text: 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 . 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 precisely control power delivered to a given load . 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 . the circuit diagram shown in fig2 illustrates one preferred embodiment of a phase - shift , full - bridge , zero - voltage - switching power converter of the present invention . essentially , the circuit shown in fig2 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 . 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 . preferably , 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 preferably is 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 is preferably 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 . while not wishing to be bound by example , in this embodiment , b_drive is preferably 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 ), it is preferred that 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 . 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 . the preferred switching operation of the overlapping switches 80 is shown with reference to the timing diagrams of fig2 a - 2 f . switch_c is turned off at certain period of the conduction of both switches b and c ( fig2 f ). the current flowing in the tank ( refer to fig2 ) is now flowing through diode d 4 ( fig2 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 ( fig2 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 ( fig2 a ), the current diverts to diode d 1 associated with switch_a before switch_a is turned on ( fig2 e ). likewise , switch_d is turned off ( fig2 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 ( fig2 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 fig2 f . in this embodiment , fig2 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 , preferably includes 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 ). 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 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 . for example : referring to fig2 b and 2 c and the feedback loop 40 of fig2 , 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 fig2 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 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 . 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 ). 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 is also preferably 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 preferably 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 fig2 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 ). to protect the circuit from an over - voltage condition , the present embodiment preferably includes 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 . preferably , voltage signal is derived from the voltage divider c 2 and c 3 ( i . e ., in parallel with load 20 ), as shown in fig2 . 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 . preferably , a timer 64 is initiated once the ovp exceeds the threshold , thereby initiating a time - out sequence . the duration of the time - out is preferably 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 . fig3 and 3 a - 3 f depict another preferred embodiment of the dc / ac circuit of the present invention . in this embodiment , the circuit operates in a similar manner as provided in fig2 and fig2 a - 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 fig3 . the pll circuit 70 preferably includes 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 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 . preferably , 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 . the operation of the feedback loop of fig3 is similar to the description above for fig2 . however , as shown in fig3 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 . referring briefly to fig3 a - 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 fig3 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 fig2 a - 2 f . referring now to fig4 a - 4 f , the output circuit of fig2 or 3 is emulated . for example , fig4 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 fig4 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 fig4 c . as another example , fig4 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 . fig4 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 fig4 f . although not shown , the emulation of the circuit of fig3 behaves in a similar manner . 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 fig3 ) will not effect the overall operational parameters set forth in fig4 a - 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 fig2 . also , the addition of the flip - flop permits the removal of the constant current circuit , described above . 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 preferably 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 is preferably 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 .