Abstract:
An inverter circuit includes an input section configured to receive voltage from a voltage source and to input the voltage to the circuit. A switching network is connected to receive the input voltage from the input section. A controller controls operation of the switching network and load connections are connected to the resonant switching circuit. A variable capacitance network is series-connected to the load connection to provide a variable capacitance during circuit operation. 
     A method includes passing a supplied voltage to a switching network which is controlled by a controller, and which delivers a lamp voltage to a lamp. A voltage in a capacitor series-connected to the lamp is clamped at predetermined levels, acting to remove a fixed capacitor from the circuit or at least a portion of a cycle of operation of the circuit, wherein an effective variable circuit capacitance is obtained by operation of the clamping action.

Description:
BACKGROUND OF THE INVENTION 
     The present application is directed to inverter circuits used in the powering of discharge lamps, and more particularly to a third order high Q impedance matching inverter circuit with automatic line regulation electronic ballast for use with high power discharge lamps operating on a low input voltage. 
     Turning to FIG. 1, shown is a known, rapid start, second-order inverter circuit topology used for powering high power, low impedance discharge lamps. Such a circuit will have a 1 to 1.5 second delay between application of a starting signal and lamp ignition. Circuit  10  includes a full bridge input section  12  which receives an input from AC source  14 . The output of the full bridge section  12  is provided to a half bridge switching circuit network  16 , comprised of a first transistor switch  18 , a second transistor switch  20 , and a controller  21 . Output voltage from the half bridge switching circuit  16  is delivered to a resonant LC network  22 , including a resonant inductor  24  and a resonant capacitor  26 . The output from LC circuit  22  is provided to a lamp  28 , which is further connected to capacitive voltage divider network  30 , composed of capacitor  32  and capacitor  34 . A starting voltage of approximately 600 volts may be used as the ignition voltage. In this type of circuit, since the striking voltage is commonly only 600 volts, a preheat circuit (not shown) may be included to preheat the lamp prior to supplying the ignition voltage. 
     A drawback of the circuit depicted in FIG. 1 is that it is not designed to operate efficiently with high impedance lamps. This is due, in part, to the use of lower input voltage. For example, when the input is a standard 120 volts, the circuit bus voltage may be about 150-160 volts. The AC voltage is approximately halved, due to the operation of switching network  18 , causing the AC output at the half-bridge switching network  18  to be approximately 75 volts. This voltage is sufficient to efficiently operate a low impedance lamp. However, if the lamp is a high impedance lamp, circuit  10  will need to draw an increased current, causing inefficient operation and stress on the components within the circuit. 
     Another drawback of the circuit in FIG. 1, is that in order to obtain an acceptable Q rating, if attempting to drive a high impedance lamp, a significantly higher voltage needs to be supplied to the lamp. In this situation, to obtain the desired Q rating, a larger sized resonant capacitor  26  and resonant inductor  24  is needed. 
     Further, the rapid start circuit  10  of FIG. 1, will maintain the preheat circuit active even after ignition of the lamp, resulting in a loss of about 1 to 1.5 watts of power. 
     If circuit  10  is attempted to be operated as an instant start lighting system, then the lamp starting voltage will be approximately 1300 volts. This higher voltage will need a higher resonant current, approximately 5 amps. The higher the current, the greater the stress on the inductor  24 , requiring a larger sized component. Increasing the size of the magnetics (i.e., inductor  24 ) increases the cost of the magnetics, and increases the size of the housing in which the magnetics are held. The same switching current will also be seen by the half-bridge switching network  16 , which includes transistors  18  and  20 . To handle these higher currents, larger sized dies will be necessary, and therefore larger packages for transistors  18  and  20  will be used (the transistors may be FET, CMOS, bipolar or other appropriate transistor type). These larger, more robust transistors and capacitors carry an increased economic cost, require a larger physically sized lamp lighting system, as well resulting in decreased circuit efficiency. 
     Thus, if the second order inverter circuit  10  of FIG. 2 is attempted to be used to drive high impedance lamps, a large starting current would be needed. It is known that when the starting current is higher, larger magnetics (i.e., inductor  24 ), and transistors will be needed to handle the higher current, resulting in a less efficient lamp lighting system. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present application, an inverter circuit includes an input section configured to receive voltage from a voltage source and to input the voltage to the circuit. A switching network is connected to receive the input voltage from the input section. A controller is placed in operational connection with the switching network and is designed to control operation of the switching network. A resonant switching circuit is configured to receive an output from the switching network. Load connections are connected to the resonant switching circuit. A variable capacitance network is connected to the load connection to provide a variable capacitance during circuit operation. 
     In accordance with another aspect of the present application, a method is provided for operating an inverter circuit, including supplying a voltage from a voltage source to an input section. The received voltage is passed from the input section to a switching network. Operation of the switching network is being controlled by a controller, wherein a prescribed voltage is transmitted to a resonant circuit and a lamp voltage is delivered to a lamp connected to the resonant circuit. A voltage in a capacitor is clamped at predetermined levels. The clamping action acts to remove a fixed capacitor from the circuit or at least a portion of a cycle of operation of the circuit, wherein an effective variable circuit capacitance is obtained by operation of the clamping action. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 shows a second-order inverter circuit topology; 
     FIG. 2 is a block circuit diagram of a circuit in accordance with the concepts of the present application; 
     FIG. 3 is a first embodiment of a circuit topology for a third-order inverter circuit with automatic line regulation in accordance with the present application; 
     FIG. 4 is the voltage across a capacitor in the circuit of the present application to illustrate a lamp&#39;s current sensitivity in the present circuit; 
     FIG. 5 shows a second embodiment of a third-order inverter circuit with integrated circuit control for open- or closed-loop operation; 
     FIG. 6 depicts a third embodiment of a third-order inverter circuit with a complementary pair of FETs; 
     FIG. 7 depicts a fourth embodiment of a third-order inverter circuit employing bipolar transistors; 
     FIG. 8 depicts a full-bridge switching network circuit in accordance with the concepts of the present application; and 
     FIG. 9 depicts a single switch network incorporating the concepts of the presents application. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The second-order inverter circuit  10  of FIG. 1 may be attempted to be used as a third-order inverter circuit if the values of capacitors  32  and  34  are made much smaller in size, or removed from the circuit. Particularly, when operating as a second-order circuit, the capacitor network  30 , including capacitors  32  and  34  act as a voltage divider to the lamp, and to store energy of the circuit. In one embodiment, which uses a 120 volt input, this may be accomplished by using capacitive values above approximately 100 nanofarads. However, if capacitors  32  and  34  are within a range from approximately 100 nanofarads down to about 5 nanofarads, the capacitor network  30  acts not only as a voltage divider/energy storage circuit, but it also becomes part of the resonant circuit (including resonant inductor  24  and resonant capacitor  26 ). This will change the circuit of FIG. 1 from a second-order inverter circuit to a third-order inverter circuit. 
     However, a circuit configured in this manner will have poor regulation during operation. For example, with an input voltage change of 10 percent, the power change may be from 20 to 25 percent. This instability continues to increase as the changes in the circuit input increase, causing stress on the circuit components, and wasting of energy. Additionally, operating the circuit  10  of FIG. 1 as a third-order inverter will result in a circuit highly sensitive not only to voltage input fluctuations but also to component variations. Particularly, a component out of specification, or even within the tolerance rating, may undesirably vary circuit operation. To control the undesirable variations, complex controls implementing IC controllers or other components would be needed to obtain some circuit stability. These drawbacks have limited practical applications of a third-order inverter operation for a circuit designed as shown in FIG. 1 in low-cost environments. This again is due to the sensitivity of the circuit to powerline variations, component variations, as well as impedance variations. 
     However, it is understood from this discussion that a third-order circuit has desirable aspects, including the benefit of being able to efficiently drive a high impedance lamp, with a low starting current. This is, in part, due to the use a resonant capacitance, much smaller than used in a second order circuit. The smaller capacitances result in smaller current values, which permit the use of a smaller inductor  24  and transistors  18  and  20 . 
     Turning to FIG. 2, depicted is a circuit block diagram  40  which represents a third-order inverter circuit according to the concepts of the present application. Signals, such as from a full bridge diode bridge (not shown) or other appropriate network, are delivered to a switching circuit block  42 . This switching network may be a single, half-bridge, full-bridge or other appropriate network designed to implement the concepts of the present application. Circuit block  42  provides a voltage to an inductor/capacitor/capacitor high Q inverter block  44 . The capacitors of block  44 , have significantly smaller values than the capacitors in a second order system. 
     The capacitive network of block  44  is designed to provide a variable capacitance as identified in variable capacitor control block  46 . By this configuration, ascribed values of voltage, power and current are delivered to a high impedance load  48  such as a high impedance lamp. The network of block  44 , also provides feedback signals to feedback gate control block  50 , used to control operation of the circuit within designed parameters. Variable capacitor control block  46  compensates for line voltage input or other component changes of the circuit, improving power regulation provided to load  48 . An operational concept of circuit block diagram  40  is to cause a capacitor component having a fixed value to act as an effective variable circuit capacitance over the cycle of circuit operation. 
     Turning to FIG. 3, illustrated is a third-order inverter circuit  60  with automatic line regulation in accordance with concepts of the present application. This design maintains many of the characteristics of the previously discussed circuit  10 . However, the present circuit design permits the efficient driving of a high-impedance lamp with a low starting current, as well as providing a low operating current, in a circuit having stable operation. 
     Circuit  60  includes a full-bridge rectifier, comprised of diodes  62   a,    62   b,    62   c  and  62   d,  connected to positive bus  63   a,  and common bus  63   b,  and supplied via an input source  64 . A switching circuit  66  is shown in this figure as a half-bridge network with a first transistor  68  and a second transistor  70 , controlled via a controller  72 . It is to be appreciated that, while the switching network in the following embodiments are shown as a half-bridge designs, these embodiments are equally applicable and are intended to encompass other input arrangements, including single and full-bridge switching networks, with a variety of control mechanisms. Therefore, switching circuit block  42  of FIG. 2 is intended to represent a variety of the known switching elements and control mechanisms. 
     As previously discussed, the output voltage generated by switching circuit  66  is supplied to a resonant circuit including of resonant inductor  74 , and resonant capacitor  76 . A second resonant capacitor  78  is connected in series with a load  80 , such as a high impedance lamp connected in the circuit by load connections  80   a,    80   b.  The present circuit further includes an impedance matching capacitor  82  also in series with lamp  80 . Matching capacitor  82  which may also be considered part of the resonant circuit acts to increase the Q factor of the circuit without the need for a higher value for resonant capacitor  76 , as would for example be needed in a second-order inverter circuit. Therefore, the starting current, is reduced allowing the use of smaller sized inductors and capacitors than otherwise possible. 
     However, it is appreciated that during operation, this high Q circuit  60  would be sensitive to line voltage and system component variations. To address these issues, circuit  60  employs impedance matching capacitor  82  to provide an effective variable capacitance, even though it has a fixed capacitor value. This is accomplished through the use of switching elements  84  and  86  in combination with impedance matching capacitor  82 . Switching element  86  is placed in parallel with impedance matching capacitor  82  and switch  84  is connected at one end to switch  86  and at its other end to the positive bus of circuit  60 . In one embodiment, switches  84  and  86  may be to high-speed, fast-recovery diodes. 
     Turning to FIG. 4, depicted is a graph illustrating a current sensitivity analysis of the lamp in accordance with the circuit shown in FIG. 3, and the effect of the arrangement of matching capacitor  82  and diodes  84 ,  86 . Voltage waveform  90  depicts the voltage across capacitor  82 . 
     As may be observed, waveform  90  is clamped at its positive going side  92  at approximately 150 volts, and at its negative going side  94  at approximately 0 volts. Particularly, waveform  90  is clamped to common on its negative side and to the positive bus voltage on its positive side. During operation in the linear range  95 , capacitor  82  acts as a component with a fixed capacitive value. Above the range from about 150 volts or below the range from about 0 volts, capacitor  82  is essentially removed from circuit operation. By this design, over an entire cycle of operation, an effective variable capacitive value is obtained. 
     When higher or lower current goes through capacitor  82 , this will indicate that higher or lower current is also going through the lamp. The lamp current and capacitor current are the same (assuming the diodes  84  and  86  are not clamping the circuit) since the capacitor  82  is in series with lamp  80 . Therefore, the current in the lamp  80  changes as the line voltage changes, or as component variations occur. 
     These variations also result in the voltage across the capacitor  82  changing. When the voltage across capacitor  82  diodes  84 ,  86  reaches a predetermined amount (e.g., 150 or 0 v), diodes  84 ,  86  clamp the voltage across capacitor  82 . Once the diodes  84  and  86  clamp capacitor  82 , it is effectively bypassed during that portion of the conduction time. By this action, the circuit substantially automatically changes the equivalent capacitor value of the circuit. Thus, the capacitor  82  and diodes  84  and  86  function as a variable capacitance control circuit, such as block  46  of FIG.  2 . This capacitance adjustment feature reduces the sensitivity of the circuit to variations, such as the mentioned input voltage variations or variations due to components. 
     A reason the described process is effective is because every line change, inductor change, capacitor change, frequency change, translate or have an effect on the lamp current, causing it to change. By controlling lamp current, it is possible to make the circuit less sensitive to such variations. This design and process permits regulation similar to that as may be obtained by a second-order inverter circuit, while gaining the benefits of a third-order circuit, such as the applicability to high-impedance lamps, use of low starting current, and high starting voltage, less stress on the components, as well as being able to construct a device with a smaller physical footprint due to the use of smaller sized components. This design also gains the benefits of a third-order inverter by having a higher efficiency operation than the second order inverter circuits when driving high impedance lamps. 
     As previously mentioned, the current through the lamp is dependant upon various factors. The following formula illustrates this concept:          Δ                   I   Lamp       =           (               L            I   Lamp       )     ·   Δ                   L     +         (                 C   Lamp              I   Lamp       )     ·   Δ                     C   Lamp       +         (                 R   Lamp              I   Lamp       )     ·   Δ                     R   Lamp                                
     Particularly, the formula emphasizes that total lamp current change (ΔI Lamp ) is comprised of three components. The first component is the lamp current change (dI Lamp ) versus the resonant inductor change (dL) of the total change in inductance (ΔL). The second component consists of the lamp current change (dI Lamp ) versus the resonant capacitor change (dC Lamp ) for the total resonant capacitive change (ΔC Lamp ). The third component is the current lamp change (dI Lamp ) versus the lamp impedance change (dR Lamp ) for a total lamp change (ΔR Lamp ). The impedance change in the lamp may be due to manufacturing variabilities of particular lamps where lamps may change from lot to lot, or even from lamp to lamp, in their inherent impedance. 
     Turning to FIG. 5, illustrated is a second embodiment of a third-order inverter circuit  100 . In this design, the switching network  102  uses two FETs  104 ,  106  controlled by an integrated control circuit  108 . The integrated control circuit  108  permits the design to operate as either an open loop or a closed loop system. The remaining components of the system are similar to that of circuit  60  in FIG.  3 . 
     Turning to FIG. 6, a third embodiment of a third-order inverter circuit  110  includes a switching network  112 , which is a complementary switching circuit design implementing a complementary pair of switches (e.g., FETs)  114 ,  116 , driven via an input of inductors  118 ,  120  and capacitor  122  (alternative designs of the complimentary pair switching are shown in U.S. Pat. Nos. 5,408,403; 5,796,214; 5,874,810; and 5,877,595 to Nerone et al., each hereby incorporated by reference in their entirety). This topology illustrates a self-oscillating, low cost system design. The remaining circuit portions are similar to the circuit of FIG.  3 . It is noted that inductor coil  118  is also part of the resonant circuit design. 
     Turning to FIG. 7 illustrated is a fourth embodiment of a third-order inverter circuit  130 , which uses bipolar transistors as the switching elements. Particularly, drive circuit  132  includes bipolar transistors  134 ,  136  and diodes  138 ,  140  attached across each respectively. Transistors  134  and  136  are driven via inductor coils  142 ,  144 , which are in electrical communication to inductor coils  146 . 
     Turning to FIG. 8, illustrated is a further embodiment of a circuit  148  in accordance with the present application, wherein the switching network  150  is particularly defined as having a full-bridge switching network consisting of transistors  152 ,  154 ,  156  and  158 . The controller is shown as a generic controller  160 , which may be any of the previously described or other existing controllers used to operate a full-bridge network. This design would allow for a much higher power operation such as 1 kw. 
     FIG. 9 illustrates a circuit  168  similar to those previously described with a switching network  170  designed for a single switch  172  controlled by a controller  174 . 
     The third-order inverter circuit embodiments illustrated in FIGS.  3  and  5 - 9 , as well as the block circuit diagram of FIG. 2, describe circuits where effective variable capacitance values are obtained from a fixed capacitor value and act as a feedback control (i.e., block  50  of FIG. 2) to stabilize circuit operation. Particularly, the capacitor adjustments are operationally opposite to variations of the input to the circuit and/or the circuit components. For example, when positive voltage changes occur (i.e., voltage increases) above a certain value, the variable capacitance acts to negate this change and/or other component changes. Action of the effective variable capacitance created by capacitor  82 , diodes  84  and  86 , combination function to counteract circuit fluctuations (i.e., increases/decreases). In this manner, the system is provided with a negative feedback control, which inherently has a stabilization feature. 
     Operation of the third-order inverter circuits of the present application increases the Q factor obtainable by this design to a range of 2-5, whereas the Q factor operation in a second-order system would substantially be a 1 to 1.5 range. Also, the physical size of a light system (such as a compact fluorescent lamp) maybe decreased by as much as 30 percent as compared to compact fluorescent lamp systems implementing existing inverter circuit designs. For one example, while the values of inductors used in second-order and third-order inverter circuits powering similar sized lamps may be substantially the same, the second-order systems would need to carry potentially twice as much current as the presently disclosed circuits, therefore, a larger core size would be necessary. Further, the diameter of the glass envelope for such a compact fluorescent lamp system, and the spacing between the loops of the glass envelope may also be significantly smaller than that for existing lamps, due to the features described herein. 
     While the present system may be embodied in a number of different alternatives, and with different values, in one embodiment implementing a half-bridge rectifier system such as maybe known in the art, used at a 125 volt input, specific values for one particular implementation such as shown in FIG. 3, would include: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Diodes 62a, 62b, 62c, 62d 
                 1N5395 
               
               
                   
                 Switch 68 
                 FQU 9N25 
               
               
                   
                 Switch 70 
                 FQU 9N25 
               
               
                   
                 Inductor 74 
                 470uh 
               
               
                   
                 Capacitor 76 
                 6.8nf 
               
               
                   
                 Capacitor 78 
                 22nf 
               
               
                   
                 Lamp 80 
                 42W 
               
               
                   
                 Capacitor 82 
                 10nf 
               
               
                   
                 Diode 84 
                 1N4937 
               
               
                   
                 Diode 86 
                 1N4937 
               
               
                   
                   
               
             
          
         
       
     
     Other numbered components set forth in this application but not included in this listing may have values similar to those described. It is to be understood the provided values are given simply as examples and are not intended to be limiting of the claims. The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.