Abstract:
A multi-lamp protection circuit for an electronic ballast includes an inverter circuit of various types, and a transistor switch provided in the inverter circuit and connected to a plurality of lamp circuits, each lamp circuit connected respectively in series to a transistor switch and a disorder checking circuit. The signal got by the disorder checking circuits mixed with a control signal starts resonance transistors so as to checking disorder and separating the damaged lamp(s), lessening its producing cost and achieving normal efficiency of the lamps.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a multi-lamp protection circuit for an electronic ballast. The multi-lamp protection circuit, which is realized with solid-state devices and hence can be of low cost and small size, is particularly used to isolate the abnormal lamp(s) from the normal lamp(s).  
           [0003]    2. Description of the Prior Art  
           [0004]    Electronic ballasts have gradually taken place of traditional heavy electromagnetic ballasts to drive fluorescent lamps because of their light, small size and better performances. In order to reduce the product cost, the electronic ballast is designed to drive multi-lamps when two or more lamps are installed in a same fixture.  
           [0005]    At present, an electronic ballast mainly uses a half-bridge series-resonant-inverter to drive the fluorescent lamps at a high frequency. For such an electronic circuit, an excessive large current may flow through power devices of the inverter and the filament of the lamp, in case that the lamp fails to be started up or comes to its life end. Therefore, difficulties surely arise from how to get rid of abnormal lamps and how to maintain normal operation of lamps when a plurality of lamps are driven by a ballast. The present method of an electronic ballast with a lamp protection circuit is to use a common electronic ballast for a plurality of lamps for separating a damaged lamp with a relay. But using a relay with mechanical contacts not merely reduces reliability but has disadvantage of a large dimension, a loud noise, and low efficiency, forming an adverse consequence against demand for light small electronic ballasts.  
         SUMMARY OF THE INVENTION  
         [0006]    The objective of the invention is to offer a multi-lamp protection circuit for an electronic ballast, which can be manufactured with less cost than the conventional ones, and possible to cut off the abnormal lamp(s) instantly to protect the ballast. As a result, the normal lamp(s) can be operated normally free from the outage disturbance.  
           [0007]    The discriminate feature of the invention is that all lamp circuits are parallel-connected and energized by an inverter circuit, lamp current of every lamp is detected and referred to its abnormality-detect circuit, respectively. The control signal of each switch is AND gated by the output of each abnormality-detect circuit. Therefore, the inverter operation of the malfunctioning lamp may be suppressed by the abnormality-detect circuit output, while the other lamps will keep on operating normally. Thus, the multi-lamp ballast protection can be achieved at a reduced cost. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    This invention will be better understood by referring to the accompanying drawings, wherein:  
         [0009]    [0009]FIG. 1 is a block diagram of a mult-lamp protection circuit for an electronic ballast in the present invention;  
         [0010]    [0010]FIG. 2 is a diagram of the mult-lamp protection circuit for an electronic ballast in the present invention; and,  
         [0011]    [0011]FIG. 3 is a diagram of the voltage of lamps and transistor switches in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    A preferred embodiment of a multi-lamp protection circuit for an electronic ballast in the present invention, as shown in FIGS. 1 and 2, includes a rectifying and filtering circuit  10 , a DC-AC inverter circuit  20 , a control-signal generating circuit  30 , lamp circuits  41  and  42 , abnormality-detect circuit  51  and  52 , and AND gates  61  and  62 . If a plurality of lamp circuits is required, the number of lamp circuits, abnormality-detect circuit, and AND gates can be increased.  
         [0013]    The inverter circuit  20  which is DC energized by the rectifier circuit  10 , consists of a common switch S, four diodes D 1 , D 2 , D 3  and D 4 , and switches S 1 , S 2 . Where S joined with S 1 , S joined with S 2  make up two half-bridge inverters. The switches are gated by the separately-excited control circuit  30 .  
         [0014]    The first lamp circuit  41  is connected to the ends of the common switch S and the diode D 3 . The second lamp circuit  42  is parallel-connected to S and D 4 . If more than two lamps circuits are required, the additional lamp circuits are parallel-connected.  
         [0015]    A signal taken from lamp circuits  41 ,  42 , is sent through the switches S 1  and S 2  respectively to resistors R 1  and R 2  to generate voltage signals. The voltage signals will serve as the gate signals of thyristors SCR 1  and SCR 2 . The anodes of SCR 1  and SCR 2  are connected to a DC-bias and compared by comparators OP 1  and OP 2  with a reference voltage Vref, generating an output signal. The output signal together with the square wave driving signals triggered by control circuits  30  are logically AND gated by gates  61  and  62 . The signals are then fed back to driving S 1  and S 2  for cutting off an abnormal lamp circuit(s) immediately.  
         [0016]    The main principle of this multi-lamp protection circuit for an electronic ballast is that the control signals for switches S 1 , S 2  are in-phase, that for switch S is out-phase. Take lamp Lp 1  as an example to illustrate the operation of this circuit. When Lp 1  operates normally, the lamp voltage is a sine wave because of the alternately switching of switches S and S 1  together with the series resonant circuit Cs 1  and Ls 1 . At first, switch S OFF and S 1  ON, current flows from lamp Lp 1 , capacitor Cs 1 , inductor Ls 1 , switch S 1 , and resistor R 1 , then returns to ground. When the resonant voltage gradually declines to zero, switch S 1  turns off and current flows directly from diode D 1  back to Lp 1 , with the common switch S turning ON.  
         [0017]    When the lamp voltage is in negative half cycle, with switch S ON, and S 1  OFF, current flows reversely from Ls 1 , Cs 1  to Lp 1 . When Lp 1  has its voltage recovering from negative half to zero crossing, S 1  turns ON, current flows back to the lamp Lp 1  through the capacitor Cdc, resistor R 1 , S 1 , Ls 1 , and Cs 1 . Switches S and S 1  will turn on under zero-voltage-transition, therefore the circuit efficiency can be promoted and causing smaller voltage stress on the switches.  
         [0018]    Provided that lamp Lp 1  should become damaged, voltage across resistor R 1  rises up, and the thyristor SCR 1  is turned ON, forcing the comparator OP 1  giving a low output. After the AND gate  61 , the gate control signal is kept at low, therefore operation of switch S 1  is suppressed. The resonant inverter circuit of Lp 1  is dismounted because of the absence of S 1 , lamp Lp 1  ceases. However, at the same time, switch S 2  still operates normally as the operation of gate  62  remains unaffected. If the lamp Lp 2  should become damaged, the scenario will be the same as the case of lamp Lp 1 .  
         [0019]    The output of abnormality detect circuit adopted in this invention is low whenever any abnormality occurred. If applied in other circuit, and that of abnormality detect circuit is high, only an additional NOT gate is required at the input terminal of AND gate. This invention needs only one signal control unit to control the two switches in the half bridge inverter. In addition, the upper arm only claims a common switch, one for a single lamp respectively. Consequently, compared to the number of lamps, that of switches is only one more. Although the common switch S might flow a comparatively larger current, control circuits and other necessary components can be saved conspicuously. Above all, to prolong service life of lamps, preheating of lamp is usually essential at the cost of additional control IC required. Consequently, upon the application of multi-lamps, the component saving by using this invention is very remarkable.  
         [0020]    To sum up, this invention makes use of a single controller controlling various types of current inverters (such as of half-bridge, quasi-half bridge, full-bridge, push-pull, buck, boost, buck-boost, fly-back, or hybrid type) in driving multi-lamps at the same time. In addition, the switches are turned on at zero voltage to lessen switching loss and stress of the components and thus enhance the efficiency of whole ballast circuit. Furthermore, provided any individual lamp is investigated as damaged, the control circuit is capable of cutting off the spoiled lamp immediately, without any impact to other normal lamps. Thus the invention not only achieves protection function for multi-lamps, but largely lessens its producing cost.  
         [0021]    While the preferred embodiment of the invention has been described above, it will be recognized and understood that various modifications may be made therein and the appended claims are intended to cover all such modifications that may fall within the spirit and scope of the invention.