PWM power converter

Details PWM power converter PWM power converter

1999-09-30

2001-07-10

Han, Jessica (Department: 2838)

Electric power conversion systems

Current conversion

With means to introduce or eliminate frequency components

Reexamination Certificate

active

06259611

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a PWM power converter employing a pulse width modulation (PWM) method, which converter is constructed using switching devices, and provides a desired output voltage by controlling the ON duration and OFF duration of the switching devices. In particular, the present invention is concerned with such a PWM power converter that includes a PWM signal generating portion consisting of a digital hardware, and is able to generate a low output voltage with reduced ripple or errors in the output voltage.
BACKGROUND OF THE INVENTION
In power converters, a PWM method has been widely used as a control method for obtaining a desired output voltage. The PWM method is a method of turning on switching devices for a time interval or duration proportional to a voltage command, and may be of a carrier comparison method or a space vector method as widely known in the art. The carrier comparison method may be used for controlling a general power converter with pulse with modulation, in which a desired pulse width is obtained by comparing a carrier signal with a voltage command value. The space vector method may be used for generating polyphase ac through a power converter such as an inverter, in which the ON duration of the switchg devices per unit time is determined based on a space vector representing voltage or magnetic flux.
The carrier comparison method, which is widely known in the art, is described, for example, at “5.2.3 PWM CONTROL” on pp 727-728 in “Electrical Engineering Handbook” published on Feb. 28, 1987 by I.E.E.J. The space vector method, which is also widely known, is described on pp 47-53 of “Theory and Design of AC Servo System” published on Aug. 19, 1991 by Sougou Electronics Publishing Co. Ltd. The details of these methods will not be describe herein.
With recent developments in the digital technology, digital hardware has been increasingly used in a PWM portion for the purpose of reducing the cost. In the digital hardware of the carrier comparison type, for example, instantaneous values of a voltage command signal and a carrier signal are treated as discrete values, and the carrier waveform and voltage command waveform are subjected to D-A (digital-to-analog) conversion, to provide stepped waveforms when displayed in an analog format. Here, the size (value) of one step, when expressed in terms of the output voltage, will be called “PWM resolution”.
FIG. 6
shows the waveform of output voltage V
out
within one period of carrier signal when the resolution is d
n
, wherein the output voltage V
out
changes in steps for every value of d
n
. As the step size d
n
decreases, quantization errors occurring when obtaining discrete values can be accordingly reduced.
A carrier signal having a desired carrier frequency can be obtained, for example, by changing the number of counting a CPU clock with an updown counter. Where the carrier frequency is to be increased, the number of counting is reduced, and, consequently, the PWM resolution is reduced with a result of an increase in quantization errors. If the step size d
n
is sufficiently small and the output voltage is sufficiently large, resulting quantization errors are insignificant or trivial. Where a low output voltage is to be generated, particularly during low-speed rotation of a motor, voltage errors in the output voltage are increased, thus causing torque ripple or nonuniform rotation.
FIG. 7
shows a known technique of reducing errors in the output voltage.
In order to reduce errors in the output voltage, output voltage detecting means
12
detects the output voltage, and the detected voltage and a command value for the output voltage are fed to a voltage regulator (AVR)
11
so as to correct a command value for generating PWM pulses, thereby to provide a desired output voltage. Here, the PWM method is of a carrier comparison type in which a carrier generating means
5
and a comparator
6
are used.
The use of feedback control as shown in
FIG. 7
in order to reduce quantization errors results in an increase in the cost due to the provision of a sensor portion including the voltage detecting means. In particular, the voltage detecting means needs to be insulated from a main circuit, and thus requires an expensive insulating amplifier, with a result of a significant increase in the cost. Also, an expensive CPU is needed in the method in which the resolution d
n
is reduced by increasing the frequency of the CPU clock.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a PWM converter that operates under open-loop control without utilizing voltage feedback control, so as to provide a desired voltage at a reduced cost even when the output voltage is relatively low.
To accomplish the above object, the present invention provides a PWM power converter wherein a high-frequency signal is superimposed on a first command signal so that a second command signal is generated, the high-frequency signal having a higher frequency than the first command signal, and wherein a pulse width modulating means as a hardware is provided for digitally performing pulse width modulation (PWM) so as to provide desired output voltage, the pulse width modulating means generating PWM pulses based on the second command signal.
According to a second aspect of the present invention, a PWM power converter is provided wherein a high-frequency signal is superimposed on a first carrier signal so that a second carrier signal is generated, the high-frequency signal having a higher frequency than a command signal, and wherein pulse width modulating means as a hardware is provided for digitally performing pulse width modulation (PWM) so as to provide desired output voltage, the pulse width modulating means generating PWM pulses by comparing the command signal with the second carrier signal.
According to a third aspect of the present invention, a PWM polyphase power converter is provided wherein a high-frequency signal is superposed on a first command signal corresponding to each phase of the power converter, so that a second command signal is generated, the high-frequency signal having a higher frequency than the first command signal, and wherein pulse width modulating means as a hardware is provided for digitally performing pulse width modulation (PWM) so as to provide desired output voltage, the pulse width modulating means generating PWM pulses for each phase by comparing the second command signal with a carrier signal.
According to a fourth aspect of the present invention, a PWM polyphase power converter is provided wherein a high-frequency signal having a higher frequency than a command signal is superimposed on a first carrier signal, so that a second carrier signal is generating, and wherein pulse width modulating means as a hardware is provided for digitally performing pulse width modulation (PWM) so as to provide desired output voltage, the pulse width modulating means generating PWM pulses for each phase of the power converter by comparing the command signal of the corresponding phase with the second carrier signal.
According to a fifth aspect of the present invention, a PWM polyphase power converter is provided wherein a high-frequency signal is superimposed on a first command signal of each phase as a zero-phase-sequence component thereof, so that a second command signal is generated, the high-frequency signal having a higher frequency than the first command signal, and wherein pulse width modulating means as a hardware is provided for digitally performing pulse width modulation (PWM) so as to provide desired output voltage, the pulse width modulating means generating PWM pulses for each phase by comparing the second command signal of the corresponding phase with a carrier signal.
In the PWM power converters constructed as described above, the high-frequency signal to be superimposed preferably has a period that is a multiple of that of the carrier signal. Also, the high-frequency signal to be superimposed is preferably in the form of a chopping wave. Furthermore, the

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