Abstract:
An electronic ballast includes an inductor, an output transformer, at least two switching elements, a control circuit, a clamping circuit, and at least two return circuits. The inductor is electrically coupled to a DC power supply. The control circuit is electrically connected to the inductor, the output transformer and the switching elements for controlling on/off statuses of the switching elements. The clamping circuit is electrically connected to the inductor, and limits a node voltage among the inductor, the control circuit, and the clamping circuit below a threshold value and generates an output current on condition that the node voltage is larger than the threshold value. Each of the return circuits is electrically connected to the clamping circuit and coupled to both terminals of one of the switching elements for transmitting the output current to the output transformer, thereby permitting a reverse voltage of the switching elements within a maximum allowable range of the switching elements.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an electronic ballast, and more particularly to an electronic ballast having a reduced reverse voltage at the transient moment of start. 
   BACKGROUND OF THE INVENTION 
   Generally, the luminous efficacy of a fluorescent lamp is about 80 luminance/watt, which is approximately five times of the conventional incandescent lamp (e.g. 16 luminance/watt). The average life of the fluorescent lamp is about 6,000 hours, which is approximately three times of the conventional incandescent lamp (e.g. 2,000 hours). In addition, heat generated from the incandescent lamp is about 61% of the input power. In contrast, heat generated from the fluorescent lamp is about 37% of the input power, which is much less than that of the incandescent lamp. Since the fluorescent lamp has many advantages over the incandescent lamp, the fluorescent lamp is widely used in offices or homes. 
   As known, in a case that the lamp tube of the fluorescent lamp is driven to illuminate at a high frequency, a higher light output ratio is achieved when identical output power is applied. In other words, such illuminating approach is less power consuming. With increasing development of electrical and electronic technologies, electronic ballasts are widely employed to replace the starters of the fluorescent lamp system and the conventional ballasts. 
   Referring to  FIG. 1(   a ), a schematic circuit diagram of a conventional electronic ballast is shown. The electronic ballast  10  is not preheated but uses a push-pull parallel resonant circuit. The electronic ballast  10  is powered by a DC power supply  11  and comprises a push-pull parallel resonant circuit  14  including an output transformer  12 , a fluorescent lamp  13  and two switching elements Q 1  and Q 2 . By controlling the turning on/off statuses of the switching elements Q 1  and Q 2 , the DC voltage provided by the DC power supply  11  is converted into a high-frequency AC voltage so as to activate several sets of fluorescent lamps  13 . The winding T 1  is used to monitor a current change in the primary winding of the output transformer  12  so as to control the operation of the switching elements Q 1  and Q 2 , where the winding T 1  is a part of the output transformer  12 . 
   The electronic ballast  10  of  FIG. 1(   a ) is popular because of some advantages. For example, the mechanism for driving the circuit is simple. In addition, the lamp tubes can be used in parallel. Since both output ends of the electronic ballast can be operated in an open or close circuit mode, no additional protection circuit is required. The lamp tube may be lighted without restarting the electronic ballast when the lamp tube is exchanged. Afterward, the mechanism for starting the electronic ballast is diverse and includes for example rapid start, instant start and program start. On the other hand, the electronic ballast  10  still has some drawbacks. For example, the switching elements Q 1  and Q 2  have large voltage stresses, the production process of the output transformer  12  is complicated, the volume of the inductor T 2  is bulky, and so on. In views of the large voltage stresses of the switching elements Q 1  and Q 2 , the node voltage V 1  between the inductor T 2  and the parallel resonant circuit  14  will has large transient voltage induced by the parallel resonant circuit  14  at the instant moment when the electronic ballast  10  is started. The transient voltage is slowly reduced and then reaches a steady state. Since the collector-to-emitter voltage (VCE) for the switching element Q 1  or Q 2  and the node voltage V 1  is in a linear relationship. If the node voltage V 1  is very large, the switching elements Q 1  or Q 2  is subjected to a large voltage at the instant moment when the electronic ballast  10  is started, as shown in  FIG. 1(   b ). 
   Accordingly, if the switching element Q 1  or Q 2  is not conducted, the region between the collector and the emitter thereof should sustain a large output voltage. Otherwise, the switching element Q 1  or Q 2  may have a breakdown, and thus the electronic ballast  10  fails to normally function. For example, when the electronic ballast  10  is started, the switching element Q 1  is conducted but the switching element Q 2  is shut, the region between the collector and the emitter of the switching element Q 2  should sustain a large transient voltage generated from the node voltage V 1 . In contrast, when the switching element Q 2  is conducted but the switching element Q 1  is shut, the region between the collector and the emitter of the switching element Q 1  should sustain a large transient voltage generated from the node voltage V 1 . Generally, the switching element Q 1  or Q 2  used in the electronic ballast  10  is a transistor capable of sustaining a high voltage such as 1.6 KV (according to the specification of Bipolar Junction Transistor, the sustainable largest transient voltage of VCE is 1 KV or 1.6 KV). However, these transistors are low in selectivity and not cost-effective. 
   In order to have the region between the collector and the emitter of the transistor sustain a large transient voltage, another conventional electronic ballast is developed, as can be seen in  FIG. 2(   a ). The electronic ballast  20  of  FIG. 2(   a ) uses a clamping circuit  22  for limiting the node voltage V 2  between the inductor T 2  and the parallel resonant circuit  24  at the moment when the electronic ballast  20  is started. The node voltage V 2  has large transient voltage induced by the parallel resonant circuit  24  at the instant moment when the electronic ballast  20  is started. In such manner, the collector-to-emitter voltage (VCE) for the switching element Q 1  or Q 2  is reduced, and thus the damage probability of the switching element Q 1  or Q 2  is reduced. For example, if the node voltage V 2  is 1.6 KV at the moment when the electronic ballast  20  is started, the clamping circuit  22  connected to the inductor T 2  will limit the node voltage V 2  from 1.6 KV down to about 1 KV, wherein the clamping circuit  22  can be a transient voltage suppressor such as Type P6KE400A available from ST Microelectronics or Type P6KE400A available from VISHAY. Accordingly, the transistor to be used in the electronic ballast  20  may have a lower rated voltage, for example 1.0 KV or less, wherein the transistor can be a transistor Type BUL1102E available from ST Microelectronics or Type BUJ403A available from Philips. As known, this transistor has higher selectivity and is cost-effective. In addition, the switching speed of this transistor is very fast and the switching loss thereof is small. 
   Since the clamping circuit  22  limits the node voltage V 2  at the moment when the electronic ballast  20  is started, the reduced voltage is converted into the current Iz, as shown in  FIG. 2(   b ). In a case that the switching element Q 1  is shut but the switching element Q 2  is conducted, the current Iz may flow through the emitter E and the collector C of the switching element Q 2 . At the moment when the switching element Q 2  is conducted, the current generated from the clamping circuit  22  is very large, and thus the reverse voltage between the base B and the emitter E of the switching element Q 2  may exceed the maximum allowable range. Under this circumstance, the switching element Q 2  may be damaged or the average life thereof may be decreased, and the performance of the electronic ballast  20  is impaired. 
     FIG. 2(   b ) is a timing waveform diagram illustrating the current Iz from the clamping circuit, the emitter-to-base voltage (VEB) and the collector-to-emitter voltage (VCE) of the switching element Q 1  or Q 2 . An example of the switching elements Q 1  or Q 2  is a BUL1102E transistor commercial available from ST Microelectronics. The maximum reverse voltage between the base B and the emitter E of the switching element Q 1  or Q 2  is 12V, i.e. the maximum allowable range of VEB is 12V. The maximum reverse voltage between the collector C and the emitter E of the switching element Q 1  or Q 2  is 1.1 KV, i.e. the maximum allowable range of VCE is 1.1 KV. As shown in  FIG. 2(   b ), when the electronic ballast  20  is started, the transient response of the collector-to-emitter voltage (VCE) of the switching element Q 1  or Q 2  is limited below 1.1 KV, and the clamping circuit  22  outputs the current Iz. Since the current Iz is very large at the instant moment when the electronic ballast  10  is started, the reverse voltage between the base B and the emitter E of the switching element Q 1  or Q 2  exceeds 12V. Accordingly, the switching element Q 1  or Q 2  is readily damaged and the average life thereof will be shortened. 
   Accordingly, the above-described prior art electronic ballasts are not perfect designs and have still many disadvantages to be solved. In views of the above-described disadvantages resulted from the conventional electronic ballasts, the applicant keeps on carving unflaggingly to develop an electronic ballast according to the present invention through wholehearted experience and research. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an electronic ballast having a reduced reverse voltage at the transient moment of start. 
   In accordance with a first aspect of the present invention, there is provided an electronic ballast. The electronic ballast comprises an inductor, an output transformer, at least two switching elements, a control circuit, a clamping circuit, and at least two return circuits. The inductor is electrically coupled to a DC power supply. The control circuit is electrically connected to the inductor, the output transformer and the switching elements for controlling on/off statuses of the switching elements. The clamping circuit is electrically connected to the inductor, and limits a node voltage among the inductor, the control circuit and the clamping circuit below a threshold value and generates an output current on condition that the node voltage is larger than the threshold value. Each of the return circuits is electrically connected to the clamping circuit and coupled to both terminals of one of the switching elements for transmitting the output current to the output transformer, thereby permitting a reverse voltage of the switching elements within a maximum allowable range of the switching elements. 
   In an embodiment, the output transformer is a centre-tapped transformer. 
   In an embodiment, the clamping circuit is a transient voltage suppressor for limiting the node voltage below the threshold value and generating the output current on condition that the node voltage is larger than the threshold value. 
   Preferably, the transient voltage suppressor is a Zener diode. 
   In an embodiment, each of the switching elements is a NPN-type bipolar junction transistor having a base, a collector and an emitter. 
   In an embodiment, each of the two return circuits is coupled to the collector and the emitter of one of the switching elements. 
   Preferably, the return circuits are diodes. 
   In an embodiment, the control circuit comprises two resistors, a winding of the output transformer and a resonant capacitor. 
   In an embodiment, the electronic ballast further comprises a plurality of ballast capacitors electrically connected between a secondary winding of the output transformer and at least two lamps. 
   In an embodiment, the electronic ballast further comprises a preheat circuit electrically connected to the ballast capacitors and the lamps for preheating the lamps. 
   In an embodiment, the electronic ballast further comprises at least two capacitors, each of which is electrically connected to one of the return circuits and a collector terminal and an emitter terminal of one of the switch elements. 
   In accordance with a second aspect of the present invention, there is provided an electronic ballast for energizing at least two lamps. The electronic ballast comprises an inductor, at least two switching elements, a clamping circuit, and at least two return circuits. The inductor is electrically connected to a power supply. The clamping circuit is electrically connected to the inductor, and limits a node voltage across both terminals thereof below a threshold value and generates an output current on condition that the node voltage is larger than the threshold value. Each of the return circuits is electrically connected to the clamping circuit and coupled to both terminals of one of the switching elements for providing a path of transmitting the output current, thereby permitting a reverse voltage of the switching elements within a maximum allowable range of the switching elements. 
   The above contents of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1(   a ) is a schematic circuit diagram of a conventional electronic ballast; 
       FIG. 1(   b ) is a timing waveform diagram illustrating the collector-to-emitter voltage (VCE) of the switching element Q 1  or Q 2  of  FIG. 1(   a ); 
       FIG. 2(   a ) is a schematic circuit diagram of another conventional electronic ballast; 
       FIG. 2(   b ) is a timing waveform diagram illustrating the current Iz from the clamping circuit, the emitter-to-base voltage (VEB) and the collector-to-emitter voltage (VCE) of the switching element Q 1  or Q 2  of  FIG. 2(   a ); 
       FIG. 3(   a ) is a schematic circuit diagram of an electronic ballast according to a preferred embodiment of the present invention; 
       FIG. 3(   b ) is a timing waveform diagram illustrating the current Iz from the clamping circuit, the emitter-to-base voltage (VEB) and the collector-to-emitter voltage (VCE) of the switching element Q 1  or Q 2  of  FIG. 3(   a ); and 
       FIG. 4  is a schematic circuit diagram of an electronic ballast according to another preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
   Referring to  FIG. 3(   a ), a schematic circuit diagram of an electronic ballast according to a preferred embodiment of the present invention is shown. The electronic ballast  30  is powered by a DC power supply  31  and comprises an inductor T 2 , a clamping circuit  32 , a control circuit  33 , an output transformer  34 , at least two switching elements Q 1  and Q 2 , and at least two return circuits  351  and  352 . 
   The inductor T 2  is electrically connected to the DC power supply  31 . The control circuit  33  is electrically connected to the inductor T 2 , the switching elements Q 1  and Q 2 , the clamping circuit  32  and the output transformer  34  so as to control the turning on/off statuses of the switching elements Q 1  and Q 2 . The control circuit  33  comprises resistors R 1  and R 2 , a winding T 1  of the output transformer  34 , and a resonant capacitor C 1 . When the switching element Q 1  is conducted, the switching element Q 2  is shut. Whereas, when the switching element Q 2  is conducted, the switching element Q 1  is shut. By controlling the turning on/off statuses of the switching elements Q 1  and Q 2 , there is voltage change in the primary winding of the output transformer  34 , and the DC voltage provided by the DC power supply  31  is converted into a high-frequency AC voltage so as to activate several sets of fluorescent lamps  36 . 
   In this embodiment, the output transformer  34  is a centre-tapped transformer. Some ballast capacitors (e.g. C 2  and C 3 ) are interconnected between the output transformer  34  and the fluorescent lamps  36  so as to adjust luminance of the respective fluorescent lamp  36 . Each of the switching elements Q 1  and Q 2  is a NPN-type bipolar junction transistor having a base B, a collector C and an emitter E. 
   A linear relationship exists between the collector-to-emitter voltage (VCE) for the switching element Q 1  or Q 2  and the node voltage V 3 , which is the voltage across both terminals of the clamping circuit  32 . If the node voltage V 3  is larger than a preset limiting voltage at the moment when the electronic ballast  30  is started and the node voltage V 3  has large transient voltage at the instant moment when the electronic ballast  30  is started, the transient voltage will be limited below the limiting voltage by the clamping circuit  32 . Under this circumstance, the collector-to-emitter voltage (VCE) of the switching element Q 1  or Q 2  is reduced, and thus the switching element Q 1  or Q 2  is not readily damaged. 
   Since the clamping circuit  32  limits the node voltage V 3  at the transient moment when the electronic ballast  30  is started, the reduced voltage is converted into the current Iz. In order to prevent that the reverse voltage between the base B and the emitter E of the switching element Q 1  or Q 2 , i.e. VEB, exceeds the maximum allowable voltage when the current Iz flows through the emitter E and the collector C of the switching element Q 1  or Q 2 , the return circuit  351  is connected to the collector C and the emitter E of the switching element Q 1  and the return circuit  352  is connected to the collector C and the emitter E of the switching element Q 2 . In such manner, the current Iz outputted from the clamping circuit  32  may be transmitted to the output transformer  34  via the return circuit  351  or  352  without passing through the base B and emitter E of the switching element Q 1  or Q 2 . Accordingly, the current Iz outputted from the clamping circuit  32  will no longer affect the reverse voltage of the switching element Q 1  or Q 2  because the reverse voltage is within the maximum allowable range. 
   In the above embodiments, the clamping circuit  32  used in the electronic ballast of the present invention can be a transient voltage suppressor, which is preferably implemented by a Zener diode. Each of the return circuit  351  and  352  is preferably a diode. It is noted that, however, those skilled in the art will readily observe that numerous modifications and alterations of the clamping circuit and the return circuit may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be limited only by the bounds of the following claims. 
   For example, during the period from starting of the electronic ballast  30  to a stable state, when the switching element Q 2  is conducted and the switching element Q 1  is shut under control of the control circuit  33 , the current Iz outputted from the clamping circuit  32  will be transmitted to the output transformer  34  via the return circuit  351 , and afterwards return to the clamping circuit  32 . Similarly, when the switching element Q 1  is conducted and the switching element Q 2  is shut under control of the control circuit  33 , the current Iz outputted from the clamping circuit  32  will be transmitted to the output transformer  34  via the return circuit  352 , and afterwards return to the clamping circuit  32 . Since the current Iz outputted from the clamping circuit  32  will not pass through the base B and emitter E of the switching element Q 1  or Q 2  when the current Iz is transmitted to the output transformer  34 , the reverse voltage between the base B and the emitter E of the switching element Q 2 , i.e. VEB, will lie in the range of the maximum allowable voltage. 
     FIG. 3(   b ) is a timing waveform diagram illustrating the current Iz from the clamping circuit, the emitter-to-base voltage (VEB) and the collector-to-emitter voltage (VCE) of the switching element Q 1  or Q 2 . Each of the switching elements Q 1  and Q 2  is a BUL1102E transistor commercial available from ST Microelectronics. Since the return circuit  351  is connected to the collector C and the emitter E of the switching element Q 1  and the return circuit  352  is connected to the collector C and the emitter E of the switching element Q 2  according to the present invention, the reverse voltage between the base B and the emitter E of the switching element Q 1  or Q 2  (i.e. VEB) is within the maximum allowable range. 
   In the above embodiment, BUJ403A transistors from Koninklijke Philips Electronics N.V. may also be selected as the switching elements Q 1  and Q 2 . In addition, each of the return circuit  351  and  352  is preferably a BA159 diode commercial available from Vishay Intertechnology, Inc. Alternatively, chips having similar functions can be used as the return circuit  351  and  352 . 
   Referring to  FIG. 4 , a schematic circuit diagram of a pre-heat type electronic ballast according to another preferred embodiment of the present invention is shown. The electronic ballast  40  is powered by a DC power supply  41  and comprises an inductor T 2 , a clamping circuit  42 , a control circuit  43 , an output transformer  44 , at least two switching elements Q 1  and Q 2 , at least two return circuits  451  and  452 , a preheat circuit  46  and at least two capacitors  471 ,  472 . 
   The inductor T 2  is electrically connected to the DC power supply  41 . The control circuit  43  is electrically connected to the inductor T 2 , the switching elements Q 1  and Q 2 , the clamping circuit  42  and the output transformer  44  so as to control the turning on/off statuses of the switching elements Q 1  and Q 2 . The control circuit  43  comprises resistors R 1  and R 2 , a winding T 1  of the output transformer  44  and a resonant capacitor C 1 . When the switching element Q 1  is conducted, the switching element Q 2  is shut. Whereas, when the switching element Q 2  is conducted, the switching element Q 1  is shut. By controlling the turning on/off statuses of the switching elements Q 1  and Q 2 , there is voltage change in the primary winding of the output transformer  44 , and the DC voltage provided by the DC power supply  41  is converted into a high-frequency AC voltage so as to activate several sets of fluorescent lamps  48 . 
   In this embodiment, the output transformer  44  is a centre-tapped transformer. Some ballast capacitors (e.g. C 2  and C 3 ) are interconnected between the output transformer  44  and the fluorescent lamps  48  so as to adjust luminance of the respective fluorescent lamp  48 . Each of the switching elements Q 1  and Q 2  is a NPN-type bipolar junction transistor having a base B, a collector C and an emitter E. 
   A linear relationship exists between the collector-to-emitter voltage (VCE) for the switching element Q 1  or Q 2  and the node voltage V 4 , which is the voltage across both terminals of the clamping circuit  42 . If the node voltage V 4  is larger than a preset limiting voltage at the moment when the electronic ballast  40  is started and the node voltage V 4  has large transient voltage at the instant moment when the electronic ballast  40  is started, the transient voltage will be limited below the limiting voltage by the clamping circuit  42 . Under this circumstance, the collector-to-emitter voltage (VCE) of the switching element Q 1  or Q 2  is reduced, and thus the switching element Q 1  or Q 2  is not readily damaged. 
   Since the clamping circuit  42  limits the node voltage V 4  at the transient moment when the electronic ballast  40  is started, the reduced voltage is converted into the current Iz. In order to prevent the reverse voltage between the base B and the emitter E of the switching element Q 1  or Q 2 , i.e. VEB, exceeds the maximum allowable voltage when the current Iz flows through the emitter E and the collector C of the switching element Q 1  or Q 2 , the return circuit  451  is connected to the collector C and the emitter E of the switching element Q 1  and the return circuit  452  is connected to the collector C and the emitter E of the switching element Q 2 . In such manner, the current Iz outputted from the clamping circuit  42  may be transmitted to the output transformer  44  via the return circuit  451  or  452  without passing through the base B and emitter E of the switching element Q 1  or Q 2 . Accordingly, the current Iz outputted from the clamping circuit  42  will no longer affect the reverse voltage of the switching element Q 1  or Q 2  because the reverse voltage is within the maximum allowable range. 
   In the above embodiments, the clamping circuit  42  used in the electronic ballast of the present invention can be a transient voltage suppressor, which is preferably implemented by a Zener diode. Each of the return circuits  451  and  452  is preferably a diode. It is noted that, however, those skilled in the art will readily observe that numerous modifications and alterations of the clamping circuit and the return circuit may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be limited only by the bounds of the following claims. 
   For example, during the period from starting of the electronic ballast  40  to a stable state, when the switching element Q 2  is conducted and the switching element Q 1  is shut under control of the control circuit  43 , the current Iz outputted from the clamping circuit  42  will be transmitted to the output transformer  44  via the return circuit  451 , and afterwards return to the clamping circuit  42 . Similarly, when the switching element Q 1  is conducted and the switching element Q 2  is shut under control of the control circuit  43 , the current Iz outputted from the clamping circuit  42  will be transmitted to the output transformer  44  via the return circuit  452 , and afterwards return to the clamping circuit  42 . Since the current Iz outputted from the clamping circuit  42  will not pass through the base B and emitter E of the switching element Q 1  or Q 2  when the current Iz is transmitted to the output transformer  44 , the reverse voltage between the base B and the emitter E of the switching element Q 2 , i.e. VEB, will lie in the range of the maximum allowable voltage. 
   In addition, the preheat circuit  46  is electrically connected to the ballast capacitors (e.g. C 2  and C 3 ) and the fluorescent lamps  48  for preheating the fluorescent lamps  48 . When the electronic ballast  40  is started, the fluorescent lamps  48  are preheated by the preheat circuit  46 . Due to the operation of the preheat circuit  46 , the switching elements Q 1  and Q 2  may enter into over-saturation state, which will damage the switching elements Q 1  and Q 2  at the transient moment when the electronic ballast  40  is started. In order to prevent the switching elements Q 1  and Q 2  from entering into over-saturation state, two capacitors  471 ,  472  are respectively employed and coupled to the return circuits  451  and  452  and the collectors C and emitters E of the switching elements Q 1  or Q 2 . 
   From the above description, since the current outputted from the clamping circuit will be directly transmitted to the output transformer without passing through the base and the collector of the switching element, the reverse voltage of the switching element of the electronic ballast is within the maximum allowable range. Accordingly, the yield, average life and performance of the electronic ballast are enhanced. 
   While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.