Abstract:
A ballast circuit for driving a fluorescent lamp is provided. The ballast circuit comprises: a self-oscillating circuit; and a series resonant circuit. The series resonant circuit comprises: an inductor; a capacitor; and two diodes. The arrangement of the series resonant circuit: a) causes less power to be dissipated by first and second lamp cathodes when a lamp is coupled to the ballast circuit and increases lamp life, b) protects the ballast circuit from self-destruction when no lamp is coupled to the ballast circuit, and protects the ballast circuit from self-destruction when either the first, second, or both cathodes of a lamp coupled to the ballast circuit have failed.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates generally to a ballast circuit for fluorescent lamps. More particularly, this invention relates to a self-oscillating electronic ballast circuit with cathode protection during normal operation and ballast protection during no-lamp and cathode failure conditions.  
           [0002]    [0002]FIG. 1 shows a ballast circuit  100  with a series-resonant parallel-loaded electronic ballast having an inherent open-cathode protection function. The open-cathode protection function is provided by placing a resonant capacitor  112  between the two cathodes  114 ,  116  of the fluorescent lamp  118 . If the fluorescent lamp  118  is removed from the ballast circuit  100 , or if one or two of the cathodes  114 ,  116  fail (i.e., cathode current path opens), the resonant inductor  120  is disconnected from the resonant capacitor  112 . With the resonant circuit disconnected, the self-oscillating electronic ballast is disabled. Upon replacing the lamp  118 , the ballast circuit  100  will resume its normal operation. However, the resonant current that flows through the resonant capacitor  112  during normal operation also flows through each of the cathodes  114 ,  116 . The continuous resonant current can cause overheating of the cathodes  114 ,  116 , reduces the life of the cathodes  114 ,  116 , and reduces the lumens per watt (LPW) of the lamp  118 .  
           [0003]    [0003]FIG. 2 shows another ballast circuit  200  with a series-resonant parallel-loaded electronic ballast with reduced cathode current and a corresponding reduction in power dissipation by the cathodes during normal operation. The ballast circuit  200  achieves reduced cathode current by splitting the resonant capacitance between two capacitors (i.e., capacitor  212  and capacitor  214 ). Capacitor  212  is between the two cathodes  216 ,  218  of the fluorescent lamp  220 , like in FIG. 1. However, capacitor  214  is in parallel with the two cathodes  216 ,  218 . In this arrangement, the current that flows through cathodes  216 ,  218  during normal operation of the lamp  220  is reduced. Likewise, the corresponding power dissipated by the cathodes  216 ,  218  during normal operation is reduced. However, under a no-lamp condition the resonant circuit formed by capacitor  214  and resonant inductor  222  is still intact and continues to conduct current. Furthermore, with the lamp  220  removed, the resonant circuit will have a higher voltage and higher current than with the lamp  220  installed. This could result in damage to the ballast under the no-lamp condition.  
         SUMMARY OF INVENTION  
         [0004]    In one aspect of the present invention a ballast circuit for driving a fluorescent lamp is provided. The ballast circuit comprises: a self-oscillating circuit; and a series resonant circuit.  
           [0005]    In another aspect of the present invention a ballast circuit for driving a fluorescent lamp is provided. The ballast circuit comprises: a self-oscillating circuit; a resonant inductor; a resonant capacitor; a first diode; and a second diode.  
           [0006]    In another aspect of the present invention a series resonant circuit for a ballast circuit, wherein the ballast circuit is adapted for driving a fluorescent lamp is provided. The series resonant circuit comprises: a resonant inductor; a resonant capacitor; a first diode; and a second diode. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0007]    [0007]FIG. 1 is a schematic diagram of a ballast circuit with ballast protection in a no-lamp and lamp cathode failure conditions.  
         [0008]    [0008]FIG. 2 is a schematic diagram of a ballast circuit with lamp cathode protection during normal operation.  
         [0009]    [0009]FIG. 3 is a schematic diagram of a ballast circuit in accordance with the present invention during normal operation.  
         [0010]    [0010]FIG. 4 is a schematic diagram of the ballast circuit of FIG. 3 in a no-lamp condition.  
         [0011]    [0011]FIG. 5 is a schematic diagram of the ballast circuit of FIG. 3 with a lamp cathode failure condition.  
         [0012]    [0012]FIG. 6 is a schematic diagram of an alternate embodiment of a ballast circuit in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    The present invention provides a cost-effective no-lamp and lamp cathode failure protection schemes for a series-resonant parallel-loaded electronic ballast. The invention also reduces the power dissipation of lamp cathodes during normal operation.  
         [0014]    [0014]FIG. 3 shows a ballast circuit  300  for fluorescent lamps in normal operation. In this embodiment, the invention adds two diodes  312 ,  314  to the ballast circuit  100  of FIG. 1. Each diode (e.g.,  312  or  314 ) is across a cathode (e.g.,  316  or  318 ) of the lamp  320 . The anode of diode  312  is coupled to a first lead of resonant capacitor  322  and the cathode of diode  312  is coupled to the resonant inductor  324 . The anode of diode  314  is coupled to a second lead of resonant capacitor  322  and the cathode of diode  314  is coupled to the half-bridge formed by the junction of capacitor  326  and capacitor  328 . As described and shown, both of the diodes  312 ,  314  are added to a series resonant circuit in a serial fashion. The series resonant circuit is comprised of a resonant inductor  324 , a first diode  312 , a resonant capacitor  322 , and a second diode  314 . The specific arrangement of the two diodes  312 ,  314  is referred to as a back-to-back arrangement with respect to the resonant capacitor  322 . In an alternate embodiment, both diodes  312 ,  314  can be reversed. In other words, the cathodes of both diodes  312 ,  314  can be coupled to opposing leads of the resonant capacitor  322  in a cathode-to-cathode arrangement. In this arrangement, the anode of diode  312  is coupled to the resonant inductor  324  and the anode of diode  314  is coupled to the junction of capacitor  326  and capacitor  328 .  
         [0015]    In either embodiment of the diodes, the ballast circuit  300  is protected from over voltage and current stress, when the lamp  320  is removed from the circuit  300  (i.e., no-lamp condition) or when one or both cathodes  316 ,  318  of the lamp  320  fail. Under no-lamp or cathode failure conditions, the invention causes the self-oscillating circuit  329  formed by semiconductor switch  330 , semiconductor switch  332 , and gate control  334  to be disabled and placed in a sleeping mode. Upon replacing the lamp  320 , the circuit automatically returns to its normal operating mode.  
         [0016]    As shown in FIG. 3, the two diodes  312 ,  314 , one across each cathode  316 ,  318  of the lamp  320 , are added to a self-oscillating series-resonant parallel-loaded electronic ballast circuit  300 . In this arrangement, during normal operation, each cathode  316 ,  318  carries operating current during alternating half cycles of current through the resonant circuit. Accordingly, the corresponding diode  312  or  314 , rather than the cathode  316  or  318  carries the resonant circuit current during the opposite alternating half cycle. This reduces power dissipation for each cathode  316 ,  318  of the fluorescent lamp by approximately an inverse of the square root of two. Cathode life and system efficacy are increased because less power is dissipated by each of the cathodes  316 ,  318  at steady-state conditions during normal operation.  
         [0017]    [0017]FIG. 4 shows the ballast circuit  300  in a no-lamp condition. If the no-lamp condition occurs (i.e., lamp  320  removed or both cathodes  316 ,  318  fail) the self-oscillating condition of the ballast circuit  300  is not met because the back-to-back arrangement of the diodes  312 ,  314  substantially blocks operating current from flowing in the resonant circuit. Therefore, the ballast circuit  300  is protected from self-destruction during the no-lamp condition.  
         [0018]    [0018]FIG. 5 shows the ballast circuit  300  with a lamp cathode failure condition. When one cathode (e.g.,  316 ) fails or breaks, the filament in the cathode  316  opens and the associated diode  312  is the only path for operating current through the resonant circuit. Since the diode  312  will only permit operating current to flow when it is forward biased, when the oscillating circuit voltage reverse biases the diode  312 , the diode  312  prevents operating current through the resonant circuit and prevents the ballast circuit from self-oscillating. If cathode  318  fails, the diode  314  and cathode  318  arrangement operates in the same fashion for the opposite cycle of operating current through the resonant circuit.  
         [0019]    [0019]FIG. 6 shows an alternate embodiment of a ballast circuit  400  employing the present invention. The present invention operates the same in this embodiment as described in the previous embodiment of FIGS.  3 - 5 . In the embodiment shown in FIG. 6, the invention adds two diodes  412 ,  414  to a self-oscillating ballast circuit. Each diode (e.g.,  412  or  414 ) is across a cathode (e.g.,  416  or  418 ) of the lamp  420 . The anode of diode  412  is coupled to a first lead of resonant capacitor  422  and the cathode of diode  412  is coupled to the resonant inductor  424 . The anode of diode  414  is coupled to a second lead of resonant capacitor  422  and the cathode of diode  414  is coupled to a first lead of capacitor  426 . As described and shown, both of the diodes  412 ,  414  are added to a series resonant circuit in a serial fashion. The series resonant circuit is comprised of a resonant inductor  424 , a first diode  412 , a resonant capacitor  422 , and a second diode  414 . The specific arrangement of the two diodes  412 ,  414  is referred to as a back-to-back arrangement with respect to the resonant capacitor  422 . In an alternate embodiment, both diodes  412 ,  414  can be reversed. In other words, the cathodes of both diodes  412 ,  414  can be coupled to opposing leads of the resonant capacitor  422  in a cathode-to-cathode arrangement. In this arrangement, the anode of diode  412  is coupled to the resonant inductor  424  and the anode of diode  414  is coupled to a capacitor  426 .  
         [0020]    In either embodiment of the diodes, the ballast circuit  400  is protected from over voltage and current stress, when the lamp  420  is removed from the circuit  400  (i.e., no-lamp condition) or when one or both cathodes  416 ,  418  of the lamp  420  fail. Under no-lamp or cathode failure conditions, the invention causes the self-oscillating circuit  429  formed by semiconductor switch  430 , semiconductor switch  432 , and gate control  434  to be disabled and placed in a sleeping mode. Upon replacing the lamp  420 , the circuit automatically returns to its normal operating mode.  
         [0021]    As shown in FIG. 6, the two diodes  412 ,  414 , one across each cathode  416 ,  418  of the lamp  420 , are added to a self-oscillating series-resonant parallel-loaded electronic ballast circuit  400 . In this arrangement, during normal operation, each cathode  416 ,  418  carries the operating current during alternating half cycles of current through the resonant circuit. Accordingly, the diode  412 ,  414 , rather than the cathode  416 ,  418 , carries the resonant circuit current during the opposite alternating half cycle. This reduces power dissipation for each cathode  416 ,  418  of the fluorescent lamp by approximately an inverse of the square root of two. Cathode life and system efficacy are increased because less power is dissipated by each of the cathodes  416 ,  418  at steady-state conditions during normal operation.  
         [0022]    While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope and spirit of the invention.