Abstract:
A nearly 2 to 1 boosting mufti-phase AC-to-DC converter may include a main rectifier, an auxiliary rectifier; and an autotransformer connected to the main rectifier and the auxiliary rectifier. The autotransformer may include a plurality of interconnected windings arranged in a plurality of legs, with one of the legs for each phase and with each leg including a plurality of windings, wherein all but one of the windings of each leg have equal turns ratios and one of the windings of each leg has a turns ratio that is less than the turns ratio of all of the other windings of the respective leg.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to AC-to-DC converters and more particularly to passive AC-to-DC converters with voltage boosting capability. 
     AC-to-DC converters play a significant role in the modern aerospace/military industry. This is particularly true in the area of more electric architecture (MEA) for aircraft and spacecraft. Power quality is a major concern for MEA aircraft because of the large number of electric power systems and equipment installed on the same bus. The power quality of these systems and equipment has stringent requirements to ensure that all power supplies/utilization equipment function together properly. 
     The term “composite AC-to-DC converter” has been coined to distinguish a converter using two or more conversion methods in parallel. The concept for a composite AC-to-DC converter originated as a further improvement towards smaller size, lower weight, and higher efficiency. 
     While composite AC-to-DC converters present a large step toward performance improvement they have not incorporated efficient boosting capabilities. They typically provide rectification of a three phase 115-Vac system resulting in a typical output voltage value of 270 Vdc. There are many applications where the output voltage is desired to be much higher for a better performance of a consecutive power conditioning. Typical values used in some power distribution systems are 540 Vdc, +/−270 Vdc and 610 Vdc. That means that it would be desirable for a composite AC-to-DC converter, used in a three phase 115-Vac system, to produce output voltage about two times higher at its rectified output. In other words, it would be desirable to provide voltage boosting capability in a composite AC-to-DC converter. Additionally, it would be desirable to achieve such voltage boosting passively, employing an autotransformer, as compared to an active semiconductor-based boosting circuit. In the context of aerospace applications inherent reliability of a passive system as compared to an active system is an important consideration. 
     Within an autotransformer of a composite AC-to-DC converter, interior winding turn ratios are responsible for its voltage boost factor. However a typical autotransformer&#39;s conversion ratio (ACR) will begin to decrease when used for voltage boosting. The main cause of a decreasing ACR in a boost topology can be viewed as the autotransformer winding volts*amperes (VA) sum is going up while the autotransformer output power is remaining constant. A high ACR is desirable in an autotransformer used in an aerospace vehicle because such an autotransformer may be constructed with smaller windings and with less weight than an autotransformer having a low ACR. 
     As can be seen, there is a need for a passive composite AC-to-DC converter with voltage boosting capability. More particularly, there is a need for such a converter that may produce voltage boosting passively with an autotransformer which may operate with a high ACR. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a multi-phase AC-to-DC converter may comprise: a main rectifier; an auxiliary rectifier; and an autotransformer connected to the main rectifier and the auxiliary rectifier, an autotransformer connected to the main rectifier and the auxiliary rectifier, the autotransformer including a plurality of interconnected windings arranged in a plurality of legs with one of the legs for each phase and with each leg including a plurality of windings, wherein all but one of the windings of each leg have equal turns ratios and one of the windings of each leg has a turns ratio that is less than the turns ratio of all of the other windings of the respective leg. 
     In another aspect of the present invention, a three phase voltage-boosting autotransformer with windings arranged in three legs may comprise: windings with turns ratios determined in accordance with a vector diagram constructed using line-to-line voltage vectors connecting tips positioned on projections of sides of an equilateral triangle; wherein a constructor arc is swung between the tips with a radius equal to a length of the line-to-line voltage vector; and wherein the length of the line-to-line voltage vector is selected so that are a resultant number of four of the windings in each of the legs of the autotransformer. 
     In still another aspect of the invention, a method for performing three phase AC-to-DC power conversion with a voltage boost may comprise the steps of: passing a first portion AC power through phase-dedicated windings of an autotransformer directly to a main rectifier with the voltage of the first portion boosted; passing a second portion of AC power through a plurality of windings of the autotransformer, other than the phase-dedicated windings, to an auxiliary rectifier with voltage of second portion boosted; rectifying the first and second portions using the main and the auxiliary rectifiers respectively; and combining outputs of the main and auxiliary rectifiers to produce a single rectified boosted DC voltage output. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an AC-to-DC power converter in accordance with an embodiment of the invention; 
         FIG. 2  is a vector diagram of an analytical technique employed to determine a winding configuration of an auto transformer in accordance with an embodiment of the invention; 
         FIG. 3  is a graphical illustration of a simplified schematic diagram of the converter of  FIG. 1  reflecting an autotransformer configuration in vector format; and 
         FIG. 4  is a flowchart of a method for performing AC-to-DC power conversion in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. 
     Broadly, embodiments of the present invention generally provide a passive composite AC-to-DC converters with voltage boosting capability. More particularly, such converters may produce voltage boosting passively with an autotransformer that operates with a high ACR. 
     Referring now to  FIG. 1 , it may be seen that an exemplary AC-to-DC converter  10  may include an autotransformer  12 , a main rectifier  14  and an auxiliary rectifier  16 , In  FIG. 1  the converter  10  is shown interconnected between a 3-phase power supply  18  and a DC load  20 . The main rectifier  14  and the auxiliary rectifier  16  may be conventional 6 pulse rectifiers. The autotransformer  12  may be constructed with three legs X, Y and Z. Each of the legs X, Y and Z may include four windings A, B, C and D. The twelve windings of the autotransformer  12  may be interconnected among one another and between the power supply  18  and the rectifiers  14  and  16  as shown in  FIG. 1 . 
     Turn ratios of the windings and their interconnection arrangement may be selected to provide for both a high voltage boost and a high autotransformer conversion ratio (ACR). 
     Referring now to  FIG. 2 , a vector diagram  30  displays an analytical technique that may be employed to determine turn ratios and an interconnection configuration for the windings of the autotransformer  12 . The vector diagram  30  may be constructed using line-to-line voltage vectors  42  connecting tips  32  positioned on projections  34  along sides  36  of an equilateral triangle  38 . A constructor arc  40  is swung between these tips  32  equal to a length of the line-to-line voltage span vector  42 . The length of the vector  42  is selected so that there are a resultant number of four of the windings A, B, C and D in each of the legs X, Y and Z (See  FIG. 1 ). Four is a minimum number of windings per leg that may work effectively in a voltage boosting  12  pulse rectification system. When only four winding are used in each leg, the overall weight and size of the autotransformer  12  may be minimized 
     One of the vectors  42  that is angularly equally spaced from the other vectors  42  may be drawn from an opposite one of the vector tips  32  to midpoint of  37  the arc  40 . The tips  32  or Intersection points of the vectors  42  with the arc  40  may then be used are used to design the voltage ratios and interconnections of the windings. For example, a vector  44  for winding YD may be drawn between the midpoint  37  of the arc  40  and a left side of the triangle  38  in a direction parallel to a bottom side of the triangle. A vector  46  for winding ZC may be drawn from the midpoint  37  to the left side of the triangle in a direction parallel to a right side of the triangle  38 . 
     The analytical process described above may be repeated for all three phases of the power input  18  so that turns ratios for all of the twelve windings of the autotransformer  12  may be determined. 
     For, example, on the diagram  30 , the sides of the triangle may be normalized to a value of 1. The length of the vector  42  may correspond to desired voltage boost and may be selected with a value of about 2. The resultant turns ratios for windings XA, YD and ZC may be about 0.57. The resultant turns ratio for the winding XB may be about 0.42. It may be seen that as the above described process is repeated for all three phases, a resultant pattern of turns ratios for each of the legs X, Y and Z may comprise three windings with equal turns ratios and a fourth winding with a turns ratio being between about 70% and 75% (e.g., 0.42/0.57) of the turns ratios of any of the other three windings of the leg. 
     Referring now to  FIG. 3 , it may be seen that the autotransformer  12  and the interconnections of  FIG. 1  may graphically displayed in an alternative simplified schematic format that may reflect the outcome of the vector analysis shown in  FIG. 2 . An AC input from phase P 1  may be connected at power input terminal  50 , an electrical intersection of windings XA, YD and ZB. An AC input from phase P 2  may be connected at power input terminal  52 , an electrical intersection of windings YA, ZD and XB. An AC input from phase P 3  may be connected at terminal  54 , an electrical intersection of windings ZA, YB and XD. The windings XA, YA and ZA may be directly connected to inputs of the main rectifier  14  at output terminals  56 ,  58  and  60  respectively. In this regard, the windings XA, YA and ZA may be considered to be phase-dedicated windings. The windings YD, ZC and XD may be connected to one another between the terminals  50  and  52 . The windings ZD, XC and YB may be connected to one another between the terminals  52  and  54 . The windings XD, YC and ZB may be connected to one another between the terminals  54  and  50 . 
     It may be seen that the portion of current fed to auxiliary rectifier  16  is less than the portion fed to the main rectifier  14  and the portion of current to the main rectifier  16  follows a low impedance path. The significance of this split is that the autotransformer  12  may have less losses if the larger current portion takes a “short” path within the autotransformer  12  to its output. As a consequence of the configuration of windings in the autotransformer  12 , the converter  10  may be provided with a voltage boost of almost 2 to 1 while the ACR of the autotransformer  12  may remain as high as about 1.5. Additionally, because the auxiliary rectifier  16  may experience lower RMS current, relative to the main rectifier  14 , the auxiliary rectifier  16  may be selected to be a smaller device than that used for the main rectifier  14 . Thus, overall weight and size of the converter  10  may remain desirable small. 
     Referring now to  FIG. 4 , flowchart  400  illustrates a method for performing AC-to-DC power conversion with a voltage boost In a step  402 , a first portion AC power may be passed through phase-dedicated windings of an autotransformer directly to a main rectifier with voltage of the first portion of AC current boosted. For example, AC power may be passed through the windings XA, YA and ZA from the AC power source  18  into the main rectifier  14 . In a step  404 , a second portion of AC power may be passed through a plurality of windings of the autotransformer, other than the phase-dedicated windings, to an auxiliary rectifier with voltage of second portion of AC power boosted. For example, AC power may be passed from the AC power source  18  through the series connected windings shown in  FIG. 3  to the auxiliary rectifier  16 . In step  406  and  408 , the first and second portions of power may be rectified in the main and the auxiliary rectifiers respectively. In a step  410 , outputs of the main and auxiliary rectifiers may be combined to produce a single DC output. 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.