Patent Publication Number: US-6661182-B2

Title: Lamp ballast system having improved power factor and end-of-lamp-life protection circuit

Description:
FIELD OF THE INVENTION 
     The present invention relates to fluorescent lamps, and more particularly, to ballast circuits for fluorescent lamps including a self-oscillation circuit having a high power factor and an end-of-lamp-life protection circuit. 
     BACKGROUND OF THE INVENTION 
     In the lighting of fluorescent lamps, a gas enclosed within a glass tube is caused to become ionized, thus reducing a breakdown voltage between electrodes placed at opposite ends of the glass tube. Ionization is initiated by heating of the electrodes. Once the gas is sufficiently ionized, a voltage at or above the breakdown voltage is placed across the lamp electrodes to thereby cause a current arc to form across the electrodes. The arc produces a bright glow within the lamp tube and produces radiation that activates a fluorescent coating on the inner surface of the glass tube, to thereby produce a bright light. 
     In controlling the turning on and off of fluorescent lamps, it is necessary to control the current to the lamp and to provide a starting voltage. In fluorescent lamps, this task is performed by a circuit called a ballast, also referred to as a ballasting circuit. There are generally two types of ballasts: magnetic ballasts and electronic ballasts. 
     Presently, most low wattage fluorescent lamps utilize magnetic ballasts that include magnetic chokes or suitable magnetic transformers and glow bulb starters. The magnetic choke limits current flow to the lamp while the glow bulb starter creates a voltage spike across the lamp after sufficiently preheating the electrodes. These magnetic ballasts are considered inefficient because of considerable power dissipation in the magnetic components. Moreover, these ballasts exhibit low power factors because of the highly inductive reactances of the magnetic chokes. The power factor is the ratio of the average (or active) power to the apparent power (root-mean-square [rms] voltages times rms current) of an alternating circuit. 
     Further, the glow bulb starters associated with these ballasts exhibit random starting times that produce unpleasant flashes or flickering as an arc attempts to be established across the electrodes of the lamp. This is especially true at low line voltages because the ballasts permit too much voltage to be applied to the bulbs, due to the inadequacies in the ballast design. Arcs are then produced across the bimetal components of the bulbs as the voltage will be nearly high enough to sustain arcing, and annoying flickering and restriking occurs. As a result, the performances of glow bulb starters are not predictable and this results in unreliable starting times of the fluorescent lamps. 
     Electronic ballasts are very expensive and can suffer from poor reliability due to the larger number of components involved. In these ballasts, a variety of electronic components are utilized to heat up the electrodes of the lamp and to establish the breakdown voltage across the electrodes. In addition, in conventional electronic ballasts a large number components, including integrated circuit components, are required to control the power factor of the conventional electronic ballasts (i.e., See Wang et al., U.S. Pat. No. 6,300,723). 
     Magnetic ballasts have reliability problems after 6,000 cycles because of contact wearout in the associated glow bulb starters therewith. Electronic ballasts suffer from similar reliability problems because of the larger number of discrete components used. 
     In addition, to meet Underwriters Laboratory, Inc. safety standards for current leakage of an electronic ballast while replacing a lamp (i.e., relamping), there is a need for an electronic circuit that is able to sufficiently protect against current leakage during relamping without using a large number of components and while still having a high power factor correction. 
     SUMMARY OF THE INVENTION 
     To overcome the above identified problems of a conventional ballast circuit, a ballast system consistent with the present invention is provided that has an improved power factor resulting in a more efficient operation of a lamp. Furthermore, the ballast system also detects end-of-lamp-life of a lamp to protect against high voltage conditions that may occur as the lamp fails to draw sufficient current. 
     In accordance with articles of manufacture consistent with the present invention, a ballast system for use with a fluorescent lamp having two filaments disposed at opposite ends of the lamp is provided. The ballast system includes a DC input terminal for connection to a DC voltage source or for receiving a rectified DC signal, a capacitor operably connected between the DC input terminal and the lamp, and an inductor. The lamp operably connects the capacitor in series with the inductor. The ballast system also includes a switching means that is operably connected to the DC input terminal and to the capacitor for sensing a change in voltage across the inductor and for controlling current from the DC voltage source to the capacitor in response to the change in voltage across the inductor. The switching means controls the current to the capacitor such that the current has a waveform and a frequency that is preferably approximately equal to a series resonant frequency defined by the capacitor and the inductor. 
     The ballast system may also include an electronic starter circuit operably connected between the switching means and the DC input terminal such that the electronic starter circuit triggers the operation of the switching means when the predetermined voltage level is present on the DC input terminal. 
     The ballast system may also include a startup capacitor operably connected between the two filaments of the lamp and a startup resistor operably connected in parallel to the startup capacitor. In this implementation, the ballast system may further include an end-of-lamp-life sensor operably connected across one of the two filaments of the lamp. The end-of-lamp-life sensor is operably configured to detect when a second predetermined voltage level is present at one end of the one filament of the lamp and to momentarily substantially short the one filament causing a pulse with a predetermined magnitude to be sent through the inductor to the switching means when the second predetermined voltage level is detected. 
     The ballast system may further include an end-of-lamp-life cutoff circuit and a transformer that has the inductor as a primary winding and that has a secondary winding. In this implementation, the end-of-lamp-life cutoff circuit is operably connected to the secondary winding and to the switching means. The end-of-lamp-life cutoff circuit also has means for causing the switching means to inhibit current flow to the lamp when the pulse with the predetermined magnitude is sent through the inductor and sensed via the secondary winding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the present invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings: 
     FIG. 1 depicts a block diagram of an exemplary ballast system embodying aspects of the present invention for energizing a fluorescent lamp and detecting an end-of-life for the fluorescent lamp; 
     FIG. 2 depicts an exemplary schematic diagram of the ballast system in FIG. 1; and 
     FIG. 3 depicts a schematic diagram of another exemplary ballast system for two or more fluorescent lamps embodying aspects of the present invention for energizing two or more fluorescent lamps and detecting an end-of-life for the two or more fluorescent lamps. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A ballast system embodying principles of the present invention has an improved power factor resulting in a more efficient operation of a lamp. Furthermore, the ballast system may also detect the end-of-lamp-life of the lamp to protect against high voltage conditions that may occur as the lamp fails to draw sufficient current. 
     FIG. 1 depicts a block diagram of an exemplary ballast system  100  embodying aspects of the present invention. The ballast system  100  is operably connected, such as through electrical and/or optical connections, to a fluorescent lamp  10  having electrodes or filaments  12  and  14 . Lamp  10  may be any standard interchangeable fluorescent lamp, such as ones configured to meet the known designation F8T5 (i.e., fluorescent 8 Watt, ⅝ inch diameter) or F13T5 (i.e., fluorescent 8 Watt, ⅝ inch diameter). 
     The ballast system  100  includes terminals  102  and  104  that are configured to be operably connected to an incoming alternating current (AC) source for powering the lamp  10 . The ballast system  100  may also include another terminal  106  for operably connecting the ballast system  100  to house or earth ground. 
     The ballast system  100  further includes a self-oscillating circuit  108  that is operably connected to the lamp  10  and that has a DC input terminal  111 . The ballast system  100  may also include a rectifier filter  110  that is operably connected to terminals  102  and  104  and to the DC input terminal  111  of the self-oscillating circuit  108  as shown in FIG.  1 . As described in greater detail below, the self-oscillating circuit  108  provides a starting voltage and current limitation to the lamp  10  while exhibiting a high power factor correction, such as 95% or more. The rectifier filter  110  converts an AC signal when present on terminals  102  and  104  to a DC signal having substantially small amount of ripple or periodic variations in voltage for input (i.e., via DC input terminal  111 ) into self-oscillating circuit  108 . 
     In another implementation, self-oscillating circuit  108  may be operably connected to a DC voltage source (not shown in figures), such as a battery, in lieu of the rectifier  110 . 
     The ballast system  100  may also include a startup capacitor  112  operably connected in series with filaments  12  and  14  of the lamp  10 , and a resistor  114  coupled across the capacitor  112 . The startup capacitor  112  determines the level of preheat current through filaments  12  and  14  when the starting voltage is provided by the self-oscillating circuit  108  to the lamp  10  and current is permitted to flow through the filaments  12  and  14  to light the lamp  10  or between the filaments  12  and  14  within the lamp once the lamp  10  is lit in response to an arc struck between the filaments  12  and  14 . 
     The resistor  114  acts a bleeder resistor to discharge or reduce the voltage held by the capacitor  112  to a safe level when the starting voltage to the lamp  10  is removed (e.g., AC power source switched or turned off) or the lamp  10  itself is removed. The resistor  114  preferably has a significantly higher level of resistance than the lamp  10  when lit such that current flows substantially through the lamp  10  between filaments  12  and  14  and not through resistor  114  when the lamp is lit. In one implementation, the resistor  114  is a thermistor that changes, preferably increases, resistance with a change, preferably a positive change, in temperature. 
     In one implementation, the ballast system  100  may also include an electromagnetic interference (EMI) filter  116  operably connected between the rectifier filter  110  and terminals  102  and  104 . The EMI filter  116  is preferably configured to prevent electromagnetic radiation frequencies or transient power surges on terminals  102  and  104  from interrupting the operation of self-oscillating circuit  102  and from degrading the high power correction of the self-oscillating circuit  108 . 
     In addition, the ballast system  100  may also include an end-of-lamp-life sensor  118  and an end-of-lamp-life cutoff circuit  120 . The end-of-lamp-life sensor  118  is operably connected across one of the filaments  12  or  14  and is configured to detect an over-voltage condition or second predetermined voltage level (e.g., at or above 30 V) that indicates the lamp  10  is no longer drawing a sufficient amount of current through the lamp. When the over-voltage condition is detected, the end-of-lamp-life sensor  114  shorts the one filament (e.g., filament  12 ) of the lamp  10  and sends a pulse to the self-oscillating circuit  102  via resistor  114  to indicate that the over-voltage condition has been detected. 
     As explained in greater detail below, the end-of-lamp-life cutoff circuit  120  is operably connected to the self-oscillating circuit  102  such that end-of-lamp-life cutoff circuit  120  is able to monitor for the pulse from the end-of-lamp-life sensor and to cutoff or disable current flow from the self-oscillating circuit  102  to the lamp  10  in response to receiving the pulse. 
     FIG. 2 depicts an exemplary schematic diagram of the ballast system  100 . As shown in FIG. 2, the rectifier filter  110  includes a full-wave rectifier  202 , which may be a full-wave bridge rectifier using a common arrangement of diodes D 1 -D 4  as shown in FIG. 2, that is operably connected to terminals  102  and  104 . The rectifier filter  110  may also include a low-pass filter  204  operably connected between the full-wave rectifier  202  and the self-oscillating circuit  108 . The full-wave rectifier  202  and the low-pass filter  204  combine to output (i.e., via DC input terminal  111 ) to the self-oscillating circuit  108  a rectified DC signal having little or no ripple voltage when an AC signal is present on terminals  102  and  104 . 
     Self-oscillating circuit  102  includes an electronic starter circuit  210  and an oscillation control circuit  220 . The electronic starter circuit  210  is operably connected to the DC input terminal  111  and to the oscillation control circuit  208  such that the electronic starter circuit  210  triggers the operation of the oscillation control circuit  220  when the rectified DC signal reaches a predetermined voltage level, which is preferably the starting voltage of the lamp  10 . 
     Electronic starter circuit  210  includes a trigger circuit  212 , such as a silicon controlled rectifier (SCR), DIAC, TRIAC, or SIDAC, that operably connects the DC input terminal  111  to a base  242  of a power transistor  240  of the oscillation control circuit  220  when the rectified DC signal reaches the predetermined voltage level so that the power transistor  240  turns on, allowing current to be supplied to the lamp  10  as described in detail below. In the implementation shown in FIG. 2, the electronic starter circuit  210  may also include a resistor  214  operably connected to the DC input terminal  111  and a capacitor  216  operably connected in series with the resistor  214 . In this implementation, the triggering circuit  212  is operably connected to a junction  218  between the resistor  214  and the capacitor  216  such that the capacitor  216  charges to the predetermined voltage in a predetermined time based on the value of the resistor  214  and the value of the capacitor  216 . 
     As shown in FIG. 2, the oscillation control circuit  220  has a choke circuit  222  that includes a capacitor  224  operably connected in series with the lamp  10  (or load for the ballast system  100 ) and a ballasting choke or inductor  226 , which acts to choke or prevent any rapid change in the flow of current to the lamp  10  from the power source. The capacitor  224  and the inductor  226  form a series resonant circuit having a low resonant impedance such that the capacitor  224  compensates for the inductance of the inductor  226  and the resistance of the lamp  10  when the lamp  10  is lit, resulting in a high power factor correction of 95% or more for the oscillation control circuit  220 . In addition, because the electronic starter circuit  210  and the startup capacitor  112  allow for a short or rapid turn-on time of about 200 milliseconds for lighting the lamp  10 , any power factor phase shift is eliminated, which contributes to the high power factor correction. To achieve a high power factor correction of 95% or more, capacitor  224  may have a value in the approximate range of 47 nF±5%, while inductor  226  has a corresponding value in the approximate range of 5.6 mH±5%. In one implementation, capacitor  224  may a value of approximately 47 nF and inductor  226  may have a value of approximately 5.7 mH. In this implementation, capacitor  224  and inductor  226  may be rated to handle voltages up to 600 V as the oscillation control circuit  220  steps up the voltage level supplied by the AC source on terminals  102  and  104  while stepping down the current supplied to the lamp  10 . 
     The self-oscillating circuit  108  may also include a clamp  228 , such as a zener diode, that is operably connected between the DC input terminal  111  and the capacitor. The clamp  228  limits the maximum voltage level presented to capacitor  224  such that other commercially available capacitors with voltage ratings less than 600 V (e.g., 400V) may be used for capacitor  224 . 
     The oscillation control circuit  220  also includes a first power transistor  230  operably connected to a junction joining the DC input terminal  111  to the capacitor  224  and the second power transistor  240  operably connected to drive the first power transistor  230  such that the first power transistor  230  in combination with the second power transistor  240  rapidly switch or oscillate current (and thus power) to the lamp  10  in a substantially sinusoidal waveform at a frequency that corresponds to the series resonant frequency formed by capacitor  224  and inductor  226 . In the example implementation shown in FIG. 2, the first and the second power transistors  230  and  240  each have a respective base  232  and  242  that is operably connected to the inductor  226 , such that each power transistor  230  and  240  switch or oscillate current to the lamp  10  in association with the current through the inductor  226  and corresponding change in voltage across the inductor  226 . 
     To facilitate self-oscillation of the oscillation control circuit  220  (once the second power transistor  240  has been triggered by the electronic starter circuit  210  to turn on) in the implementation shown in FIG. 2, a first terminal or collector  234  of the first power transistor  230  is operably connected to the junction joining the DC input terminal  111  to the capacitor  224 , a first terminal or collector  244  of the second power transistor  240  is operably connected to a second terminal or an emitter  236  of the first power transistor, and a second terminal or an emitter  246  of the second power transistor is operably connected to ground. 
     To further facilitate self-oscillation, the oscillation control circuit  220  also includes a transformer  250  having a primary winding  252  and two secondary windings  254  and  256 . The primary winding  252  is operably connected in series between the inductor  226  and a junction joining the emitter  236  of the first power transistor  230  to the collector of the second power transistor. The first  254  of the two secondary windings  254  and  256  is operably connected to the base  232  of the first power transistor  230  such that an output signal from the collector  234  oscillates in association with the change in voltage across the inductor  226 . In addition, the second  256  of the two secondary windings  254  and  256  is operably connected to the base  242  of the second power transistor  230  such that an output signal from the collector  244  also oscillates in association with the change in voltage across the inductor  226 . Thus, the respective collector output ( 234  and  244 ) of each power transistor  230  and  240  continues to oscillate as the current to each respective base input ( 232  and  242 ) is driven through respective secondary windings  254  and  256  when the lamp  10  is lit as described above. 
     The oscillation control circuit  220  may also include a resistor  247  operably connected in series between the power transistor base  232  and the secondary winding  254 , and a resistor  248  operably connected in series between the power transistor base  242  and the secondary winding  256 . The resistors  247  and  248  each have a respective predetermined value to limit current to the respective base  232  and  242  such that the respective collector output ( 234  and  244 ) of each power transistor  230  and  240  continues to oscillate once the electronic starter circuit  210  triggers the operation of the oscillation control circuit  220 . 
     FIG. 2 also depicts an exemplary implementation of the end-of-lamp-life sensor  118  and the end-of-lamp-life cutoff circuit  120  of the ballast system  100 . In this implementation, the end-of-lamp-life sensor  118  includes a first switch  260  that has a control input  262 , such as a TRIAC, and a second switch  264  that is normally open, such as a DIAC. The first switch  260 , which is also normally open, is operably coupled across the filament  12  of the lamp  10 . The second switch  264  is operably connected to the control input  262  of the first switch  260  and to a junction joining the capacitor  224  of the oscillation control circuit  220  to one end of the filament  12 . In this implementation, the second switch  264  is configured to detect an over-voltage condition or second predetermined voltage level (e.g., at or above 30 V) at the one end of the filament  12  of the lamp  10  that indicates the lamp  10  is no longer drawing a sufficient amount of current through the lamp. When the over-voltage condition is detected, the second switch  264  closes causing the first switch  260  to close momentarily so that the filament  12  of the lamp  10  is substantially shorted, allowing a pulse having a predetermined magnitude to be sent via resistor  114  to the inductor  226  which is then sensed by the end-of-lamp-life circuit  120 . 
     In another implementation, in lieu of the first and second switches, the end-of-lamp-life sensor may include a single switch, such as a SIDAC, operably connected across the filament  12  and that is capable of momentarily closing when an over-voltage condition is detected. 
     As shown in FIG. 2, the oscillation control circuit  220  may also include a transformer  270  in which the inductor  226  is a primary winding of the transformer  270 . The transformer  270  has a secondary winding  272  or inductor that is operably connected to the end-of-lamp-life cutoff circuit  120 , such that the end-of-lamp-life cutoff circuit  120  is able to monitor for the pulse having the predetermined magnitude. 
     Continuing with FIG. 2, the end-of-lamp-life cutoff circuit  120  has a third switch  280 , operably connected between the secondary winding  272  of transformer  270  and the base  242  of the power transistor  240 . The third switch  280  may be any switch (such as a DIAC, TRIAC, or other SCR) that is capable of switching to a closed state upon detection of a pulse having the predetermined magnitude. In this implementation, when the third switch  280  is in the closed state, the power transistor  240  turns off causing the power transistor  230  to also turn off, which inhibits current flow from the oscillation control circuit  120  to the lamp  10 . 
     The end-of-lamp-life cutoff circuit  120  may also include a capacitor  282  operably connected to a junction joining the third switch  280  to the secondary winding  272  and a resistor  284  connected across the capacitor  282 . The capacitor  282  charges to the predetermined magnitude of a detected pulse and discharges at a rate associated with the value of the resistor  284 . Thus, the capacitor  282  and the resistor  284  may combine to hold the third switch  280  in the closed state for a predetermined time sufficient to turn off the power transistor  240 . 
     In a preferred implementation, the end-of-lamp-life cutoff circuit  120  includes a transistor  290 , preferably a small signal transistor, that is operably connected between the third switch  280  and the base  242  of the power transistor  240 , such that the transistor  290  sufficiently lowers the voltage on (e.g., grounds) the base  242  of the power transistor  240  when the third switch  280  is in the closed state. In this implementation, the end-of-lamp-life cutoff circuit  120  may include the capacitor  282  and the resistor  284  operably configured to hold the third switch  280  in the closed state for a predetermined time sufficient to turn on the transistor  290  so that the power transistor  240  is turned off as described above. 
     As shown in FIG. 2, the end-of-lamp-life cutoff circuit  120  may also include a fourth switch  294  having a control input  296  operably connected to the third switch  280  such that the fourth switch is operable when the third switch is closed (i.e., when a pulse having the predetermined magnitude is detected). The fourth switch  294  is operably connected to the collector  234  of the power transistor  230  and to a junction joining the third switch  280  to base of the transistor  290 . The fourth switch  294  is operably configured to close and rapidly direct current flow away from the lamp  10  to ground via transistor  290  when a pulse having the predetermined magnitude is detected as described above, causing the third switch to momentarily close. 
     FIG. 3 depicts a schematic diagram of another exemplary ballast system  300  embodying aspects of the present invention for energizing two or more fluorescent lamps and detecting an end-of-life for the two or more fluorescent lamps. As shown in FIG. 3, the ballast system  300  incorporates the ballast system  100  in FIG.  2 . In addition, the ballast system  300  is operably connected to a second fluorescent lamp  20  having filaments  22  and  44  such that the ballast system  300  supplies current to both lamps  10  and  20  once the electronic starter circuit  210  triggers the operation of the oscillation control circuit  220 . 
     The oscillation control circuit  220  of the ballast system  300  has a second ballasting choke or inductor  326  operably connected in series with the lamp  20 . The inductor  326 , which may be of the same type and have the same value as the inductor  226 , acts to choke or prevent any rapid change in the flow of current to the lamp  20  from the power source. In addition, the inductor  226  and lamp  10  are connected in parallel with the inductor  326  and the lamp  20  so that combination of inductors  226  and  326  and lamps  10  and  20  forms a series resonant circuit with the capacitor  224 . The series resonant circuit has a low resonant impedance such that the capacitor  224  compensates for the inductance of both inductors  226  and  326  making the current more or less in phase with the voltage provided to the lamps  10  and  20  when the lamps  10  and  20  are lit. As a result, the oscillation control circuit  220  has a high power factor correction of 95% or more when supplying power to the lamps  10  and  20 . 
     Once the electronic starter circuit  210  triggers the operation of the oscillation control circuit  220  in the ballast system  300 , collector outputs  234  and  244  of power transistors  230  and  240  of ballast system  300  oscillate current supplied to lamps  10  and  20  as described above in reference to ballast system  100  as the current to base inputs  232  and  242  is driven through respective secondary windings  254  and  256 . 
     As shown in FIG. 3, the ballast system  300  may also include a second startup capacitor  312  operably connected in series with filaments  22  and  24  of the lamp  20 , and a resistor  314  coupled across the capacitor  312 . The startup capacitor  312 , which may be of the same type and have the same value as the capacitor  212 , determines the level of preheat current through filaments  22  and  24  when the starting voltage is provided by the self-oscillating circuit  108  to the lamp  20  and current is permitted to flow through the filaments  22  and  24  to light the lamp  20  or between the filaments  22  and  24  within the lamp once the lamp  20  is lit in response to an arc struck between the filaments  22  and  24 . 
     The resistor  314  operates similarly to resistor  114 , acting as a bleeder resistor to discharge or reduce the voltage held by the capacitor  312  to a safe level when the starting voltage to the lamp  20  is removed (e.g., AC power source switched or turned off) or the lamp  20  itself is removed. The resistor  314  preferably has a significantly higher level of resistance than the lamp  20  when lit such that current flows substantially through the lamp  20  between filaments  22  and  24  and not through resistor  314  when the lamp  20  is lit. 
     As shown in FIG. 3, the ballast system  300  may also include another end-of-lamp-life sensor  318  operably coupled across one (e.g., filament  22 ) of the filaments  22  and  24 . The end-of-lamp-life sensor  318 , which operates in the same manner as the end-of-lamp-life sensor  118 , is operably configured to detect an over-voltage condition or second predetermined voltage level (e.g., at or above 30 V) at the one end of the filament  22  of the lamp  20 , indicating that the lamp  20  is no longer drawing a sufficient amount of current through the lamp  20 . When the over-voltage condition is detected by the end-of-lamp-life sensor  318 , the end-of-lamp-life sensor  318  generates a pulse having a predetermined magnitude to be sent via resistor  314  to the inductor  326  which is then sensed by the end-of-lamp-life circuit  120 . 
     To allow the end-of-lamp-life cutoff circuit  120  to sense the pulse from the end-of-lamp-life sensor  318 , the oscillation control circuit  220  may also include a second transformer  370  in which the inductor  326  is a primary winding of the transformer  370 . The transformer  370  has a secondary winding  372  or inductor that is operably connected to the oscillation control circuit  220 , such that the that the end-of-lamp-life cutoff circuit  120  is able to monitor for or sense a pulse from the end-of-lamp-life sensor  318  that has the predetermined magnitude. In the implementation shown in FIG. 3, the secondary winding  372  of transformer  370  is operably connected to the third switch  280  of the end-of-lamp-life cutoff circuit  120 , such that a pulse from either end-of-lamp-life sensors  118  or  318  that has the predetermined magnitude causes the third switch  280  to close, such that the second power transistor turns off and current flow to both lamps  10  and  20  is inhibited. Thus, when either lamp  10  or lamp  20  is removed or reaches an end-of-lamp-life state (i.e., an over voltage condition exists is association with the respective lamp  10  or  20 ), the ballast system  300  advantageously inhibits current flow to both lamps  10  and  20  until either the missing lamp or lamp at end-of-lamp-life state is replaced. 
     In an alternative implementation, the ballasting system  300  may include another oscillation control circuit (not shown in figures) operably configured to independently provide current to lamp  20  from the AC current source in the same manner as in ballasting system  100 . In this implementation, the ballasting system may also include another end-of-lamp-life cutoff circuit (not shown in figures) to inhibit current flow to lamp  20 , independent of current flow to lamp  10 , when a pulse having the predetermined magnitude is received from the end-of-lamp-life sensor  318 . 
     While various embodiments of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.