JP2001069761A - Pulse width modulation control converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pulse width modulation control converter capable of attaining high output power without degrading efficiency. SOLUTION: In this pulse width modulation control converter, its rectifying circuit is connected with a freewheeling diode 6 through a cable 7. The cable 7 serves as a protective DC reactor which protects the freewheeling diode 6 from surge voltage and the cable 7 restrains reverse recovery current flowing through the freewheeling diode 6 at the moment of mode shift, so that backward serge voltage applied to the free-wheeling diode 6 decreases. It is thus possible to eliminate an increase in the number of the freewheeling diodes 6 connected in series and attain high power by a rise in power supply voltage without degrading efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse width modulation control converter for converting AC power into DC power in order to supply DC power to a load such as an inverter device.

[0002]

2. Description of the Related Art Conventionally, AC power has been used to adjust the rotational speed of an induction motor in accordance with a load, or to perform induction heating to quench carbon steel such as a blade of a machine tool. And an inverter device whose output frequency can be adjusted according to the load. In general, an inverter device includes a converter that converts AC power into DC power in order to use AC power supplied by a power company. The converter provided in the inverter device includes a bridge circuit formed of a self-extinguishing element such as a transistor, a DC reactor for smoothing DC power including pulsation rectified by the bridge circuit, and a self-extinguishing element of the bridge circuit. A control circuit for controlling ignition is provided. In such a converter, as disclosed in Japanese Patent Publication No. H8-221145, when the self-extinguishing element of the bridge circuit is controlled to fire, the width of the DC voltage pulse output from the bridge circuit is determined by the power to be output. PWM control that adjusts according to the following conditions can be adopted.

FIG. 4 shows an example of a converter in which PWM control is applied to rectification of three-phase alternating current. 4, a converter 51 includes a bridge circuit 52 for full-wave rectification of three-phase AC, a DC reactor 53 for smoothing the current rectified by the bridge circuit 52, and a self-extinguishing element of the bridge circuit 52.
And a pulse width control circuit 55 for controlling the firing of the pulse 54. Among these, the bridge circuit 52 is provided between each of the U-W phases on the primary side and the positive electrode on the secondary side, and U-W on the primary side in order to rectify the half-wave of each phase of the three-phase AC. There is a self-extinguishing element 54 provided between each of the phases and the negative electrode on the secondary side. Each of these self-extinguishing elements 54 is connected in series with a blocking diode 56 so that a large surge voltage due to the switching operation is not applied to the self-extinguishing element 54.

[0004] The output side of the converter 51 is connected to a load 57 such as an inverter for inversely converting DC power into AC current of an appropriate frequency. A free-wheeling diode 58 is connected between the output terminals of the bridge circuit 52. Here, the plurality of self-extinguishing elements 54 provided in the converter 51 are provided in a rectifying mode in which the current i1 rectified by the bridge circuit 52 is supplied to the load 57 via the DC reactor 53, and in the free-wheeling diode 58 and the load. 57 forms a closed circuit, and switching is performed so that the closed circuit mode in which the induced current i2 of the DC reactor 53 circulates in the closed circuit is alternately repeated.

[0005]

In the converter 51 performing such an operation, the rectification mode in which the free wheeling diode 58 enters the reverse blocking state from the closed circuit mode in which the forward current i2 flows through the free wheeling diode 58. , The reverse recovery current i3 instantaneously flows through the free-wheeling diode 58 at the moment of transition to the rectification mode. Then, the direction of the current flowing through the free-wheeling diode 58 is instantaneously changed to the opposite direction, so that dI / dt, which is a temporal change of the current, increases, and a large reverse surge voltage is applied to the free-wheeling diode 58. Is added. Even if a snubber circuit is connected in parallel with the free wheeling diode 58 in order to reduce the reverse surge voltage applied to the free wheeling diode 58, the reverse surge voltage cannot be reduced. For this reason, there is a problem that the reverse breakdown voltage performance of the free-wheeling diode 58 is insufficient when an attempt is made to increase the output by increasing the power supply voltage. On the other hand, in order to ensure the reverse withstand voltage performance, it is necessary to increase the number of the free-wheeling diodes 58 connected in series. The efficiency of the process is reduced.

SUMMARY OF THE INVENTION An object of the present invention is to provide a pulse width modulation control type converter capable of increasing the output without lowering the efficiency.

[0007]

SUMMARY OF THE INVENTION The present invention provides a bridge type rectifier circuit having a self-extinguishing element as a main element and rectifying AC power from an AC power supply, and a pulse-like intermittent output from the rectifier circuit. Reactor connected to the output of the rectifier circuit, a DC reactor for smoothing the current to be applied and a DC current, a pulse width control circuit for controlling the conduction time of the self-extinguishing element according to the power to be output, A pulse width modulation control type converter including a wheeling diode, wherein a protection DC reactor for protecting the free wheeling diode from a surge voltage is connected between the rectifier circuit and the free wheeling diode. It is characterized by having been done. In the present invention as described above, when shifting from the closed circuit mode to the rectification mode, the reverse recovery current flowing instantaneously through the free wheeling diode is suppressed by the protection DC reactor, so that the current flows through the free wheeling diode. Even if the direction of the current instantaneously changes to the opposite direction, dI / dt, which is the temporal change of the current, does not increase as in the related art, and the reverse surge voltage applied to the free-wheeling diode decreases. For this reason, it is not necessary to increase the number of free-wheeling diodes connected in series when increasing the power supply voltage to increase the output, and the efficiency decreases when increasing the power supply voltage and increasing the output. There is no.

In the above, it is desirable that the cable connecting the free wheeling diode and the rectifier circuit be the protective DC reactor. Here, the free-wheeling diode usually fixes each element of the rectifier circuit and is connected to a bus bar (bus) that electrically connects these elements. The bus bar has a small inductance. Therefore, as described above, if a cable of an appropriate length is used for connection with the free-wheeling diode and the rectifier circuit, an appropriate-sized inductance is formed between the free-wheeling diode and the rectifier circuit, This cable acts as a reactor to suppress the reverse recovery current and reduce the reverse surge voltage applied to the free-wheeling diode.

It is preferable that an inverter for generating high-frequency power for performing induction heating is connected to the output side. In order to finely adjust the power to the inverter device that generates such high-frequency power, it is necessary to switch the self-extinguishing element on the converter side at high speed. When the self-extinguishing element is switched at high speed, the reverse surge voltage applied to the free-wheeling diode tends to increase. If the present invention is applied to a converter that supplies DC power to an inverter device that generates such high-frequency power, the protective DC reactor suppresses the reverse recovery current, so that the reverse surge voltage applied to the free-wheeling diode is reduced. As a result, the output of the converter can be increased, and the output of the inverter device that generates high-frequency power for induction heating can be increased.

Further, the self-extinguishing element is an IGBT (Insulated
Gate Bipolar mode Transistor, MOSFET (MOS field effect transistor) and GTO (Gate Turn Off Thyristo)
It is desirable to use any one of the high-speed switching elements of r). If a rectifier circuit is formed by such a high-speed switching element, the conduction time of the self-extinguishing element can be finely adjusted in accordance with the power to be output to the load, and a fine-grained pulse width control circuit becomes possible. Thereby, the optimal amount of power for the load is always supplied to the load.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a converter 1 according to an embodiment of the present invention. This converter 1
Is a current-type converter that converts three-phase AC power supplied from the three-phase AC power supply 2 into DC power. Converter 1
Is connected to an inverter device 3 that converts DC power into AC power. Thus, the DC power converted by the converter 1 is converted by the inverter device 3 into high-frequency power required for induction heating. The converter 1 has a plurality of self-extinguishing elements 4 as main elements, a bridge-type rectifying circuit 10 for rectifying AC power with these self-extinguishing elements 4, and an output from the rectifying circuit 10. There are provided a DC reactor 20 for smoothing a pulse-shaped intermittent current to make it a DC current, and a pulse width control circuit 30 for controlling the conduction time of the self-extinguishing element 4 according to the power to be output.

In order to rectify the three-phase alternating current supplied from the AC power supply into direct current, between each of the U-phase, V-phase and W-phase on the primary side of the rectifier circuit 10 and the positive electrode on the secondary side, Positive electrode self-extinguishing elements 4U +, 4V +, 4W + are provided, respectively, and between the primary U-phase, V-phase, and W-phase and the secondary negative electrode, self-extinguishing elements 4U-, 4V-, 4W- are provided. In order to protect these self-extinguishing elements 4 from surge voltage, a blocking diode 5 is connected to each self-extinguishing element 4 in series. The connection and fixation of the self-extinguishing element 4 and the blocking diode 5 are performed by a bus bar 11 forming a conductive path of the rectifier circuit 10. Here, as the self-extinguishing element 4 of the rectifier circuit 10, an IGBT for power control, which is a high-speed switching element, is employed.

A free wheeling diode 6 is connected between the positive output and the negative output of the rectifier circuit 10. For connection to the rectifier circuit 10 and the free-wheeling diode 6, a cable 7 having a larger inductance than the bus bar 11 is used. This cable 7
It is a protective DC reactor for protecting the free wheeling diode 6 from surge voltage. further,
The AC input portion of the rectifier circuit 10 includes a capacitor 8 that absorbs an overvoltage generated during the switching operation of the self-extinguishing element 4, and a high-frequency current generated during the switching operation of the self-extinguishing element 4, ie, a carrier. An AC reactor 9 for preventing the high-frequency component current of the wave from flowing back to the AC power supply side is connected.

The pulse width control circuit 30 intermittently fires the self-extinguishing element 4 and adjusts the width of the DC voltage pulse output from the rectifier circuit 10 according to the power to be output. Control means for performing the following. The pulse width control of the pulse width control circuit 30 detects and monitors the input three-phase AC voltage and the DC voltage output from the rectifier circuit 10 so that the DC voltage to the inverter device 3 has a constant value. To
This is a method of expanding the width of the DC voltage pulse output from the rectifier circuit 10. As a result, the power consumption of the inverter device 3 increases, the output current of the converter 1 increases, the voltage drop due to the internal resistance increases, and when the output voltage to the inverter device 3 is about to decrease, the width of the DC voltage pulse is increased. Is extended, the decrease in the output voltage is suppressed, and the power supplied to the inverter device 3 is increased.

In this pulse width control, a rectification mode and a closed circuit mode described later are set. In other words, the pulse width control circuit 30 generates a carrier wave that is a pulse-like control signal to be applied to the gate of the self-extinguishing element 4 so that the rectification mode and the closed circuit mode are alternately repeated.
The self-extinguishing element 4 is appropriately fired by this carrier wave.

Next, the operation of the converter 1 of the present embodiment will be described. Here, as shown in FIG. 2, the output voltage of the AC power supply 2 is a three-phase AC whose phases are shifted from each other by 120 °. This three-phase output voltage is one cycle T
When attention is paid to the periods T1 to T6 obtained by dividing into six equal parts, in each of the periods T1 to T6, the two phases have the same polarity and the remaining phases have the opposite polarities. Therefore, it can be understood that DC power can be obtained by rectifying each of the two phases with reference to one phase having the opposite polarity. Further, the waveforms in the periods T1, T3, and T5 have the same shape as each other, and the waveforms in the periods T2, T4, and T6 are only opposite in polarity to the waveforms of the periods T1, T3, and T5. . Therefore, the operation of converter 1 in period T1 will be described, and the operation in periods T2 to T6 will be the same as the operation in period T1, and a description thereof will be omitted. Note that the waveforms in the periods T1, T3, and T5 have the same shape as each other, and the waveforms in the periods T2, T4, and T6 have the same shape as the waveforms in the periods T1, T3, and T5, although they have opposite signs. Therefore, here, the operation in the period T4 will be described, and the periods T1 to T3, T5, T6
The operation in is the same as in the period T4, and thus the description is omitted.

As shown in FIG. 2, the converter 1
It is assumed that a three-phase AC voltage that changes in a sine wave shape is applied. Then, in the period T4, the self-extinguishing element on the V-phase positive electrode side
4V + maintains a conductive state from the start to the end of the period T4. In this state, the self-extinguishing element on the negative side of the V phase
When 4V- is conducted, as shown in FIG. 3A, the rectifier circuit 10 and the inverter device 3, and the free wheeling diode 6 and the inverter device 3 form a closed circuit, and the closed circuit has a direct current. The induction current of the reactor 20 circulates, whereby the closed circuit mode is performed. Here, the current i4 flowing through the inverter device 3 is
On the side, a current i5 flowing through the rectifier circuit 10 and a current i6 flowing through the freewheeling diode 6 are divided.

Next, when the self-extinguishing element 4U- on the negative side of the U-phase is turned on, as shown in FIG.
The current rectified by the ten self-arc-extinguishing elements 4V + and the self-arc-extinguishing element 4U− is supplied to the inverter device 3 via the DC reactor 20.
, Whereby the first rectification mode is performed. At this time, the voltage of the V-phase of the three-phase AC is positive and the voltage of the U-phase is negative, so that the current flows from the V-phase to the U-phase. Subsequently, when the self-extinguishing element 4W− on the negative side of the W phase is turned on, the self-extinguishing element 4V + and the self-extinguishing element 4W− of the rectifier circuit 10 rectify as shown in FIG. The supplied current flows through the DC reactor 20 to the inverter device 3, whereby the second rectification mode is performed. At this time, the voltage of the V-phase of the three-phase AC is positive and the voltage of the W-phase is negative, so that the current flows from the V-phase to the W-phase. Such a closed circuit mode, the first rectification mode, and the second rectification mode include:
Until the period T4 ends, it is repeated alternately a plurality of times.

In the operation of the converter 1 as described above, the mode shifts from the closed circuit mode in which the forward current i6 flows through the freewheeling diode 6 to the rectification mode in which the freewheeling diode 6 is in the reverse blocking state. Then
At the moment of the transition to the rectification mode, a reverse recovery current i3 flows instantaneously through the free-wheeling diode 6, but this reverse recovery current i3 is suppressed by the cable 7, which is a protective DC reactor. Does not. As a result, even if the direction of the current flowing through the free-wheeling diode is instantaneously changed to the opposite direction, dI / dt, which is a temporal change of the current, is suppressed, and the free-wheeling diode 58
No large reverse surge voltage is applied to the motor. Moreover, the current i4 flowing through the inverter device 3 in the closed circuit mode
Is divided into a current i5 on the rectifier circuit 10 side and a current i6 on the freewheeling diode 6 side, and the current value flowing through the freewheeling diode 6 in the closed circuit mode is reduced. DI / dt which is a temporal change
Is suppressed.

According to the above-described embodiment, the following effects can be obtained. That is, at the moment of transition to the rectification mode, the reverse recovery current i3 flowing instantaneously through the free wheeling diode 6 is suppressed by the cable 7 which is a protection DC reactor. Instantaneously turns in the reverse direction, dI / dt, which is a temporal change of the current, is suppressed, and a large reverse surge voltage is not applied to the free-wheeling diode 58. For this reason, even if the power supply voltage is increased to increase the output, it is not necessary to increase the number of free-wheeling diodes 6 connected in series, and the output of the converter 1 can be increased without lowering the efficiency. Can be.

In addition, the current i4 flowing through the inverter device 3 in the closed circuit mode is divided into the current i5 on the rectifier circuit 10 side and the current i6 on the free wheeling diode 6 side. DI / dt, which is the temporal change of the current,
From this point, a large reverse surge voltage is not applied to the free-wheeling diode 58, and the output of the converter 1 can be increased without lowering the efficiency.

The cable 7 for connecting the free wheeling diode 6 and the rectifier circuit 10 to each other is a protective DC reactor, and the cable 7 protects the free wheeling diode 6 from surge voltage.
An ordinary converter having no protective DC reactor can be easily modified.

An inverter 3 for generating high-frequency power for performing induction heating is provided on the output side of the converter 1.
In order to finely adjust the power to the inverter device 3 even when the self-extinguishing element 4 on the converter 1 side is switched at a high speed, the cable 7 serving as the protection DC reactor suppresses the reverse recovery current. Therefore, the surge voltage in the reverse direction applied to the free-wheeling diode 6 is reduced, the output of the converter 1 can be increased, and the output of the inverter device 3 that generates high-frequency power for induction heating can be increased.

Further, as the self-extinguishing element 4, an IGBT
Is adopted, the conduction time of the self-extinguishing element 4 can be finely adjusted in accordance with the power to be output to the inverter device 3, fine pulse width control becomes possible, and the optimum power for the inverter device 3 is always obtained. The quantity can be supplied to the inverter device 3.

Although the present invention has been described with reference to preferred embodiments, the present invention is not limited to these embodiments, and various improvements and design changes may be made without departing from the spirit of the present invention. It is possible. For example, the self-extinguishing element is not limited to the IGBT but may be any of a MOSFET and a GTO. In short, any element that can perform high-speed switching and has a small DC resistance value during conduction may be used. In addition, as a DC reactor for protection,
Not only the cable but also a coil-shaped reactor may be used, and a specific configuration of the protection DC reactor may be appropriately set in practice.

Further, the converter is not limited to a converter that rectifies a three-phase AC, but may be a converter that rectifies a single-phase AC. Further, the converter is not limited to a current type converter having a relatively large output impedance, but may be a voltage type converter having a relatively small output impedance.

[0027]

As described above, according to the present invention, it is possible to increase the output of the converter without lowering the efficiency.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a converter according to an embodiment of the present invention.

FIG. 2 is a diagram showing an input voltage waveform of the converter of the embodiment.

FIG. 3 is a diagram for explaining the operation of the embodiment.

FIG. 4 is a diagram corresponding to FIG. 1 showing a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Converter 2 Three-phase AC power supply 3 Inverter device 4 Self-extinguishing element 5 Blocking diode 6 Free-wheeling diode 7 Cable 10 Rectifier circuit 20 DC reactor 30 Pulse width control circuit

Continuation of the front page (72) Inventor Satoshi Kano 5983 Tamura, Hiratsuka-shi, Kanagawa F-term smelting company F-term (reference) 5H006 AA05 CA01 CB01 DB01 DC05

Claims (4)

[Claims] 1. A bridge type rectifier circuit having a self-extinguishing element as a main element and rectifying AC power from an AC power supply, and a DC current smoothing a pulse-like intermittent current output from the rectifier circuit. A pulse width control circuit that controls the conduction time of the self-arc-extinguishing element according to the power to be output, and a free-wheeling diode connected between the outputs of the rectifier circuit. A width modulation control type converter, wherein a protection DC reactor for protecting the free wheeling diode from a surge voltage is connected between the rectifier circuit and the free wheeling diode. Pulse width modulation control converter. 2. The pulse width modulation control type converter according to claim 1, wherein a cable connecting the free-wheeling diode and the rectifier circuit to each other is the protective DC reactor. Width modulation control type converter. 3. The pulse width modulation control converter according to claim 1, wherein an inverter device for generating high frequency power for performing induction heating is connected to an output side of the converter. A pulse width modulation control converter. 4. The pulse width modulation control converter according to claim 1, wherein said self-turn-off device is a high-speed switching device of any of IGBT, MOSFET and GTO. A pulse width modulation control converter.