Abstract:
An electric power converter includes an inverter, an insulating transformer, and a rectifier. The inverter converts an input DC voltage, supplied from a DC power supply, to an AC voltage outputted at an AC output side of the inverter, and includes at least one semiconductor switching device made of wide bandgap semiconductor material configured to carry out turning-on and turning-off operations at a specified frequency to thereby invert the DC voltage to the AC voltage at the specified frequency; and at least one freewheeling diode made of silicon-based semiconductor material respectively connected to the at least one switching device in inverse parallel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims foreign priority to Japanese Patent Application No. 2016-096705, filed May 13, 2016 in the Japanese Patent Office, the content of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
     1. Field 
       [0002]    The present disclosure relates to an electric power converter which converts a DC voltage to a high-frequency AC voltage by the operation of an inverter, supplies the converted high-frequency AC voltage to a transformer to obtain an AC voltage insulated from the AC voltage on the inverter side before converting thus insulated AC voltage to a DC voltage with a specified magnitude by using a rectifier circuit. 
       2. Related Art 
       [0003]    In an electric power converter such as an auxiliary power supply for rolling stock, for downsizing the converter, a technology is actualized by which an AC output voltage of an inverter is changed to a DC voltage with a specified magnitude by using a resonant circuit, a high-frequency insulating transformer and a rectifier circuit to supply the DC voltage to loads such as lighting facilities and air-conditioning facilities. 
         [0004]      FIG. 3  is a diagram showing the configuration of an electric power converter of such kind described in JP-A-2013-110786 (paragraphs [0022] to [0049] and FIG. 1, etc.) as an example of a related art. 
         [0005]    The electric power converter is provided with a DC power supply  10 , an inverter  20  of a half-bridge type including a resonant circuit, a transformer  30  for insulation, a rectifier circuit (rectifying and smoothing circuit)  40  and a control circuit  100  for controlling semiconductor switching devices in the inverter  20  and the rectifier circuit  40 . Onto the output side of the rectifier circuit  40 , a load  50  is connected. 
         [0006]    Here, the inverter  20  is provided with a series circuit with capacitors  21  and  22 , a series circuit with semiconductor switching devices  23  and  24  such as IGBTs and the series resonant circuit formed with a capacitor  25  and inductor  26 . In addition, the rectifier circuit  40  is provided with a bridge circuit with semiconductor switching devices  41  to  44  each allowing bidirectional current conduction and a capacitor  45  connected onto the output side of the bridge circuit. 
         [0007]    In the foregoing configuration according to the related art, the series resonant circuit formed of the capacitor  25  and inductor  26  on the primary winding side of the transformer  30  carries out a high-frequency series resonant operation by alternating turning-on and -off operations between the switching devices  23  and  25 , by which a high-frequency AC voltage is outputted from the secondary winding side of the transformer  30 . The high-frequency AC voltage is rectified and smoothed by the rectifier circuit  40  to be a DC voltage with a specified magnitude, which is supplied to the load  50 . In particular, by making the semiconductor switching devices  41  to  44  in the rectifier circuit  40  selectively turned-on and -off in synchronization with the turning-on and -off of the semiconductor switching devices  23  and  24 , the semiconductor switching devices  23 ,  24  and  41  to  44  are made to carry out switching with zero voltages and zero currents to thereby reduce the switching losses thereof. 
         [0008]    In the next,  FIG. 4  is a diagram showing the configuration of an electric power converter described in JP-A-2014-233121 (paragraphs [0009] to [0018] and FIG. 1, etc.) as another example of the related art. 
         [0009]    In  FIG. 4 , reference numerals  61 ,  62  and  63  designate a pantograph, wheels of an electric car and a step-up reactor, respectively. The electric power converter is provided with a step-up chopper  70 , a half-bridge inverter  80  and a rectifying and smoothing circuit  90 . The step-up chopper  70  is made up of a switching device  71  and a diode  72  and the inverter  80  is made up of a series circuit with capacitors  81  and  82  and a series circuit with switching devices  83  and  84 . The rectifying and smoothing circuit  90  is made up of diodes  91  to  94  in a bridge connection, a reactor  95  and a capacitor  96 . The other constituents are designated by reference numerals being the same as those shown in  FIG. 3 . 
         [0010]    In the configuration according to the foregoing related art, the DC voltage between the pantograph  61  and the wheels  62  is stepped up by the step-up chopper  70  and the DC voltage after being stepped up is inverted to an AC voltage by the inverter  80  before being supplied to the rectifying and smoothing circuit  90  through the transformer  30  to be converted into a DC voltage with a specified magnitude, which is then supplied to a load. 
         [0011]    In JP-A-2014-233121, there is described that a wide bandgap semiconductor device made of material such as SiC (silicon carbide) is used for the switching device  71  in the step-up chopper  70  or for each of the switching devices  83  and  84  in the inverter  80  to thereby allow the loss therein to be reduced. The use of a wide bandgap semiconductor device such as an SBD (Schottky barrier diode) also for a freewheeling diode connected in inverse parallel to each of the switching device  71 ,  83  and  84  can reduce the reverse recovery loss thereof. 
         [0012]    However, the use of wide bandgap semiconductor devices for all of the switching devices and freewheeling diodes forming a system such as an inverter causes an increase in the cost of each chip having the switching devices and diodes formed therein to result in an increase in the price of the whole system. 
         [0013]    In addition, in the electric power converter described in JP-A-2013-110786, each of the switching devices  23 ,  24 , and  41  to  44  carries out zero voltage and zero current switching to cause no reverse recovery current to flow in the freewheeling diode, for example, connected in inverse parallel to each of the switching devices  23  and  24 . Therefore, the use of wide bandgap semiconductor devices also for the freewheeling diodes results in so-called excessive improvement in quality to cause high cost of the system. 
         [0014]    Accordingly, this disclosure provides an electric power converter in which useless cost is reduced to permit a reduction in the price thereof. 
       SUMMARY 
       [0015]    For achieving the aforementioned benefits, a first aspect of the disclosure is that, in an electric power converter including an inverter in which at least one semiconductor switching device connected to a DC power supply carries out turning-on and -off operations at a specified frequency, thereby inverting a DC voltage supplied from the DC power supply to an AC voltage at the frequency to output the inverted AC voltage, an insulating transformer the primary winding of which is connected to the AC output side of the inverter, a rectifier circuit converting the AC voltage outputted from the secondary winding of the transformer to a DC voltage to supply the converted DC voltage to a load, the semiconductor switching device is made of wide bandgap semiconductor material and a freewheeling diode made of silicon-based semiconductor material is connected to the switching device in inverse parallel thereto. 
         [0016]    A second aspect of the disclosure is that, in the electric power converter in the first aspect of the disclosure, the inverter has a configuration in which a series circuit of a first and second ones of the semiconductor switching devices is connected across the DC power supply while being connected in parallel to a series circuit of first and second capacitors and, along with this, a resonant circuit, formed of a series connection of a capacitor and an inductor to carry out a resonant operation with the resonant frequency thereof determined as the specified resonant frequency, and the primary winding of the transformer are connected in series between the connection point of the first and second semiconductor switching devices and the connection point of the first and second capacitors, the first and second semiconductor switching devices being alternately turned-on and -off with a duty ratio of 50% by the resonant frequency of the resonant circuit. 
         [0017]    A third aspect of the disclosure is that, in the electric power converter in the first or the second aspect of the disclosure, the semiconductor switching device is an FET made of one of SiC, GaN and diamond as wide bandgap semiconductor material. 
         [0018]    In the disclosure, the switching devices in the configuration of the inverter are made of wide bandgap semiconductor material and the freewheeling diodes connected in inverse parallel to their respective switching devices are made of silicon-based semiconductor material. 
         [0019]    This reduces losses in the switching devices and, along with this, by making the switching devices alternately turned-on and -off by the resonant frequency of the resonant circuit with a duty ratio of 50%, makes it possible to prevent reverse recovery currents from flowing in the freewheeling diodes. Therefore, by the use of freewheeling diodes made of relatively low-priced silicon-based semiconductor material, the electric power convertor can be made to be less expensive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a diagram showing the configuration of the main circuit of the electric power converter according to an embodiment of the disclosure; 
           [0021]      FIG. 2  is a diagram showing the operation of the main circuit of the electric power converter according to the embodiment of the disclosure shown in  FIG. 1 ; 
           [0022]      FIG. 3  is a diagram showing the configuration of an electric power converter described in JP-A-2013-110786 as an example of a related art; and 
           [0023]      FIG. 4  is a diagram showing the configuration of an electric power converter described in JP-A-2014-233121 as another example of a related art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    In the following, an embodiment of the disclosure will be explained with reference to the attached drawings. 
         [0025]      FIG. 1  is a diagram showing the configuration of the main circuit of an electric power converter according to the embodiment of the disclosure. As is shown in the diagram, the electric power converter is provided with an inverter INV, a high-frequency insulating transformer Tr connected to the output side of the inverter INV and a rectifier circuit (rectifier and smoothing circuit) REC and carries out a DC-AC-DC conversion to feed a DC voltage with a specified magnitude to a load R. 
         [0026]    The inverter INV is provided with a DC power supply E d  (the voltage thereof is also designated as E d ), a series circuit of a first capacitor C dc1  and a second capacitor C dc2  and a series circuit of a first semiconductor switching device Q 1  and a second semiconductor switching device Q 2  both being made of any one of wide bandgap semiconductor materials such as SiC (silicon carbide), GaN (gallium nitride) and diamond. The series circuits are connected in parallel to each other across the DC power supply E d . Here, the switching devices Q 1  and Q 2  are FETs (Field Effect Transistors), for example, to which freewheeling diodes D 1  and D 2  of silicon-based semiconductor material are connected in inverse parallel, respectively. 
         [0027]    The capacitors C dc1  and C dc2  have capacitance values equal to each other with their respective shared voltages being E d /2. 
         [0028]    Between the connection point of the switching devices Q 1  and Q 2  and the connection point of the capacitors C dc1  and C dc2 , a capacitor C r , an inductor L r  and the primary winding N 1  of the transformer Tr are connected in series. Both ends of the secondary winding N 2  of the transformer Tr are connected to the rectifier circuit REC. Here, the capacitor C r  and inductor L r  form an LC resonant circuit. 
         [0029]    For the inductor L r , the leakage inductance of the primary winding N 1  of the transformer Tr can be utilized. 
         [0030]    The rectifier circuit REC is provided with a bridge circuit formed of diodes D 3  to D 6  and a smoothing capacitor C o  connected between the DC output terminals of the bridge circuit. The AC input side of the bridge circuit is connected to both ends of the secondary winding N 2  and, across the smoothing capacitor C o , a load R is connected. 
         [0031]    In the next, the operation of the embodiment will be explained with reference to  FIG. 2  as a waveform diagram showing the operation of the main circuit of the electric power converter according to the embodiment of the disclosure shown in  FIG. 1 . The positive directions of the currents and voltages shown in  FIG. 2  are determined as the directions of arrows attached to the signs of the corresponding currents and voltages shown in  FIG. 1 . 
         [0032]    First, the switching devices Q 1  and Q 2  forming the inverter INV are alternately switched with a duty ratio of 50% as is shown in  FIG. 2  by the resonant frequency of the LC resonant circuit formed of the capacitor C r  and the inductor L r . 
         [0033]    This provides the waveforms as are shown in  FIG. 2  for the current I c1  and voltage V CE1  of the switching device Q 1  and the current I c2  and the voltage V CE2  of the switching device Q 2 , by which a rectangular-wave-like voltage V Tr1  is applied to the primary winding N 1  of the transformer Tr to make a sinusoidal-wave-like current I r1  flow therein. The waveforms of the voltage V Tr1  and current I r1  are in phase with the waveforms of the currents I c1  and I c2  and the voltages V CE1  and V CE2  as the fundamental waves. 
         [0034]    In more detail, when the voltage V CE1  becomes 0V by the turning-on of the switching device Q 1  to allow the voltage E d /2 across the capacitor C dc1  to be applied to the LC resonant circuit, the current I c1  flows by the applied voltage E d /2 through the path of the capacitor C dc1 →the switching device Q 1 →the capacitor C r →the inductor L r →the primary winding N 1  of the transformer Tr→the capacitor C dc1 . 
         [0035]    In addition, when the voltage V CE2  becomes 0V by the turning-on of the switching device Q 2  to allow the voltage E d /2 across the capacitor C dc2  to be applied to the LC resonant circuit, the current I c2  flows by the applied voltage E d /2 through the path of the capacitor C dc2 →the primary winding N 1  of the transformer Tr→the inductor L r →the capacitor C r →the switching device Q 2 →the capacitor C dc2 . 
         [0036]    Thus, the current I r1  flowing in the primary winding N 1  of the transformer Tr becomes a current which is provided as a combination of the currents I c1  and I c2  (their respective current values are also designated as I c1  and I c2 ) to have a value I C1 -I C2  as is shown in  FIG. 2 . 
         [0037]    Moreover, the voltage V Tr2  and current I r2  of the secondary winding N 2  of the transformer Tr become in phase with the voltage V Tr1  and current I r1  of the primary winding N 1 , respectively. The current I r2  is subjected to full-wave rectification in the rectifier circuit REC to be a DC output current I o . Then, the DC output current I o  is supplied to a load R, from both ends of which a DC output voltage E o , which is smoothed by the smoothing capacitor C o , is outputted with a specified magnitude. 
         [0038]    By the foregoing operation, zero-current switching can be carried out at the timings of turning-on and -off the switching devices Q 1  and Q 2 . In addition, the use of devices made of wide bandgap semiconductor material for the switching devices Q 1  and Q 2  allows the devices to reduce switching losses, to be operated at high-speeds and to have high breakdown voltages. 
         [0039]    Further, at the turning-on of the switching devices Q 1  and Q 2 , their respective freewheeling diodes D 1  and D 2  have no reverse recovery currents flowing therein. Thus, even in the case of using inexpensive devices made of silicon-based semiconductor material for the freewheeling diodes D 1  and D 2 , there is no possibility of causing losses. 
         [0040]    Therefore, switching losses in the devices can be ideally made to be approximately zero. 
         [0041]    Although not shown in  FIG. 1 , a step-up chopper provided with a switching devices made of wide bandgap semiconductor material and a freewheeling diode made of silicon-based semiconductor material may be inserted as necessary between the DC power supply E d  and the series circuit of the capacitors C dc1  and C dc2  to raise the DC power supply voltage supplied from the DC power supply E d  and apply the raised DC voltage across the series circuit of the switching devices Q 1  and Q 2 . 
         [0042]    The electric power converter according to the disclosure can be utilized for various kinds of electric power converters and power supplies on which downsizing is strongly required like on auxiliary power supplies for rolling stock. 
         [0043]    Inclusion in this disclosure of any characterization of any product or method of the related art does not imply or admit that such characterization was known in the prior art or that such characterization would have been appreciated by one of ordinary skill in the art at the time a claimed was made, even if the product or method itself was known in the prior art at the time of invention of the present disclosure. For example, if a related art document discussed in the foregoing sections of this disclosure constitutes prior art, the inclusion of any characterization of the related art document does not imply or admit that such characterization of the related art document was known in the prior art or would have been appreciated by one of ordinary skill in the art at the time a claimed was made, especially if the characterization is not disclosed in the related art document itself. 
         [0044]    While the present disclosure has been particularly shown and described with reference to embodiment thereof, such as those discussed above, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the present invention.