Abstract:
A shut-down circuit configured for use with an electronic ballast coupled to a lamp in a control path includes a device for sensing the electrical energy associated with the control path, and a sensing circuit for shutting down the ballast in the event that the energy does not conform to a predetermined condition. The sensed energy may be current indicative of lamp installation or short circuit, or voltage indicative of arcing or open circuit. The device for sensing the electrical energy associated with the control path may be an isolation transformer, or alternative devices such as optical isolators may be employed. The circuitry may further include electronic componentry to disable the sensing circuit during initial energization of the lamp. In one disclosed example, the sensing circuit includes a node that should be at or near a predetermined electrical potential when the lamp is operating properly, and a switch such as a Schmitt trigger coupled to the node that turns on or off to shut down the ballast if the node is not at or near the predetermined electrical potential. In an implementation used to detect voltage fluctuations indicative of arcing, the circuitry may include a high-pass filter or differentiator and detector to detect high-frequency noise. Alternatively, a phase-locked loop may be coupled to a low-pass filter to detect high-frequency noise indicative of arcing. In another alternative implementation, a microprocessor performing a digital signal processing algorithm may determine the presence of an abnormal condition.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/649,885, filed Aug. 26, 2003, the entire content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to electronic ballasts and, in particular, to improvements relating to shutdown of the lighting circuit with abnormal load conditions, such as short circuit, open circuit, or arcing.  
       BACKGROUND OF THE INVENTION  
       [0003]     Existing ballast circuits of the type used with fluorescent and, in some cases, neon lamps suffer from certain technical problems. Particularly with respect to single-pin lamps in DC-powered mobile applications, a single interconnection carries both DC input current and AC output current. This causes AC noise and ripple to feed back into the DC power bus distribution system, which, in turn, can affect performance and operation of other systems connected to the DC buss.  
         [0004]      FIG. 1  is a diagram of an existing single-pin lamp system. A 24-volt DC supply is connected to a ballast  102  and one side of the lamp  110 . Typical of such systems, a safety switch  112  is provided to remove the DC power from the ballast if the lamp is removed. The connections are on either side of the pin, such that when the lamp is removed, the circuit is broken. The other pin  114  of the lamp  110  is connected to the ballast through line  116 .  
         [0005]     The configuration just described establishes two current paths, including a first current path  120  which includes noisy and high-frequency lamp current, and a second DC ballast input current path  122  which should not include any noise. However due to the use of the safety switch  112 , a single line  130  is common to both the lamp current and DC input current paths. As a result, noise or ripple may be coupled into the DC path, causing AC noise to infiltrate the DC power buss distribution system, leading to performance degradation and failure of other equipment powered by the DC buss, in some cases.  
         [0006]     Given that it is not technically straightforward to filter out the conducted electromagnetic interference (EMI) prevalent in existing designs, the need remains for an improved filtering and control circuit configuration.  
       SUMMARY OF THE INVENTION  
       [0007]     This invention resides in a shutdown circuit configured for use with an electronic ballast coupled to a lamp in a control path. In broad and general terms, the circuit includes a device for sensing the electrical energy associated with the load path, and a control circuit for shutting down the ballast in the event that the energy does not conform to a predetermined condition. The sensed energy may be current indicative of lamp installation, short circuit, or removal, or voltage indicative of arcing.  
         [0008]     In the preferred embodiment, the device for sensing the electrical energy associated with a load path is an isolation transformer which performs a high pass filter function; however in other embodiments alternative devices such as optical isolators may be employed. The circuitry may further include electronic circuitry to disable the sensing circuit during initial energization of the lamp. In one disclosed example, the sensing circuit includes a node that should be at or near a predetermined electrical potential when the lamp is operating properly, driving a switch such as a Schmitt trigger that turns on or off to shut down the ballast if the node is not at or near the predetermined electrical potential.  
         [0009]     In an implementation used to detect voltage fluctuations indicative of arcing, the circuitry may include a second stage high-pass filter or differentiator and detector to detect higher frequency noise. Alternatively, a phase locked loop may be coupled to a low-pass filter to detect high-frequency noise indicative of arcing.  
         [0010]     In yet another implementation, a microprocessor performing a digital signal processing algorithm may analyze the energy profile with respect to frequency and make sure that it conforms within a template representative of normal operation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic diagram illustrating the problem associated with existing ballast circuits where an electrical path is common to both the DC and AC circuits;  
         [0012]      FIG. 2  is a simplified block diagram illustrating important components associated with a preferred embodiment of this invention;  
         [0013]      FIG. 3A  is a detailed schematic diagram of the preferred embodiment;  
         [0014]      FIG. 3B  is a redraw of the detailed schematic diagram to assist with understanding the way in which the circuits function;  
         [0015]      FIG. 4  is an alternative sensing circuit incorporating a phase-locked loop;  
         [0016]      FIG. 5  is a different alternative sensing circuit involving a highpass filter and Schmitt trigger;  
         [0017]      FIG. 6  is yet a further alternative sensing circuit incorporating a module to differentiate arcing noise;  
         [0018]      FIG. 7  is yet a different alternative sensing circuit involving an opto isolator and high pass filter;  
         [0019]      FIG. 8  represents a microprocessor based instrumentation of the invention enabling control and sensing to be communicated over a buss; and  
         [0020]      FIG. 9  represents yet a different alternative sensing circuit involving a microprocessor performing a digital signal processing algorithm to identify an undesired lamp load condition. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The system of this invention broadly prevents ripple and ballast-generated noise from feeding back into the buss power system of a lamp control circuit. This is broadly carried out by eliminating the common DC path and lamp current hookup connection to the lamp. This is more particularly carried out by eliminating the safety switching action of the pin and socket interlock currently typical of single pin systems.  
         [0022]     According to the invention, ballast shutdown is performed electronically when the lamp is removed. The circuit of the invention detects the presence of the lamp current to keep the ballast running, but if the lamp is removed or short-circuited, the ballast shuts down to address safety issues. The invention further includes apparatus and methods to detect arcing that occur if a wire becomes loose, foreign material interferes with a good connection, or a socket becomes defective. In the preferred embodiment, the circuit detects arcing conditions that last longer than a predetermined amount, such as 500 milliseconds, or thereabouts and shuts down the ballast if arcing is detected for this extended time. Broadly, because the output of the ballast is entirely isolated from the DC input, filtering inside the ballast can now attenuate noise produced by the ballast to acceptable levels.  
         [0023]     Reference is now made to  FIG. 2 , which depicts in block-diagram form a circuit configuration according to the invention. 24 VDC is provided to a ballast inverter  210 , preferably through a line filter  208 . The output of the ballast inverter  210  is fed to an isolation output transformer  212 , which forms a lamp current path  214  to operate lamp  220 . This configuration allows relatively little noise to couple back into the DC line, because the inverter ripple and noise current  209  are separate from the lamp current path  222 .  
         [0024]     The use of a ballast inverter coupled to an isolation output transformer in a fluorescent lamp control circuit is well-known, but previously limited to the mechanical type of a system shutoff described with respect to  FIG. 1 . Novel to this invention is the use of an architecture featuring a lamp current frequency profile sensing circuit and abnormal condition detection circuit  250  to control the ballast inverter  210 . In the preferred embodiment, isolation sense transformer  240  is used to couple a portion of the lamp current to the sensing circuit  250 . However, as will be described in further detail below, different types of arc detection may be utilized in accordance with this invention.  
         [0025]     Given the simplified block diagram of  FIG. 2 , the reader&#39;s attention is now directed to  FIG. 3 , which shows an actual schematic for a circuit constructed in accordance with this invention.  FIG. 3B  is a redraw of the detailed schematic diagram to assist with understanding the way in which the circuits function In  FIG. 3A , T 1  is the isolation power output current transformer, T 2  is an isolation current sense and voltage step up transformer, and T 3  is a line filter. The DC input voltage (24 VDC) gets connected at the plus and minus terminals (pins  1  and  2  of J 1  respectively) at the bottom of the drawing.  
         [0026]     Q 1  and Q 2  function as inverters. Although they are preferably implemented using MOSFETs, they may be bipolar transistors or other types of switching devices as appropriate. Inverters Q 1  and Q 2  are connected in push-pull fashion into the primary winding of transformer T 1 , that is, the lower windings shown in the diagram having a center tap. L 1  is connected to the center tap via a circuit breaker shown lower in the diagram.  
         [0027]     The MOSFET inverter control element feedback signal is connected via the upper windings of T 1  through R 1 . An initial turn-on bias is provided by resistors R 2  and R 3 . This configuration is known in the art as a Royer type oscillator. Once oscillation commences, D 4  serves to clamp the gate voltages from going too negative. Also, a circuit comprising C 1 , C 2 , R 3 , and R 4  reduces the turn on bias coupled to Q 1  and Q 2  once oscillation begins, by reducing ringing due to parasitic reactance in T 1 . Diodes D 5  and D 6  serve to rapidly discharge remaining gate voltage during switch off of each MOSFET. Diode D 3  provides positive MOSFET gate protection. Conversely, if the load is short-circuited, the harmonic content in the output of the ballast will be negligible. Because T 2  acts to filter out the lamp inverter fundamental frequency, a low output level will occur when harmonic content is low. This has the same effect on the detection circuit as for lamp removal, and the shutdown circuit will activate also.  
         [0028]     Oscillation may be terminated by means external to the inverter by turning on Q 4 , a PNP device which pulls down the voltage at its emitter and shuts off the oscillator circuit when asserted. Q 5  and Q 6  represent a Schmitt trigger used to turn on and off Q 4  very rapidly, causing it to function as a switch as opposed to a linear amplifier. If operated in a linear region of the load line, the power devices Q 1  and Q 2  controlled by its output could become overheated.  
         [0029]     A point “X” has been labeled in the diagram. When the voltage at this point rises to a level of approximately a volt or more, the Schmitt trigger comprised of Q 5  and Q 6  turns on which causes Q 4  to rapidly shut off. During power initialization to the ballast, however, the control capability of the circuit must be temporarily disabled or overridden to allow the lamp to fire. This is accomplished through the use of capacitor C 10  along with R 14 , R 15 , and Q 7 . On initialization, or first turn on, DC is applied to the filter through the circuit breaker device CB 1  and through the diode D 13  which isolates that portion of the circuit which is sensitive to noise, filtered by C 12 . In particular this isolates the Schmitt trigger and Q 4  from false activation.  
         [0030]     In other words, the capacitor C 10  upon acting temporarily as a short circuit during initialization, causes the rise in supply voltage approach to 24 volts to turn Q 7  on, energizing point “X”, which then triggers the Schmitt trigger immediately, allowing the ballast to come on. The capacitor then charges through resistors R 14  through R 16  and R 19  via the base-emitter junction of Q 6 . The interval for disabling the out of tolerance detection circuit during the initial start-up is determined with a time constant set by C 10 , R 14  through R 16 , and R 19 . After this window, in case of detection of an abnormal condition, a low voltage at point “X” causes Q 6  to turn off, which causes Q 5  and Q 4  to turn on, which causes the inverter to shut down.  
         [0031]     Once the capacitor C 10  charges through the resistor series, the lamp current flow detection circuits become enabled. T 2  supports abnormal condition detection by its design with the proper choice of core material to provide a stepped-up AC voltage at frequencies above the fundamental lamp operating frequency. The T 2  secondary voltage is peak limited by Zener diodes D 17  and D 18  to prevent damage to the MOSFETs used. Conveniently, the internal parasitic diode of the MOSFET Q 8  is utilized as a rectifying device, with one end connected to ground. Because the diode normally goes from source to drain with the cathode being connected to the drain, in this case the diode is pointing up at the diagram, thereby serving as a rectifying device to produce a DC voltage at the secondary output of T 2 .  
         [0032]     R 10  provides a current limiting function when the capacitor C 9  filters ripple from the Q 8  parasitic diode rectifier, so that the current through Q 8  does not become excessive.  
         [0033]     R 12  provides loading for T 2 , which is operating as a current transformer. Otherwise, the voltage fed into the Schmitt trigger circuit might be excessive, and the value of R 12  may be adjusted for optimum output voltage swing as the rectified DC level changes from T 2 .  
         [0034]     R 13  in conjunction with C 11  provides a time delay filter feature so that the rectified DC voltage level produced by the arc detection circuit does not prematurely cause inverter shutoff. D 14  and R 11  provide for a discharge time constant smaller than the charging time constant, to allow the detection filter to reset promptly.  
         [0035]     However, if the rectified DC running voltage were not applied through R 13 , C 10  would gradually charge and the reduced voltage at point “X” would cause system shut off.  
         [0036]     R 16  transforms the voltage at point X into a current to be fed into the base of Q 6 .  
         [0037]     Now if the system is operating properly, and there is a load present, the running voltage present at the point “X”, will be approximately one and a half to three volts. This value could vary in accordance with component selection tolerances, the lamp used, and other factors. In any case this running voltage will remain relatively consistent as long as a particular lamp is connected. If the lamp is pulled out and removed however, this current would be terminated, and the voltage generated through the internal MOSFET diode discussed above would disappear as well. As the Schmitt trigger input voltage would then drop to zero in a predetermined amount of time, the system will shut down. This aspect of the system provides a safety feature during re-lamping. That is, when the lamp is installed, it will not become operational until the power is turned off and then turned back on again.  
         [0038]     Given that the lamp current is already being detected according to this invention, circuits are added to sense arcing and to perform desirable functions if arcing is present. Such arcing is a problem, particularly with single-pin lamps since when the pins are pushed in, they may not seat properly because they are spring loaded. On certain interconnections in the system, a high voltage is connected to the operational current source, such that if a gap does exist it may produce enough voltage to jump the gap and cause problems. In typical systems, which operate at 1000 volts on start-up, with current on the order of 200 milliamps or thereabouts, this represents a great deal of power which can generate heat and even a fire.  
         [0039]     Thus according to the invention, the circuit senses arcing and conveniently uses the transformer T 2  for that purpose. In particular the system looks for a characteristic frequency component and amplitude which is produced when arcing occurs across a gap that is larger than one typically encountered when a lamp is operating properly. When the lamp is operating properly, not that much voltage is required to maintain its light output, so that lower voltages are generally acceptable, with a higher voltage being indicative of a problem. Additionally, frequencies higher than the first few harmonics of the fundamental drive frequency are indicative of a problem.  
         [0040]     In the preferred embodiment, a first stage filter uses T 2  to remove the fundamental, and a second stage high frequency noise and filter function is provided by C 6  to perform signal differentiation. C 6  along with R 7  are connected to dual diodes D 8 , to form a differentiator and detector, which converts noise to a DC voltage presented to the gate of the MOSFET Q 8 . To confirm the noise remains present beyond a reasonable time window to cause shutdown, C 7  and R 9  comprise an integrator that will not build the gate voltage sufficiently until the noise has been present for sufficient duration. Very little charge is required on the gate of the MOSFET to turn the “diode” function into a transistor, which now switches on to conduct in the opposite direction. This conduction overcomes the conduction of the device operating as a diode, and the net voltage across the device either becomes zero or some negative value which causes discharge of C 11  through R 11  and D 14  to occur even more rapidly, thereby reacting relatively quickly to an arcing condition. A slight delay at the onset of arcing is provided, because too fast a trigger might falsely fault a relatively high integrity connection. This minimizes false shutdowns.  
         [0041]     D 11  is a clamp on the gate of the MOSFET so that it does not receive a damagingly high voltage. R 8  trickles charge off the gate to allow for reset. When the point “X” is raised, the Schmitt trigger turns on rapidly, turning off Q 4 , but the system continues to operate. R 7  works in cooperation with C 6  as a differentiator to make sure that only rapidly changing noise is detected, again, preventing false shutdowns from occurring.  
         [0042]     An additional protection means is provided by D 12  and R 9  which turn the MOSFET Q 8  on to bring the control point “X” down and shut down the ballast, when excessive supply voltage is present.  
         [0043]     To provide external indication of the state of the ballast functioning, J 2  pin  1  may be connected to an LED to show that external power is applied to the ballast, and J 2  pin  3  may be connected to an LED to show that the protection circuit has activated.  
         [0044]      FIG. 3B  is a redrawing of the more complete schematic diagram of  FIG. 3A , with component illustration perhaps better suited to understanding the operation of the various functions. Note the arrows labeled A and B. Flow in the direction of A passes through the secondary of transformer T 2  and Q 8 , which functions as a parasitic diode to generate DC voltage to keep the inverter running. T 2  current path B through Q 8 , when the device is turned on, removes the DC voltage from the base of Q 6  and thus shuts down the inverter. The device of C 6 , R 7 , and D 8  (note which is a dual diode package) differentiate and detect high slew-rate currents coupled through T 2  during arcing condition to turn on Q 8 .  
         [0045]     As mentioned above, alternative techniques may be used to monitor the lamp current path in accordance with this invention.  FIGS. 4-7 , and  9  illustrate some of these alternative techniques. In  FIG. 4 , a transformer is still used as a sensing element, with a Royer-type oscillator. Because without load, referring back to  FIG. 3 , the output frequency is determined by the series inductor L 1  in the primary circuit along with the snubber capacitors C 1  and C 2 , as the load condition changes with arcing, this change in frequency will in turn change the frequency of a phase locked loop, the output voltage of which could be fed into a Schmitt trigger preferably being used once again to activate the shutdown circuit.  
         [0046]     In  FIG. 5 , a transformer is once again used, but a high pass filter (HPF) is used in conjunction with an inverter, which is less sensitive than the circuit of  FIG. 4 . In general the circuit of  FIG. 5  generates higher DC voltage to the Schmitt trigger to bring about shutdown.  
         [0047]     The circuit of  FIG. 6  uses a separate isolation transformer that detects a higher change in current with respect to time in conjunction with a differentiator circuit, the output of which is indicative of arcing. Broadly, the circuit responds to a noise component above a certain threshold level such that if such noise is detected, it is concluded to be arcing.  
         [0048]     The circuit of  FIG. 7  represents yet a further alternative embodiment utilizing an optical isolator to detect an increase in voltage. Under normal operating conditions, the lamp would otherwise clamp the output circuit voltage, but if there is an arcing condition, this will allow the voltage to rise much higher. A Zener diode is used, the breakdown voltage of which causes the opto-isolator to activate, which in turn generates the shutoff signal.  
         [0049]      FIG. 8  is a drawing which shows an alternative embodiment of the invention, controlled by microprocessor  802 , enabling inputs and outputs to be conducted over a bidirectional bus  810 . Since the CPU  802  generates the control signals over lines  804  for the MOSFETs  806 , it will be appreciated by one of skill in the art of electrical engineering that other functions may be controlled, including a dimming operation.  
         [0050]      FIG. 9  is a drawing which shows yet another alternative embodiment of the invention, controlled by microprocessor  902 , making use of digital signal processing technology to monitor the frequency characteristics of the lamp drive signal and to cause shutdown when the characteristics are outside of proper tolerance.  
         [0051]     Note that although the invention has been described in terms of arc detection with respect to fluorescent tubes, the invention and embodiments described herein on not limited to fluorescent tubes, but may be used with higher voltage systems such as neon signs so long as would be advantageous to sense lamp removal and/or high frequency noise of the type generated by arcing.