Abstract:
A ballast circuit operable with a triac based controller circuit, the ballast circuit ( 100 ) includes a rectifier ( 115 ) configured for operative connection with an associated triac based circuit ( 110 ) for converting AC current to DC current, a capacitor assembly ( 137 ) coupled to the rectifier ( 115 ), a first connection ( 150 ) between the rectifier and the capacitor assembly ( 137 ), a converter ( 153 ) coupled to the rectifier ( 115 ) for converting the DC current to AC current, a gate drive arrangement coupled to the converter for controlling the converter ( 153 ), a resistance-inductance circuit ( 163 ) coupled to the converter ( 153 ), and a second connection ( 165 ) between the capacitor assembly ( 137 ) and the resistance-inductance circuit ( 163 ). The converter ( 153 ) induces AC current in the resistance-inductance circuit ( 163 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application relates to an electrical circuit, and in particular, to a converter circuit which is compatible with a triac based circuit. 
     2. Discussion of the Art 
     Incandescent lamps have widespread use in a variety of applications. Typically, incandescent lamps use mains or line voltage for power although in selected circumstances, low voltage is desired. For example, color rendering characteristics and light beam control are distinct advantages with low voltage lamps. Low voltage lamps, however, require a voltage lower than the line voltage because of the voltage rating of the lamp filaments. An exemplary line voltage is about 120 V, but certain lamp filaments, such as those found in MR16 lamps, have voltage ratings of only approximately 12 V. Thus, low voltage lamps require converters to reduce the line voltage to match the requirements of the lamp filament. 
     Although low voltage lamps have better optical light quality than high voltage lamps, the use of low voltage lamps in the business area (e.g., restaurants, commercial establishments, etc.) has not yet found widespread adoption. One reason for this may be attributed to the fact that many business establishments also desire dimmable lamps. For example, many restaurants want brighter light output during lunch hours to accommodate business lunches and want to have the capability to dim the lights during dinner hours for a more personal and private ambiance. 
     In order to use the low voltage lamps in traditional lamp sockets, it is known in the art to place lamps having small, integral electronic converters within existing fixtures. Typical electronic converters, however, are not readily compatible with the wide variety of commercially available triac based circuits which are prevalent in the consumer, retail, restaurant, and hotel lighting markets. Common triac based control circuits include wall dimmers and solid state switches activated by photo sensors, motion sensors, occupancy detectors, and timer controls. 
     Common mode chokes and resistors have been used to damp oscillations which are otherwise caused by a triac based phase dimmer circuit. While this approach provides dimming capability, it presents other problems. First, the dimensions of the outer lamp envelope constrain the size of converter circuits. Use of an inductor of 50 mH, for example, is not practical since it is a fairly large component. Additionally, the resistors compromise the efficiency of the circuit by introducing an additional (i.e., non-light producing) load to discharge the resistive-capacitive (RC) element in the dimmer circuit. 
     Yet another approach is to design a custom dimmable converter circuit for a low voltage lamp. This solution, however, fails to take advantage of the many available dimmer circuits already in existence. Further, if the custom design requires that the dimmer circuit be integral with the lamp, each lamp to would have to be dimmed individually. The lamp would not have the capability of being dimmed by traditional dimmer circuits, which generally have the capability control entire light fixtures, not just a single lamp. Thus, more time and labor would be required to dim the lamps, and the dimming amount may not be uniform throughout the establishment. 
     Accordingly, a need exists for a converter circuit compatible with commercially available triac based circuits. 
     BRIEF SUMMARY OF THE INVENTION 
     A high frequency electronic power converter, which allows a low voltage lamp to be connected to standard consumer mains AC voltage through a triac based circuit, is disclosed. 
     An exemplary embodiment of the present invention concerns a ballast circuit operable with a triac based controller. The ballast circuit includes a rectifier configured for operative connection with an associated triac based circuit for converting AC current to DC current, a capacitor assembly coupled to the rectifier, a first connection between the rectifier and the capacitor assembly, a converter coupled to the rectifier for converting the DC current to AC current, a gate drive arrangement coupled to the converter for controlling the converter, a resistance-inductance circuit coupled to the converter, and a second connection between the capacitor assembly and the resistance-inductance circuit. The converter induces AC current in the resistance-inductance circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a converter circuit embodying the present invention; 
     FIG. 2 is a schematic diagram of a second converter circuit embodying the present invention; and 
     FIG. 3 is a schematic diagram of a third converter circuit embodying the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the Figures, several embodiments of the present invention are shown and will now be described. Like reference numerals are used to indicate the same element throughout the specification. 
     A ballast or converter circuit  100  includes an AC source  105  coupled to a triac based controller  110  in the embodiment of FIG.  1 . The AC source is generally the standard consumer mains or line voltage. The triac based controller  110  is typically a commercially available phase controlled triac dimmer prevalent in consumer, retail, restaurant, and hotel lighting markets, such as General Electric Company incandescent light dimmer, part number DIT261 M5. The triac based controller circuit may also be a solid state switch which controls the on-off operation of a lamp, such as a dusk-to-dawn controller. 
     The triac based controller  110  is coupled to a rectifier  115 , such as a full-wave bridge rectifier, which converts AC current to DC current. An electromagnetic interference (EMI) filter  120  is preferably interposed between the triac based controller  110  and the bridge rectifier  115 . The EMI filter  120  suppresses EMI from adjacent electrical devices. The EMI filter includes a resistor  125 , a capacitor  130 , and an inductor  135 , where the capacitor and resistor are serially connected as shown in FIG. 1, and the inductor is coupled between the triac based controller  110  and rectifier  115 . 
     The bridge rectifier  115  is coupled in parallel to a capacitor assembly  137 . The capacitor assembly  137  includes capacitors  140  and  145 , which are standard half-bridge. The bridge rectifier  115  is further coupled to the capacitor assembly  137  via a first or direct electrical connection  150 . For example, a jumper connection suitably interconnects the bridge rectifier  115  at connection node N 1  between the half-bridge capacitors  140  and  145 . The half-bridge capacitors  140  and  145 , which maintain the connection node N 1  at about one-half bus voltage V BUS , are connected in parallel to a DC-to-AC converter  153 . 
     The DC-to-AC converter  153 , which includes first and second switches  155  and  160 , converts the DC current received from the output of the bridge rectifier  115  to an AC current. The AC current is received by a resistance-inductance circuit  163  via a second or capacitive connection  165 , shown as a decoupling capacitor in FIG.  1 . The resistance-inductance circuit  163  includes a high frequency transformer  170 , which has a primary winding  175  and a secondary winding  180 , and a load  185 , shown as a lamp in FIG.  1 . The lamp may be any number of low voltage lamps, such as a low voltage incandescent lamp. 
     The first and second switches  155  and  160  are complementary to each other in the sense that the first switch  155  may be an n-channel enhancement mode device as shown, and the second switch  160  is a p-channel enhancement mode device, or what are common referred to as MOSFET switches. Each of the first and second switches  155 ,  160  has a respective gate (or control terminal) G 1  or G 2 , respectively. The voltage from gate G 1  to source (reference terminal) S 1  of the first switch  155  controls the conduction state of that switch. Similarly, the voltage from gate G 2  to source S 2  of the second switch  160  controls the conduction state of that switch. As illustrated, sources S 1  and S 2  are connected together at a common node N 2  and the gates G 1  and G 2  are interconnected at the common control node N 3 . The gates G 1  and G 2  may be coupled to gate resistors  187  and  189  to prevent over rating of the gate-to-source resonance and improve reliability of the converter circuit  100 . Drains D 1  and D 2  of the first and second switches  155  and  160  are connected to a bus conductor  190  and a reference conductor  195 , respectively. The reference conductor  195  is shown for convenience as a ground. DC bus voltage V BUS  exists between the bus conductor  190  and the reference conductor  195 . 
     The DC-to-AC converter  153  is coupled to a gate drive circuit, which comprises a driving inductor  200 , a second inductor  205 , and a blocking capacitor  210 . The gate drive circuit is coupled to three starting resistors  211 ,  212 , and  213 . Together, the starting resistors  211 ,  212 , and  213  and the first switch  155  form a self-starting circuit as is well-known in the art. 
     A bi-directional voltage clamp  215 , is disposed in parallel relation with the gate drive circuit between common control node N 3  and the common node N 2 . The bi-directional voltage clamp  215  is preferably comprised of back-to-back Zener diodes  217 ,  218 . The bi-directional voltage clamp  215  clamps positive and negative excursions of gate-to-source voltage ratings of the first and second switches  155  and  160  so that gate-to-source maximum ratings of the switches are not exceeded. 
     A snubber capacitor  220  is preferably connected between the connection node N 1  and the common node N 2  to protect the first and second switches  155  and  160  from exceeding maximum gate-to-source voltage ratings during a dead time interval when the first and second switches  155  and  160  are both off. The converter circuit  100  may also include a gate or swamp capacitor  225  across the DC bus. The gate capacitor  225  supports the converter operation following a zero-crossing of the line voltage. 
     The converter circuit  100  may further include a second EMI filter  230 , shown as an inductor, connected in series between the bridge rectifier  115  and the bridge capacitor  140  to further suppress EMI at the output of the bridge rectifier  115 . Depending upon the other EMI control measures taken, it may also be desirable to have a third connection  235  between the secondary winding  180  of the high frequency transformer  170  and the node N 1  between the half-bridge capacitors  140  and  145 . The third connection  235  may be a direct electrical connection. The third connection may also be a capacitive connection in applications where a direct electrical connection would be detrimental to the performance of the product. 
     The converter circuit  100  operates as follows. The bridge rectifier  115  converts AC current from the source  105  to DC current. The first and second switches  155  and  160  are alternately switched at high frequency by the self-resonating converter circuit  153  to drive the primary winding  175  of the high frequency transformer  170 . The secondary winding  180  of the high frequency transformer  170  drives the load  185 . The capacitive connection  165  is used to decouple high frequency switching from the low frequency line. The half-bridge midpoint voltage is referenced to the line to provide a low frequency current path back to the load  185  to maintain compatibility with the triac based controller  110 . The EMI filter  120  at the circuit input and the secondary winding  180  of the high frequency transformer  170  are referenced to the half-bridge midpoint voltage to help further suppress EMI. 
     The self-starting circuit provides a path for input from a source to start inductor action. The blocking capacitor  210  becomes initially charged upon energizing of the AC source  105 , via the resistors  211 ,  212 , and  213 . At this instant, the voltage across the blocking capacitor  210  is zero. During the starting process, the driving inductor  200  and the primary winding  175  of the high frequency transformer  170  act essentially as a short circuit due to the relatively long time constant for charging of the blocking capacitor  210 . Upon initial bus energizing, the voltage on the common node N 2  is approximately one-third of the bus voltage V BUS  with resistors  211 ,  212 , and  213  being of equal value, for instance. The voltage at the common control node N 3  between the resistors  211 ,  212 , and  213  is one-half of the bus voltage V BUS . In this manner, the blocking capacitor  210  becomes increasingly charged, from left to right, until it reaches the threshold voltage of the gate-to-source voltage of the first switch  155  (e.g., 2-3 volts). At this point, the first switch  155  switches into a conduction mode, which then results in current being supplied by the first switch  155  to the primary winding  175  of the high frequency transformer  170 . The secondary winding  180  of the high frequency transformer  170  drives the load  185 . In turn, the resulting current in the transformer  170  causes regenerative control of the first and second switches  155  and  160  in the manner described above. 
     During steady state operation of the converter circuit  100 , the voltage of the common node N 2  between the first and second switches  155  and  160  becomes approximately ½ of the bus voltage V BUS . The voltage at the common control node N 3  also becomes approximately ½ of the bus voltage V BUS  so that the blocking capacitor  210  cannot again, during steady state operation, become charged and create another starting pulse for turning on the first switch  155 . The capacitive reactance of the blocking capacitor  210  is much smaller than the inductive reactance of the driving inductor  200  and the second inductor  205  so that the blocking capacitor  210  does not interfere with the operation of the driving inductor  200  and second inductor  205 . 
     The driving inductor  200  of the gate drive circuit is mutually coupled to the primary winding  175  of the high frequency transformer  170  in such a manner that a voltage is induced therein which is proportional to the instantaneous rate of change of an AC load current. The driving inductor  200  is further connected at one end to the common node N 2 . The driving inductor  200  provides the driving energy for operation of the gate drive circuit. The second inductor  205 , which is serially connected to the blocking capacitor  210  and the common control node N 3 , is used to adjust the phase angle of the gate-to-source voltage appearing between common control node N 3  and common node N 2 . 
     The converter circuit  100  continues to operate at low line conditions and restarts quickly when the triac based controller  110  triggers. A current pathway is provided which allows a resistor-capacitor (RC) network of the triac based controller  110  to discharge and provide consistent operation of the triac based controller  110 . The passive EMI filter  120 , which limits the line current and damps oscillations which might be caused by the firing of the triac based controller  110 , is applied. Thus, the converter circuit  100  is compatible with a wide variety of commercially available triac based controllers. 
     Exemplary component values for the converter circuit  100  are as follows, with a line voltage of 120V: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Zener diodes 215 
                 10 V - ¼ Watt 
               
               
                   
                 Diodes of full-wave bridge rectifier 115 
                 1N4006 
               
               
                   
                 Gate capacitor 225 
                 100 nanofarads, 200 V 
               
               
                   
                 Half-bridge capacitors 140, 145 
                 100 nanofarads, 100 V 
               
               
                   
                 Decoupling capacitor 165 
                 100 nanofarads, 100 V 
               
               
                   
                 Gate resistors 187, 189 
                 10 ohms 
               
               
                   
                 EMI filter capacitor 130 
                 100 nanofarads, 200 V 
               
               
                   
                 Blocking capacitor 210 
                 100 nanofarads, 25 V 
               
               
                   
                 Snubber capacitor 220 
                 680 picofarads, 200 V 
               
               
                   
                 Starting resistors 211, 212, 213 
                 270 k ohms 
               
               
                   
                 EMI filter resistor 125 
                 100 ohms 
               
               
                   
                   
               
             
          
         
       
     
     Additionally, the first switch  155  may be an IRFU214, n-channel MOSFET, and the second switch  160  may be an IRFU9214, p-channel MOSFET, both of which are sold by International Rectifier Company, of El Segundo, Calif. 
     FIG. 2 is a schematic diagram of a second converter circuit  250  embodying the present invention. The second converter circuit  250  functions in the same manner as the converter circuit  100  of FIG. 1 described above. The primary difference between the second converter circuit  250  and the converter circuit  100  resides in the placement of the direct electrical connection  150  and the capacitive connection  165 . As shown in FIG. 2, the capacitive connection  165  of the converter circuit  250  is located between the connection node N 1  and the bridge rectifier  115  while the direct electrical connection  150  is located between the connection node N 1  and the primary winding  175 . Thus, the placement of the direct electrical connection  150  and the capacitive connection  165  are opposite of that shown in FIG.  1 . 
     FIG. 3 is a schematic diagram of a third converter circuit  300  embodying the present invention. Again, the third converter circuit  300  functions in the same manner as the converter circuit  100  of FIG.  1  and uses the identical components, with the exception of the direct electrical connection  150  between the bridge rectifier  115  and the node N 1  shown in FIG.  1 . In the converter circuit  300 , the direct electrical connection  150  is replaced with a capacitive connection  305 , shown as a second decoupling capacitor in FIG.  3 . This capacitive connection  305  replaces the direct electrical connection  150  of the converter circuit  100 , which may be desirable for small performance variations, such as reducing the peak filament voltage in a lamp. Thus, the converter circuit  300  has two capacitive connections  165  and  305  instead of one capacitive connection and one direct electrical connection, as in the converter circuit  100  described above. 
     In summary, the present invention provides a manner of efficiently using integrated circuit components with a commercially available triac based controller. The circuit is small enough such that it may be easily integrated within the lamp housing itself, thereby providing a low voltage lamp which is easily retrofitted to existing incandescent lamp fixtures. This converter also provides a low current crest factor at high frequency to the lamp filament, which is conducive to long lamp life. 
     Furthermore, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired that the present invention be limited to the exact construction and operation illustrated and described herein. Accordingly, all suitable modifications and equivalents which may be resorted to are intended to fall within the scope of the claims.