Abstract:
A Light Emitting Diode (LED) lighting circuit has a passive valley-fill type power factor correction (PFC) circuit. A boost converter stage is coupled to the PFC circuit.

Description:
RELATED APPLICATION 
       [0001]    The present patent application is related to U.S. Provisional Application Ser. No. 61/598,543, filed Feb. 14, 2012, in the name of the same inventor listed above, and entitled, “BOOST Converter Assisted Valley-Fill Power Factor Correction Circuit”. 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 Diodes (LEDs) and, more specifically, to a passive valley-fill Power Factor Correction (PFC) circuit that provides an improved Power Factor (PF). 
         [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. 
         [0004]    Passive valley-fill power factor correction (PFC) circuits are commonly used in lighting ballasts to improve the input power factor (PF). Referring now to  FIG. 1 , a basic prior-art valley-fill PFC circuit is shown. The circuit of  FIG. 1  receives AC mains power from an AC source  101  and delivers DC voltage to the load  100 . The circuit of  FIG. 1  consists of a bridge rectifier  102 , energy storage capacitors  108  and  109 , and commutating diodes  110 ,  111  and  112 . Power factor is defined as the ratio between the active power drawn from the AC source  101  and the product of its AC current and voltage. 
         [0005]    The circuit shown in  FIG. 1  can achieve power factors ranging from approximately 0.70˜0.85. The typical waveforms of current  202  and voltage  201  in the AC source  101  are shown in  FIG. 2 . 
         [0006]    Referring now to  FIG. 3 , an improved prior-art valley-fill PFC circuit is shown. The circuit of  FIG. 3  is similar to that shown in  FIG. 1 . The circuit shown in  FIG. 3  may be able to achieve power factor close to 0.9 by addition of a resistor  118  in series with the commutating diode  110 . The fundamental deficiency of the circuit in  FIG. 3  still remains: it does not address the current dropouts in the AC source  101  around zero crossings of its voltage. The improvement is also limited by power dissipation in the resistor  118 . 
         [0007]    The passive PFC circuits of  FIGS. 1 and 3  are attractive due to their high efficiency and low cost. However, a power factor greater than 0.9 is typically needed for modern general lighting applications such as ones employing high-brightness light-emitting diodes (LED). 
         [0008]    Therefore, it would be desirable to provide a circuit and method that overcomes the above problems. 
       SUMMARY 
       [0009]    A Light Emitting Diode (LED) lighting circuit has a passive valley-fill power factor correction (PFC) circuit. A boost converter stage is coupled to the PFC circuit. 
         [0010]    A Light Emitting Diode (LED) lighting circuit has a passive valley-fill power factor correction (PFC) circuit. A boost converter stage is coupled to the PFC circuit. The boost converter stage comprises a boost element coupled to an input of the PFC circuit. A control switch coupled to the boost element. A control circuit is coupled to the control switch, wherein the control circuit comprises a current sense element which detects current flow in energy storage devices of the PFC circuit; and a switching modulator circuit for driving the control switch. 
         [0011]    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 
         [0012]    Embodiments of the disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a prior art passive valley-fill power factor correction (PFC) circuit; 
           [0014]      FIG. 2  shows typical waveforms of current and voltage in the AC source used in  FIG. 1 ; 
           [0015]      FIG. 3  is another prior art PFC circuit; 
           [0016]      FIG. 4  is a circuit diagram showing a PFC circuit of the present invention; 
           [0017]      FIG. 5  is a circuit diagram showing another embodiment of the PFC circuit of the present invention; 
           [0018]      FIG. 6  is a circuit diagram showing one implementation of the PFC circuit shown in  FIG. 5 ; 
           [0019]      FIG. 7  shows waveforms of current and voltage for the circuit of  FIG. 6 ; and 
           [0020]      FIG. 8  is a circuit diagram showing another implementation of the PFC circuit shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring now to  FIG. 4 , a generalized PFC circuit of the present invention is shown. The PFC circuit of  FIG. 4  may be similar to the circuit of  FIG. 3 . The PFC circuit of  FIG. 4  may receive AC mains power from an AC source  101  and delivers DC voltage to the load  100 . The circuit of  FIG. 4  consists of a bridge rectifier  102 , energy storage capacitors  108  and  109 , and commutating diodes  110 ,  111  and  112 . A resistor  118  in series with the commutating diode  110  may be coupled to the energy storage capacitor  108  or  109 . 
         [0022]    The circuit of  FIG. 4  may further include a boost converter stage. The boost converter stage may allow the circuit of  FIG. 4  to generate a power factor greater than 0.9. The boost converter stage may comprise a boost inductor  104 , a controlled switch  107 , a boost diode  105 , an input smoothing capacitor  103 , and an output smoothing capacitor  117 . The boost stage is bypassed by a diode  106 . The circuit of  FIG. 4  may also comprise a control circuit of the switch  107  consisting of a current sense element  113 , a detector circuit  114  detecting presence of positive-polarity current in the capacitors  108 - 109 , a switching modulator circuit for driving the switch  107  on and off at a high frequency rate, and a gate circuit  116  gating the drive signal  115  to the switch  107  when the positive-polarity current in the capacitors  108 - 109  is detected. 
         [0023]    Referring now to  FIG. 5 , another embodiment of the circuit of the present invention is shown. The circuit of  FIG. 5  is similar to that shown in  FIG. 4 . However, in addition to the elements of the circuit of  FIG. 4 , the circuit of  FIG. 5  may further include a blocking diode  119  in series with the diode  112 , wherein the switch  107  is connected at the common node of the diodes  112  and  119 . In this circuit, current in the switch  104  is limited by the positive-polarity current in the capacitors  108 - 109 . 
         [0024]    One possible implementation of the circuit of  FIG. 5  is shown in  FIG. 6 . The switch  107  is replaced by a bipolar junction transistor  120  with its base drive circuit  121 . The diode  119  consisting of multiple series-connected diodes  119   a,    119   b  also serves the function of the current sense  113  and the detector  114 . A MOSFET  122  serves a function of the gate circuit  116  driving the transistor  120  at the switching frequency rate of  115  when the diodes  119   a  and  119   b  are forward-biased. The boost stage therefore draws input current near the zero crossings of the input AC voltage  101 , effectively filling in the dropouts of the input current inherent to the prior-art valley-fill circuits. The boost converter stage does not affect operation of the valley-fill circuit outside these dropouts. 
         [0025]    Thus, the circuits of the present invention improve power factor and harmonic distortion of the input AC current without significant reduction in the overall power efficiency. The typical voltage  301  and current  302  waveforms of the PFC circuit of  FIG. 6  are shown in  FIG. 7 . The circuit achieves power factor significantly greater than 0.9, as well as low harmonic distortion of the input AC current  302 . 
         [0026]    Another possible implementation of the circuit of  FIG. 5  is shown in  FIG. 8 . The circuit of  FIG. 8  is similar to that in  FIG. 6 . When the load  100  is a switching converter  199 , diodes  119   a  and  119   b  conduct pulsed input current of the converter  199 . Therefore, the switch  122  of  FIG. 6  may be eliminated. 
         [0027]    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.