Abstract:
A harmonic input current reduction and power factor correction circuit for three phase, power supplies. The circuit includes passive elements including a series inductance and capacitor connected between each AC line of a three phase voltage source, and each input phase of the uncorrected power supply. The inductance and capacitor are designed and chosen to meet linearity and volt ampere requirements to achieve total harmonic current levels of less than 10%, and power factors greater than 0.98. This is achieved with less than 1% loss in line operating input voltage range and overall efficiency of greater than 99.5%. Further, the dynamic response of a circuit to power supply load transient demands is limited in voltage overshoot or undershoot effects.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This disclosure relates to and claims the benefit of the filing date of commonly-owned, U.S. Provisional Patent Application No. 61/362,559, filed Jul. 8, 2010, the entire contents and disclosure of which is incorporated by reference as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to regulated DC power supplies generally, and specifically, an apparatus and circuit for reducing current at harmonic frequencies while increasing power factor correction for DC supplies. 
       BACKGROUND 
       [0003]      FIG. 1  illustrates is a schematic diagram of a conventional uncorrected switching regulator power supply  10  which converts unregulated AC high voltage from an AC power line to regulated DC low voltage for powering circuitry in electronic equipment such as computer terminals, radar transmitters, machine tools, motors, and controllers. 
         [0004]    The uncorrected switching regulator power supply  10  shown in  FIG. 1  includes a full wave rectifier component  15  adapted to receive the 3 phase 120/240/480V, 50/60 HZ power line voltage. The rectifier input is connected to a filter  17 , capacitor filter, in turn, this is coupled to a DC to DC inverter circuit  20 . The inverter  20  supplies a transformer  25  having a primary winding N 1  and a secondary winding N 2 . The electrical output waveform  50 , referred to as E o , at the secondary of the transformer, is shown. This waveform  50  is applied to a low pass filter  60  constituting the output circuit of power supply  10 , comprising inductance Lo and output capacitance Co. 
         [0005]    The AC line voltage is full wave rectified by rectifier  15 , and filtered by  17 , resulting in a high voltage unregulated DC bus voltage at the output of the rectifier. This filtered voltage is applied to the inverter circuit  20 . The latter is a DC to DC inverter which typically operates from 20 to 500 KHZ, and a control circuit provides pulse width modulation to the switching devices for control of the output  75  DC level. The DC output component  75  of the input waveform from the AC power line is the output voltage of the low pass filter circuit  60  (and, thus, of power supply  10 ), it is the ratio of the width of each pulse (t on ) to a full cycle (T) of the train multiplied by the pulse amplitude, or mathematically as shown in  FIG. 1 : Output voltage=(t on /T)(N 2 /N 1 )*E DC  where t on  is pulse width; T is pulse cycle time; N  1  and N  2  are the number of windings on the primary and secondary, respectively, of the transformer; and E DC  is the DC level of the rectified AC input voltage to the power supply across capacitor  17 . 
         [0006]    As shown in the plot of  FIG. 2 , the amplitude of the input current waveform  80  consists of a periodic series of quasi sinusoidal current pulses, each pulse corresponds to the conduction interval of the input voltage  12 , full wave rectifier  15 , capacitor  17  (as shown in  FIG. 1 ). That is, the input current will flow whenever the input voltage per phase is greater than the capacitor voltage  17 . As such the input current will contain intervals of zero  85  current and spike intervals of current  87 . The resultant input current per phase is shown in  FIG. 2 . As seen, this is rich in harmonic current levels and the total harmonic content can exceed 50%. Each phase input current will be the same as  FIG. 2 , shifted by 120 degrees in time. 
         [0007]    This output voltage from power supply  10  is a suitable low voltage supply for any of a number of electronic equipment applications, such as computer systems, medical instrumentation, telephone switching systems, machine control systems, or other apparatus employing semiconductor devices, motors or integrated circuitry or that requires supply voltages. 
         [0008]    The output voltage of the supply  10  is employed as the supply voltage for any device operating on DC power. The power supply efficiency is the ratio of power out to power in, and can be high—for example, greater than 80% for 150 volt outputs and greater than 75% for 48 volt outputs. The power factor, which is a measure of how well such a power supply utilizes the AC line voltage, however, is typically relatively low. A power factor of 0.70 is not unusual for supplies above 10,000 watts. 
         [0009]    Low power factor is attributable to the fact that the input current drawn by the rectifier and filter capacitor of the power supply is not sinusoidal and is not in phase with the input voltage. For example, as shown in the plot of  FIG. 2 , one phase current  80  of the three phase line current drawn by the supply, from the source (e.g., a 7.5 KW power supply load, 480V AC/60 HZ operation), is drawn only in periodic pulses  87  to recharge the input capacitor. A power factor improvement can be realized by increasing the conduction angle φ, but this capability is limited by the ripple current rating of the input filter capacitor. For a typical conventional power supply with a conduction angle “φ” of ¼ of T/2 seconds, the demand is four times the RMS value of the input current. 
         [0010]    As shown in the plot  80  of  FIG. 2 , the large peak current loading produces stress on the facility source, and may result in loss of peak AC voltage because of reactive and resistive regulation losses. It is not unusual to measure harmonic current values of greater than 50%. This far exceeds good design practice for AC loading of a source and generators in particular, for long life. 
         [0011]    In a further example, a three phase power supply having a power factor of 0.75 draws 25% more input current than a comparable power supply having a unity power factor. For example, a conventional 7500 watt, 425 volt power supply operating with a 0.75 power factor off a 480 volt three-phase AC input line will draw 12 amps. The harmonic current content will exceed 50%. This harmonic current per phase is further detrimental if a neutral is present since harmonic currents will add on the return neutral connection. 
         [0012]    The susceptibility of other loads to deterioration of performance is reduced in the presence of a power supply operating with the higher power factor, at least partly because harmonic current is substantially reduced or virtually eliminated. 
         [0013]    It would be highly desirable to provide an improved regulated DC power supply with high efficiency of power conversion, reduction of line harmonic current and providing near unity power factor. 
       SUMMARY 
       [0014]    In one embodiment, there is provided an improved regulated AC-DC power supply with high efficiency of power conversion, reduction of line harmonic current and providing near unity power factor. 
         [0015]    Further, there is provided a regulated AC to DC converter having unity or near-unity power factor and relatively low input current demand. 
         [0016]    Still further, there is provided an AC-DC power supply that achieves high power conversion efficiency without adversely affecting the operating line range. 
         [0017]    Still yet further, there is provided a circuit for use with or addition to a regulated AC to DC power supply, that reduces and lowers harmonic input current, improve the power factor, conversion efficiency, and operating line range impact of the power supply. 
         [0018]    According to one aspect, there is provided a passive power factor correction circuit for an AC to DC power supply that receives 3-phase current from a connected 3 phase AC power source, the power supply having respective inputs associated with phase, the correction circuit comprising: a linear inductor having an inductance and, a capacitor having a capacitance, with the inductor having a winding arranged and adapted for electrical connection in series with the capacitor, the circuit connecting each respective phase the source and a respective power supply input associated with each respective phase, the inductance in combination with the capacitance of values reducing odd harmonic frequency current components from the line current drawn by the power supply in response to a load being placed on the AC power source; and, the inductance in combination with the capacitance of values providing a resonant frequency of the circuit set below a lowest frequency of the AC power source. 
         [0019]    According to a further aspect, there is provided an AC to DC power supply apparatus comprising: an AC to DC power supply that receives 3-phase current from a connected 3-phase AC power source and generates DC current output, the power supply having a respective input terminal associated with each phase for receiving line current of the phase; a power factor correction circuit associated with each respective phase, each the correction circuit including a linear inductor having an inductance and, a capacitor having a capacitance, with the inductor having a winding arranged and adapted for electrical connection in series with the capacitor, each the circuit connecting a respective AC source line current source of a respective phase and a respective power supply input terminal; the inductance in combination with the capacitance of values reducing odd harmonic frequency current components from the line current drawn by the power supply in response to a load being placed on the AC power source; and, the inductance in combination with the capacitance of values setting a resonant frequency of the circuit below a lowest frequency of the AC power source current. 
         [0020]    Further to this aspect, the inductance and capacitance values are selected to provide low real power loss to the power supply line operation, while providing high impedance to harmonic currents otherwise present. In one embodiment, a high impedance is provided to odd harmonic frequencies currents of the power source. 
         [0021]    Further to this aspect, the inductance and capacitance values are selected to provide low real power loss to the power supply line operation results in increased power factor while maintaining low voltage drop loss at the fundamental frequency current flow from the said three phase source to the power supply. 
         [0022]    Further, harmonic current content at odd frequencies is reduced to less than 10% in the source current when in place with said power supply. 
         [0023]    Further, while achieving reduction of harmonic current content at odd frequencies, there is exhibited improved power factor to 0.98, with loss of the power supply operating voltage range of less than 1%, while obtaining efficiency of better than 99.5%. 
         [0024]    In addition, the characteristic impedance may be adjusted to achieve an overall critically or over damped transient response. 
         [0025]    By implementing the circuit in the embodiments described, the source line current also experiences a line current phase shift on a cycle by cycle basis to greatly improve the source loading for power supply load transients often seen in certain types of radar power supplies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Other aspects, features and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which similar elements are given similar reference numerals. 
           [0027]      FIG. 1  illustrates an example circuit configuration of a conventional uncorrected power supply  10 ; 
           [0028]      FIG. 2  illustrates an example plot depicting an input current waveform of any phase drawn by the conventional uncorrected power supply connected to the three phase source of  FIG. 1 ; 
           [0029]      FIG. 3  illustrates the inventive harmonic current correction circuit  140  for a power supply  100  in accordance with one embodiment; and, 
           [0030]      FIG. 4  illustrates an example plot depicting a corrected input current waveform with harmonic current correction circuit situated between source and supply of the power supply shown in  FIG. 3 ; 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    In one embodiment, there is provided an apparatus and circuit for improving the power factor of switching regulator or electronic voltage power supplies. 
         [0032]    Particularly, according to a preferred embodiment, a circuit, alternately referred to herein as a harmonic current or power factor “correction circuit”, includes a linear inductor and capacitor, the capacitor being electrically connected in series with the inductor, and an identical (repeating) circuit is placed in each phase between the source and the uncorrected power supply input. 
         [0033]    In view of  FIG. 3 , a circuit  140  is provided for enhancing power factor of a switched regulated electronic power supply  100 . 
         [0034]    In one embodiment, power supply  100  is similar to the embodiment of power supply  10  of  FIG. 1  that converts a 3-phase alternating current AC signal from a source  120 , e.g., a 3-phase power line, into a DC current. The source  120  may include, for example, a 3-phase generator output, having line current of respective phases φ A , φ B  and φ C , each 120° degrees apart at approximately from 57 to 63 HZ on respective voltage lines  105   A ,  105   B , and  105   C.    
         [0035]    The embodiment shown in  FIG. 3 , includes connection of the power factor correction circuit  140  between each line  105   A ,  105   B  and  105   C  of a respective a 3-phase line source and respective terminals  110   A,B,C  forming inputs of power supply  100  that receives the input line current of each phase, and presents an impedance to the AC signal source  120  for reducing harmonic current of each respective phase in the manner according to the embodiments described in greater detail herein below. 
         [0036]    As further shown in  FIG. 3 , power supply inputs  110  of each phase are connected to rectifier component  150  that includes a number of diodes in a specific arrangement for converting received 3-phase AC current to a DC current which is fed through capacitor filter  170 . The power supply further includes an inverter component  200  that receives the filtered DC current at a primary winding N 1  of transformer  250 . Secondary winding N 2  of transformer  250  is connected after rectification to a low pass filter  260  of an inductance Lo and output capacitance Co forming the output circuit  275  of power supply  100 . The output D.C. Voltage is developed in a similar manor as described with respect to  FIG. 1 . 
         [0037]    In one non-limiting example, power supply  100  may include a Switching Power, Inc. Remo model ITT-150 Power Supply or model MD-10 KW (Switching Power Inc., Ronkonkoma N.Y. USA). Other switched regulated power supplies may benefit from the power factor correction circuit of the present invention. 
         [0038]    The apparatus includes, more particularly, a power factor correction circuit  140  having a linear inductor and capacitor connected in a series between each respective phase of the source voltage and the corresponding phase of the power supply input. As shown in  FIG. 3 , the inductor L and capacitor C represents the embodiment of this structure with the inductor L having a winding arranged and adapted for electrical connection to capacitor C. In the embodiment shown in  FIG. 3 , connected between each line  105   A ,  105   B  and  105   C  of a respective a 3-phase line source and respective terminals  110   A,B,C  forming input of power supply  100  receiving each line current are respective individual correction circuits  140   A,B,C ; correction circuit  140   A  includes series connection of inductance L  101   A  and capacitor C  102   A ; correction circuit  140   B  includes series connection of inductance L  101   B  and capacitor C  102   B ; and, correction circuit  140   C  includes series connection of inductance L  101   C  and capacitor C  102   C . 
         [0039]    In one embodiment, each inductance L and C values are designed and selected to attenuate the odd harmonics that otherwise distort the current I AC  from the AC power line. That is, for example, harmonics attributable principally to the rectifier  150  and the capacitor filter  170  of the power supply  100 . In the method, by proper selection of the value of inductance L and capacitance C, the odd harmonic currents may be reduced to less than approximately ten cent of their uncorrected value, e.g., at an AC power line frequency ranging from 57 to 63 HZ. 
         [0040]      FIG. 4  illustrates a resultant waveform  180  of the corrected input line current due to incorporating power factor corrector circuit  140 . Particularly,  FIG. 4  shows the greatly improved generator line current  185  resulting from implementation of the power supply  100  having the harmonic current correction circuit  140  of  FIG. 3 , connected between the source and the power supply input terminals  110   A ,  110   B , and  110   C  for each phase. In the generator line current  185  shown in  FIG. 4 , total harmonic distortion of less than 10% is demonstrated. 
         [0041]    In power supply operation using the embodiment of power supply  100  having power factor correction circuit  140  shown and described herein which, in one embodiment, comprises a linear current transformer, L, in series with a capacitor, C, at the front end input section of the power supply  100  for direct connection to the AC power line, there has been achieved a power factor improvement ranging from 20% to 30%. These improvements are achieved at least in part by the effect of this circuit  140  to enhance the input waveform to the power supply  100 , reduce harmonics attributable to other circuitry within the power supply, and enhance the load demand. 
         [0042]    With respect to the impedance presented by the power factor correction circuits to the AC signal source  120  for reducing harmonic current of each respective phase in the manner as described herein, the impedance introduced as a function of frequency is calculated as follows: 
         [0043]    For a series R L C circuit as shown in  FIG. 3 , the series connection of inductor and capacitor, exhibits a magnitude of impedance (|Z|) according to: 
         [0000]      | Z|=R+ωLj+ 1/(ω Cj )
 
         [0000]    where ωL is the inductive reactance component (|X l |) and 1/+C is the capacitive reactance component (|X c |) of the impedance. 
         [0044]    Impedance |Z|=√{square root over ((R 2 +(Xl−Xc) 2 ))} is present to current flow from the source. If X l =X c  then the loss due to line current flow at the fundamental frequency is Iac 2 *R, and can be minimized by design. In one example embodiment, use of an inductor and capacitor near resonance but below the fundamental frequency achieves efficiency of greater than 99.5%, the |Z| being low at the generator fundamental frequency less than 1.0 ohm. While increasing |Z| with frequency reduces harmonic current amplitudes that would flow due to the power supply input rectifier  150  and bulk storage capacitor  170 . The uncorrected line current, for each phase, can be represented by its Fourier Series as follows: 
         [0000]    
       
         
           
             
               
                 i 
                 ϑ 
               
                
               
                 ( 
                 t 
                 ) 
               
             
             = 
             
               
                 ∑ 
                 
                   
                     n 
                     = 
                     1 
                   
                   , 
                   3 
                   , 
                   5 
                   , 
                   7 
                   , 
                   9 
                 
                 ∞ 
               
                
               
                 
                   i 
                    
                   
                     ( 
                     t 
                     ) 
                   
                 
                  
                 
                   sin 
                    
                   
                     ( 
                     
                       
                         
                           w 
                           o 
                         
                          
                         nt 
                       
                       + 
                       ϑ 
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    where i(t)=∫ 0   2π i 1 (t)sin(w o nt) are the Fourier coefficients;
 
I RMS  is the line current: I RMS =√{square root over (I 1   2+ I 3   2 +I 5   2+ I 7   2+  . . . +I n   2 )}; I 1 =RMS value of fundamental current; and, the line current=I RMS =I 1  when all harmonics are 0.
 
         [0045]    From the above equations, it is can be seen that the harmonic current is reduced as impedance to these higher frequency components is increased. 
         [0046]    Thus, referring to  FIG. 3 ,  140  circuit including Inductance L and C are designed and selected at a value calculated to attenuate the odd harmonics that otherwise distort the current IAC from the AC power line. The harmonics are attributable principally to the rectifier and the capacitor filter of the power supply  100 . Particularly, by proper selection of the value of inductance L and C, the odd harmonic currents may be reduced to less than approximately ten cent of their uncorrected value. 
         [0047]    This results in an input current to the power supply  100  having a virtually distortion free sinusoidal characteristic. 
         [0048]    The correction circuit  140  exploits the constant efficiency characteristics of the switching regulator power supply load. The product of line current and line voltage is constant, and, consequently, the input current is greatest at low line voltage. At the same time the voltage drop across the inductor winding is at a maximum. Nevertheless, the choice of resonance for the circuit  140  ranging from between 0.91 and 0.94 of the fundamental source frequency, assures that less than 1% of the line range is sacrificed, to produce a power factor of 0.98 or better. Also, the impedance of this circuit  140  increases as above for all harmonics and results in less than 10% total harmonic content in the corrected source line current. 
         [0049]    More particularly, the capacitor of the power factor correction circuit is selected to have a value suitable to provide the reactive power (volt amperes) demanded by the load presented by power supply  100 . The circuit voltage drop V 1  which is proportional to its impedance at the power line frequency (i.e., the demand current of the load represented by the power supply  10 ) is Iac*|Z| at the fundamental source frequency. With the inductive reactance=X l  and the capacitance reactance=X c , and with X l −X c  approaching zero, the voltage drop is Iac*R where R is essentially the loss in L. 
         [0050]    An efficiency of 99.5% with loss of less than 1% of the operating line range of the power supply  100  have been achieved with the correction circuit  140  configured in or operable with power supply, while producing a power factor of 0.98 for the power supply  100 . 
         [0051]    Using the power factor correction circuit  140  of the present invention with a switching dc power supply, e.g., Switching Power, Inc. Remo model ITT-1500S power supply. The supply tested was loaded to 7.5 KW at 480V AC at 60 HZ. Data was recorded with and without the configuration of circuit present. 
         [0052]    Table 1 illustrates performance of ITT-1500S Power Supply without the present power factor correction circuit  140  according to one example, and particularly provides example data representing various AC Line Voltage VOLTS and corresponding AC Line Current AMPS, a percent of total harmonic current; and, the resultant power factor achieved: 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1.0 
               
             
             
               
                   
               
               
                 ITT-1500S Power Supply Without The Present Invention 
               
               
                 7.5 KW load, Efficiency = 85.4% 
               
             
          
           
               
                   
                 AC Line Voltage 
                 AC Line Current 
                 Total Harmonic 
                 Power 
               
               
                   
                 VOLTS 
                 AMPS 
                 Current (%) 
                 Factor 
               
               
                   
                   
               
               
                   
                 430 
                 13.2 
                 &gt;55 
                 0.79 
               
               
                   
                 480 
                 12.8 
                 &gt;58 
                 0.78 
               
               
                   
                 530 
                 12.4 
                 &gt;64 
                 0.75 
               
               
                   
                   
               
             
          
         
       
     
         [0053]    Table 2 illustrates performance of an ITT-1500S Power Supply incorporating or otherwise used with the present power factor correction circuit  140  according to one example, and particularly provides example data representing various AC Line Voltage VOLTS and corresponding AC Line Current AMPS, a percent of total harmonic current; and, the resultant power factor achieved: 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 ITT-1500S Power Supply With The Present 
               
               
                 Invention 7.5 KW load, Efficiency = 85.1% 
               
             
          
           
               
                   
                 AC Line 
                 AC Line 
                 Total 
                   
               
               
                   
                 Voltage 
                 Current 
                 Harmonic 
                 Power 
               
               
                   
                 VOLTS 
                 AMPS 
                 Current (%) 
                 Factor 
               
               
                   
                   
               
               
                   
                 430 
                 8.9 
                 9.1 
                 0.99 
               
               
                   
                 480 
                 8.3 
                 9.5 
                 0.98 
               
               
                   
                 530 
                 7.9 
                 9.8 
                 0.98 
               
               
                   
                   
               
             
          
         
       
     
         [0054]    In a further aspect, the inductor L and capacitor C values of the power factor correction circuit  140  are selected to provide: a low real power loss (i.e., less than 0.1% of total power loss) to the power supply line operation, while providing high impedance (Xl−Xc) to harmonic currents otherwise present, e.g., odd harmonic frequencies of the power source, thereby increasing the power factor while maintaining low voltage drop loss at the fundamental frequency current flow from the three-phase source to the power supply. Thus there is obtained less the 10% harmonic current content in the source current when in place with the power supply, while achieving, power factor improvement to 0.98, with loss of the power supply operating voltage range of less than 1%, while obtaining efficiency of better than 99.5% for the inductance capacitor art. 
         [0055]    In addition, the characteristic impedance of said art is adjusted so as to get an overall: critically or over damped transient response. The source line current with the power factor correction circuit in place also experiences a line current phase shift on a cycle by cycle basis; this greatly improves the source loading for power supply load transients, e.g., as often experienced in radar transmitter power supplies. Instantaneous load changes produces large magneto-motive forces, which in turn can damage the rotor bearings of source generators and high harmonic current content is responsible for low generator life. 
         [0056]    The inductor and capacitor circuit values may further be selected to provide a resonant frequency set below the lowest frequency of source operation, wherein a setting ranges from between 0.91 and 0.94 the lowest frequency of source operation. 
         [0057]    Advantageously, the harmonic input current reduction and power factor correction circuit for three phase, power supplies provides a simpler, lower cost circuit alternative for enhancing power factor in an uncorrected electronic voltage power supply. 
         [0058]    Although a few examples of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.