Abstract:
A method of controlling an output current of a power supply circuit comprising a first converter stage for receiving an input voltage, a second converter stage delivering a regulated output current, a controlled switch for conducting the input current and the output current, the method comprising: detecting a portion of the input current conducted by the controlled switch; detecting a current in the controlled switch; deriving a difference between the current in the controlled switch and the portion of the input current conducted by the controlled switch; and maintaining the difference at a reference level.

Description:
RELATED APPLICATION 
       [0001]    The present patent application is a continuation of U.S. patent application Ser. No. 12/269,512, filed Nov. 12, 2008, in the name of the same inventors listed above, and entitled, “LED DRIVER WITH LOW HARMONIC DISTORTION OF INPUT CURRENT AND METHOD OF CONTROLLING THE SAME”. The present patent application is further related to U.S. Provisional Application Ser. No. 61/047,286, filed Apr. 23, 2008, in the name of the same inventors listed above, and entitled, “LED DRIVER WITH LOW HARMONIC DISTORTION OF INPUT CURRENT AND METHOD OF CONTROLLING THE SAME”. The present patent application claims the benefit under 35 U.S.C. §119(e). 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to a Light Emitting Diode (LED) driver and, more specifically, to a single-stage non-isolated switching converter capable of high-efficiency operation, low harmonic distortion of the AC line current, high reliability and long life in the presence of an elevated ambient temperature. 
         [0003]    Recent developments of high-brightness light emitting diodes (LED) have opened new horizons in lighting. Highly efficient and reliable LED lighting continuously wins recognition in various areas of general lighting, especially in areas where cost of maintenance is a concern. One example of such application is in street lighting where LED lighting is becoming increasingly popular throughout the world. 
         [0004]    These applications have created demand for a special LED driver, a current-regulated power supply circuit, which can match the long life of LEDs. A typical set of requirements to such LED driver includes high power efficiency, power factor correction (PFC) and low distortion of the input AC current. Due to high power dissipation within LEDs themselves, the LED driver must be capable of continuous operation at elevated ambient temperature. Non-isolated LED driver topologies are typical for these types of applications, since galvanic safety isolation of the LED load from AC mains is not generally required. 
         [0005]    However, prior art LED drivers generally employ two-stage power conversion, wherein the first stage, typically a boost converter, is responsible for AC-to-DC rectification featuring power factor correction and low harmonic distortion of the AC line current, and the second stage is a constant output current DC-to-DC converter. One obvious disadvantage of such approach is its higher cost and component count compared to a single-stage power supply. Since the overall efficiency of such an LED driver is the product of the efficiency of each conversion stage, achieving high efficiency can be difficult with a two-stage approach. 
         [0006]    Therefore, it would be desirable to provide a circuit and method that overcomes the above problems. The circuit would be a single-stage non-isolated switching converter capable of high-efficiency operation, low harmonic distortion of the AC line current, high reliability and long life in the presence of an elevated ambient temperature. 
       SUMMARY 
       [0007]    A power supply circuit for powering a load at constant current has a rectifier stage for receiving an AC voltage input and for producing a first substantially DC voltage. A first capacitor is attached to the load. A charge-pump is attached to an output of the rectifier stage and to the load for providing power factor correction and for converting the first substantially DC voltage to a second substantially DC voltage at the first capacitor. The charge pump is prevented from conducting energy back into the output of the rectifier stage. The charge pump delivers energy to a charge pump output, the energy being delivered directly instead of being stored. A converter stage is attached to the load and the first capacitor. The converter stage is used for converting voltages at the first capacitor and the charge pump to an output DC current. The converter stage has a switch for periodically connecting a first series-coupled circuit of the charge pump to the output of the rectifier stage. 
         [0008]    A method of controlling an output current of a power supply circuit comprising a first converter stage for receiving an input voltage, a second converter stage delivering a regulated output current, a controlled switch for conducting the input current and the output current, the method comprising: detecting a portion of the input current conducted by the controlled switch; detecting a current in the controlled switch; deriving a difference between the current in the controlled switch and the portion of the input current conducted by the controlled switch; and maintaining the difference at a reference level. 
         [0009]    The features, functions, and advantages can be achieved independently in various embodiments of the disclosure or may be combined in yet other embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Embodiments of the disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0011]      FIG. 1  shows a simplified schematic of an LED driver of the present invention for powering an LED load at constant current; 
           [0012]      FIG. 2  shows a simplified schematic of one embodiment of the summation circuit shown in the LED driver depicted in  FIG. 1 ; 
           [0013]      FIG. 3  shows the LED driver of  FIG. 1  using second-harmonic voltage injection in the oscillator circuit; 
           [0014]      FIG. 4  shows the LED driver of  FIG. 3  wherein the oscillator includes a low-pass filter to create phase shifted modulation of its output frequency with respect to the output voltage of the rectifier; and 
           [0015]      FIG. 5  depicts an LED driver of  FIG. 1  wherein the flying capacitor is distributed over a plurality of output buck stages. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 1 , an LED driver  10  of the present invention is shown. The LED driver  10  is used for powering an LED load  200  at constant current wherein the LED driver  10  receives input power from an AC voltage source  101  and delivers constant-current output to the LED load  200 . The LED driver  10  has a rectifier circuit  191  attached to the AC voltage source  101 . A charge pump circuit  120  is attached to the rectifier circuit  191 . In the embodiment depicted in  FIG. 1 , the charge pump circuit  120  comprises a flying capacitor  109 , a resonant inductor  107 , diodes  106  and  108 . The resonant inductor  107  has a first terminal attached to the rectifier circuit  191 . A second terminal of the resonant inductor  107  is attached to a first terminal of the diode  106 . A second terminal of the diode  106  is attached to a first terminal of the flying capacitor  109  and to the first terminal of the diode  108 . A second terminal of the diode  108  is attached to the load  200 . 
         [0017]    A buck converter stage  202  is coupled to a second terminal of the flying capacitor  109 , the load  200  and to the second terminal of the diode  108 . In the embodiment depicted in  FIG. 1 , the buck converter stage  202  includes a power switch  102 , a freewheeling diode  104  and an output smoothing inductor  103 . A first terminal of the output smoothing inductor  103  is coupled to the load  200 . A second terminal of the output smoothing inductor  103  is attached to the second terminal of the flying capacitor  109 . A first terminal of the freewheeling diode  104  is coupled to the second terminals of the output smoothing inductor  103  and the flying capacitor  109 . A second terminal of the freewheeling diode  104  is attached to the load  200  and to the first terminal of a hold-up capacitor  105  which is grounded. The power switch  102  has a first terminal attached to the first terminal of the freewheeling diode  104 , and the second terminals of the flying capacitor  109  and the output smoothing inductor  103 . A second terminal of the power switch  102  is attached to a switching control circuit  100 . A third terminal of the power switch  102  is grounded. 
         [0018]    In the embodiment depicted in  FIG. 1 , the switching control circuit  100  comprises current sensors  131  and  133 , a summation element  132 , a current comparator  134  with a current reference REF, an oscillator circuit  136  and a latch  135 . The current sensor  131  is attached to the third terminal of the power switch  102  which is grounded. The current sensor  133  is attached to the second terminals of the output smoothing inductor  103  and the flying capacitor  109 . The summation element has a first terminal attached to the current sensor  133 , a second terminal attached to the current sensor  131  and a third terminal attached to an input of the current comparator  134 . A second input of the current comparator  134  is attached to a reference current REF. The output of the current comparator  134  is attached to the latch  135 . In the embodiment shown in  FIG. 1 , the latch  135  is a Pulse Width Modulation (PWM) latch wherein the S input is attached to the oscillator circuit  136  and the R input is attached to the output of the current comparator  134 . The output of the RS latch  134  is attached to the second terminal of the power switch  102 . 
         [0019]    In operation, rectifier  191  receives the AC line voltage  101  and delivers the full-wave rectified voltage VIN. There are two modes of operation within the half-cycle of the AC voltage  101 . The first mode occurs when VIN is less than or equal to half the voltage at the hold-up capacitor  105 , the second mode occurs when VIN is more than half of said voltage. 
         [0020]    In the first mode of operation, when the switch  102  is turned on by the control circuit  100 , capacitor  109  charges resonantly via inductor  107  and diode  106  to approximately double of VIN. Subsequently, the current in inductor  107  falls to zero, and diode  106  becomes reverse-biased. At the same time while switch  102  is on, LED load  200  receives current from capacitor  105  via output smoothing inductor  103 . The diodes  108  and  104  are reverse-biased in this operating mode. When control circuit  100  turns off power switch  102 , the inductor  103  current is diverted from switch  102  into capacitor  109  via diode  108 . As soon as this capacitor  109  has been discharged fully, diode  104  becomes forward-biased and conducts the inductor  103  current. 
         [0021]    In the second mode of operation, the voltage on capacitor  109  does not double, but instead will be clamped to the voltage on the hold-up capacitor  105  by the action of diode  108 . The remaining energy of inductor  107  causes delivery of energy from inductor  107  and the AC source  101  directly into capacitor  105  via diodes  106  and  108 , until the current in inductor  107  has, at more or less constant rate, fallen to zero, and diode  106  becomes reverse-biased. The remainder of the operation is similar to that disclosed above for the first mode of operation. 
         [0022]    In operation of the control circuit  100 , oscillator  136  sets latch  135  and switch  102  is activated. At this moment, the current registered by current sensor  131  is the superposition of the current in inductor  103  and the current in capacitor  109 . Summation element  132  subtracts the capacitor  109  current signal registered by sensor  133  from the one registered by sensor  131  and delivers a current signal reflecting the inductor  103  current only. When this signal exceeds the reference level REF, the output of comparator  134  changes state, and latch  135  turns switch  102  off. The cycle repeats itself upon the next clock pulse of oscillator  136   
         [0023]    Since charge-pump current is largely proportional to VIN, the LED driver  10  of  FIG. 1  achieves low harmonic distortion of the input AC current. In case the charging of capacitor  109  ends before the switch  102  turns off, the charge pump circuit does not increase switching loss in switch  102 . Moreover, the turn-off of switch  102  occurs at zero voltage, since the voltage slew rate is limited by relatively slow discharging rate of capacitor  109 . 
         [0024]    Referring now to  FIG. 2 , one embodiment of the summation circuit  132  depicted in  FIG. 1  is shown. The summation circuit  132  comprises a current sense resistor  141 , a current sense transformer  142  and a pair of resistors  141  and  143 . The current sense resistor  141  is placed in series with the power switch  102 . The current sense transformer  142  is placed so that the primary winding of the current sense transformer  142  is in series with the flying capacitor  109  and the secondary winding is burdened with the resistor  143 . Resistors  141  and  143  are connected in series, and the total voltage across the resistors  141  and  143  is applied at the signal input of current sense comparator  134 . The polarity of the current sense transformer  142  winding connection is such that the comparator  134  input voltage is independent of the current in the capacitor  109 . 
         [0025]    Referring now to  FIG. 3 , another embodiment of the LED driver  10 A is shown. The LED driver  10 A is similar to the LED driver  10  of  FIG. 1 . The LED driver  10 A of the present embodiment uses second-harmonic voltage injection in the oscillator circuit  136 . In the present embodiment, the oscillator  136  comprises a voltage controlled oscillator (VCO) block  138  and a summation element  137 . The VCO block  137  is attached to the S input of the latch  135 . The summation element  136  has a first terminal attached to the VCO block  137 , a second terminal attached to the rectifier  191  and the inductor  107 , and a third terminal attached to the reference signal REF 1 . In operation, the oscillator  136  outputs switching frequency proportional to the difference between a reference signal REF 1  and voltage at the output of rectifier  191 . 
         [0026]    Referring now to  FIG. 4 , another embodiment of the oscillator  136  is shown. The oscillator  136  in this embodiment is similar to that shown in  FIG. 3 . In the present embodiment, the oscillator  136  includes a low-pass filter  139  at the input of VCO  138 . The low-pass filter creates a phase shifted modulation of its output frequency with respect to the output voltage of rectifier  191 . 
         [0027]    Referring now to  FIG. 5 , another embodiment of the LED driver  10 B is shown. The LED driver  10 B is similar to that shown in  FIG. 1 . In  FIG. 5 , the flying capacitor  109  is distributed over a plurality of output buck stages  300 . Each output buck stage  300  comprises a power switch  102 , a freewheeling diode  104  and a smoothing inductor  103 . Each output buck stage  300  drives a portion of the LED load  200 . All control circuits  100  are synchronized with respect to one oscillator  136  in such a way that all power switches  102  turn on simultaneously. The operation of the LED driver  10 B of  FIG. 5  is identical to the one of  FIG. 1 . 
         [0028]    While embodiments of the disclosure have been described in terms of various specific embodiments, those skilled in the art will recognize that the embodiments of the disclosure can be practiced with modifications within the spirit and scope of the claims.