Abstract:
A plurality of lighting ballasts ( 12, 14, 16 ) draw power from a single DC bus signal. A power factor correction circuit ( 10 ) rectifies and smoothes AC power to produce the DC bus signal. In order to prevent damage to the ballast ( 12 ) when a lamp ( 18 ) dies or is removed, the ballast ( 12 ) includes an AC switch that senses damaging conditions and responds by changing a resonant frequency of the ballast ( 12 ). The AC switch operates in 2-3 second cycles. While it is operative, it shunts current away from inductors ( 38, 40 ) of the ballast ( 12 ) causing a resonant frequency of the ballast ( 12 ) to change. At the end of the cycle, the switch turns off, but if a load fault is still present in the ballast ( 12 ) it activates again. Preferably, the AC switch has a response time of approximately 500 μs.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the artificial illumination arts. It finds particular application in protecting lamp ballasts from open circuit load faults and will be described with particular reference thereto. It is to be appreciated, however, that the present invention is also applicable to protecting vital components of other circuits from voltage and current surges, and is not limited to the aforementioned application.  
           [0002]    Often, it is desirable to operate fluorescent lamps independently of each other. In many existing systems, if one lamp fails, others will also cease to function. To aid in the re-lamping process, it is desirable to have all functioning lamps operating, so that a repairman can easily discern which lamps need to be replaced. Also, it is easier for the repairman to work if he has light from adjacent lamps to aid him.  
           [0003]    When a lamp fails, or is outright removed from the circuit while the ballast is in operation, open circuit voltages can become so high as to damage sensitive circuit components, rendering the ballast useless for further lighting applications. However in order to replace the lamp while other lamps are in operation, it is necessary to remove the lamp while power is being supplied to the ballast.  
           [0004]    Often, ballast circuits include a plurality of transistors, such as bi-polar junction transistors, (BJTs), for switching purposes. In these applications, transistors may be stressed beyond their rated values. In existing ballasts, an open circuit load fault causes greatly increased current flow. With such current flowing through the BJTs, the maximum rated current may be exceeded, resulting in failure of the transistors.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0005]    In accordance with one aspect of the present invention, a lighting system powered by an alternating current source is provided. A power factor correction circuit creates a direct current bus, which is used to power a lamp ballast. The ballast includes an alternating current switch that protects circuit components when the lamp is removed or fails to ignite.  
           [0006]    In accordance with another aspect of the present invention, a method of protecting a DC lighting ballast from high open circuit voltages is provided. In response to a load being removed from the circuit, an alternating current switch is activated that raises the operating frequency of the ballast, thereby lowering voltage and current flow through the circuit.  
           [0007]    In accordance with another aspect of the present invention, an alternating current switch for use in conjunction with a lamp ballast is provided. First and second field effect transistors draw current away from the phase shifting inductor of the ballast. An inductive tap provides a signal to control the gates of the transistors. First and second charge pumps store charge from the inductive tap. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.  
         [0009]    [0009]FIG. 1 is a diagrammatic illustration of a lighting system including multiple lamp ballasts operating off of a direct current bus, in accordance with an embodiment of the present invention;  
         [0010]    [0010]FIG. 2 is a circuit diagram of a lamp ballast circuit, in accordance with an embodiment of the present invention;  
         [0011]    [0011]FIG. 3 is a circuit diagram of an alternating current switch incorporated into the ballast circuit, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    With reference to FIG. 1, a power factor correction circuit  10  is connected to an alternating current (AC) power source. The power factor correction circuit  10  includes rectifying and smoothing components, as are well known in the art, to convert the AC signal into a direct current (DC) bus signal. A plurality of lamp ballasts  12 ,  14 ,  16  are connected in parallel to the power factor correction circuit. It is to be understood that any number of ballasts may draw power from the DC bus, to a capacity of the AC source. Associated with each ballast is a lamp  18 ,  20 ,  22 . In the preferred embodiment, the ballasts  12 ,  14 ,  16  are independently operable. Though they draw power from the same source, any combination of lamps  18 ,  20 ,  22  may be lit at any given time. Preferred control methods include wall switch controls, RF remote control, audible/voice recognition control, and others.  
         [0013]    Preferably, each ballast operates at a distinct resonance frequency. That is, there is a single frequency or small range of frequencies at which the ballast circuit will light the lamp. Some ballasts are selected to have broad ranges of operating frequencies, and some have relatively narrow ranges. Knowing the respective frequency ranges and the overlap among the ballasts allows a frequency to be selected that lights the desired lamps.  
         [0014]    With reference to FIG. 2, in a preferred embodiment of the ballast circuit, the lamp  18  is connected between external contacts  24 ,  26 . Should the lamp  18  be removed from the circuit, or fail to ignite, the area between points  24  and  26  looks open to the rest of the circuit. With no load lamp present in the ballast, the remainder of the circuit is overdriven, which results in increased current flow to bi-polar junction transistors (BJTs)  28 ,  30  which can overload the BJTs and damage them to a point where the circuit is useless. Diodes  32 ,  34  are present to help prevent reverse current flow through the BJTs.  
         [0015]    As briefly stated previously, the ballast operates at a certain frequency or range of frequencies. The resonant frequency of the ballast, that is, the frequency at which the ballast yields optimal performance, is defined by a set of inductors  36 ,  38 ,  40  and the capacitor  35 . During normal operation, the operating frequency is slightly above the resonant frequency determined by the capacitor  35  and the inductor  40 . In the preferred embodiment, an AC switch is connected between points a and b. When activated, the AC switch creates a low impedance between points a and b, thereby bypassing the inductors  38  and  40 , which help to define the resonant frequency of the ballast. When the AC switch begins conducting, it shunts most of the current away from the inductor  40 . This reduces the ability of the capacitor  35  and inductor  40  to control the operating frequency and the oscillations cease, causing the current flow through transistors  28  and  30  to cease. Every two to three seconds, the switch opens again, introducing the inductors  38 ,  40  back into the ballast circuit. If the lamp  18  is not present or is not functioning, the switch conducts repeating the cycle until an operating lamp is present between contacts  24  and  26 . Back to back Zener diodes  42  clamp the voltage across the inductor  40 . This clamps the voltage between nodes  24  and  26  until the AC switch begins to conduct.  
         [0016]    With reference to FIG. 3 the AC switch includes a pair of metal oxide semi-conducting field effect transistors (MOSFETs)  44 ,  46 . It is to be understood that other transistor types, such as IGFETs, JFETs, or BJTs, all of either the p or n junction variety can also be used. When turned conductive, the MOSFETs  44 ,  46  provide a current shunt away from the inductors  38 ,  40 , that is, they provide a low impedance between points a and b. In order for the MOSFETs  44 ,  46  to turn conductive, a positive voltage must be applied to the gate of each MOSFET. It is to be understood that although n-channel MOSFETs are illustrated, the polarities of this circuit can be reversed to allow for p-channel devices as well. An inductive tap  48  provides voltage for the AC switch. The inductive tap  48  is a transformer winding in which voltage is induced by the inductor  36 . There is a large voltage difference between the voltage across the inductor  36  when the lamp  18  is connected and when it is disconnected. Macroscopically, the AC switch senses the difference and activates, thereby protecting the ballast circuit.  
         [0017]    During normal operation of the ballast circuit, i.e., when the lamp is engaged and functioning, the gates of the MOSFETs  44 ,  46 , are reverse biased, rendering them non-conductive. The reverse bias is caused by the lower charge pump, comprising the capacitor  56  and Zener diode  58 . Additionally, the upper charge pump comprises the capacitor  52 , diode  54 , diode  55 , and capacitor  50  also operates, establishing a voltage across the capacitor  50 . In one preferred embodiment, the reverse bias is approximately five volts with respect to the sources of the MOSFETs  44  and  46 . The reverse bias is to ensure that the MOSFETs  44 ,  46  do not activate when the lamp  18  is operating normally. If the lamp is removed from the circuit, the voltage across the inductor  36  rises. Thus, the voltage produced by the inductive tap  48  likewise rises. This rise in voltage causes the voltage across the capacitor  50  to increase while the voltage produced across the diode  58  is clamped by the zenering function. Thus, the voltage across the capacitor  50  continues to rise while the voltage across diode  58  does not change. This action turns on the MOSFETs  44  and  46 .  
         [0018]    The upper charge pump includes a capacitor  50  that stores charge to overcome the reverse bias applied to the gates of the MOSFETs  44 ,  46 . In one embodiment, this capacitor is a 1 μF capacitor. During one-half cycle of the inverter operating cycle, the inductive tap charges capacitor  52 . During the next half cycle, the inductive tap  48  charges a second capacitor  50  of the first charge pump. Essentially, charge is transferred from capacitor  52  to capacitor  50 . Since capacitor  52  is much smaller than capacitor  50 , the rate at which capacitor  50  charges is controlled. The capacitor  52  may for example be a 47 nF capacitor. Smaller values will charge capacitor  50  at a slower rate, causing more of a delay to turn on MOSFETs  44  and  46 . During the next half cycle, the inductive tap  48  once again charges the capacitor  52 . The charging of capacitor  50  is repeated on the next half cycle. Thus, capacitor  50  continues to charge until MOSFETs  44  and  46  are turned on. Over several charging cycles, capacitor  50  accumulates sufficient charge to overcome the reverse bias on the gates. The voltage applied to the gates of the MOSFETs  44  and  46  is determined by the average voltage across Zener diode  58 , the voltage across capacitor  50  and the voltage divider, comprising resistors  62  and  64 . Since the voltage appearing across the winding  48  is a square wave. The average voltage across the Zener diode  58  will be ½ of its Zener value. For example, if the Zener voltage is 10 volts, the average voltage will be 5 volts. The voltage at the gates of the MOSFETs  44  and  46  is given by:  
         v   gs     =           v   C50     ·   R64     -       v   B     ·   R62         R64   +   R62                             
 
         [0019]    where v gs  is the gate-source voltage of the MOSFETs  44  and  46 , v C50  is the voltage across the capacitor  50  and v B  is the average voltage developed across the Zener diode  58 , that is, the bias voltage.  
         [0020]    V gs  will increase until the threshold of MOSFETs  44  and  46  is reached. At this time,  44  and  46  begin to conduct, shunting current away from the inductors  38  and  40 .  
         [0021]    Once a forward bias is applied to the gates, that is, once the threshold voltage has been reached, the MOSFETS  44 ,  46  turn conductive. As previously discussed, this diverts current away from the inductors  38  and  40 , causing inverter oscillations to cease. As a result, the voltage across the inductor  36  is reduced to zero, and consequently, the voltage induced in the winding  48  is reduced to zero. Thus, the negative bias is removed and the voltage developed across the capacitor  66  (also the gate-source voltage) rises. This means, although the capacitor  50  had to accumulate enough charge to overcome the threshold voltage, it has to discharge back to the threshold to render MOSFETs  44  and  46  non-conductive. Utilizing one set of component values, the capacitor  50  takes approximately 500 μs to charge to the threshold voltage, and approximately two to three seconds to discharge.  
         [0022]    When MOSFETs  44 ,  46  again become non-conductive, the inverter re-initiates oscillations. If the lamp  18  is still not functional, the cycle repeats until the lamp  18  is replaced. If the lamp is functional, the ballast resumes steady state operation with the AC switch off.  
         [0023]    In the time period when the capacitor  50  is charging, relatively high voltages and currents are present within the AC switch. To protect the MOSFETs, a Zener diode  60  clamps the voltage to a safe potential. Preferably, and with specific reference to FIG. 3, the resistor on the top rail  62  is nominally 510 kΩ and the resistor on the bottom rail  64  is 270 kΩ. Also included in the AC switch is a capacitor  66  that averages the voltage produced by the bias circuit. In the present embodiment, the capacitor  66  is a 1 nF capacitor.  
         [0024]    Following is a list of exemplary components and component values for the preferred embodiment.  
                                                           BJT   28   13003           BJT   30   93003           Diode   32   1N4937           Diode   34   1N4937           Capacitor   35   100 nanofarads           Inductor   36   3.5 millihenries           Inductor   38   350 millihenries           Inductor   40   150 microhenries           Zener Diodes   42   1N5240           First MOSFET   44   IRLML2502           Second MOSFET   46   IRLML6401           Inductor   48   3.5 millihenries           Capacitor   50   1 microfarad           Capacitor   52   47 nanofarads           Diode   54   1N4148           Diode   55   1N4148           Capacitor   56   1 nanofarad           Zener Diode   58   1N5240           Resistor   62   510KΩ           Resistor   64   270 KΩ           Capacitor   66   1 nanofarad                      
 
         [0025]    The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.