Abstract:
A method of controlling a series-resonant, half-bridge inverter includes turning off the bottom switch and turning on the top switch the inverter when the current through the resonant inductor crosses the zero axis while the current is increasing, thereby insuring zero voltage switching of the inverter switches and increases the overall switching period so that the actual inverter frequency is closer to the resonant frequency of the series-resonant circuit. Using an on-time control circuit, the method further includes controlling the current delivered to the load (such as a gas discharge lamp) by varying the on-time of the top inverter switch.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to resonant inverters, such as those used in an electronic ballast to provide power to a gas discharge lamp. More particularly, the present invention pertains to methods of operating a resonant inverter-type electronic ballast to provide stable operation of, and to control power to, a gas discharge lamp. 
     Electronic ballasts are commonly used to power gas discharge lamps, such as a fluorescent lamp. A typical electronic ballast will include a half-bridge resonant inverter driven by an inverter drive circuit. The inverter drive circuit controls the switching of the top and bottom inverter switches so that the inverter operates at or near the self-resonant frequency of the inverter. Using a driven resonant inverter allows for the lamp power to be controlled (for dimming and/or stable power regulation) by varying the inverter frequency and/or by varying the pulse width of the inverter output. 
     Stable operation of the lamp is important in the design and operation of an electronic ballast. For many applications, an ideal resonant inverter would act as an ideal current source in which the open loop output impedance is infinite so that all of the current generated by the inverter flows to the lamp. Also, to achieve optimal efficiency while lowering component stresses, a preferred resonant inverter design would insure zero-voltage switching of the inverter transistors. 
     Unfortunately, prior art resonant inverters that operate as current sources suffer from several problems caused by deficiencies in the inverter switching methods. For example, use of frequency modulation to control the inverter output does not guarantee zero voltage switching. Moreover, with frequency modulation, the higher frequencies and higher lamp voltages associated with lamp dimming create undesirable circulating currents through the component and parasitic capacitances in the circuit. Resonant inverters that use symmetric pulse width modulation (also known as dead time modulation) also have a high likelihood of non-zero voltage switching. 
     What is needed, then, is a method of operating a resonant inverter that maximizes the open loop output impedance by operating the inverter close to self-resonance and that guarantees zero voltage switching of the inverter transistors. 
     BRIEF SUMMARY OF THE INVENTION 
     The method of this invention is implemented in an inverter that has top and bottom inverter switches arranged in a half-bridge configuration and a series-resonant circuit connected across the output of the inverter. The inverter is also connected to a drive circuit that controls commutation of the top and bottom inverter switches. The series-resonant circuit includes a resonant capacitor in series with a resonant inductor. A load, such as a gas discharge lamp, is connected across the resonant capacitor. 
     The method of the present invention includes turning off the bottom switch and turning on the top switch of a series-resonant half-bridge inverter when the current through the resonant inductor crosses the zero axis while the current is increasing. This insures zero voltage switching of the inverter switches. This operation also increases the overall switching period so that the actual inverter frequency is closer to the resonant frequency of the series-resonant circuit. By operating the inverter close to resonance, the open loop output impedance of the inverter is increased, providing a more stable operation of a load such as gas discharge lamp. 
     Using an on-time control circuit, the method of this invention further includes controlling the current delivered to the load (such as a gas discharge lamp) by varying the on-time of the top inverter switch. This is accomplished in one embodiment by feeding back—cycle-by-cycle—a measurement of the current through the resonant inductor so that the top switch is commutated when the resonant inductor current reaches a reference value. In a second embodiment, the on-time of the top inverter switch is controlled by commutating the top switch based on a timer that is triggered when the top switch is turned on. 
     In one embodiment of the invention, the inverter drive circuit includes (a) a switching circuit coupled to the top and bottom inverter switches and (b) a zero-crossing detector circuit coupled to the inverter to monitor the AC load current through the resonant inductor. The zero-crossing detector circuit compares the measured AC load current with a first reference current value. The zero-crossing detector circuit also compares a derivative of the measured AC load current with a rate of change reference value. When the measured AC load current is at or near the first reference current value, and when the rate of change of the measured AC load current is one of either above or below the rate of change reference value, zero-crossing detector circuit generates an indicator signal. Preferably, the first reference current value is zero. In addition, the rate of change reference value is preferably zero so that the indicator signal is generated when the current is positive. 
     The first indicator signal is delivered to an input to the switching circuit. The switching circuit is operable to commutate the top and bottom inverter switches. Using the first indicator signal, the switching circuit causes the top inverter switch to turn on, and the bottom inverter switch to turn off, when the current in the resonant inductor crosses the zero-axis, while the inductor current is increasing. This insures zero-current switching of the inverter switches and increases the overall switching period so that the inverter operates closer to resonance. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a first embodiment of the invention. 
         FIG. 2  is a schematic diagram of a second embodiment of the invention. 
         FIG. 3A  is a graph showing the timing and magnitude relationships among the AC load current (I LOAD ), the resonant inductor current (I LSer ), and commutation of the top inverter switch (S TOP ) in accordance with one aspect of the invention. 
         FIG. 3B  is a graph showing the timing and magnitude relationships among the voltage across the DC blocking capacitor (V Cser ), the voltage across the resonant inductor (V LSer ), and the voltage across the resonant capacitor (V CPar ). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of the inverter circuit  10  of the present invention is shown in  FIG. 1 . The inverter circuit  10  is particularly useful when the load  12  is a gas discharge lamp. The preferred embodiment stabilizes the operation of the lamp by maximizing the open loop output impedance of the inverter circuit. 
     The inverter circuit  10  includes a half-bridge inverter  14  coupled to a DC source  16  and a series-resonant circuit  22 . The inverter  14  and series resonant circuit  22  conventionally operate to convert a DC voltage from DC source  16  to a high frequency, high voltage AC current that is supplied to a load  12 , such as a gas discharge lamp. In this embodiment, the inverter  14  is a driven inverter in which commutation of the top and bottom inverter switches  28 ,  20  is controlled by an inverter drive circuit. 
     A DC blocking capacitor  19  may be placed between the inverter  14  and the resonant circuit  22 . The inverter output  18  may have varying time, frequency, and envelope characteristics depending on the characteristics of resonant circuit  22  and the desired application. 
     In a preferred embodiment, the inverter  14  has a top switch  28  and bottom switch  30  arranged in a half-bridge inverter topology. Preferably, the series-resonant circuit  22  includes a resonant inductor  24  and resonant capacitor  26 . The load  12  is connected across the resonant capacitor  26  to receive an AC load current  23 . (Labeled I LOAD  on  FIG. 3(   a ).) 
     The inverter drive circuit includes a zero-crossing detector circuit  32  coupled to the resonant circuit  22 . The zero-crossing detector circuit  32  monitors the current I LSer  through the resonant inductor  24  so that commutation of the inverter switches  28 ,  30  occurs (a) at a frequency that is close to the resonant frequency of the resonant circuit  22  and (b) in a manner that is synchronized to a particular rate of change of the current through resonant inductor  24 . By selecting certain current and current rate of change reference values, the zero-crossing detector circuit  32  can assure zero voltage switching of the top and bottom inverter transistors  28 ,  30 . 
     The zero-crossing detector circuit  32  receives an AC current signal  25  corresponding to the current I LSer  through resonant inductor  24 . The AC current signal  25  may also be any current signal associated with the AC load current  23  so long as the AC current signal  25  has a frequency and amplitude approximately proportionally related to the frequency and amplitude of the AC load current  23 . Preferably however, the AC current signal  25  corresponds to the same frequency as the AC load current  23 . Those of skill in the art will recognize that the AC current signal  25  can be generated in a variety of conventional ways, such as by inductively coupling to the resonant inductor or by measuring a voltage across a current sensing resistor (not shown). 
     The zero-crossing detector circuit  32  is operable to compare the AC current signal  25  with a first reference current value  34 . As shown in  FIGS. 1-2 , this may be accomplished using a first comparator  35 . The first comparator  35  produces a current value crossing signal  37  when the AC current signal  25  is at or near the first reference current value  34 . 
     The zero-crossing detector circuit  32  is also operable to compare a rate of change of the AC current signal  25  with a rate of change reference value  36 . This may be accomplished by utilizing a current derivative (rate of change) detector  42  and a second comparator  44 . The current derivative detector  42  generates a signal that corresponds to a rate of change of the AC current signal  25 . The current derivative detector  42  may be a combination of passive components or an active device. For example, the current derivative detector  42  may include an inductor that is coupled to other passive or active components. Alternatively, the current derivative detector  42  may be a processor device that calculates the derivative of the AC current signal  25  and outputs the appropriate rate of change current signal. This signal is then coupled to one input of second comparator  44 . 
     The second comparator  44  compares the rate of change of the AC current signal  25  with a rate of change reference value  36 . When the rate of change of the AC current signal  25  is one of either approximately above or below the rate of change reference value  36 , the zero-crossing detector circuit  32  generates a rate of change crossing signal  39 . A logic device  33  is coupled to the outputs of comparators  35 ,  44 . The logic device  33  is functional to generate a first indicator signal  49  when the current value crossing signal  37  and the rate of change crossing signal  39  are generated by the first and second comparators  35 ,  44 . Preferably, the logic device  33  is an AND gate. 
     The zero-crossing detector circuit  32  provides all of the measurements required for zero voltage switching. Zero voltage switching occurs when the inverter switches  28 ,  30  are commutated near or at a zero current crossing and when the current rate of change is approximately positive. Consequently, the first reference current value  34  and the rate of change reference value  36  are at or near zero. In addition, the first indicator signal  49  is produced when the rate of change of the AC current signal  25  is approximately above the rate of change reference value  36 . In this manner, the indicator signal  49  is produced when the AC load current  23  is at or near a zero crossing and has a positive rate of change. 
     The AC current signal  25  does not need to have the same frequency as the AC load current  23 . The AC current signal  25  needs to have a frequency approximately proportional to that of the AC load current  23 . Thus, additional devices (not shown) may be included in the inverter circuit  10  so that the indicator signal  49  is produced according to a known proportion between the frequencies of the AC load current  23  and the AC current signal  25 . For example, the circuit may cause the indicator signal  49  to be produced at every other zero crossing if the frequency of the AC current signal  25  is approximately double the frequency of the AC load current  23 . However, in the preferred embodiment the AC current signal  25  has the same frequency as the AC load current  23 . 
     Referring again to  FIGS. 1-2 , the inverter drive circuit includes a switching circuit  38  coupled to the inverter  14  and to the zero crossing detector circuit  32 . The switching circuit  38  is responsive to the first indicator signal  49  from the output from the zero crossing detector circuit  32  to commutate the top and bottom inverter switches  28 ,  30 . As described above, the first indicator signal  49  is preferably generated at a zero crossing of the AC current signal  25  and when the rate of change of the AC current signal  25  is positive. 
     As shown in  FIGS. 1-2 , the switching circuit  38  is coupled to the top and bottom inverter switches  28 ,  30  such that when one switch  28 ,  30  is open the other switch  30 ,  28  is closed. Therefore, the switching circuit  38  may be a conventional S-R flip-flop device in which the set input S receives the first indicator signal  49 . 
     The power delivered to the load  12  can be controlled by varying the on-time of the top inverter switch  28 , using an on-time control circuit. In the embodiment of  FIG. 1 , the on-time of top inverter switch  28  is controlled by a comparator circuit  40  coupled to the resonant circuit  22 . The comparator circuit  40  has a signal input coupled to the resonant inductor  24  and a reference input connected to a second reference current value  41 . The comparator circuit  40  compares the AC current signal  25  with second reference current value  41 . When the AC current signal  25  is at or near the second reference current value  41 , a second indicator signal  50  is sent to the reset input R of switching circuit  38 . The switching circuit  38  responds to the second indicator signal  50  to cause commutation of the inverter switches  28 ,  30 . More particularly, the output Q of switching circuit  38  is coupled to the top inverter switch  28 . The complementary output Q′ is coupled to the bottom switch  30 . In one embodiment, the top and bottom switches  28 ,  30  can be switching transistors in which the bases (or gates) of the transistors are coupled to the flip-flop outputs Q and Q′. When the set input S receives the first indicator signal  49 , the normal output Q is switched high while the complementary output Q′ is switched low. This signal is represent as S TOP  on  FIG. 3A . This turns on the top inverter switch  28  and turns off the bottom inverter switch  30 . When the second indicator signal  50  is received by the reset input R, the normal output Q is switched low while the complementary output Q′ is switched high. This causes the top inverter switch  28  to turn off and the bottom inverter switch  30  to turn on. 
     In a second embodiment of the inverter circuit  10  shown in  FIG. 2 , the on-time of the top inverter switch  28  is controlled by a timer circuit  41 . The timer circuit  41  is preferably activated when the top inverter switch  28  is turned on. Thereafter, the second indicator signal  50  is generated after a time delay, causing the top inverter switch  28  to turn off. Preferably, and as shown in  FIG. 3B , the time delay is synchronized so that the top inverter switch  28  is turned off at or near a peak current value for the AC current signal  25 . As described above, the zero-crossing detector circuit  32  insures commutation at or near zero current crossing when the load current is increasing. 
     A graphical illustration of the operation of the inverter circuit  20  is shown in  FIGS. 3A-3B . Referring specifically to  FIG. 3A , the graph depicts the output  31  from the normal output Q of switching circuit  38 . When the output  31  is low, top inverter switch  28  is turned off. When the zero crossing detector circuit  32  determines that the rate of change of the AC current signal  25  is positive, and that the AC current signal  25  is at or near zero, the first indicator signal  49  is generated at set input S and the output  31  is switched high. This turns top switch  28  on. When the second indicator signal  50  is received by the reset input R, the top inverter switch  28  is turned off. As shown in  FIG. 3A , the output  31  is switched back to low at or near the peak value of the AC current signal  25 . The resultant AC load current  23  is illustrated in the graph. 
       FIG. 3B  shows a graphical illustration of the resultant voltage outputs from the inverter circuit  10 , including the voltage  59  across DC blocking capacitor  19 . The voltage  60  across the resonant inductor  24  is also shown. The voltage  61  across resonant capacitor  26 , which is the same as the AC voltage across the load  12 , is also shown. 
     Throughout this disclosure, the words “approximately” and “near” have been to describe when a various actions of the inverter circuit  10  are triggered or performed. These words recognize that no electronic device can perform a particular action precisely at a particular moment. While electronic devices are intended to be as accurate as possible, one of ordinary skill in the art recognizes that in practice no device performs at 100% accuracy. The words “approximately” and “near” are intended to recognize this inaccuracy in all electronic devices. Standard errors in electronic circuits and techniques utilized for correcting these errors should be considered when interpreting whether a particular action of the inverter circuit  10  is performed “approximately” and “near” a reference value. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful Method of Operating a Resonant Inverter Using Zero Current Switching and Arbitrary Frequency Pulse Width Modulation, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.