Power converting apparatus

In a power converting apparatus which converts AC power into DC power, an inverter circuit including at least one series-connected single-phase inverter is connected in a downstream of a stage in which an AC input is rectified in series therewith. In the downstream stage of the inverter circuit, there are provided a smoothing capacitor connected via a rectifier diode and a short-circuiting switch for bypassing the smoothing capacitor. The short-circuiting switch is set to an ON state only in each of short-circuiting phase ranges of which midpoint matches each of zero-crossing phases and an output of the inverter circuit is controlled by using a current command so that a DC voltage of the smoothing capacitor follows a target voltage and an input power factor is improved.

TECHNICAL FIELD

The present invention relates to a power converting apparatus which converts AC power into DC power, the power converting apparatus being provided with a circuit for improving an input power factor.

BACKGROUND ART

A conventional power converting apparatus is configured to full-wave rectify input AC power by a diode bridge with a reactor connected to one end of the diode bridge and a switching device connected between a downstream end of the reactor and the other output end of the diode bridge. Connected downstream of this configuration via a diode is an output stage to perform input current control for improving an input power factor and voltage control of the output stage by turning on and off the aforementioned switching device (refer to Patent Document 1, for example).

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

A great deal of loss and noise occur in the aforementioned kind of power converting apparatus because an AC voltage is switched at high frequency by a semiconductor switch for controlling a current fed from an AC power supply. Also, if the switching frequency is lowered to avoid this problem, there would arise the need for a large current-limiting reactor in order to achieve a satisfactory input power factor.

The present invention has been made to solve the aforementioned problems. Accordingly, it is an object of the invention to lower power loss and noise and achieve a reduction in system structure by making it unnecessary to employ a large current-limiting circuit in a power converting apparatus which converts AC power into DC power by performing input current control for improving an input power factor and voltage control of an output stage.

Means for Solving the Problem

A power converting apparatus according to this invention includes a rectification circuit for rectifying an input from an AC input power supply, an inverter circuit in which AC sides of at least one single-phase inverter are connected in series and connected to an output of the rectification circuit in series, each single-phase inverter having a plurality of semiconductor switching devices and a DC voltage source, the inverter circuit being configured to superimpose the sum of an output of each single-phase inverter on the output of the rectification circuit, a smoothing capacitor connected to a downstream end of the inverter circuit via a rectifier diode for smoothing an output of the inverter circuit, and a short-circuiting switch of which one end is connected to the inverter circuit and the other end is connected to one end of the smoothing capacitor. The output of the inverter circuit is controlled by using a current command so that a voltage of the smoothing capacitor follows a target voltage and an input power factor of the AC input power supply is improved.

Another power converting apparatus according to this invention includes an inverter circuit in which AC sides of at least one single-phase inverter are connected in series and connected to a first terminal of an AC input power supply in series, each single-phase inverter having a plurality of semiconductor switching devices and a DC voltage source, the inverter circuit being configured to superimpose the sum of an output of each single-phase inverter on an AC input, a smoothing capacitor disposed in a downstream stage of the inverter circuit for smoothing an output of the inverter circuit, and first and second series circuits each of which is configured with a short-circuiting switch and a rectifier diode which are connected in series between both terminals of the smoothing capacitor. A halfway point of the first series circuit is connected to an AC output line of the inverter circuit at a downstream end thereof whereas a halfway point of the second series circuit is connected to a second terminal of the AC input power supply. The output of the inverter circuit is controlled by using a current command so that a voltage of the smoothing capacitor follows a target voltage and an input power factor of the AC input power supply is improved.

Advantageous Effects of the Invention

According to the present invention, it is not necessary for a short-circuiting switch to perform high-frequency switching and an inverter circuit configured to improve an input power factor and control a voltage in an output stage can decrease a voltage handled in switching operation to a relatively low voltage. For this reason, it is possible to reduce switching loss and noise without the need for a large current-limiting circuit, and this makes it possible to provide a power converting apparatus which permits a reduction in power loss and noise as well as a reduction in system structure.

BEST MODES FOR CARRYING OUT THE INVENTION

First Embodiment

A power converting apparatus according to a first embodiment of this invention is now described below. FIG.1is a general configuration diagram of the power converting apparatus according to the first embodiment of the invention.

As depicted inFIG. 1, an AC voltage source1(hereinafter referred to simply as the AC power supply1) which serves as an AC input power supply is connected to a diode bridge2which serves as a rectification circuit. An output of the diode bridge2is connected to a reactor3which serves as a current-limiting circuit and an AC side of an inverter circuit100configured with a single-phase inverter is series-connected to a downstream end of the reactor3. The single-phase inverter constituting the inverter circuit100includes semiconductor switching devices4,5, diodes6,7and a DC voltage source8. An insulated-gate bipolar transistor (IGBT) in which diodes are connected in reverse parallel or a metal oxide semiconductor field effect transistor (MOSFET) incorporating a diode connected between a source and a drain is used as each of the semiconductor switching devices4,5. The diodes6,7may also be configured with semiconductor switching devices like the semiconductor switching devices4,5. Also, the reactor3may be series-connected to the inverter circuit100at a downstream stage thereof.

In the downstream stage of the inverter circuit100, there are connected a short-circuiting switch9and a rectifier diode10of which cathode is connected to a positive electrode of a smoothing capacitor11provided in an output stage. Here, a connecting point between the short-circuiting switch9and an anode of the rectifier diode10is connected to an AC output line at the downstream stage of the inverter circuit100and the other end of the short-circuiting switch9is connected to a negative electrode of the smoothing capacitor11. While the short-circuiting switch9is illustrated as being configured with a semiconductor switching device in which diodes are connected in reverse parallel, the invention is not limited to this configuration but the short-circuiting switch9may be a mechanically acting switch, for instance.

The working of the power converting apparatus thus configured is now described with reference to waveforms at individual parts depicted inFIG. 2.

An input from the AC power supply1is full-wave rectified by the diode bridge2and a voltage Vin and a current Iin observed at a downstream end of the diode bridge2have waveforms as depicted inFIG. 2. Indicated by vdc is a DC voltage of the smoothing capacitor11that is controlled to match a specific target voltage Vdc*. In this case, a peak voltage of the voltage Vin is made higher than the DC voltage Vdc of the smoothing capacitor11.

The inverter circuit100produces an output by controlling the current Iin by pulse width modulation (PWM) control so that an input power factor of the AC power supply1becomes approximately 1, and superimposes a voltage generated on the AC side on the voltage Vin at the downstream end of the diode bridge2. When the semiconductor switching devices4,5are off, a current flowing in the inverter circuit100charges the DC voltage source8through the diode6and is output through the diode7as depicted inFIGS. 3 to 5. When only the semiconductor switching device4is turned on, the current is output by way of the semiconductor switching device4and the diode7. Similarly, when only the semiconductor switching device5is turned on, the current is output by way of the diode6and the semiconductor switching device5. Also, when the semiconductor switching devices4,5are turned on at the same time, the DC voltage source8is discharged through the semiconductor switching device4and the current is output through the semiconductor switching device5. The inverter circuit100is controlled by PWM control by controlling the semiconductor switching devices4,5by combinations of the above-described four kinds of control operation.

Expressing the phase of an input voltage from the AC power supply1by θ and the phase when the voltage Vin becomes equal to the target voltage Vdc* of the smoothing capacitor11by θ=θ2(0<θ2<π/2), the short-circuiting switch9is kept in an ON state from a point of phase θ=0 to a point of a specific phase θ1where 0<θ1<θ2. In this case, a current from the AC power supply1flows through a path routed along the AC power supply1, the diode bridge2, the reactor3, the inverter circuit100, the short-circuiting switch9, the diode bridge2and the AC power supply1in this order as depicted inFIG. 3. Since the short-circuiting switch9is in the ON state, no current flows through the rectifier diode10and the smoothing capacitor11in the output stage. The inverter circuit100produces the output by controlling the current Iin by PWM control so that the input power factor becomes approximately 1 while generating a voltage that is approximately equal to a reversal of the voltage Vin using a combination of the case in which both of the semiconductor switching devices4,5are off and the case in which only the semiconductor switching device4is on, for example. Energy is charged into the DC voltage source8of the inverter circuit100during this period.

Next, if the short-circuiting switch9is turned off when the phase θ=θ1, the current from the AC power supply1flows through a path routed along the AC power supply1, the diode bridge2, the reactor3, the inverter circuit100, the rectifier diode10, the smoothing capacitor11, the diode bridge2and the AC power supply1in this order as depicted inFIG. 4.

When the phase θ satisfies θ1≦θ≦θ2, the inverter circuit100produces the output by PWM control using a combination of the case in which the semiconductor switching devices4,5are on at the same time and the case in which only the semiconductor switching device4is on, for example. At this time, the inverter circuit100produces the output by controlling the current Iin so that the input power factor becomes approximately 1 while generating a voltage that is approximately equal to Vdc*−Vin in order that the DC voltage Vdc of the smoothing capacitor11can be maintained at the target voltage Vdc*. Since the polarity of the voltage generated by the inverter circuit100and the polarity of the current Iin are the same during this period, the DC voltage source8of the inverter circuit100is discharged.

Next, if the voltage Vin becomes equal to the DC voltage Vdc* of the smoothing capacitor11when the phase θ=θ2, the inverter circuit100operates in a different way although the short-circuiting switch9remains in an OFF state.

Specifically, when the phase θ satisfies θ2≦θ≦π/2, the current from the AC power supply1flows through a path routed along the AC power supply1, the diode bridge2, the reactor3, the inverter circuit100, the rectifier diode10, the smoothing capacitor11, the diode bridge2and the AC power supply1in this order as depicted inFIG. 5. Also, the inverter circuit100produces the output by PWM control using a combination of the case in which both of the semiconductor switching devices4,5are off and the case in which only the semiconductor switching device5is on, for example. At this time, the target voltage Vdc* of the smoothing capacitor11and the voltage Vin have a relationship expressed by Vdc*≦Vin and the inverter circuit100produces the output by controlling the current Iin so that the input power factor becomes approximately 1 while generating a voltage that is approximately equal to Vin−Vdc*, the voltage having polarity opposite to that of the voltage Vin, in order that the DC voltage Vdc of the smoothing capacitor11can be maintained at the target voltage Vdc*. Since the polarity of the voltage generated by the inverter circuit100and the polarity of the current Iin are opposite to each other during this period, the DC voltage source8of the inverter circuit100is charged.

As depicted in theFIG. 2, the power converting apparatus operates in symmetrical patterns during a phase period of π/2≦θ≦π and during a phase period of 0≦θ≦π/2 while the power converting apparatus operates in the same pattern during a phase period of π≦θ≦2π and during a phase period of 0≦θ≦π.

The power converting apparatus switches the short-circuiting switch9at specified phases which are defined as zero-crossing phases (θ=0, π) of the phase θ of the input voltage from the AC power supply1±θ1. Specifically, the power converting apparatus sets the short-circuiting switch9to the ON state to bypass the smoothing capacitor11only in each of phase ranges of ±θ1(hereinafter referred to as short-circuiting phase ranges20) of which midpoint matches each of the zero-crossing phases. At this time, the inverter circuit100produces the output by controlling the current Iin so that the input power factor becomes approximately 1 while generating a voltage that is approximately equal to a reversal of the voltage Vin, thereby charging the DC voltage source8. At phases not falling within the aforementioned short-circuiting phase ranges20, the inverter circuit100maintains the DC voltage Vdc of the smoothing capacitor11at the target voltage Vdc* and produces the output by controlling the current Iin so that the input power factor becomes approximately 1. In this case, the DC voltage source8is discharged if the voltage yin is equal to or lower than the target voltage Vdc* of the smoothing capacitor11whereas the DC voltage source8is charged if the voltage Vin is equal to or higher than the target voltage Vdc*.

If θ1is made larger, energy charged into the DC voltage source8increases, making it possible to superimpose the generated voltage on the voltage Vin in a high voltage range and increase the amount of discharged energy in a subsequent discharging process. For this reason, it is possible to increase the DC voltage Vdc (target voltage Vdc*) of the smoothing capacitor11.

In the phase period of 0≦θ≦π/2, as aforementioned, the DC voltage source8of the inverter circuit100is charged during periods 0≦θ≦θ1and θ2≦θ≦π/2 and discharged during a period θ1≦θ≦θ2. If the amount of energy charged into the DC voltage source8of the inverter circuit100and the amount of energy discharged therefrom are equal to each other, the following equation is satisfied, where Vp is the peak voltage of the voltage Vin and Ip is a peak current of the current Iin:
∫θθ1Vpsin θ·Ipsin θ·dθ+∫θ2π/2(Vpsin θ−Vdc*)·Ipsin θ·dθ=∫θ1θ2(Vdc*−Vpsin θ)·Ipsin θ·dθ(1)

If Vin=Vp·sin θ and Iin=Ip·sin θ, Vdc*=Vp·π/(4 cos θ1). This indicates that the target voltage Vdc* of the smoothing capacitor11is determined by θ1which defines the short-circuiting phase range20and, thus, the target voltage Vdc* can be controlled by varying θ1. Then, the DC voltage Vdc of the smoothing capacitor11is controlled to follow the target voltage Vdc*.

Expressing the voltage of the DC voltage source8of the inverter circuit100by Vsub, the inverter circuit100can reliably perform the above-described preferable control operation if the voltage Vsub is set to become equal to or higher than a voltage desirably generated by the inverter circuit100in each of the phase periods 0≦θ≦θ1, θ1≦θ≦θ2and θ2≦θ≦π/2. Specifically, if the voltage Vsub is set to satisfy three conditions expressed by Vp·sin θ1≦Vsub, (Vdc*−Vp·sin θ1)≦Vsub and (Vp−Vdc*)≦Vsub, it is possible to maintain the DC voltage Vdc of the smoothing capacitor11at the target voltage Vdc* and causes the inverter circuit100to reliably perform the control operation for controlling the current Iin in such a manner that the input power factor becomes approximately 1 at all phases of the AC power supply1. Incidentally, the voltage Vsub of the DC voltage source8is set to become equal to or lower than the peak voltage Vp of Vin.

Described next in the following are details of the control operation performed by the inverter circuit100for maintaining the voltage Vdc of the smoothing capacitor11at the target voltage Vdc* and controlling the current Iin so that the input power factor becomes approximately 1.

The inverter circuit100is controlled by control blocks as depicted inFIG. 6. First, using a difference21abetween the DC voltage Vdc of the smoothing capacitor11in the output stage and the target voltage Vdc* as a feedback quantity, an output22ais calculated by proportional-integral (PI) feedback control. Also, to maintain the voltage Vsub of the DC voltage source8of the inverter circuit100at a constant level, an output22bis calculated by PI feedback control using a difference21bbetween the voltage Vsub and a target voltage Vsub* thereof as a feedback quantity, and an amplitude target value23of the current Iin is determined from the sum of the two outputs22a,22b. Then, on the basis of this amplitude target value23, a sine-wave current command Iin* synchronized with the voltage Vin is generated. Next, using a difference24between the current command Iin* and the detected current Iin as a feedback quantity, an output is obtained by PI feedback control as a voltage command25which serves as a target value of the voltage generated by the inverter circuit100. At this time, the voltage command25is corrected by adding thereto a feedforward correction voltage ΔV which is synchronized with timings of turning on and off the short-circuiting switch9. Then, using a corrected voltage command26(the uncorrected voltage command25at other than the timings of turning on and off the short-circuiting switch9), driving signals to be fed into the individual semiconductor switching devices4,5of the inverter circuit100are generated in order to operate the inverter circuit100.

The short-circuiting switch9is switched between the ON and OFF states at the specified phases which are defined as the zero-crossing phases (θ=0, π) of the input voltage from the AC power supply1±θ1. The inverter circuit100switches from control mode in which the DC voltage source8is charged to control mode in which the DC voltage source8is discharged when the short-circuiting switch9is switched from the ON state to the OFF state, whereas the inverter circuit100switches from the control mode in which the DC voltage source8is discharged to the control mode in which the DC voltage source8is charged when the short-circuiting switch9is switched from the OFF state to the ON state. As the voltage command25is corrected by adding thereto the feedforward correction voltage ΔV which is synchronized with the timings of turning on and off the short-circuiting switch9as described above, it is possible to avoid a delay in control operation by as much as a feedback control response time. Incidentally, the feedforward correction voltage ΔV is a voltage having positive polarity when the short-circuiting switch9is switched from the ON state to the OFF state and is a voltage having negative polarity when the short-circuiting switch9is switched from the OFF state to the ON state.

In this embodiment, the power converting apparatus is controlled so that the DC voltage Vdc of the smoothing capacitor11follows the target voltage Vdc* and the input power factor of the AC power supply1is improved by controlling the inverter circuit100by using the current command mentioned above. It is not necessary for the short-circuiting switch9to perform high-frequency switching and the inverter circuit100configured to improve the input power factor and control the DC voltage Vdc in the output stage can decrease a voltage handled in switching operation to a level significantly lower than a peak voltage of the AC power supply1. For this reason, it is possible to reduce switching loss and noise without the need for a reactor3having a large capacity. Also, when the short-circuiting switch9is in the ON state, the DC voltage source8of the inverter circuit100can be charged while bypassing the smoothing capacitor11and, therefore, it is possible to flow the current Iin through the AC power supply1without causing the inverter circuit100to generate a high voltage and charged energy can be used for discharging into the smoothing capacitor11. For this reason, it is possible to further decrease the voltage handled in the switching operation and achieve a higher efficiency and a further reduction in noise.

It is to be noted that the reactor3is not an element for storing energy but serves as a current-limiting circuit for limiting a current to thereby improve the reliability of current control.

Also, advantageous effects of the higher efficiency and the reduction in noise mentioned above are obtained in a reliable fashion with the voltage Vsub of the DC voltage source8that is a DC voltage of the inverter circuit100set at a level equal to or lower than the peak voltage Vp of Vin.

Also, as the short-circuiting switch9is operated only at the specified phases of the input voltage from the AC power supply1, it is possible to control the power converting apparatus in a stable fashion while producing almost no loss caused by the switching operation. Also, since the smoothing capacitor11is bypassed by keeping the short-circuiting switch9in the ON state only in each of the short-circuiting phase ranges20of ±θ1of which midpoint matches each of the zero-crossing phases θ=0, π, it is not necessary to deliver any output to the smoothing capacitor11in regions where the voltage Vin is low and the power converting apparatus can be configured with the DC voltage of the inverter circuit100set low. This makes it possible to obtain advantageous effects including the higher efficiency and the reduction in noise in a reliable fashion.

Additionally, because the target voltage Vdc* of the smoothing capacitor11is controlled by θ1in each of the short-circuiting phase ranges20, it is possible to easily control the target voltage Vdc*, thereby providing improved degrees of freedom in design and control.

Also, as the inverter circuit100is controlled to switch between operations for charging and discharging the DC voltage source8by using feedforward control at the timings of turning on and off the short-circuiting switch9, it is possible to avoid a delay in control operation by as much as the feedback control response time and thereby achieve high-speed control.

Additionally, since the power converting apparatus is controlled to maintain the voltage Vsub of the DC voltage source8at a constant level by varying the current command, it is possible to control the power converting apparatus in a stable fashion. It is also possible to balance the amounts of energy charged into and discharged from the DC voltage source8and this makes it unnecessary to supply DC power from an external source and serves to provide a simplified system structure.

Incidentally, the voltage of the DC voltage source8may be controlled from an external source and, in this case, operation for controlling the output of the inverter circuit100need not involve control operation for maintaining the voltage Vsub at a constant level.

While the peak voltage of the voltage Vin is made higher than the DC voltage Vdc of the smoothing capacitor11in the foregoing embodiment, the former may be made lower than the latter. In this case, the power converting apparatus performs no operation in the aforementioned phase range θ2≦θ≦π/2 but performs operation for charging the DC voltage source8in a phase range 0≦θ≦θ1and operation for discharging the DC voltage source8in a phase range θ1≦θ≦π/2.

It is also possible to keep the short-circuiting switch9constantly in the OFF state with01set to satisfy θ1=0 and, in this case, the power converting apparatus performs operation for discharging the DC voltage source8in a phase range050502and operation for charging the DC voltage source8in a phase range θ2≦θ≦π/2.

Also, while the foregoing embodiment has been described with reference to an arrangement in which the cathode of the rectifier diode10is connected to the positive electrode of the smoothing capacitor11in the output stage, the rectifier diode10may be arranged on a negative electrode side of the smoothing capacitor11so that the negative electrode thereof is connected to the anode of the rectifier diode10, yet obtaining the same operation as in the foregoing embodiment.

Also, while the foregoing embodiment has been described with reference to an arrangement in which the inverter circuit100is configured with one single-phase inverter, the power converting apparatus may be reconfigured to include an inverter circuit200in which AC sides of a plurality of single-phase inverters100a,100bare connected in series as depicted inFIG. 7. In this case, the sum of outputs of the individual single-phase inverters100a,100bproduces an output of the inverter circuit200and the power converting apparatus is controlled such that the DC voltage of the smoothing capacitor11follows a target voltage and the input power factor of the AC power supply1is improved by using a current command as in the foregoing embodiment. Then, the inverter circuit200superimposes a voltage generated on the AC side on the voltage Vin at the downstream end of the diode bridge2. In this case, the inverter circuit200may produce an output by gradational control operation in which a steplike voltage waveform is generated from the sum of the outputs of the plurality of single-phase inverters or by performing PWM control of a specified one of the plurality of single-phase inverters.

Second Embodiment

Although one end of the short-circuiting switch9is connected to the AC output line of the inverter circuit100in the above-described first embodiment, one end of a short-circuiting switch9ais connected to a negative electrode side of the DC voltage source8which forms part of the inverter circuit100in this second embodiment as illustrated inFIG. 8. The other end of the short-circuiting switch9ais connected to the negative electrode side of the smoothing capacitor11, or one end of the diode bridge2, in the same fashion as in the foregoing first embodiment.

The inverter circuit100and the short-circuiting switch9aare controlled in the same way as in the foregoing first embodiment. When the short-circuiting switch9ais in the ON state, or when the phase θ of the input voltage from the AC power supply1falls within any of the short-circuiting phase ranges20of the zero-crossing phases (θ=0, π) ±θ1, however, there is formed a current path as indicated inFIG. 9in this embodiment. The current from the AC power supply1flows through the path routed along the AC power supply1, the diode bridge2, the reactor3, the semiconductor switching device4of the inverter circuit100, the short-circuiting switch9a, the diode bridge2and the AC power supply1or along the AC power supply1, the diode bridge2, the reactor3, the diode6of the inverter circuit100, the DC voltage source8, the short-circuiting switch9a, the diode bridge2and the AC power supply1. At this time, energy is charged into the DC voltage source8of the inverter circuit100until the short-circuiting switch9ais turned off at a point where θ=θ1as in the foregoing first embodiment. After the short-circuiting switch9ahas been turned off, there are formed current paths that are the same as those indicated inFIGS. 4 and 5in the foregoing first embodiment.

The second embodiment discussed heretofore provides the same advantageous effects as obtained from the foregoing first embodiment. In addition, as the short-circuiting switch9ais connected to the negative electrode side of the DC voltage source8, it is possible to reduce the number of elements through which the current flows when the short-circuiting switch9ais turned on, decrease conduction loss and improve conversion efficiency of the entirety of a power converting apparatus.

In a case where an inverter circuit200is configured with AC sides of a plurality of single-phase inverters100a,100bconnected in series-as depicted inFIG. 7, the power converting apparatus operates in the same way and produces the same effects if the short-circuiting switch9ais connected to the negative electrode side of the DC voltage source8of the single-phase inverter100bwhich is one of the plurality of single-phase inverters100a,100bconnected at a downstream end thereof.

Third Embodiment

Next, a power converting apparatus according to a third embodiment of this invention is described with reference toFIG. 10.

As depicted inFIG. 10, an output from a first terminal of the AC power supply1is connected to the reactor3and an AC side of an inverter circuit300configured with a single-phase inverter is series-connected to a downstream end of the reactor3. The single-phase inverter provided in the inverter circuit300includes semiconductor switching devices4,5,16,17each of which is made up of an IGBT in which diodes are connected in reverse parallel or a MOSFET incorporating a diode connected between a source and a drain as well as a DC voltage source8.

Also, a halfway point of a first series circuit15awhich constitutes an inverter in which a short-circuiting switch12amade up of a semiconductor switching device and a rectifier diode13aare connected in series is connected to an AC output line at a downstream end of the inverter circuit300. Further, a halfway point of a second series circuit15bwhich constitutes an inverter in which a short-circuiting switch12bmade up of a semiconductor switching device and a rectifier diode13bare connected in series is connected to a second terminal of the AC power supply1. The first and second series circuits15a,15bare connected parallel to each other between both terminals of the smoothing capacitor11provided in the output stage.

In this case, the individual short-circuiting switches12a,12bare not limited to the semiconductor switching devices but may be mechanically acting switches. Diodes14aand14bshould however be connected in reverse parallel with the short-circuiting switches12aand12b, respectively.

In the working of the power converting apparatus thus configured, the inverter circuit300produces an output by controlling the current Iin by PWM control so that the DC voltage Vdc of the smoothing capacitor11is maintained at the specific target voltage Vdc* and the input power factor of the AC power supply1becomes approximately 1, and superimposes a voltage generated on the AC side on the voltage Vin input from the AC power supply1in the same fashion as in the foregoing first embodiment. The power converting apparatus switches the short-circuiting switches12a,12bat specified phases which are defined as zero-crossing phases (θ=0, π) of the phase θ of the input voltage from the AC power supply1±θ1. Specifically, the power converting apparatus sets the short-circuiting switches12a,12bin the ON state to bypass the smoothing capacitor11only in each short-circuiting phase range20of ±θ1of which midpoint matches each of the zero-crossing phases.

If the short-circuiting switches12a,12bare set to the ON state when the polarity of the AC power supply1is positive and the phase θ of the voltage Vin falls within a range 0≦θ≦θ1in the short-circuiting phase range20, for example, a current flows through a path routed along the AC power supply1, the reactor3, the inverter circuit300, the short-circuiting switch12a, the short-circuiting switch12band the AC power supply1in this order as depicted inFIG. 11. If the short-circuiting switches12a,12bare set to the ON state when the polarity of the AC power supply1is negative and the phase θ of the voltage Vin falls within a range π≦θ≦π+θ1, for example, a current flows through a path routed along the AC power supply1, the short-circuiting switch12b, the short-circuiting switch12a, the inverter circuit300, the reactor3and the AC power supply1in a direction reversed with respect to the path depicted inFIG. 11. At this time, the inverter circuit300produces the output by controlling the current Iin by PWM control so that the input power factor becomes approximately 1 while generating a voltage that is approximately equal to a reversal of the voltage Vin. Energy is charged into the DC voltage source8of the inverter circuit300during this period.

While the short-circuiting switches12a,12bare set to the ON state at the same time within the short-circuiting phase range20as discussed above, this approach may be so modified as to set only the short-circuiting switch12ato the ON state when the polarity of the AC power supply1is positive and set only the short-circuiting switch12bto the ON state when the polarity of the AC power supply1is negative. In the latter case, a current flows through one of the diodes14a,14bconnected to the other one of the short-circuiting switches12a,12b.

If the short-circuiting switches12a,12bare turned off when the phase θ of the voltage Vin falls within a range of any of the zero-crossing phases (θ=0, π) ±θ1, a current flows in a below-described manner. When the polarity of the AC power supply1is positive, the current flows through a path routed along the AC power supply1, the reactor3, the inverter circuit300, the rectifier diode13a, the smoothing capacitor11, the diode14bof the short-circuiting switch12band the AC power supply1in this order as depicted inFIG. 12. When the polarity of the AC power supply1is negative, the current flows through a path routed along the AC power supply1, the rectifier diode13b, the smoothing capacitor11, the diode14aof the short-circuiting switch12a, the inverter circuit300, the reactor3and the AC power supply1in this order. At this time, the inverter circuit300maintains the DC voltage Vdc of the smoothing capacitor11at the target voltage Vdc* and produces the output by controlling the current Iin so that the input power factor becomes approximately 1. In this case, the DC voltage source8is discharged if the absolute value of the voltage Vin is equal to or lower than the target voltage Vdc* of the smoothing capacitor11whereas the DC voltage source8is charged if the absolute value of the voltage Vin is equal to or higher than the target voltage Vdc*.

The target voltage Vdc* of the smoothing capacitor11is determined by θ1which defines the short-circuiting phase range20and, thus, the target voltage Vdc* can be controlled by varying θ1in the present embodiment in the same fashion as in the foregoing first embodiment. Then, the DC voltage Vdc of the smoothing capacitor11is controlled to follow the target voltage Vdc*.

If the voltage Vsub of the DC voltage source8is set at a level equal to or lower than the peak voltage Vp of Vin to satisfy three conditions expressed by Vp·sin θ1≦Vsub, (Vdc*−Vp·sin θ1)≦Vsub and (Vp−Vdc*)≦Vsub, it is possible to maintain the DC voltage Vdc of the smoothing capacitor11at the target voltage Vdc* and causes the inverter circuit300to reliably perform the control operation for controlling the current Iin in such a manner that the input power factor becomes approximately 1 at all phases of the AC power supply1.

The inverter circuit300generates a current command in the same fashion as in the foregoing first embodiment and is controlled by a voltage command calculated on the basis of the current command. Here, the inverter circuit300is controlled to switch between operations for charging and discharging the DC voltage source8with the voltage command corrected by adding thereto a feedforward correction voltage ΔV which is synchronized with timings of turning on and off the short-circuiting switches12a,12bin the same fashion as in the foregoing first embodiment. This arrangement makes it possible to avoid a delay in control operation by as much as a feedback control response time and thereby achieve high-speed control.

In the present third embodiment, the input power factor is improved and the DC voltage Vdc in the output stage is controlled by the control operation of the inverter circuit300as in the foregoing first embodiment, so that the inverter circuit300can decrease a voltage handled in switching operation to a level significantly lower than the peak voltage of the AC power supply1, making it possible to reduce switching loss and noise without the need for a reactor3having a large capacity. Also, with the provision of the short-circuiting switches12a,12b, the DC voltage source8of the inverter circuit300can be charged while bypassing the smoothing capacitor11when the short-circuiting switches12a,12bare in the ON state. Therefore, it is possible to further decrease the voltage handled in the switching operation, achieve a higher efficiency and a further reduction in noise, and thus obtain the same advantageous effects as in the foregoing first embodiment.

Furthermore, since the diode bridge2employed in the foregoing first embodiment is made unnecessary, it is possible to reduce the number of components and provide a simplified system structure. Also, as the number of elements through which the current flows can be reduced, it is possible to decrease conduction loss and improve conversion efficiency of the entirety of the power converting apparatus.

Incidentally, the inverter circuit300may be reconfigured such that AC sides of a plurality of single-phase inverters are connected in series as depicted inFIG. 7in this third embodiment as well.

Also, while one or more of the rectifier diodes10,13a,13bare connected to the smoothing capacitor11in the foregoing individual embodiments, semiconductor switching devices may be connected instead of these rectifier diodes10,13a,13bto perform the same operation by ON/OFF control thereof.