Abstract:
A power converter includes a small-sized inductor connected to an AC voltage input line for power factor correction and a filter for suppressing conduction noise. The inductor is connected to a rectifier and comprises first and second windings and that are wound on a common magnetic core and loosely coupled with each other. A leakage inductance component of the inductor functions as an energy storage element in a main conversion operation and an excitation inductance component of the inductor functions as a noise reduction element for suppressing an conduction noise caused by on-off operation of a switching element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on, and claims priority to, Japanese Patent Application No. 2008-091029, filed on Mar. 31, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power converter that outputs a DC voltage from an AC voltage by on-off controlling a switching element, the DC voltage being generated with energy stored in a an inductor. In particular, the invention relates to a power converter operated as a step-up or step-down chopper. 
     2. Description of the Related Art 
       FIG. 5  shows an example of a conventional power converter using a switching element in which a power factor correction circuit is provided in the output side of a rectifying circuit. In the power converter of  FIG. 5 , an AC power supply  1  supplies a DC voltage to a load  20  through a switching power supply  10 . 
     The switching power supply  10  comprises a noise filter  2  consisting of capacitors  2   a ,  2   b ,  2   c  and a common mode choke coil  3 , a rectifying circuit  4  consisting of diodes  4   a ,  4   b ,  4   c ,  4   d , an inductor  5  for correcting a power factor disposed in the side of an output line of the rectifying circuit  4 , a switching element  6  on-off controlled by a PWM control circuit  6   a , and a smoothing circuit consisting of a diode  7  and a capacitor  8 . The noise filter  2  is provided for reducing the conduction noise from the power converter to the AC power supply  1 . A MOSFET is used for the switching element  6 . 
     Operation of the power converter  10  is described below. 
     When the switching element  6  is turned on, the inductor  5  is supplied with an electric current and stores energy. Then, when the switching element  6  is turned off, the stored energy is transferred to the load  20 . In this stage, the control circuit  6   a  controls pulse widths of on-off operation of the switching element  6  to make an input current from the AC power supply to be a sinusoidal waveform for correcting the power factor. 
     Switching operation of a MOSFET used in a rectifier circuit or a inverter circuit composing a power converter such as a PWM inverter is carried out based on driving signals that are pulse wave modulated (PWM) with a carrier frequency in the range from several kHz to ten and several kHz. It is known that the switching operation generates common mode noises containing components at frequencies higher than several tens of kHz between the main circuit conductor and the ground in the power converter. 
     In order to reduce the conduction noise generated in this type of power factor correction circuit, Patent Document 1 discloses a circuit construction in which a choke coil is divided into two coils each having an equal half number of windings wound on a single core in the opposite polarity, and inserted and connected to the both ends of an output line of the rectifying circuit. In this circuit construction, the high frequency noises such as ripple current generated at the voltage supplying side line and the negative side line from the power factor correction circuit by on-off operation of the switching element appear in the opposite phases, cancelling normal mode noise generated between the voltage supplying side line and the negative side line of the rectifying circuit and diminishing the noise level. 
     In this type of power converter, the current in the inductor  5  provided for suppressing the normal mode noise has an AC waveform of a superposition with a low frequency component of a sinusoidal waveform and high frequency components due to on-off operation of the switching element  5 . The instantaneous current at the peak in the low frequency component is substantially large. A magnetic flux density in the magnetic core at an average current value needs to be so small that the core does not saturate at the moment of this peak. Accordingly, the core needs to be designed in a large size. Further, in a devise that is to permit an overload in a short duration, the core size is necessarily still larger, entailing a problem of enlarged size of an overall power converter. 
     In the case of connection to an external power source such as a commercial power line, a common mode choke coil  3  is required in a nose filter  2  as shown in  FIG. 5 . However, the conventional device as disclosed in Japanese Unexamined Patent Application Publication No. 5-191976, FIG. 1) restrains only normal mode noise and cannot suppress common mode noise. Accordingly, a means for suppressing the common mode noise must be provided in addition to the inductor  5  for power factor correction. This is a large obstacle against minimization of a power converter. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problems, it is an object of the present invention to provide a power converter that has minimized sizes of an inductor for power factor correction and a filter for managing conduction noise. 
     To solve the above problem, a power converter includes a rectifier circuit for rectifying an AC voltage, a smoothing capacitor for smoothing an output from the rectifying circuit, a switching element controlling on-off operation of the AC voltage, an inverter circuit, and an inductor including a first set of windings and a second set of windings both wound on a common core and loosely coupled with each other, a leakage inductance component storing energy in a main conversion operation to be output as the DC voltage, and an excitation inductance component suppressing an conduction noise generated by the on-off operation of the switching element. 
     According to the invention, the leakage magnetic flux does not saturate the core of the inductor. Consequently, saturation of the core is avoided even when energy is stored in the leakage inductance of the inductor. The magnetic core does not need an enlarged size and can be minimized, achieving a small size of an overall power converter. 
     The excitation inductance of the inductor is utilized as a common mode choke coil, thereby eliminating or minimizing a common mode choke coil, which has been conventionally provided as an additional component. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing an example of a power converter according to an embodiment of the present invention; 
         FIG. 2(   a ) is a sectional view showing a coil configuration of an inductor; 
         FIG. 2(   b ) is a perspective view showing a configuration of a magnetic core of the inductor; 
         FIG. 3  is a sectional view showing another coil configuration of the inductor in  FIG. 1 ; 
         FIG. 4  is a circuit diagram showing second embodiment of a power converter; and 
         FIG. 5  is a circuit diagram showing an example of a conventional power converter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment according to the invention will be described in the following with reference to accompanying drawings. 
       FIG. 1  is a circuit diagram showing an example of a power converter according to an embodiment of the present invention. The same parts as those in the conventional power converter of  FIG. 5  are given the same symbols and description thereon is omitted. 
     The power converter shown in  FIG. 1  comprises an inductor  9  having first and second sets of windings  9   a  and  9   b  wound on a common magnetic core and is disposed between a rectifier circuit  4  and a switching element  6 . The inductor functions as a step up coil. The first set of windings  9   a  is arranged in a voltage supplying side line of the rectifying circuit  4  connecting to a diode  7 ; the second set of windings  9   b  is arranged in a negative side line of the rectifying circuit  4  connecting to a ground side of a load  20 . The first and second sets of windings  9   a  and  9   b  here are wound on a single core with the same number of turns. 
     When the switching element  6  is turned on, an electric current flows in the set of windings  9   a  and the set of windings  9   b  generating a magnetic flux in the core. The magnetic flux generated by the set of windings  9   a  and the magnetic flux generated by the set of windings  9   b  are in a opposite direction with each other. In the inductor  9 , the major magnetic fluxes cancel with each other and the leakage inductance component stores energy. 
       FIGS. 2(   a ) and  2 ( b ) illustrate an inductor as shown in  FIG. 1 .  FIG. 2(   a ) is a sectional view showing a coil configuration of the inductor, and  FIG. 2(   b ) is a perspective view showing the configuration of the magnetic core. 
     Referring to  FIG. 2(   a ), a separator made of an insulator  22  is disposed in the middle of the bobbin  21  and separates the first and the second sets of windings  9   a , and  9   b  wound on the bobbin  21  in the left and the right sides. The core  23  has a configuration of the letter  8  as shown in  FIG. 2(   b ), and includes a leg portion  23   a  of the core penetrating the bobbin  21 , and accommodates the first and second sets of windings  9   a  and  9   b  in the core  23 . The core  23  can be constructed by combining a portion  23   e  with a shape of the letter E and a portion  23   i  with a shape of the letter I, or combining two portions each having a shape of the letter E. 
     The sets of windings  9   a  and  9   b  of the inductor  9  correspond to the windings indicated in  FIG. 1 . A leakage inductance value required for the inductor  9  can be obtained by the sets of windings  9   a  and  9   b  that are loosely coupled through the separator  22  at an appropriately loose coupling. The loose coupling means that a magnetic coupling coefficient between the first set of windings  9   a  and the second set of windings  9   b  is less than 1 for example, 0.8. The coupling coefficient of less than 1 means existence of a magnetic flux that interlinks with the first set of windings  9   a  and does not interlink with the second set of windings  9   b , and existence of a magnetic flux that interlinks with the second set of windings  9   b  and does not interlink with the first set of windings  9   a . A magnitude of the leakage inductance can be varied by adjusting numbers of turns of the sets of windings  9   a  and  9   b , and by adjusting a distance between the two sets of windings modifying the dimensions of the separator. Thus, the leakage inductance value can be increased to a magnitude in a range of several hundred μH to several mH, which is required to suppress a common mode current. 
       FIG. 3  is a sectional view showing another coil configuration composing the inductor indicated in  FIG. 1 . While the sets of windings  9   a  and  9   b  are separated longitudinally (in the direction of the left and the right) in the coil configuration in  FIGS. 2(   a ) and  2 ( b ), the two sets of windings  9   a  and  9   b  can be loosely coupled concentrically in a core  33  including a leg portion  33   a  as shown in  FIG. 3  to increase the leakage inductance value. 
     As shown in  FIG. 3 , the first set of windings  9   a  is wound in the inner circumferential side of a common bobbin  31  and the second set of windings  9   b  is wound in the outer circumferential side of the bobbin coaxially with the first set of windings  9   a  with a separator  32  made of an insulator interposed therebetween. In this structure, the coupling between the first and second sets of windings  9   a  and  9   b  of the inductor  9  can be made loose by increasing a thickness of the separator  32  to attain an enlarged leakage inductance value. 
       FIG. 4  is a circuit diagram showing a second embodiment of a power converter. This embodiment of a converter of the invention is constructed as a step down type chopper. The same parts as those in  FIG. 1  are given the same symbols and description is omitted. 
     Input terminals  1   a  and  1   b  are connected to a rectifying circuit  4  (not shown in  FIG. 4 , but shown in  FIG. 1 ), and output terminals  20   a  and  20   b  are connected to a load  20  (not shown in  FIG. 4 , but shown in  FIG. 1 ). The embodiment of  FIG. 4  is different from the embodiment of  FIG. 1  in that a switching element  6  is connected to the input terminal  1   a  that is connected to the rectifying circuit  4 , and a first set of windings  9   a  of an inductor  9  is arranged in the output side of the switching element  6 . 
     The thus constructed power converter utilizes a leakage inductance of the inductor  9  as an energy storage element as in the inverter of  FIG. 1 , and allows minimization of the coil composing the inductor  9 . Therefore, the power converter as a whole can be made small, too. 
     While the embodiments as described heretofore use a MOSFET for the switching element  6 , the switching element  6  can be composed using other switching devices including a bipolar transistor and an IGBT. 
     Yet in the power converters as shown in  FIG. 1  and  FIG. 4 , an auxiliary coil can be provided corresponding to the common mode choke coil  3  in the conventional converter shown in  FIG. 5 . Such an auxiliary coil in a converter of the invention has a smaller scale than a common mode choke coil  3  in the conventional technology, and effectively suppresses the common mode noise. Therefore, a size of a power converter can be reduced.