Patent Publication Number: US-7211964-B2

Title: Lamp including phase-control power controller with digital RMS load voltage regulation

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a phase-control power controller that supplies a specified power to a load, and more particularly to a voltage converter for a lamp that converts line voltage to a voltage suitable for lamp operation. 
     Some loads, such as lamps, operate at a voltage lower than a line (or mains) voltage of, for example, 120V or 220V, and for such loads a voltage converter that converts line voltage to a lower operating voltage must be provided. The power supplied to the load may be controlled with a phase-control circuit that typically includes an RC circuit. Moreover, some loads operate most efficiently when the power is constant (or substantially so). However, line voltage variations are magnified by these phase-control circuits due to their inherent properties (as will be explained below) and the phase-control circuit is desirably modified to provide a (nearly) constant RMS load voltage. 
     When the phase-control power controller is used in a voltage converter of a lamp, the voltage converter may be provided in a fixture to which the lamp is connected or within the lamp itself. U.S. Pat. No. 3,869,631 is an example of the latter, in which a diode is provided in the lamp base for clipping the line voltage to reduce RMS load voltage at the light emitting element. U.S. Pat. No. 6,445,133 is another example of the latter, in which transformer circuits are provided in the lamp base for reducing the load voltage at the light emitting element. 
     Factors to be considered when designing a voltage converter that is to be located within a lamp include the sizes of the lamp and voltage converter, costs of materials and production, production of a potentially harmful DC load on a source of power for installations of multiple lamps, and the operating temperature of the lamp and an effect of the operating temperature on a structure and operation of the voltage converter. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a novel phase-control power controller that converts a line voltage to an RMS load voltage and incorporates digital load regulation. 
     A further object is to provide power controller with a phase-control circuit having a digital potentiometer whose resistance determines clipping of a load voltage to control an RMS load voltage, a load sensing circuit that senses the load voltage and provides a DC signal that is different from but related to the RMS load voltage, and a comparator circuit that compares the DC signal to a reference and adjusts the resistance of the potentiometer to control the RMS load voltage. 
     A yet further object is to provide a lamp with this power controller in a voltage conversion circuit that converts a line voltage at a lamp terminal to the RMS load voltage usable by a light emitting element of the lamp. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross section of an embodiment of a lamp of the present invention. 
         FIG. 2  is a schematic circuit diagram of a phase-controlled dimming circuit of the prior art. 
         FIG. 3  is a schematic circuit diagram of the phase-controlled dimming circuit of  FIG. 2  showing an effective state in which the triac is not yet triggered. 
         FIG. 4  is a schematic circuit diagram of the phase-controlled dimming circuit of  FIG. 2  showing an effective state in which the triac has been triggered. 
         FIG. 5  is a graph illustrating current clipping in the phase-controlled dimming circuit of  FIG. 2 . 
         FIG. 6  is a graph illustrating voltage clipping in the phase-controlled dimming circuit of  FIG. 2 . 
         FIG. 7  is a graph showing the conduction angle convention adopted herein. 
         FIG. 8  is a graph showing the relationship of load voltage to conduction angle for several RMS line voltages. 
         FIG. 9  is a graph showing the relationship of line voltage to conduction angle for fixed RMS load voltages. 
         FIG. 10  is a schematic circuit diagram of a phase-controlled dimming circuit illustrating the concept of the present invention. 
         FIG. 11  is a schematic circuit diagram of an embodiment of the present invention. 
         FIG. 12  is a schematic circuit diagram of a load sensing circuit of the embodiment of  FIG. 11 . 
         FIG. 13  is a schematic circuit diagram of a comparator circuit of the embodiment of  FIG. 11 . 
         FIG. 14  is a schematic circuit diagram of a reference generator of the embodiment of  FIG. 11 . 
         FIG. 15  is a schematic circuit diagram of a further embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , a lamp  10  includes a base  12  with a lamp terminal  14  that is adapted to be connected to line (mains) voltage, a light-transmitting envelope  16  attached to the base  12  and housing a light emitting element  18  (an incandescent filament in the embodiment of  FIG. 1 ), and a voltage conversion circuit  20  for converting a line voltage at the lamp terminal  14  to a lower operating voltage. The voltage conversion circuit  20  is within the base  12  and connected between the lamp terminal  14  and the light emitting element  18 . The voltage conversion circuit  20  may be an integrated circuit in a suitable package as shown schematically in  FIG. 1 . 
     While  FIG. 1  shows the voltage conversion circuit  20  in a parabolic aluminized reflector (PAR) halogen lamp, the voltage conversion circuit  20  may be used in any incandescent lamp when placed in series between the light emitting element (e.g., filament) and a connection (e.g., lamp terminal) to a line voltage. Further, the voltage conversion circuit described and claimed herein finds application other than in lamps and is not limited to lamps. 
     The voltage conversion circuit  20  includes a phase-controlled dimming circuit, derived from a conventional phase-controlled dimming circuit such as shown in  FIG. 2  that has a capacitor  22 , a diac  24 , a triac  26  that is triggered by the diac  24 , and resistor  28 . In a conventional dimming circuit, the resistor  28  may be a potentiometer that sets a resistance in the circuit to control a phase at which the triac  26  fires. A dimming circuit is a two terminal device intended to reside in series with a relatively small resistive load. 
     In operation, a dimming circuit such as shown in  FIG. 2  has two states. In the first state the diac  24  and triac  26  operate in the cutoff region where virtually no current flows. Since the diac and triac function as open circuits in this state, the result is an RC series network such as illustrated in  FIG. 3 . Due to the nature of such an RC series network, the voltage across the capacitor  22  leads the line voltage by a phase angle that is determined by the resistance and capacitance in the RC series network. The magnitude of the capacitor voltage is also dependent on these values. 
     The voltage across the diac  24  is analogous to the voltage drop across the capacitor  22  and thus the diac will fire once breakover voltage is achieved across the capacitor. The triac  26  fires when the diac  24  fires. Once the diac has triggered the triac, the triac will continue to operate in saturation until the diac voltage approaches zero. That is, the triac will continue to conduct until the line voltage nears zero crossing. The virtual short circuit provided by the triac becomes the second state of the dimming circuit as illustrated in  FIG. 4 . 
     Triggering of the triac  26  in the dimming circuit is phase-controlled by the RC series network and the leading portion of the mains voltage waveform is clipped until triggering occurs as illustrated in  FIGS. 5–6 . A load attached to the dimming circuit experiences this clipping in both voltage and current due to the relatively large resistance in the dimming circuit. 
     Accordingly, the RMS load voltage and current are determined by the resistance and capacitance values in the dimming circuit since the phase at which the clipping occurs is determined by the RC series network and since the RMS voltage and current depend on how much energy is removed by the clipping. 
     Line voltage may vary from location to location up to about 10% and this variation can cause a variation in RMS load voltage in the load (e.g., a lamp) by an amount that can vary light levels, shorten lamp life, or even cause immediate failure. For example, if line voltage were above the standard for which the voltage conversion circuit was designed, the triac  26  may trigger early thereby increasing RMS load voltage. In a halogen incandescent lamp, it is particularly desirable to have a constant RMS load voltage. As will be explained below, there are several options for dealing with this problem. 
     By way of background and with reference to  FIG. 7 , clipping is characterized by a conduction angle α and a delay angle θ. The conduction angle is the phase between the point on the load voltage/current waveforms where the triac begins conducting and the point on the load voltage/current waveform where the triac stops conducting. Conversely, the delay angle is the phase delay between the leading line voltage zero crossing and the point where the triac begins conducting. 
     Define V irrms  as RMS line voltage, V ip  as peak line voltage, V orms  as RMS load voltage, V op  as peak load voltage, T as period, and ω as angular frequency (rad) with ω=2πf. The RMS voltage is determined from the general formula: 
     
       
         
           
             
               V 
               orms 
             
             = 
             
               
                 
                   1 
                   T 
                 
                 ⁢ 
                 
                   
                     ∫ 
                     0 
                     T 
                   
                   ⁢ 
                   
                     
                       
                         v 
                         2 
                       
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     ⁢ 
                     
                       ⅆ 
                       t 
                     
                   
                 
               
             
           
         
       
     
     Applying the conduction angle defined above yields: 
     
       
         
           
             
               
                 
                   
                     V 
                     orms 
                   
                   = 
                     
                   ⁢ 
                   
                     
                       
                         1 
                         
                           2 
                           ⁢ 
                           π 
                         
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             
                               ∫ 
                               
                                 π 
                                 - 
                                 α 
                               
                               π 
                             
                             ⁢ 
                             
                               
                                 V 
                                 ip 
                                 2 
                               
                               ⁢ 
                               
                                 
                                   sin 
                                   2 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   ω 
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 ⅆ 
                                 ω 
                               
                             
                           
                           + 
                           
                             
                               ∫ 
                               
                                 
                                   2 
                                   ⁢ 
                                   π 
                                 
                                 - 
                                 α 
                               
                               
                                 2 
                                 ⁢ 
                                 π 
                               
                             
                             ⁢ 
                             
                               
                                 V 
                                 ip 
                                 2 
                               
                               ⁢ 
                               
                                 
                                   sin 
                                   2 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   ω 
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 ⅆ 
                                 ω 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
             
             
               
                 
                   
                     V 
                     orms 
                   
                   = 
                     
                   ⁢ 
                   
                     
                       
                         1 
                         
                           2 
                           ⁢ 
                           π 
                         
                       
                       ⁢ 
                       
                         
                           ( 
                           2 
                           ) 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               ∫ 
                               
                                 π 
                                 - 
                                 α 
                               
                               π 
                             
                             ⁢ 
                             
                               
                                 V 
                                 ip 
                                 2 
                               
                               ⁢ 
                               
                                 
                                   sin 
                                   2 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   ω 
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 ⅆ 
                                 ω 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
             
             
               
                 
                   
                     V 
                     orms 
                   
                   = 
                     
                   ⁢ 
                   
                     
                       
                         
                           V 
                           ip 
                           2 
                         
                         π 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             α 
                             - 
                             
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               α 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               cos 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               α 
                             
                           
                           2 
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 
                   
                     V 
                     orms 
                   
                   = 
                     
                   ⁢ 
                   
                     
                       V 
                       ip 
                     
                     ⁢ 
                     
                       
                         
                           α 
                           - 
                           
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             α 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             cos 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             α 
                           
                         
                         
                           2 
                           ⁢ 
                           π 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     This relationship can also be used to define V ip  in terms of V orms  and α: 
     
       
         
           
             
               V 
               ip 
             
             = 
             
               
                 V 
                 orms 
               
               ⁢ 
               
                 
                   
                     2 
                     ⁢ 
                     π 
                   
                   
                     α 
                     - 
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       α 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       α 
                     
                   
                 
               
             
           
         
       
     
     Using these equations, the relationship between peak line voltage, RMS line voltage, RMS load voltage, and conduction angle α may be displayed graphically.  FIG. 8  shows V orms  as a function of conduction angle α for line voltages 220V, 230V and 240V. Note that small changes in line voltage result in larger changes in RMS load voltage.  FIG. 9  shows the relationship of line voltage to conduction angle for fixed RMS load voltages. A lamp light emitting element (e.g., filament) is designed to operate at a particular load voltage, such as 120 Vrms. As seen these graphs, the conduction angle required to achieve this load voltage depends on the RMS line voltage and the relationship is not linear. Changes in the line voltage are exaggerated at the load. 
     With reference to  FIG. 10  that illustrates the concept of the present invention, one option for solving the problem of varying line voltages is to provide the voltage conversion circuit  20  that includes an RC series network with a resistance element  30  and a capacitor  32  whose resistance and capacitance cause a conduction angle that provides the RMS load voltage appropriate for the lamp. 
     Recall that the conduction angle of triac triggering is dependent on the RC series portion of the dimming circuit. When selecting the resistance and capacitance for the voltage conversion circuit, it is preferable to pick an appropriate capacitance and vary the resistance. Consider how varying resistance affects triggering. In a simple RC series circuit (e.g.,  FIG. 3 ), the circuit resistance R T  will be load resistance plus the resistance of the resistor. In application, the load resistance is very small compared to the resistance of the resistor and may be ignored. Using Kirchoff&#39;s voltage law the line source voltage V s  can be written in terms of loop current I and element impedances: 
     
       
         
           
             
               V 
               S 
             
             = 
             
               I 
               ⁡ 
               
                 [ 
                 
                   
                     R 
                     T 
                   
                   + 
                   
                     1 
                     
                       j 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       C 
                     
                   
                 
                 ] 
               
             
           
         
       
         
         
           
             which may be rewritten: 
           
         
       
    
     
       
         
           
             I 
             = 
             
               
                 j 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 ω 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   CV 
                   S 
                 
               
               
                 
                   j 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   ω 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     R 
                     T 
                   
                 
                 + 
                 1 
               
             
           
         
       
     
     This equation may be used to write an expression for the voltage across the capacitor: 
     
       
         
           
             
               
                 
                   
                     V 
                     C 
                   
                   = 
                     
                   ⁢ 
                   
                     I 
                     ⁢ 
                     
                       1 
                       
                         j 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         C 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       
                         j 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           CV 
                           S 
                         
                       
                       
                         
                           j 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             T 
                           
                           ⁢ 
                           C 
                         
                         + 
                         1 
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         1 
                         
                           j 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           C 
                         
                       
                       ] 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       
                         V 
                         S 
                       
                       ⁡ 
                       
                         ( 
                         
                           1 
                           - 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             ω 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               R 
                               T 
                             
                             ⁢ 
                             C 
                           
                         
                         ) 
                       
                     
                     
                       
                         
                           ω 
                           2 
                         
                         ⁢ 
                         
                           R 
                           T 
                           2 
                         
                         ⁢ 
                         
                           C 
                           2 
                         
                       
                       + 
                       1 
                     
                   
                 
               
             
           
         
       
     
     The magnitude and phase relation of capacitor voltage with respect to reference line voltage can be calculated: 
     
       
         
           
             
               
                 
                   
                     Im 
                     ⁢ 
                     
                       { 
                       
                         V 
                         c 
                       
                       } 
                     
                   
                   = 
                     
                   ⁢ 
                   
                     
                       
                         - 
                         
                           V 
                           s 
                         
                       
                       ⁢ 
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         R 
                         t 
                       
                       ⁢ 
                       C 
                     
                     
                       
                         
                           ω 
                           2 
                         
                         ⁢ 
                         
                           R 
                           T 
                           2 
                         
                         ⁢ 
                         
                           C 
                           2 
                         
                       
                       + 
                       1 
                     
                   
                 
               
             
             
               
                 
                   
                     Re 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       { 
                       
                         V 
                         c 
                       
                       } 
                     
                   
                   = 
                     
                   ⁢ 
                   
                     
                       V 
                       S 
                     
                     
                       
                         
                           ω 
                           2 
                         
                         ⁢ 
                         
                           R 
                           T 
                           2 
                         
                         ⁢ 
                         
                           C 
                           2 
                         
                       
                       + 
                       1 
                     
                   
                 
               
             
             
               
                 
                   
                     | 
                     
                       V 
                       C 
                     
                     | 
                   
                   = 
                     
                   ⁢ 
                   
                     
                       
                         
                           Im 
                           2 
                         
                         ⁢ 
                         
                           { 
                           
                             V 
                             C 
                           
                           } 
                         
                       
                       + 
                       
                         
                           Re 
                           2 
                         
                         ⁢ 
                         
                           { 
                           
                             V 
                             C 
                           
                           } 
                         
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       V 
                       S 
                     
                     
                       
                         
                           
                             ω 
                             2 
                           
                           ⁢ 
                           
                             R 
                             T 
                             2 
                           
                           ⁢ 
                           
                             C 
                             2 
                           
                         
                         + 
                         1 
                       
                     
                   
                 
               
             
             
               
                 
                   
                     ∠Θ 
                     C 
                   
                   = 
                     
                   ⁢ 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           Im 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             { 
                             
                               V 
                               C 
                             
                             } 
                           
                         
                         
                           Re 
                           ⁢ 
                           
                             { 
                             
                               V 
                               C 
                             
                             } 
                           
                         
                       
                       ] 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           - 
                           ω 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           R 
                           T 
                         
                         ⁢ 
                         C 
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     The equations for capacitor voltage magnitude and phase delay show how the value of R T  affects triggering. Diac triggering occurs (and thus triac triggering also occurs) when V C  reaches diac breakover voltage. If capacitance and circuit frequency are fixed values, then R T  and V S  are the only variables that will affect the time required for V C  to reach the diac breakover voltage. 
     With reference now to  FIG. 11 , an embodiment of the phase-control power controller  38  of the present invention converts a line voltage at the line terminals  40  to an RMS load voltage. The controller  38  includes a phase-control circuit  42  that is connected to the line terminals  40  and load terminals  44  and that includes a potentiometer  46  ( FIG. 13 , connected across RH and RL) and the capacitor  32  that clip the load voltage in the manner described above. A load sensing circuit  48  connected across the load terminals  44  senses the load voltage (at LampH and LampL) and provides a DC signal (Vlamp) that is related to the RMS load voltage. A comparator circuit  50  is connected to the phase-control circuit and to the load sensing circuit  48  and compares the DC signal from the load sensing circuit  48  to a reference (Vref) and adjusts a resistance of the potentiometer  46  in response to the comparison to control the RMS load voltage. 
     The DC signal (Vlamp) is different from but related to an RMS load voltage. In a preferred embodiment and with reference to  FIG. 12 , the load sensing circuit  48  may be digital and include a full-wave bridge  52  that sets the DC signal (Vlamp) to correspond to a peak of the clipped load voltage. Current limiting resistor  52 ′ ensures that minimal current is drawn from the load. The full-wave bridge  52  and filter capacitor  52 ″ set the DC signal level as the peak of the clipped load voltage waveform. Note that this is not the as RMS load voltage, but this value is related to RMS load voltage and thus the reference (Vref) may be set accordingly. 
     With reference to  FIG. 13 , the comparator circuit  50  may include a comparator  54  that receives the DC signal (Vlamp) and the reference (Vref) and provides an output to the potentiometer  46  to control the resistance provided by the potentiometer  46 . The potentiometer  46  is, in a preferred embodiment, a digital potentiometer. The potentiometer increases resistance if the DC signal is greater than the reference, thereby decreasing conduction angle and decreasing RMS load voltage. Conversely, the potentiometer decreases resistance if the DC signal is less than the reference, thereby increasing the delay angle and increasing RMS load voltage. 
     With reference to  FIGS. 11 and 14 , the controller  38  further include a trigger circuit  56  that establishes the DC power level (Vcc) that is used by the comparator circuit  50  and potentiometer  46 , a common voltage (Com), a trigger signal level (Trig) for the potentiometer  46  (a digital potentiometer uses a pulsing trigger signal for sampling), and the reference (Vref). 
     By way of further explanation, a preferred embodiment of the invention includes voltage conversion circuit  38  that has (a) digital load voltage sensor  48  that provides an output different from but related to an RMS load voltage and (b) phase-controlled dimming circuit  42  that has digital potentiometer  46  that varies a resistance in the phase-controlled dimming circuit  42  responsive to the output from load voltage sensor  48 . 
     The phase-controlled power controller may, in an alternative embodiment, include an insulated gate bipolar transistor (IGBT)  60  instead of the diac  24  and triac  26  as illustrated schematically in  FIG. 15 . The operation of the IGBT  60  corresponds to that of the combination of the diac  24  and triac  26  and may be suitable for high voltage operation (e.g., above 300V). 
     The description above refers to use of the present invention in a lamp. The invention is not limited to lamp applications, and may be used more generally where resistive or inductive loads (e.g., motor control) are present to convert an unregulated AC line or mains voltage at a particular frequency or in a particular frequency range to a regulated RMS load voltage of specified value. 
     While embodiments of the present invention have been described in the foregoing specification and drawings, it is to be understood that the present invention is defined by the following claims when read in light of the specification and drawings.