Power circuit

Abstract

A power supply circuit according to an embodiment of the present invention comprises: a three-phase reactor connected at one end to phase terminals of a three-phase AC power source; a converter main circuit formed by connecting six switching elements on a three-phase bridge basis; a smoothing capacitor connected between output terminals of the converter main circuit; a voltage detection unit that detects a terminal voltage of the smoothing capacitor; a phase detection unit that detects the phase of the AC power source; and a control unit that divides one cycle of the AC power source into a plurality of periods, and exclusively switches the six switching elements in accordance with a predetermined on-off pattern during the periods. The control unit corrects the on-off pattern in the switching so as to suppress fluctuation in the terminal voltage detected by the voltage detection unit.

Description

[Technical Field]

[0001] An embodiment of the present invention relates to a power supply circuit that converts power from a three-phase AC power supply into DC. [Background technology]

[0002] For example, Patent Document 1 discloses a technique for suppressing the effects of input current imbalance caused by voltage imbalance in a three-phase power supply by using a switching element provided on the DC side of a rectifier circuit, while Patent Document 2 discloses a technique for dealing with voltage imbalance by using a PWM converter. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-18751 [Patent Document 2] Japanese Patent Application Publication No. 1-99478 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0004] The configuration of Patent Document 1 can resolve the current imbalance of the three-phase power supply, but requires measures to deal with the high-frequency current that accompanies the switching of the boost chopper. Also, in the configuration of Patent Document 2, all six switching elements are switched within one power supply cycle, which causes an increase in switching loss. Therefore, a power supply circuit is provided that can reduce switching loss while simultaneously taking measures against voltage imbalance and high frequency current. [Means for solving the problem]

[0005] A power supply circuit according to an embodiment includes a three-phase reactor having one end connected to each phase terminal of a three-phase AC power supply, a converter main circuit formed by connecting six switching elements in a three-phase bridge configuration, a smoothing capacitor connected between output terminals of the converter main circuit, a voltage detection unit that detects the terminal voltage of the smoothing capacitor, a phase detection unit that detects the phase of the AC power supply, and a control unit that divides one cycle of the AC power supply into a plurality of periods and switches each of the six switching elements exclusively in these periods according to a predetermined on / off pattern, and the control unit corrects the on / off pattern in the switching so as to suppress fluctuations in the terminal voltage detected by the voltage detection unit. When the fluctuation of the terminal voltage in a half cycle of the AC power supply exceeds a threshold value, a correction is made so as to reduce the on-continuation time in the on-off pattern of the switching element that is operated at a point in time when a certain period of time has elapsed since the point in time when the fluctuation reaches a maximum value. . [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a diagram showing the configuration of a motor drive system using the power supply circuit of the first embodiment. [Figure 2] FIG. 2 is a functional block diagram showing the internal configuration of the control unit of the embodiment, focusing on a part that controls the converter main circuit. [Figure 3] FIG. 3 is a diagram showing an example of data stored in the memory of the embodiment. [Figure 4] FIG. 4 is a diagram showing waveforms at various parts of the power supply circuit within one cycle of the AC power supply when the voltage of the three-phase AC power supply is in a balanced state. [Figure 5] FIG. 5 is a diagram showing waveforms of the power supply voltage, the power supply current, and the terminal voltage V of the smoothing capacitor when the voltage of the three-phase AC power supply is in an unbalanced state. [Figure 6] FIG. 6 is a flowchart showing the processing contents of the on-time correction unit. [Figure 7] FIG. 7 is a diagram illustrating the phase timing for performing correction. [Figure 8] FIG. 8 is a waveform diagram illustrating the correction of the on-duration time. [Figure 9] FIG. 9 is a flowchart showing the processing performed by the on-time correction unit according to the second embodiment. [Figure 10]FIG. 10 is a diagram for explaining the phase timing for performing correction. [Figure 11] FIG. 11 is a waveform diagram illustrating the correction of the duty ratio. BEST MODE FOR CARRYING OUT THE INVENTION

[0007] (First embodiment) Figure 1 shows the configuration of a motor drive system. The motor drive system 1 includes a power supply circuit 3 that converts AC power from a three-phase AC power supply 2 into DC power, an inverter main circuit 4 to which the DC power is supplied, and a motor 5 that is operated by the inverter main circuit 4. The power supply circuit 3 includes a converter main circuit 7 in which switching elements, such as IGBTs (Insulated Gate Bipolar Transistors) 6U to 6Z, are connected in a three-phase bridge configuration. A freewheel diode 8 is connected between the collector and emitter of the IGBTs 6U to 6Z. Hereinafter, the IGBTs 6U to 6Z will be collectively referred to as IGBTs 6.

[0008] The U, V, and W terminals of the three-phase AC power supply 2 are connected to the input terminals of each phase of the converter main circuit 7 via reactors L1, L2, and L3. The U-phase line U of the three-phase AC power supply 2 is connected to the interconnection of IGBT6U and IGBT6X via reactor L1, the V-phase line V is connected to the interconnection of IGBT6V and IGBT6Y via reactor L2, and the W-phase line W is connected to the interconnection of IGBT6W and IGBT6Z via reactor L3. A smoothing capacitor 9 is connected between the power supply circuit 3 that outputs DC and the inverter main circuit 4. Like the converter main circuit 7, the inverter main circuit 4 also has a configuration in which IGBTs 15 are connected in a three-phase bridge.

[0009] The converter main circuit 7 and the inverter main circuit 4 are controlled by a common control unit 10. The control unit 10 is composed of a microcomputer with a memory and its peripheral circuits. A voltage zero-cross detection unit 11 is composed of a photocoupler or the like that detects the presence or absence of voltage, detects the zero-cross points of the AC voltage waveform at each phase terminal of the three-phase AC power supply 2, and inputs the detection result to the control unit 10. A voltage detection unit 12 detects a terminal voltage V, which is the voltage across the smoothing capacitor 9, and inputs the detection result to the control unit 10. The terminal voltage V is also the output voltage of the power supply circuit 3.

[0010] FIG. 2 is a functional block diagram showing the internal configuration of the control unit 10, focusing on the part that controls the converter main circuit 7. In practice, the functions of these blocks are implemented by a microcomputer executing a pre-stored program. These functional blocks may also be assembled using logic circuits. The AC phase detector 21 receives timing signals indicating the zero-crossing points of each phase voltage waveform from the voltage zero-cross detector 11. The AC phase detector 21 calculates the phase angle θ of the power supply voltage based on these timing signals and inputs the calculated values ​​to the ON / OFF pattern generator 22 and the ON-time corrector 23. The phase angle θ is determined based on one specific phase of the three phases. The voltage detector 12 inputs the terminal voltage of the smoothing capacitor 9, i.e., the DC output voltage V of the power supply circuit 3, to the power supply voltage unbalance detector 24. The power supply voltage unbalance detector 24 detects the voltage imbalance of the three-phase AC power supply 2 based on fluctuations in the DC output voltage V. That is, when an imbalance occurs in the three-phase power supply voltage, a fluctuation occurs in the output DC voltage, and this is used to detect the voltage imbalance in the three-phase AC power supply 2. This detection signal is input to the on-time correction unit 23.

[0011] A switching element ON / OFF pattern memory 25, which corresponds to a storage unit, stores in advance as data a pattern for periodically turning on and off the six IGBTs 6U to 6Z that constitute the converter main circuit 7. The switching element ON / OFF pattern memory 25 is also simply referred to as memory 25. FIG. 3 shows an example of data stored in the memory 25. For each of the IGBTs 6U to 6Z, an ON phase angle and an ON duration are stored as a set of data. The ON / OFF pattern generation unit 22 reads out the ON / OFF pattern data from the memory 25 in accordance with the phase angle θ and inputs it to the switching element drive unit 26.

[0012] That is, when the phase angle θ becomes the ON phase angle stored in the memory 25, the ON / OFF pattern generation unit 22 outputs a signal to turn on the corresponding switching element, maintains the ON state for the ON duration, and then outputs a signal to turn off the switching element. For example, when θ reaches θ=θv3, an ON signal is output to IGBT 6V, maintains the ON state for tv3, and then outputs an OFF signal to IGBT 6V. Based on the signal input from the ON / OFF pattern generation unit 22, the switching element drive unit 26 inputs drive signals to the gates of the IGBTs 6 of the U, V, W, X, Y, and Z phases that constitute the converter main circuit 7. Here, the ON / OFF patterns stored in the memory 25 are set so that none of the IGBTs 6U to 6Z are turned ON simultaneously. That is, each of the six switching elements IGBTs 6U to 6Z is switched exclusively.

[0013] When an imbalance detection signal is input from the power supply voltage imbalance detection unit 24, the on-time correction unit 23 inputs to the ON / OFF pattern generation unit 22 a signal for correcting the on-duration of the ON / OFF pattern data that the ON / OFF pattern generation unit 22 inputs to the switching element drive unit 26, according to the phase angle θ of the detected timing, together with the phase of the IGBT 6 to be corrected.

[0014] FIG. 4 shows waveforms of various parts within one AC power supply cycle when the voltage of the three-phase AC power supply 2 is in a balanced state. The top of the figure shows the voltage and current waveforms of the three-phase AC power supply 2 and the voltage waveform of the smoothing capacitor 9. The L1-L3 power supply voltages and input currents are waveforms of the voltages and currents input to the power supply circuit 3 from the three-phase AC power supply 2 via the reactors L1-L3. The bottom section, which shows the switching operation of the power supply circuit 3, lists the IGBTs 6U-6Z of each phase on the vertical axis, and the X1, Z2, W1, V2, etc. on the horizontal axis indicate periods during which only one of the IGBTs 6U-6Z of each phase constituting the converter main circuit 7 is switched exclusively. For example, during periods X1 and X2, only the IGBT 6X is switched, and during periods Z2 and Z1, only the IGBT 6Z is switched. In this way, one cycle of the three-phase AC power supply 2 is divided into 12 periods, and each of the IGBTs 6U to 6Z for each phase is switched exclusively in two of the 12 periods.

[0015] Furthermore, the period during which the IGBTs 6U to 6Z of each phase are switched corresponds to the rising or falling period of the rectified waveform of the connected AC voltage. By controlling the IGBTs 6 in this way, the current of each phase of the three-phase AC power supply 2 becomes closer to a sine wave, as shown in Figure 4, thereby improving the power factor and reducing harmonics.

[0016] Regarding the ON / OFF pattern data for each IGBT 6U-6Z stored in memory 25, the number of ON times in each cycle within the 12-period period is preferably approximately 1 to 10 times, and more preferably approximately 3 to 10 times. If the number of ON times is small, the current rise and fall will be small, and the current waveform will deviate from a sine wave. On the other hand, if the number of ON times is large, switching noise will increase and efficiency will decrease. Therefore, the optimal ON timing (phase angle, number of ON times, and ON duration of each ON time) is determined in advance by testing when the power supply voltage of the three-phase AC power supply 2 is balanced. This optimized ON / OFF pattern data is stored in memory 25. Furthermore, because the response to current changes on the rising edge of the rectified voltage waveform of the three-phase AC power supply 2 is slower than that on the falling edge, the number of switching operations on the rising edge of the rectified voltage waveform is made greater than the number of switching operations on the falling edge. Similarly, with regard to the on-duration, the on-duration on the rising edge of the rectified voltage waveform is made longer than that on the falling edge.

[0017] Note that the voltage waveforms of the U, V, and W phases of the three-phase AC power supply 2 are similar, with each phase angle shifted by 120°, i.e., 2π / 3. Therefore, the ON phase angle and ON duration of the IGBTs 6U-6Z in each phase can also be shifted by 120°. In this case, the data stored in memory 25 may be the ON phase angle and ON duration of only one specific IGBT 6, rather than the data for each IGBT 6U-6Z, as shown in FIG. 3. For example, only the ON phase angle and ON duration of the U-phase IGBT 6U may be stored, and the drive pattern for the V-phase IGBT 6V may be calculated by adding 2π / 3 to the ON phase angle of the U-phase IGBT 6U stored in memory 25. The ON duration is used as is. That is, referring to FIG. 3, for example, the ON phase angle of the V-phase IGBT 6V is θvn = θun + 2π / 3 (n = 1, 2, 3, etc.). Similarly, the ON phase angle θwn of the W-phase IGBT 6W is θun=θun−2π / 3. According to this method, the amount of stored data can be reduced, and the capacity of the memory 25 can be reduced.

[0018] Figure 5 shows the waveform of the terminal voltage V in an unbalanced state. The example shown in Figure 5 shows the waveforms of each part when the switching operation shown in Figure 4 is performed when the W-phase voltage drops. When this unbalanced state occurs, even if the IGBT 6 is controlled as described above, the currents in each phase become unbalanced, and the current waveforms deviate from a sinusoidal waveform, increasing harmonics. This also causes differences in the magnitude of the currents in each phase, with the peak value of the reactor current L1 in the U-phase of the three-phase AC power supply 2 being the largest and the reactor current L3 in the W-phase being the smallest. Furthermore, the difference in the magnitude of the currents in each phase increases the ripple in the terminal voltage V, i.e., the DC ripple.

[0019] Next, the operation of this embodiment will be described. Fig. 6 is a flowchart showing the processing contents of the on-time correction unit 23. This processing is executed every half cycle of the AC power supply. Note that "DC voltage" in the figure indicates the terminal voltage V. First, at 1 / 2 of the power supply cycle T, the maximum value DC_MAX and minimum value DC_MIN of the terminal voltage V are recorded, and the phase θ_MAX at the time when the maximum value DC_MAX is recorded is also recorded (S1).

[0020] Next, it is determined whether the difference between the maximum value DC_MAX and the minimum value DC_MIN exceeds a threshold value Va (S2). The difference is the fluctuation range of the voltage ripple, and the threshold value Va is set to a value that determines that the voltage of the AC power supply 2 is in a balanced state. If the determination here is "YES," the duty ratio of the switching pulse is corrected to be smaller in the switching period corresponding to the phase (θ_MAX+3 / 8×2π), i.e., in one of the 12 periods U1 to Z2 (S4).

[0021] Here, we will explain the phase (θ_MAX+3 / 8×2π). As shown in FIG. 7, the ripple of the terminal voltage V has a period of 1 / 2 of the power supply period T. As a result of testing by the inventors, we found that in order to correct the duty ratio of the switching pulse, i.e., the on-duty duration, so as to reduce the maximum value of the fluctuating terminal voltage V, it is optimal to correct the duty ratio in a decreasing direction at a timing that is 1 / 8 of a period ahead of the phase indicating the maximum value DC_MAX based on the above fluctuation period. By controlling in this way, it was possible to reduce the next maximum value, which occurs 1 / 8 of a period later.

[0022] Therefore, by correcting the on-duration to decrease at the timing 3 / 8T after the point in time when the maximum value DC_MAX is indicated, the on-time ratio of the IGBT 6 that performs switching operation during that period is reduced, and the maximum value of the terminal voltage V for the next half cycle can be corrected. Here, 3 / 8T after is nothing more than the timing that is 1 / 8 cycle ahead of the phase indicating the next maximum value DC_MAX. Based on this, in step S3, the phase (θ_MAX+3 / 8×2π) is set.

[0023] In the example shown in FIG. 7, the period showing the maximum value DC_MAX is V2, and 3 / 8T after that is the period Z1 in which IGBT6Z is driven, so the duty ratio of the ON / OFF pattern for IGBT6Z output during the period Z1, i.e., the ON duration, is reduced. Specifically, for the timing of the ON phase angle θz1 shown in FIG. 3, the ON duration tz1 is changed to (tz1×α). Here, α is a correction coefficient and takes a positive value smaller than 1. This is multiplied by all ON durations tzn during that period. Furthermore, the period showing the next maximum value DC_MAX is Y2, and 3 / 8T after that is the period W1 in which IGBT6W is driven, so the ON duration of the ON / OFF pattern for IGBT6W output during the period W1 is reduced as shown in FIG. 8. In the same figure, w1 ,θ w2 ,θ w3 , is the on-timing before correction on-duration t w1 ,t w2 ,t w3 is multiplied by a correction coefficient α to reduce the on duration.

[0024] If the determination in step S2 is "NO," the process then determines whether the difference between the maximum value DC_MAX and the minimum value DC_MIN falls below a threshold value Vb (S3). The threshold value Vb is set to a value smaller than the threshold value Va to provide hysteresis for suppressing frequent fluctuations in control. If the difference is equal to or greater than the threshold value Vb, the "NO" process is terminated. If the difference is below the threshold value Vb (YES), the correction coefficient α used in the correction performed in step S4 is gradually increased toward "1" to extend the on-duration of the IGBT 6 (S5). The correction coefficient α is increased little by little each time step S5 is performed, and once the correction coefficient α reaches "1," no processing is performed. In other words, the amount of correction to the on-duration decreases as the correction coefficient α is gradually increased.

[0025] As described above, according to this embodiment, the power supply circuit 3 includes reactors L1 to L3, one end of which is connected to each phase terminal of the three-phase AC power supply 2, a converter main circuit 7, a smoothing capacitor 9 connected between output terminals of the converter main circuit 7, a voltage detector 12 that detects the terminal voltage V of the smoothing capacitor 9, an AC phase detector 21 that detects the phase of the AC power supply 2, and a control unit 10 that divides one cycle of the AC power supply 2 into 12 periods and switches each of the six IGBTs 6 exclusively for two periods each according to a predetermined on / off pattern. The control unit 10 then corrects the on / off pattern of the IGBTs 6 to suppress fluctuations in the terminal voltage V. This configuration makes it possible to achieve both voltage imbalance and high-frequency current countermeasures while reducing the switching loss of the IGBTs 6.

[0026] The control unit 10 includes a memory 25 that stores the on / off patterns in advance and uses the stored on / off patterns to select and control the on / off of the IGBTs 6. When the fluctuation of the terminal voltage V in a half cycle of the AC power supply 2 exceeds a threshold value Va, the control unit 10 corrects the IGBTs 6 to operate after a certain period of time has elapsed since the fluctuation reached its maximum value DC_MAX by reducing the on-time ratio in the on / off pattern. The certain period of time is set to 3 / 8 of the cycle of the AC power supply 2. This reliably suppresses voltage imbalance. When the fluctuation falls below the threshold value during correction, the control unit 10 reduces the amount of correction, thereby terminating the correction without increasing the fluctuation. This suppresses fluctuations in the terminal voltage V even when a voltage imbalance occurs in the three-phase AC power supply 2. Once the voltage imbalance in the three-phase AC power supply 2 is resolved, the original control can be restored.

[0027] (Second embodiment) Hereinafter, the same parts as those in the first embodiment are denoted by the same reference numerals and their explanations are omitted, and only the differences will be described. In the first embodiment, correction was performed using the maximum value DC_MAX as the base point, but in the second embodiment, correction is performed using the minimum value DC_MIN as the base point. As shown in FIG. 9, in step S6, which replaces step S1, the phase θ_MIN at the time when the minimum value DC_MIN was recorded is recorded instead of the maximum value DC_MAX. Then, in step S7, which replaces step S4, a correction is made to increase the on duration of the switching pulse in one of the 12 switching periods, as shown in FIG. 11.

[0028] As shown in FIG. 10, the phase relationship remains the same even when the maximum value DC_MAX is replaced with the minimum value DC_MIN. By correcting the ON duration by increasing it at 3 / 8T from the time when the minimum value DC_MIN is reached, the minimum value of the terminal voltage V for the next half cycle can be increased. Increasing the minimum value of the terminal voltage V reduces the difference with the maximum value, thereby reducing the ripple of the terminal voltage V of the smoothing capacitor 9. In the example shown in the figure, the period when the minimum value DC_MIN is reached is X1, and 3 / 8T after that is the period Y1, so the ON duration of the ON / OFF pattern output during the period Y1 is increased. Furthermore, the period when the next minimum value DC_MIN is reached is U1, and 3 / 8T after that is the period V1, so the ON duration of the ON / OFF pattern output during the period V1 is increased.

[0029] The ON duration can be increased by multiplying the ON duration of the ON / OFF pattern by a correction coefficient β, where β is a number equal to or greater than 1. In the example of IGBT6Y shown in FIG. 11, the phase θ y」1 ,θ y2 ,θ y3 , is the on-timing before correction on-duration t y1 ,t y2 ,t y3 is multiplied by a correction coefficient β to increase the on duration.

[0030] As described above, the second embodiment provides the same effects as the first embodiment. However, when a correction is made to increase the on-duration as in the second embodiment, the power supply current of a specific phase increases, which may cause problems such as overcurrent. For this reason, depending on the device, a correction to decrease the on-duration as in the first embodiment may be preferable.

[0031] (Other embodiments) The thresholds Va and Vb may be set to the same value. The switching element is not limited to an IGBT, but may be a MOSFET, a bipolar transistor, or the like. The power supply circuit 3 and the inverter main circuit 4 may be controlled by separate control units. Although one cycle of the AC voltage of the three-phase AC power supply 2 is divided into 12 periods, each period does not need to be equal and spaced at 30° intervals. Generally, extending the rising edge of the waveform of the rectified AC voltage and shortening the falling edge has a greater effect on improving the power factor and reducing harmonics. For this reason, the rising edge period may be set to 35° and the falling edge period to 25°. These periods can be easily changed by rewriting the ON phase angle θ in the memory data shown in Figure 3. Note that the total of both periods is 60°, and this cannot be changed.

[0032] The on-duration times stored in memory 25 are corrected by multiplying each on-duration time by the correction coefficient α or β. Alternatively, when the on-duration time is to be increased, a correction value having a positive value may be added to the on-duration time, and when the on-duration time is to be decreased, a correction value having a positive value may be subtracted from the on-duration time. Furthermore, since it is sufficient to suppress fluctuations in the terminal voltage V, when the fluctuations in the terminal voltage V in a half cycle of the AC power supply 2 exceed the threshold value Va, the on / off pattern of the IGBT 6 corresponding to the period in which the IGBT 6 should be operated from the point at which the fluctuation reaches its maximum value DC_MAX to the point at which the period is 3 / 8 of the cycle of the AC power supply 2 may be left unchanged, and correction may be made to increase the proportion of the on time of the IGBT 6 in periods other than the relevant period. In this way, the on duration of the IGBT 6 in periods other than the relevant period increases, which increases the current and ultimately suppresses fluctuations in the terminal voltage V. Similarly, when the fluctuation of the terminal voltage V in a half cycle of the AC power supply 2 exceeds the threshold value Va, the ON duration of the ON / OFF pattern may be reduced in a period other than the period 3 / 8T (cycle) after the point in time when the minimum value DC_MIN is shown. However, these methods result in a decrease in the utilization rate of the ON / OFF pattern that has been optimized and stored in the memory 25, and there is a possibility that harmonics will increase compared to the first and second embodiments.

[0033] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0034] In the drawing, 1 indicates a motor drive system, 2 indicates a three-phase AC power supply, 3 indicates a power supply circuit, 4 indicates an inverter main circuit, 5 indicates a motor, 6 indicates an IGBT, 7 indicates a converter main circuit, 9 indicates a smoothing capacitor, 10 indicates a control unit, 11 indicates a voltage zero-cross detection unit, 12 indicates a voltage detection unit, 23 indicates an on-time correction unit, 24 indicates a power supply voltage imbalance detection unit (imbalance detection unit), and 25 indicates a switching element ON / OFF pattern memory (storage unit).

Claims

1. a three-phase reactor having one end connected to each phase terminal of a three-phase AC power supply; a converter main circuit formed by connecting six switching elements in a three-phase bridge; a smoothing capacitor connected between the output terminals of the converter main circuit; a voltage detection unit that detects the terminal voltage of the smoothing capacitor; a phase detection unit that detects a phase of the AC power supply; a control unit that divides one cycle of the AC power supply into a plurality of periods and switches each of the six switching elements exclusively in each of these periods according to a predetermined on / off pattern, the control unit corrects an on-off pattern in the switching so as to suppress fluctuations in the terminal voltage detected by the voltage detection unit; When the fluctuation in the terminal voltage in a half cycle of the AC power supply exceeds a threshold, the power supply circuit corrects the on-off pattern of a switching element that is operated at a point in time when a certain period of time has elapsed since the fluctuation reached its maximum value, so as to reduce the on-continuation time of the switching element.

2. A three-phase reactor having one end connected to each phase terminal of a three-phase AC power supply; a converter main circuit formed by connecting six switching elements in a three-phase bridge; a smoothing capacitor connected between the output terminals of the converter main circuit; a voltage detection unit that detects the terminal voltage of the smoothing capacitor; a phase detection unit that detects a phase of the AC power supply; a control unit that divides one cycle of the AC power supply into a plurality of periods and switches each of the six switching elements exclusively in each of these periods according to a predetermined on / off pattern, the control unit corrects an on-off pattern in the switching so as to suppress fluctuations in the terminal voltage detected by the voltage detection unit; When the fluctuation in the terminal voltage in a half cycle of the AC power supply exceeds a threshold, the power supply circuit corrects the on-off pattern of a switching element that is operated at a point in time when a certain period of time has elapsed since the fluctuation reached its minimum value, so as to increase the on-continuation time.

3. the control unit includes a storage unit that stores the on / off pattern in advance, 3. The power supply circuit according to claim 1, wherein the on / off pattern stored in the storage unit is used to select and control the on / off of the switching elements.

4. 4. The power supply circuit according to claim 3, wherein the storage unit stores, as the on / off pattern, an on-timing and an on-duration of a specific switching element.

5. The power supply circuit according to claim 1 , wherein the control unit reduces an amount of correction when the fluctuation falls below a threshold while the correction is being performed.

6. 6. The power supply circuit according to claim 1, wherein the predetermined period is 3 / 8 of a cycle of the AC power supply.

7. 7. The power supply circuit according to claim 1, wherein the plurality of periods dividing one cycle of the AC power supply are 12 periods.

8. 8. The power supply circuit according to claim 7, wherein the period during which each of the six switching elements is switched exclusively occurs twice out of the twelve periods.

9. 9. The power supply circuit according to claim 8, wherein the control unit controls the switching elements to be exclusively switched on and off in the two periods, the switching elements corresponding to rising or falling periods of an AC voltage waveform obtained by full-wave rectifying the three-phase AC power supply.

10. 10. The power supply circuit according to claim 7, wherein the length of each of the twelve periods is variable.

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