, a frequency sweeper  22  generates a 50% duty-cycle pulse signal, starting with an upper frequency and sweeping downwards at a pre-determined rate and at pre-determined steps (i.e., square wave signal of variable pulse width). The frequency sweeper  22  can be a programmable frequency generator, as is known in the art. The pulse signal  90  (from the sweeper  22 ) is delivered to B_Drive (which drives the Switch_B, i.e., controls the gate of Switch_B), and is delivered to A_Drive, which generates a complementary pulse signal  92  and a ramp signal  26 . The complementary pulse signal  92  is approximately 180° out of phase with pulse signal  90 , and the ramp signal  26  is approximately 90° out of phase with pulse signal, as will be described below. The ramp signal can be a sawtooth signal, as shown in the Figure. The ramp signal  26  is compared with the output signal  24  (referred to herein as CMP) of the error amplifier  32 , through comparator  28 , thus generating signal  94 . The output signal  94  of the comparator  28  is likewise a 50% duty pulse delivered to C_Drive to initiate the turning on of Switch_C which, in turn, determines the amount of overlap between the switches B and C, and switches A and D. Its complimentary signal (phased approximately 180°) is applied to Switch_D, via D_Drive. It will be understood by those skilled in the art that circuits Drive_A-Drive_D are connected to the control lines (e.g., gate) of Switch_A-Switch_D, respectively, which permits each of the switches to controllably conduct, as described herein. By adjusting the amount of overlap between switches B, C and A, D, lamp-current regulation is achieved. In other words, it is the amount of overlapping in the conduction state of the pairs of switches that determines the amount of power processed in the converter. Hence, switches B and C, and switches A and D, will be referred to herein as overlapping switches. 
         [0029]    While not wishing to be bound by example, in this embodiment, B_Drive can be formed of a totem pole circuit, generic low-impedance op-amp circuit, or emitter follower circuit. C_Drive is likewise constructed. Since both A-Drive and D_Drive are not directly connected to ground (i.e., floating), in one embodiment, these drives are formed of a boot-strap circuit, or other high-side drive circuitry known in the art. Additionally, as stated above, A_Drive and D_Drive include an inverter to invert (i.e., phase) the signal flowing from B_Drive and C_Drive, respectively. 
         [0030]    High-efficiency operation is achieved through a zero-voltage-switching technique. The four MOSFETs (Switch_A-Switch_D)  80  are turned on after their intrinsic diodes (D 1 -D 4 ) conduct, which provides a current flowing path of energy in the transformer/capacitor (TX 1 /C 1 ) arrangement, thereby ensuring that a zero voltage is across the switches when they are turned on. With this controlled operation, switching loss is minimized and high efficiency is maintained. 
         [0031]    The switching operation of the overlapping switches  80  is shown with reference to the timing diagrams of  FIG. 2   a - FIG. 2   f . Switch_C is turned off at certain period of the conduction of both switches B and C ( FIG. 2   f ). The current flowing in the tank (refer to  FIG. 2 ) is now flowing through diode D 4  ( FIG. 2   e ) in Switch_D, the primary of transformer, C 1 , and Switch_B, after Switch_C is turned off, thereby resonating the voltage and current in capacitor C 1  and the transformer as a result of the energy delivered when switches B and C were conducting ( FIG. 2   f ). Note that this condition must occur, since an instantaneous change in current direction of the primary side of the transformer would violate Faraday&#39;s Law. Thus, current must flow through D 4  when Switch_C turns off. Switch_D is turned on after D 4  has conducted. Similarly, Switch_B is turned off ( FIG. 2   a ), the current diverts to Diode D 1  associated with Switch_A before Switch_A is turned on ( FIG. 2   e ). Likewise, Switch_D is turned off ( FIG. 2   d ), and the current is now flowing now from Switch_A, through C 1 , the transformer primary and Diode D 3 . Switch_C is turned on after D 3  has conducted ( FIG. 2   e ). Switch_B is turned on after Switch_A is turned off which allows the diode D 2  to conduct first before it is turned on. Note that the overlap of turn-on time of the diagonal switches B,C and A,D determines the energy delivered to the transformer, as shown in  FIG. 2   f.    
         [0032]    In this embodiment,  FIG. 2   b  shows that the ramp signal  26  is generated only when Switch_A is turned on. Accordingly, Drive_A, which generates the ramp signal  26 , can include a constant current generator circuit (not shown) that includes a capacitor having an appropriate time constant to create the ramp signal. To this end, a reference current (not shown) is utilized to charge the capacitor, and the capacitor is grounded (via, for example a transistor switch) so that the discharge rate exceeds the charge rate, thus generating the sawtooth ramp signal  26 . Of course, as noted above, this can be accomplished by integrating the pulse signal  90 , and thus, the ramp signal  26  can be formed using an integrator circuit (e.g., op-amp and capacitor). 
         [0033]    In the ignition period, a pre-determined minimum overlap between the two diagonal switches is generated (i.e., between switches A,D and B,C). This gives a minimum energy from the input to the tank circuit including C 1 , transformer, C 2 , C 3  and the CCFL load. Note that the load can be resistive and/or capacitive. The drive frequency starts at a predetermined upper frequency until it approaches the resonant frequency of the tank circuit and equivalent circuit reflected by the secondary side of the transformer, a significant amount of energy is delivered to the load where the CCFL is connected. Due to its high-impedance characteristics characteristics before ignition, the CCFL is subjected to high voltage from the energy supplied to the primary side. This voltage is sufficient to ignite the CCFL. The CCFL impedance decreases to its normal operating value (e.g., about 100 Kohm to 130 Kohm), and the energy supplied to the primary side based on the minimum-overlap operation is no longer sufficient to sustain a steady state operation of the CCFL. The output of the error amplifier  26  starts its regulating function to increase the overlap. It is the level of the error amplifier output determines the amount of the overlap. 
         [0034]    Referring to  FIG. 2   b  and  FIG. 2   c  and the feedback loop  40  of  FIG. 2 , it is important to note that Switch_C is turned on when the ramp signal  26  (generated by Drive_A) is equal to the value of signal CMP  24  (generated by error amplifier  32 ), determined in comparator  28 . This is indicated as the intersection point  36  in  FIG. 2   b . To prevent a short circuit, switches A,B and C,D must never be ON simultaneously. By controlling the CMP level, the overlap time between switches A,D and B,C regulates the energy delivered to the transformer. To adjust the energy delivered to the transformer (and thereby adjust the energy delivered to the CCFL load), switches C and D are time-shifted with respect to switches A and B, by controlling the error amplifier output, CMP  24 . As can be understood by the timing diagrams, if the driving pulses from the output of comparator  28  into switches C and D are shifted to the right by increasing the level of CMP, an increase in the overlap between switches A,C and B,D is realized, thus increasing the energy delivered to the transformer. In practice, this corresponds to the to the higher-lamp current operation. Conversely, shifting the driving pulses of switches C and D to the left (by decreasing the CMP signal) decreases the energy delivered. 
         [0035]    To this end, error amplifier  32  compares the feedback signal FB with a reference voltage REF. FB is a measure of the current value through the sense resistor Rs, which is indicative of the total current through the load  20 . REF is a signal indicative of the desired load conditions, e.g., the desired current to flow through the load. During normal operation, REF=FB. If, however, load conditions are intentionally offset, for example, from a dimmer switch associated with an LCD panel display, the value of REF will increase/decrease accordingly. The compared value generates CMP accordingly. The value of CMP is reflective of the load conditions and/or an intentional bias, and is realized as the difference between REF and FB (i.e., REF-FB). 
         [0036]    To protect the load and circuit from an open circuit condition at the load (e.g., open CCFL lamp condition during normal operation), the FB signal can also be compared to a reference value (not shown and different from the REF signal described above) at the current sense comparator  42 , the output of which defines the condition of switch  28 , discussed below. This reference value can be programmable, and/or user-definable, and reflects the minimum or maximum current permitted by the system (for example, as may be rated for the individual components, and, in particular, the CCFL load). If the value of the feedback FB signal and the reference signal is within a permitted range (normal operation), the output of the current sense comparator is 1 (or, HIGH). This permits CMP to flow through switch  38 , and the circuit operates as described herein to deliver power to the load. If, however, the value of the FB signal and the reference signal is outside a predetermined range (open circuit or short circuit condition), the output of the current sense comparator is 0 (or, LOW), prohibiting the CMP signal from flowing through the switch  38 . (Of course, the reverse can be true, in which the switch triggers on a LOW condition). Instead a minimal voltage Vmin is supplied by switch  38  (not shown) and applied to comparator  28  until the current sense comparator indicates permissible current flowing through Rs. Accordingly, switch  38  includes appropriate programmable voltage selection Vmin for when the sense current is 0. Turning again to  FIG. 2   b , the effect of this operation is a lowering of the CMP DC value to a nominal, or minimum value (i.e., CMP=Vmin) so that a high voltage condition is not appearing on the transformer TX 1 . Thus, the crossover point  36  is shifted to the left, thereby decreasing the amount of overlap between complementary switches (recall Switch_C is turned ON at the intersection point  36 ). Likewise, current sense comparator  42  is connected to the frequency generator  22  to turn the generator  22  off when the sense value is 0 (or some other preset value indicative of an open-circuit condition). The CMP is fed into the protection circuit  62 . This is to shut off the frequency sweeper  22  if the CCFL is removed during operation (open-circuit condition). 
         [0037]    To protect the circuit from an over-voltage condition, the present embodiment includes a protection circuit  60 , the operation of which is provided below (the description of the over current protection through the current sense comparator  42  is provided above). The circuit  60  includes a protection comparator  62  which compares signal CMP with a voltage signal  66  derived from the load  20 . In one embodiment, voltage signal is derived from the voltage divider C 2  and C 3  (i.e., in parallel with load  20 ), as shown in  FIG. 2 . In the open-lamp condition, the frequency sweeper continues sweeping until the OVP signal  66  reaches a threshold. The OVP signal  62  is taken at the output capacitor divider C 2  and C 3  to detect the voltage at the output of the transformer TX 1 . To simplify the analysis, these capacitors also represent the lump capacitor of the equivalent load capacitance. The threshold is a reference and circuit is being designed so that the voltage at the secondary side of the transformer is greater than the minimum striking voltage (e.g., as may be required by the LCD panel) while less than the rated voltage of the transformer. When OVP exceeds the threshold, the frequency sweeper stops the frequency sweeping. Meanwhile, the current-sense  42  detects no signal across the sense resistor Rs. Therefore the signal at  24 , the output of a switch block  38 , is set to be at minimum value so that minimum overlap between switches A,C and B,D is seen. In one embodiment, a timer  64  is initiated once the OVP exceeds the threshold, thereby initiating a time-out sequence. The duration of the time-out can be designed according to the requirement of the loads (e.g., CCFLs of an LCD panel), but could alternately be set at some programmable value. Drive pulses are disabled once the time-out is reached, thus providing safe-operation output of the converter circuit. That is, circuit  60  provides a sufficient voltage to ignite the lamp, but will shut off after a certain period if the lamp is not connected to the converter, so that erroneous high voltage is avoided at the output. This duration is necessary since a non-ignited lamp is similar to an open-lamp condition. 
         [0038]      FIG. 3  and  FIG. 3   a - FIG. 3   f  depict another embodiment of the DC/AC circuit of the present invention. In this embodiment, the circuit operates in a similar manner as provided in  FIG. 2  and  FIG. 2   a - FIG. 2   f , however this embodiment further includes a phase lock loop circuit (PLL)  70  for controlling the frequency sweeper  22 , and a flip-flop circuit  72  to time the input of a signal into C_Drive. As can be understood by the timing diagrams, if the 50% driving pulses of switches C and D are shifted to the right by increasing the level of CMP, an increase in the overlap between switches A,C and B,D is realized, thus increasing the energy delivered to the transformer. In practice, this corresponds to the higher-lamp current operation (as may be required, e.g., by a manual increase in the REF voltage, described above). Conversely, shifting the driving pulses of switches C and D to the left (by decreasing the CMP signal) decreases the energy delivered. The phase-lock-loop circuit  70  maintains the phase relationship between the feedback current (through Rs) and tank current (through TX 1 /C 1 ) during normal operation, as shown in  FIG. 3 . The PLL circuit  70  can include input signals from the tank circuit (C 1  and the primary of TX 1 ) signal  98  and Rs (FB signal, described above). Once the CCFL is ignited, and the current in the CCFL is detected through Rs, the PLL  70  circuit is activated which locks the phase the phase between the lamp current and the current in the primary resonant tank (C 1  and transformer primary). That is, the PLL is provided to adjust the frequency of the frequency sweeper  22  for any parasitic variations such as temperature effect, mechanical arrangement like wiring between the converter and the LCD panel and distance between the lamp and metal chassis of LCD panel that affect the capacitance and inductance. In one embodiment, the system maintains a phase difference of 180 degrees between the resonant tank circuit and the current through Rs (load current). Thus, irrespective of the particular load conditions and/or the operating frequency of the resonant tank circuit, the system finds an optimal operation point. 
         [0039]    The operation of the feedback loop of  FIG. 3  is similar to the description above for  FIG. 2 . However, as shown in  FIG. 3   b , this embodiment times the output of an initiating signal through C_Drive through flip-flop  72 . For instance, during normal operation, the output of the error amplifier  32  is fed through the controlled switch block  38  (described above), resulting in signal  24 . A certain amount of overlap between switches A,C and B,D is seen through comparator  28  and flip-flop  72  which drives switches C and D (recall D_Drive produces the complementary signal of C_Drive). This provides a steady-state operation for the CCFL (panel) load. Considering the removal of the CCFL (panel) during the normal operation, CMP rises to the rail of output of the error amplifier and triggers the protection circuit immediately. This function is inhibited during the ignition period. 
         [0040]    Referring briefly to  FIG. 3   a - FIG. 3   f , the triggering of switches C and D, through C-Drive and D_Drive, is, in this embodiment, alternating as a result of the flip-flop circuit  72 . As is shown in  FIG. 3   b , the flip-flop triggers every other time, thereby initiating C_Drive (and, accordingly, D_Drive). The timing otherwise operates in the same way as discussed above with reference to. 
         [0041]    Referring now to  FIG. 4   a - FIG. 4   f , the output circuit of  FIG. 2  or  FIG. 3  is emulated. For example,  FIG. 4   a  shows that at 21V input, when the frequency sweeper approaches 75.7 KHz (0.5 us overlapping), the output is reaching 1.67 KVp-p. This voltage is insufficient to turn on the CCFL if it requires 3300 Vp-p to ignite. As the frequency decreases to say 68 KHz, the minimum overlap generates about 3.9 KVp-p at the output, which is sufficient to ignite the CCFL. This is illustrated in  FIG. 4   b . At this frequency, the overlap increases to 1.5 us gives output about 1.9 KVp-p to operate the 130 Kohm lamp impedance. This has been shown in  FIG. 4   c . As another example,  FIG. 4   d  illustrates the operation while the input voltage is 7V. At 71.4 KHz, output is 750Vp-p before the lamp is striking. As the frequency decreases, the output voltage increases until the lamp ignites.  FIG. 4   e  shows that at 65.8 KHz, the output reaches 3500Vp-p. The regulation of the CCFL current is achieved by adjusting the overlap to support 130 Kohm impedance after ignition. The voltage across the CCFL is now 1.9 KVp-p for a 660Vrms lamp. This is also illustrated in  FIG. 4   f . Although not shown, the emulation of the circuit of  FIG. 3  behaves in a similar manner. 
         [0042]    It should be noted that the difference between the first and second embodiments (i.e., by the addition of the flip flop and the PLL in  FIG. 3 ) will not effect the overall operational parameters set forth in  FIG. 4   a - FIG. 4   f . However, the addition of the PLL has been determined to account for non-ideal impedances that develop in the circuit, and may be added as an alternative to the circuit shown in  FIG. 2 . Also, the addition of the flip-flop permits the removal of the constant current circuit, described above. 
         [0043]    Thus, it is evident that there has been provided a high efficiency adaptive DC/AC converter circuit that satisfies the aims and objectives stated herein. It will be apparent to those skilled in the art that modifications are possible. For example, although the present invention has described the use of MOSFETs for the switched, those skilled in the art will recognize that the entire circuit can be constructed using BJT transistors, or a mix of any type of transistors, including MOSFETs and BJTs. Other modifications are possible. For example, the drive circuitry associated with Drive_B and Drive_D may be comprised of common-collector type circuitry, since the associated transistors are coupled to ground and are thus not subject to floating conditions. The PLL circuit described herein is can be a generic PLL circuit  70 , as is known in the art, appropriately modified to accept the input signal and generate the control signal, described above. The pulse generator  22  can be a pulse width modulation circuit (PWM) or frequency width modulation circuit (FWM), both of which are well known in the art. Likewise, the protection circuit  62  and timer are constructed out of known circuits and are appropriately modified to operate as described herein. Other circuitry will become readily apparent to those skilled in the art, and all such modifications are deemed within the spirit and scope of the present invention, only as limited by the appended claims.