Abstract:
A restart circuit for causing an electronic ballast to perform a restart in response to reconnecting any lamp of a multiple lamp configuration of the electronic ballast to the electronic ballast is disclosed. The electronic ballast includes a filament health check circuit for providing a first current through a monitored filament of the lamps to a controller of the ballast. The controller restarts the electronic ballast when a determined ratio of the first current to a reference current indicates that the monitored filament has been disconnected or broken (i.e., the first current substantially decreases) and is subsequently replaced or reconnected to the ballast (i.e., the first current returns to a predetermined level). The ballast further comprises a dv/dt circuit for reducing the first current for a transient time period in response to reconnecting a filament other than the monitored filament to the ballast, causing the controller to restart the ballast.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Co-invented and co-owned U.S. patent application Ser. No. ______, filed simultaneously herewith, entitled Electronic Ballast Control Circuit, is incorporated herein by reference in its entirety. In addition, co-invented and co-owned U.S. patent application Ser. No. ______, filed simultaneously herewith, entitled Resetting An Electronic Ballast In The Event Of Fault, is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to electronic ballasts for providing power to a pair of lamps. More particularly, the invention is concerned with causing the ballast to restart in response to replacing either of the lamps. 
       BACKGROUND OF THE INVENTION 
       [0003]    Ballasts for powering two fluorescent lamps simultaneously start the lamps when power is received from a power supply such as a household power switch (i.e., 120V AC). Starting the ballast includes checking for fault conditions and, upon finding no faults, driving a switching operation of an inverter of the ballast to provide power to the lamps via a resonant circuit of the ballast. When a lamp is disconnected from the ballast or a fault occurs with one of the lamps (e.g., a filament breaks or becomes nonconductive), the ballast prevents the inverter from performing the switching operation. That is, the inverter is shut down. The inverter remains shut down until the power to the ballast from the power supply is disconnected and subsequently reconnected, or until a monitored filament of the two lamps is disconnected from the ballast and subsequently reconnected, causing a restart (i.e., relamping) of the ballast. For example, if a user removes and replaces the lamp having the monitored filament, the ballast automatically restarts when the lamp is reconnected to the ballast (e.g., reinserted into a fixture containing the ballast). If the user instead removes and replaces the other lamp, not having the monitored filament, the ballast shuts off when the lamp is removed, and remains off even after the lamp is reconnected to the ballast. The user must remove and replace the lamp having the monitored filament, or cycle the power to the ballast (i.e., turn the power to the ballast off and back on) in order to restart the ballast. 
       SUMMARY OF THE INVENTION 
       [0004]    Aspects of the invention include an electronic ballast and method for causing a restart (i.e., relamping) of the ballast in response to a user replacing either of a first lamp or a second lamp powered by the ballast. The ballast includes a controller, an inverter, a resonant circuit, a filament health check circuit, and a dv/dt (voltage rate of change or voltage slope) circuit. The controller compares a first current, representative of a current through a second filament of the second lamp, to a second current, wherein the second current is a reference current. If a determined ratio of the first current to the second current is less than or equal to a predetermined ratio, then the controller prevents a switching operation of the inverter. If the determined ratio is greater than the predetermined ratio, then the controller drives the switching operation of the inverter. The controller restarts the ballast in response to the determined ratio transitioning from below the predetermined ratio to equal to or above the predetermined ratio. The dv/dt circuit reduces the first current for a transient time period in response to a disturbance of a direct current (DC) component of a current through a second filament of the first lamp, causing the ballast to relamp or restart when either of the first lamp or the second lamp is reconnected to the ballast. The determined ratio may be the ratio of the second current to the first current. Additionally, or alternatively, the controller may prevent the switching operation of the inverter if the determined ratio is greater than a predetermined ratio, and drive the switching operation of the inverter if the determined ratio is less than the predetermined ratio, without deviating from the scope of the invention. 
         [0005]    Other objects and features will be in part apparent and in part pointed out hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic diagram, partially in block form, of an electronic ballast to power a lamp according to one embodiment of the invention. 
           [0007]      FIG. 2  is a partial schematic diagram of a controller of the electronic ballast of  FIG. 1  according to one embodiment of the invention. 
           [0008]      FIG. 3  is a schematic diagram of the ballast of  FIG. 1  according to one embodiment of the invention. 
       
    
    
       [0009]    Corresponding reference characters indicate corresponding parts throughout the drawings. 
       DETAILED DESCRIPTION 
       [0010]    Referring to  FIG. 1 , an electronic ballast  100  receives AC power from an alternating current (AC) power supply  102  (e.g., standard 120V AC household power). The ballast  100  comprises a rectifier  104 , an inverter  110 , a resonant circuit  112 , a controller  114 , a dv/dt (voltage rate of change or voltage slope) circuit  116 , a filament health check circuit  160 , and miscellaneous resistors, capacitors, and terminals. The rectifier  104  converts the AC power, received from the AC power supply  102 , to direct current (DC) power. Various optional components may be connected to or internal to the rectifier  104  for conditioning and/or altering the DC voltage output of the rectifier  104 . These include, but are not limited to, a bus capacitor (shown as element C 1  in  FIG. 1 ), a voltage regulator (not shown), a voltage limiter (not shown), a power factor correction circuit (not shown), and a DC-to-DC converter (not shown). The rectifier  104  outputs a DC voltage on a DC voltage bus  106  and a ground  108  for the ballast  100 . The optional capacitor C 1 , connected between the DC voltage bus  106  and the ground  108 , conditions the DC voltage transmitted via the DC voltage bus  106 . The inverter  110  is connected to the DC voltage bus  106  and the ground  108 . When driven by the controller  114 , the inverter  110  provides an AC output to the resonant circuit  112 . In some embodiments, the inverter may be a half bridge inverter. 
         [0011]    During steady state operation, the controller  114  drives a switching operation of the inverter  110  by using a pulse width modulation unit  214 , which is part of the controller  114 . The controller  114  driving the switching operation of the inverter  110  results in the inverter  110  providing power to the resonant circuit  112 . The resonant circuit  112 , in turn, uses the provided power to power a first lamp L 1  and a second lamp L 2 . Each of the lamps L 1 , L 2  includes a first filament and a second filament, and each of the filaments includes a first terminal and a second terminal. The resonant circuit includes a first output pair  121 , a second output pair  122 , and a third output pair  123 . In some embodiments, the resonant circuit may include a resonant inductor (e.g., inductor L 4 -A shown in  FIG. 3 ) and a resonant capacitor (e.g., capacitor C 16  shown in  FIG. 3 ). 
         [0012]    The output pairs of the resonant circuit  114  are connected to the lamps L 1 , L 2  as follows. The first output pair  121  is connected across a first filament  130  of the first lamp L 1 . That is, the first output pair  121  is connected to the first terminal  144  and the second terminal  146  of the first filament  130  of the first lamp L 1 . The second output pair  122  is connected to the second terminal  142  of the second filament  132  of the first lamp L 1  and to the first terminal  150  of the first filament  134  of the second lamp L 2 . The ballast  100  also connects the first terminal  148  of the second filament  132  of the first lamp L 1  to the second terminal  152  of the first filament  134  of the second lamp L 2 . The third output pair  123  is connected across the second filament  136  of the second lamp L 2 . That is, the third output pair  123  is connected to the first terminal  156  of the second filament  136  of the second lamp, and to the second terminal  154  of the second filament of the second lamp. Each of the first output pair  121 , second output pair  122 , and third output pair  123  has a first terminal and a second terminal for connecting to the corresponding first or second terminals of the lamps L 1 , L 2 , such that the terminals  144 ,  146 ,  142 ,  148 ,  150 ,  152 ,  154 , and  156  can be referred to as the terminals of the output pairs or of the filaments. 
         [0013]    The controller  114  prevents the switching operation of the inverter  110  if the controller determines that the second filament  136  of the second lamp L 2  is not electrically conductive. For example, the second lamp L 2  may be broken, not intact, or may otherwise be disconnected from the third output pair  123 . A filament health check circuit  160  is for detecting a fault in the second filament  136  of the second lamp L 2 . The filament health check circuit  160  includes a resistance R 25 . The filament health check circuit  160  provides the first current to the controller  114  when the second filament  136  of the second lamp L 2  is connected to the third output pair  123  regardless of whether the other filaments are connected to the other output pairs. In the electronic ballast  100  shown in  FIG. 1 , the filament health check circuit also includes resistors R 31 , R 21 , and R 23 . The resistance R 25  is connected between the DC voltage bus  106  and the first terminal  156  of the third output pair  123 . The second terminal  154  of the third output pair  123  is connected to the first current input  160  of the controller  114  via resistors R 31 , R 21 , and R 23 . Thus, the first current is at least in part representative of a DC current from the DC bus to the controller through the second filament  136  of the second lamp L 2 . A resistive network comprising resistors R 29 , R 33 , and R 22  provides a reference current to a second current input  162  of the controller  114 . Thus, the reference current may herein be referred to interchangeably as the second current. The controller  114  compares the first current to the second current and determines a ratio of the first current to the second current. If the determined ratio is less than or equal to a predetermined ratio, the controller  114  prevents the switching operation of the inverter  110 . That is, the controller  114  prevents the inverter  110  from powering the resonant circuit  112  and the lamps L 1  and L 2 . If the determined ratio is more than the predetermined ratio, the controller  114  drives the switching operation of the inverter  110  to provide power to the resonant circuit  112  and the lamps L 1  and L 2 . In some embodiments, the predetermined ratio may be 3/4. When the determined ratio, as determined by the controller  114 , transitions from below the predetermined ratio to the predetermined ratio, the controller  114  checks the electronic ballast  100  and the lamps L 1  and L 2  for faults. Faults may include, but not limited to, end of lamp life, filaments not intact, and rectifier effect. The controller  114  restarts the electronic ballast  100  if the controller  114  finds no faults. 
         [0014]    Referring to  FIG. 2 , the controller  114  of  FIGS. 1 ,  2 , and  3 B receives the first current at the first current input  160 . The anode of a first controller diode  206  is connected to the first current input  160 , and the cathode of the first controller diode  206  is connected to a first side a first controller resistor  208 . A second side of the first controller resistor  208  is connected to an operating voltage node  216  of the controller  114 . The anode of a second controller diode  202  is connected to the second current input  162 , and the cathode of the second controller diode  202  is connected to a first side a second controller resistor  204 . A second side of the second controller resistor  204  is connected to the operating voltage node  216  of the controller  114 . In some embodiments, a capacitor (not shown in  FIG. 2 ) may be connected between the operating voltage node  216  and the ground  108 . The controller also includes a comparator  210  having a negative input connected to the cathode of the second controller diode  202  and a positive input connected to the cathode of the first controller diode  206 . An output of the comparator  210  is connected to a logic circuit  212  of the controller  114 . The logic circuit  212  determines whether to prevent or drive the switching operation of the inverter  110 . The logic circuit  212  loads parameters into a pulse width modulation (PWM) unit  214  of the controller  114  for driving or preventing the switching operation of the inverter  110 . The PWM unit  214  drives the inverter as a function of the loaded parameters. When the first and second currents are supplied to the controller  114 , the operating voltage node  216  develops an operating voltage for the controller  114 , and the controller draws an operating current from the node, enabling start up of the electronic ballast  100 . In some embodiments, the controller may be an OS2331418 or ICB2FLOSRAM available from Infineon Technologies, AG of Neubiberg, Germany. The controller  114  also analyzes the first current and the second current to determine other lamp problems, such as but not limited to end of lamp life and rectifier effect. 
         [0015]    Referring again to  FIG. 1 , the dv/dt circuit  116  reduces the first current for a transient time period in response to replacement of the first lamp L 1  or the second lamp L 2 . The dv/dt circuit  116  comprises a first resistor R 44 , a second resistor R 46 , a first capacitor C 28 , a third resistor R 45 , a second capacitor C 27 , and a switch Q 5 . The first resistor R 44  is connected between the first terminal  156  of the third output pair  123  and the first terminal  152  of the second output pair  122 . The second resistor R 46  has a high side connected to the second terminal  142  of the second output pair  122  and a low side connected to the ground  108 . The first capacitor C 28  has an input side connected to the high side of the second resistor R 46 . The output side of the first capacitor C 28  is connected to a high side of third resistor R 45 , and a low side of the third resistor R 45  is connected to the ground  108 . The second capacitor C 27  is connected in parallel with the third resistor R 45 . The switch Q 5  has an input connected to the output side of the first capacitor C 28 , a low side connected to the ground  108 , and a high side connected to the first current input  160  of the controller  114 . In the electronic ballast  100  shown in  FIG. 1 , the dv/dt circuit  116  also includes, and in some embodiments may optionally include, a second capacitor C 27  connected in parallel with the third resistor R 45 , a first diode D 12  connected in parallel with the third resistor R 45  with its anode connected to the ground  108 , a second diode D 13  connected in parallel with the second resistor R 46  with its anode connected to the ground  108 , and a third capacitor C 33  connected in parallel with the second resistor R 46 . Also in the electronic ballast  100  shown in  FIG. 1 , the input side of the first capacitor C 28  is connected to the first terminal  142  of the second output pair  122  via a fourth resistor R 47 , and the high side of the switch Q 5  is connected to the first current input  160  via a fifth resistor R 34 . 
         [0016]    In operation, the dv/dt circuit  116  monitors a voltage of the second output pair  122  connected to the second terminal  142  of the first lamp L 1  for a rapid voltage change. Such a rapid voltage change activates a switch Q 5  when a voltage change with respect to time exceeds a threshold. The time that the switch remains activated (i.e., the transient time period) is a function of the values of the resistors and capacitors that form the dv/dt circuit  116  and the time rate of change of the monitored voltage. In the electronic ballast  100  shown in  FIG. 1 , the dv/dt circuit activates the switch Q 5  when the second filament  132  of the first lamp L 1  or the first filament  134  of the second lamp L 2  is reconnected to the electronic ballast  100  after a period of being disconnected. The activation of the switch Q 5  causes the first current to dip, and the determined ratio of the first current to the second current, as determined by the controller  114 , falls below the predetermined ratio. When the transient time period passes, the first current returns to approximately the same level as before activation of the switch Q 5 , and the determined ratio of the first current to the second current, as determined by the controller  114 , now meets or exceeds the predetermined ratio. The controller  114 , in response, restarts the electronic ballast  100  by driving the switching operation of the inverter  110 . 
         [0017]      FIGS. 3A ,  3 B and  3 C illustrate in detail an embodiment of a light source that includes the electronic ballast  100  shown in detail in  FIG. 1 , lamps L 1  and L 2 , and the power supply  102 . The light source illustrated by  FIGS. 3A ,  3 B and  3 C includes the inverter  110 , the rectifier  104 , the resonant circuit  112 , and various other components of the electronic ballast  100  according to one embodiment of the invention. 
         [0018]    In  FIG. 3A , transformer T 1  steps up the AC line voltage provided by power supply  102  and provides the stepped up voltage to the rectifier  104 . The rectifier  104  including diodes D 1 -D 4  provides the rectified voltage to a power factor correction circuit  310 , including transformers T 2  and T 3  and switches Q 1 , Q 2 , and Q 2 A (see  FIG. 3B ). 
         [0019]    In  FIG. 3B , the inverter  110  includes switches Q 3  and Q 4  controlled by the controller  114  to generate the rectified, inverted voltage provided to the resonant circuit  112 . 
         [0020]    In  FIG. 3C , the resonant circuit  112  is illustrated and includes inductor L 4 -A and C 16  which cooperate with miscellaneous other inductors and capacitors illustrated in  FIG. 3C  to determine the resonant frequency of the resonant circuit  112 . The dv/dt circuit  116  includes resistors R 45 , R 46 , and R 47 , capacitors C 27 , C 28 , and C 35 , diodes D 12  and D 13 , and switch Q 5 . 
         [0021]    When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0022]    In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
         [0023]    Having described aspects of the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of aspects of the invention as defined in the appended claims. As various changes could be made in the above constructions, products, and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.