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 removal, or voltage indicative of arcing. 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.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to an electronic ballast and, in particular, to lamp-unseating and arc-detecting shut-down circuit and related improvements.  
       BACKGROUND OF THE INVENTION  
       [0002]     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 buss distribution system, which, in turn, can affect performance and operation of other systems connected to the DC buss.  
         [0003]      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 .  
         [0004]     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 bus distribution system, leading to performance degradation and failure of other equipment powered by the DC buss, in some cases.  
         [0005]     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  
       [0006]     This invention resides in a shut-down 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 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 removal, or voltage indicative of arcing.  
         [0007]     In the preferred embodiment, the device for sensing the electrical energy associated with the control path is an isolation transformer; however, in other embodiments 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.  
         [0008]     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.  
         [0009]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic diagram illustrating the problem associated with existing ballast circuits wherein an electrical path is common to both the DC and AC circuits;  
         [0011]      FIG. 2  is a simplified block diagram illustrating important components associated with a preferred embodiment of this invention;  
         [0012]      FIG. 3A  is a detailed schematic diagram of the preferred embodiment;  
         [0013]      FIG. 3B  is a redraw of the detailed schematic diagram to assist with understanding the way in which the circuits function;  
         [0014]      FIG. 4  is an alternative sensing circuit incorporating a phase-locked loop;  
         [0015]      FIG. 5  is a different alternative sensing circuit involving a highpass filter and Schmitt trigger;  
         [0016]      FIG. 6  is yet a further alternative sensing circuit incorporating a module to differentiate arcing noise;  
         [0017]      FIG. 7  is yet a different alternative sensing circuit involving an opto-isolator and highpass filter; and  
         [0018]      FIG. 8  represents a microprocessor-based instrumentation of the invention, enabling control and sensing to be communicated over a buss. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     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 hook-up connection to the lamp. This is more particularly carried out by eliminating the safety switching action of the pin and the socket currently typical of single-pin systems.  
         [0020]     According to the invention, ballast shut down 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, 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, or if the socket is defective. In the preferred embodiment, the circuit detects arcing that lasts longer than a predetermined amount, such as 500 milliseconds, or thereabouts, and shuts down the ballast if such arcing is detected. Broadly, because the output of the ballast is entirely isolated from the DC input, filtering inside the ballast can now attenuate the noise produced by the ballast to acceptable levels.  
         [0021]     Reference is now made to  FIG. 2 , which depicts in block-diagram form a circuit configuration according to the invention. Twenty-four volts DC 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 .  
         [0022]     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 system shutoff described with respect to  FIG. 1 . Novel to this invention, is the use of an arc detection feature and a lamp current/arc sensing circuit  250  to control the ballast inverter  210 . In the preferred embodiment, isolation output transformer  240  is used to cover 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.  
         [0023]     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. In this diagram, T 1  is the isolation current transformer, T 2  is a step-up transformer and L 2  is a line filter. The DC input voltage (24 Vdc) gets connected at the plus and minus terminals shown at the bottom of the drawing.  
         [0024]     Q 1  and Q 2  function as inverters. Although they are preferably implemented utilizing MOSFETs, they may be bi-polar transistors or other types of switching devices, as appropriate. Inverters Q 1  and Q 2  are connected in push/pull fashion to the primary winding of transformer T 1 , that is, the lower windings shown in the diagram have a center tab. L 1  is connected to the center tab to a circuit breaker shown lower in the diagram.  
         [0025]     The feedback signal is connected to the upper windings of T 1  through R 1  and R 2 . An initial turn-on bias is provided by the center tap winding of T 1  through R 4 . This is known in the art as a Royer-type oscillator. Once oscillation commences, a circuit comprises D 5 , C 1  and R 3  serves to clamp the voltage and reduces the turn-on bias coupled to Q 1  and Q 2  once oscillation begins. Oscillation may be terminated by turning on Q 3 , a PNP device which pulls down the voltage at the emitter and shuts off the oscillator circuit when asserted. Q 4  and Q 5  represent a Schmitt trigger used to turn off Q 3  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 device could become overheated.  
         [0026]     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, a Schmitt trigger turns on which causes Q 3  to rapidly turn off. This is intentional, since when power is initialized to the ballast, it is desirable to have a lamp come on as soon as possible. During this time, however, the control capability of the circuit must be temporarily disabled or overwritten to allow the lamp to fire. This is accomplished through the use of capacitor C 9 . On initialization, or first turn on, DC is applied through the filter, through the circuit breaker device, and through the diode D 10  which isolates that portion of the circuit which is sensitive to noise. In particular, this isolates the Schmitt trigger and Q 3 , which is filtered by C 10 .  
         [0027]     In other words, the capacitor C 9  upon initialization, causes the rise in voltage approaching 24 volts to energize point “X,” which then triggers the Schmitt trigger immediately, allowing the ballast to come on. The capacitor is then discharged through the resistor, and the base-emitter junction of Q 5 . The disabling of the initial start-up is determined by a time constant set by C 9  and R 8  when the voltage at the base-emitter junction of Q 5  drops to a sufficiently low value, allowing the circuit to be disabled for a portion of a second so that the lamp has the opportunity to energize. A low voltage at point “X” causes Q 5  to turn off, which causes Q 4  and Q 3  to turn on, which causes the inverter to shut down.  
         [0028]     Once the capacitor C 9  is discharged through the resistor series, thereby disabling the arc detection circuit, the system becomes active to detect current flow through the lamp. T 2  serves this function as well, by providing a stepped-up AC voltage which 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 is utilized as a rectifying device, with one end being connected to ground. Whereas 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 for the DC voltage generated at the bottom of T 2 .  
         [0029]     R 16  provides a current-limiting function when the capacitor is charged so that the voltage across the junction does not become excessive. However, if there were no running voltage through R 9 , C 9  would simply discharge and the system would cycle through shut-off.  
         [0030]     Now, if the system is operating properly, and there is a load present, the running voltage will be present at the point X, approximately 3 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 the lamp is connected. If the lamp is pulled out and removed, however, this current would be terminated, and the voltage generated through the internal diode discussed above would disappear as well. If the Schmitt trigger 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 relamping. That is, when the lamp is installed, it will not become operational until the power is turned off and then turned back on again.  
         [0031]     Given that the lamp current is already being detected according to this invention, circuits are added to sense arcing and 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 might 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, thereabouts, representing a great deal of power which can generate heat and even a fire.  
         [0032]     Thus, according to the invention, the circuit senses arcing and, conveniently, uses the transformer D 2  for that purpose. In particular, the system looks for a characteristic 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, so that lower voltages are generally acceptable, with a higher voltage being indicative of a problem.  
         [0033]     In the preferred embodiment, high-frequency noise and filter function is provided by C 6  to perform this differentiation. C 6  is connected to diodes D 11  and D 12 , which form a detector, which converts noise to a DC voltage presented to the gate of the MOSFET Q 6 . 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 9  through R 10  and D 14  to occur even more rapidly, thereby reacting relatively quickly to an arcing condition. Although there is a slight delay at the onset of arcing, too fast a trigger might be indicative of a relatively high-integrity connection, so shut-downs are minimized.  
         [0034]     D 16  is a clamp on the gate of the MOSFET so that it does not receive a damagingly high voltage. R 13  trickles off the gate to allow for reset. When the point X is raised, the Schmitt trigger turns on rapidly, turning off Q 3 , but the system continues to operate. R 17  works in cooperation with C 6  as a differentiator to make sure that only rapidly changing noise is detected, again, preventing false shut downs from occurring.  
         [0035]      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 through the secondary of transformer T 2  through Q 6  functions as a parasitic diode to generate DC voltage to keep the inverter running. T 2  current B through Q 6 , when the device is turned on, removes the DC voltage from the base of Q 5  and thus shuts down the inverter. The device of C 6 , R 17 , D 11  and D 12  differentiate and detect high slew-rate currents coupled through T 2  during arcing condition to turn on Q 6 .  
         [0036]     As mentioned above, alternative techniques may be used to monitor the lamp current path in accordance with this invention.  FIGS. 4-7  illustrate some of these alternative techniques. In  FIG. 4 , a transformer is still used as a sensing element, but this change in frequency changes the frequency of a phased-lock loop. A Royer-type oscillator is still used, but because the output is a series inductor circuit determinative of frequency, if there was a change in load condition such as arcing, this changes the frequency which can be detected by a phase-lock loop, with a Schmitt trigger preferably being used once again to activate the shut down circuit.  
         [0037]     In  FIG. 5 , a transformer is once again used, but a highpass 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 shut down.  
         [0038]     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 is response to a noise component above a certain level of threshold such that if such noise is detected, it is concluded to be arcing. The circuit of  FIG. 7  represents yet a further alternative embodiment utilizing an optical isolator to detect and increase in voltage. Under normal operating conditions, the lamp would otherwise clamp the lamp 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 shut-off signal.  
         [0039]      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 bi-directional buss  810 . The CPU  802 , not only generates the control signals over lines  804  for the MOSFETs  806 , but also inputs information regarding arc detection through sensor  808  and input  809 . Use of a microprocessor also facilitates other inputs and outputs, including, for example, a temperature input at  812 . Since the processor  802  generates control signals 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.  
         [0040]     Note that although the invention has been described in terms of arc detection with respect to fluorescent tubes, the invention and embodiments described herein are not limited to fluorescent tubes, but may be used with higher-voltage systems such as neon signs, and the like, so long as it would be advantageous to sense lamp removal and/or high-frequency noise of the type generated by arcing.