Abstract:
A parallel feed-forward compensation type three-phase power factor correction circuit for a three-phase power supply comprises a primary rectifying circuit I, and a feed-forward compensation circuit II which is in parallel with the primary rectifying circuit I. The primary bridge rectifier circuit I comprises a three-phase bridge and a filter capacitor. The feed-forward compensation circuit II comprises a plurality of bi-direction switches, a rectifying circuit, a boost converter, an output current sampler and a control circuit. In the corresponding phase interval the feed-forward compensation circuit sequentially closes the phase of that whose absolute value of the voltage is the higher one of the two phases at same polarity. The other two phases are rectified by bridge rectifying circuit and forced to export a compulsive current waveform. In this configuration, it can amend the current waveform of each phase, reduce harmonic distortion and improve the efficiency of the power supply with very little power.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This Application is a Section 371 National Stage Application of International Application No. PCT/CN02/00828, filed Nov. 19, 2002 and published as WO 03/044933 on May 30, 2003, which is based upon Chinese Application No. 01140014.5, the contents of which are hereby incorporated by reference in their entirety. 
   FIELD OF THE INVENTION 
   The present invention relates, in general, to three-phase active power factor correction (APFC), more specifically, to a parallel, feed-forward and compensation three-phase APFC. 
   BACKGROUND OF THE INVENTION 
   U.S. Pat. No. 006,043,997A disclosed a two stage, three-phase power boost factor correction circuit comprising a primary rectifier and a primary boost switch. The correction circuit is coupled between an input of three-phase power supply and an output of a three-phase boost converter. In the three-phase boost converter, a method of reducing input current total harmonic distortion (THD) at the input of the three-phase boost converter is to use an auxiliary stage including first, second and third pair auxiliary boost inductors coupled to corresponding inputs of the three phase power supply and the primary rectifier. The auxiliary stage also includes an auxiliary boost switch interposed between the first, second and third auxiliary boost inductors and the output so as to draw corresponding phase currents through the first, second and third auxiliary boost inductors, and thereby reducing input current THD at the input of the three-phase boost converter. The circuit has to process the whole power of the power supply, so that the efficiency is reduced and the cost is high. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a parallel feed-forward and compensation three-phase active power factor correction (APFC) circuit, which can operate selectively so as to meet the needs of low cost and high efficiency, while reducing input current total harmonic distortion (THD). 
   In the present invention, an auxiliary compensation circuit II, which is referred to as an auxiliary compensation circuit II hereinafter, is in parallel with the common three-phase rectifier I. 
   The primary rectifying circuit I comprises a three-phase bridge rectifier  12  and a filter capacitor  13 , the feed-forward compensation circuit II comprises a plurality of bi-direction switches  15 , a rectifier  16 , a boost converter  17 , an output current sampler  18  and a control circuit  19 . As shown in  FIG. 2 , the feed-forward compensation circuit II works in such a way that, a whole period of three-phase is divided into 12 parts, the control circuit  19  makes the bi-direction switches  15  cut off the phase C in a-b interval, the phase A in b-c interval, the phase B in c-d interval, the phase C in d-e interval, the phase A in e-f interval, the phase B in f-g interval, the phase C in g-h . . . . That is for the two phases at same polarity, the phase with higher absolute value is cut off by the bi-direction switches  15 , then the phase with lower absolute value is forced to conduct to the third phase, such as the phase A with phase B in a-b interval, the phase C with phase B in b-c interval, the phase C with phase A in c-d interval, the phase B with phase A in d-e interval, the phase B with phase C in e-f interval, the phase A with phase C in f-g interval, the phase A with phase B in g-h . . . , so as to generate a DC voltage through the rectifier  16  of the feed-forward compensation circuit II. Then the boost converter  17  forces the output DC current of rectifier  16  to inject into the output  14  of the primary three-phase bridge rectifier  12 . In other words, in turns of 12 steps in each of 12 intervals of a whole three-phase line period, the feed-forward compensation circuit II forces the phase with lower absolute value of the two phases having same polarity to conduct with the phase at the opposite polarity by cutting off the phase with higher absolute value, and rectifies and boosts a DC current with suitable waveform which is then injected into the output  14  of the primary three-phase rectifier  12 . 
   The feed-forward compensation circuit II in parallel with the primary rectifier circuit I is used in present invention, so the THD is decreased greatly. It can produce a compulsive current waveform (for example the sine waveform of ±π/6 shown in  FIG. 3  and  FIG. 4 ) in ±π/6 interval across zero of each of the three phases, in which the phase is not conducted without application of this circuit. After the compulsive process of this circuit, the waveform of voltage in the primary rectifier circuit does not change, and the output waveforms of the current and the voltage do not change too. For instance, in the b-c interval the feed-forward compensation circuit II cuts off the phase A which is at same polarity with phase C but has higher absolute value, so the phase C and B are rectified by rectifier  16 . The compulsive current waveform by boost converter  17  is shown in  FIG. 3 , the sine current waveform in the interval of 5π/6 to π, which is in proportion to the output current, is boosted and injected into the output of the primary three-phase rectifier  14 . In this interval, the phase B, at the opposite polarity, conducts with both phase A and phase C through the primary and the auxiliary rectifies respectively, so that its input current will be maintained as it was before. The phase C has no current through the primary bridge rectifier circuit, its current forced by the feed-forward compensation circuit II will cut down the current of phase A being at same polarity. The working principles of the rest  11  intervals are the same as it. It is because that, the feed-forward compensation circuit II is not only to give out the current with suitable waveform in its current gap between ±π/6 phase interval, but also cut down the current of the phase with higher absolute value. As a result, neither the waveform of the output voltage and current of the rectifying circuit nor the waveforms of the phases having different polarity changes, of the two phases at same polarity, the lower one has a suitable waveform and the current of the higher one is decreased. That is the reason that the input current total harmonic distortion (THD) can be reduced thoroughly with very high efficiency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates the phase relationship of common three-phase main line power supply. 
       FIG. 2  illustrates a schematic diagram of the invention. 
       FIG. 3  and  FIG. 4  illustrate a possible current waveforms selected to be forced by boost converter. 
       FIG. 5  and  FIG. 6  illustrate the output voltage and current waveforms of a three-phase low filter capacitor rectifier with constant power load, which are normalized by the magnitude of phase peak voltage and output power. 
       FIG. 7  and  FIG. 8  illustrate the phase current waveforms with and without APFC, that the filter capacitance is negligible. 
       FIG. 9  and  FIG. 10  illustrate the phase current waveforms with and without APFC, that normalized filter capacitance is 0.2 (by phase peak voltage and output power, period is 2π) 
       FIG. 11  and  FIG. 12  illustrate two embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Referring initially to  FIG. 1 , illustrated is the phase relationship of common three-phase main line power supply. 
     FIG. 2  is a schematic diagram of the present invention. In one embodiment, a primary rectifying circuit I and a feed-forward compensation circuit II are parallel. The primary rectifying circuit I comprises a three-phase bridge rectifier  12  and a filter capacitor  13 . A three-phase power supply  11  is connected to the inputs of the three-phase bridge rectifier  12  of the primary rectifying circuit I. The outputs of the three-phase bridge rectifier  12  are connected to the filter capacitor  13 . The feed-forward compensation circuit II comprises a plurality of bi-direction switches  15 , a rectifier  16 , a boost converter  17 , an output current sampler  18  and a control circuit  19 . The inputs of the bi-direction switches  15  of the feed-forward compensation circuit II are connected to the three-phase power supply  11  of the primary rectifying circuit I. The outputs of the bi-direction switches  15  are connected to the inputs of the rectifier  16 . The outputs of the rectifier  16  are connected to the boost converter  17 . The control circuit  19  is connected to the three-phase power supply  11 , one terminal of the output current sampler  18 , the bi-direction switches  15  and the boost converter  17 . The other terminal of the output current sampler  18  is connected to the outputs  14 . As far as the current gaps between ±π/6 interval in the primary rectifier I is concerned, in the 12 steps in the feed-forward compensation circuit II the control circuit  19  makes the bi-direction switches  15  cut off the control circuit  19 , makes the bi-direction switches  15  cut off the phase C in a-b interval, the phase A in b-c interval, the phase B in c-d interval, the phase C in d-e interval, the phase A in e-f interval, the phase B in f-g interval, the phase C in g-h . . . . That is for the two phases at same polarity, the phase with higher absolute value is cut off by the bi-direction switches  15 , then the phase with lower absolute value is forced to conduct to the third phase, such as the phase A with phase B in a-b interval, the phase C with phase B in b-c interval, the phase C with phase A in c-d interval, the phase B with phase A in d-e interval, the phase B with phase C in e-f interval, the phase A with phase C in f-g interval, the phase A with phase B in g-h . . . , so as to generate a DC voltage through the rectifier  16  of the feed-forward compensation circuit II. Then the boost converter  17  forces the output DC current of rectifier  16  to inject into the output  14  of the primary three-phase bridge rectifier  12 , and make the phase have a current with suitable waveform. In such a way, for instance in the b-c interval shown in  FIG. 1 , the phase C has suitable current waveform in its current gap, the current of phase A is decreased and its waveform becomes more close to sine waveform, only the input current of phase B is maintained as it was. The method processes only a small part of the whole power, but can make the current waveforms much better and decrease the THD greatly. 
   The effect of the feed-forward compensation circuit II to decrease the THD is illustrated from  FIG. 5  to  FIG. 10 . 
   The common voltage waveform of the output of a three-phase bridge rectifier is shown in  FIG. 5 , and the current waveform is shown in  FIG. 6  if the load is constant. 
     FIG. 7  shows the phase current waveform, without feed-forward compensation circuit II and the filter capacitance is negligible, the THD is obviously. 
     FIG. 8  shows the phase current waveform, with feed-forward compensation circuit II and the filter capacitance is negligible, the THD is decreased. 
     FIG. 9  shows the phase current waveform, without feed-forward compensation circuit II and the filter capacitance is not negligible, the THD is obviously. 
     FIG. 10  shows the phase current waveform, with feed-forward compensation circuit II and the filter capacitance is not negligible, the THD is decreased. 
   In the Figures above, the curves A is a sine to compare and all of the current waveforms are normalized by the output power. 
   As the first embodiment of the invention, in the  FIG. 11  the primary rectifying circuit I and the feed-forward compensation circuit II are parallel. The primary rectifying circuit I is a common three-phase rectifying circuit comprising a three-phase bridge rectifier  12  and a filter capacitor  13 . In the feed-forward compensation circuit II, the bi-direction switches  15  are TRIACs  21 ,  22 ,  23 , the rectifying circuit is a rectifier  16 , a boost converter  17  comprises boost inductors  28 ,  29 , high frequency diodes  24 ,  25  and switching transistor  27  as a switching unit. Also an output current sampler  18  and a control circuit  19  are included. 
   The phases A, B and C of three-phase power supply  11  are connected to the inputs of the three-phase bridge rectifier  12 . The outputs of the three-phase bridge rectifier  12  are connected to the outputs  14  paralleled with the filter capacitor  13 . The inputs of TRIACs  21 ,  22 ,  23  of the feed-forward compensation circuit II are connected to the phases A, B and C of three-phase power supply  11  and their outputs are connected to the inputs of the rectifier  16 . The outputs of the rectifier  16  are connected to the boost inductors  28 ,  29 . The outputs of the boost inductors  28 ,  29  are connected to the positive terminal of the diode  25  and negative terminal of the diode  24 . The negative terminal of the diode  25  is connected to the positive terminal of the output  14 , and the positive terminal of diode  24  to the negative terminal of the output  14 . The outputs of the boost inductors  28 ,  29  are respectively connected to the collector and emitter of the switching transistor  27  also. In the feed-forward compensation circuit II, the control circuit  19  comprises a toggle circuit  30 , three phase detectors  31 ,  32 ,  33 , three TRIAC control terminals  34 ,  35 ,  36  and an output current detect terminal  37 . The toggle circuit  30  is connected to the gate of the switching transistor  27 . Three phase detectors  31 ,  32 ,  33  are connected to the phases A, B and C of three-phase power supply. Three TRIAC control terminals  34 ,  35 ,  36  are connected to the control terminals of three TRIACs  21 ,  22 ,  23 , and the current detect terminal  37  is connected to one terminal of the output current sampler  18 . The other terminal of the output current sampler  18  is connected to the positive polarity of the output  14 . The control circuit  19  get the magnitude of the output current through the output current sampler  18  to determine the magnitude of the output current of the boost converter  17 . 
   The control circuit  19  of the feed-forward compensation circuit II gets the information of the phase from input of the three-phase power supply  11 , to turn off the TRIAC  21  and cut off the phase C in a-b interval so that phase A and phase B are rectified to supply a DC voltage through rectifier  16  and feed it into the boost converter  17 , to turn off the TRIAC  23  and cut off the phase A in b-c interval so that phase C and phase B are rectified to supply a DC voltage through rectifier  16  and feed it into the boost converter  17 , to turn off the TRIAC  22  and cut off the phase B in c-d interval so that phase A and phase C are rectified to supply a DC voltage through rectifier  16  and feed it into the boost converter  17  . . . , to turn off the TRIAC  22  and cut off the phase B in l-m interval shown in  FIG. 1  so that phase A and phase C are rectified to supply a DC voltage through rectifier  16  and feed it into the boost converter  17 . By repeating above steps, the information of the phase determines the current phase of the boost converter  17  also. That is for the two phases at same polarity, the phase with higher absolute value is cut off, then the rest phases are rectified by rectifier  16  and the boost converter  17  outputs suitable compulsive current waveform. 
   By the way, in each phase interval, only one of the inductors  28 ,  29  works in boost state, the voltage cross the other is zero. They change their states at the points a, c, e, g, i, k, m shown in the  FIG. 1 , the compulsive current is just zero at these points. And they do not change their states at the points b, d, f, h, j, l shown in the  FIG. 1 , the inductor in the boost state series with the TRIAC need to be turned off and turned on, it makes the TRIAC turn off easy. 
   The second embodiment of the invention shown in  FIG. 12  has almost the same principle with the one above described. In the  FIG. 12  a primary rectifying circuit I and a feed-forward compensation circuit II are parallel. The primary rectifying circuit I is a common three-phase rectifying circuit comprising a three-phase bridge rectifier  12  and a filter capacitor  13 . In the feed-forward compensation circuit II, bi-direction switches  15  are TRIACs  21 ,  22 ,  23 , rectifying circuit is rectifier  16 , boost converter  17  is a fly-back insulated converter comprising a IGBT 27 , a transformer  28 , a diode  26 . And also an output current sampler  18  and a control circuit  19  are included. 
   The phases A, B and C of three-phase power supply  11  are connected to the inputs of the three-phase bridge rectifier  12 , the outputs of the three-phase bridge rectifier  12  are connected to the outputs  14  paralleled with the filter capacitor  13 . The inputs of TRIACs  21 ,  22 ,  23  of the feed-forward compensation circuit II are connected to the phases A, B and C of the three-phase power supply  11 . The outputs of the TRIACs  21 ,  22 ,  23  are connected to the inputs of the rectifier  16 , the positive output of the rectifier  16  is connected to one terminal of the first coil of the transformer  28 . The other terminal of the first coil of the transformer  28  is connected to the collector of the boost IGBT. Then the emitter of the boost IGBT is connected to the negative terminal of the rectifier  16 . The negative terminal of the diode  26  is connected to positive terminal of the output  14 . And positive terminal of the diode  26  is connected to one terminal of the second coil of the transformer  28 . The other terminal of the second coil of the transformer  28  is connected to the negative terminal of the output  14 . The control circuit  19  comprises a toggle circuit  30 , three-phase detectors  31 ,  32 ,  33 , three TRIAC control terminals  34 ,  35 ,  36  and an output current detect terminal  37 . The toggle circuit  30  is connected to the gate of the switching transistor  27 . The three phase detectors  31 ,  32 ,  33  are connected to three phases A, B and C of three-phase power supply  11 . Three TRIAC control terminals  34 ,  35 ,  36  are connected to the control terminals of the three TRIACs  21 ,  22 ,  23  respectively, and the current detect terminal  37  is connected to one terminal of an output current sampler  18 . The other terminal of the output current sampler  18  is connected to the positive terminal of the output  14 . The control circuit  19  gets the magnitude of the output current through output current sampler  18  to determine the magnitude of the output current of the boost converter  17 . 
   Although the present invention has been described in some details, those skilled in the art should understand that they can make various changes, for example, other forms of bi-direction switches  15 , rectifier  16 , boost converter  17 , output current sampler  18 , control circuit  19 , substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.