Abstract:
A control circuit for a discontinuous conduction mode power factor correction converter using harmonic modulation includes: a first difference circuit configured to calculate and output a difference between an output voltage of a discontinuous conduction mode power factor correction converter and a reference voltage; a PI converter configured to perform a proportional integral control on an output signal of the first difference circuit, and output a signal having an arbitrary duty ratio; a second difference circuit configured to output a difference between a rectified input voltage, which is input to the discontinuous conduction mode power factor correction converter, and a harmonic modulation factor DC voltage; and a multiplication circuit configured to multiply an output of the PI controller and an output of the second difference circuit, and output a PFC control signal to a switch of the discontinuous conduction mode power factor correction converter.

Description:
CROSS-REFERENCE(S) TO RELATED APPLICATION 
     This application claims priority of Korean Patent Application No. 10-2012-0031787, filed on Mar. 28, 2012, in the Korean Intellectual Property Office, which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control circuit for a discontinuous conduction mode power factor correction converter using harmonic modulation, and more particularly, to a control circuit for a discontinuous conduction mode power factor correction converter using harmonic modulation, which improves a power factor and an inductor current in a discontinuous conduction mode power factor correction converter by using harmonic modulation with respect to the discontinuous conduction mode power factor correction converter. 
     2. Description of the Related Art 
     A power factor correction (PFC) circuit is used for reducing harmonics on a power line. In particular, the PFC circuit includes an accessory load so that the circuit substantially appears as a pure resistive load. The purpose of the PFC circuit is to make AC voltage and current become substantially in-phase. This improves efficiency and removes the generation of harmful harmonics. 
     For example, the PFC circuit operates in a frequency range from tens of kHz to hundreds of kHz, and enables a wide-range variation in input power supply and load. Therefore, the PFC circuit can suppress most of harmonic distortion and have unity power factor. 
     A basic circuit configuration of a DC/DC converter may be classified into six basic types according to relative positions of an inductor and an active switch: a buck converter, a boost converter, a buck-boost converter, a Cuk converter, a SEPIC converter, and a Zeta converter. The boost and buck-boost circuit configurations are suitable for execution of PFC. 
     Since the inductor can operate in a continuous conduction mode (CCM) or a discontinuous conduction mode (DCM), high power factor correction can be achieved. In the same output power, the inductor operating in the DCM has a larger peak current than the inductor operating in the CCM. Since power becomes higher and peak current becomes larger, the switching loss of the circuit is increased. 
     Thus, the CCM is suitable for high power output. However, when the inductor operates in the CCM, a control circuit needs to detect a relationship of an input voltage, an inductor current, and an output voltage in real time. Therefore, the circuit becomes complicated. In addition, a switching frequency and a duty ratio of a switch need to change constantly in each cycle of an input voltage. 
     When it is necessary to integrate the PFC circuit and the two-stage converter in a single-stage structure, switching elements of the PFC circuit and the two-stage converter should have the same switching frequency and duty ratio. 
     Therefore, when the PFC circuit operates in the CCM, the PFC circuit is unsuitable for integration into the two-stage converter. On the contrary, in the buck-boost PFC converter, in case where the switching frequency and the duty ratio of the switching element are constantly maintained at each input power supply period, the PFC function can be easily achieved when the inductor operates in the DCM. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is directed to provide a control circuit for a discontinuous conduction mode power factor correction converter using harmonic modulation, which improves a power factor and an inductor current in a discontinuous conduction mode power factor correction converter by using harmonic modulation with respect to the discontinuous conduction mode power factor correction converter. 
     According to an embodiment of the present invention, a control circuit for a discontinuous conduction mode power factor correction converter using harmonic modulation includes: a first difference circuit configured to calculate and output a difference between an output voltage of a discontinuous conduction mode power factor correction converter and a reference voltage; a PI converter configured to perform a proportional integral control on an output signal of the first difference circuit, and output a signal having an arbitrary duty ratio; a second difference circuit configured to output a difference between a rectified input voltage, which is input to the discontinuous conduction mode power factor correction converter, and a harmonic modulation factor DC voltage; and a multiplication circuit configured to multiply an output of the PI controller and an output of the second difference circuit, and output a PFC control signal to a switch of the discontinuous conduction mode power factor correction converter. 
     A magnitude of the harmonic modulation factor DC voltage may be 1+K/2 (where K is a maximum value of a magnitude of the rectified input voltage). 
     The output of the second difference circuit may have a value in a range from 1−K/2 to 1+K/2. 
     The PFC control signal may be a result value to which a duty ratio of the output of the PI controller and a harmonic modulation factor of the output of the second difference circuit are reflected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for describing a control circuit for a DCM PFC converter using harmonic modulation according to an embodiment of the present invention. 
         FIG. 2  is a diagram for describing the operation of the control circuit for the DCM PFC converter using harmonic modulation according to the embodiment of the present invention. 
         FIG. 3  is a diagram for describing improvement of an inductor current and an AC input current in a PFC converter by the control circuit for the DCM PFC converter using harmonic modulation according to the embodiment of the present invention. 
         FIG. 4  is a diagram for describing improvement of an inductor current in the PFC converter by the control circuit for the DCM PFC converter using harmonic modulation according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                  10: AC power supply 
                  20: filter 
               
               
                   
                  30: rectifier 
                  40: PFC converter 
               
               
                   
                  41: boost inductor 
                  42: boost diode 
               
               
                   
                  43: input capacitor 
                  44: PFC switch 
               
               
                   
                  45: output capacitor 
                  46: output load 
               
               
                   
                 100: PFC control circuit 
                 110: first difference circuit 
               
               
                   
                 120: PI controller 
                 130: second difference circuit 
               
               
                   
                 140: multiplication circuit 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. These embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. In the drawings, the widths, lengths and thicknesses of elements may be exaggerated for clarity. Throughout the drawings and description, like reference numerals will be used to refer to like elements. 
       FIG. 1  is a diagram for describing a control circuit for a DCM PFC converter using harmonic modulation according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the control circuit for the DCM PFC converter includes an AC power supply  10 , a filter  20 , a rectifier  30 , a PFC converter  40 , and a PFC control circuit  100  for controlling the PFC converter  40 . 
     The filter  20  is implemented with a filter inductor and a filter capacitor, and may be used for removing high-frequency components from an input current of the converter. The input current has a low-frequency form because of a wave having the same phase as the input voltage. 
     The rectifier  30  is implemented with at least one rectifier. For example, the rectifier  30  may be a full-bridge rectification circuit implemented with four diodes. It is apparent that other types of rectification circuits can also be used. The rectifier  30  may be implemented with various elements, such as BJTs, MOSFETs, and SCRs, as well as diodes. 
     The PFC converter  40  may include a boost inductor  41 , a boost diode  42 , an input capacitor  43 , a PFC switch  44 , an output capacitor  45 , and an output load  46 . 
     The boost inductor  41  is connected to one terminal of the rectifier  30  and receives a rectified voltage from the rectifier  30 . The PFC switch  44  is connected in series to the boost inductor  41 , and both output terminals of the rectifier  30  are connected to the boost inductor  41 . The boost diode  42  is connected in series to the boost inductor  41 . The input capacitor  43  is connected to both output terminals of the rectifier  30  at the front stage of the boost inductor  41 , and the output capacitor  45  is connected to a cathode of the boost diode  42 . 
     The PFC control circuit  100  may include a first difference circuit  110 , a PI controller  120 , a second difference circuit  130 , and a multiplication circuit  140 . 
     The first difference circuit  110  calculates and outputs a difference between an output voltage of the DSM PFC converter and a reference voltage. 
     The PI controller  120  performs a proportional integral control on the output signal of the first difference circuit  110 , and outputs a signal having an arbitrary duty ratio. 
     The second difference circuit  130  outputs a difference between a rectified input voltage, which is input to the DCM PFC converter  40 , and a harmonic modulation factor DC voltage. 
     The multiplication circuit  140  multiplies the output of the PI controller  120  and the output of the second difference circuit  130 , and outputs a PFC control signal to the PFC switch  44  of the DCM PFC converter  40 . 
     The PFC control signal output from the multiplication circuit  140  is a result value to which the duty ratio of the output of the PI controller  120  and the harmonic modulation factor of the output of the second difference circuit  130  are reflected. 
     The magnitude of the harmonic modulation factor DC voltage input to the second difference circuit  130  is 1+K/2. Herein, K represents a maximum value of the magnitude of the rectified input voltage. Therefore, the output of the second difference circuit  130  may have a value in a range from 1−K/2 to 1+K/2. 
       FIG. 2  is a diagram for describing the operation of the control circuit for the DCM PFC converter using harmonic modulation according to the embodiment of the present invention. 
     Referring to  FIGS. 1 and 2 , the first difference circuit  110  receives the output voltage V o  applied to the output load  46  of the PFC converter  40  and the reference voltage Vo−ref. The difference between the reference voltage Vo−ref and the output voltage Vo is output from the first difference circuit  110  and is input to the PI controller  120 . The PI controller  120  receives the difference between the reference voltage Vo−ref and the output voltage Vo from the first difference circuit  110 , performs the proportional integral control, and outputs the resultant signal to the multiplication circuit  140 . The signal output from the PI controller  120  has an arbitrary duty ratio D. The waveform of the output signal of the PI controller  120  is illustrated in  FIG. 2B . 
     On the other hand, the second difference circuit  130  receives the harmonic modulation factor DC voltage V level  and the rectified input voltage |V in |, which is input to the PFC converter  40  through the rectifier  30 . 
     The rectified input voltage |V in | is illustrated in  FIG. 2E , and the harmonic modulation factor DC voltage V level  is illustrated in  FIG. 2D . 
     The magnitude of the harmonic modulation factor DC voltage V level  input to the second difference circuit  130  is 1+K/2. Herein, K represents the maximum value |V in | of the magnitude of the rectified input voltage. Therefore, the output of the second difference circuit  130  may have a value in a range from 1−K/2 to 1+K/2. The output of the second difference circuit  130  is illustrated in  FIG. 2C . 
     The multiplication circuit  140  receives the output of the PI controller  120  and the output of the second difference circuit  130 . The output of the PI controller  120  and the output of the second difference circuit  130  are multiplied by the multiplication circuit  140 , and the resultant signal is output to the PFC switch  44  of the PFC converter  40  as the PFC control signal. The PFC control signal output from the multiplication circuit  140  is illustrated in  FIG. 2A . 
     The PFC control signal output from the multiplication circuit  140  is a result value to which the duty ratio D of the output of the PI controller  120  and the harmonic modulation factor Kh of the output of the second difference circuit  130  are reflected. 
     The peak current applied to the boost inductor  41  can be expressed as Equation (1) below.
 
| I   ac   [n]|=V   in   [n]V   o ( Kh[n]D ) 2   T   s /(2 L ( V   o   −V   in   [n ]))  Equation (1)
 
     where n represents the time, and D represents the duty ratio. Kh represents the harmonic modulation factor. 
     On the other hand, the peak current applied to the boost inductor by the conventional PFC control method, which reflects no harmonic modulation factor, can be expressed as Equation (2) below.
 
| I   ac   [n]|=V   in   [n]V   o   D   2   T   s /(2 L ( V   o   −V   in   [n ]))  Equation (2)
 
     Comparing Equations (1) and (2), it can be seen that the peak current is reduced according to the harmonic modulation factor Kh. 
     The harmonic modulation factor Kh of the output of the second difference circuit  130 , which is obtained from Equation (1), can be expressed as Equation (3) below. 
     
       
         
           
             
               
                 
                   
                     Kh 
                     ⁡ 
                     
                       [ 
                       n 
                       ] 
                     
                   
                   = 
                   
                     
                       
                         
                           V 
                           o 
                         
                         ⁢ 
                         
                           V 
                           
                             in 
                             , 
                             DC 
                           
                         
                       
                       
                         
                           V 
                           
                             in 
                             , 
                             rms 
                           
                         
                         ⁢ 
                         
                           
                             
                               V 
                               o 
                               2 
                             
                             - 
                             
                               
                                 V 
                                 o 
                               
                               ⁢ 
                               
                                 V 
                                 
                                   in 
                                   , 
                                   DC 
                                 
                               
                             
                           
                         
                       
                     
                     ⁢ 
                     
                       
                         1 
                         - 
                         
                           
                             
                               
                                 2 
                               
                               ⁢ 
                               
                                 V 
                                 
                                   in 
                                   , 
                                   rms 
                                 
                               
                             
                             
                               V 
                               o 
                             
                           
                           ⁢ 
                           
                              
                             
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 nT 
                                 s 
                               
                             
                              
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
     The maximum value of the harmonic modulation factor Kh the output of the second difference circuit  130  has can be expressed as Equation (4) below.
 
 Kh   max   =V   o   V   in,DC /( V   in,rms √{square root over ( V   o   2   −V   o   V   in,DC )})  Equation (4)
 
     The minimum value of the harmonic modulation factor Kh the output of the second difference circuit  130  has can be expressed as Equation (5) below.
 
 Kh   min   =Kh   max √{square root over (1−√{square root over (2)} V   in,rms   /V   o )}  Equation (5)
 
       FIG. 3  is a diagram for describing the improvement of the inductor current and the AC input current in the PFC converter by the control circuit for the DCM PFC converter using harmonic modulation according to the embodiment of the present invention. 
     Referring to  FIG. 3 , the waveform of the current flowing through the boost inductor  41  within the PFC converter  40  of the present invention is illustrated in  FIG. 3A . The waveform of the AC input current applied to the PFC converter  40  of the present invention is illustrated in  FIG. 3B . The PFC power factor (PF) of the PFC converter  40  is 0.996. 
     Meanwhile, the waveform of the current flowing through the boost inductor within the conventional PFC converter is illustrated in  FIG. 3C . The waveform of the AC input current applied to the conventional PFC converter is illustrated in  FIG. 3D . The PFC power factor (PF) of the PFC converter  40  is 0.965. That is, it can be seen that the PFC control method using harmonic modulation according to the present invention improves the PFC power factor (PF) of the PFC converter, as compared with the conventional PFC control method. 
     Referring to  FIG. 3 , it can be seen that the current peak waveform of the present invention illustrated in  FIGS. 3A and 3B  is reduced as compared with the conventional current peak waveform illustrated in  FIGS. 3C and 3D , and thus, the waveform becomes smoother. 
       FIG. 4  is a diagram for describing the improvement of the inductor current in the PFC converter by the control circuit for the DCM PFC converter using harmonic modulation according to the embodiment of the present invention. 
     Referring to  FIG. 4 , it can be clearly seen that the current peak waveform of the present invention illustrated in  FIG. 4A  is reduced as compared with the conventional current peak waveform illustrated in  FIG. 4B , so that the waveform becomes more smooth. 
     If the peak current flowing through the boost inductor is reduced, the size of the inductor can be reduced. A core of the inductor is determined by the product of a window area of the core and a cross-sectional area of the core. The window area of the core is reduced as the magnitude of the peak current is reduced. 
     According to the present invention, the peak current of the inductor in the DCM PFC converter is significantly reduced, and the power factor by the modulation of the inductor current can be ensured. Therefore, the effect of reducing the size of the inductor can be obtained. 
     While the embodiments of the present invention have been described with reference to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.