CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority of U.S. Provisional Application Ser. No. 61/267,638, filed Dec. 8, 2009, the entire contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to electronics, and more specifically, to electronic ballasts for light sources. 
       BACKGROUND 
       [0003]    Typically, a ballast provides power to a lamp and regulates the current and/or power provided to the lamp. Lamps, such as high intensity discharge (HID) lamps and fluorescent lamps, use a ballast to provide the proper starting voltage for the lamp and to limit the operating current once the lamp is ignited. A ballast generally includes power factor control (PFC) circuitry for sinusoidal input current control and generation of a regulated direct current (DC) bus voltage. A lamp driver, which comprises an inverter, converts the high DC voltage into a suitable AC voltage for energizing the lamp. A commutation period occurs each time the inverter changes the polarity of the voltage provided to the lamp. In conventional ballasts, the commutation period has a duration of around 100 microseconds. 
       SUMMARY 
       [0004]    The duration of the commutation periods in conventional ballasts can lead to problems related to lamp operation. Such problems include, but are not limited to, high Spectral Power Ratio (SPR), re-ignition spikes, poor lumen-maintenance, and audible humming noise originating from the lamp. Embodiments of the present invention provide a lamp driver circuit that limits the commutation period to a duration of less than 100 microseconds, and in so doing, removes and/or limits many of these problems. 
         [0005]    In an embodiment, there is provided a driver circuit. The driver circuit includes a plurality of switching components, wherein the plurality of switching components includes a first switching leg and a second switching leg, each connected between a first direct current voltage bus and a second direct current voltage bus, wherein the first switching leg and the second switching leg each include at least a first switching component and a second switching component, and wherein the first switching component of the first switching leg is connected to the second switching component of the second switching leg to form a first diagonal pair and the second switching component of the first switching leg is connected to the first switching component of the second switching leg to form a second diagonal pair. The driver circuit also includes a load circuit connecting the first switching leg and the second switching leg. The load circuit includes a first inductor connected to a terminal between the first switching component and the second switching component of the first switching leg; a second inductor connected to a terminal between the first switching component and the second switching component of the second switching leg; and lamp terminals connected between the first inductor and the second inductor and connected in series with the second inductor. The driver circuit also includes a capacitor connected in parallel with the series connected lamp terminals and the second inductor, and a control circuit connected to the plurality of switching components. During a commutation period, the control circuit is configured to operate the first diagonal pair in a non-conductive state and to operate the second diagonal pair in a conductive state, until a current through the first inductor reaches a predefined value, and then to operate the second diagonal pair in a non-conductive state until the current through the first inductor reaches zero. 
         [0006]    In a related embodiment, the first switching leg may include: a first switching component connected to the first direct current voltage bus; a first diode connected across the first switching component; a second switching component connected between the first switching component and the second direct current voltage bus; and a second diode connected across the second switching component; and the second switching leg may include: a third switching component connected to the first direct current voltage bus; a third diode connected across the third switching component; a fourth switching component connected between the third switching component and the second direct current voltage bus; and a fourth diode connected across the fourth switching component; such that the first switching component and the fourth switching component may form the first diagonal pair and the second switching component and the third switching component may form the second diagonal pair. 
         [0007]    In another related embodiment, the control circuit may be further configured to operate the plurality of switching components in a first operation mode for a first time period during which the current through the first inductor has a first polarity, and the control circuit may be configured to operate the switching components in a second operation mode for a second time period during which a current through the second inductor has a second polarity, and the commutation period may occur between the first and second time periods. 
         [0008]    In yet another related embodiment, the control circuit may be configured to operate the plurality of switching components so that the commutation period has a duration of less than 50 microseconds. In still another related embodiment, the driver circuit may further include a ballast and a high intensity discharge lamp connected across the lamp terminals. The ballast may include: an electromagnetic interference filter configured to receive alternating current voltage from a power source; a rectifier connected to the electromagnetic interference filter to convert the alternating current voltage to direct current voltage; and a power factor control circuit connected to the rectifier, the power factor control circuit having a first output connected to the first direct current voltage bus and a second output connected to the second direct current voltage bus, wherein the power factor control circuit may be configured to produce a high direct current voltage output across the first and second outputs. 
         [0009]    In another embodiment, there is provided a driver circuit. The driver circuit includes: a plurality of switching components, wherein the plurality of switching components includes a first switching leg and a second switching leg, each connected between a first direct current voltage bus and a second direct current voltage bus, wherein the first switching leg and the second switching leg each include at least a first switching component and a second switching component, and wherein the first switching component of the first switching leg is connected to the second switching component of the second switching leg to form a diagonal pair and the second switching component of the first switching leg is connected to the first switching component of the second switching leg to form a diagonal pair. The driver circuit also includes a load circuit connecting the first switching leg and the second switching leg, wherein the load circuit includes: a first inductor connected to a terminal between the first switching component and the second switching component of the first switching leg; a second inductor connected to a terminal between the first switching component and the second switching component of the second switching leg; and lamp terminals connected between the first inductor and the second inductor and connected in series with the second inductor. The driver circuit also includes a capacitor connected in parallel with the series connected lamp terminals and the second inductor; and a control circuit having a plurality of control outputs, wherein each control output in the plurality of control outputs is connected to a corresponding switching component in the plurality of switching components. The control circuit is configured to operate the plurality of switching components: (i) in a first operation mode for a first time period, during which a current through the first inductor has a first polarity; (ii) in a second operation mode for a second time period, during which the current through the first inductor has a second polarity that is opposite of the first polarity; and (iii) in a third operating mode during a commutation period, during which a polarity of a current through the capacitor is being reversed; wherein during the commutation period, the control circuit is configured to operate a first diagonal pair in a non-conductive state and to operate a second diagonal pair in a conductive state, until the current through the first inductor reaches a predefined value, and then to operate the second diagonal pair in a non-conductive state until the current through the first inductor reaches zero. 
         [0010]    In a related embodiment, the first switching leg may include: a first switching component connected to the first direct current voltage bus; a first diode connected across the first switching component; a second switching component connected between the first switching component and the second direct current voltage bus; and a second diode connected across the second switching component; and the second switching leg may include: a third switching component connected to the first direct current voltage bus; a third diode connected across the third switching component; a fourth switching component connected between the third switching component and the second direct current voltage bus; and a fourth diode connected across the fourth switching component; such that the first switching component and the fourth switching component may form a diagonal pair and the second switching component and the third switching component may form a diagonal pair. 
         [0011]    In another related embodiment, the first operation mode may be a positive cycle operation mode during which the current through the first inductor has a positive polarity, and the second operation mode may be a negative cycle operation mode during which the current through the first inductor has a negative polarity, and wherein the control circuit may be configured to operate in the third operating mode during transitions from the positive cycle operation mode to the negative cycle operation mode and from the negative cycle operation mode to the positive cycle operation mode. 
         [0012]    In yet another related embodiment, the third operation mode may be a positive-to-negative transition mode during a commutation period in which the polarity of the current through the capacitor is being changed from positive to negative, and during the positive-to-negative transition mode the first diagonal pair may be the diagonal pair of switching components comprising the first switching component of the first switching leg and the second switching component of the second switching leg, and the second diagonal pair of switching components may be the diagonal pair of switching components comprising the second switching component of the first switching leg and the first switching component of the second switching leg. 
         [0013]    In still another related embodiment, the third operation mode may be a negative-to-positive transition mode during a commutation period in which the polarity of the current through the capacitor is being changed from negative to positive, and during the negative-to-positive transition mode the first diagonal pair of switching components may be the diagonal pair of switching components comprising the second switching component of the first switching leg and the first switching component of the second switching leg and the second diagonal pair of switching components may be the diagonal pair of switching components comprising the first switching component of the first switching leg and the second switching component of the second switching leg. 
         [0014]    In yet still another related embodiment, the control circuit may be configured to operate the plurality of switching components so that the commutation period has a duration of less than 50 microseconds. 
         [0015]    In still yet another related embodiment, the driver circuit may further include a ballast and a high intensity discharge lamp connected across the lamp terminals, wherein the ballast may include: an electromagnetic interference filter configured to receive alternating current voltage from a power source; a rectifier connected to the electromagnetic interference filter to convert the alternating current voltage to direct current voltage; and a power factor control circuit connected to the rectifier, the power factor control circuit having a first output connected to the first direct current voltage bus and a second output connected to the second direct current voltage bus, wherein the power factor control circuit may be configured to produce a high direct current voltage output across the first and second outputs. 
         [0016]    In another embodiment, there is provided a ballast. The ballast includes: an electromagnetic interference filter configured to receive alternating current voltage from a power source; a rectifier connected to the electromagnetic interference filter to convert the alternating current voltage to direct current voltage; a power factor control circuit connected to the rectifier, the power factor control circuit having a first output and a second output, wherein the power factor control circuit is configured to produce a high direct current voltage output across the first and second outputs; a first direct current voltage bus connected to the first output of the power factor control circuit; a second direct current voltage bus connected to the second output of the power factor control circuit; a plurality of switching components, wherein the plurality of switching components includes a first switching leg and a second switching leg, each connected between a first direct current voltage bus and a second direct current voltage bus, wherein the first switching leg and the second switching leg each include at least a first switching component and a second switching component, and wherein the first switching component of the first switching leg is connected to the second switching component of the second switching leg to form a diagonal pair and the second switching component of the first switching leg is connected to the first switching component of the second switching leg to form a diagonal pair; a load circuit connecting the first switching leg and the second switching leg, wherein the load circuit includes: a first inductor connected to a terminal between the first switching component and the second switching component of the first switching leg; a second inductor connected to a terminal between the first switching component and the second switching component of the second switching leg; and lamp terminals connected between the first inductor and the second inductor and connected in series with the second inductor; a capacitor connected in parallel with the series connected lamp terminals and the second inductor; and a control circuit connected to the plurality of switching components, wherein during a commutation period the control circuit is configured to operate a first diagonal pair in a non-conductive state and to operate a second diagonal pair in a conductive state, until a current through the first inductor reaches a predefined value, and then to operate the second diagonal pair in a non-conductive state until the current through the first inductor reaches zero. 
         [0017]    In a related embodiment, the first switching leg may include: a first switching component connected to the first direct current voltage bus; a first diode connected across the first switching component; a second switching component connected between the first switching component and the second direct current voltage bus; and a second diode connected across the second switching component; and the second switching leg may include: a third switching component connected to the first direct current voltage bus; a third diode connected across the third switching component; a fourth switching component connected between the third switching component and the second direct current voltage bus; and a fourth diode connected across the fourth switching component; such that the first switching component and the fourth switching component may form a diagonal pair and the second switching component and the third switching component may form a diagonal pair. 
         [0018]    In another related embodiment, the ballast may further include an electrolytic capacitor connected in a shunt configuration across the first and second outputs of the power factor control circuit. In yet another related embodiment, the ballast may further include a high intensity discharge (HID) lamp connected across the lamp terminals. 
         [0019]    In still another related embodiment, the control circuit may be further configured to operate the plurality of switching components in a positive operation mode for a first time period during which the current through the first inductor has a positive polarity, and the control circuit may be configured to operate the plurality of switching components in a negative operation mode for a second time period during which a current through the second inductor has a negative polarity, and the commutation period may occur between the first and second time periods. 
         [0020]    In yet still another related embodiment, during a commutation period in which the polarity of a current through the capacitor is being changed from positive to negative, the first diagonal pair may be the diagonal pair of switching components comprising the first switching component of the first switching leg and the second switching component of the second switching leg, and the second diagonal pair of switching components may be the diagonal pair of switching components comprising the second switching component of the first switching leg and the first switching component of the second switching leg. 
         [0021]    In still yet another related embodiment, during a commutation period in which a polarity of a current through the capacitor is being changed from negative to positive, the first diagonal pair of switching components may be the diagonal pair of switching components comprising the second switching component of the first switching leg and the first switching component of the second switching leg, and the second diagonal pair of switching components may be the diagonal pair of switching components comprising the first switching component of the first switching leg and the second switching component of the second switching leg. 
         [0022]    In yet another related embodiment, the control circuit may be configured to operate the plurality of switching components so that the commutation period has a duration of less than 50 microseconds. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The foregoing and other objects, features and advantages disclosed herein will be apparent from the following description of particular embodiments disclosed herein, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles disclosed herein. 
           [0024]      FIG. 1  is a schematic of a lamp system including a ballast with an inverter for use with an input power source to energize a lamp according to embodiments disclosed herein. 
           [0025]      FIGS. 2A-5B  each illustrate exemplary current paths through an inverter of the ballast of the lamp system of  FIG. 1 . 
           [0026]      FIG. 6  is a flow chart illustrating an exemplary sequence of operating modes according to embodiments disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 1  illustrates a lamp system  100  according to embodiments described herein. The lamp system  100  includes an input power source  102 , such as an alternating current (AC) power source, an electronic ballast  104 , and a lamp  106 . The lamp system  100  described herein is used to energize, for example, one or more HID lamps  106 . Exemplary HID lamps include, but are not limited to, mercury vapor, metal halide, high-pressure sodium, and low-pressure sodium lamps. The lamp system  100  may be used to energize other types of lamps, such as a fluorescent lamp, without departing from the scope of the invention. 
         [0028]    The electronic ballast  104  includes one or more input terminals adapted to connect to the input power source  102  and a ground terminal connectable to ground potential. In some embodiments, the input power source  102  includes a first voltage source and a second voltage source, and the electronic ballast  104  is operatively connected to either the first voltage source or the second voltage source. Thus, the electronic ballast  104  may selectively receive power from either the first voltage source (e.g., 208 volts AC) or the second voltage source (e.g., 347 volts, 480 volts). Other input power sources  102  known in the art may be used without departing from the scope of the present invention. 
         [0029]    The electronic ballast  104  receives an input AC power signal from the input power source  102  via the input terminal. In some embodiments, the electronic ballast  104  includes an electromagnetic interference (EMI) filter and a rectifier (e.g., full-wave rectifier), illustrated generally at  110 . The EMI filter prevents noise, which may be generated by the electronic ballast  104 , from being transmitted back to the input power source  102 . The rectifier converts AC voltage of the input power signal to DC (direct current) voltage. 
         [0030]    The electronic ballast  104  includes a power stage to convert power supplied by the input power source  102  to drive the lamp  106 . In  FIG. 1 , the electronic ballast  104  includes a first power stage comprising a power factor control circuit  112 . The power factor control circuit  112 , such as a boost converter, receives the rectified input power signal and produces a high DC voltage (e.g., 460 volts DC). The electronic ballast  104  also includes a second power stage comprising a lamp driver (broadly, lamp driver and ignition circuit)  114 . In particular, as described below, the lamp driver  114  includes an inverter circuit that converts the high DC voltage into a suitable AC voltage to energize the lamp  106 . A capacitor  118 , such as an electrolytic capacitor, is connected in a shunt configuration between the first power stage (e.g., power factor control circuit  112 ) and the second power stage (e.g., lamp driver  114 ) to provide a low impedance source of voltage to the inverter. The electronic ballast  104  includes a controller  120  (broadly referred to throughout as a “control circuit”) to control the operations of the components of the electronic ballast  104 . In some embodiments, the controller  120  has one or more control outputs which electrically connect the controller  120  to the lamp driver  114 . For example, the controller  120  may be a microcontroller that may have control outputs that are electrically connected to the lamp driver  114  via driver control circuits. 
         [0031]    In  FIG. 1 , the lamp driver (broadly referred to throughout as a “driver circuit”)  114  comprises a first DC voltage bus  122  and a second DC voltage bus  124  (e.g., high DC voltage bus and low DC voltage bus, respectively) connected to the power factor control circuit  112  and the capacitor  118 . A first switching leg and a second switching leg are each connected between the first DC voltage bus  122  and the second DC voltage bus  124 . The first switching leg includes a first switching component Q 1  connected to the first DC voltage bus  122 , and a first diode D 1  connected (e.g., shunted) across the first switching component Q 1 . The first switching leg also includes a second switching component Q 2  connected between the first switching component Q 1  and the second DC voltage bus  124 , and a second diode D 2  connected (e.g., shunted) across the second switching component Q 2 . Thus, the first switching component Q 1  in combination with the first diode D 1  is connected in series with the second switching component Q 2  in combination with the second diode D 2 . 
         [0032]    Similarly, the second switching leg includes a third switching component Q 3  connected to the first DC voltage bus  122 , and a third diode D 3  connected (e.g., shunted) across the third switching component Q 3 . The second switching leg also includes a fourth switching component Q 4  connected between the third switching component Q 3  and the second voltage bus  124 , and a fourth diode connected (e.g., shunted) across the fourth switching component Q 4 . Thus, the third switching component Q 3  in combination with the third diode D 3  is connected in series with the fourth switching component Q 4  in combination with the fourth diode D 4 . In some embodiments, each of the switching components Q 1 , Q 2 , Q 3 , and Q 4  is a metal-oxide-semiconductor field-effect transistor (MOSFET). However, the scope of the present invention is not limited to a particular type of switch. 
         [0033]    A load circuit  126  connects the first switching leg to the second switching leg. In particular, a first inductor L buck  is connected to a terminal  128  between the first switching component Q 1  and the second switching component Q 2 . A second inductor (e.g., igniter) is connected to a terminal  130  between the third switching component Q 3  and the fourth switching component Q 4 . Load terminals  132  are connected between the first inductor L buck  and the second inductor (e.g., igniter) and are adapted for connecting a load, such as a lamp  106 , in series with the first inductor L buck  and the second inductor (e.g., igniter). A capacitor C buck  is connected in parallel with the series arrangement of the lamp terminals  132  and the second inductor (e.g., igniter). 
         [0034]    As shown in  FIG. 1 , the controller  120  includes a first control output  134  connected to the first switching component Q 1 , a second control output  136  connected to the second switching component Q 2 , a third control output  138  connected to the third switching component Q 3 , and a fourth control output  140  connected to the fourth switching component Q 4 . In operation, the controller  120  operates the first, second, third, and fourth switching components Q 1 , Q 2 , Q 3 , Q 4  via the first, second, third, and fourth control outputs  134 ,  136 ,  138 ,  140 , respectively, in diagonal pairs in order to convert the high DC voltage generated by the power factor control circuit  112  to an AC voltage signal suitable to energize the lamp  106 . The AC voltage signal is comprised of positive cycles and negative cycles. In particular, the first switching component Q 1  and the fourth switching component Q 4  form a diagonal pair that is operated to generate the positive cycles of the AC voltage signal. Similarly, the second switching component Q 2  and the third switching component Q 3  form a diagonal pair that is operated to generate the negative cycles of the AC voltage signal. 
         [0035]    Accordingly, the controller  120  is configured to operate the switching components Q 1 , Q 2 , Q 3 , and Q 4  according to a plurality of operation modes. More particularly, the controller  120  is configured to operate the switching components Q 1 , Q 2 , Q 3 , and Q 4  in a first operation mode for a first time period (e.g., T 1 ) during which the current through or voltage across the first inductor L buck  has a first polarity (e.g., positive, negative). The controller  120  is configured to operate the switching components Q 1 , Q 2 , Q 3 , and Q 4  in a second operation mode for a second time period (e.g., T 2 ) during which the current through or voltage across the first inductor L buck  has a second polarity (e.g., positive, negative) that is opposite of the first polarity. For example, the first and second operation modes may be the positive cycle operation mode and the negative cycle operation mode described below. The controller  120  is configured to operate the switching components Q 1 , Q 2 , Q 3 , and Q 4  in a third operation mode for a third time period (e.g., T 3 ). The third time period is a commutation period during which a polarity of the current through or voltage across the capacitor is being reversed. The third time period (e.g., commutation period) represents a transition time needed to switch the polarity of the current through or voltage across the capacitor C buck . Thus, there will be a commutation period each time the controller  120  transitions from the first time period T 1  to the second time period T 2  and from the second time period T 2  back to the first time period T 1 . 
         [0036]    More particularly, in some embodiments, the controller  120  is configured to operate the switching components in a positive cycle operation mode, a negative cycle operation mode, a positive-to-negative transition operation mode, and a negative-to-positive transition mode operation. During the positive cycle operation mode, the controller  120  operates the inverter (i.e., switching components Q 1 , Q 2 , Q 3 , Q 4 ) so that a positive portion of an AC voltage signal is provided to the lamp  106 . During the negative cycle operation mode, the controller  120  operates the inverter (i.e., switching components Q 1 , Q 2 , Q 3 , Q 4 ) so that a negative portion of an AC voltage signal is provided to the lamp  106 . During the positive-to-negative transition operation mode, the controller  120  operates the inverter (i.e., switching components Q 1 , Q 2 , Q 3 , Q 4 ) to transition the polarity of the AC voltage signal provided to the lamp  106  from positive to negative. Similarly, during the negative-to-positive transition operation mode, the controller  120  operates the inverter (i.e., switching components Q 1 , Q 2 , Q 3 , Q 4 ) to transition the polarity of the AC voltage signal provided to the lamp  106  from negative to positive. The switching sequence described below employed by the controller  120  during the positive-to-negative transition operation mode and during the negative-to-positive transition mode minimizes commutation time. As such, embodiments of the present invention reduce Spectral Power Ratio (SPR), re-ignition spikes, poor lumen-maintenance, and audible humming noise produced by the lamp  106 . 
         [0037]    Referring to  FIGS. 2A and 2B , during the positive cycle operation mode, the controller  120  operates the inverter (i.e., switching components Q 1 , Q 2 , Q 3 , Q 4 ) between two different states. In the first state (i.e., positive cycle state  1 ), illustrated in  FIG. 2A , the diagonal pair of switching components comprising the second switching component Q 2  and the third switching component Q 3 , is non-conductive. The diagonal pair of switching components comprising the first switching component Q 1  and the fourth switching component Q 4 , is conductive. As such, current flows in a path from the first DC voltage bus and through the first switching component Q 1  and the first inductor L buck . The current path then divides so that current flows through the capacitor C buck  and the lamp  106  and second inductor (i.e., igniter). The current path continues through the fourth switching component Q 4  to the second DC voltage bus. Accordingly, the current through the first inductor L buck  rises and a positive voltage is generated across the lamp  106 . 
         [0038]    In the second state during the positive cycle operation mode (i.e., positive cycle state  2 ), illustrated in  FIG. 2B , the second switching component Q 2  and the third switching component Q 3  remain non-conductive. The first switching component Q 1  is turned OFF so that it is non-conductive. Energy stored by the first inductor L buck  dissipates to generate a decreasing current that flows in a closed loop through the capacitor C buck , the lamp  106 , and the second inductor (i.e., igniter) to the fourth switching component Q 4 , and then from the anode to the cathode of the second diode D 2 . As such, a positive voltage is maintained across the lamp  106 . When the current through the first inductor L buck  reaches zero, the controller  120  returns the inverter to the positive cycle state  1 . As such, the first switching component Q 1  is turned ON so that it is conductive. 
         [0039]    Subsequent to the positive cycle operation mode, which, in some embodiments, includes either positive cycle state  1  or positive cycle state  2 , and in other embodiments, includes both positive cycle state  1  and positive cycle state  2 , the positive-to-negative transition mode is initiated. Referring to  FIGS. 3A and 3B , during the positive-to-negative transition mode, the controller  120  operates the inverter (i.e., switching components Q 1 , Q 2 , Q 3 , Q 4 ) between two different states. In the first state (i.e., positive-to-negative state  1 ) illustrated in  FIG. 3A , the diagonal pair of switching components comprising the first switching component Q 1  and the fourth switching component Q 4 , is non-conductive. The diagonal pair of switching components comprising the second switching component Q 2  and the third switching component Q 3 , is conductive. As such, current flows in path from the first DC voltage bus and through the third switching component Q 3 . The current path then divides so that current flows through capacitor C buck , the lamp  106 , and the second inductor (i.e., igniter). The current path continues through the first inductor L buck , through the second switching component Q 2  to the second DC voltage bus. Accordingly, the current through the first inductor L buck  rises. 
         [0040]    When the current through the first inductor L buck  reaches a predefined value (e.g., peak value), the second state (i.e., positive-to-negative state  2 ) of the positive-to-negative transition mode is initiated. In the positive-to-negative state  2 , illustrated in  FIG. 3B , each of the switching components Q 1 , Q 2 , Q 3 , and Q 4  are non-conductive. As such, energy stored by the first inductor L buck  dissipates to generate a decreasing current that flows in a path from the second DC voltage bus and through the fourth diode D 4 . The current path divides so that current flows through the capacitor C buck , the lamp  106 , and the second inductor (i.e., igniter). The current path then continues through the first inductor L buck , through the first diode D 1 , to the first DC voltage bus. When the current through the first inductor L buck  reaches zero, the controller  120  returns the inverter to the positive-to-negative state  1 . 
         [0041]    Referring to  FIGS. 4A and 4B , during the negative cycle operation mode, the controller  120  operates the inverter (i.e., switching components Q 1 , Q 2 , Q 3 , Q 4 ) between two different states. In the first state (i.e., negative cycle state  1 ), illustrated in  FIG. 4A , the diagonal pair of switching components comprising the first switching component Q 1  and the fourth switching component Q 4 , is non-conductive. The diagonal pair of switching components comprising the second switching component Q 2  and the third switching component Q 3 , is conductive. As such, current flows from the first DC voltage bus and through the third switching component Q 3 . The current path then divides so that current flows through the capacitor C buck , the lamp  106 , and the second inductor (i.e., igniter). The current path continues through the first inductor L buck , and the second switching component Q 2  to the second DC voltage bus. Accordingly, the current through the first inductor L buck  rises and a negative voltage is generated across the lamp  106 . 
         [0042]    In the second state during the negative cycle operation mode (negative cycle state  2 ), illustrated in  FIG. 4B , the first switching component Q 1  and the fourth switching component Q 4  remain non-conductive. The second switching component Q 2  is turned OFF so that it is non-conductive. Energy stored by the first inductor L buck  dissipates to generate a decreasing current that flows in a closed loop from the anode to the cathode of the first diode D 1 , through the third switching component Q 3 , and then through the capacitor C buck , the lamp  106 , and the second inductor (i.e., igniter). As such, a negative voltage is maintained across the lamp  106 . When the current through the first inductor L buck  reaches zero, the controller  120  returns the inverter to the negative cycle state  1 . As such, the second switching component Q 2  is turned ON so that it is conductive. 
         [0043]    Subsequent to the negative cycle operation mode, which, in some embodiments, includes either negative cycle state  1  or negative cycle state  2 , and in other embodiments, includes both negative cycle state  1  and negative cycle state  2 , the negative-to-positive transition mode is initiated. Referring to  FIGS. 5A and 5B , during the negative-to-positive transition mode, the controller  120  operates the inverter (i.e., switching components Q 1 , Q 2 , Q 3 , Q 4 ) between two different states. In the first state (i.e., negative-to-positive state  1 ), illustrated in  FIG. 5A  the diagonal pair of switching components comprising the second switching component Q 2  and the third switching component Q 3 , is non-conductive. The diagonal pair of switching components comprising the first switching component Q 1  and the fourth switching component Q 4 , is conductive. As such, current flows in path from the first DC voltage bus through the first switching component Q 1 , and through the first inductor L buck . The current path then divides so that current flows through capacitor C buck , the lamp  106  and the second inductor (i.e., igniter). The current path continues through the fourth switching component Q 4  to the second DC voltage bus. Accordingly, the current through the first inductor L buck  rises. 
         [0044]    When the current through the first inductor L buck  reaches a predefined value (e.g., peak value), the second state (i.e., negative-to-positive state  2 ) of the negative-to-positive transition mode is initiated. In the negative-to-positive state  2 , illustrated in  FIG. 5B , each of the switching components Q 1 , Q 2 , Q 3 , and Q 4  is non-conductive. As such, energy stored by the first inductor L buck  dissipates to generate a decreasing current that flows in a path from the second DC voltage bus, through the second diode D 2 , and through the first inductor L buck . The current path is then divided so that current flows through the capacitor C buck , the lamp  106  and the second inductor (i.e., igniter). The current path continues through the third diode D 3  to the first DC voltage bus. When the current through the first inductor L buck  reaches zero, the controller  120  returns the inverter to the negative-to-positive state  1 . 
         [0045]      FIG. 6  is a flow chart illustrating exemplary operations modes implemented by the controller  120  for providing an AC voltage signal to the lamp  106  in accordance with an embodiment of the invention. At  202 , the controller  120  initiates operation of the inverter in the positive cycle operation mode. The positive cycle operation mode has a predefined duration period, T positive . In an exemplary embodiment, T positive  has a predefined value of 3.2 milliseconds. When the positive cycle operation mode is initiated, the controller  120  operates the inverter, indicated at  202 , in the positive cycle state  1  for a time period of t pos     —     cycle     —     state1 . In an exemplary embodiment, the time period t pos     —     cycle     —     state1  has a pre-defined value of 5.0 μsec. As indicated at  204  if the time period T positive  for the positive cycle operation mode has not expired, the controller  120  then, at  206 , operates the inverter in the positive cycle state  2  for a time period of t pos     —     cycle     —     state2 . In an exemplary embodiment, the time period t pos     —     cycle     —     state2  has a pre-defined value of 12.0 μsec which is based on the amount of time that it takes for the current through the first inductor L buck  to reach zero. After operating the inverter in the positive cycle state  2  for the time period t pos     —     cycle     —     state2 , the controller  120  returns the inverter to the positive cycle state  1  at  202 . The controller  120  continues to alternately operate the inverter between the positive cycle state  1  and the positive cycle state  2  for the duration of the positive cycle operation mode time period T positive . As such, according to the exemplary embodiment, during the positive cycle operation mode, the first switching component Q 1  is switched at a frequency of 58.8 kHz and the fourth switching component Q 4  is switched at a frequency of 156.25 Hz. 
         [0046]    When the positive cycle operation mode time period T positive  expires, the controller  120  initiates a positive-to-negative transition operation mode. The positive-to-negative transition operation mode has a predefined duration period, T pos-to-neg . In an exemplary embodiment, T pos-to-neg  has a predefined value of around 46 microseconds. When the positive-to-negative transition operation mode is initiated, the controller  120  operates the inverter, indicated at  208 , in the positive-to-negative state  1  for a time period of t pos-neg     —     state1 . The time period t pos-neg     —     state1  is a function of the peak value I MAX  for the current that is passed through the first inductor L buck  causing the saturation of the first inductor L buck . In particular the value of the time period t pos-neg     —     state1  is given as follows 
         [0000]    
       
         
           
             
               t 
               
                 pos 
                 - 
                 
                   
                     neg 
                      
                     _ 
                      
                     state 
                   
                    
                   
                       
                   
                    
                   1 
                 
               
             
             = 
             
               
                 
                   L 
                   BUCK 
                 
                  
                 
                   I 
                   MAX 
                 
               
               
                 
                   DC 
                    
                   
                       
                   
                    
                   Bus 
                    
                   
                       
                   
                    
                   Voltage 
                 
                 + 
                 
                   Lamp 
                    
                   
                       
                   
                    
                   Voltage 
                 
               
             
           
         
       
     
         [0047]    The peak value I MAX  for the current is based on the inductance value of the first inductor L buck . In an exemplary embodiment, the inductance value of the first inductor L buck  is 285 micro Henrys, and the peak value I MAX  for the current is accordingly about 9 Amps. In the exemplary embodiment, the DC bus voltage is 460 Volts and the lamp voltage is 135 Volts. As such, in accordance with the exemplary embodiment, the duration for the positive-to negative state  1  t pos-neg     —     state1  is about 4.3 microseconds. 
         [0048]    As indicated at  210  if the time period T pos-to-neg  for the positive-to-negative transition operation mode has not expired, the controller  120  then, at  212 , operates the inverter in the positive-to-negative state  2  for a time period of t pos-neg     —     state2 . In an exemplary embodiment, the time period t pos-neg     —     state2  has a pre-defined value of 7.9 μsec, which is based on the amount of time that it takes for the current through the first inductor L buck  to reach zero. After operating the inverter in the positive-to-negative state  2  for the time period t pos-neg     —     state2 , the controller  120  returns the inverter to the positive-to-negative state  1  at  208 . The controller  120  continues to alternately operate the inverter between the positive-to-negative state  1  and the positive-to-negative state  2  for the duration of the positive-to-negative transition operation mode time period T pos-to-neg . 
         [0049]    When the positive-to-negative transition operation mode time period T pos-to-neg  expires, the controller  120  initiates the negative cycle operation mode. The negative cycle operation mode has a predefined duration period, T negative . In an exemplary embodiment, T negative  has a predefined value of 3.2 milliseconds. When the negative cycle operation mode is initiated, the controller  120  operates the inverter, indicated at  214 , in the negative cycle state  1  for a time period of t neg     —     cycle     —     state1 . In an exemplary embodiment, the time period t neg     —     cycle     —     state1  has a pre-defined value of 5.0 μsec. As indicated at  216  if the time period T negative  for the negative cycle operation mode has not expired, the controller  120  then, at  218 , operates the inverter in the negative cycle state  2  for a time period of t neg     —     cycle     —     state2 . In an exemplary embodiment, the time period t neg     —     cycle     —     state2  has a pre-defined value of 12.0 μsec which is based on the amount of time that it takes for the current through the first inductor L buck  to reach zero. After operating the inverter in the negative cycle state  2  for the time period t neg     —     cycle     —     state2 , the controller  120  returns the inverter to the negative cycle state  1  at  214 . The controller  120  continues to alternately operate the inverter between the negative cycle state  1  and the negative cycle state  2  for the duration of the negative cycle operation mode time period T negative . As such, according to the exemplary embodiment, during the negative cycle operation mode, the second switching component Q 2  is switched at a frequency of 58.8 kHz and the third switching component Q 3  is switched at a frequency of 156.25 Hz. 
         [0050]    When the negative cycle operation mode time period T negative  expires, the controller  120  initiates negative-to-positive transition operation mode. The negative-to-positive transition operation mode has a predefined duration period, T neg-to-pos . In an exemplary embodiment, T neg-to-pos  has a predefined value of around 46 microseconds. When the negative-to-positive transition operation mode is initiated, the controller  120  operates the inverter, indicated at  220 , in the negative-to-positive state  1  for a time period of t neg-pos     —     state1 . The time period t neg-pos     —     state1  is a function of the peak value I MAX  for the current that is passed through the first inductor L buck  causing the saturation of the first inductor L buck . As similarly described above in connection with the positive-to-negative state  1 , the value of the time period t neg-pos     —     state1  is given as follows 
         [0000]    
       
         
           
             
               t 
               
                 neg 
                 - 
                 
                   
                     pos 
                      
                     _ 
                      
                     state 
                   
                    
                   
                       
                   
                    
                   1 
                 
               
             
             = 
             
               
                 
                   L 
                   BUCK 
                 
                  
                 
                   I 
                   MAX 
                 
               
               
                 
                   DC 
                    
                   
                       
                   
                    
                   Bus 
                    
                   
                       
                   
                    
                   Voltage 
                 
                 + 
                 
                   Lamp 
                    
                   
                       
                   
                    
                   Voltage 
                 
               
             
           
         
       
     
         [0051]    The peak value I MAX  for the current is based on the inductance value of the first inductor L buck . In an exemplary embodiment, the inductance value of the first inductor L buck  is 285 micro Henrys, and the peak value I MAX  for the current is accordingly about 9 Amps. In the exemplary embodiment, the DC bus voltage is 460 Volts and the lamp voltage is 135 Volts. As such, in accordance with the exemplary embodiment, the duration for the negative-to-positive state  1  t neg-pos     —     state1  is about 4.3 microseconds. 
         [0052]    As indicated at  222  if the time period T neg-to-pos  for the negative-to-positive transition operation mode has not expired, the controller  120  then, at  224 , operates the inverter in the negative-to-positive state  2  for a time period of t pos-neg     —     state2 . In an exemplary embodiment, the time period t pos-neg     —     state2  has a pre-defined value of 7.9 μsec, which is based on the amount of time that it takes for the current through the first inductor L buck  to reach zero. After operating the inverter in the positive-to-negative state  2  for the time period t pos-neg     —     state2 , the controller  120  returns the inverter to the positive-to-negative state  1  at  220 . The controller  120  continues to alternately operate the inverter between the negative-to-positive state  1  and the negative-to-positive state  2  for the duration of the negative-to-positive transition operation mode time period T neg-to-pos . 
         [0053]    When the negative-to-positive transition operation mode time period T neg-to-pos  expires, the controller  120  returns to the positive cycle operation mode. The controller  120  continues to cycle through the positive cycle operation mode, positive-to-negative transition mode, negative cycle operation mode, and negative-to-positive transition mode as described above in order to provide the AC voltage signal to the lamp  106 . 
         [0054]    Unless otherwise stated, use of the word “substantially” may be construed to include a precise relationship, condition, arrangement, orientation, and/or other characteristic, and deviations thereof as understood by one of ordinary skill in the art, to the extent that such deviations do not materially affect the disclosed methods and systems. 
         [0055]    Throughout the entirety of the present disclosure, use of the articles “a” and/or “an” and/or “the” to modify a noun may be understood to be used for convenience and to include one, or more than one, of the modified noun, unless otherwise specifically stated. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0056]    Elements, components, modules, and/or parts thereof that are described and/or otherwise portrayed through the figures to communicate with, be associated with, and/or be based on, something else, may be understood to so communicate, be associated with, and or be based on in a direct and/or indirect manner, unless otherwise stipulated herein. 
         [0057]    Although the methods and systems have been described relative to a specific embodiment thereof, they are not so limited. Obviously many modifications and variations may become apparent in light of the above teachings. Many additional changes in the details, materials, and arrangement of parts, herein described and illustrated, may be made by those skilled in the art.
A driver circuit includes a plurality of switches, forming two switching legs, each of at least two switches and connected between two DC voltage buses. The switches are matched to form diagonal pairs. The driver circuit also includes a load circuit connecting the legs, with a first inductor connected between one leg's switches, and a second inductor connected between the other leg's switches, and lamp terminals between the inductors and in series with the second inductor. The driver circuit also includes a capacitor in parallel with the series-connected lamp terminals and the second inductor, and a control circuit connected to the plurality of switches. During a commutation period, a diagonal pair operates in a non-conductive state and the other in a conductive state, until a current through the first inductor reaches a predefined value. Then the other operates in a non-conductive state until the current through the first inductor reaches zero.