Abstract:
A harmonic-suppressing AC/DC power converter employs a control method for permitting the AC/DC power converter only to detect an input AC current and an output DC voltage. The control method can control the AC side of the AC/DC power converter to generate a voltage which is proportional to the input AC current. Thereby, the AC/DC power converter acts as a virtual resistor having a linear resistance characteristic. Accordingly, the input AC current of the AC/DC power converter can be controlled to approximate nearly as a sinusoidal wave with the performance of high input power factor and low input harmonic current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control method of an AC/DC power converter for input current harmonic suppression. More particularly, the present invention relates to the control method for applying to the AC/DC power converter without detecting an AC voltage so as to adjust an input AC current to approximate nearly as a sinusoidal wave and to obtain an unity power factor, and to supply an output of an adjustable DC voltage. 
     2. Description of the Related Art 
     Power converters have been widely used in many areas recently. These power converters include AC/DC, DC/DC and DC/AC converters. Conventionally, the AC/DC power converter is configured by a diode rectifier. There are advantages of simplified configuration and reduced cost in using the diode rectifier. However, a DC side of the diode rectifier cannot be controlled, and a great amount of input harmonic components and poor input power factors occur in its AC side. 
     In order to improve the problems of harmonic pollution effectively, many harmonic control standards, such as IEEE519-1992, IEC1000-3-2, and IEC1000-3-4 etc., have been established. In this way, the modern power electronic equipment need to meet the requirements for low input harmonic distortion and high input power factor. Recently, a variety of power factor correctors are developed to solve the harmonic problems caused by the AC/DC power converter. 
     Referring to  FIG. 1 , a schematic circuitry of a conventional power factor corrector is illustrated. Generally, the power factor corrector includes a diode rectifier  10 , an inductor  11 , a power electronic switch  12 , a diode  13 , a DC capacitor  14  and a controller  15 . Control methods for the power factor corrector are well known and described in U.S. Pat. No. 6,650,554 and U.S. Pat. No. 6,388,429, for example. The output DC voltage of the power factor corrector can be controlled by controlling the power electronic switch  12 . The output DC voltage of the power factor corrector is higher than a peak value of an input AC voltage. An input current approximated nearly as a sinusoidal wave and an unity input power factor can be obtained at an input AC side of the power factor corrector. The conventional control method for controlling the power factor corrector employs a detected output DC voltage for regulating the output DC voltage so as to determine a reference amplitude of the input AC current. Subsequently, a detected AC voltage is employed to determine a reference waveform of the input AC current. The reference waveform multiplies the reference amplitude, thereby obtaining a reference signal of the input AC current. Subsequently, the reference signal and a detected input AC current are operated in a closed-loop control to produce a modulation signal. Finally, the modulation signal is sent to a pulse-width-modulation circuit and a driving circuit to produce a driving signal for the power electronic switch  12 . In this way, the conventional control method for the power factor corrector disadvantageously requires to detect three signals, including the output DC voltage, the input AC voltage and the input AC current. 
     Generally, an AC/DC power converter must employ a power converter having a bridge configuration. Referring now to  FIGS. 2   a  and  2   b , schematic circuitry of conventional single-phase AC/DC power converters applied to a single-phase AC power system in accordance with the prior art are illustrated. 
     Still referring to  FIG. 2   a , the conventional single-phase AC/DC power converter having a half-bridge configuration is disclosed. The half-bridge configuration of the single-phase AC/DC power converter includes a power electronic switch set  20 , a pair of capacitors  21 ,  22 , an inductor  32  and a controller  24 . The power electronic switch set  20  has two power electronic switches. The capacitors  21 ,  22  have the same capacitance. The controller  24  can control the power electronic switch set  20 , thereby controlling the AC/DC power converter to receive an input AC current supplied from an AC power source through the inductor  23 . Advantageously, the input AC current is approximated nearly as a sinusoidal wave and in phase with the input voltage of the AC power source. Consequently, the harmonics in the AC/DC power converter can be suppressed, the power factor is nearly unity, and the output DC voltage can be controlled. 
     Referring again to  FIG. 2   b , the conventional single-phase AC/DC power converter having a full-bridge configuration is disclosed. The full-bridge configuration of the single-phase AC/DC power converter includes a power electronic switch set  30 , a capacitor  31 , an inductor  32  and a controller  33 . The power electronic switch set  30  has four power electronic switches. The controller  33  can control to switch the power electronic switch set  30 , thereby controlling the AC/DC power converter to receive an input AC current supplied from an AC power source through the inductor  23 . Advantageously, the input AC current is approximated nearly as a sinusoidal wave and in phase with the input voltage of the AC power source. Consequently, the harmonics in the AC/DC power converter can be suppressed, the power factor is nearly unity, and the output DC voltage can be controlled. 
     Referring to  FIG. 3 , a schematic circuitry of a conventional three-phase AC/DC power converter applied to a three-phase AC power system in accordance with the prior art is illustrated. The three-phase AC/DC power converter includes a power electronic switch set  40 , a capacitor  41 , a three-phase inductor set  42  and a controller  43 . The power electronic switch set  40  has six power electronic switches. The controller  43  can control to switch the power electronic switch set  40 , thereby controlling the AC/DC power converter to produce a balanced three-phase sine-wave currents on the three-phase inductor set  42 . Advantageously, phases of the three-phase sine-wave currents are identical with those of the input voltages of a three-phase power source. Consequently, the harmonics in the three-phase AC/DC power converter can be suppressed, and the power factor can be improved to nearly unity. 
     The conventional control method for both the single-phase AC/DC power converter and the three-phase AC/DC power converter employs a detected output DC voltage for regulating the output DC voltage so as to determine a reference amplitude of the input AC current. Subsequently, a detected AC voltage of the AC power source is employed to determine a reference waveform of the input AC current. The reference waveform multiplies the reference amplitude, thereby obtaining a reference signal of the input AC current. Subsequently, the reference signal and the detected input AC current are operated in closed-loop control to produce a modulation signal. Finally, the modulation signal is sent to a pulse-width-modulation/driving circuit to produce a set of driving signals for the power electronic switch sets  20 ,  30 ,  40 . In this way, the conventional control method for the single-phase AC/DC power converter and the three-phase AC/DC power converter disadvantageously require to detect three signals, including the output DC voltage, the input AC voltage and the input AC current. 
     Even though the conventional control methods of the AC/DC power converters can suppress the harmonic components of the input AC current and improve the power factor, the controller must detect the output DC voltage and the input AC voltage to determine the reference signal. Subsequently, the input AC current is detected and operated in closed-loop control to obtain a sine-wave input AC current. Advantageously, the sine-wave input AC current is in phase with the input voltage of the AC power source. However, the conventional control methods for the AC/DC power converter disadvantageously require detecting three signals, including the output DC voltage, the input AC voltage and the input AC current. Accordingly, the control circuit can be complicated and cannot be normally operated due to fluctuations in frequency of the AC power system. 
     The present invention intends to provide a simplified control method of an AC/DC power converter for suppressing the input current harmonics. The control circuit only detects two signals from the output DC voltage and the input AC current. Additionally, the control method can be normally operated under fluctuations in frequency of the AC power system for controlling an input AC current to approximate nearly as a sinusoidal wave and a unity power factor, and to supply an adjustable output DC voltage. 
     SUMMARY OF THE INVENTION 
     The primary objective of this invention is to provide a simplified control method of an AC/DC power converter for suppressing the input current harmonics. The AC/DC power converter can convert energy of an AC power source into a regulated output DC voltage to supply to a DC load. The control method permits the AC/DC power converter without detecting a voltage of an AC power source for simplifying the entire structure. Accordingly, the AC/DC power converter can be normally operated under the variable frequency of the AC power source for controlling an input AC current to approximate nearly as a sinusoidal wave with the performance of high input power factor and low input harmonic current. Consequently, the purposes of harmonic suppression and power factor improvement can be achieved. 
     The AC/DC power converter in accordance with the present invention employs a control method for permitting the AC/DC power converter only to detect an input AC current and an output DC voltage. The control method can control the AC side of the AC/DC power converter to generate a voltage which is proportional to the input AC current. Thereby, the AC/DC power converter acts as a virtual resistor having a linear resistance characteristic. The detected output DC voltage is used for regulating the output DC voltage so as to determine a value of the virtual resistor for operation of the AC/DC power converter. Accordingly, the input AC current of the AC/DC power converter can be controlled to approximate nearly as a sinusoidal wave with the performance of high power factor and low harmonic distortion. Since the AC/DC power converter acts as a virtual resistor, frequency of the input AC current can be synchronously changed in response to the change in frequency of the AC power source. Consequently, the AC/DC power converter can be normally operated under the variable frequency of an AC power source for controlling the input AC current to approximate nearly as a sinusoidal wave with the performance of high power factor and low harmonic distortion. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic circuitry of a conventional power factor corrector in accordance with the prior art; 
         FIG. 2   a  is a schematic circuitry of a conventional single-phase AC/DC power converter applied to a single-phase AC power system in accordance with the prior art; 
         FIG. 2   b  is a schematic circuitry of another conventional single-phase AC/DC power converter applied to a single-phase AC power system in accordance with the prior art; 
         FIG. 3  is a schematic circuitry of a conventional three-phase AC/DC power converter applied to a three-phase AC power system in accordance with the prior art; 
         FIG. 4  is a control block diagram illustrating a control circuitry of a harmonic-suppressing AC/DC power converter applied to a power factor corrector in accordance with a first embodiment of the present invention; 
         FIG. 5  is a control block diagram illustrating a control circuitry of a harmonic-suppressing single-phase AC/DC power converter employing a half-bridge or full-bridge structure in accordance with a second embodiment of the present invention; and 
         FIG. 6  is a control block diagram illustrating a control circuitry of a harmonic-suppressing single-phase AC/DC power converter in accordance with a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 4 , a control block diagram of a harmonic-suppressing AC/DC power converter applied to a power factor corrector in accordance with a first embodiment of the present invention is illustrated. The power factor corrector in accordance with the preferred embodiment has similar configuration and similar function as that of the conventional power factor corrector, as shown in  FIG. 1 , and detailed descriptions may be omitted. In the first embodiment, the control block diagram of the AC/DC power converter includes a voltage-regulation circuit, a current-detecting circuit, a multiplier circuit and a pulse-width-modulation/driving circuit. 
     Still referring to  FIG. 4 , the voltage-regulation circuit includes a voltage detector  50 , a first subtracter  51 , a controller  52  and a second subtracter  53 ; the current-detecting circuit includes a current detector  54 ; the multiplier circuit includes a multiplier  55 ; and the pulse-width-modulation/driving circuit includes a third subtracter  56 , a pulse-width-modulation circuit  57  and a driving circuit  58 . 
     Referring back to  FIGS. 1 and 4 , the voltage detector  50  detects an output DC voltage of the power factor corrector, and then sends to the first subtracter  51  which subtracts the detected output DC voltage from a first predetermined value. Subsequently, the result is sent to the controller  52  to obtain an output, and the output of controller  52  is sent to the second subtracter  53  which subtracts the output of the controller  52  from a second predetermined value. Accordingly, the second subtracter  53  can generate a control signal V R  which provides a value acting as a virtual resistor for the power factor corrector. Preferably, the first predetermined value of the first subtracter  51  is set at an expected value of the output DC voltage, and it can be changed as the desired output DC voltage is changed. Since the power factor corrector is acted as the virtual resistor, the power factor corrector can absorb lesser real power as the value of the resistor is greater; namely, the resistance of the virtual resistor is inversed-proportional to the conversion real power of the power factor corrector. Accordingly, the output of the controller  52  must be subtracted from the second predetermined value by the second subtracter  53 . Under these conditions the second predetermined value of the second subtracter  53  equals a maximum value of the virtual resistor as well as a minimum value of the conversion real power of the power factor corrector. Consequently, this ensures a positive value for the input real power of the power factor corrector. 
     Still referring to  FIGS. 1 and 4 , the current detector  54  is used to detect an input AC current passing through the inductor  11  of the power factor corrector, as best shown in  FIG. 1 . Subsequently, the input AC current and the control signal V R  of the second subtracter  53  are sent to the multiplier  55 , and then the result is sent to the pulse-width-modulation/driving circuit. With reference to  FIG. 1 , when the power electronic switch  12  of the power factor corrector is turned on, a voltage V 1  across the power electronic switch  12  is nearly zero; conversely, when the power electronic switch  12  of the power factor corrector is turned off, a voltage V 1  across the power electronic switch  12  is the same with the output DC voltage of the power factor corrector. Accordingly, an average value of the voltage V 1  is reduced as a duty ratio of the power electronic switch  12  is increased, wherein the duty ratio is the ratio of a conduction time to a switching period of the power electronic switch  12 ; namely, the voltage V 1  is inversed-proportional to the duty ratio of the power electronic switch  12 . Prior to sending to the pulse-width-modulation/driving circuit, the output of the multiplier  55 , must be sent to the third subtracter  56  which can subtract the output of the multiplier  55  from a third predetermined value. Subsequently, the result of the third subtracter  56  is sent to the pulse-width-modulation circuit  57  to operate as a modulation signal. Typically, the pulse-width-modulation circuit  57  can select a high-frequency triangular or saw-tooth wave acting as a carrier wave. In the pulse-width-modulation circuit  57 , the modulation signal is compared with the carrier wave so as to generate a high-frequency pulse-width-modulation signal. Finally, an output of the pulse-width-modulation circuit  57  is sent to the driving circuit  58  so as to generate a driving signal for the power electronic switch  12  of the power factor corrector. Preferably, the third predetermined value of the third subtracter  56  is set for a peak value of the high-frequency carrier wave of the pulse-width-modulation circuit  57 . When the driving circuit  58  sends the driving signal to drive the power electronic switch  12  of the power factor corrector, the voltage V 1  across the power electronic switch  12  is obtained and proportional to the input AC current. Consequently, the power factor corrector can be acted as the virtual resistor, and used to absorb real power from the AC power source and to convert it into a DC power with an adjustable output DC voltage. Furthermore, a current waveform identical with the voltage waveform of the AC power source is generated at the AC side of the power factor corrector so as to adjust the input AC current to be approached to the unity power factor. Since the AC power source supplies an AC voltage with sinusoidal waveform, the input AC current is approximated nearly as a sinusoidal wave which has low harmonic distortion. 
     Turning now to  FIG. 5 , a control block diagram of a harmonic-suppressing single-phase AC/DC power converter employing a half-bridge or full-bridge configuration in accordance with a second embodiment of the present invention is illustrated. The half-bridge or full-bridge configuration of the single-phase AC/DC power converter in accordance with the preferred embodiment has similar configuration and similar function as that of the conventional single-phase AC/DC power converter, as shown in  FIGS. 2   a  and  2   b , and detailed descriptions may be omitted. In the second embodiment, the control block diagram of the single-phase AC/DC power converter includes a voltage-regulation circuit, a current-detecting circuit, a multiplier circuit and a pulse-width-modulation/driving circuit. 
     Still referring to  FIG. 5 , the voltage-regulation circuit includes a voltage detector  60 , a first subtracter  61 , a controller  62  and a second subtracter  63 ; the current-detecting circuit includes a current detector  64 ; the multiplier circuit includes a multiplier  65 ; and the pulse-width-modulation/driving circuit includes a pulse-width-modulation circuit  66  and a driving circuit  67 . 
     Referring back to  FIGS. 2   a ,  2   b  and  5 , the voltage detector  60  detects an output DC voltage of the single-phase AC/DC power converter, and then sends to the first subtracter  61  which subtracts the detected output DC voltage from a first predetermined value. Subsequently, the result is sent to the controller  62  to obtain an output, and the output of controller  62  is sent to the second subtracter  63  which subtracts the output of the controller  62  from a second predetermined value. Accordingly, the second subtracter  63  can generate a control signal V R  which provides a value acting as a virtual resistor for the single-phase AC/DC power converter. Preferably, the first predetermined value of the first subtracter  61  is set at an expected value of the output DC voltage, and it can be changed as the desired output DC voltage is changed. Since the single-phase AC/DC power converter acts as the virtual resistor, the single-phase AC/DC power converter can absorb lesser real power as the value of virtual resistor is greater; namely, the resistance of the virtual resistor is inversed-proportional to the conversion real power of the single-phase AC/DC power converter. Accordingly, the output of the controller  62  must be subtracted from the second predetermined value by the second subtracter  63 . Under these conditions the second predetermined value of the second subtracter  63  equals a maximum value of the virtual resistor as well as a minimum value of the conversion real power of the single-phase AC/DC power converter. Consequently, this ensures a positive value for the input real power of the single-phase AC/DC power converter. 
     Still referring to  FIGS. 2   a ,  2   b  and  5 , the current detector  64  is used to detect an input AC current passing through the inductor  23  or  32  of the single-phase AC/DC power converter, as best shown in  FIGS. 2   a  and  2   b . The input AC current of the single-phase AC/DC power converter is detected. Subsequently, the input AC current and the control signal V R  of the second subtracter  63  are sent to the multiplier  65 , and then the result is sent to the pulse-width-modulation circuit  66  to obtain a modulation signal. Typically, the pulse-width-modulation circuit  66  can select a high-frequency triangular or saw-tooth wave acting as a carrier wave. In the pulse-width-modulation circuit  66 , the modulation signal is compared with the carrier wave so as to generate a high-frequency pulse-width-modulation signal. Finally, an output of the pulse-width-modulation circuit  66  is sent to the driving circuit  67  so as to generate the driving signals for the power electronic switch set  20  or  30  of the single-phase AC/DC power converter, as shown in  FIGS. 2   a  and  2   b . When the driving circuit  67  sends the driving signals to drive the power electronic switch set  20  or  30  of the single-phase AC/DC power converter, the voltage across the output of power electronic switch set  20  or  30  is proportional to the input AC current. Consequently, the single-phase AC/DC power converter acts as the virtual resistor, and used to absorb real power from the AC power source and to convert it into a DC power with an adjustable output DC voltage. Furthermore, a current waveform identical with the voltage waveform of the AC power source is generated at the AC side of the single-phase AC/DC power converter so as to adjust the input AC current to be approached to the unity power factor. Since the AC power source supplies an AC voltage with sinusoidal waveform, the input AC current is approximated nearly as a sinusoidal wave which has low harmonic distortion. 
     Turning now to  FIG. 6 , a control block diagram of a harmonic-suppressing three-phase AC/DC power converter in accordance with a third embodiment of the present invention is illustrated. The three-phase AC/DC power converter in accordance with the preferred embodiment has similar configuration and similar as that of the conventional three-phase AC/DC power converter, as shown in  FIG. 3 , and detailed descriptions may be omitted. In the third embodiment, the control circuitry of the three-phase AC/DC power converter includes a voltage-regulation circuit, a current-detecting circuit, a multiplier circuit and a pulse-width-modulation/driving circuit. 
     Still referring to  FIG. 6 , the voltage-regulation circuit includes a voltage detector  70 , a first subtracter  71 , a controller  72  and a second subtracter  73 ; the current-detecting circuit includes a first current detector  74   a  and a second current detector  74   b ; the multiplier circuit includes a first multiplier  75   a  and a second multiplier  75   b ; and the pulse-width-modulation/driving circuit includes an inverting adder  76 , a three-phase pulse-width-modulation circuit  77  and a driving circuit  78 . 
     Referring back to  FIGS. 3 and 6 , the voltage detector  70  detects an output DC voltage of the three-phase AC/DC power converter, and then sends to the first subtracter  71  which subtracts the detected output DC voltage from a first predetermined value. Subsequently, the result is sent to the controller  72  to obtain an output, and the output of controller  72  is sent to the second subtracter  73  which subtracts the output of the controller  72  from a second predetermined value. Accordingly, the second subtracter  73  can generate a control signal V R  which provides a value acting as a virtual resistor for the three-phase AC/DC power converter. Preferably, the first predetermined value of the first subtracter  71  is set at an expected value of the output DC voltage, and it can be changed as the desired output DC voltage is changed. Since the three-phase AC/DC power converter acts as the virtual resistor, the three-phase AC/DC power converter can absorb lesser real power as the value of the virtual resistor is greater; namely, the resistance of the virtual resistor is inversed-proportional to conversion real power of the AC/DC power converter. Accordingly, the second subtracter  73  must subtract the output of the controller  72  from the second predetermined value. Under these conditions the second predetermined value of the second subtracter  73  equals a maximum value of the virtual resistor as well as a minimum value of the conversion power of the three-phase AC/DC power converter. Consequently, this ensures a positive value for the input real power of the three-phase AC/DC power converter. 
     Still referring to  FIGS. 3 and 6 , the three-phase AC/DC power converter in accordance with the present invention is applied to a three-phase AC power system which supplies a three-phase current, including a first-phase input AC current, a second-phase input AC current and a third-phase input AC current. In the third embodiment, the first current detector  74   a  and the second current detector  74   b  are used to detect two of the three-phase input AC currents passing through the three-phase inductor set  42  of the three-phase AC/DC power converter, as shown in  FIG. 3 , are detected. Subsequently, the input AC currents and the control signal V R  of the second subtracter  73  are sent to the first multiplier  75   a  and the second multiplier  75   b  to obtain a first modulation signal and a second modulation signal respectively. The first and second modulation signals are then sent to the pulse-width-modulation/drive. Concretely, the summation of three phase AC currents is zero in the three-phase three-wire AC power system. In order to obtain a third modulation signal for a third phase of the three-phase AC/DC power converter, the first and second modulation signals are sent to the inverting adder  76 . Subsequently, the first modulation signal, the second modulation signal and the third modulation signal are sent to the three-phase pulse-width-modulation circuit  77  to obtain pulse-width-modulation signals. Typically, the three-phase pulse-width-modulation circuit  77  can select a high-frequency triangular or saw-tooth wave acting as a carrier wave. In the three-phase pulse-width-modulation circuit  77 , the modulation signals are compared with the carrier wave so as to generate high-frequency pulse-width-modulation signals. Finally, outputs of the three-phase pulse-width-modulation circuit  77  are sent to the driving circuit  78  so as to generate the driving signals for the power electronic switch set  40  of the three-phase AC/DC power converter, as shown in  FIG. 3 . When the driving circuit  78  sends the driving signals to drive the power electronic switch set  40  of the three-phase AC/DC power converter, the voltages across the power electronic switch set  60  are proportional to the input AC currents. Consequently, the single-phase AC/DC power converter acts as the virtual resistor, and used to absorb real power from the AC power source and to convert it into a DC power with an adjustable output DC voltage. Furthermore, a current waveform identical with the voltage waveform of the AC power source is generated at the AC side of the three-phase AC/DC power converter so as to adjust the input AC currents to be approached to the unity power factor. Since the AC power source supply three-phase voltages with sinusoidal waveform, the input AC currents are approximated nearly as sinusoidal wave which has low harmonic distortion. 
     As has been discussed above, the harmonic-suppressing AC/DC power converter in accordance with the present invention can produce a voltage proportional to the input AC current. This permits the AC/DC power converter acting as a virtual resistor which can be used to absorb real power from the AC power source and to convert it into an adjustable output DC voltage of the output DC voltage to supply to a DC load. Consequently, the purposes of harmonic suppression and power factor improvement can be achieved. Conversely, the conventional control circuit must detect the output DC voltage and the input AC voltage to generate a reference signal. Subsequently, the input AC current is detected and operated in closed-loop control to obtain a sine-wave input AC current, and the input AC current is in phase with the AC voltage of the AC power source. However, the conventional control method for the AC/DC power converter must disadvantageously require detecting the output DC voltage, the input AC voltage and the input AC current. Accordingly, the control circuit can be sophisticated and cannot be normally operated due to the frequency variation of the AC power system. 
     The control method in accordance with the present invention permits the AC/DC power converter to detect only the input AC current and the output DC voltage for simplifying the entire structure. Additionally, the control method for the AC/DC power converter can omit to detect a voltage of an AC power source, and acts as the virtual resistor which can be normally operated under the power source with frequency variation. 
     Although the invention has been described in detail with reference to its presently preferred embodiment, it will be understood by one of ordinary skill in the art that various modifications can be made without departing from the spirit and the scope of the invention, as set forth in the appended claims.