Abstract:
A power circuit is applicable to a Direct Current (DC) to DC converter. The power circuit includes a gate driver circuit and a High Electron Mobility Transistor (HEMT). The gate driver circuit functions as a Sigmoid (S) function and controls a gate and a source of the HEMT with a cross voltage of the sigmoid (S) type function. Accordingly, an overall characteristic curve of the HEMT and the gate driver circuit is like a characteristic curve of a single rectifier diode, so as to achieve a rectifying, freewheeling, or reversing effect. In addition, since an energy loss is low when the HEMT is conducted, the energy loss of the whole power circuit is much less than that of a conventional diode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 099142886 filed in Taiwan, R.O.C. on Dec. 8, 2010, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present disclosure relates to a power circuit and a Direct Current (DC) to DC converter thereof, and more particularly to a power circuit and a DC to DC converter thereof with a low circulation loss. 
         [0004]    2. Related Art 
         [0005]    A diode is quite widely applied to an electronic product. Usually, a rectifying function of the diode is used. That is to say, a rectifying effect is achieved with a characteristic that the diode is in a conducted state when in a forward bias condition and is in a cut-off state and not conducted when in a reverse bias condition. 
         [0006]    When the diode is in the conducted state, all currents flow through the diode. At this time, energy consumed by the diode is a product obtained by multiplying a cut-off voltage of the diode by the current. That is to say, the larger the cut-off voltage of the diode or the current is, the larger the consumed energy (usually referred to as a circulation loss) will be. Therefore, in order to reduce the circulation loss of the diode, the current flowing through the diode and the cut-off voltage of the diode may be reduced. When the diode is used as a rectifier element, the current flowing through the diode usually is a main current, which has little chance to be reduced. The cut-off voltage of the diode is related to a structure and a material of the diode, for example, a forward bias (cut-off voltage) of a diode with silicon as the main material is approximately 0.7 volts (V), a cut-off voltage of a silicon carbide diode applicable to a high voltage is approximately from 1.0 V to 1.2 V, and a forward bias of a germanium diode is approximately 0.2 V. 
         [0007]    In addition, when being used, the diode also generates a switching loss in addition to the circulation loss, in which the switching loss refers to a loss that occurs at the moment when the diode is switched to be conducted or cut-off. When the rectifier diode is applied to a current supplier, a bridge rectifier, a flyback DC to DC converter, or a forward DC to DC converter, the circulation loss and the switching loss usually occupy almost a half of the total energy loss of a power source supplier. Therefore, under the energy-saving trend, how to reduce the energy loss of the rectifier diode remains a subject continuously concerned by the industry. 
       SUMMARY 
       [0008]    Accordingly, the present disclosure is a power circuit (the power circuit is applicable to power conversion, so as to form a rectifier circuit or a freewheeling circuit) and a DC to DC converter thereof, which may reduce a conducting loss (or referred to as a circulation loss) and a switching loss as well. 
         [0009]    According to an embodiment, the present disclosure provides a power circuit, which comprises a High Electron Mobility Transistor (HEMT) and a gate driver circuit. An anode, a cathode, and a driving end of the gate driver circuit are respectively electrically connected to a drain, a source, and a gate of the HEMT. The gate driver circuit satisfies the following equation: 
         [0000]        v   GS   =V   + (1− e   −v     DS     /β )/1+ e   −v     DS     /β .
 
         [0010]    V GS  is a voltage between the driving end and the cathode, V DS  is a voltage between the anode and the cathode, and β is a characteristic constant of the gate driver circuit. 
         [0011]    According to an embodiment, the gate driver circuit comprises a first Zener diode, a second Zener diode, and a resistor. An anode of the first Zener diode is electrically connected to the source, a cathode of the second Zener diode is electrically connected to a cathode of the first Zener diode, and both ends of the resistor are respectively electrically connected to the drain and an anode of the second Zener diode. The HEMT satisfies the following characteristic equations: 
         [0000]    
       
         
           
             
               
                 i 
                 D 
               
               = 
               
                 
                   I 
                   Dmax 
                 
                  
                 
                   1 
                   
                     1 
                     + 
                     
                        
                       
                         - 
                         
                           
                             
                               v 
                               GS 
                             
                             - 
                             
                               α 
                                
                               
                                   
                               
                                
                               
                                 V 
                                 T 
                               
                             
                           
                           γ 
                         
                       
                     
                   
                 
               
             
             , 
             and 
           
         
       
       
         
           
             γ 
             = 
             
               
                 ( 
                 
                   1 
                   + 
                   α 
                 
                 ) 
               
                
               
                 
                   V 
                   T 
                 
                 / 
                 6. 
               
             
           
         
       
     
         [0012]    i D  is a current flowing through the drain, γ is a scaling factor, V T  is a cut-off voltage, I Dmax  is a maximum value of the current flowing through the drain, and α is a characteristic constant of the HEMT. 
         [0013]    According to an embodiment, the present disclosure provides a DC to DC converter, which comprises a control circuit, a primary side coil, a secondary side coil, a capacitor, and a rectifier circuit. The control circuit receives a power source and converts the power source into a voltage signal with a predetermined frequency. The primary side coil receives the voltage signal. The secondary side coil has a first end and a second end. The secondary side coil is configured corresponding to the primary side coil, so as to generate a secondary side signal at the first end and the second end in response to the voltage signal. The capacitor has a first end and a second end, and the second end is electrically connected to the second end of the secondary side coil. The rectifier circuit has an anode and a cathode. The anode is electrically connected to the first end of the secondary side coil, and the cathode is electrically connected to the first end of the capacitor. The rectifier circuit comprises an HEMT and a gate driver circuit. An anode, a cathode, and a driving end of the gate driver circuit are respectively electrically connected to a drain, a source, and a gate of the HEMT. The gate driver circuit satisfies the following equation: 
         [0000]        v   GS   =V   + (1− e   −v     DS     /β )/1+ e   −v     DS     /β 
 
         [0014]    V GS  is a voltage between the driving end and the cathode, V DS  is a voltage between the anode and the cathode, β is a characteristic constant of the gate driver circuit. 
         [0015]    Through the features of the power circuit (or referred to as the rectifier circuit), the gate driver circuit forms a cross voltage of a sigmoid (S) type function between the gate and the source of the HEMT, and after being operated according to the characteristic equations, the HEMT generates a rectification characteristic similar to the diode between the drain and the source thereof. In addition, the energy loss of the HEMT is directly proportional to a region area of the gate, and the energy loss thereof is much less than that of a conventional diode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present disclosure will become more fully understood from the detailed description given herein below for illustration only, and thus are not limitative of the present disclosure, and wherein: 
           [0017]      FIG. 1  is a schematic circuit block diagram of a rectifier circuit according to an embodiment of the present disclosure; 
           [0018]      FIG. 2  is a schematic view of a voltage-current curve of an HEMT feature according to an embodiment of the present disclosure; 
           [0019]      FIG. 3  is a schematic view of a V GS -V DS  characteristic curve of a gate driver circuit according to an embodiment of the present disclosure; 
           [0020]      FIG. 4  is a schematic view of a desired characteristic curve of a rectifier circuit according to an embodiment of the present disclosure; 
           [0021]      FIG. 5  is another schematic circuit block diagram of a rectifier circuit according to an embodiment of the present disclosure; 
           [0022]      FIG. 6  is a schematic view of a characteristic curve of a rectifier circuit according to an embodiment of the present disclosure; 
           [0023]      FIG. 7  is a schematic circuit block diagram of a DC to DC converter according to an embodiment of the present disclosure; and 
           [0024]      FIG. 8  is a schematic circuit block diagram of a three-phase motor control circuit according to an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Firstly, referring to  FIG. 1 , it is a schematic circuit block diagram of a power circuit according to an embodiment of the present disclosure. It can be known from  FIG. 1  that, a power circuit  20  comprises an HEMT  30  and a gate driver circuit  40 . The power circuit  20  may generate a rectifying effect like a rectifier diode or a freewheeling function like a freewheeling diode (described in detail hereinafter). The power circuit  20  comprises an anode  20 A and a cathode  20 C. When a voltage applied to the anode  20 A and the cathode  20 C is a forward bias (that is, a voltage value of the anode  20 A minus a voltage value of the cathode  20 C is a positive value), the power circuit  20  conducts the anode  20 A and the cathode  20 C. On the contrary, when a voltage applied to the anode  20 A and the cathode  20 C is a reverse bias (that is, the voltage value of the anode  20 A is smaller than the voltage value of the cathode  20 C), the power circuit  20  cuts off an electrical connection between the anode  20 A and the cathode  20 C. 
         [0026]    The HEMT  30  has a drain  30 D, a gate  30 G, and a source  30 S.  FIG. 2  is a schematic view of a voltage-current curve of an HEMT  30  feature according to an embodiment of the present disclosure. Referring to  FIG. 2 , a horizontal axis in  FIG. 2  is a voltage value V DS  between the drain  30 D and the source  30 S, a unit is volt (V), a vertical axis is a current value I DS  between the drain  30 D and the source  30 S, and the curves in  FIG. 2  respectively represent the I DS -V DS  characteristic curves at different voltages V GS  between the gate  30 G and the source  30 S. It can be known from  FIG. 2  that, the HEMT  30  is cut off (that is, a conducted state between the drain  30 D and the source  30 S is cut off) only when the voltage difference V GS  between the gate and the source is smaller than −4V. On the contrary, as long as the voltage difference V GS  between the gate and the source is larger than approximately −4V, the HEMT  30  is in a conducted state (that is, the drain  30 D and the source  30 S are conducted). 
         [0027]    The HEMT  30  may be, but is not limited to, a GaN HEMT or an AlGaN HEMT, and may be a depletion mode HEMT, a normally-off HEMT, a depletion mode Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), or a normally-off MOSFET. 
         [0028]    Taking the GaN HEMT as an example, the GaN HEMT has a high breakdown voltage and can effectively reduce a conducting loss generated when being conducted, for which a reason is that a transistor with a high breakdown voltage may be designed to be very close to an electrode, so as to reduce a conducting loss when the current flows. The conducting loss of the GaN HEMT is reduced to be ⅕ than a silicon transistor, and the GaN HEMT has a fast switching characteristic at the same time, and thus, a switching loss thereof also can be reduced to be 1%. The GaN HEMT has characteristics of a high output power density, a high operating voltage, and a low switching loss. Under an operation of a pulse, an element is cut off for the first time, and in the case of a high voltage electric field, an electron is injected into a defect (a surface or a buffer layer defect). When a channel is opened, the bound electron cannot make response in time, and thus, an instant energy of a power transistor of the GaN HEMT is reduced. Because of a polarization phenomenon, the AlGaN HEMT or GaN HEMT has a two-dimensional electron gas (2DEG), and the electron has a high electron mobility, a low conducting resistance, and a high switching speed. 
         [0029]    The characteristic curve shown in  FIG. 2  is a typical DC characteristic curve of a depletion mode GaN HEMT of 20 millimeters (mm). When a voltage difference V GS  between the gate  30 G and the source  30 S is zero, a maximum current is 20 amperes (A), a drain knee voltage thereof may be controlled between 1 V and 5 V according to differences in a process, and a cut-off voltage is approximately 200 V. 
         [0030]    Characteristic Equations (1) and (2) of the HEMT are as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         i 
                         D 
                       
                       = 
                       
                         
                           I 
                           Dmax 
                         
                          
                         
                           1 
                           
                             1 
                             + 
                             
                                
                               
                                 - 
                                 
                                   
                                     
                                       v 
                                       GS 
                                     
                                     - 
                                     
                                       α 
                                        
                                       
                                           
                                       
                                        
                                       
                                         V 
                                         T 
                                       
                                     
                                   
                                   γ 
                                 
                               
                             
                           
                         
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
                    
                   and 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
             
               
                 
                   γ 
                   = 
                   
                     
                       ( 
                       
                         1 
                         + 
                         α 
                       
                       ) 
                     
                      
                     
                       
                         V 
                         + 
                       
                       / 
                       6. 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
         [0031]    i D  is a current flowing through the drain, γ is a scaling factor, V T  is a cut-off voltage, I Dmax  is a maximum value of the current flowing through the drain, α is a characteristic constant of the HEMT and can be obtained by parameter fitting, and V+ is a constant. 
         [0032]    The gate driver circuit  40  has an anode  40 A, a cathode  40 C, and a driving end  40 D. The driving end  40 D, the anode  40 A, and the cathode  40 C are respectively electrically connected to the gate  30 G, the drain  30 D, and the source  30 S of the transistor  30 . The gate driver circuit  40  satisfies the following Equation (3): 
         [0000]        v   GS   =V   + (1− e   −v     DS     /β )1+ e   −v     DS     /β   Equation (3).
 
         [0033]    V GS  is a voltage difference between the driving end  40 D and the cathode  40 C (also referred to as a voltage difference between the gate  30 G and the source  30 S, and briefly referred to as a voltage difference V GS  between the gate and the source), V DS  is a voltage between the anode  40 A and the cathode  40 C (also referred to as a voltage difference between the drain  30 D and the source  30 S, and briefly referred to as a voltage difference V DS  between the drain and the source), and β is a characteristic constant of a gate driver circuit  40  and may be obtained by parameter fitting. 
         [0034]      FIG. 3  is a schematic view of a V GS -V DS  characteristic curve of a gate driver circuit according to an embodiment of the present disclosure. When the gate driver circuit  40  satisfies the characteristic Equation (2), a characteristic curve between the voltage difference V GS  between the gate and the source and the voltage difference V DS  between the drain and the source is as shown in  FIG. 3 . As shown in  FIG. 3 , when the voltage difference V DS  between the drain and the source is larger than zero, the gate driver circuit  40  outputs the voltage difference V GS  between the gate and the source as +V. On the contrary, when the voltage difference V DS  between the drain and the source is smaller than zero, the gate driver circuit  40  outputs the voltage difference V GS  between the gate and the source as −V. Thus, a characteristic curve of an S type function is formed. In the S type function, when β in the Equation (3) approaches zero, the driving end  40 D has no transitional region when being turned on or off. 
         [0035]    Therefore, by combining the HEMT  30  and the S type gate driver circuit  40 , the characteristic curve of the power circuit  20  is as shown in  FIG. 4 .  FIG. 4  is a schematic view of a desired characteristic curve of a power circuit according to an embodiment of the present disclosure. Referring to  FIG. 4 , a horizontal axis is a voltage difference between the anode  20 A and the cathode  20 C of the power circuit  20  (also referred to as a voltage difference V DS  between the drain and the source of the transistor), and a vertical axis is a current value I DS  between the anode  20 A and the cathode  20 C of the power circuit  20 . It can be known from  FIG. 4  that, when a forward bias is applied between the anode  20 A and the cathode  20 C of the power circuit  20 , the anode  20 A and the cathode  20 C are conducted, and when the current between the anode  20 A and the cathode  20 C reaches a maximum value I MAX , the voltage is directly proportional to the current. 
         [0036]    Then,  FIG. 5  is another schematic circuit block diagram of a power circuit according to an embodiment of the present disclosure. As shown in  FIG. 5 , the power circuit  20  comprises an HEMT  30  and a gate driver circuit  40 . The gate driver circuit  40  comprises a first Zener diode  42 , a second Zener diode  44 , and a resistor  46 . 
         [0037]    The first Zener diode  42  has an anode  42 A and a cathode  42 C. The second Zener diode  44  has an anode  44 A and a cathode  44 C. The anode  42 A of the first Zener diode  42  is electrically connected to the source  30 S of the HEMT  30  (that is, the cathode  20 C of the power circuit  20 ). The cathode  42 C of the first Zener diode  42  is electrically connected to the cathode  44 C of the second Zener diode  44 . Both ends  46   a  and  46   b  of the resistor  46  are respectively electrically connected to the drain  30 D of the HEMT  30  and the anode  44 A of the second Zener diode  44 . 
         [0038]    Referring to  FIG. 5 , according to the Kirchhoffs Current Law (KCL), the current flowing from the anode  20 A of the power circuit  20  is equal to the current flowing into the HEMT  30  and the resistor  46 , that is, i=i D +i R . Similarly, i R =i Z +i G . It may be obtained that V=i R R1+V GS  according to the Kirchhoffs Voltage Law (KVL), in which R1 is a resistance value of the resistor  46 , and V is a voltage difference between the anode  20 A and the cathode  20 C of the power circuit  20 . They are applied to the Equations (1) and (2), and the following Equations (4) and (5) are obtained with the cut-off voltage (V T ) of the GaN HEMT: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       γ 
                       = 
                       
                         
                           ( 
                           
                             1 
                             + 
                             α 
                           
                           ) 
                         
                          
                         
                           
                             V 
                             T 
                           
                           / 
                           6 
                         
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
                    
                   and 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
             
               
                 
                   
                     i 
                     D 
                   
                   = 
                   
                     
                       I 
                       Dmax 
                     
                      
                     
                       
                         1 
                         
                           1 
                           + 
                           
                              
                             
                               6 
                                
                               
                                 ( 
                                 
                                   
                                     α 
                                     
                                       1 
                                       + 
                                       α 
                                     
                                   
                                   - 
                                   
                                     
                                       v 
                                       GS 
                                     
                                     
                                       
                                         ( 
                                         
                                           1 
                                           + 
                                           α 
                                         
                                         ) 
                                       
                                        
                                       
                                         V 
                                         T 
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
         [0039]    A current-voltage relationship of the first Zener diode  42  and the second Zener diode  44  which are serially connected may be combined by the Gompertz equation, which is: 
         [0000]    
       
         
           
             
               
                 
                   
                     v 
                     GS 
                   
                   = 
                   
                     
                       V 
                       
                         Z 
                          
                         
                             
                         
                          
                         2 
                       
                     
                      
                     
                       
                         
                           
                             i 
                             Z 
                           
                           / 
                           β 
                         
                         
                           
                             1 
                             + 
                             
                               
                                 ( 
                                 
                                   
                                     i 
                                     Z 
                                   
                                   / 
                                   β 
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     6 
                     ) 
                   
                 
               
             
           
         
       
     
         [0040]    In the equation, V Z2 =V Z +V γ , and i Z =(V−V GS )/R1−i G , in which i Z  is a current flowing through the second Zener diode  44 , V Z  is a reverse breakdown voltage of the Zener diode, and V γ  is a forward conducting bias of the Zener diode. It is assumed that i G  is very small and can be ignored, i Z =(V−V GS )/R1, and a cross voltage from the gate to the source is calculated according to the following Equation (7): 
         [0000]      (β R   1   v   GS ) 2 +( v−v   GS ) 2 ( v   GS   2   −V   Z2   2 )=0  Equation (7).
 
         [0041]      FIG. 6  is a schematic view of a characteristic curve of a power circuit according to an embodiment of the present disclosure. As shown in  FIG. 6 , a result coincides with a desired output result in  FIG. 4 . In addition, a comparative analysis is performed with respect to energy consumption between this embodiment and a conventional rectifier diode, and the energy consumption of the conventional diode may be denoted as P≈i 2 R. For example, if a rectifier diode with a current of 4 A is taken as an example and a diode of a PSF10A40 type is used, the power consumption thereof is approximately 4 W, and for another example, if a common silicon carbide diode is used, the power consumption thereof is approximately 4.8 W. On the contrary, in this embodiment, if the GaN HEMT of 3 mΩ-cm 2  is used, and a gate region thereof is 5 mm 2 , a conducting impedance of 0.06Ω and an energy loss of 0.96 W may be deduced and obtained. By comparison, the energy loss of this embodiment is approximately ¼ to ⅕ of that of the conventional one. 
         [0042]    In addition,  FIG. 7  is a schematic circuit block diagram of a DC to DC converter according to an embodiment of the present disclosure. It can be known from  FIG. 7  that, a DC to DC converter  50  comprises a control circuit  52 , a primary side coil  54 , a secondary side coil  56 , a rectifier circuit  20  (also referred to as a power conversion circuit, that is, the above power circuit  20  is applied as a rectifier circuit), and a capacitor  58 . 
         [0043]    The control circuit  52  receives a power source Vin and converts the power source into a voltage signal with a predetermined frequency. The primary side coil  54  receives the voltage signal. The secondary side coil  56  has a first end  56   a  and a second end  56   b . The secondary side coil  56  is configured corresponding to the primary side coil  54 , so as to generate a secondary side signal at the first end  56   a  and the second end  56   b  in response to the voltage signal of the primary side coil  54 . The secondary side signal is generated in response to the predetermined frequency of the voltage signal of the primary side. 
         [0044]    The second end  58   b  of the capacitor  58  is electrically connected to the second end  56   b  of the secondary side coil  56 . The rectifier circuit  20  has an anode  20 A and a cathode  20 C. The anode  20 A is electrically connected to the first end  56   a  of the secondary side coil  56 , and the cathode  20 C of the rectifier circuit  20  is electrically connected to the first end  58   a  of the capacitor  58 . 
         [0045]    The rectifier circuit  20  comprises an HEMT  30  and a gate driver circuit  40 . The gate driver circuit  40  comprises a first Zener diode  42 , a second Zener diode  44 , and a resistor  46 . After being serially connected as shown in  FIG. 7 , the resistor  46 , the second Zener diode  44 , and the first Zener diode  42  are connected in parallel with the HEMT  30 , so as to form the above rectifier circuit  20 . As the description made for the rectifier circuit  20 , the rectifier circuit  20  may be regarded as a rectifier diode according to the efficacies thereof. Therefore, if the rectifier circuit  20  is matched with a suitable capacitor  58 , the secondary side signal may be successfully rectified into a DC signal. Since the rectifier circuit  20  of this embodiment has an advantage of significantly reducing the circulation loss, the energy consumed by the DC to DC converter  50  may be effectively reduced if the rectifier circuit  20  is applied to the DC to DC converter  50 . 
         [0046]    Although the rectifier circuit  20  according to the present disclosure is applied to the DC to DC converter  50  in a manner shown in  FIG. 7 , it is not used to limit an application field of the rectifier circuit  20 , and the rectifier circuit  20  according to the present disclosure may be adopted in any occasion where the rectifier circuit is necessary, such as a power converter, a flyback converter, a forward converter, or a transformer. 
         [0047]    In addition, the power circuit  20  according to the present disclosure may also replace a freewheeling diode, for example, but is not limited to that both ends of the power circuit  20  are connected to two ends of an inductor and a resistor which are serially connected, and thus a freewheeling diode (also referred to as a flyback diode, a suppressor diode, and a catch diode) is formed, so as to eliminate a flyback phenomenon and a sudden voltage spike. 
         [0048]    Refer to  FIG. 8  for the application of the power circuit  20  as a freewheeling circuit.  FIG. 8  is a schematic circuit block diagram of a three-phase motor control circuit according to an embodiment of the present disclosure. It can be known from  FIG. 8  that, the three-phase motor control circuit comprises a DC power source  60 , a gate driver circuit  62 , a power transistor  64 , a freewheeling circuit  20  (that is, the power circuit  20  is applied as a freewheeling circuit), and a load  66 . 
         [0049]    The DC power source  60  generates a DC. The gate driver circuit  62  controls a gate of the power transistor  64 , so that the whole circuit generates a control signal of the three-phase motor for the load  66 . A detailed structure of the freewheeling circuit  20  is that of the above power circuit  20 . By applying the power circuit  20  to the three-phase motor control circuit, the power circuit  20  is a freewheeling circuit  20 . In the above example in which the power circuit  20  is applied to power conversion, the power circuit  20  is the above rectifier circuit  20  or the freewheeling circuit  20 .