Abstract:
The present invention is an overvoltage protection and automatic re-strike circuit for an electronic ballast. The electronic ballast has an inverter, a shut-down circuit, a safety circuit, a monitoring circuit, and an overvoltage protection circuit. The inverter provides an appropriate alternating current power supply to operate the lamp. The shut-down, safety, monitoring, and overvoltage protection circuits are coupled to the inverter and provide the overvoltage protection and automatic re-striking functions. During an overvoltage condition, the overvoltage protection circuit will temporarily disable the inverter. Subsequent to the overvoltage condition, the overvoltage protection circuit permits the inverter to attempt to re-ignite the lamp. After a predetermined number of unsuccessful re-ignition attempts, the safety circuit will permanently disable the inverter to avoid damage to the ballast.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to electronic ballasts used for powering gas discharge lamps. More particularly, the present invention pertains to methods and circuits for providing overvoltage protection and automatic lamp re-striking in an electronic ballast. 
     Electronic ballasts for gas discharge lamps, e.g. fluorescent lights, are well known in the prior art. Electronic ballasts can provide, among others, the means to ignite and operate the gas discharge lamps. 
     Gas discharge lamps are lit through a variety of methods. For exemplary purposes, one method requires the lamp, having an elongated tube with a phosphor coating on the inside surface, to be subjected to a large voltage differential between its terminals. This large voltage differential is sufficient to generate an electrical pathway between the terminals (the voltage differential is greater than the breakdown voltage between the lamp terminals). The resultant current flowing between the terminals excites gaseous atoms, already present in the tube of the lamp, which in turn causes the gaseous atoms to release photons. These photons are outside of the visible spectrum, typically, in the ultraviolet range. These ultraviolet photons bombard the atoms comprising the phosphor coating of the tube and cause the phosphor coating to release photons which are in the visible spectrum. In this way visible light is produced. 
     The ballast occupies an integral role in this process. The ballast supplies the means to ignite the lamp through the process detailed above. Once the lamp is ignited, the ballast also regulates the electrical current that flows through the lamp. Without the regulation efforts of the ballast, the current demanded by the lamp would be significant because once the lamp begins to operate it presents very little electrical resistance. If there was not a mechanism to curtail the current demanded by the lamp, the lamp would be impractical to use. 
     Of particular import is the ability of the ballast to reliably ignite, or re-ignite, the lamp after the lamp malfunctions or is replaced. Ideally, the ballast should successfully ignite the lamp with only one attempt but it is not unusual for a ballast to make a series of ignition attempts before the lamp actually ignites. This succession of ignition pulses engenders the ballast ignition system with a degree of robustness. 
     However, it is also desirable for the ballast to recognize when a lamp is faulty and cannot be lit or when no lamp is present. In either case it would be advantageous for the ballast to appreciate that further ignition attempts will be fruitless. Unfettered re-ignition attempts can pose safety risks to those exposed to the lamp fixture because the ballast must generate a significant voltage to induce the lamp to ignite. Moreover, continuous ignition or re-ignition attempts needlessly stress the ballast and can lead to premature component fatigue and eventual failure. Consequently, a ballast that can generate a series of ignition pulses to effectively ignite a lamp and can also diagnose when further ignition attempts are ill advised is desirable. 
     No less crucial than ignition concerns is a the ability of the ballast to guard against potentially damaging overvoltage conditions, such as when the lamp experiences input arcing or unsuccessful ignition attempts. To effectively forestall damage from overvoltage conditions, the ballast must expeditiously recognize and suppress the overvoltage condition before irreparable damage occurs. As with unnecessary ignition attempts, overvoltage conditions are deleterious to the ballast because the ballast&#39;s components are stressed. Prolonged and/or excessive overvoltage conditions can stress the components until they fail. 
     As discussed above, when a ballast attempts to ignite a lamp, a large voltage differential is presented across the lamp terminals. Typically, this voltage differential is applied across the terminals by an inverter. For a myriad of reasons a lamp may not ignite even with a sufficient voltage differential across its terminals—alternatively, ignition may not even be possible if no lamp is present. If the differential were allowed to build beyond this point the ballast may be damaged, in addition to posing dangers for individuals working around these ballasts. To prevent this from happening the ballast needs an overvoltage protection mechanism to disable the inverter or otherwise safely dissipate the accumulated voltage differential. Additionally, to effectively protect the ballast, the overvoltage protection mechanism must rapidly address this overvoltage condition. 
     Thus, a contentious relationship exists between providing a voltage differential large enough to effectively ignite the lamp, overstressing the ballast by exposing the ballast to extreme voltages or high voltages for prolonged periods of time, and mitigating potential hazards to persons dealing with the ballast. As such, a ballast capable of expertly managing these concerns, particularly any associated overvoltage conditions that may arise, is paramount to safe and reliable ballast operation. 
     The prior art has not left these concerns unaddressed. Conventional ballasts disclosed in the prior art handle overvoltage conditions by completely disabling the inverter or retarding the output of the inverter. Prior art ballasts also teach systems having multiple re-strike ignition capabilities that can be limited to a predetermined number of attempts. For example U.S. Pat. No. 7,015,652 issued to Shi discloses one such ballast. Shi teaches a ballast having an overvoltage protection system with multiple re-strike capabilities that can be controlled. However, the prior art does not include a ballast that has a reliable, safe, automatic re-striking capability following an overvoltage shutdown condition, an overvoltage protection mechanism that responds with sufficient speed to protect the ballast regardless of the cause of the overvoltage condition, and the ability to recognize when further re-ignition attempts should cease, e.g. a faulty lamp. 
     What is needed, then, is a ballast that provides overvoltage protection and re-ignition systems that cooperate to produce an effective, reliable ballast in a simple implementation so that measured automatic re-ignition attempts are made after the ballast has reacted to an overvoltage condition. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is an electronic ballast for a gas discharge lamp having an overvoltage protection system and an automatic re-striking function. The electronic ballast has an inverter, a shut-down circuit, a safety circuit, a monitoring circuit, and an overvoltage protection circuit. The inverter provides an appropriate alternating current power supply to operate the lamp. The shut-down, safety, monitoring, and overvoltage protection circuits are coupled to the inverter and provide the overvoltage protection and automatic re-striking functions. 
     The overvoltage protection circuit is able to detect an overvoltage condition in the inverter. In one embodiment, this detection is accomplished by a sensor magnetically coupled to a resonant circuit of the inverter. The overvoltage may be the result of a ballast or lamp failure condition. If an overvoltage condition is detected, the overvoltage protection circuit will temporarily disable both the inverter and the monitoring circuit via the shut-down circuit, which is operably connected to the power supply for the inverter. This temporary disablement allows the overvoltage condition to dissipate. When the overvoltage condition is no longer present the overvoltage protection circuit will permit the inverter to institute re-ignition efforts. 
     The safety circuit operates to permanently disable the inverter when a safety threshold has been exceeded. The safety threshold is exceeded if the inverter experiences more than a predetermined number of overvoltage events or conditions. This threshold can be adjusted by the selection of ballast circuit components. The threshold corresponds to a state indicating that the ballast has failed, the lamp has failed, or no lamp is present. Thus, when an overvoltage condition is present, or immediately thereafter, and the safety threshold is exceeded, the safety circuit, via the shut-down circuit, will prevent the inverter from attempting to re-ignite the lamp. Subsequent to this scenario, the ballast will function only after the lamp has been replaced or the power to the ballast has been recycled. Accordingly, the final state of the inverter, i.e. its ability to attempt re-ignition, hinges on whether, during the overvoltage event, the safety threshold was exceeded. 
     To ensure that the safety circuit does not prematurely or inadvertently disable the inverter, the monitoring circuit prevents the safety circuit from functioning under normal inverter operating conditions. Thus, in order for the safety circuit to activate, the overvoltage protection circuit must first disable the monitoring circuit, as occurs during an overvoltage condition, and the safety threshold must be exceeded. The interaction between the safety, monitoring, shut-down, and overvoltage protection circuits engender the ballast with the ability to rapidly detect and correct overvoltage conditions, re-ignite the lamp after an overvoltage condition, and recognize that an anomaly exists with the ballast or lamp and cease re-ignition attempts. 
     For example, if a new lamp is inserted into the ballast and the lamp is not lit by the first attempt, the inverter may encounter an overvoltage condition. To prevent damage to the lamp or the ballast, the overvoltage protection circuit, via the shut-down circuit, will temporarily disable the inverter and the monitoring circuit until the overvoltage condition passes. After the overvoltage condition subsides the inverter will be free to attempt to ignite the lamp again, assuming the safety threshold was not exceeded. If a re-ignition attempt is successful and the inverter is within normal operating parameters, the monitoring circuit will obviate the safety circuit&#39;s ability to disable the inverter. 
     Now consider that the ballast contains a faulty lamp. In this scenario, the inverter will unsuccessfully attempt to light the lamp, which results in an overvoltage condition that that is corrected by the overvoltage protection circuit. During each overvoltage condition, the overvoltage protection circuit disables the monitoring circuit, in addition to the inverter, so that the safety circuit may evaluate the state of the ballast and/or lamp. After some number of unsuccessful attempts, and during or immediately after the overvoltage event, the safety threshold will be exceeded and the safety circuit will permanently disable the inverter. The inverter will be disabled, or locked-up, until either the power to the ballast is cycled or the lamp is removed. 
     Accordingly it is an object of the invention to provide an electronic ballast having an overvoltage protection circuit. 
     It is another object of the invention to provide an electronic ballast with an automatic re-striking capability. 
     It is yet another object of the invention to provide an electronic ballast with an overvoltage protection circuit that temporarily disables the inverter to correct overvoltage conditions and permanently disable the inverter after predetermined number or sequence of ignition attempts. 
     It is still another object of the invention to provide an electronic ballast that can rapidly respond to overvoltage conditions to avoid damage to the ballast. 
     It is also an object of the invention to provide an electronic ballast that can reliably re-start after an overvoltage condition has subsided. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a block diagram of one embodiment of the present invention. 
         FIG. 2  is a schematic drawing of one embodiment of the invention shown in  FIG. 1 . 
         FIG. 3  is a flow chart describing a sequence of steps implemented by the method of the invention to address overvoltage conditions. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Functional Overview 
       FIG. 3  illustrates a sequence of steps in which the method of the invention evaluates and corrects overvoltage conditions and provides automatic re-striking capabilities. Initially, it is determined if an overvoltage condition exists, as shown in step  100 . If no overvoltage condition is detected then step  102  instructs that no action is taken. However, if an overvoltage condition is detected step  104  then determines if the safety threshold has been exceeded. If the safety threshold has been exceeded, indicating that an anomaly with the lamp or ballast exists, then the inverter is disabled, as depicted in step  106 . Conversely, if the safety threshold has not been exceeded then step  108  proffers that the inverter be temporarily disabled so that the overvoltage condition may subside. Finally, after the overvoltage condition has dissipated, the inverter will automatically attempt to ignite or re-ignite the lamp, as described in step  110 . 
     Now referring to  FIGS. 1 and 2 , the electronic ballast  10  for a gas discharge lamp has an inverter  12  that receives a rectified DC rail voltage and generates a relatively high frequency AC voltage suitable to operate a gas discharge lamp. The ballast  10  also includes a shut-down circuit  14  coupled to the inverter  12 . Preferably, the shut-down circuit  14  is coupled to the power supply node  16  of the inverter  12  so that when the shut-down circuit  14  is activated, the shut-down circuit  14  will deny the inverter  12  sufficient power to operate—causing the inverter  12  to be disabled. It is also envisioned that the shut-down circuit  14  may be connected to an enabling node on the inverter  12 , which must be set for proper operation, thereby permitting the shut-down circuit  14  to prevent the inverter  12  from continuing to supply power to the lamp. 
     It is further contemplated that the shut-down circuit  14  may indirectly control the operation of the inverter  12  by manipulating ballast circuit components that condition and supply the signals received by the inverter  12  or otherwise facilitate the operation of the inverter  12 . For instance, a power factor correction circuit (not shown) may supply the inverter  12  with a conditioned signal and if the shut-down circuit  14  disables the power factor correction circuit the inverter  12  is also restricted from properly functioning. Regardless of the mechanism, the shut-down circuit  14  superintends the inverter  12 . 
     The ballast  10  also includes a safety circuit  18  coupled to the inverter  12  and the shut-down circuit  14 . The safety circuit  18  evaluates the state of the inverter  12  and functions to instruct the shut-down circuit  14  to disable the inverter  12  if a safety threshold is exceeded. Once the inverter  12  has been disabled at the direction of the safety circuit  18 , the inverter can only be restarted if the ballast  10  is reset. This may occur if the power to the ballast  10  is cycled or a lamp is removed and replaced in the ballast  10 . 
     The safety circuit  18  is designed to permanently disable (until the ballast power is cycled or a lamp is replaced) the inverter  12  when the safety threshold has been exceeded. The safety threshold may be exceeded if the ballast or lamp is faulty or if no lamp is connected to the ballast  10 . As will be discussed in greater detail below, the inverter  12  will attempt to ignite, or re-ignite the lamp in any of the preceding conditions, i.e. faulty lamp, ballast, or no lamp. At some point it is desirable to prohibit any further attempts by the inverter  12  to re-strike (re-ignite) the lamp. The safety threshold serves to set this point. The safety threshold correlates to a predetermined number or cumulative duration of overvoltage conditions/events or a similar measure. 
     The desirability to restrict re-ignition attempts stems from the inexpedient results that may accompany limitless re-ignition efforts. These results include, among others, unnecessary stress on the ballast circuit components and shock hazards to individuals associating with the ballast. The crux of these undesirable effects is the significant voltage that must be developed by the inverter  12  to successfully ignite the lamp. The safety circuit  18  recognizes when additional ignition attempts are ill advised and stifles any such efforts by the inverter  12 . 
     The monitoring circuit  20  is operably engaged to the inverter  12 , the shut-down circuit  14 , and the safety circuit  18 . The monitoring circuit  20  prevents the safety circuit  18  from activating, and permanently disabling the inverter  12 , during normal operating conditions (or normal inverter operating conditions). Normal operating conditions are those conditions in which the ballast  10  is functioning within acceptable parameters. More specifically, normal operating conditions are those other than overvoltage conditions/events and, potentially, immediately thereafter. An overvoltage condition may occur when the lamp or ballast malfunctions or no lamp is present and the inverter  12  generates a large voltage differential in an endeavor to re-strike or re-ignite the lamp. Thus, as long as the inverter  12 , or the ballast  10  in general, is operating within acceptable limits, the monitoring circuit  20  will preclude the safety circuit  18  from activating. 
     The overvoltage protection circuit  22  is capable of detecting overvoltage conditions in the inverter  12  or ballast  10 . Furthermore, once an overvoltage condition has been detected, the overvoltage protection circuit  22  will temporarily disable the inverter  12  via the shut-down circuit  14 . By temporarily disabling the inverter  12 , the overvoltage protection circuit  22  allows any unwanted overvoltage conditions to dissipate. Following the elimination of the overvoltage condition, the overvoltage protection circuit  22  will allow the inverter  12  to attempt re-ignition of the lamp or otherwise resume normal operation. 
     The overvoltage protection circuit  22  will also disable the monitoring circuit  20  during overvoltage conditions thereby allowing the safety circuit  18  to evaluate the state of the inverter  12  and ascertain if a permanent shut-down is in order, i.e. has the safety threshold been exceeded? If the threshold has been exceeded the safety circuit  18  will instruct the shut-down circuit  14  to disable the inverter  12 . If the safety threshold was not exceeded and the overvoltage condition has ended, the overvoltage protection circuit  22  will permit the monitoring circuit  20  to reactivate which, in turn, disables the safety circuit  18  and allows the inverter  12  resume its operation. 
     In sum, the interaction between the shut-down circuit  14 , the safety circuit  18 , the monitoring circuit  20 , and the overvoltage protection circuit  22  bestow the present invention with the ability to provide rapid overvoltage protection, automatic re-strike capabilities, and the faculties to recognize when to permanently disable the ballast because further re-strike attempts would be detrimental to the ballast or persons around the ballast. 
     Particularly, when the overvoltage protection circuit  22  detects an overvoltage it temporarily disables the monitoring circuit  20  and the inverter  12  through the shut-down circuit  14  until the condition has subsided. While the monitoring circuit  20  is disabled the safety circuit  18  is free to evaluate the state of the inverter/ballast and if the safety circuit  18  determines that the safety threshold has been exceeded, it will permanently disable the inverter  12  via the shut-down circuit  14 . If the threshold has not been exceeded, the safety circuit will permit the inverter  12 , and the monitoring circuit  20 , to activate. Once activated, the monitoring circuit  20  will frustrate any efforts by the safety circuit  18  to disable the inverter  12  as long normal operating conditions persist. However, if the overvoltage protection circuit  22  detects another overvoltage, the above sequence repeats giving the safety circuit  18  another chance to determine if the safety threshold has been exceeded and disable the inverter  12 . 
     II. Detailed Circuit Operation 
     The inverter  12  may have an inverter power supply node  16  (Vcc) with an operating supply potential, a potential sufficient to allow the inverter  12  to properly function. In one embodiment shown in  FIG. 2 , the inverter drive circuit (IC)  28  is powered by capacitor C 15 , which is charged through power supply node  16  (Vcc). Power supply node  16  is fed by V_rail through resistors R 20 , R 21 , R 16 , R 19 , R 1 , diode D 12  and lamp filaments Rf_ 1  and Rf_ 3 . When C 15  is sufficiently charged, the inverter drive circuit  28  will generate inverter switching signals, allowing inverter  12  to commence normal operation. 
     The ballast  10  may also have a disabling node  32  with a potential lower than that of the operating supply potential. The disabling node potential does not meet the demands required to power the inverter  12 . As shown in  FIG. 2 , the shut-down circuit  14  includes a switch  30 , Q 5 , having a pair of terminals. One of the pair of terminals is coupled to the power supply node  16 , and consequently C 15 , and the other of the pair of terminals is coupled to the disabling node  32 , electrical ground in this embodiment. 
     Once the shut-down circuit  14  has been activated, by the monitoring circuit  20  or the safety circuit  18 , the shut-down circuit  14 , via switch  30 , will rapidly discharge capacitor C 15  through the disabling node  32  (essentially short circuiting C 15  to ground). This will effectively disable the inverter  12  by deactivating the inverter drive circuit  28 . As long as switch  30  is activated, i.e. the gate threshold voltage of Q 5  is exceeded, C 15  will not charge up and power the inverter drive circuit  28 . Although the shut-down circuit  14  has been described through a transistor implementation, it would be obvious to one of ordinary skill in the art that a plethora of other implementations may serve to satisfy the same or similar ends. 
     The overvoltage protection circuit  22  may include a sensor  24  coupled to the inverter  12 . The sensor  24  is capable of sensing overvoltage conditions in the inverter  12 . In one embodiment shown in  FIG. 2 , the sensor  24  is a magnetically coupled secondary winding, T_resonant_A, of the inductor T_resonant. However, capacitive and resistive coupling are also within the scope of the invention as is the location of the coupling. T_resonant is coupled to the parallel resonant LC tank circuit (C_preheat and T_preheat). Any overvoltage conditions in the tank circuit will be reflected in the sensor  24 . The overvoltage protection circuit  22  also includes Zener diodes D 30  and D 29 , resistor R 14 , and capacitor C 14  (protecting capacitor) depicted in  FIG. 2 . 
     As the voltage across T_resonant_A increases, such as from an overvoltage condition in the tank circuit, the voltage will cause D 30  to break down and start conducting. Accordingly, D 30  sets the overvoltage condition for the circuit. This will allow C 14  to begin to charge through D 30  and D 22 . Once the voltage across C 14  reaches the turn-on threshold of switch  30 , i.e. Q 5 , the switch  30  will conduct and discharge C 15 . As C 15  is discharged, the inverter  12  will be disabled. As the inverter  12  is not contributing to the overvoltage condition, the condition will subside. 
     Eventually, D 30  will stop conducting, because the biasing voltage relayed through T_resonant_A will fall in accordance with the dissipation of the overvoltage condition, and C 14  will begin to discharge through R 14 . With C 14  no longer supplying an adequate turn-on voltage for the switch  30 , it will stop conducting and allow C 15  to start charging. Once sufficiently charged, C 15  will allow drive circuit  28  to start the inverter  12  and lamp ignition efforts will begin. 
     The actions of the overvoltage protection circuit  22  also impact the operation of the monitoring circuit  20 . The monitoring circuit  20  includes a monitoring switch  34 , also referred to as a second switch (Q 4  in  FIG. 2 ). Referring to  FIG. 2 , the gate of Q 4 , i.e. the control terminal, is coupled to C 15  (and hence Vcc). Accordingly, during the response of the overvoltage protection circuit to an overvoltage condition, i.e. discharging C 15 , Q 4  turns off. This occurs because as C 15  is being discharged through Q 5 , the gate voltage on Q 4  is pulled down below the gate threshold voltage thereby turning off Q 4 . As with the inverter  12 , once the overvoltage condition is over, and C 14  cannot bias Q 5 , C 15  will charge up and eventually turn on Q 4 . Consequently, Q 4  will be conducting during normal operating conditions. 
     The ballast  10  also includes a safety circuit  18  operably coupled to the monitoring circuit  20 , the inverter  12 , and the shut-down circuit  14 . As shown in  FIG. 2 , the safety circuit may include a capacitor C 6 . One terminal of C 6  is coupled to the gate of Q 5  so that when C 6  is sufficiently charged, it may activate Q 5  so that C 15  will discharge—disabling the inverter  12 . The charge level at which the voltage across C 6  is adequate to turn on transistor Q 5  is referred to as the safety threshold or activation level. However, C 6  is only permitted to charge when Q 4  is turned off. When Q 4  is conducting it will prevent C 6  from charging because Q 4  presents a less resistive path than that offered by path including C 6 . Thus, C 6  will be prevented from turning on Q 5  to disable the inverter  12  while Q 4  is conducting. This prevents the safety circuit  18  from permanently disabling the inverter  12  during normal operating conditions. 
     As the overvoltage protection circuit  22  reacts to an overvoltage condition and turns Q 4  off, C 6  is allowed to charge through R 13 , D 25 , and D 18 . When the overvoltage condition has passed and C 15  sufficiently charges to turn Q 4  on, C 6  will once again be precluded from further charging. As long as the safety threshold has not been exceeded, the inverter  12  will be able to attempt re-ignition of the lamp after the overvoltage condition has been corrected. However, after a predetermined sequence of overvoltage correction cycles C 6  will incrementally charge to the extent that it is able to turn on Q 5  and permanently disables the inverter  12 . This sequence can be determined by careful selection of the ballast circuit components. The inverter  12  will be permanently disabled because once C 6  is charged beyond the safety threshold, the inverter  12  will only reactivate if the power to the ballast  10  is cycled or the lamp is removed and replaced. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful OVER-VOLTAGE PROTECTION AND AUTOMATIC RE-STRIKE CIRCUIT FOR AN ELECTRONIC BALLAST, 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.