Patent Publication Number: US-8525438-B1

Title: Load driver with integrated power factor correction

Description:
RELATED APPLICATION 
     This patent application claims the priority benefit of commonly owned U.S. provisional patent application Ser. No. 61/362,835 filed Jul. 9, 2010 entitled “LED Driver With Integrated Power Factor Correction”, which provisional patent application is hereby incorporated by reference in its entirety into the present utility patent application. 
    
    
     TECHNICAL FIELD 
     This invention pertains to the field of driver circuits, such as IC (integrated circuit) drivers, for driving variable DC (direct current) loads, such as LEDs (light emitting diodes). 
     BACKGROUND ART 
     The use of high-brightness LEDs in lighting applications is growing rapidly as a result of inherent benefits to LED technology, such as long lifetimes, good efficiency, and use of non-toxic materials. LED lighting solutions, however, still need to offer better performance at better value. Because LEDs prefer to be driven in a more sophisticated fashion as compared to traditional incandescent bulbs, performance is heavily dependent on the LED driver circuit. 
     Traditional LED driver ICs (integrated circuits) suffer in performance and supported features in several ways. First, the driver efficiency generally falls well short of the desired targets. Similarly, the power factor for existing solutions can be quite poor, especially while in a dimming configuration. Finally, when using the triac-based wall dimmers that are typical in existing installations, conventional solutions may cause annoying flicker while dimming, and are often bulky and unreliable. 
     When trying to address these concerns, existing solutions can grow substantially in solution complexity, size, and cost, thereby limiting the adoption of such approaches. 
     The present invention addresses and solves these and other concerns. 
     DISCLOSURE OF INVENTION 
     Methods and apparati for forcing the current through a load ( 11 ) in a variable DC (direct current) electrical circuit to be proportional to the input voltage (V(in)). A circuit embodiment of the present invention comprises a source ( 27 ) of input AC (alternating current); a rectifier ( 23 ) coupled to the input AC source ( 27 ), said rectifier ( 23 ) producing a variable DC input voltage; coupled to the rectifier ( 23 ), a load ( 11 ) having a variable direct current flowing therethrough; and means ( 12 - 16 ) for forcing the current through the load ( 11 ) to be proportional to the variable DC input voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other more detailed and specific objects and features of the present invention are more fully disclosed in the following specification, reference being had to the accompanying drawings, in which: 
         FIG. 1  is circuit diagram of a general embodiment of the present invention. 
         FIG. 2  is a circuit diagram of a non-isolated embodiment of the present invention. 
         FIG. 3  is a circuit diagram of an isolated embodiment of the present invention. 
         FIG. 4  is a circuit diagram of an embodiment of the present invention in which an integrated circuit  20  is used. 
         FIG. 5  is a circuit diagram showing components within integrated circuit  20  of  FIG. 4 . 
         FIGS. 6   a  through  6   d  constitute a set of waveforms showing voltages and currents at various points in the  FIG. 5  circuit of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a method embodiment of the present invention, an integrated approach to power factor correction is achieved by sampling the rectified line input V(in) from the AC mains  27 , and by using that waveform to modulate an on-chip reference  21  used to control the current flowing through the load  11 . In this way, the load  11  current is forced to follow the line input voltage V(in) waveform, thereby yielding a good power factor. 
       FIG. 1  shows this method implemented in a general embodiment, in which the modulating step comprises sensing the current flowing through a variable current source  12  that is coupled to the load  11 , thereby producing a sensed current signal; and sending the sensed current signal through a feedback loop  15 ,  16  back to the variable current source  12  to modulate the current flowing through variable current source  12 . 
     Load  11  can be any variable current load, such as any combination of any one or more of the following: a single LED (light emitting diode), any series or parallel combination of LEDs, a capacitor, a motor, a compressor, a refrigerator, an air conditioner, etc. In many applications, load  11  comprises an LED  44  plus a capacitor  45  in parallel with the LED (see  FIG. 4 ). 
     The embodiment of the present invention that is illustrated in  FIG. 1  features a three-terminal variable current source  12  that has two of its terminals respectively coupled to the load  11  and to a rectified AC input voltage V(in). V(in) is graphically illustrated on  FIG. 1  as an absolute value of a sine wave. A current sensor  14  senses the current flowing through variable current source  12 . A summer  15  having two inputs, a first input coupled to current sensor  14  and a second input coupled to V(in), has its output amplified by post-summing amplifier  16  and fed back to the third terminal of variable current source  12 . 
     In  FIG. 1 , variable current source  12  is coupled in series with the load  11 . In other embodiments, source  12  can be in parallel with load  11 , or both in series and in parallel with the load  11 . When source  12  is in series with the load  11 , there may be a second variable current source  13  that is coupled in parallel with the load  11 . Second variable current source  13  may be a two terminal device or a three terminal device. If a three terminal device, its third terminal is coupled to the third terminal of variable current source  12 , as illustrated in  FIG. 1 . The purpose of optional current source  13  is to improve the system efficiency. 
     This invention eliminates the need for traditional bulky power factor correction (PFC) circuit components (shown within dashed lines as item  19  in  FIG. 1 ), thereby improving the efficiency of the driver circuit, and reducing its size. 
       FIG. 2  illustrates a non-isolated embodiment of the present invention, which may be used, for example, for value-conscious lighting solutions in those embodiments where load  11  comprises one or more LEDs. A source  27  of input AC is processed by rectifier  23 , producing a variable DC input voltage V(in). Rectifier  23  may be a full bridge rectifier comprising four diodes in a standard bridge configuration. Load  11 , which has a variable direct current flowing therethrough, is coupled to bridge rectifier  23  via a power inductor  26 . Inductor  26  is particularly useful in smoothing the current that flows through load  11  when current source  12  is a switching power supply. 
     An EMI (electromagnetic interference) filter  24  is optionally coupled between input AC source  27  and rectifier  23 . A triac dimmer  25  may also be optionally coupled between input AC source  27  and rectifier  23 . When both EMI filter  24  and triac dimmer  25  are present, triac dimmer  25  is typically placed between input AC source  27  and EMI filter  24 . 
     The remainder of the circuitry illustrated in  FIG. 2  is the circuitry that forces the current through the load  11  to be proportional to the variable DC input voltage V(in). 
     In  FIG. 2 , current sensing means  14  comprises a resistor  14 ; and the summer  15  and post-summing amplifier  16  are embodied in a single error amplifier  21 . Variable current source  12  may be a switching FET (field effect transistor), as illustrated in  FIG. 4 . A switching power supply is desirable from the standpoint of efficiency. In general, variable current source  12  can be any power device that can be modulated, such as a three-terminal power device (FET, bipolar transistor, silicon controlled resistor), or a complementary two-terminal power device. 
     In  FIG. 2 , a second amplifier  22  is coupled to sensing resistor  14  as shown; and the error amplifier  21  and the second amplifier  22  are embodied within a single integrated circuit  20 . 
       FIG. 3  illustrates an isolated embodiment of the present invention, in which a transformer  30  takes the place of power inductor  26 . The  FIG. 3  embodiment is suitable for higher-end performance lighting applications, where electrical isolation is needed or desired, e.g., for reasons of safety. In  FIG. 3 , three-terminal variable current source  12  is located on the rectifier  23  side of transformer  30 , while optional second variable current source  13  is located on the load  11  side of transformer  30 . 
       FIG. 4  illustrates an embodiment of the present invention in which a linear regulator  40  is positioned between rectifier  23  and integrated circuit  20 . This can be useful in embodiments where it is desired to more closely control the voltages of the components within integrated circuit  20 . In  FIG. 4 , resistors  41  and  42  constitute a voltage divider, serving to set the input voltage PFIN of IC  20  to a voltage for which IC  20  has been designed. In  FIG. 4 , variable current source  12  is a switching FET, and second variable current source  13  is a Zener diode. An additional resistor  43  is positioned between the FREQ pin of IC  20  and ground. If a higher level of integration is desired, it is possible to put all of the components of  FIG. 4 , except for inductor  26  and load  11 , into a single integrated circuit  20 . 
       FIG. 5  illustrates components that are typically encompassed within IC  20 : voltage comparator  51 , oscillator  52 , and latch  53 . Comparator  51  has two inputs, a negative input coupled via the PFIN pin of IC  20  to voltage divider  41 ,  42 ; and a positive input coupled to current sensing resistor  14  and FET  12  via pin SNS of IC  20 . The output of comparator  51  is coupled to the reset input of latch  53 . The set input of latch  53  is coupled to the output of oscillator  52 , which has an arbitrary frequency of oscillation, e.g., 100 KHz. The output of latch  53  is coupled to the gate of FET  12  via the GATE pin of IC  20 . 
       FIGS. 6   a  through  6   d  are a series of waveforms showing various voltages and currents in the  FIG. 5  circuit as a function of a common time t. The  FIG. 6   a  waveform shows the voltages V(LEDP) (which is the same as V(in)) and V(PFIN). The latter voltage has the same periodicity, but typically a different amplitude as a function of time, as the former voltage. 
     The  FIG. 6   b  waveform shows the voltage at the SNS (sense) pin of IC  20 . This voltage is equal to the current flowing through switching FET  12  times the resistance of current sensing resistor  14 . This voltage has the same envelope as V(PFIN), except it is chopped up at the frequency of oscillation (switching) defined by oscillator  52 . 
     The  FIG. 6   c  waveform shows the current flowing through diode  13 . This current is chopped at the same frequency as V(SNS), since diode  13  is in series with FET  12 , which is switched at the frequency dictated by oscillator  52 . 
     Finally, the  FIG. 6   d  waveform shows the current through inductor  26  (and hence the current through load  11 ), which is equal to the current flowing through FET  12  plus the current flowing through diode  13 . Note that this current is proportional to the input voltage V(in) as desired. There is an AC ripple on this load  11  current, at the frequency of oscillation, but this is usually not a problem. The ripple is due to the fact that the power supply  12  is a switching power supply, typically an FET or a variable resistance power supply. For example, when the load  11  comprises an LED or a series of LEDs, the human eye does not notice the ripple because of the eye&#39;s innate property of persistence. 
     The goal of the prior art is to keep a steady current flowing through the load. On the other hand, the goal of the present invention is to make the output current flowing through the load  11  to be proportional to the input voltage V(in), while disregarding AC ripple on the load  11  current when the power supply  12  is a switching power supply. 
     The present invention exhibits excellent efficiency and power factor, even when a triac dimmer  25  is used. 
     The above description is included to illustrate the operation of the preferred embodiments, and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention.