Abstract:
A lamp ballast for energizing a lamp has short circuit protection. The lamp ballast contains a transformer that provides an output voltage to the lamp. The transformer has a primary winding, a secondary winding, and a third sense winding. The third sense winding has an output that provides an indication of transformer output voltage. A reference voltage circuit creates a reference voltage that provides an indication of a non-short circuited lamp ballast condition. A comparator compares the reference voltage and the output of the third sense winding to provide an indication if the third sense winding output is below the reference. This indicates if there is a short in the output of the lamp ballast. The secondary is wound on multiple sections of a bobbin with one section being a low voltage section. The sense winding is wound under the secondary in the bobbin secondary low voltage section. The ballast additionally has a track and hold circuit to prevent an indication of a short in the lamp voltage when the lamp voltage is dimmed with a dimming circuit. The ballast also has a preheat logic signal to prevent a short circuit indication during the preheat mode at startup.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates to a ballast for driving multiple lamps in a liquid crystal display (LCD). More specifically, the invention relates to a method for sensing a short circuit condition in the ballast transformers outputs to the multiple lamps. 
     2. Description of the Related Art 
     In some lamp ballasts, it is desirable to sense a short circuit condition on the output and take an action. This action could be to reduce the power level or shut down entirely in order to avoid circuit breakdown or other possible catastrophic situations. The conventional method to achieve this is to sense the current in the output (hence the lamp current) and compare to a threshold. When the reference level is exceeded, the action can be taken. This current sensing can be implemented in several ways, for instance by inserting a resistor in series with the load and sensing the voltage drop across it. 
     A difficulty with the state of the art is that it requires a measurement of current. It may be undesirable because the sensing resistor dissipates too much power. It may also be difficult if the circuit inherently limits the current to the load. An example would be an LCD backlight inverter based on the Alpha IC (UBA2010). The secondary winding of the transformers has enough leakage inductance that its impedance limits the current if the load is shorted. The circuit is self protecting, but one may still wish to sense the shorted condition and shut down for safety reasons. 
     What is needed is a ballast where a short circuit condition is sensed by a method other than measuring lamp (load) current or output voltage directly. 
     SUMMARY OF THE INVENTION 
     The invention is a lamp ballast for energizing a lamp which has a new method for sensing a short circuit condition. The lamp ballast has a transformer that provides an output voltage to the lamp. The transformer has a primary winding, a secondary winding, and a third sense winding. The third winding has an output which provides an indication of transformer output voltage. A reference voltage circuit creates a reference voltage providing an indication of a non-short circuited lamp ballast condition. A comparator compares the reference voltage and the output of the third sense winding. The comparator provides an indication if the third sense winding output voltage is below the reference voltage thus indicating a short in the output of the lamp ballast. 
     The method of winding the transformer improves the third sense winding output voltage signal. The transformer is wound on a bobbin with multiple sections. The transformer secondary is wound on multiple sections of the bobbin with one section being a low voltage section. The sense winding is wound under the secondary in the bobbin secondary low voltage section. 
     Several circuits are necessary to implement the above circuit for indicating a short in the output of the lamp ballast. First, the ballast has a dimming circuit during which time the lamp voltage is reduced substantially. The ballast has a track and hold circuit which cooperates with the voltage of the sense winding at the comparator to hold the voltage above the reference voltage so as to prevent an indication of a short in the lamp voltage. 
     Secondly, during startup the transformer output is zero and would have an indication of a short circuit. A special circuit is provided to delay any indication of a short circuit until the lamp ballast circuit is powered up. During startup the sense winding voltage at the comparator gradually increases during a preheat mode. A preheat logic signal applies a low voltage to the reference voltage circuit during the preheat mode. This prevents the reference voltage from exceeding the sense winding voltage thus preventing a short circuit indication during the preheat mode. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic drawing of an LCD backlight inverter with a series structure and short circuit protection. 
     FIG. 2 shows a block diagram of the LCD backlight inverter with short circuit protection of FIG.  1 . 
     FIG. 2 a  shows an enlarged view of the pinout of the Alpha IC U 2  from FIG.  2 . 
     FIG. 2 b  shows a detailed schematic drawing of one embodiment of a block FIG. 2 b  as shown in FIG.  2 . 
     FIG. 2 c  shows a detailed schematic drawing of one embodiment of a block FIG. 2 c  as shown in FIG.  2 . 
     FIG. 3 shows block diagram of the LCD backlight inverter with short circuit protection of FIG.  2 . 
     FIG. 3 a  shows a detailed schematic drawing of one embodiment of a block FIG. 3 a  as shown in FIG.  3 . 
     FIG. 3 b  shows a detailed schematic drawing of one embodiment of a block FIG. 3 b  as shown in FIG.  3 . 
     FIG. 4 is a signal vs time chart for the various signals involved with the circuit for startup, preheating and prevention of a short circuit indication for the lamp during startup. 
     FIG. 5 a  is a schematic of the primary, secondary and sense windings of the transformers. 
     FIG. 5 b  is a drawing of the mechanical structure of a bobbin wound with the primary, secondary and sense windings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a block diagram of short circuit protection for a liquid crystal display (LCD) backlight ballast with PWM dimming. Element  10  shows a prior art liquid crystal display (LCD) backlight inverter with a series structure. In this structure, one inverter is used to power two Cold Cathode Fluorescent Lamps (CCFL). Over voltage, over-current protection circuitry  12  is the basis for the present invention and provides a means for sensing a short circuit in the CCFL lamps by sensing transformer voltage instead of lamp current or output voltage directly. Dimming Logic  14  is for dimming the LCD display in cooperation with the overvoltage protection circuit. Start sequence logic  16  is for enabling the LCD display in cooperation with the overvoltage protection circuit. Control integrated circuit  18 , (Alpha IC(U 2 )), is a state of the art control IC (model UBA  2010 ) which controls the functions and timing of LCD backlight inverter  10 , over voltage, over-current protection circuitry  12 , dimming logic  14  and sequence logic  16 . Power is provided to the circuit by a voltage source Vin ( 22 ) connected across capacitor C 2  ( 24 ) to provide a VDD ( 26 ) to power the inverter circuit. 
     In prior art liquid crystal display (LCD) backlight inverter  10 , control integrated circuit (IC) ( 18 ) controls switches M 2  ( 28 ) and M 3  ( 30 ) which are connected across capacitor C 2  ( 24 ). When switch M 2  ( 28 ) is closed, switch M 3  ( 30 ) is open and the opposite is true when switch M 2  ( 28 ) is open. Inductor T 3  ( 34 ) has one terminal connected to a common terminal of switches M 2  ( 28 ) and M 3  ( 30 ) and the other terminal to primary ( 36 ) of transformer T 1  ( 38 ) which is connected in series to primary ( 40 ) of transformer T 2  ( 42 ). The other terminal of primary ( 40 ) is connected to one terminal of capacitor C 3  ( 44 ) which has its other terminal connected to VDD ( 26 ). The secondary ( 46 ) of transformer T 1  ( 38 ) has one terminal connected to lamp ( 48 ) and the other terminal to ground. The secondary ( 50 ) of transformer T 2  ( 42 ) has one terminal connected to lamp ( 52 ) and the other terminal to ground. Capacitor C 1  ( 54 ) has one terminal connected to capacitor C 3  ( 44 ) and the other terminal connected to ground. Sense resistor Rsense ( 56 ) has one terminal connected to transformer secondaries ( 46 ) and ( 50 ) and the other terminal connected to ground. Sense resistor Rsense ( 56 ) is used to sense the current in secondaries ( 46 ) and ( 50 ), respectively. The sensed voltage across Rsense ( 56 ) current is provided to control IC ( 18 ) through line ( 60 ). Control IC ( 18 ) also provides control lines ( 62 ) and ( 64 ) to switches M 3  ( 30 ) and M 2  ( 28 ), respectively, to open or close the switches so that one switch is on while the other switch is off and vice versa. 
     In operation, external voltage source Vin ( 22 ) provides a voltage across capacitor C 2  ( 24 ) which builds up to a voltage VDD ( 26 ). To backlight the LCD screen, control IC ( 18 ) provides a control signal on control line ( 64 ) to turn on switch M 2  ( 28 ). This creates one-half VDD ( 26 ) between points N and M along with a voltage divider circuit made up of capacitor C 3  ( 44 ) and C 1  ( 54 ). Inductor T 3  ( 34 ) and transformer primaries ( 36 ), ( 40 ) have one-half VDD applied across them between points N and M. Transformer primaries ( 36 ), ( 40 ) provide the signal applied to transformer secondaries ( 46 ) and ( 50 ) to drive lamps ( 48 ) and ( 52 ) respectively. In the second half of the high frequency switching cycle, the control signal from control IC ( 18 ) is applied to switch M 3  ( 30 ) to turn it on. At the same time switch M 2  ( 28 ) is turned off. Sense resistor Rsense ( 56 ) is used to sense the current in secondaries ( 46 ) and ( 50 ), and provide that information to control IC ( 18 ) through lines ( 60 ). 
     FIGS. 2 b  and  2   c  illustrate a detailed schematic showing the circuitry of each of the blocks of FIG.  1 . Over voltage, over-current protection circuitry  12  and liquid crystal display (LCD) backlight inverter  10  shown in FIG. 1 are shown in detail in blocks  70  and  72 , respectively, in FIGS. 2 b  and  2   c , and contain the circuitry which forms the basis of the invention. Both of these blocks will be described in extensive detail. Start-up sequencing logic  16  shown in FIG. 1 is made up of blocks  74  and  76  in FIGS. 2 b  and  2   c , respectively. The circuitry shown in these blocks cooperates with the short circuit protection circuit. PWM dimming logic  14  shown in FIG. 1 is made up of blocks  78  and  80  in FIG. 2 b . The circuitry shown in these blocks which cooperates with the short circuit protection circuit. Control integrated circuit  18 , Alpha IC(U 2 ), shown in FIG. 1 is shown in detail in FIG. 2 b . FIG. 2 a  shows an enlarged view of the pinout of the Alpha IC U 2 . This circuit is known in the art and provides control and timing to the LCD backlight inverter. 
     FIGS. 3A and 3B shows an enlarged view of over voltage, over-current protection circuitry ( 70 ) and liquid crystal display (LCD) backlight inverter ( 72 ). Looking first at backlight inverter ( 72 ), VDD ( 26 ) represents the same VDD as in FIG.  1  and powers the backlight inverter circuit. Control integrated circuit ( 18 )(shown in FIG. 2 b ) controls switches M 2  ( 28 ) and M 3  ( 30 ). These switches are connected across VDD  26 . When switch M 2  ( 28 ) is closed, switch M 3  ( 30 ) is open and the opposite is true when switch M 2  ( 28 ) is open. Inductor T 3  ( 34 ) has one terminal connected to a common terminal of switches M 2  ( 28 ) and M 3  ( 30 ) and the other terminal to primary ( 36 ) of transformer T 1  ( 38 ) which is connected in series to primary ( 40 ) of transformer T 2  ( 42 ). Transformers T 1  ( 38 ) and T 2  ( 42 ) are shown in more detail in FIG. 3 a  and contain the sensing elements ( 142 ) and ( 144 ) which are part of the invention. 
     The other terminal of primary ( 40 ) is connected to one terminal of capacitor C 3  ( 120 ) which has its other terminal connected to VDD ( 26 ). The secondary ( 122 ) of transformer T 1  ( 38 ) has one terminal connected to lamp J 1  ( 124 ) and the other terminal to sense resistor R 1  ( 126 ) and then to ground. The secondary ( 130 ) of transformer T 2  ( 42 ) has one terminal connected to lamp ( 132 ) and the other terminal to sense resistors R 1  ( 126 ), R 59  ( 127 ), R 60  ( 128 ) and then to ground. Capacitor C 1  ( 134 ) has one terminal connected to capacitor C 3  ( 120 ) and the other terminal connected to ground. Sense resistors R 1  ( 126 ), R 59  ( 127 ), R 60  ( 128 ) are used to sense the current in secondaries ( 122 ) and ( 130 ), respectively. The sensed voltage across Rsense including R 1  ( 126 ), R 59  ( 127 ), R 60  ( 128 ) is provided to control IC ( 18 )(shown in FIG.2) through line ( 60 ). 
     The power stage  72  is a half bridge having resonant inductor T 3  ( 34 ) and transformers T 1  ( 38 ) and T 2  ( 42 ). The transformer primary windings are in series, and connect to the split buffer capacitors C 1  ( 134 ) and C 3  ( 120 ). Note that there is no explicit resonant capacitor. However, the parasitic capacitors of the shield and the transformers are significant due to the large output transformer turns ratio. In fact, depending upon the LCD panel construction and turns ratio, the equivalent primary capacitance can reach 1 μF. This capacitance is the resonant element that creates the high voltage. The Q factor and therefore the reactive power have been kept to a minimum in the resonant tank by not adding an explicit capacitor. The current in the inductor and switches is lower than in a high Q system. 
     The over-voltage, over-current protection circuitry  70  enlarged in FIGS. 3A and 3B includes a number of inventive circuits which solve problems associated with sensing a short circuit condition in the transformers T 1  ( 38 ) and T 2  ( 42 ). 
     The first advancement of the invention is to sense a short circuit condition by sensing transformer voltage instead of lamp (load) current or output voltage directly. This is accomplished by sensing the voltage of the third sense winding ( 142 ) and ( 144 ) on the transformers T 1  ( 38 ) and T 2  ( 42 ). 
     One challenge of the implementation of the first advancement is that dimming of the lamps is implemented by periodically turning off the oscillations in the power stage with the circuitry in PWM Dimming Block  78  and extinguishing the lamps. During that time, the voltage of lamps J 1  ( 124 ) and J 2  ( 132 ) and transformers T 1 ( 38 ) and T 2  ( 42 ) goes to zero and without further modification the zero voltage output of the third sense windings  142 ,  144  would be interpreted as a shorted lamp. The second innovation of the invention is to create a track and hold circuit for the filtered lamp third sense winding voltage, so no indication of zero voltage is given during dimming. 
     The third innovation involves the initial startup of the inverter when the lamp voltage is zero. The third innovation prevents a short circuit signal from occurring while the lamp voltage is less than a threshold signal indicating a short circuit. 
     A fourth innovation is a method of winding the third sense winding on the transformer that improves the signal sensed. 
     Looking at the first advancement of the invention, a short circuit condition is detected by sensing transformer voltage instead of lamp (load) current or output voltage directly. A representation of the lamp voltage is sensed by third sense windings ( 142 ) and ( 144 ) of transformers T 1  ( 38 ) and T 2  ( 42 ) shown in backlight inverter ( 72 ). The sensing continues for both third windings ( 142 ) and ( 144 ) while the lamp circuits J 1  ( 124 ) and J 2  ( 132 ) reach their steady state operating modes. Over-voltage, over-current protection circuitry  70  shows a circuit to rectify and filter each of the sense winding voltages and compare them to a threshold which indicates if a short circuit condition is occurring. In the upper right hand of the schematic corner, the components labeled D 18  ( 150 ), R 43  ( 152 ), R 45  ( 154 ), and C 30  ( 156 ) rectify and filter the winding voltage from transformer T 2  ( 42 ), while D 13  ( 160 ), R 42  ( 162 ), R 52  ( 164 ), and C 34  ( 166 ) do the same for the other transformer T 1  ( 38 ). A comparison is made with a reference or threshold level voltage in IC U 7  ( 170 ) in comparators ( 172 ) and ( 174 ) respectively. The threshold setting indicates when a short circuit has occurred. The threshold setting circuit consists of R 55  ( 176 ), R 56  ( 178 ), D 17  ( 180 ), and C 35  ( 182 ). 
     One challenge of this implementation is that dimming of the lamps J 1  ( 124 ) and J 2  ( 132 ) is implemented by periodically turning off the oscillations in the power stage and extinguishing the lamps. The oscillator is made up of R 8 , C 24 , C 8 , R 9  (shown in FIG. 2 b ) connected to the DIM pin in control IC  18 . During that time, the lamp voltage goes to zero and without further modification the voltage sensing circuit, including third windings ( 142 ) and ( 144 ), would interpret the zero voltage as a shorted lamp. A second innovation of the invention is to create a track and hold circuit for the filtered lamp voltage so it will not go to zero during dimming. Switches, such as transistors Q 7  ( 158 ) and Q 6  ( 159 ), are placed in series with the filter resistors R 43  ( 152 ) and R 52  ( 164 ) respectively so that there is no DC current path from the filter capacitor to ground. During oscillation, switchs Q 7  ( 158 ) and Q 6  ( 159 ) are on, and the filters operate normally. When the oscillation is interrupted, the switches Q 7  and Q 6  are off, and the capacitors C 30  and C 34  hold their voltage. Since the voltage does not decay, an erroneous short circuit signal is avoided. When oscillation resumes, the capacitor voltages are already at their proper value, and circuit operation begins exactly where it left off. 
     When the backlight inverter initially starts up, the lamp and third windings ( 142 ) and ( 144 ) voltages are zero. A third innovation prevents a short circuit signal from occurring while the lamps and third windings ( 142 ) and ( 144 ) voltages are less than the threshold level ( 174 ) established by threshold setting circuit consisting of R 55  ( 176 ), R 56  ( 178 ), D 17  ( 180 ), and C 35  ( 182 ). This resistive divider is connected to a logic signal instead of a fixed power supply voltage. This logic signal, “preheatb” ( 184 ) is zero volts during the time the control IC ( 18 )(shown in FIG.2) is in preheat mode, and rises to the zener voltage of D 17  ( 180 ) when the preheat mode ends. 
     Startup waveforms of this circuit are shown in FIG.  4 . The power circuit oscillates during preheat mode, so voltage builds up upon C 30  ( 156 ) and C 34  ( 166 ). At the end of preheat mode, the C 30  ( 156 ) and C 34  ( 166 ) voltages are greater than the threshold ( 174 ), and continue to rise as the control IC ( 18 ) attempts to ignite the lamp. The threshold voltage ( 174 ) also rises during ignition, but the presence of C 35  ( 182 ) slows its rise. Therefore we are assured that the short circuit sensing voltages C 30  ( 156 ) and C 34  ( 166 ) are always greater than their threshold level ( 174 ), and the circuit starts properly. FIG. 4 shows the complete operation of the circuit during startup, although the time axis is not to scale. The power supply VDD (FIG. 1) rises to 12V when the user plugs in the unit and turns on the power supply. When the ENABLE signal (J 3  in FIG. 2) is raised to true (5V) by the user or after 100 msec delay, a charge pump circuit turns on chip_VDD (from the source of M 1  in block  76 ) and boosts it above 12V. The VL pin of the control IC  18  follows chip_VDD. When VL crosses 1.5V, U 7  pulls preheatb ( 184 ) low. When chip_VDD crosses the startup voltage of the undervoltage lockput circuit (about 11.5), oscillation of the power circuit begins, and the preheat and protection circuit of the control IC  18  begins to oscillate and count triangular pulses at the CP node. During the preheat period voltage builds up across the lamps and the sensing circuits for short circuit protection also build up voltage. The preheat mode ends after 16 pulses at CP. A this time, the VL node drops and remains less than 1 V during ignition and normal operation of the lamp. The preheatb signal goes high, turning on Q 4  and increasing the CP capacitance. The ignition period lasts 8 CP pulses, each much longer than before due to the increased capacitance. When preheatb goes high, the threshold voltage for short circuit protection is released from zero and begins to charge. It always remains less than the sensed voltages (unless there is a short circuit). If the lamp has not ignited at the end of the ignition period, the control IC  18  latches in standby mode. 
     A fourth innovation is a method of winding transformers T 1  ( 38 ) and T 2  ( 42 ) that improves the signal from third sense windings ( 142 ) and ( 144 ). A schematic for the windings of transformers T 1  ( 38 ) and T 2  ( 42 ) is shown in FIG. 5 a . The primary winding is labeled  5 - 7  ( 200 ), the secondary is labeled  3 - 1  ( 202 ), and the sense winding is labeled  9 - 10  ( 204 ). A schematic of the bobbin on which each of transformers Ti and T 2  are wound is shown in FIG. 5 b.    
     Looking more closely at FIG. 5 b , thin diameter Cold Cathode Fluorescent Lamps (CCFL), such as those used as LCD backlights, require a high voltage. In prior art CCFL lamps in order to minimize the voltage between turns of the transformer secondary, this winding is usually wound on a long bobbin with one section for the primary and several sections for the secondary somewhat similar to the inventive bobbin shown in FIG. 5 b . Such a construction leads to a relatively high leakage inductance or equivalently relatively poor coupling between the windings. However, in the prior art the sensing winding is connected to the low voltage control circuitry and is normally wound in the same section as the primary winding. In the invention shown in FIG. 5 b , the sense winding  9 - 10  ( 204 ) is placed in the low voltage section of the secondary winding  3 - 1  ( 202 ) underneath the winding. This low voltage section is the one that should be closest to the primary. The sense winding  9 - 10  ( 204 ) is coupled more tightly to the secondary than in the normal prior art case. This means that the sensed voltage will be a more accurate representation of the secondary voltage. When one of the outputs (lamps J 1 ( 124 ) or J 2  ( 132 )) is shorted, the sensed voltage from one of the third sense windings ( 142 ) or (  144 ) will decrease more than when the winding is in the primary section ( 200 ). Of course, the sensed voltage will not decrease to zero, because there will still be magnetic flux that is coupled by the secondary winding ( 202 ) but not by the sense winding ( 204 ). The voltage across the sense winding will be the difference between the output voltage (zero) and that across the secondary leakage inductance. The sense winding can also be connected to the secondary winding in a tapped or autotransformer configuration. 
     Looking at the remaining blocks of FIGS. 2 b  and  2   c , blocks  74 , 76  make up the start sequence logic  16  of FIG.  1 . Block  74  is made up of Q 4 , C 11 , C 12 , D 10  and R 36  connected as shown in FIG. 2 b . Block  76  is made up of logic circuit U 8  manufactured by Philips Semiconductor, R 47 , R 46 , Q 5 , R 49 , D 15 , C 32 , C 21 ,C 31 , R 29 , D 14 , D 11  land M 1  connected as shown in FIG. 2 c.    
     The backlight ballast is dimmed with ON-OFF (PWM) dimming. Blocks  78  and  80  make up dimming logic  14  shown in FIG.  1 . Block  78  is made up of logic circuit U 3  manufactured by Philips Semiconductor, R 50 , R 35 , M 3 , Q 3 , R 3 , R 4 , D 16 , D 4 , C 1 , C 13 ,C 14 , R 17 , R 18  and D 14 , connected as shown in FIG. 2 b . Dimming is achieved by stopping full power operation for variable intervals at a frequency of about 150-200 Hz. The logic circuitry of U 3  takes a pulse width modulated signal (PWM)), DIM, from an external control device, i.e. control IC  18 . When the “DIM” signal is low, the power stage oscillates, when it is high, the oscillation stops. The PWM signal must be synchronized to the power stage oscillation so that the switching stops the same way from cycle to cycle. Synchronization ensures that flicker in the display is minimized. The “LampOn” output of V 3  goes low when DIM rises AND “G2in”, the M 2  gate drive output of the control integrated circuit  18 , rises. LampOn goes high when the PWM signal falls, and oscillation resumes. 
     PWM control block  80  is made up of D 5 , M 2  and R 15  (shown in FIG. 2 b ). When the PWM “OFF” signal comes in to R 15 , instead of shutting down control integrated circuit  18 , the oscillation is halted by injecting current from Q 1  in block  80  into the CF pin of control integrated circuit  18 . After injecting enough current, the control integrated circuit  18  is unable to discharge C 8 , and the control integrated circuit  18  oscillator stops with G 2 in high (shown in block  78 ). In block  78  the LampOn output is logically ANDed with G 2 in to create G 2 out, ensuring that the both gate drives are off when oscillation is blocked. Large power MOSFETS, Q 2  and Q 3  are used to boost the output current. 
     Integrated circuit U 8  in block  76  (shown in FIG. 2 c ) and its surrounding components perform a power-on reset function. They ensure that the DIM signal does not stop oscillation while the power supply to the control integrated circuit  18  is rising and the lamps ignite. Only after a time delay is the dimming enabled. At that time PWM dimming can begin. 
     Integrated circuits U 7  and U 8  in blocks  70  and  76 , respectively, contain the circuitry that allows the microprocessor board to enable or disable the backlight inverter. A 5 V ENABLE input from J 3  is shifted to 12 V and delayed by the inverter of Q 5  in block  76  and the following RC filter. A diode, D 15  is used to reduce the delay to disable the inverter. The shifted and delayed enable signal turns on a p-channel MOSFET, M 1 , providing power to control integrated circuit  18 . 
     While the preferred embodiments of the invention have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.