JPH11196579A - Control device for semiconductor power converter - Google Patents

Abstract

(57) [Problem] To achieve good control characteristics by minimizing the generation of a minimum pulse width (inter-pulse off-width) limit during low-output (or high-output) operation. An absolute value circuit and a low-pass filter are provided.
To calculate the amplitude of the signal wave. When the calculated amplitude is small (or large), the function generator 14 reduces the voltage of the carrier with decreasing (or increasing) amplitude toward the lowest frequency, and sets the voltage so that the frequency becomes constant at the maximum frequency in the middle range. An input control voltage for oscillating the control oscillator 16 is generated. The function generator may generate a function such that the frequency of the carrier changes according to the amplitude and frequency of the signal wave.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for a semiconductor power conversion device for performing pulse width modulation (PWM) using a carrier wave and a signal wave to convert AC power to DC power or DC power to AC power. In particular, the present invention relates to a control device for a semiconductor power conversion device that performs good conversion during low-output or high-output operation.

The present invention can be used in all power application fields to which a PWM-modulated semiconductor power converter is applied, for example, a power supply for driving a current of a superconducting coil having zero electric resistance.

[0003]

2. Description of the Related Art Conventionally, PWM (Pu
The lse Width Modulation (pulse width modulation) converter and the PWM inverter are roughly classified as shown in FIG. 6 depending on whether they are voltage type or current type. FIG. 6 shows the case of a converter. FIG. 6A shows a configuration example of a typical voltage type converter, and FIG. 6B shows a configuration example of a typical current type converter. Although a single phase is shown here for simplicity, the principle of operation is the same for three phases. In FIG. 6, S1 to S4 are switching elements, which are represented by transistors here. In the voltage type converter of FIG. 6A, the diode D1 is often inserted in parallel to form the freewheel mode. In the current-type converter shown in FIG. 6B, the diode D1 is often inserted because the reverse breakdown voltage of the self-erasing element is low.
Note that an inverter refers to an operation in the case where the flow of electric power is changed from a DC side to an AC power supply, that is, an operation in the case where DC power is converted to AC power. Abbreviate. Further, the other components shown in FIG. 6 and their operations are well known to those skilled in the art, and thus description thereof is omitted.

[0004] The PWM control is to give an ON / OFF command to these switching elements S1 to S4 shown in FIG. The PWM modulation is a method of determining an ON / OFF pattern of a switching element by comparing a control signal with a carrier, and there are many methods of the method. Even if only the used carrier wave is used, there are a sawtooth wave, a triangular wave, a sine wave, and the like, each of which has a characteristic. The simplest example is PWM using a single-phase PWM inverter circuit.
This will be described with reference to FIG. 7, which is an explanatory diagram of the modulation method. FIG. 7A shows a configuration example of a single-phase voltage-type inverter circuit having a DC voltage source E as a power supply and performing pulse width modulation (PWM). The capacitor C shown in FIG. This corresponds to the case where E is replaced. Therefore, the switching element S
Depending on ON / OFF from 1 to S4, three voltages of + E, 0, and -E can be applied to the loads L and R. FIG.
(B) is a diagram for explaining the principle of pulse width modulation (PWM). In this example, a triangular wave is used as a carrier. At this time, a command value for the load voltage (hereinafter referred to as a signal wave)
Has a waveform (substantially a sine wave shape) shown in FIG. 7B. That is, PWM is a method of determining the ON / OFF pattern of each switching element by comparing the magnitude of a signal wave with the magnitude of a carrier wave, as shown in FIG. 7B, and averaging the output to the signal wave. By making the waveforms similar to each other and controlling the ratio from the three voltages of + E, 0, and -E to the on / off time, +
It refers to a modulation method in which an output between E and -E is continuously obtained as a time average.

[0005] However, in the conventional PWM control, the frequency of the carrier is given as a constant value from the outside, or the output of a frequency multiplier for synchronizing with the power supply voltage is used. In any case, it was not envisaged that the frequency of the carrier wave would vary significantly as a function of the signal wave. In other words, the conventional PWM control is designed on the condition that the frequency and phase of the carrier are constant in principle. Therefore, depending on the operating conditions, assuming a constant change in frequency and phase,
As a result, an undesirable control state may occur. Next, this point will be described.

For example, as shown in FIG. 8A, an example of a current-type converter for driving a current of a superconducting coil is assumed. Here, an AC filter is inserted on the AC side to remove harmonics. To pass a certain current through the superconducting coil, the switching elements S1
This is done by appropriately turning on and off S4. After a constant current flows through the superconducting coil, the superconducting coil does not require power in principle to maintain the coil current. In other words, ignoring the resistance of the switching element and the circuit, it is sufficient to set a circulating (or return) current mode in which the current of the superconducting coil continues to flow through the switching elements S4 and S2 as shown in FIG. Yes,
The coil current can be kept constant without repeatedly turning on and off. However, in actual operation of the superconducting coil, the switching elements S1 to S4 are often turned on and off in FIG. That is, regardless of the switching pattern, the power supply and the load are connected for a certain period of time, but the current type converter is in a low output operation state.

PWM using comparison circuit for carrier wave and signal wave
In the control circuit, the system is designed on the assumption that there is always an intersection in one carrier wave. For this reason, when trying to control the superconducting coil at a constant current in the low output operation state of the current type converter, a pattern with a very short pulse width (an impulse train pattern) is generated as shown in FIG. Results.

Semiconductor power conversion elements used as switching elements include power transistors, thyristors, gate turn-off thyristors GTO, insulated gate bipolar transistors IGBT, and the like. Therefore,
Switching the off state requires a certain amount of time determined by the element. However, when a pattern having a short pulse width is given as described above, for example, in the case of a power transistor, the transistor is operated in the active region, resulting in an increase in power loss, and in the worst case, destruction of the element.
Further, in the thyristor, the reverse pressure application time required to turn off the element, that is, the turn-off time cannot be guaranteed, and in some cases, commutation may fail. So, in order to avoid operationlessly short on / off operation (from the viewpoint of power conversion),
As shown in FIG. 10, a maximum / minimum limiter is provided for a modulation signal input (signal wave) so that a signal wave always crosses within one carrier wave, thereby providing a minimum pulse width. And a maximum on / off period.

If the on / off operation is always generated within one carrier wave, the number of switching times of the element becomes exactly equal to the frequency of the carrier wave. As a result, there is an advantage that the switching loss can be accurately evaluated. However, in the case where the current of the superconducting coil is driven, a PWM pattern with zero DC output average voltage is generated, and only the reactive current flows to the AC side. Moreover, if the pulse width per carrier is equally limited by the limiter, the advantages of PWM control are no longer available. That is, an impulse-shaped current equal to the magnitude of the DC output current flows to the AC side, resulting in a large waveform distortion in the AC-side current waveform. This causes pulsation of the DC output current, and as a result of the feedback control response, the AC current waveform is distorted.

[0010]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a control device for overcoming the above-mentioned difficulties and allowing a semiconductor power conversion device to perform a good conversion during a low-output or high-output operation. is there.

[0011]

In order to solve the above problems, a pulse width modulation (PWM) using a carrier wave and a signal wave is provided.
In the control device for a semiconductor power conversion device of the present invention, which performs AC power conversion from DC power to DC power or from DC power to AC power, the frequency of the carrier is determined as an amplitude of the signal wave or a function of both of them. It is characterized by the following.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has been made to overcome the above-mentioned difficulties, and the switching frequency, that is, the frequency of a carrier wave, is changed in principle according to the average pulse width of a switching element, that is, the amplitude of a signal wave. This can be overcome and this is the gist of the present invention. Figure 1 below
The basic principle of the present invention will be described with reference to FIG. 1A and 1B are diagrams for explaining a PWM pattern when the frequency of a carrier is changed. FIG. 1A illustrates a case where the frequency of the carrier is constant, and FIG. 1B illustrates a case where the frequency of the carrier is reduced. Each is shown schematically. As shown in FIG. 1A, when the amplitude of the signal wave becomes small, the pulse width becomes short, and the pulse width becomes smaller than the allowable minimum pulse width. Therefore, as shown in FIG. 1B, the carrier is switched to a carrier wave having the same amplitude and appropriately lowering only the frequency. Naturally, the number of pulses decreases in proportion to the frequency, but the time per pulse is extended. As a result, the pulse whose pulse width is less than the allowable value decreases and the harmonic component increases, but the fundamental component approaches the signal wave.

From the above, the frequency of the carrier may be automatically determined by using the amplitude of the signal wave as a function. At that time, it is necessary to set the maximum frequency and the minimum frequency as before. Further, assuming that the carrier is used as a converter, the frequency of the carrier may be set to an integral multiple of the power supply frequency. However, in order to avoid unstable operation, it is necessary to provide a hysteresis characteristic as described later.
Hereinafter, a configuration example of a preferred control device for a semiconductor power conversion device according to the present invention will be described.

FIG. 2 shows an example of a circuit configuration of a control device for generating the frequency of a carrier wave as a function of a signal wave, specifically, a function of its amplitude when the present invention is applied to a control device for a single-phase inverter. ing. In FIG. 2, an absolute value circuit 10 detects, for example, a peak of a signal wave, and a low-pass filter 12 calculates the amplitude of the signal wave by smoothing the detected waveform, and outputs an output representing the amplitude of the signal wave. In the case of a three-phase inverter or a converter, the amplitude of the signal wave may be calculated in a vector. Voltage controlled oscillator 16
Is an oscillator whose oscillation frequency changes according to the magnitude of the input control voltage, as is well known. FIG. 3 shows an example of a change in the frequency of a carrier with respect to the amplitude of a signal wave according to a preferred embodiment of the present invention. Since the oscillation frequency of the voltage controlled oscillator 16 changes according to the input control voltage, it can be read that the vertical axis in FIG. 3 indicates the magnitude of the input control voltage. That is, the function generator 14 generates a voltage having the shape shown in FIG. 3 according to the amplitude of the signal wave calculated from the low-pass filter 12. When the semiconductor power converter is operated with a pulse width that allows the switching element to be reliably turned on or off, the amplitude of the signal wave in FIG. 3 corresponds to a range having an intermediate magnitude, and the function generator 14 has a constant value. A maximum voltage is generated, and accordingly, the voltage controlled oscillator 16 oscillates at a reference (highest) frequency. On the other hand, when the operation of the semiconductor power converter reduces the output and approaches zero, the frequency of the carrier wave of the voltage controlled oscillator 16 is reduced to reduce the amplitude of the signal wave so that the width of the pulse for turning on the switching element does not become too narrow. And lower it so that the output becomes the lowest frequency when the output is zero. Therefore, when such a signal wave has a small amplitude, the function generator 1
4 decreases from the maximum voltage to the minimum voltage as the amplitude of the signal wave decreases. Conversely, even when the operation of the semiconductor power converter increases the output and approaches the maximum output, the frequency of the carrier wave of the voltage controlled oscillator 16 is changed so that the pulse width for turning off the switching element does not become too narrow. With the increase of the amplitude of the signal so that the output becomes the lowest frequency when the output is the highest. Therefore, at such a large amplitude of the signal wave, the output voltage of the function generator 14 decreases from the maximum voltage to the minimum voltage as the amplitude of the signal wave increases. In FIG. 3, the manner in which the frequency changes in a region where the frequency of the carrier changes with respect to the change in the amplitude of the signal wave is linear,
The present invention is not limited to this. The carrier wave frequency may be reduced on the side where the amplitude of the signal wave is small, in accordance with the decrease in the amplitude of the signal wave, and on the side where the amplitude of the signal wave is large, in accordance with the increase in the amplitude of the signal wave.

As shown in FIG. 3, switching is performed by a well-known comparison circuit and a pulse generation circuit (not shown) based on the carrier wave of the voltage controlled oscillator 16 having a frequency that varies according to the amplitude of the signal wave and the signal wave. A pulse is generated that is applied to the device. Since the frequency of the carrier decreases as the amplitude of the signal wave decreases as the amplitude decreases and as the amplitude increases as the amplitude of the signal wave increases, the on-pulse width (at the time of small amplitude) applied between the switching elements and the pulse The off-width (at the time of large amplitude) is maintained at or above an allowable value as shown in FIG. As a result, the switching element operates stably, so that the semiconductor power converter can stably perform power conversion even in low-output or high-output operation. Although the embodiment shown in FIGS. 2 and 3 is applied to a converter, it is apparent that the embodiment can be applied to an inverter in the same manner.

As known to those skilled in the art, in order for the PWM semiconductor power converter to operate stably, the ratio between the frequency of the signal wave and the frequency of the carrier needs to be within a certain range. Therefore, when the frequency of the signal wave changes, it is desirable to change the frequency of the carrier wave according to not only the amplitude of the signal wave but also the frequency. Such a case will be described below. FIG. 4 shows an example of a circuit configuration of a control device for a PWM converter that determines a frequency of a carrier wave as a function of two variables of amplitude and frequency of a signal wave according to a preferred embodiment of the present invention. 4, components denoted by the same reference numerals as those in FIG. 2 indicate the same components as those in FIG. 2, and description thereof will not be repeated. In FIG. 4, the zero point detection and counter 20 detects a zero point of the amplitude of the signal wave and counts the number of zero points per cycle of the signal wave.
An output representing the frequency of the signal wave is generated. The two-variable function generator 22 converts a carrier wave having a desired frequency into a frequency voltage based on an output representing the amplitude of the signal wave from the absolute value circuit 10 and an output representing the frequency of the signal wave from the zero point detection and the counter 20. It generates an input control voltage to the voltage controlled oscillator 16 for the control oscillator 16 to generate.

FIG. 5 shows an example of the characteristics of the two-variable function generator 22. FIG. 5A shows the voltage-controlled oscillator 1 with respect to the frequency of the signal wave.
6 shows the change of the input control voltage as a change in the frequency of the carrier wave, and (b) shows the change in the amplitude of the signal wave (a).
Shows the selection of the function shown in FIG. In this example, as shown in FIG. 5A, the frequency of the carrier wave is controlled in a rectangular wave with respect to the frequency of the signal wave, and as shown in FIG. Let me. In FIG. 5A, when the amplitude of the signal wave is of a standard magnitude other than at the time of the small amplitude and the large amplitude (in FIG. 5B, the function A is set in the middle range of the amplitude of the signal wave). Is selected), the two-variable function generator 22 for the frequency of the signal wave
Are shown by solid lines. Since the carrier frequency must be between the highest and lowest frequencies,
As the frequency of the signal wave increases, the frequency of the carrier wave changes from the lowest frequency to the highest frequency, shifts from the highest frequency to the lowest frequency at some point of the frequency of the signal wave, and so on. The reason why the shift has hysteresis in the increasing and decreasing directions of the frequency of the signal wave is to stabilize the operation in the vicinity thereof. The signal wave when the amplitude of the signal wave is small and large (corresponding to the case where the function B is selected in the range of the small amplitude and the large amplitude of the amplitude of the signal wave in FIG. 5B) The change of the frequency of the two-variable function generator 22 with respect to the frequency is indicated by an alternate long and short dash line. Note that the frequency of the signal wave is the same as that of the above-described standard magnitude outside the range indicated by the dashed line, and the frequency of the carrier changes along the solid line with respect to the frequency of the signal wave.

When the amplitude of the signal wave is standard, the two-variable function generator 22 selects the function A as shown in FIG. 5B and responds to the change in the frequency of the signal wave. Then, an input control voltage is generated such that the frequency of the carrier changes as shown by the solid line in FIG. in this case,
As described above, since the ratio between the frequency of the signal wave and the frequency of the carrier wave may be within a certain range, the change in the frequency transition of the carrier wave is large, and the signal when the carrier frequency is the highest frequency is changed. The range of wave frequencies is wide. On the other hand, when the amplitude of the signal wave is small or large, the two-variable function generator 22 selects the function B as shown in FIG. Correspondingly, the input control voltage is generated such that the frequency of the carrier changes as shown by the dashed line in FIG. In this case, in addition to ensuring that the ratio relationship between the frequency of the signal wave and the frequency of the carrier wave is within a certain range, too narrow pulse width or too narrow off between pulses during low output or high output operation In order to avoid the occurrence of width, the transition of the carrier frequency changes slowly. In addition, when the magnitude of the amplitude of the signal wave changes while the frequency of the signal wave changes, and there is a change in selection between the function A and the function B, FIG.
The carrier frequency changes so as to shift between the solid line and the dashed line in accordance with the selection change. It should be noted that, as shown in FIG. 5 (b), the reason why the function selection has hysteresis in response to the change in the amplitude of the signal wave is that the frequency of the carrier wave is changed in the vicinity of the change in the selection as shown in FIG. This is to avoid flapping between the solid line and the one-dot chain line in a).

In FIG. 5, the manner in which the frequency of the carrier wave changes with respect to the change in the frequency of the signal wave is linear, but the present invention is not limited to this. What is necessary is just to reduce the frequency of the carrier according to the reduction.

As shown in FIG. 5, a voltage controlled oscillator 1 having a frequency that varies according to the amplitude and frequency of a signal wave
Based on the carrier wave and the signal wave of No. 6, a pulse to be applied to the switching element is generated by a well-known comparison circuit and a pulse generation circuit (not shown). In this case, since the frequency of the carrier wave is reduced along with the frequency of the signal wave, the ON pulse width applied to the switching element (at a small amplitude)
The off-width between pulses (at the time of large amplitude) is maintained at a value equal to or larger than the allowable value as shown in FIG. Moreover,
The ratio relationship between the frequency of the signal wave and the frequency of the carrier is kept within a certain range. As a result, since the switching element operates stably, the semiconductor power converter can stably perform power conversion even in low-output and high-output operations. Therefore, the optimum carrier frequency can be determined in consideration of all information related to the input and output of the PWM converter. Although the embodiment shown in FIGS. 4 and 5 is applied to a converter, it is apparent that the embodiment can be applied to an inverter in the same manner.

In the present invention, the signal wave as the command value may be generated by a signal wave generating circuit, but it is also possible to use the input side or output side voltage or current of the semiconductor power converter. The signal wave in the present invention includes them.

[0022]

The present invention is constructed as described above. Since the frequency of the carrier is determined as a function of the amplitude of the signal wave or the frequency or both of them, the pulse width is zero or infinity in principle. In such an operating state as described above, it is possible to select an optimum carrier frequency corresponding to the rating of the semiconductor power converter, and stabilization by optimal control can be realized. That is, the occurrence of the minimum pulse width limitation or the minimum off-width limitation between pulses during low-output or high-output operation can be minimized, and good control characteristics can be realized.

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams for explaining a PWM pattern when the frequency of a carrier is changed, wherein FIG. 1A illustrates a case where the frequency of the carrier is constant, and FIG. 1B illustrates a case where the frequency of the carrier is reduced. Each is shown schematically.

FIG. 2 shows an example of a circuit configuration created as a function of the amplitude of a carrier frequency when the present invention is applied to a control device for a single-phase inverter.

FIG. 3 shows an example of a change in the frequency of a carrier with respect to the amplitude of a signal wave according to a preferred embodiment of the present invention.

FIG. 4 shows an example of a circuit configuration for a control device of a PWM converter for determining a frequency of a carrier wave as a function of two variables of amplitude and frequency of a signal wave according to a preferred embodiment of the present invention.

5 shows an example of characteristics of the two-variable function generator 22 shown in FIG. 4, wherein (a) shows the voltage-controlled oscillator 1 with respect to the frequency of the signal wave;
6 shows the change of the input control voltage as a change in the frequency of the carrier wave, and (b) shows the change in the amplitude of the signal wave (a).
The function selection shown in FIG.

FIG. 6A shows a configuration example of a typical voltage-type converter, and FIG. 6B shows a configuration example of a typical current-type converter.

FIG. 7A shows a configuration example of a single-phase voltage-type inverter circuit having a DC voltage source E as a power source and performing pulse width modulation (PWM), and FIG. 7B shows a comparison of the magnitude of a signal wave and a carrier wave; FIG. 4 is a diagram for explaining a method of determining an on / off pattern of each switching element.

FIG. 8 (a) shows an example of a current type converter for driving a current of a superconducting coil, and FIG. 8 (b) can keep a constant coil current without repeating on / off in a circulating current mode. FIG.

FIG. 9 shows a current-type converter in a low-power operation state.
When trying to control a constant current in a superconducting coil, a pattern with a very short pulse width (impulse train pattern)
FIG. 4 is a diagram for explaining that the occurrence of the error occurs.

FIG. 10 provides a maximum / minimum limiter for a modulation signal input (signal wave) so that a signal wave always intersects within one carrier wave. It is a figure showing that an OFF period is guaranteed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Absolute value circuit 12 Low-pass filter 14 Function generator 16 Voltage controlled oscillator 20 Zero detection and counter 22 Two-variable function generator

Claims (1)

[Claims] 1. A control device for a semiconductor power converter that performs pulse width modulation (PWM) using a carrier wave and a signal wave to convert AC power to DC power or DC power to AC power, wherein the frequency of the carrier wave is A control device for a semiconductor power conversion device, wherein the control device is determined as a function of an amplitude or a frequency of a signal wave or both of them.