Abstract:
A lighting ballast and associated methods balance current through resonant inductors that have inductance variation, and are further effective to balance lamp currents in the range from full brightness to full dimming. The ballast includes a lighting power source, a balancing transformer having a plurality of windings, a first resonant tank circuit having one or more transformer windings and a second resonant tank circuit having a like number of transformer windings. Each of the windings for the first resonant tank are reversed in direction in association with a corresponding winding for the second resonant tank, such that the only current passing through the windings is a current difference between the two windings.

Description:
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of the following patent application(s) which is/are hereby incorporated by reference: None 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to gas discharge lighting ballasts for powering multiple lamps in parallel. More particularly, the present invention relates to a lamp ballast topology and associated method to match multiple independent resonant tanks for parallel lamp operation. 
     An electronic ballast with multiple parallel independent lamp operation is generally desirable so that if one lamp fails the remaining lamps will still be functional. This feature allows for significantly reduced maintenance costs because there is correspondingly no need to replace the failed lamp immediately if such replacement is inconvenient or impractical under the circumstances. 
     One convention ballast topology that provides multiple parallel independent lamp operation is to use multiple independent resonant tanks, as in the circuit  110  shown in  FIG. 1 . Multiple lamp applications may be easily expanded based on the two-lamp application shown. 
     Referring to  FIG. 1 , an equivalent AC input voltage source V_in may typically be the output of a half-bridge inverter circuit. The frequency of the input voltage V_in is adjustable for dimming applications. Inductors L_res_ 1 , L_res_ 2  are resonant inductors for the respective resonant tanks. Capacitors C_res_ 1 , C_res_ 2  are resonant capacitors for the respective resonant tanks. DC blocking capacitors C 1 , C 2  are coupled between the resonant inductors and the lamps in the respective resonant tanks, with L_res_ 1 , C_res_ 1 , C 1  and Lamp 1  forming a first series resonant tank  112   a  and L_res_ 2 , C_res_ 2 , C 2  and Lamp 2  forming a second series resonant tank  112   b . Bidirectional switches S 1  and S 2  can be turned on or turned off for single-lamp and two-lamp applications, or alternatively where Lamp 1  or Lamp 2  have failed. 
     For series resonant tanks, the lamp current (I_lamp) is dependent on the resonant circuit quality factor (Q) and operating frequency (f). Represented in  FIG. 2  are typical output characteristics (lamp current- vs. operating frequency curve) for a series resonant circuit. Output curve  1  represents the output characteristic for the first resonant tank  112   a , and output curve  2  represents the output characteristic for the second resonant tank  112   b . Because the resonant components will not generally be exactly the same in the resonant tanks, the two output curves will accordingly be different as well. For the same operating frequency (f_steady), the lamp currents I_lamp 1 , I_lamp 2  will not be the same. The higher the Q of the resonant tank, the bigger the difference between the lamp currents. 
     A conventional lamp current balancing method as represented in  FIG. 3  will not be sufficient to balance the lamp current and resonant inductor current for two independent resonant tanks. If a lamp current balancing transformer T 1  is designed to be sufficiently large, the transformer T 1  will be able to balance the lamp current. However, the difference in resonant inductor current will be amplified by the transformer T 1 , as described below:
 
 V 2 =V   —   c 2 +V _lamp2 +V   —   T 1 B;  
 
 V 1 =V   —   c 1+ V _lamp1+ V   —   T 1 A;  
 
     In the above equations, V 1  and V 2  are the voltages across the resonant inductors L_res 1  and L_res 2 , respectively. If the lamp currents are balanced to be the same by the transformer T 1 , then:
 
 V   —   c 1 =V   —   c 2;
 
 V _lamp1 =V _lamp2; and
 
 V 2 −V 1 =V   —   T 1 B−V   —   T 1 A  
 
     Because the voltages V_T 1 B and V_T 1 A are different by 180 degrees due to the transformer design,
 
 V 2 −V 1=2*( V   —   T 1 A )
 
     As demonstrated herein, a large voltage difference will therefore be seen across the resonant inductors L_res 1  and L_res 2 . The large voltage difference will further cause a large current difference through the resonant inductors. This current differential makes design of the resonant inductors exceedingly difficult because the current could be almost any value depending on the voltage across the transformer T 1 . This feature also makes the ballast thermal design very difficult, as the increased current results in a measurably increased temperature for the inductor as well. 
     If the Q or output characteristic of the two resonant tanks are sufficiently close, the lamp currents I_lamp 1  and I_lamp 2 , respectively, would also be very close so that the voltage across the transformer T 1  would correspondingly be quite small. As a result the current imbalance for the respective resonant inductors would be substantially reduced. 
     In practice, the resonant capacitors typically have very low variation (e.g., 1-3%). The inductance of the resonant inductor may however vary across a typical range of about 5-10%. Therefore, balancing of the inductor current or resonant inductance is an important consideration for balancing of the lamp currents and thereby solving the thermal imbalance for resonant inductors. 
     BRIEF SUMMARY OF THE INVENTION 
     A resonant tank topology and associated methods are herein provided in accordance with the present invention to match multiple independent resonant tanks in a lamp ballast for parallel lamp operation. 
     In another aspect of the present invention, a resonant current and lamp current balancing method is provided for multiple independent resonant tanks. 
     In another aspect, a method is provided for disabling associated balancing transformer windings in multiple resonant tanks. 
     In a particular embodiment of the present invention, a lighting ballast and associated methods are provided to balance current through resonant inductors that have inductance variation, and further effective to balance lamp currents in the range from full brightness to full dimming. The ballast includes a lighting power source, one or more balancing transformers having a plurality of windings, a first resonant tank circuit having one or more transformer windings and a second resonant tank circuit having a like number of transformer windings. Each of the windings for the first resonant tank are reversed in direction in association with a corresponding winding for the second resonant tank, such that the only current passing through the windings is a current difference between the two windings. 
     In another embodiment, a lighting ballast in accordance with the present invention includes a lighting power source, a balancing transformer with a plurality of windings, a first resonant tank circuit having a plurality of the transformer windings and a second resonant tank circuit having at least as many of said transformer windings as are present in the first tank circuit. Each of the windings for the first resonant tank is reversed in direction in association with a corresponding winding for the second resonant tank. 
     In another embodiment, a lighting ballast in accordance with the present invention includes a lighting power source, first and second balancing transformers each having a plurality of windings, a first resonant tank circuit having one or more windings from each of the first and second transformers, and a second resonant tank circuit having one or more windings from each of the first and second transformers. Each of the windings for the first resonant tank is reversed in direction in association with a corresponding winding for the second resonant tank. 
     In various embodiments, the lighting ballast may further include a transformer disabling control circuit with one or more switching elements and transformer windings coupled to a large capacitor. Operation of the switching elements either causes the balancing transformer to operate normally or to effectively short, wherein one or more of the resonant tanks are disabled. In this manner the ballast may properly operate with fewer lamps than available resonant tanks. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit diagram representing a resonant tank topology as previously known in the art. 
         FIG. 2  is a graphical diagram representing typical output characteristics for the resonant tank topology of  FIG. 1 . 
         FIG. 3  is a circuit diagram representing a lamp balancing resonant tank topology as previously known in the art. 
         FIG. 4  is a circuit diagram representing an embodiment of a resonant tank topology of the present invention. 
         FIG. 5  is a circuit diagram representing an alternative embodiment of the topology of  FIG. 4  with a disabling control circuit. 
         FIG. 6  is a circuit diagram representing another embodiment of a resonant tank topology of the present invention. 
         FIG. 7  is a circuit diagram representing an alternative embodiment of the topology of  FIG. 6  with a disabling control circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. 
     The term “coupled” means at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” as used herein may include any meanings as may be understood by those of ordinary skill in the art, including at least one current, voltage, charge, temperature, data or a state of one or more memory locations as expressed on one or more transmission mediums. 
     The terms “switching element” and “switch” may be used interchangeably and may refer herein to at least: a variety of transistors as known in the art (including but not limited to FET, BJT, IGBT, JFET, etc.), a switching diode, a silicon controlled rectifier (SCR), a diode for alternating current (DIAC), a triode for alternating current (TRIAC), a mechanical single pole/double pole switch (SPDT), or electrical, solid state or reed relays. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa. 
     Terms such as “providing,” “processing,” “supplying,” “determining,” “calculating” or the like may refer at least to an action of a computer system, computer program, signal processor, logic or alternative analog or digital electronic device that may be transformative of signals represented as physical quantities, whether automatically or manually initiated. 
     Referring generally to  FIGS. 4-7 , various embodiments are described herein for a lighting ballast having multiple independent resonant tank circuits and associated methods for parallel lamp operation. Where the various figures may describe embodiments sharing various common elements and features with other embodiments, similar elements and features are given the same reference numerals and redundant description thereof may be omitted below. 
     Referring first to an exemplary embodiment as represented in  FIG. 4 , a lighting ballast  10  in accordance with the present invention is provided with first and second independent resonant tank circuits  12   a ,  12   b , respectively, coupled across positive and negative terminals of an input voltage source V_in which may generally but without express limitation be the output from an inverter circuit (not shown) associated with the ballast  10 . The first resonant tank  12   a  as shown includes a resonant inductor L_res 1  coupled on a first end to the positive terminal of the voltage source V_in via a switching element S 2 , a resonant capacitor C_res 1  coupled between a second end of the resonant inductor L_res 1  and the negative terminal of the voltage source V_in, and a capacitor C 1  coupled in series with first and second lamp connection terminals  16   a ,  16   b  across (in parallel with) the resonant capacitor C_res 1 . 
     A balancing transformer T 1  is provided to substantially match the two independent resonant tanks  12   a ,  12   b . The first resonant tank  12   a  includes transformer windings T 1 _B 1 , T 1 _B 2 , T 1 _B 3  which are each coupled on a first end to a common node and further coupled on a second end to the resonant inductor L_res 1 , the first lamp connection terminal  16   a  for the first tank, and the resonant capacitor C_res 1 , respectively. The second resonant tank  12   b  includes transformer windings T 1 _A 1 , T 1 _A 2 , T 1 _A 3  which are each coupled on a first end to a common node and further coupled on a second end to the resonant inductor L_res 2 , the first lamp connection terminal  16   a  for the second tank, and the resonant capacitor C_res 2 , respectively. 
     In various embodiments, each of the windings for the first resonant tank  12   a  is reversed in direction in association with a corresponding winding for the second resonant tank  12   b . The only current flowing through any corresponding set of windings may therefore be defined as a current differential between that set of windings. 
     As represented in  FIG. 4 , transformer windings T 1 _A 1  and T 1 _B 1  define a first set of windings which may be used to balance the resonant inductor current. Transformer windings T 1 _A 2  and T 1 _B 2  define a second set of windings which may be used to balance the current through lamps connected to the respective lamp connection terminals (the lamp currents-I_lamp 1 , I_lamp 2 ). Transformer windings T 1 _A 3  and T 1 _B 3  define a third set of windings which may be used to balance the resonant capacitor current. The voltage across the balancing transformer T 1  caused by whatever relatively small unbalanced current is generated through the independent resonant tanks may, in accordance with embodiments as described above, automatically balance the resonant inductor current and the lamp current. 
     When there is only one lamp coupled to the lamp connection terminals of one of the resonant tanks  12   a ,  12   b , due, for example, to end-of-life failure or other like reasons, the switching elements S 1  or S 2  coupled to the resonant tank associated with the failed lamp may be opened to disable the tank. The switching element may be driven to turn on and off by, for example, a controller which is effective to determine an end-of-life failure or an open circuit across the associated lamp connection terminals and to control the switch state accordingly. Such processes are known in the art and further description may accordingly be omitted herein. 
     However, in an embodiment of the present invention, a resonant tank disabling control circuit  14  may be provided to disable the balancing transformer T 1  during such conditions and facilitate proper single-lamp operation for the ballast  10 . Referring to  FIG. 5 , in one embodiment the control circuit  14  includes a switching element S 3  coupled across positive and negative terminals of a voltage source, which may be, for example, a rail voltage (V_rail) and ground terminal for the ballast. A diode D 5  may have its anode coupled to the switch S 3  and its cathode coupled to the rail voltage terminal V_rail. A seventh balancing transformer winding T 1 _C is coupled in series with a capacitor C 3  across (in parallel with) the switching element S 3 . The switching element S 3  may be driven in accordance with the turning on or off of the other two switching elements S 1 , S 2 , or alternatively may be driven independently of the other switches in a literal sense but still turned on and off based, for example, on the detection of either a multi-lamp or single-lamp operating condition for the ballast. Driving circuitry for the switching element S 3  is not shown but is well known in the art. 
     When the switching element S 3  is driven to be in a first switch state (e.g., open), the balancing transformer T 1  is allowed to function normally. However, when the switching element S 3  is driven to be in a second switch state (e.g., closed), the transformer winding T 1 _C is shorted with the capacitor C 3  so that the voltage across the winding T 1 _C is limited to a value defined by the capacitance of the capacitor C 3  and the turns ratio N between the transformer windings T 1 _C and T 1 _A. If the capacitance of the capacitor C 3  is sufficiently large, the voltage drop across the capacitor C 3  will be small enough that the transformer T 1  is substantially shorted when the switching element S 3  is closed. 
     In embodiments of the present invention as described above and more particularly with reference to  FIGS. 4-5 , a core size for the balancing transformer and associated conductor sizes may generally be designed to be sufficiently large to accommodate large currents flowing passing through the transformer. Alternative embodiments may be provided with reference to  FIGS. 6-7  which may accordingly reduce the size of the balancing transformer T 1 . 
     Referring first to an embodiment as represented in  FIG. 6 , a first transformer T 1  may be dedicated for balancing of the lamp current and a second transformer T 2  may be dedicated for balancing of the resonant inductor current. With the resonant tank components otherwise disposed substantially the same as described with respect to the embodiment of  FIG. 4 , a first winding T 1 _A from the first transformer T 1  is coupled between lamp connection terminal  16   b  of the first tank  12   a  and the negative power source terminal (e.g., ground). A second winding T 1 _B from the first transformer T 1  is coupled between lamp connection terminal  16   b  of the second tank  12   b  and the negative power source terminal. The first and second windings T 1 _A, T 1 _B from the first transformer T 1  may be magnetically coupled to each other but reversed in direction with respect to each other as demonstrated in  FIG. 6  and similarly described above. 
     A first winding T 2 _A from the second transformer T 2  is coupled between the resonant inductor L_res 1  of the first tank  12   a  and a node between the resonant capacitor C_res 1  and the capacitor C 1 . A second winding T 2 _B from the second transformer T 2  is coupled between the resonant inductor L_res 2  of the second tank  12   b  and a node between the resonant capacitor C_res 2  and the capacitor C 2 . The first and second windings T 2 _A, T 2 _B from the second transformer T 2  may be magnetically coupled to each other but reversed in direction with respect to each other as shown in  FIG. 6  and further as similarly described above. 
     Referring further to  FIG. 7 , in one embodiment a resonant tank disabling control circuit  14  may be provided in association with the topology of  FIG. 6 . A first control loop is defined substantially as described above with respect to  FIG. 5 , and includes a switching element S 3  coupled in series with a diode D 5  across a positive rail terminal and a negative rail terminal, and a supplemental winding T 1 _C from the first transformer T 1  coupled in series with a capacitor C 3  across (in parallel with) the switching element S 3 . A second control loop further includes a switching element S 4  coupled in series with a diode D 6  across the positive rail terminal and the negative rail terminal, and a supplemental winding T 2 _C from the second transformer T 2  coupled to a node between the capacitor C 3  and the other supplemental winding T 2 _C, the supplemental winding T 2 _C together with the capacitor C 3  forming a series circuit coupled across (in parallel with) the switching element S 4 . 
     The control circuit is effective (in similar manner to the control circuit represented in  FIG. 5  and as described above) when each of the switching elements are in a first switch state (e.g., open) to operate the first and second resonant tanks. When at least one of the switching elements are in a second switch state (e.g., closed) the control circuit is effective to substantially short and disable the associated balancing transformers. 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of the present invention of a new and useful “Lighting Ballast and Method for Balancing Multiple Independent Resonant Tanks,” 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.