Power conversion device

Abstract

The present invention provides a power conversion device in which a control unit adjusts a gate signal for turning a switching element on and off based on a difference between a waveform command for outputting a current having a desired waveform from an inverter unit and an output current from the inverter unit to a load, and, if the output current is an overcurrent, a value corresponding to the magnitude of the overcurrent.

Description

Technical Field

[0001] The present invention relates to a power conversion device, and more particularly to a power conversion device including a control unit that controls an inverter unit. Background Art

[0002] Conventionally, a power conversion device including a control unit that controls an inverter unit is known. Such a power conversion device is disclosed in, for example, Japanese Patent Application Laid-Open No. 2016-194993.

[0003] Japanese Patent Application Publication No. 2016-194993 discloses an induction melting furnace comprising a power conversion unit (inverter unit) including switching elements, a control circuit (control unit) for controlling the power conversion unit, and a heating coil provided on the output side of the power conversion unit. In this induction melting furnace, a current detector is provided between the power conversion unit and the heating coil, and the current detector detects the current supplied from the power conversion unit to the heating coil. Furthermore, an automatic current regulator (ACR) is provided in the control circuit. The ACR performs proportional-integral feedback control based on the current detected by the current detector. Based on the output from the ACR, a gate signal is generated for controlling the on / off switching of the power conversion unit's switching elements. Based on the generated gate signal, the power conversion unit's switching elements are driven. Consequently, the current supplied from the power conversion unit to the heating coil is controlled to be constant.

[0004] In an induction melting furnace such as that described in Japanese Patent Application Laid-Open No. 2016-194993, if the load impedance of the heating coil changes dramatically, the current flowing through the heating coil may change dramatically. In other words, the current flowing through the heating coil may become an overcurrent. However, the response speed of the ACR is generally slow. Therefore, control using the ACR (proportional-integral feedback control) presents the following problem: if the current flowing through the heating coil becomes an overcurrent, it is difficult to quickly stabilize the current supplied to the heating coil (load) (to within a specified range). Summary of the Invention

[0005] The present invention has been made to solve the above-mentioned problems, and one object of the present invention is to provide a power conversion device that can quickly bring the current supplied to the load within a predetermined range even when the current flowing through the load becomes an overcurrent.

[0006] In order to achieve the above-mentioned object, an electric power conversion device according to one aspect of the present invention comprises: a rectifier unit which rectifies AC power supplied from a power supply into DC power; a smoothing unit which is arranged on the output side of the rectifier unit and smoothes the DC power obtained after rectification in the rectifier unit; an inverter unit which converts the DC power smoothed by the smoothing unit into AC power; and a control unit which controls the on and off of a switching element of the inverter unit, wherein the control unit is configured to adjust a gate signal for turning the switching element on and off based on a difference between a waveform instruction for outputting a current of a desired waveform from the inverter unit and an output current output from the inverter unit to a load, and a value corresponding to the magnitude of the overcurrent when the output current is an overcurrent.

[0007] In a power conversion device according to one aspect of the present invention, as described above, the control unit is configured to adjust the gate signal for turning the switching element on and off based on the difference between the waveform command for outputting a current of a desired waveform from the inverter unit and the output current from the inverter unit to the load, and, in the event of an overcurrent, a value corresponding to the magnitude of the overcurrent. Thus, unlike feedback control based on the difference (deviation) between the waveform command and the fed-back output current, the gate signal is adjusted based on a value corresponding to the magnitude of the overcurrent in addition to the deviation. That is, the deviation used for feedback control can be adjusted using a value corresponding to the magnitude of the overcurrent. As a result, even in the event of an overcurrent, the current supplied to the load can be quickly brought within a specified range.

[0008] In the power conversion device according to the aforementioned aspect, the control unit is preferably configured to adjust the gate signal by subtracting a value corresponding to the magnitude of the overcurrent from a value based on the difference between the waveform command and the output current. With this configuration, the value corresponding to the magnitude of the overcurrent is subtracted from the value based on the difference between the waveform command and the output current (the deviation used for feedback control), thereby immediately reducing the deviation used for feedback control. Consequently, even if the current flowing through the load exceeds the specified range, the current supplied to the load can be more quickly brought within the specified range.

[0009] In this case, the control unit is preferably configured to adjust the gate signal by subtracting an amount corresponding to the difference between the output current and the threshold value from a value based on the difference between the waveform command and the output current (the deviation used for feedback control) when the output current exceeds a predetermined threshold. With this configuration, a value corresponding to the magnitude of the output current (the magnitude of the overcurrent) is subtracted from the value based on the difference between the waveform command and the output current (the deviation used for feedback control). As a result, the current supplied to the load can be quickly brought within a predetermined range based on the magnitude of the overcurrent.

[0010] In the power conversion device according to the above aspect, the waveform command preferably includes a sinusoidal wave command. This sinusoidal wave command is calculated based on the frequency of the gate signal, the phase of the output current of the inverter unit, and the effective value of the current command for outputting the desired current from the inverter unit, with the phase of the zero-crossing point of the output current of the inverter unit as the starting point. Here, the frequency of the appropriate gate signal (carrier frequency) changes with load fluctuations. Therefore, as described above, the sinusoidal wave command is constructed based on the frequency of the gate signal, the phase of the output current, and the effective value of the current command, with the phase of the zero-crossing point of the output current as the starting point, thereby enabling the sinusoidal wave command to be set to match the carrier frequency.

[0011] In this case, it is preferable to further include a first filter for attenuating a desired frequency component of the output current of the inverter unit, and the control unit is configured to obtain the phase of the zero-crossing point of the output current based on the output current of the inverter unit obtained by the first filter. Oscillation (vibration of the output current) may occur near the zero-crossing point of the output current. Therefore, by attenuating the desired frequency component of the output current of the inverter unit using the first filter, the phase of the zero-crossing point of the output current can be appropriately obtained.

[0012] The power conversion device including the above-described first filter preferably further includes a second filter for attenuating a desired frequency component of the difference between the waveform command and the output current, with the first and second filters being configured to attenuate the same frequency component. With this configuration, the first filter for determining the phase of the zero-crossing point and the second filter for the difference between the waveform command and the output current (deviation used for feedback control) have identical characteristics, enabling highly accurate control to keep the current supplied to the load within a specified range.

[0013] In the power conversion device according to the aforementioned aspect, the control unit is preferably configured to adjust the frequency of the gate signal based on the difference between the waveform command and the output current and, in the event of an overcurrent, a value corresponding to the magnitude of the overcurrent. With this configuration, even in the event of an overcurrent, even in the event of a load current exceeding the specified range, the current supplied to the load can be quickly brought within a specified range during pulse frequency modulation (PFM) control, which controls the on / off switching of the switching element by varying the frequency of the gate signal.

[0014] In the power conversion device of one aspect described above, it is preferred that the load includes a heating coil, which is provided on the output side of the inverter unit and heats the object to be heated. Here, in the heating coil that heats the object to be heated, the load impedance of the heating coil is prone to change rapidly due to changes in the shape of the heated object that is heated and melted. Therefore, the current flowing through the heating coil is prone to become an overcurrent. Therefore, in the power conversion device that supplies power to the heating coil that heats the object to be heated, it is particularly effective to quickly converge the current supplied to the load within a specified range by configuring it as the power conversion device of one aspect described above.

[0015] According to the present invention, as described above, even when the current flowing through the load becomes an overcurrent, the current supplied to the load can be quickly brought within a predetermined range. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a circuit diagram of an induction heating device (power conversion device) according to one embodiment.

[0017] Figure 2 This is a block diagram for explaining generation of a reference waveform in a power conversion device according to one embodiment.

[0018] Figure 3 This is a control block diagram of a control unit of a power conversion device according to one embodiment.

[0019] Figure 4 Graph showing the relationship between the frequency of a gate signal and the output current.

[0020] Figure 5 It is a graph showing the relationship between frequency and output current. DETAILED DESCRIPTION

[0021] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0022] Reference Figures 1 to 5 The structure of the induction heating device 1 according to this embodiment will be described.

[0023] like Figure 1 As shown, the induction heating device 1 includes a power conversion device 100, a current detector 2, resonant capacitors C1 and C2, an induction heating coil 3, and a resistor 4. The induction heating coil 3 is an example of a "load" and a "heating coil" in the claims.

[0024] Power conversion device 100 includes a rectifier 10. Rectifier 10 is configured to rectify AC power supplied from a three-phase commercial power supply 200 into DC power. Specifically, rectifier 10 includes a plurality of diodes forming a three-phase full-wave rectifier circuit. Commercial power supply 200 is an example of a "power supply" as defined in the claims.

[0025] Furthermore, the power conversion device 100 includes a smoothing capacitor 20. The smoothing capacitor 20 is provided on the output side of the rectifier 10. The smoothing capacitor 20 is configured to smooth the DC power rectified by the rectifier 10. The smoothing capacitor 20 is an example of a "smoothing unit" in the claims.

[0026] The power conversion device 100 also includes an inverter unit 30 (high-frequency inverter circuit). The inverter unit 30 is configured to convert DC power smoothed by the smoothing capacitor 20 into AC power. The inverter unit 30 includes a plurality of switching elements S (semiconductor switching elements). The plurality of switching elements S include switching elements S1 and S3 forming an upper arm, and switching elements S2 and S4 forming a lower arm.

[0027] The power conversion device 100 also includes a control unit 40. The control unit 40 is configured to control the on / off switching of the switching elements S1 to S4 of the inverter unit 30. The detailed configuration of the control unit 40 will be described later.

[0028] In the induction heating device 1 , a plurality of (two in the present embodiment) sets of the rectifier unit 10 and the inverter unit 30 are provided in parallel with each other.

[0029] The current detector 2 is configured to detect a current flowing from the AC output side of the inverter unit 30 to the resonant capacitor C1 .

[0030] The resonant capacitors C1 and C2 are provided on the AC output side of the inverter unit 30. The resonant capacitors C1 and C2, the induction heating coil 3, and the metal as the object to be heated constitute a resonant circuit.

[0031] In this embodiment, the induction heating coil 3 is provided on the output side of the inverter unit 30 and is configured to heat the heated object (metal) to melt it. For example, the metal is an aluminum ingot. Furthermore, the AC output side of the inverter unit 30 is connected to one electrode of the resonant capacitor C1 (resonant capacitor C2). Furthermore, the other electrode of the resonant capacitor C1 (resonant capacitor C2) is connected to the induction heating coil 3.

[0032] Furthermore, the induction heating coil 3 is connected to a resistor 4. The resistor 4 is disposed between the induction heating coil 3 and the resonant capacitor C2.

[0033] (Detailed Structure of Control Unit)

[0034] Next, the detailed configuration of the control unit 40 will be described.

[0035] First, refer to Figure 2 The generation of the reference waveform by the control unit 40 is described below. The reference waveform is a standardized waveform (sine wave). In addition, a sine wave instruction is generated by multiplying the reference waveform by the execution value of the current instruction flowing to the induction heating coil 3. The sine wave instruction is used to generate a gate signal for controlling the on and off of the switching element S included in the inverter unit 30. In addition, when the frequency (carrier frequency) of the gate signal changes, it is necessary to change the reference waveform so that the reference waveform corresponds to the frequency of the changed gate signal. The following is a detailed description.

[0036] The current value of the output current of the inverter unit 30 detected by the current detector 2 is input to the control unit 40. Furthermore, the current value detected by the current detector 2 is input to the control unit 40 after analog-to-digital conversion. The analog-to-digital conversion is performed at a relatively high speed. Furthermore, the current detector 2 detects the output current of the inverter unit 30 at every predetermined sampling period.

[0037] Here, the output current (at least near the zero-crossing point of the output current) sometimes oscillates (vibrates). Therefore, a filter 41 is provided between the current detector 2 and the control unit 40. The filter 41 is configured to attenuate the desired frequency component of the output current of the inverter unit 30. For example, the filter 41 is configured by an LPF (low-pass filter). Then, the current value that has passed through the filter 41 is input to the control unit 40. In addition, the filter 41 is an example of the "first filter" in the claims.

[0038] In this embodiment, the control unit 40 is configured to obtain the phase of the zero-cross point of the output current based on the output current of the inverter unit 30 obtained through the filter 41. Specifically, the phase of the zero-cross point is obtained based on the current value of the output current obtained through the filter 41.

[0039] Furthermore, in this embodiment, the control unit 40 calculates a sinusoidal wave command based on the phase of the zero-crossing point of the inverter unit 30's output current as a starting point, the frequency of the gate signal, the phase of the inverter unit 30's output current (the phase of the zero-crossing point of the output current), and the effective value of the current command for outputting the desired current from the inverter unit 30. Specifically, the frequency of the gate signal, the detection period (sampling period) of the inverter unit 30's output current, and the phase of the zero-crossing point of the output current are input to the reference waveform generation unit 42 of the control unit 40. The frequency of the gate signal is the current frequency of the gate signal driving the switching element S. This current frequency of the gate signal is used as the frequency of the generated reference signal. Furthermore, a counter (not shown) counts the phase of the inverter unit 30's output current every detection period (sampling period) of the inverter unit 30 using the phase of the zero-crossing point of the output current as a starting point. The reference waveform generation unit 42 calculates the phase position of the reference waveform based on the count value obtained by the counter. The reference waveform generation unit 42 then generates the reference waveform (sinθ) using the phase of the zero-crossing point of the output current as a starting point.

[0040] Then, if Figure 3 As shown, the generated reference waveform is multiplied by the effective value (target value, A) of the current command for outputting the desired current from the inverter unit 30 by a multiplier 43. The effective value (A) of the current command is input to the control unit 40 from an ARP (AC power regulator) (not shown). Furthermore, the reference waveform (sinθ) is multiplied by the output power command value (A) to generate a sinusoidal wave command (A×sinθ). Generating a sinusoidal wave command in this manner generates a sinusoidal wave command that corresponds to changes in the frequency (carrier frequency) of the gate signal.

[0041] The difference (deviation) between the generated sine wave command and the current value (feedback current value) detected by the current detector 2 is then calculated by the subtractor 44. Furthermore, the current value detected by the current detector 2 is converted from analog to digital, and the desired frequency component is attenuated by the filter 41.

[0042] In this embodiment, a filter 45 is provided to attenuate the desired frequency component of the difference (deviation) between the sine wave command and the output current. Filters 41 and 45 are configured to attenuate the same frequency component. The desired frequency component of the output (deviation) of subtractor 44 is attenuated by filter 45. The deviation, after attenuation of the desired frequency component by filter 45, is then multiplied by the gain. Filter 45 is an example of a "second filter" in the claims.

[0043] Here, in this embodiment, the control unit 40 is configured to adjust the gate signal for turning the switching element S on and off based on the difference (deviation, I2) between the sine wave instruction for outputting a current of a desired waveform from the inverter unit 30 and the output current (I1) output from the inverter unit 30 to the induction heating coil 3, and a value corresponding to the magnitude of the overcurrent when the output current is an overcurrent. Specifically, the control unit 40 subtracts the value corresponding to the magnitude of the overcurrent from the value based on the difference between the sine wave instruction and the output current. In detail, when the output current exceeds a predetermined threshold value (I TH ), the control unit 40 subtracts an amount corresponding to the difference between the output current and a predetermined threshold value (I1-I TH ). In addition, although not shown in the figure, an amount corresponding to the difference between the output current and the predetermined threshold value (I1-I TH ) multiplied by the gain, or may be obtained by subtracting an amount corresponding to the difference between the output current and a predetermined threshold value from the deviation after the desired frequency component is attenuated by the filter 45 (I1-I TH ) and then multiplied by the gain.

[0044] That is, a predetermined threshold value is subtracted from the current value of the output current detected by the current detector 2 by a subtractor 46. Then, a subtractor 47 subtracts the output of the subtractor 46 (the amount I1-I2 corresponding to the difference between the output current and the predetermined threshold value) from the deviation (kI2') obtained by attenuating the desired frequency component (referred to as the deviation I2') by the filter 45 and multiplying it by the gain k. TH ). In addition, only values greater than 0 among the values obtained by the subtraction operation of the subtractor 46 are input to the subtractor 47. Thus, when the current output from the inverter unit 30 is an overcurrent, the value corresponding to the overcurrent (the output value of the subtractor 46) is immediately subtracted from the deviation multiplied by the gain. In addition, the predetermined threshold value is, for example, a value 1.1 times the rated current of the power conversion device 100. In addition, the deviation (kI2') after the desired frequency component is attenuated by the filter 45 and multiplied by the gain k is an example of "a value based on the difference between the sine wave command and the output current" in the claims.

[0045] Then, the output of the subtractor 47 (kI2'-(I1-I TH )) is input to the PI regulator 48. That is, even when the current output from the inverter unit 30 is an overcurrent, the value obtained by subtracting the value corresponding to the overcurrent (the output value of the subtractor 46) from the above-mentioned deviation is input to the PI regulator 48.

[0046] Then, the output of the PI regulator 48 is input to the frequency converter 49. That is, in this embodiment, the control unit 40 is configured to adjust the output current based on the difference I2 between the sinusoidal waveform command and the output current and the value (I1-I2) corresponding to the magnitude of the overcurrent when the output current is an overcurrent. TH ) to adjust the frequency of the gate signal. That is, in the power conversion device 100, pulse frequency modulation (PFM) control is performed to control the on and off of the switching element S by changing the frequency of the gate signal.

[0047] Then, the frequency converter 49 generates a gate signal for controlling the on / off of the switching element S. As described above, even when the current output from the inverter unit 30 is an overcurrent, a value corresponding to the overcurrent (the output value from the subtractor 46 (I1-I2)) is subtracted. TH The value obtained after )) is input to the PI regulator 48, so the gate signal generated by the frequency converter 49 takes into account the value corresponding to the overcurrent. This can quickly suppress the output current of the inverter unit 30 from becoming an overcurrent.

[0048] Next, refer to Figure 4 and Figure 5 The relationship between the frequency of the gate signal and the output current of the inverter unit 30 will be described.

[0049] As the frequency of the gate signal decreases, the magnitude (amplitude) of the output current of the inverter unit 30 increases. In addition, in the induction heating device 1, resonance is generated by the induction heating coil 3 and the resonant capacitors C1 and C2. Figure 5 As shown, the output current of the inverter unit 30 reaches its maximum at the resonant frequency f1. Furthermore, in the induction heating device 1, the gate signal is controlled within a frequency range of f2 to f3, which is higher than the resonant frequency f1. By having the gate signal have a frequency higher than the resonant frequency f1, it is possible to suppress damage to the switching element S caused by through-current flowing through the switching element S.

[0050] [Effects of this embodiment]

[0051] In this embodiment, the following effects can be obtained.

[0052] In this embodiment, as described above, the control unit 40 is configured to adjust the gate signal for turning the switching element S on and off based on the difference between the sine wave command for outputting a current of a desired waveform from the inverter unit 30 and the output current output from the inverter unit 30 to the induction heating coil 3, and a value corresponding to the magnitude of the overcurrent when the output current is an overcurrent. Thus, unlike the case where feedback control is performed based on the difference (deviation) between the sine wave command and the fed-back output current, the gate signal is adjusted based on a value corresponding to the magnitude of the overcurrent in addition to the deviation. That is, the deviation used for feedback control can be adjusted using a value corresponding to the magnitude of the overcurrent. As a result, even when the current flowing through the induction heating coil 3 becomes an overcurrent, the current supplied to the induction heating coil 3 can be quickly brought within a specified range.

[0053] Furthermore, in this embodiment, as described above, the control unit 40 is configured to adjust the gate signal by subtracting a value corresponding to the magnitude of the overcurrent from a value based on the difference between the sine wave command and the output current. This subtracts the value corresponding to the magnitude of the overcurrent from the value based on the difference between the sine wave command and the output current (the deviation used for feedback control), immediately reducing the deviation used for feedback control. Consequently, even if the current flowing through the induction heating coil 3 exceeds the specified range, the current supplied to the induction heating coil 3 can be more quickly brought within the specified range.

[0054] Furthermore, in this embodiment, as described above, the control unit 40 is configured to adjust the gate signal by subtracting an amount corresponding to the difference between the output current and the threshold value from a value based on the difference between the sine wave command and the output current, if the output current exceeds a predetermined threshold. This subtracts a value corresponding to the magnitude of the output current (the magnitude of the overcurrent) from the value based on the difference between the sine wave command and the output current (the deviation used for feedback control). As a result, the current supplied to the induction heating coil 3 can be quickly brought within a predetermined range based on the magnitude of the overcurrent.

[0055] In addition, in this embodiment, as described above, the sinusoidal wave command includes a sinusoidal wave command calculated based on the frequency of the gate signal, the phase of the output current of the inverter unit 30, and the effective value of the current command for outputting the desired current from the inverter unit 30, with the phase of the zero-crossing point of the output current of the inverter unit 30 as the starting point. Here, the frequency (carrier frequency) of the appropriate gate signal changes with changes in the load (the load impedance of the induction heating coil 3). Therefore, as described above, the sinusoidal wave command is constructed based on the frequency of the gate signal, the phase of the output current, and the effective value of the current command, with the phase of the zero-crossing point of the output current as the starting point, thereby enabling the sinusoidal wave command to be set to match the carrier frequency.

[0056] Furthermore, in this embodiment, as described above, the control unit 40 is configured to obtain the phase of the zero-crossing point of the output current based on the output current of the inverter unit 30 obtained via the filter 41. Oscillation (vibration of the output current) may occur near the zero-crossing point of the output current. Therefore, by attenuating the desired frequency component of the output current of the inverter unit 30 using the filter 41, the phase of the zero-crossing point of the output current can be appropriately obtained.

[0057] In addition, in this embodiment, as described above, the filter 41 and the filter 45 are configured to attenuate the same frequency component. Thus, the filter 41 for obtaining the phase of the zero-crossing point and the filter 45 for the difference between the sine wave command and the output current (deviation for feedback control) have the same characteristics, thereby enabling high-precision control to keep the current supplied to the induction heating coil 3 within a specified range.

[0058] Furthermore, in this embodiment, as described above, the control unit 40 is configured to adjust the frequency of the gate signal based on the difference between the sinusoidal wave command and the output current, and, in the event of an overcurrent, a value corresponding to the magnitude of the overcurrent. Consequently, even in the event of an overcurrent, the current supplied to the induction heating coil 3 can be quickly brought within a predetermined range during pulse frequency modulation (PFM) control, which controls the on / off switching of the switching element S by varying the frequency of the gate signal.

[0059] In addition, in the present embodiment, as described above, the induction heating coil 3 that heats the object to be heated is provided on the output side of the inverter unit 30. Here, in the induction heating coil 3 that heats the object to be heated, the load impedance of the induction heating coil 3 tends to change rapidly due to changes in the shape of the object to be heated and melted. Therefore, the current flowing through the induction heating coil 3 tends to become an overcurrent. Therefore, in the power conversion device 100 that supplies power to the induction heating coil 3 that heats the object to be heated, it is particularly effective to quickly converge the current supplied to the induction heating coil 3 within a prescribed range by configuring it as the power conversion device 100 of the present embodiment.

[0060] [Modification]

[0061] The embodiments disclosed herein are to be considered in all respects as illustrative and non-restrictive. The scope of the present invention is not indicated by the description of the embodiments described above, but by the claims, and includes all modifications (variations) within the meaning and scope equivalent to the claims.

[0062] For example, in the above embodiment, the present invention is applied to the power conversion device 100 used in the induction heating device 1 , but the present invention is not limited thereto and can also be applied to the power conversion device 100 used in devices other than the induction heating device 1 .

[0063] In the above embodiment, an example is shown in which the gate signal is adjusted by subtracting a value corresponding to the magnitude of the overcurrent (the difference between the sine wave command and a predetermined threshold value) from a value based on the difference between the sine wave command and the output current. However, the present invention is not limited to this. In the present invention, the gate signal can also be adjusted based on a value corresponding to the magnitude of the overcurrent using methods other than subtraction. For example, a value obtained by multiplying the difference between the sine wave command and the predetermined threshold value by a predetermined constant can be subtracted from the difference between the sine wave command and the output current.

[0064] In the above embodiment, an example is shown in which a sine wave command is generated based on the frequency of the gate signal, the phase of the output current of the inverter unit 30, and the effective value of the current command, with the phase of the zero-crossing point of the output current of the inverter unit 30 as the starting point. However, the method for generating a sine wave command is not limited to this method. Furthermore, commands other than sine wave commands may be used as commands for outputting a current of a desired waveform from the inverter unit 30.

[0065] In the above embodiment, the filter 41 and the filter 45 are configured as LPFs, but the present invention is not limited thereto. For example, the filter 41 and the filter 45 may be configured as filters other than LPFs.

[0066] In the above embodiment, pulse frequency modulation (PFM) control is used as an example for controlling the on / off switching of the switching element S. However, the present invention is not limited thereto. For example, pulse width modulation (PWM) control may also be used as the on / off control of the switching element S.

[0067] In the above embodiment, the induction heating coil 3 is used as the "load" of the present invention. However, the present invention is not limited to this. The present invention can also be applied to a power conversion device that supplies power to a load other than the induction heating coil 3. In addition, heating coils other than the induction heating coil 3 can also be used.

Claims

1. A power conversion device comprising: a rectifier that rectifies AC power supplied from a power source into DC power; a smoothing unit provided on an output side of the rectifying unit and smoothing the DC power obtained after rectification in the rectifying unit; an inverter unit that converts the DC power smoothed by the smoothing unit into AC power; as well as a control unit that controls on and off the switching element of the inverter unit, In which, the control unit is configured to adjust the gate signal for turning on and off the switching element by subtracting a value corresponding to the magnitude of the overcurrent when the output current is an overcurrent from a value based on a waveform instruction for outputting a current of a desired waveform from the inverter unit and an output current output from the inverter unit to the load, or subtracting a value obtained by multiplying the value corresponding to the magnitude of the overcurrent by a prescribed constant.

2. The power conversion device according to claim 1, wherein: The control unit is configured to adjust the gate signal by subtracting an amount corresponding to the difference between the output current and the predetermined threshold from a value based on the difference between the waveform command and the output current when the output current exceeds a predetermined threshold.

3. The power conversion device according to claim 1 or 2, characterized in that: The waveform instruction includes a sinusoidal wave instruction, which is an instruction calculated based on the frequency of the gate signal, the phase of the output current of the inverter unit, and the effective value of the current instruction for outputting the desired current from the inverter unit, and takes the phase of the zero crossing point of the output current of the inverter unit as the starting point.

4. The power conversion device according to claim 3, wherein: further comprising a first filter for attenuating a desired frequency component of the output current of the inverter unit, The control unit is configured to acquire a phase of a zero-cross point of the output current based on the output current of the inverter unit acquired via the first filter.

5. The power conversion device according to claim 4, characterized in that further comprising a second filter for attenuating a desired frequency component of the difference between the waveform command and the output current, The first filter and the second filter are configured to attenuate the same frequency component.

6. The power conversion device according to claim 1 or 2, characterized in that: The control unit is configured to adjust the frequency of the gate signal based on a difference between the waveform command and the output current and, when the output current is an overcurrent, a value corresponding to a magnitude of the overcurrent.

7. The power conversion device according to claim 1 or 2, characterized in that: The load includes a heating coil provided on an output side of the inverter unit and heating an object to be heated.

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