Patent Publication Number: US-8994342-B2

Title: Switching apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a switching apparatus which includes a first loop circuit and a second loop circuit that share an inductive component, and controls current flowing through the inductive component. 
     BACKGROUND ART 
     Conventionally, a control apparatus for an electric motor has been known which reduces high-frequency noise such as radio noise by disposing a capacitor, used for absorbing the high-frequency noise, between a gate of a switching element used for PWM control of the electric motor and an upstream side-terminal of the electric motor (see, e.g., Patent Document 1). Patent Document 1: Japanese Laid-Open Patent Application No. H09-42096 
     Since the control apparatus for the electric motor reduces the noise by adding the capacitor or the like which is connected to the gate of the switching element, the number of elements used for reducing the noise is increased, and the composition of the control apparatus for the electric motor becomes complex. 
     DISCLOSURE OF INVENTION 
     It is a general object of the present invention to provide a switching apparatus which reduces high-frequency noise by improving the circuit configuration of the switching apparatus. 
     Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a switching apparatus particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an embodiment of the present invention provides a switching apparatus including a first loop circuit configured to include a switching element, an inductive component and a capacitor; and a second loop circuit configured to share the inductive component with the first loop circuit, wherein the capacitor is inserted in series with the inductive component in the first loop, wherein the switching apparatus controls respective currents flowing through the first loop circuit and the second loop circuit in an alternating manner by turning on/off the switching element in order to control the current flowing through the inductive component, and wherein a first magnetic flux generated by the current flowing through the first loop circuit as the switching element is being turned on and a second magnetic flux generated by the current flowing through the second loop circuit as the switching element is being turned off head in the same direction. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit configuration of a conventional load drive apparatus; 
         FIG. 2  shows an arrangement of components included in the conventional load drive apparatus; 
         FIG. 3  shows a circuit configuration of a load drive apparatus according to an exemplary embodiment of the present invention; 
         FIG. 4  shows a circuit configuration of a load drive apparatus according to another exemplary embodiment of the present invention; 
         FIG. 5  shows a circuit configuration of the load drive apparatus according to the present embodiment; 
         FIG. 6  shows waveforms which represent a reduced fluctuation effect of magnetic flux in a load drive apparatus including the circuit configuration shown in  FIG. 5 ; 
         FIG. 7A  shows a circuit unit U 1  including the conventional load drive apparatus; 
         FIG. 7B  shows a circuit unit U 2  including the load drive apparatus  1  according to the present embodiment; 
         FIG. 8  shows noise levels detected in the circuit units U 1  and U 2  shown in  FIGS. 7A and 7B ; 
         FIG. 9  shows a cross section of an exemplary configuration of a four-layer printed circuit board; 
         FIG. 10  shows another exemplary circuit configuration of the load drive apparatus  1 ; 
         FIG. 11  shows yet another exemplary circuit configuration of the load drive apparatus  1 ; 
         FIG. 12  shows a load drive apparatus  1  including a switching element Q 2  instead of the diode D 2  shown in  FIG. 3 ; and 
         FIG. 13  shows a load drive apparatus  2  including a switching element Q 2  instead of the diode D 2  shown in  FIG. 4 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In the following, preferred embodiments of the present invention are described with reference to the drawings. An exemplary embodiment of the switching apparatus of the present invention includes a load drive apparatus for driving an inductive load including an inductive component. 
       FIG. 1  shows a circuit configuration of a conventional load drive apparatus. A load drive circuit  11  drives an inductive load including an inductive component L by switching on and off a switching element Q. The load drive circuit  11  includes a loop circuit A 11  and a loop circuit A 12  which share the inductive component L between the loop circuit A 11  and the loop circuit A 12 . As the switching element Q is being turned on, current is flowing through the loop circuit A, i.e. through the inductive load including the inductive component L and the switching element Q in order. As the switching element Q is being turned off, current is flowing through the loop circuit B, i.e. through the inductive load including the inductive component L and a diode D in order. 
     In general, the circuit shown in  FIG. 1  is realized by disposing elements and patterned lines, which are necessary for realizing the circuit, onto a printed circuit board. In  FIG. 2 , a patterned line which is maintained at the same potential as the potential of a connecting point P 12  (shown in  FIG. 1 ) that connects the diode D and the switching element Q, and a patterned line which is maintained at the same potential as the potential of a terminal T 3  that is connected to one end of the inductive load including the inductive component L via a wiring harness, are connected with each other on the back surface of the printed circuit board via through holes H 11  and H 12 . The patterned line connected to the through holes H 11  and the patterned line connected to the through holes H 12  are maintained at the same potential. 
     In the conventional circuit shown in  FIGS. 1 and 2 , current is flowing through the loop circuits A 11  and A 12  alternately, for example, as the switching element Q is being turned on and off. Thus, a magnetic field penetrating through the loop circuit A 11  and a magnetic field penetrating through the loop circuit A 12  are generated alternately. The direction of the current flowing through the loop circuit A 11  and the direction of the current flowing through the loop circuit A 12  are opposite to each other as indicated by arrows shown in  FIGS. 1 and 2 . Thus, the direction of the magnetic field, relative to sheets of  FIGS. 1 and 2 , penetrating through the loop circuit A 11  is opposite to the direction of the magnetic field, relative to sheets of  FIGS. 1 and 2 , penetrating through the loop circuit A 12  according to the right-hand grip rule. Since the direction of the magnetic field is alternately changed by high-speed (short time) switching on/off of the switching element Q, high-frequency noise may be caused by fluctuation of the magnetic field in such a configuration. 
       FIG. 3  shows a circuit configuration of a load drive apparatus  1  according to an exemplary embodiment of the present invention. The circuit configuration of the load drive apparatus  1  according to the present embodiment is similar to the circuit configuration of the conventional load drive apparatus  11  shown in  FIG. 1 . 
     The load drive apparatus  1  includes a loop circuit A 1  and a loop circuit A 2 . The loop circuit A 1  constitutes a first loop circuit, and the loop circuit A 2  constitutes a second loop circuit. The load drive apparatus  1  drives an inductive type electric load  40 . The electric load  40  includes two ends, i.e. a first end and a second end. A first drive terminal  22  of the load drive apparatus  1  is connected to the first end of the electric load  40 , and a second drive terminal  23  of the load drive apparatus  1  is connected to the second end of the electric load  40 . The loop circuit A 1  and the loop circuit A 2  share an inductive component of the electric load  40 . 
     The loop circuit A 1  includes a switching element Q 1  and a capacitor C 1  in addition to the electric load  40  including an inductive component. In the loop circuit A 1 , a connecting point P 1 , a connecting point P 2 , an upstream side drive terminal  22 , the electric load  40 , a downstream side drive terminal  23 , a connecting point P 3 , the switching element Q 1 , a connecting point P 4  and the capacitor C 1  are connected in this order. The connecting point P 1  is connected to a power supply terminal  21  which is connected to the positive electrode of an electric power supply. The switching element Q 1  may be comprised of, for example, a semiconductor element such as MOSFET (Metal Oxide Semiconductor Field Effect Transistor), or may be comprised of, for example, a transistor such as IGBT (Insulated Gate Bipolar Transistor). The capacitor C 1  may be comprised of, for example, a multilayer ceramic capacitor (multilayer ceramic condenser) which is smaller than an electrolytic capacitor (electrolytic condenser) or a film capacitor (film condenser). 
     The drain of the switching element Q 1  is connected to the drive terminal  23 , which is connected to the downstream side of the electric load  40 , at the connecting point P 3 . The source of the switching element Q 1  is connected to a ground terminal  24 , which is connected to ground (GND), at the connecting point P 4 . 
     The capacitor C 1  which is connected to the electric load  40  in series is inserted into a current pathway of the loop circuit A 1  in series. More specifically, the capacitor C 1  is inserted in series into a looped current pathway of the current flowing through the loop circuit A 1 . One end of the capacitor C 1  is connected to a first power supply line which is disposed upstream of the switching element Q 1 . The other end of the capacitor C 1  is connected to a second power supply line which is disposed down stream of the switching element Q 1 . The electric potential of the first power supply line is higher than the electric potential of the second power supply line. For example, the first power supply line is connected to the positive electrode of a direct current power source, and the second power supply line is connected to ground. More specifically, one end of the capacitor C 1  is connected to the power supply terminal  21  at the connecting point P 1  and is further connected to the drive terminal  22 , which is connected to the upstream side of the electric load  40  at the connecting point P 2 . The other end of the capacitor C 1  is connected to ground terminal at the connecting point P 4 . 
     Since the series connection of the capacitor C 1  and the electric load  40  forms an LC circuit, and the capacitor C 1  is inserted into the current pathway of the loop circuit A 1  in series, it becomes possible to use the multilayer ceramic capacitor, which has smaller capacitance and is smaller than the electrolytic capacitor, as the capacitor C 1 . Thus, it becomes possible to produce a sufficient noise reduction effect. 
     On the contrary, the loop circuit A 2  includes a diode D 2  in addition to the electric load  40  including the inductive component. In the loop circuit A 2 , the electric load  40 , the drive terminal  23 , the connecting point P 3 , the diode D 2 , the connecting point P 2  and the drive terminal  22  are connected in this order. 
     The anode of the diode D 2  is connected to the drive terminal  23  and the drain of the switching element Q 1  at the connecting point P 3 . The cathode of the diode D 2  is connected to the drive terminal  22 , which is connected to the upstream side of the electric load  40 , at the connecting point P 2 . The diode D 2  is connected to the electric load  40  in parallel. The diode D 2  blocks the current flowing into the loop circuit A 2  from the power source (+B) as the switching transistor Q 1  is being turned on, and passes the current flowing into the loop circuit A 2  from the power source (+B) as the switching transistor Q 1  is being turned off. 
     The power supply terminal  21  is connected to the positive electrode (+B) of the power source, and the ground terminal  24  is connected to ground (GND). The power supply terminal  21  constitutes a first power supply terminal which is maintained at higher potential, the power source constitutes a first direct current power source, and the ground terminal  24  constitutes a second power supply terminal which is maintained at lower potential. Herein, the ground terminal  24  may be connected to a second direct current power source of which the rated voltage is lower than that of the first direct current power source. In this case, the rated voltage of the first direct current power source and the rated voltage of the second direct current power source may be set to any voltage as long as the rated voltage of the first direct current power source is set to be higher than the rated voltage of the second direct current power source. Hereinafter, for ease of explanation, the embodiment in which the ground terminal  24  is connected to ground will be described, unless otherwise described. 
     The main function of the capacitor C 1  is to reduce the noise of the load drive apparatus  1 . It is preferable to use a ceramic type capacitor which has greater degradation resistance as the capacitor C 1 . Further, it becomes possible to downsize the load drive apparatus  1  by using the ceramic type capacitor. 
     The switching element Q 1  is controlled to be turned on/off alternately. The cycle of turning on/off and the duty ratio of turning on/off may be set to any values. 
     According to the exemplary embodiment shown in  FIG. 3 , a current I 1  is flowing through the loop circuit A 1  in the direction of an arrow I 1  as the switching element Q 1  is being turned on. As the switching element Q 1  is being turned off, current I 2  is flowing through the loop circuit A 2  in the direction of an arrow I 2  shown in  FIG. 3 . In this way, it becomes possible to control the current flowing through the electric load  40  by controlling the on-duty ratio of the switching element Q 1  appropriately. 
     Although, as described above, the load drive apparatus  1  shown in  FIG. 3  is formed as a sink-type circuit, the switching apparatus of an embodiment of the present invention may be applied to a load drive apparatus which is formed a as source-type circuit. 
     As shown in  FIG. 4 , a load drive apparatus  2  includes a loop circuit B 1  and a loop circuit B 2 . The loop circuit B 1  constitutes the first loop circuit, and the loop circuit B 2  constitutes the second loop circuit. A drive terminal  25  of the load drive apparatus  2  is connected to an upstream side terminal of the electric load  40 . The upstream side terminal of the electric load  40  constitutes the first end. The downstream side terminal of the electric load  40  is connected to ground at a connecting point P 8 . The downstream side terminal of the electric load  40  constitutes the second end. The second end of the electric load  40  may be connected to ground terminal of the load drive apparatus  2 . The loop circuit B 1  and the loop circuit B 2  share the inductive component of the electric load  40 . 
     The loop circuit B 1  includes the switching element Q 1  and the capacitor C 1  in addition to the electric load  40  including the inductive component. In the loop circuit B 1 , a connecting point P 6 , the switching element Q 1 , a connecting point P 7 , the upstream side drive terminal  25 , the electric load  40 , the connecting point P 8 , the ground terminal  24 , a connecting point P 9 , and the capacitor C 1  are connected in this order. The switching element Q 1  may be comprised of, for example, a semiconductor element such as MOSFET, or may be comprised of, for example, a transistor such as IGBT. The capacitor C 1  may be comprised of, for example, a multilayer ceramic capacitor. 
     The source of the switching element Q 1  is connected to the drive terminal  25 , which is connected to the upstream side of the electric load  40 , at the connecting point P 7 . The drain of the switching element Q 1  is connected to the power supply terminal  21 , which is connected to the power source (+B), at the connecting point P 6 . 
     The capacitor C 1  which is connected to the electric load  40  in series is inserted into a current pathway of the loop circuit B 1  in series. More specifically, the capacitor C 1  is inserted in series into a looped current pathway of the current flowing through the loop circuit B 1 . One end of the capacitor C 1  is connected to the power supply terminal  21  and the drain of the switching element Q 1  at the connecting point P 6 . The other end of the capacitor C 1  is connected to ground at the connecting point P 9 . 
     Since the series connection of the capacitor C 1  and the electric load  40  forms an LC circuit, and the capacitor C 1  is inserted into the current pathway of the loop circuit B 1  in series, it becomes possible to use the multilayer ceramic capacitor, which has smaller capacitance and is smaller than the electrolytic capacitor, as the capacitor C 1 . Thus, it becomes possible to produce a sufficient noise reduction effect. 
     On the contrary, the loop circuit B 2  includes a diode D 2  in addition to the electric load  40  including the inductive component. In the loop circuit B 2 , the electric load  40 , the ground terminal  24 , the connecting point P 9 , the diode D 2 , the connecting point P 7  and the drive terminal  25  are connected in this order. 
     The cathode of the diode D 2  is connected to the drive terminal  25  and the source of the switching element Q 1  at the connecting point P 7 . The anode of the diode D 2  is connected to the drive terminal  24 , which is connected to the downstream side of the electric load  40 , at the connecting point P 9 . The diode D 2  is connected to the electric load  40  in parallel. 
     The main function of the capacitor C 1  is to reduce the noise of the load drive apparatus  2 . It is preferable to use a ceramic type capacitor which has greater degradation resistance as the capacitor C 1 . Further, it becomes possible to downsize the load drive apparatus  2  by using the ceramic type capacitor. 
     The switching element Q 1  is controlled to be turned on/off alternately. The cycle of turning on/off and the duty ratio of turning on/off may be set to any values. 
     According to the exemplary embodiment shown in  FIG. 4 , a current I 1  is flowing through the loop circuit B 1  in the direction of an arrow I 1 , as the switching element Q 1  is being turned on. As the switching element Q 1  is being turned off, a current I 2  is flowing through the loop circuit B 2  in the direction of an arrow I 2  shown in  FIG. 4 . In this way, it becomes possible to control the current flowing through the electric load  40  by controlling the duty ratio of the switching element Q 1  appropriately. 
     As described above with reference to  FIGS. 1 and 2 , if the load drive apparatus  1  shown in  FIG. 3  or the load drive apparatus  2  shown in  FIG. 4  is disposed in a planar configuration, a problem of the high-frequency noise caused by high-frequency fluctuation of the magnetic field penetrating through the loop circuit A 1  (B 1 ) and the magnetic field penetrating through the loop circuit A 2  (B 2 ) may be occur as the switching element Q 1  is being turned on/off at high frequency. 
     According to the present embodiment, it becomes possible to reduce the noise effectively, which noise is caused by the fluctuation of the magnetic field occurring in the loop circuits A 1  and A 2 , by arranging the circuit configuration of the load drive apparatus  1  appropriately. Hereinafter, the detailed circuit configuration of the load drive apparatus  1  will be described. Since the detailed circuit configuration of the load drive apparatus  2  is similar to the circuit configuration of the load drive apparatus  1 , the detailed description of the circuit configuration of the load drive apparatus  2  is omitted. 
       FIG. 5  shows a circuit configuration of the load drive apparatus  1  according to the present embodiment. Herein, magnetic flux, which is penetrating through the current pathway of the loop circuit A 1  on one surface of the printed circuit board, is referred to as magnetic flux φ 1 , and magnetic flux, which is penetrating through the current pathway of the loop circuit A 2  on the other surface of the printed circuit board, is referred to as magnetic flux φ 2 . According to the present embodiment, the circuit configuration of the load drive apparatus  1  is arranged so that the magnetic flux φ 1  and the magnetic flux φ 2  head in the same direction. In other words, as shown in  FIG. 5 , a plane including the current pathway of the loop circuit A 1  on the surface of the printed circuit board, and a plane including the current pathway of the loop circuit A 2  on the surface of the printed circuit board, are stacked relative to each other in the direction of normal lines of the two surfaces. In addition to this, the looped current pathways in the stacked loop circuits have the same direction. The current pathway of the loop circuit A 1  on one surface of the printed circuit board and the current pathway of the loop circuit A 2  on the other surface of the printed circuit board are arranged to be stacked relative to each other in a manner arranged by bending the current pathways along line X 1 -X 2  shown in  FIG. 3 . The circuit configuration of the load drive apparatus  2  is similar to the circuit configuration of the load drive apparatus  1 . 
     As shown in  FIG. 5 , the capacitor C 1  is connected between the connecting point P 1  connected to the power supply terminal  21 , and the connecting point P 4  connected to the ground terminal  24 . The capacitor C 1  functions as a bypass circuit for the noise occurring in the loop circuits in the load drive apparatus  1 . The capacitor C 1  suppresses leakage of the noise, in a normal operation, out of the load drive apparatus  1  via the wire harnesses connected to the power supply terminal  21  and the ground terminal  24 . 
     The load drive apparatus  1  includes a parallel section in which a first current pathway and a second current pathway are arranged in parallel. The first current pathway is located on the upstream side of the switching element Q 1 , and the current flows through it as the switching element Q 1  is being turned on. The second current pathway is located on the downstream side of the switching element Q 1 , and the current flows through it as the switching element Q 1  is being turned on. The parallel section includes an opposite current flowing section. In the opposite current flowing section, the direction of the current flowing through the first current pathway is opposite to the direction of the current flowing through the second current pathway. Thus, it becomes possible to suppress the noise occurring near the opposite current flowing section by arranging the opposite current flowing section. 
     For example, as shown in  FIGS. 3 and 5 , the first current pathway corresponds to the current pathway between the positive electrode of the power source (+B) and the drive terminal  22 . The first current pathway also includes the wire harness between the positive electrode of the power source and the power supply terminal  21 . The second current pathway corresponds to the current pathway between the downstream side terminal of the switching element Q 1  and ground. The second current pathway also includes the wire harness between the ground terminal  24  and ground. In  FIG. 4 , for example, the first current pathway corresponds to the current pathway between the positive electrode of the power source (+B) and the upstream side of the switching element Q 1 . The first current pathway also includes the wire harness between the positive electrode of the power source and the power supply terminal  21 . The second current pathway corresponds to the current pathway between the downstream side of the switching element Q 1  and ground. The second current pathway also includes the wire harness between the drive terminal  25  and ground (or the electric load  40 ). 
     As shown in  FIG. 5 , as the switching element is being turned on, the direction of the current flowing from the positive electrode of the power source to the load drive apparatus  1  is opposite to the direction of the current flowing from the load drive apparatus  1  to ground. Magnetic fields M B  and M G  are generated around the current pathways in the direction of arrows shown in  FIG. 5  by causing the currents to flow through the parallel section in opposite directions to each other. Thus, the noises (mainly switching noises generated by the switching of the switching element) generated from the respective current pathways in the parallel section are reduced by cancelling the noises with each other. 
     As shown in  FIG. 5 , in the right side area of the opposite current flowing section, the direction of the magnetic fields M B  and the direction of the magnetic fields M G  are opposite to each other, i.e. upward and downward. It becomes possible to reduce the intensity of the magnetic field in the right side area of the opposite current flowing section shown in  FIG. 5 . The same applies to the left side area of the opposite current flowing section shown in  FIG. 5 . Thus, it becomes possible to reduce the intensity of the magnetic field in the whole area around the opposite current flowing section. Accordingly, it becomes possible to produce a noise reduction effect in a case where a signal line for transmitting a predetermined signal is disposed in the area around the opposite current flowing section. For example, it becomes possible to provide the noise reduction effect to a signal line which is tied together with the wire harness connected to the power supply terminal  21  and the wire harness (ground wire harness) connected to the ground terminal  24 . 
     Herein, the opposite current flowing section may be formed outside the load drive apparatus  1 , or may be formed inside the load drive apparatus  1  by arranging two adjacent patterned lines in parallel. 
     The capacitor C 1  controls the current flowing through the electric load  40  at a predetermined switching frequency f 1 , i.e. the capacitor C 1  holds a PWM waveform of the current which controls the electric load  40  in the PWM manner, in order to cause the current flowing through the two loop circuits at a frequency f 2  which is to be cancelled. Thus, it is necessary that the capacitance C of the capacitor C 1  be set at a value which satisfies the conditional expression (1) described below. The relational expression (2) is obtained by solving the conditional expression (1). 
     
       
         
           
             
               
                 
                   
                     
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     Since most switching noise becomes a problem at higher frequency, f 1 &lt;f 2 , i.e. f 1   2 &lt;&lt;f 2   2  is established. It is possible to cause the current to flow through the loop circuits at the noise frequency f 2  while holding the PWM waveform of the current by selecting the capacitance C which satisfies the first term of the relational expression (2). Thus, it becomes possible to design a circuit which reduces noise. 
       FIG. 6  shows waveforms which represent reduced fluctuation effect of magnetic flux in the load drive apparatus  1  including the circuit configuration shown in  FIG. 5 . 
     As described above, as the switching element Q 1  is being driven at a predetermined duty ratio, the current holding the waveforms shown in  FIGS. 6(A) and 6(B)  is flowing through the loop circuits A 1  and A 2 . Then the voltage at the connecting point P 3  fluctuates and has waveforms shown in  FIG. 6(F) . Then the magnetic fluxes φ 1  and φ 2 , which are penetrating the loop circuits A 1  and A 2  respectively, are generated and have waveforms shown in  FIGS. 6(C) and 6(D) . The magnetic fluxes φ 1  and φ 2  are caused by the currents flowing through the loop circuits A 1  and A 2 . Since the switching element Q 1  is driven at a higher frequency, the magnetic fluxes φ 1  and φ 2  fluctuate greatly in a short time. According to the present embodiment, the magnetic flux φ 1  and the magnetic flux φ 2  have opposite phases. Thus, as shown in  FIG. 6(E) , composite waveforms of the magnetic flux φ 1  and the magnetic flux φ 2  do not include steep fluctuation. The magnetic flux with small scale temporal variation is obtained. According to the load drive apparatus  1  including the circuit configuration shown in  FIG. 5 , it becomes possible to reduce the noise caused by the magnetic flux φ 1  and magnetic flux φ 2 . 
       FIG. 7A  shows a circuit unit U 1  including the conventional load drive apparatus.  FIG. 7B  shows a circuit unit U 2  including the load drive apparatus  1  according to the present embodiment. In  FIGS. 7A and 7B , surface A shows the top surface of the printed circuit board, and surface B shows the bottom surface of the printed circuit board. In  FIG. 7A , elements which correspond to the elements shown in  FIG. 1  are referred to by the same reference numerals. In  FIG. 7B , elements which correspond to the elements shown in  FIG. 3  are referred to by the same reference numerals. In  FIGS. 7A and 7B , reference numerals  71 ˜ 76  show planar current pathways formed on the top surface and the bottom surface. The current pathways constitute patterned lines. 
     As shown in  FIG. 7A , the current pathway  71  corresponds to a part of the loop circuit A 11  shown in  FIG. 1 , and constitutes a patterned ground line which connects the downstream side of the switching element Q and one side terminal of the capacitor C. The current pathway  72  corresponds to a part of the loop circuit A 12  shown in  FIG. 1 , and constitutes a patterned line which connects the anode of the diode D and the upstream side of the electric load  40 . The current pathway  73  corresponds to the patterned line between the connecting point P 12  and the terminal T 3  which is shared between the loop circuit A 11  and the loop circuit A 12 . The current pathway  73  is formed on the top surface and the bottom surface of the printed circuit board via a through-hole H. 
     In  FIG. 7B , the current pathway  74  corresponds to a part of the loop circuit A 1  shown in  FIG. 3 , and constitutes a patterned ground line which connects the downstream side of the switching element Q 1  and one side terminal of the capacitor C 1 . The current pathway  75  corresponds to a part of the loop circuit A 2  shown in  FIG. 3 , and constitutes a patterned line which connects the anode of the diode D 2  and the upstream side of the electric load  40 . The current pathway  75  is formed on the top surface and the bottom surface of the printed circuit board via a through-hole H 1 . The current pathway  76  corresponds to the patterned line between the connecting point P 3  and the terminal  23  which is shared between the loop circuit A 1  and the loop circuit A 2 . The current pathway  76  is formed on the top surface and the bottom surface of the printed circuit board via a through-hole H 2 . 
     In the circuit U 1  shown in  FIG. 7A , the current is flowing through the patterned line  71  and the patterned line  72  alternately. The direction of the magnetic flux φ 1  generated by the current flowing through the patterned line  71  is opposite to the direction of the magnetic flux φ 2  generated by the current flowing through the patterned line  72 . On the contrary, in the circuit U 2  shown in  FIG. 7B , the current is flowing through the patterned line  74  and the patterned line  75  alternately. The direction of the magnetic flux φ 1  generated by the current flowing through the patterned line  74  is the same as the direction of the magnetic flux φ 2  generated by the current flowing through the patterned line  75 . 
       FIG. 8  shows noise levels detected in the circuit units U 1  and U 2  shown in  FIGS. 7A and 7B . The noise levels shown in  FIG. 8  are obtained in a condition where the switching elements Q and Q 1  are turned on and off at 20 kHz. A single scale in the horizontal axis corresponds to 200 kHz, and a single scale in the vertical axis corresponds to 10 dB. As shown in  FIG. 8 , the noise level of the circuit unit U 2  according to the present embodiment is reduced compared with the conventional circuit unit U 1 , particularly in the AM range. 
     In the circuit unit U 2 , the direction of the magnetic flux φ 1  and the direction of the magnetic flux φ 2  are the same as each other, and the patterned line  74  and the patterned line  75  of the circuit unit U 2  are closely formed on the surface A and surface B. It becomes easier to form the composite magnetic flux of magnetic fluxes φ 1  and φ 2  in the circuit unit U 2 . Thus, it becomes possible to reduce the noise generated by the magnetic fluxes. 
     It is not necessary to form the whole current pathway of the loop circuit A 1  on the surface which is opposite to the surface on which the whole current pathway of the loop circuit A 2 . A part of the current pathway included in the loop circuit A 1  or a part of the current pathway included in the loop circuit A 2  may be formed on another surface or another layer of the printed circuit board. Further, a part of the loop circuit A 1  (particularly the patterned line) or a part of the loop circuit A 2  (particularly the patterned line) may be formed on another surface or another layer of the printed circuit board. For example, as shown in  FIG. 7B , a part of the loop circuit A 2  may be formed on the same surface on which the loop circuit A 1  is formed. On the contrary, a part of the loop circuit A 1  may be formed on the same surface on which the loop circuit A 2  is formed. The printed circuit board is not limited to a specific printed circuit board. 
     The patterned lines included in loop circuits A 1  and A 2  may be formed on any layer of a multilayer printed circuit board.  FIG. 9  shows a cross section of an exemplary configuration of a four-layer printed circuit board. Adjacent four layers are insulated from each other by insulating layers  82 . For example, the patterned lines of the loop circuit A 1  may be formed on a second layer  81   b , and the patterned lines of the loop circuit A 2  may be formed on a third layer  81   c . In another exemplary embodiment, the patterned lines of the loop circuit A 1  may be formed on a first layer  81   a , and the patterned lines of the loop circuit A 2  may be formed on the third layer  81   c . Although the number of the multilayers is not limited to four, it becomes possible to obtain a greater noise reduction effect by forming the loop circuits A 1  and A 2  on adjacent layers. 
       FIG. 10  shows another exemplary circuit configuration of the load drive apparatus  1 . 
     In an exemplary circuit configuration shown in  FIG. 10 , a flexible substrate  80 , on which the patterned line  74  and the patterned line  75  are formed, is bent so that the patterned line  74  and the patterned line  75  are arranged to be stacked with each other. Herein, the patterned line  74  forms the loop circuit A 1 , and the patterned line  75  forms the loop circuit A 2 . Thus, the loop circuit A 1  and the loop circuit A 2  are arranged to be stacked with each other. The loop circuit A 1  and the loop circuit A 2  are arranged so that the direction of the magnetic flux generated from the loop circuit A 1  corresponds to the direction of the magnetic flux generated from the loop circuit A 2 . The insulating layer  82  is formed on the loop circuits A 1 , A 2  and the flexible substrate  80 . Thus, the loop circuits A 1  and A 2  (particularly the power supply terminal and ground terminal) are insulated from each other. 
       FIG. 11  shows yet another exemplary circuit configuration of the load drive apparatus  1 . As shown in  FIG. 11 , the patterned line  74  of the loop circuit A 1  and the patterned line  75  of the loop circuit A 2  are formed on different substrates  84   a  and  84   b , and the substrates  84   a  and  84   b  are stacked with each other. The loop circuit A 1  and the loop circuit A 2  are arranged so that the direction of the magnetic flux generated from the loop circuit A 1  corresponds to the direction of the magnetic flux generated from the loop circuit A 2 . The insulating layer  82   a  is formed on the patterned line  74  and the substrate  84   a . The insulating layer  82   b  is formed on the patterned line  75  and the substrate  84   b . Thus, the patterned line  74  and the patterned line  75  are insulated from each other. In this case, the patterned line  76 , which is shared between the loop circuit A 1  and the loop circuit A 2  is formed on either the substrate  84   a  or the substrate  84   b . The substrates  84   a  and  84   b  may be comprised of printed circuit board, flexible substrate or ceramic substrate. 
     In the exemplary circuit configuration shown in  FIG. 11 , the substrate  84   a , on which the patterned line  74  and the insulating layer  82   a  are formed, and the substrate  84   b , on which the patterned line  74  and the insulating layer  82   a  are formed, are stacked directly. The substrate  84   a  and the substrate  84   b  may be stacked indirectly, i.e. the substrate  84   a , one or more other layers and the substrate  84   b  may be stacked in this order. Further, one or more other layers may be stacked on the substrate  84   a . The substrate  84   a  and the substrate  84   b  may be stacked over one or more other layers. It becomes possible to improve noise resistance by forming conductive material such as copper on the whole surface of other layers stacked on the substrate  84   a  or stacked under the substrate  84   b.    
     The load drive apparatus  1  according to the above embodiments includes the loop circuits which reduce fluctuation of the magnetic fluxes generated therefrom as the switching element is being turned on/off. Thus, it becomes possible to reduce radio noise generated by switching the switching element. According to the above embodiments, two loop circuits share the same inductive component of the electric load. 
     In accordance with the present invention according to the embodiments described above, it becomes possible to reduce high-frequency noise by improving the circuit configuration of the switching apparatus. 
     Although the load drive apparatus  1  according to the above embodiments drives the electric load  40  including the inductive component, the load drive apparatus  1  may drive a motor such as a motor including brushes, a brushless motor, a stepping motor, a three-phase motor and a linear motor. Further, the load drive apparatus  1  may drive a linear solenoid and an electromagnetic valve. In a case where the load drive apparatus  1  is integrated with a control unit such as an ECU (Electronic Control Unit) and an actuator, the greater noise reduction effect is obtained. In this case, degradation of the noise resistance caused by the integration of the ECU and the actuator is suppressed. 
     The load drive apparatus  1  may be widely applied to an electronic apparatus which controls current flowing through an electric load including an inductive component. The electronic apparatus described above may include an ECU for controlling a motor of radiator cooling fan, an ECU for fuel control, an ECU for controlling a motor for electric power steering, an ECU for controlling an electric seat, an ECU for controlling electric windows, an ECU for controlling brightness of head lights, an ECU for controlling a motor of a haptic apparatus, an ECU for controlling a motor of a slide door, an ECU for controlling an air conditioner, an ECU for controlling a motor of windshield wiper, an ECU for controlling a blower motor and an ECU for controlling a motor included in a transmission. In a case where the load drive apparatus  1  is applied to a plurality of the electronic apparatus as described above, it becomes possible to obtain greater noise reduction effect in a vehicle. Since the noise reduction effect can be obtained without adding an element or a component, it becomes possible to avoid weight gain. Thus, it becomes possible to provide the load drive apparatus  1  which is easy to apply to the electronic apparatuses mounted on a vehicle. 
     Although it is preferable that the loop circuits A 1  and A 2  have the same areas through which the magnetic flux penetrates, the loop circuits A 1  and A 2  may have different areas, particularly in a case where it is difficult to obtain the same areas because of restriction of design. Similarly, it is preferable that the loop circuits A 1  and A 2  have larger overlap areas in planer view, and the loop circuits A 1  and A 2  may have an overlap area in a small part. 
     The patterned lines included in the loop circuits A 1  and/or A 2  may have a rounded portion in a turning portion in order to reduce noise generated from the turning portion. 
     Further, at least a part of the current pathway of the loop circuits A 1  and/or A 2  may be formed by a cable core of a coaxial cable. In this case, it is possible to suppress magnetic flux from the current pathway. 
     The edge portion of the printed circuit board may be coated with insulating material in a manner coated twice or coated by dipping, in a case where both of the current pathways of the upstream side portion and downstream side portion are arranged close to the edge portion of the printed circuit board. Both of the current pathways of the upstream side portion and downstream side portion may be arranged in a center portion of the printed circuit in planar view in order to lengthen the distance between the upstream side portion and the downstream side portion. Similarly, both of the current pathways of the upstream side portion and the downstream side portion are arranged apart from the through hole in planar view in order to lengthen the distance between the upstream side portion and the downstream side portion. 
     The diode D 2  included in the load drive apparatus  1  shown in  FIG. 3  may be replaced by a switching element Q 2  as shown in  FIG. 12 . The diode D 2  included in the load drive apparatus  2  shown in  FIG. 4  may be replaced by a switching element Q 2  as shown in  FIG. 13 . In these cases, the switching element Q 1  and the switching element Q 2  are alternately turned on and off in an opposite manner. The switching element Q 2  is used as a rectifier element. Control conditions of the switching elements Q 1  and Q 2  such as with or without dead time, length of the dead time or the like are arbitrarily defined. 
     The present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese Priority Application No. 2009-169383 filed on Jul. 17, 2009 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.