Patent Publication Number: US-9431915-B2

Title: Power conversion apparatus and refrigeration air-conditioning apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of International Application No. PCT/JP2013/057794 filed on Mar. 19, 2013, the disclosure of which is incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a power converter and a refrigeration air-conditioning apparatus. 
     BACKGROUND ART 
     Along with increased practical uses of variable voltage variable frequency inverter devices and other devices, application fields of various kinds of power converter have been developed. 
     For example, technologies applied to a boost/buck converter have been actively developed for a power converter in recent years. Meanwhile, wide band-gap semiconductor elements and other elements containing silicon carbide or other materials as its material have also been actively developed. In regard to such novel elements, elements having a high breakdown voltage but a small current capacity (permissible current effective value) have been put into practical use mainly for rectifiers (see, for example, Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2005-160284 (FIG. 1) 
     SUMMARY OF INVENTION 
     Technical Problem 
     Meanwhile, the practical use of novel highly efficient elements, such as elements having a large current capacity, is accompanied by a large number of challenges in terms of high cost, crystal defects, and other such problems, and it is considered that it will take some time before such elements become widespread. Accordingly, it is difficult at present to use such a novel element to achieve high efficiency of a power converter for converting electric power higher than electric power to be supplied to a motor, for example, of a compressor of an air-conditioning apparatus. 
     The present invention has been made in view of the above-mentioned problem, and provides a power converter and the like, which are capable of securing high efficiency, high reliability, and others. The present invention is also aimed at further reducing a loss due to power conversion. 
     Solution to Problem 
     According to one embodiment of the present invention, there is provided a power converter for converting electric power between a power source and a load, comprising: a voltage changing device including a rectifier configured to prevent a backflow of a current from a load to a power source, the voltage changing device being configured to change a voltage of electric power supplied from the power source to a predetermined voltage; a commutation device configured to perform a commutation operation of directing a current flowing through the voltage changing device to an other path; and a controller configured to perform control related to the voltage change of the voltage hanging device and control related to the commutation operation of the commutation device, wherein the commutation device is configured to direct, when the commutation device performs the commutation operation, a current generating a voltage causing reverse recovery of the rectifier to the commutation device. 
     Advantageous Effects of Invention 
     According to the power converter in the one embodiment of the present invention, the commutation device capable of performing the commutation operation is provided, and hence the current flowing through the voltage changing device may be commutated to the other path. Consequently, for example, in the operation of the voltage changing device, a recovery current flowing from the load side (smoothing device side) to the voltage changing device side (power source side) may be reduced, and hence, for example, irrespective of the current capacity and other characteristics of an element used for the voltage changing device, loss by the current, namely, conduction loss, may be reduced through appropriate design of the configuration. In this case, the commutation device is configured so that the current generating the voltage causing reverse recovery of the rectifier may flow through the commutation device, and hence the loss in the commutation operation not directly contributing to the power conversion (voltage change) may be further reduced. According to the commutation operation of the commutation device configured in this manner, the loss may be reduced, and the power conversion may be performed with higher efficiency in the system as a whole. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 1 of the present invention. 
         FIG. 2  is a diagram illustrating an example (first example) of an operation mode of the system according to Embodiment 1 of the present invention. 
         FIG. 3  is a diagram illustrating an example (second example) of the operation mode of the system according to Embodiment 1 of the present invention. 
         FIG. 4  is a diagram illustrating an example (third example) of the operation mode of the system according to Embodiment 1 of the present invention. 
         FIG. 5  is a diagram illustrating an example (fourth example) of the operation mode of the system according to Embodiment 1 of the present invention. 
         FIG. 6  is a diagram illustrating the flow of a recovery current. 
         FIG. 7  is a diagram illustrating the waveforms of signals and the like at the time when commutation control is performed in the system according to Embodiment 1 of the present invention. 
         FIG. 8  is a diagram illustrating a path of a recovery current at the time of reverse recovery of a boost rectifier  23  according to Embodiment 1 of the present invention. 
         FIG. 9  is a diagram illustrating a path of a recovery current at the time of reverse recovery of a commutation rectifier  42  according to Embodiment 1 of the present invention. 
         FIG. 10  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 2 of the present invention. 
         FIG. 11  is a diagram illustrating a configuration of a commutation device in a power converter according to Embodiment 3 of the present invention. 
         FIG. 12  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 4 of the present invention. 
         FIG. 13  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 5 of the present invention. 
         FIG. 14  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 6 of the present invention. 
         FIG. 15  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 7 of the present invention. 
         FIG. 16  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 8 of the present invention. 
         FIG. 17  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 9 of the present invention. 
         FIG. 18  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 10 of the present invention. 
         FIG. 19  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 11 of the present invention. 
         FIG. 20  is a configuration diagram of a refrigeration air-conditioning apparatus according to Embodiment 13 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, a power converter and the like according to embodiments of the present invention are described with reference to the drawings. In the following drawings, including  FIG. 1 , the same or corresponding parts are denoted by the same reference symbols, which is common to the entire contents of the following embodiments. Then, the modes of components described herein are merely illustrative, and are not intended to be limited to those described herein. 
     Embodiment 1 
       FIG. 1  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 1 of the present invention. First, a description is given of the system configuration illustrated in  FIG. 1  including the power converter capable of performing highly efficient power conversion. 
     In the system illustrated in  FIG. 1 , the power converter is connected between a power source  1  and a load  9 . As the power source  1 , various kinds of power sources such as a DC power source, a single-phase power source, and a three-phase power source can be used. In the following description, the power source  1  is a DC power source. Further, the load  9  is, for example, a motor and an inverter device connected to the motor. 
     The power converter includes a boosting device (boosting circuit)  2  serving as a voltage changing device for boosting an applied voltage relating to power supply from the power source  1  to a predetermined voltage, a commutation device (commutation circuit)  4  for commutating a current flowing through the boosting device  2  to a different path (an other path) at a necessary timing, and a smoothing device (smoothing circuit)  3  for smoothing a voltage (output voltage) relating to operations of the boosting device  2  and the commutation device  4 . The power converter further includes a voltage detection device  5  for detecting the voltage obtained by the smoothing device  3 , and a controller  100  for controlling the boosting device  2  and the commutation device  4  based on the voltage relating to the detection by the voltage detection device  5 . The power converter further includes a drive signal transmission device  7  for converting a drive signal sa supplied from the controller  100  into a drive signal SA suitable for the boosting device  2  and transmitting the drive signal SA to the boosting device  2 , and a commutation signal transmission device  8  for converting a drive signal (commutation signal) sb supplied from the controller  100  into a drive signal SB suitable for the commutation device  4  and transmitting the drive signal SB to the commutation device  4 . 
     The boosting device  2  in this embodiment includes, for example, a magnetic energy storage unit  21  constructed with a reactor connected to the positive side or the negative side of the power source  1 , and a boost open/close switch unit  22  (power change open/close switch  22 ) and a boost rectifier  23  (power change rectifier  23 ) constructed with a rectifier, which are connected at a subsequent stage of the magnetic energy storage unit  21 . In this case, as illustrated in  FIG. 1 , the rectifier of the boost rectifier  23  has an anode on the point B side and a cathode on the point C side. The boost open/close switch unit  22  including a switching element, for example, is opened and closed based on the drive signal SA transmitted from the drive signal transmission device  7 , and controls electrical connection and electrical disconnection between the positive side and the negative side of the power source  1  via the boost open/close switch unit  22 . The type of semiconductor element used as the switching element is not particularly limited, but a high withstand voltage element that can withstand electric power supplied from the power source  1  is used (for example, an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), etc.). In this case, although not illustrated in  FIG. 1 , the boost open/close switch unit  22  is supplied with electric power for performing the open/close operation from a switch operation power source. Further, the boost rectifier  23  constructed with a rectifier such as a pn junction diode, for example, is a backflow preventing element for rectifying a current (electric power) from the power source  1  side to the load  9  side and preventing the backflow from the load  9  side to the power source  1  side. In this embodiment, a rectifier having a large current capacity is used depending on the magnitude of electric power to be supplied from the power source  1  to the load  9 . Further, in order to suppress electric power (energy) loss in the boost rectifier  23 , an element having a low forward voltage (good Vf characteristics) is used for the rectification. 
     Further, the commutation device  4  in this embodiment includes a transformer  41 , a commutation rectifier  42 , elements of a transformer drive circuit  43  for driving the transformer  41 , and other components. In  FIG. 1 , primary-side and secondary-side windings of the transformer  41  have the same polarity. Then, the secondary-side winding of the transformer  41  is connected in series to the commutation rectifier  42 . In addition, the commutation rectifier  42  is connected in parallel to the boost rectifier  23  of the boosting device  2 . 
     The transformer  41  including a pulse transformer, for example, constructs a commutation operation device together with the transformer drive circuit  43 . A voltage is applied to the primary-side winding to cause an excitation current to flow therethrough, to thereby induce a voltage in the secondary-side winding to cause a current to flow therethrough. In this manner, the current flowing through the boosting device  2  is commutated. In this case, the transformer  41  in this embodiment has the adjusted turns ratio between the primary-side winding and the secondary-side winding, and other such adjusted parameters. For example, the turns ratio between the primary-side winding and the secondary-side winding of the transformer  41  is adjusted to be A:B (A≧B, where B=1 or more). Further, the inductance ratio between the primary-side winding and the secondary-side winding is set to be substantially A 2 :B 2  (A 2 ≧B 2 , where B=1 or more). Through the adjustment of the turns ratio and other parameters, a surplus voltage can be suppressed while a voltage (approximately several V) equal to or higher than a voltage necessary for reverse recovery of the boost rectifier  23  (rectifier) is generated, and hence the reverse recovery can be performed without causing an excessive current to flow to the commutation device  4  side, and energy can be saved. Then, the above-mentioned effect can be obtained by a relatively easy method such as the adjustment of the turns ratio. 
     Further, the transformer  41  in this embodiment includes a reset winding connected to the primary-side winding. Through the connection of the reset winding, excitation energy can be regenerated on a transformer power source unit  45  side at the time of resetting so that electric power can be recovered, and hence the efficiency can be further increased. The transformer  41  is described in detail later. 
     The commutation rectifier  42  rectifies the current relating to the commutation (the current flowing through the other path). In this case, the commutation rectifier  42  includes a rectifier including, for example, a semiconductor element which has excellent electrical characteristics (in particular, recovery characteristics), has a small current capacity, and takes a short time to complete the reverse recovery. The rectifier included in the commutation rectifier  42  is located on a path of the electric power supplied from the power source  1  to the load  9 , and hence is required to be formed of a high withstand voltage element. Thus, in this case, a Schottky barrier diode made of silicon having good recovery characteristics in particular, or an element formed of, for example, a wide band-gap semiconductor containing silicon carbide (SiC), gallium nitride (GaN), or diamond as its material is used for the commutation rectifier  42 . 
     Further, in this embodiment, the transformer drive circuit  43  is constructed with a commutation switch  44 , the transformer power source unit  45 , a transformer drive rectifier  46 , and a transformer smoothing unit  47 . The commutation switch  44  including a switching element such as a transistor, for example, is opened or closed based on the commutation signal SB transmitted from the commutation signal transmission device  8 , to thereby supply electric power from the transformer power source unit  45  to the transformer  41  (primary winding side) or stop the supply of the electric power. In this case, the switching element may include an insulating unit for insulating the gate side and the drain (collector)-source (emitter) side from each other. In this case, it is preferred that the insulating unit be constructed with a photocoupler or a pulse transformer. Because the insulating unit is formed, the commutation device can be electrically disconnected from the control side such as the controller  100 , to thereby prevent an excessive current from flowing to the control side. The transformer power source unit  45  serves as, for example, a power source for supplying electric power to the transformer  41  so as to cause the commutation device  4  to perform the commutation operation. Then, the voltage to be applied from the transformer power source unit  45  to the transformer  41  is set to be lower than the voltage (output voltage) to be applied from the boosting device  2  and the commutation device  4  to the smoothing device  3 . In this case, although not particularly illustrated in  FIG. 1 , a limiting resistor, a high frequency capacitor, a snubber circuit, or a protective circuit may be inserted as necessary into a wiring path connecting the transformer power source unit  45 , the commutation switch  44 , and the primary-side winding of the transformer  41  in consideration of noise countermeasures, circuit protection in case of failure, and other circumstances. Further, the transformer power source unit  45  may be shared with the power source used for the boost open/close switch unit  22  to perform the opening and closing operation. The transformer drive rectifier  46  rectifies a current flowing through the transformer drive circuit  43  to supply electric power to the primary-side winding of the transformer  41 . Further, the transformer smoothing unit  47  including a capacitor smoothes the electric power from the transformer power source unit  45  and supplies the smoothed electric power to the primary-side winding. Because the transformer smoothing unit  47  is provided to smooth the electric power, for example, an abrupt fluctuation of the transformer power source unit  45 , an abrupt rise of the current, and other phenomena can be suppressed. 
     The smoothing device  3  is constructed with a smoothing capacitor, for example. The smoothing device  3  smoothes a voltage relating to the operation of the boosting device  2  and other devices, and applies the smoothed voltage to the load  9 . Further, the voltage detection device  5  detects the voltage (output voltage Vdc) smoothed by the smoothing device  3 . The voltage detection device  5  is constructed with a level shift circuit including voltage dividing resistors. In this case, when needed, the voltage detection device  5  may be added with an analog/digital converter in order to generate a signal (data) that can be used for the controller  100  to perform arithmetic processing and other processing. 
     The system in this embodiment further includes a current detection element  10  and a current detection device  11 . The current detection element  10  detects a current at a node between the power source  1  and the negative side of the boost open/close switch unit  22 . As the current detection element  10 , for example, a current transformer or a shunt resistor is used. When transmitting a current relating to the detection by the current detection element  10  as a signal, the current detection device  11  converts the current into a signal of a proper value (Idc) that can be processed by the controller  100 , and inputs the converted signal to the controller  100 . Thus, the current detection device  11  is constructed with an amplifier circuit, a level shift circuit, or a filter circuit. In this case, such circuit may be omitted as appropriate if the function of the current detection device  11  can be handled by the controller  100  instead. 
     The controller  100  performs the processing of generating and transmitting the drive signals based on the voltage relating to the detection by the voltage detection device  5  and/or the current relating to the detection by the current detection element  10  and the detection by the current detection device  11 . The power converter of  FIG. 1  includes both of the voltage detection device  5  and the set of the current detection element  10  and the current detection device  11 , but only one of the voltage detection device  5  and the set of the current detection element  10  and the current detection device  11  may be provided so that the controller  100  may perform the processing of generating the drive signals and other signals based only on the current or only on the voltage. 
     The controller  100  is constructed with an arithmetic unit such as a microcomputer and a digital signal processor, a device having an internal function similar to the arithmetic unit. In this embodiment, for example, based on the voltage and the current relating to the detection by the voltage detection device  5  and the detection by the current detection element  10  and the current detection device  11 , the controller  100  generates the signals for instructing the boost open/close switch unit  22  and the commutation switch  44  to operate, to thereby control the boosting device  2  and the commutation device  4 . In this case, although not illustrated in  FIG. 1 , the controller  100  is supplied with electric power for performing the processing operations from a controller operation power source. This power source may be shared with the transformer power source unit  45 . Further, in this embodiment, the controller  100  is described as being configured to control the operations of the boosting device  2  and the commutation device  4 , but is not limited thereto. For example, two controllers may control the boosting device  2  and the commutation device  4 , respectively. 
     The drive signal transmission device  7  is constructed with a buffer, a logic IC, or a level shift circuit, for example, and converts the drive signal sa into the drive signal SA to transmit the drive signal SA to the boosting device  2 . Note that, for example, when this function is built into the controller  100 , the drive signal transmission device  7  can be omitted as appropriate. In this case, the controller  100  only needs to transmit the drive signal sa as the drive signal SA to directly control the opening and closing operation of the boost open/close switch unit  22 . Further, similarly to the drive signal transmission device  7 , the commutation signal transmission device  8  is also generally constructed with a buffer, a logic IC, or a level shift circuit, and converts the commutation signal sb into the commutation signal Sb to transmit the commutation signal Sb to the commutation device  4 . Note that, when this function is built into the controller  100 , the commutation signal transmission device  8  can be omitted as appropriate. In this case, the controller  100  only needs to transmit the drive signal sb as the drive signal SB to directly control the opening and closing operation of the commutation switch  44 . In the following description, the drive signal SA is regarded as the same as the drive signal sa transmitted from the controller  100 , and the commutation signal SB is regarded as the same as the commutation signal sb (the drive signal SA and the commutation signal SB are thus referred to as “drive signal sa” and “commutation signal sb”). 
       FIG. 2  to  FIG. 5  are diagrams illustrating examples of operation modes of the system according to Embodiment 1 of the present invention. Next, the operation relating to the system of  FIG. 1  and other drawings is described. The power conversion operation (boosting operation in this embodiment) of the power converter in this system is realized by adding the commutation operation of the commutation device  4  to a boost chopper. Thus, there are four operation modes in total based on a combination of the open/close states of the boost open/close switch unit  22  and the commutation switch  44 . 
     First, the case of the state in which the boost open/close switch unit  22  is on (closed) and the commutation switch  44  is off (opened) is considered. In general, the boost rectifier  23  uses an element having a low forward voltage as compared to the commutation rectifier  42  having good recovery characteristics. Further, the winding of the transformer  41  is an inductance component, and hence no current flows when the excitation current is not caused to flow. Accordingly, in this case where the commutation switch  44  is off, no current flows through the path in which the commutation device  4  is provided (the other path). Then, because the boost open/close switch unit  22  is on, the positive side and the negative side of the power source  1  are electrically connected to each other and the current flows through the path of  FIG. 2  (thus, no current flows through the path via the boost rectifier  23 ). In this manner, energy can be stored in the magnetic energy storage unit  21 . 
     Next, the case where the boost open/close switch unit  22  is off and the commutation switch  44  is off is considered. Also in this case, because the commutation switch  44  is off, no current flows through the path in which the commutation device  4  is provided. Further, because the boost open/close switch unit  22  is off, the energy of the magnetic energy storage unit  21  can be supplied to the load  9  side via the smoothing device  3  through the path of  FIG. 3  (the path via the boost rectifier  23 ). 
     In addition, the case where the boost open/close switch unit  22  is on and the commutation switch  44  is on is considered. In this case, the commutation switch  44  is on, but the boost open/close switch unit  22  is also in the on state, and the impedance on the power source  1  side is low. Accordingly, almost no current flows through the path in which the commutation device  4  is provided. Thus, the current flows through the path of  FIG. 4 , and the energy can be stored in the magnetic energy storage unit  21 . This operation mode is an operation mode not used for control. The operation mode may be entered instantaneously due to a transmission delay of the commutation signal sb, but there is no particular problem for use. 
     Then, the case where the boost open/close switch unit  22  is off and the commutation switch  44  is on is considered. In this case, because the boost open/close switch unit  22  is off, the current flows into the load  9  side via the boost rectifier  23  (Current Path  1 ). Further, because the commutation switch  44  is on, the transformer  41  is excited, and as illustrated in  FIG. 5 , the current flows also through the path in which the commutation device  4  is provided (Current Path  2 ). Then, when this state lasts for a predetermined period of time, the current is completely commutated so that the current flows only through the path in which the commutation device  4  is provided. 
     According to the respective operation modes described above, the commutation operation is performed when the boost open/close switch unit  22  is off and the commutation switch  44  is on, but the operation of storing the energy in the magnetic energy storage unit  21  in response to the opening and closing of the boost open/close switch unit  22  follows the operation of the boost chopper. Accordingly, when the boost open/close switch unit  22  is repeatedly switched (opened/closed) for an on time T on  and an off time T off , the point C is applied with an average voltage Ec, Ec=(T on +T off )·E 1 /T off , and the voltage is thus boosted. For simplification, the voltage of the power source  1  is represented by E 1 . 
       FIG. 6  is a diagram illustrating the flow of a recovery current. When a pn junction diode, for example, is used for the boost rectifier  23 , a short-circuit current flows through the path illustrated in  FIG. 6  until the reverse recovery of the boost rectifier  23  is completed (until a reverse current is inhibited) (this short-circuit current is hereinafter referred to as “recovery current”). Then, the circuit loss is increased due to the recovery current flowing from the load  9  (smoothing device  3 ) side to the power source  1  side. Further, this current is responsible for displacement of a common-mode current, resulting in an increase in level of noise terminal voltage and radiation noise. Accordingly, cost is required for noise countermeasures. Further, a noise filter (not shown) is upsized, and the degree of freedom of installation space is limited. 
     Further, in general, a rectifier has a tendency that the amount of stored carriers increases as the current capacity increases. Accordingly, when the current capacity increases, the recovery current also increases due to a delay of reverse recovery. Further, the recovery current also increases as the applied reverse bias voltage becomes higher. 
     In view of the above, in this embodiment, the reverse recovery is not performed in a manner that a high reverse bias voltage is applied to the boost rectifier  23  having a large current capacity, but the reverse recovery is performed in a manner that the other path for commutation is provided and at the timing immediately before the boost open/close switch unit  22  is turned on (closed), a low reverse bias voltage is applied to the boost rectifier  23  via the transformer  41  and the commutation rectifier  42  of the commutation device  4 , and thereafter the boost open/close switch unit  22  is controlled to be turned on (this control is hereinafter referred to as “commutation control”). 
     Then, the controller  100  turns on the commutation signal sb for the commutation device  4  immediately before turning on the drive signal sa, to thereby generate the signal for commutating the current flowing through the boost rectifier  23  to the commutation rectifier  42  via the transformer  41 . 
       FIG. 7  is a diagram illustrating the waveforms of signals and the like at the time when the commutation control is performed in the system according to Embodiment 1 of the present invention. In  FIG. 7 , the waveforms of the drive signal sa, the commutation signal sb, a voltage V 1  relating to the primary-side winding of the transformer  41 , a voltage V 2  relating to the secondary-side winding of the transformer  41 , and currents I 1  to I 5  at the time when the commutation device  4  is operated (the commutation signal sb is transmitted) are illustrated. 
     As described above, the drive signal sa is a drive signal to be transmitted by the controller  100  in order to operate the boost open/close switch unit  22  of the boosting device  2 . Further, the commutation signal sb is a drive signal to be transmitted by the controller  100  in order to operate the commutation switch  44  of the commutation device  4 . In this case, the drive signal sa is a PWM signal in which the HI side is the active direction (on direction). When the drive signal sa is turned on, the boost open/close switch unit  22  is turned on (closed), and when the drive signal sa is turned off, the boost open/close switch unit  22  is turned off (opened). Further, the commutation signal sb is also a PWM signal in which the HI side is the active direction (on direction). Further, the respective current waveforms represent an example in which the on time and the off time of the drive signal sa are controlled so that the output voltage Vdc, that is, the output to the load  9 , may become constant after the power source  1  is powered on, and a sufficient period of time has elapsed thereafter. Then, the duty ratio (the ratio between the on time and the off time) of the drive signal sa shows a substantially constant value. 
     The voltage V 1  represents a schematic waveform of the voltage across the primary-side winding of the transformer  41 . Further, the voltage V 2  represents a schematic waveform of the voltage across the secondary-side winding of the transformer  41 . 
     The current I 1  represents the waveform of the current flowing between the power source  1  and the boosting device  2  (magnetic energy storage unit  21 ). The current I 2  represents the waveform of the current flowing through the boost open/close switch unit  22  of the boosting device  2 . The current I 3  represents the waveform of the current flowing between the point A and the point B of  FIG. 1 . In this case, the current I 1  branches into the current I 2  and the current I 3  (I 1 =I 2 +I 3 ). 
     Further, the current I 4  represents the waveform of the current flowing through the boost rectifier  23 . The current I 5 A represents the waveform of the current flowing through the primary winding of the transformer  41 . The current I 5 B represents the waveform of the current flowing through the secondary winding of the transformer  41 . In this case, the current I 3  branches into the current I 4  and the current I 5 B (I 3 =I 4 +I 5 B). 
     In the power converter in this embodiment, the turns ratio between the primary-side winding and the secondary-side winding of the transformer  41  is adjusted, and hence, as illustrated in  FIG. 7 , the magnitudes of the voltage V 1  and the voltage V 2  can be arbitrarily set to be different from each other. Further, the magnitudes of the current I 5 A and the current I 5 B are also different from each other. Through the adjustment of the voltage V 2 , the electric power relating to the commutation can be suppressed to save the energy. 
     Next, the relationship between the drive signal sa and the commutation signal sb and the currents flowing is described with reference to  FIG. 1  and  FIG. 7 . When the commutation signal sb is turned on immediately before the drive signal sa is turned on (the boost open/close switch unit  22  is turned on), the current starts to flow through the secondary-side winding of the transformer  41  due to the excitation current. Accordingly, the current starts to flow while branching into the boost rectifier  23  side and the commutation rectifier  42  side (the other path). After that, when the on state of the commutation signal sb is maintained, the current no longer flows to the boost rectifier  23  side, and all the currents flow to the commutation rectifier  42  side (the commutation is completed). 
     At this time, the applied voltage relating to the transformer power source unit  45  is set to be sufficiently lower than the output voltage of the boosting device  2  (such as the potential between the point C and the point D). In this manner, the boost rectifier  23  can be turned off (reverse recovery) even with a low reverse bias voltage. 
     Then, in this state, the drive signal sa is turned on. At this time, the reverse recovery operation is performed in the commutation rectifier  42 . Also in this case, the recovery current is generated. However, the current supply period in the reverse recovery of the commutation rectifier  42  is significantly shorter than that for the boost rectifier  23 , and hence the value of the effective current required for the commutation rectifier  42  can be set to be small. Consequently, an element which stores a small amount of carriers and has a small current capacity can be used, and hence the recovery current can be reduced as compared to the boost rectifier  23  (note that, an element is selected in consideration of the peak current). 
       FIG. 8  is a diagram illustrating a path of the recovery current during the reverse recovery of the boost rectifier  23  according to Embodiment 1 of the present invention. When the commutation signal sb changes from off to on, the recovery current during the reverse recovery of the boost rectifier  23  flows through the path from the secondary-side winding of the transformer  41  (the side connected to the commutation rectifier  42 ) to the secondary-side winding of the transformer  41  (the point B side of  FIG. 3 ) via the commutation rectifier  42  and the boost rectifier  23  in the stated order. 
     In this case, the voltage necessary for causing the current relating to the reverse recovery of the boost rectifier  23  to flow through the commutation device  4  depends on the voltage level of the transformer power source unit  45  of the commutation device  4 . For example, in the case where the transformer power source unit  45  can supply electric power independently of the system, as exemplified by an external power source, the voltage level of the transformer power source unit  45  may be adjusted. Meanwhile, there may be a case where it is desired to use a power source for generating necessary electric power in the system due to system constraints. In such a case, for example, arbitrary one output, such as a switching power source installed in the system in order to acquire a controller power source, is used. 
     The commutation device  4  performs the commutation operation in order to suppress the generation of the recovery current in the boost rectifier  23 . Thus, if the voltage causing the reverse recovery of the boost rectifier  23  can be obtained to cause a corresponding current to flow, as the electric power relating to the commutation operation not directly contributing to power conversion becomes lower, the efficiency is increased and the energy is saved more. However, this power source cannot necessarily apply an appropriate voltage in the operation of the commutation device  4 . If an excessive voltage higher than the voltage causing the reverse recovery of the boost rectifier  23  is applied so that the current corresponding to the applied voltage flows, the recovery loss is increased by the amount of electric power expressed by the product of the applied voltage and the recovery current. Further, if the application of the appropriate voltage is attempted to be achieved by multi-output of the switching power source, such as providing an additional output, the cost of the system is increased. 
     In view of the above, in this embodiment, the winding ratio and other parameters of the transformer  41  are appropriately set depending on the voltage level of the transformer power source unit  45  so that, in the reverse recovery of the boost rectifier  23 , an appropriate voltage can be applied to the commutation device  4  side and an appropriate current can be caused to flow therethrough without being wasted. 
     When the winding ratio between the primary-side winding and the secondary-side winding of the transformer  41  is A:B, and when the commutation switch  44  is turned on so that the voltage V 1  is induced in the primary-side winding, the voltage V 2  of the secondary-side winding is V 2 =(B/A)·V 1 . When the inductance ratio is A 2 :B 2 , the voltage V 2  of the secondary-side winding is V 2 =(B 2 /A 2 )·V 1 . Because A≧B is established, the voltage V 2  can be set to be equal to or lower than the voltage V 1  through the adjustment of the windings of the transformer  41 . In this manner, the voltage relating to the secondary-side winding and the voltage relating to the primary-side winding are uniquely determined based on the winding ratio and the inductance ratio. 
     In consideration of the impedance of the circuit pattern, the on voltage of the switch, and other characteristics, the windings of the transformer  41  of the commutation device  4  are set so that an appropriate voltage causing the reverse recovery of the boost rectifier  23  of the boosting device  2  can be applied across the boost rectifier  23 . Because the appropriate voltage can be applied to the commutation device  4  side, the reverse recovery of the boost rectifier  23  can be performed with a voltage not higher than necessary, and hence the loss can be reduced. 
     Further, as illustrated in  FIG. 7 , when the current ISA flows through the primary winding and the current I 5 B flows through the secondary-side winding at the time of completion of the commutation, A·I 5 A=B·I 5 B is established in accordance with the law of equal ampere-turns. Thus, the current ISA flowing through the primary-side winding of the transformer  41  is B/A times as large as the current I 5 B flowing through the secondary-side winding, and hence the return current on the primary winding side can be suppressed to be smaller than the current flowing on the secondary winding side. Thus, a necessary voltage can be applied without the need of overspecification of the current capacity of each element connected to the primary winding. Consequently, through the setting of the turns of the windings of the transformer  41 , the recovery loss can be reduced without significantly increasing the cost. In the system in this embodiment, the values of the winding ratio and the inductance ratio between the primary winding and the secondary-side winding are each adjusted to be different from each other as a basic configuration, but this is not intended to prevent the adjustment of the winding ratio and the inductance ratio to be 1:1. 
       FIG. 9  is a diagram illustrating a path of the recovery current during the reverse recovery of the commutation rectifier  42  according to Embodiment 1 of the present invention. When the commutation signal sb changes from on to off, the recovery current flows through the path from the smoothing device  3  (positive side) to the smoothing device  3  (negative side) via the commutation rectifier  42  and the boost open/close switch unit  22  in the stated order. 
     As a result, according to the system in Embodiment 1, the commutation device  4  is provided in the power converter, and the current flowing through the boosting device  2  is commutated to the smoothing device  3  side through the other path, and hence the reverse recovery of the boost rectifier  23  is performed before the boost open/close switch unit  22  is turned on so that the recovery current, which flows in response to the turn-on of the boost open/close switch unit  22 , may flow not via the boost rectifier  23  which has a low forward voltage but through which a large amount of recovery current flows but via the commutation rectifier  42  which is short in time relating to the reverse recovery and has good recovery characteristics. Consequently, the recovery current in the power converter can be reduced. Further, the current flows through the boost rectifier  23  having a low forward voltage when the commutation operation is not performed (normal state), and hence the loss during the operation of the power conversion of the boosting device  2  can also be suppressed. Consequently, for example, even when an element having a large current capacity is used for the boost rectifier  23 , the recovery loss and the conduction loss can be reduced irrespective of the current capacity of the element, the recovery characteristics of the element, and other characteristics in the boosting device  2 . Thus, although the commutation operation of the commutation device  4  and other operations are performed, the loss and the noise amount (level of noise terminal voltage, radiation noise, etc.) caused by the recovery current can be reduced in the system as a whole. 
     Then, in this embodiment, the turns ratio and other parameters between the primary-side winding and the secondary-side winding of the transformer  41  are adjusted so that the voltage of the secondary-side winding in the commutation operation can be prevented from being surplus while securing a voltage equal to or higher than the voltage causing the reverse recovery of the boost rectifier  23 , and hence the reverse recovery can be performed without an excessive current flowing to the commutation device  4  side. Consequently, the electric power relating to the commutation operation not directly contributing to the power conversion can be reduced, and hence the loss can be reduced in the power converter as a whole to save the energy. Then, this effect can be easily realized through the adjustment of the turns ratio and other parameters of the transformer  41 . Further, an abrupt rise of the current can be suppressed due to the inductance component of the transformer  41 , and hence the generation of noise can be suppressed. Consequently, the present invention is applicable also to an apparatus for handling a large capacity in which noise is liable to be generated, irrespective of the capacity and other characteristics. 
     Further, the reset winding is provided to the primary-side winding of the transformer  41  of the commutation device  4 , and hence the electric power can be recovered at the time of resetting, and the transformer  41  can be operated with high efficiency. In addition, in the commutation device  4 , the transformer smoothing unit  47  is provided between the transformer power source unit  45  for the transformer and the primary-side winding of the transformer  41 , and hence the supply of electric power in which the abrupt fluctuation of the transformer power source unit  45  and the abrupt rise of the current are suppressed can be performed. 
     Further, the wide gap-band semiconductor is used for the commutation rectifier  42 , and hence the power converter with low loss can be obtained. Further, because the electric power loss is small, the efficiency of the element can be increased. A wide gap-band semiconductor is high in permissible current density, and hence the use of a wide gap-band semiconductor can downsize the element and also downsize the apparatus in which the element is incorporated. A wide gap-band semiconductor can also be used for another element than the commutation rectifier  42 , for example, the commutation switch  44 , which does not affect the loss in the system as a whole. 
     In this case, instead of the wide gap-band semiconductor, for example, a Schottky barrier diode having a low forward voltage and a high breakdown voltage with a small loss may be used for the commutation rectifier  42 . When such element has a larger permissible current effective value according to its specifications, crystal defects are increased and the cost is increased. According to the power converter (system) in this embodiment, the period during which the current flows through the other path is short, and hence an element having a small permissible current effective value (having a small current capacity) can be used for the rectifier in the commutation device. Consequently, the power converter with high cost performance and high efficiency can be realized. 
     Further, the boosting device  2 , the secondary-side winding of the transformer  41 , and the commutation rectifier  42  can be insulated from the transformer drive circuit  43 , the controller  100 , and the commutation signal sb via the transformer  41 , and hence the commutation signal sb (commutation signal SB) can be transmitted relatively easily. Then, the device applied with high voltage and the device operating with low voltage can be electrically separated from each other. Further, the system with high safety and high reliability can be constructed. In this embodiment, the commutation operation device is constructed with the transformer  41  and the transformer drive circuit  43 , but the device configuration can be modified as long as the commutation operation of commutating the current to the other path can be performed, although the above-mentioned effects may not be exerted. 
     Embodiment 2 
       FIG. 10  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 2 of the present invention. In  FIG. 10 , the devices and the like denoted by the same reference symbols as those in  FIG. 1  perform the same operations and the like as those described in Embodiment 1. 
     In  FIG. 10 , similarly to the commutation switch  44  described in Embodiment 1, commutation switches  44   a  and  44   b  control the supply of electric power from the transformer power source unit  45  to the primary winding of the transformer  41  and the stop of the supply based on the commutation signal sb. According to the system in this embodiment, both of the commutation switches  44   a  and  44   b  are controlled to be opened or closed based on the commutation signal sb, and hence even when one of the commutation switches  44   a  and  44   b  undergoes a short-circuit failure, for example, the commutation operation can be continued. Consequently, the reliability of the system (apparatus) can be enhanced to protect the system. 
     Embodiment 3 
       FIG. 11  is a diagram illustrating a configuration of a commutation device in a power converter according to Embodiment 3 of the present invention. In  FIG. 11 , the devices and the like denoted by the same reference symbols as those in  FIG. 1  perform the same operations and the like as those described in Embodiment 1. 
     In  FIG. 11 , a current detection unit  200  includes a current detection element, and transmits a signal relating to the current flowing through the primary-side winding of the transformer  41  (transformer drive circuit  43 ) to the controller  100 . The current detection unit  200  includes a current transformer or a resistor. When the controller  100  determines, based on the signal transmitted from the current detection unit  200 , that a current higher than a preset possible current value flows, the controller  100  stops the transmission of the commutation signal sb to turn off the commutation switch  44 . The operation of the commutation switch  44  is stopped so that no current flows through the transformer drive circuit  43 , to thereby stop the commutation operation of the commutation device  4 . In this manner, the reliability of the system (apparatus) can be enhanced to protect the system. Further, whether or not to shorten the period of the commutation operation or to stop the commutation device  4  is determined based on the detected current. In this manner, magnetic flux saturation of the transformer  41  and other phenomena can be prevented to enhance the reliability. 
     Embodiment 4 
       FIG. 12  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 4 of the present invention. In  FIG. 12 , the devices and the like denoted by the same reference symbols as those in  FIG. 1  perform the same operations and the like as those described in Embodiment 1. A current limiting unit  48  in this embodiment includes a resistor, for example, and limits the current flowing through the commutation device  4  in the commutation operation. 
     In Embodiment 1 and other embodiments described above, the transformer  41  is provided, and the winding ratio and other parameters of the transformer  41  are adjusted. Then, a voltage which is equal to or higher than the voltage causing the reverse recovery of the boost rectifier  23  and which is not excessive is applied to the secondary-side winding, to thereby prevent an excessive current from flowing to the commutation device  4  side. In this embodiment, the current limiting unit  48  is used to adjust so that the current flowing through the commutation device  4  in the commutation operation is prevented from being excessive. 
     The use of the current limiting unit  48  as in this embodiment can simplify the circuit configuration of the commutation device  4 . In this case, the current rises abruptly as compared to the case where the transformer  41  is used as in Embodiment 1 and other embodiments. Noise may be generated, but it is effective to apply this configuration to an apparatus for converting electric power having a relatively small capacity. 
     Embodiment 5 
       FIG. 13  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 5 of the present invention. In  FIG. 13 , the devices and the like denoted by the same reference symbols as those in  FIG. 12  perform the same operations and the like as those described in Embodiment 4. 
     The power converter in this embodiment includes, as illustrated in  FIG. 13 , instead of the transformer power source unit  45  in Embodiment 4, a power source generation device  6  for generating the power source for the commutation device  4  based on the electric power supplied from the power source  1 . In this case, in  FIG. 13 , the power source generation device  6  is illustrated as being independent of the commutation device  4 , but may not particularly be independent instead. 
     The power source generation device (power source generation circuit)  6  in this embodiment includes a power source generation smoothing unit  62  and a switching power source unit  63 . The switching power source unit  63  converts the supplied electric power into electric power for driving the commutation device  4 . In this embodiment, the switching power source unit  63  is constructed with a DC/DC converter for performing the conversion based on electric power supplied from the power source  1  being a DC power source to the power converter. Further, the power source generation smoothing unit  62  smoothes the electric power from the switching power source unit  63 . 
     As described above, according to the power converter in this embodiment, the electric power to be supplied to the commutation device  4  can be acquired in the system. 
     Embodiment 6 
       FIG. 14  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 6 of the present invention. In  FIG. 14 , the devices and the like denoted by the same reference symbols as those in  FIG. 1 ,  FIG. 13 , and other drawings perform the same operations and the like as those described in Embodiment 1, Embodiment 5, and other embodiments. 
     The power converter in this embodiment includes, as illustrated in  FIG. 14 , instead of the transformer power source unit  45  which constructs a part of the transformer drive circuit  43  in Embodiment 1 and other embodiments, the power source generation device  6  for generating the power source for the transformer drive circuit  43  based on the electric power supplied from the power source  1 . In this case, in  FIG. 14 , the power source generation device  6  is illustrated as being independent of the transformer drive circuit  43 , but may not particularly be independent instead. 
     The power source generation device (power source generation circuit)  6  in this embodiment includes a power source generation smoothing unit  62  and a switching power source unit  63 . The switching power source unit  63  converts the supplied electric power into electric power for driving the transformer drive circuit  43  (transformer  41 ). In this embodiment, the switching power source unit  63  is constructed with a DC/DC converter for performing the conversion based on electric power supplied from the power source  1  being a DC power source to the power converter. Further, the power source generation smoothing unit  62  smoothes the electric power from the switching power source unit  63  and supplies the smoothed electric power to the transformer drive circuit  43  (primary-side winding of the transformer  41 ). 
     As described above, according to the power converter in this embodiment, the electric power to be supplied to the commutation device  4  (transformer drive circuit  43 ) can be acquired in the system. 
     Embodiment 7 
       FIG. 15  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 7 of the present invention. In  FIG. 15 , the devices and the like denoted by the same reference symbols as those in  FIG. 14  and other drawings perform the same operations and the like as those described in Embodiment 6 and other embodiments. 
     In the power converter in this embodiment, the boosting device  2  includes a transformer unit  25 . The transformer unit  25  is constructed with a transformer. In the transformer unit  25 , a voltage is induced in a secondary-side winding based on a current flowing through a primary-side winding in response to the opening and closing of the boost open/close switch unit  22 , and the induced voltage is applied to the power source generation device  6 . Further, the power source generation device  6  includes a power source generation rectifier  61 . The power source generation rectifier  61  is constructed with a rectifier such as a diode, and rectifies the current flowing based on the voltage applied by the transformer unit  25 . Then, the power source generation smoothing unit  62  smoothes the rectified current to supply electric power to the transformer drive circuit  43  (the primary-side winding of the transformer  41 ) to the primary-side winding of the transformer  41 . Alternatively, the transformer unit  25  may be included in the magnetic energy storage unit  21 . In other words, at least a part of the magnetic energy storage unit  21  may be used like a transformer, and an auxiliary (secondary) winding may be provided to the reactor to extract a part of energy, to thereby supply electric power required for the power source generation device  6 . In this manner, the number of components may be reduced to downsize the apparatus depending on various conditions such as the system configuration and the load. 
     As described above, according to the power converter in this embodiment, the electric power to be supplied to the commutation device  4  (transformer drive circuit  43 ) can be acquired from the power converter (boosting device  2 ). The boost open/close switch unit  22  of the boosting device  2  can be used, and hence the number of elements (components) for generating the power source for the commutation device  4  can be suppressed to reduce the cost. Further, the operation of the boosting device  2  and the operation of the commutation device  4  can be synchronized with each other. For example, when the boosting device  2  is not operating, no recovery current is generated and the commutation device  4  is not required to be operated, and hence standby power can be reduced. In addition, the base circuit can be easily shared among circuit boards forming the devices except for the commutation device  4 . 
     Embodiment 8 
       FIG. 16  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 8 of the present invention. In  FIG. 16 , the devices and the like denoted by the same reference symbols as those in  FIG. 15  and other drawings perform the same operations and the like as those described in Embodiment 7 and other embodiments. 
     In this embodiment, the configuration devices and others are the same as those in Embodiment 7. In Embodiment 7, the transformer unit  25  is connected in parallel to the boost rectifier  23  (the transformer unit  25  is connected between the point A and the boost open/close switch unit  22 ). In this embodiment, the transformer unit  25  is connected in series to the boost rectifier  23  (the transformer unit  25  is connected between the magnetic energy storage unit  21  and the point A). Even when the power converter is configured as described above, the electric power to be supplied to the commutation device  4  (transformer drive circuit  43 ) can be acquired from the power converter (boosting device  2 ), and the same effects as those of the power converter in Embodiment 5 and other embodiments are exerted. 
     Embodiment 9 
       FIG. 17  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 9 of the present invention. In  FIG. 17 , the devices and the like denoted by the same reference symbols as those in  FIG. 14  and other drawings perform the same operations and the like as those described in Embodiment 6 and other embodiments. 
     In the power converter in this embodiment, the power source  1  is constructed with a single-phase AC power source  1   a  and a rectifying device  1   b  such as a diode bridge. Then, electric power supplied to the load  9  being the output of the power converter is also supplied to the power source generation device  6 . Even when the power source in the system is applied to the single-phase AC power source in this manner, the same effects as those described above in each of the embodiments can be exerted. An impedance detection unit  110  detects an impedance ZC between the single-phase AC power source  1   a  and the rectifying device  1   b , and transmits a detection signal zc to the controller  100 . 
     Embodiment 10 
       FIG. 18  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 10 of the present invention. In  FIG. 18 , the devices and the like denoted by the same reference symbols as those in  FIG. 14  and other drawings perform the same operations and the like as those described in Embodiment 6 and other embodiments. 
     In the power converter in this embodiment, the power source  1  is constructed with a three-phase AC power source  1   c  and the rectifying device  1   b  such as the diode bridge. Further, electric power supplied to the load  9  being the output of the power converter is also supplied to the power source generation device  6 . Even when the power source in the system is applied to the three-phase AC power source in this manner, the same effects as those described above in each of the embodiments can be exerted. 
     Embodiment 11 
       FIG. 19  is a diagram illustrating a system configuration mainly including a power converter according to Embodiment 11 of the present invention. In  FIG. 19 , the devices and the like denoted by the same reference symbols as those in  FIG. 1  and other drawings perform the same operations and the like as those described in Embodiment 1 and other embodiments. 
     In the power converter in this embodiment, in the boosting device  2 , a current interruption unit (interruption device)  27 , such as a fuse or a protective switch, for interrupting a circuit when an excessive current flows therethrough is connected on a path of a current flowing from the power source  1  side to the load  1  side. Consequently, the power converter (system) can be protected. 
     Embodiment 12 
     In the above-mentioned embodiments, a description has been given of the power converter in which the boosting device  2  is subjected to the commutation by the commutation device  4  and which performs power conversion by boosting the voltage of the power source  1 , but the present invention is not limited thereto. The same effects as those described above in each of the embodiments can be exerted even in a power converter in which the boosting device  2  is replaced with a voltage changing device as exemplified by a buck device and a boost/buck device, which is capable of converting electric power to be supplied to the load  9  through the change in voltage. 
     Embodiment 13 
       FIG. 20  is a configuration diagram of a refrigeration air-conditioning apparatus according to Embodiment 13 of the present invention. In this embodiment, a description is given of a refrigeration air-conditioning apparatus to be supplied with electric power via the above-mentioned power converter. The refrigeration air-conditioning apparatus of  FIG. 20  includes a heat source-side unit (outdoor unit)  300  and a load-side unit (indoor unit)  400 . The heat source-side unit  300  and the load-side unit  400  are coupled to each other via refrigerant pipes, to thereby form a main refrigerant circuit to circulate refrigerant. In the refrigerant pipes, one pipe through which gas refrigerant flows is referred to as “gas pipe  500 ”, and the other pipe through which liquid refrigerant (sometimes, two-phase gas-liquid refrigerant) flows is referred to as “liquid pipe  600 ”. 
     In this embodiment, the heat source-side unit  300  includes respective devices (units), namely, a compressor  301 , an oil separator  302 , a four-way valve  303 , a heat source-side heat exchanger  304 , a heat source-side fan  305 , an accumulator  306 , a heat source-side expansion device (expansion valve)  307 , an intermediate heat exchanger  308 , a bypass expansion device  309 , and a heat source-side controller  310 . 
     The compressor  301  compresses and discharges the sucked refrigerant. In this case, the compressor  301  can arbitrarily change an operating frequency thereof so that the capacity of the compressor  301  (the amount of refrigerant sent per unit time) can be finely changed. Then, the power converter described above in each of the embodiments is mounted between the power source  1  for supplying electric power for driving the compressor  301  (motor) and the compressor  301  and other devices serving as the load  9 . 
     The oil separator  302  separates lubricant oil which is mixed in the refrigerant and discharged from the compressor  301 . The separated lubricant oil is returned to the compressor  301 . The four-way valve  303  switches the flow of the refrigerant between a cooling operation and a heating operation based on an instruction from the heat source-side controller  310 . Further, the heat source-side heat exchanger  304  exchanges heat between the refrigerant and the air (outside air). For example, in the heating operation, the heat source-side heat exchanger  304  functions as an evaporator, and exchanges heat between low-pressure refrigerant flowing into the heat source-side heat exchanger  304  via the heat source-side expansion device  307  and the air, to thereby evaporate and gasify the refrigerant. On the other hand, in the cooling operation, the heat source-side heat exchanger  304  functions as a condensor, and exchanges heat between refrigerant flowing into the heat source-side heat exchanger  304  from the four-way valve  303  side and compressed by the compressor  301  and the air, to thereby condense and liquefy the refrigerant. The heat source-side fan  305  is provided to the heat source-side heat exchanger  304  in order to efficiently exchange heat between the refrigerant and the air. The heat source-side fan  305  may also be supplied with electric power via the power converter described above in each of the embodiments, and, for example, an operating frequency of a fan motor may be arbitrarily changed by an inverter device serving as the load  9  so that the rotation speed of the fan may be finely changed. 
     The intermediate heat exchanger  308  exchanges heat between refrigerant flowing through a main passage of the refrigerant circuit and refrigerant branching from the passage to have the flow rate adjusted by the bypass expansion device  309  (expansion valve). In particular, when the refrigerant needs to be subcooled in the cooling operation, the intermediate heat exchanger  308  subcools the refrigerant and supplies the subcooled refrigerant to the load-side unit  400 . Liquid flowing via the bypass expansion device  309  is returned to the accumulator  306  via a bypass pipe. The accumulator  306  is a unit for storing excess liquid refrigerant, for example. The heat source-side controller  310  is constructed with a microcomputer, for example. Then, the heat source-side controller  310  can communicate to and from the load-side controller  404  through wired or wireless connection, and, for example, based on data relating to detection by various kinds of detection units (sensors) included in the refrigeration air-conditioning apparatus, controls the respective devices (units) of the refrigeration air-conditioning apparatus, such as the control of the operating frequency of the compressor  301  by inverter circuit control, to thereby control the operation of the overall refrigeration air-conditioning apparatus. Further, the processing performed by the controller  100  described above in each of the embodiments may be performed by the heat source-side controller  310 . 
     Besides, the load-side unit  400  includes a load-side heat exchanger  401 , a load-side expansion device (expansion valve)  402 , a load-side fan  403 , and a load-side controller  404 . The load-side heat exchanger  401  exchanges heat between the refrigerant and the air. For example, in the heating operation, the load-side heat exchanger  401  functions as a condensor, and exchanges heat between refrigerant flowing into the load-side heat exchanger  401  from the gas pipe  500  and the air, to thereby condense and liquefy the refrigerant (or turn the refrigerant into two-phase gas-liquid state), and discharges the refrigerant to the liquid pipe  600  side. On the other hand, in the cooling operation, the load-side heat exchanger  401  functions as an evaporator, and exchanges heat between refrigerant reduced in pressure by the load-side expansion device  402  and the air, to thereby cause the refrigerant to receive the heat of the air to evaporate and gasify the refrigerant, and discharge the refrigerant to the gas pipe  500  side. Further, the load-side fan  403  for adjusting the flow of the air subjected to heat exchange is provided to the load-side unit  400 . The operating speed of the load-side fan  403  is determined based on user&#39;s setting, for example. The load-side expansion device  402  is provided in order to regulate the pressure of the refrigerant in the load-side heat exchanger  401  by being changed in opening degree. 
     Further, the load-side controller  404  is also constructed with a microcomputer, and can communicate to and from the heat source-side controller  310  through wired or wireless communication, for example. The load-side controller  404  controls the respective devices (units) of the load-side unit  400  based on an instruction from the heat source-side controller  310  or an instruction from a resident so that, for example, the indoor space may have a predetermined temperature. Further, the load-side controller  404  transmits a signal including data relating to detection by a detection unit provided to the load-side unit  400 . 
     As described above, in the refrigeration air-conditioning apparatus according to Embodiment 13, the power converter according to each of the above-mentioned embodiments is used to supply electric power to the compressor  301 , the heat source-side fan  305 , and other devices. Consequently, the highly efficient, highly reliable, and power saving refrigeration air-conditioning apparatus can be obtained. 
     INDUSTRIAL APPLICABILITY 
     In Embodiment 13 described above, a description has been given of the case where the power converter according to the present invention is applied to a refrigeration air-conditioning apparatus, but the present invention is not limited thereto. The power converter according to the present invention is applicable also to a heat pump apparatus, an apparatus using a refrigeration cycle (heat pump cycle) such as a refrigerator, a conveyance apparatus such as an elevator, and a lighting apparatus (system). 
     REFERENCE SIGNS LIST 
     power source  1   a  single-phase AC power source  1   b  rectifying device  1   c  three-phase AC power source  2  boosting device  3  smoothing device  4  commutation device  5  voltage detection device  6  power source generation device  7  drive signal transmission device  8  commutation signal transmission device  9  load  10  current detection element  11  current detection device  21  magnetic energy storage unit  22  boost open/close switch unit  22  power change open/close switch unit  23  power change rectifier  23  boost rectifier  25  transformer  27  current interruption unit  41  transformer  42  commutation rectifier  43  transformer drive circuit  44 ,  44   a ,  44   b  commutation switch  45  transformer power source unit  46  transformer drive rectifier  47  transformer smoothing unit  48  current limiting unit  61  power source generation rectifier  62  power source generation smoothing unit  63  switching power source unit  100  controller  110  impedance detection unit  200  current detection unit  300  heat source-side unit  301  compressor  302  oil separator  303  four-way valve  304  heat source-side heat exchanger  305  heat source-side fan  306  accumulator  307  heat source-side expansion device  308  intermediate heat exchanger  309  bypass expansion device  310  heat source-side controller  400  load-side unit  401  load-side heat exchanger  402  load-side expansion device  403  load-side fan  404  load-side controller  500  gas pipe  600  liquid pipe