EMI reduction circuit
DESCRIPTION 
     1. Technical Field 
     The present invention relates generally to electromagnetic interference reduction circuits and more particularly to circuitry for reducing harmonics in AC power lines caused by rectification of AC power carried thereby. 
     2. Background Art 
     Often, AC power from a generator or another AC source is rectified by a rectifier circuit to develop DC power. However, operation of the rectifier circuit causes high-amplitude harmonics to be developed on the AC power lines. These harmonics in turn generate electromagnetic interference which can adversely affect the operation of components in the vicinity of the power lines. In some installations the degree of electromagnetic interference may be considered unacceptable and must therefore be reduced. 
     In the past, harmonic currents have been limited by the use of a large filter connected in the AC power lines. However, in installations, such as in aircraft, where space is at a premium and where weight must be minimized, this has been found to be a poor solution to the problem. 
     One type of circuit for filtering undesired harmonics on an AC power line is disclosed in Peterson et al U.S. Pat. No. 4,053,820. This patent discloses a harmonic converter and a plurality of filters which together generate currents that are combined with the currents carried by the AC power lines to cancel the harmonics therein. The harmonic converter comprises a first rectifier bridge coupled to the AC power lines for rectifying the AC power and a second plurality of controlled rectifiers for inverting the rectified AC power to derive injected currents which are combined with the line currents. 
     A further type of harmonic reduction circuit is disclosed in Finney U.S. Pat. No. 4,460,950. This patent discloses a controlled bridge rectifier wherein two or more three-phase thyristor bridges are connected in parallel with one another between an AC supply and a DC output by means of interphase transformers. The thyristors of each bridge are controlled so that the harmonic content in the current drawn from the AC supply is reduced. 
     Additional harmonic reduction circuits are disclosed in Rosa et al U.S. Pat. No. 4,366,532 and Brewster et al U.S. Pat. No. 4,143,414. 
     3. Disclosure of Invention 
     In accordance with the present invention, a harmonic reduction circuit reduces harmonics on AC power lines coupled to a rectifier so that the size of the filter in the line may be reduced. 
     The harmonic reduction circuit includes means for sensing the current in the AC power line, means for developing a pulse width modulated (PWM) waveform having a duty cycle dependent upon the sensed AC line current and a switching circuit coupled to the AC power line and controlled by the PWM waveform developing means for compensating the AC line current whereby undesired harmonics therein are reduced. 
     The developing means includes means for generating a reference sine wave, means for summing a current signal representing the sensed AC line current with the reference sine wave to develop an error signal and means for deriving the PWM waveform from the error signal. The deriving means in turn includes means for summing the error signal with a constant amplitude triangle wave to develop a modified error signal and a comparator for comparing the modified error signal with a reference signal to derive the PWM waveform. 
     The switching circuit includes a transformer having a primary winding coupled to at least one switch which is controlled by the PWM waveform and a secondary winding coupled in the AC power line. The switch is controlled to compensate for variations in the line current caused by rectification thereof. 
     The harmonic reduction circuit of the instant invention is equally applicable to single phase and polyphase systems and is implemented in a simple and inexpensive fashion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block digram of a power conversion system in conjunction with the harmonic reduction circuit of the present invention; 
     FIG. 2 is a partial schematic and partial block diagram of the system shown in FIG. 1; 
     FIG. 3 is a block diagram of the control circuit shown in FIG. 2; 
     FIG. 4 is a block diagram of the synchronizing circuit and sine wave reference generator shown in FIG. 3; 
     FIG. 5 is a combined logic and block diagram of the logic and drive circuits shown in FIG. 3; 
     FIG. 6 is a series of waveform diagrams illustrating the operation of the instant invention; and 
     FIG. 7 is a block diagram illustrating an alternative embodiment of the instant invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to FIG. 1, there is illustrated a power conversion system 10 for converting AC power on AC power lines 12 into DC power on lines 14a,14b. In the embodiment illustrated, the AC power includes three phases carried on power lines 12a-12c. It should be noted, however, that the instant invention is equally applicable to single phase or N-phase power, where N is an integer. 
     The AC power on the power lines 12 is coupled to a filter 16 and thence over lines 17a-17c to a rectifier and filter circuit 18 which converts the polyphase AC power into the DC power on the lines 14a,14b. 
     High-amplitude harmonics are typically generated on the AC power lines 12 due to the rectification of the AC power by the circuit 18. These harmonics are reduced by means of an EMI or harmonic reduction circuit 20 according to the present invention in conjunction with the filter 16. The harmonic reduction circuit includes a control circuit 22, discussed in greater detail hereinafter in connection with FIGS. 3-5, that operates N switching circuits which, in the preferred embodiment, comprise three circuits 24a-24c. The control circuit 22 receives current signals developed by N current transformers 26 representing the currents in the AC power lines 12. 
     The control circuit 22 also receives a signal from a current sensor 28 that detects the current at the output of the rectifier and filter circuit 18. 
     Referring now to FIG. 2, there is illustrated in greater detail the harmonic reduction circuit 20 and the power conversion system 10. 
     All of the N switching circuits 24 are identical, and hence only the switching circuit 24a is shown and described in detail. The switching circuit 24a includes first and second power switches comprising transistors Q A  and Q B  which are coupled to first and second ends of a primary winding 30ap of a transformer 30a. The transformer primary winding 30ap also includes a mid-tap which is coupled to the line 14a at the output of the rectifier and filter circuit 18. 
     It can be seen that the AC power on the lines 12 together with the rectifier and filter circuit 18 comprises a source of DC power for the switching circuit 24a. Of course, the switching circuits 24 may be coupled to a different source of DC power, if desired. 
     Coupled across each of the transistors Q A ,Q B  is a diode D1,D2, respectively. The emitters of the transistors Q A ,Q B  are coupled together to the line 14b. 
     Generally, the switches Q A  and Q B  are operated in a fashion described more specifically below to derive compensating power which is combined by means of a secondary winding 30as of the transformer 30a with the power on the line 17a to minimize the harmonics therein. 
     Referring now to FIG. 3, there is illustrated in greater detail the control circuit 22 shown in block diagram form in FIGS. 1 and 2. 
     The portion of the control circuit 22 illustrated in FIG. 3 is the circuitry required to generate control signals for operating the switches Q A ,Q B  in the switching circuit 24a. Additional circuitry identical to that shown in FIG. 3 generates the control signals for the switches in the remaining switching circuits 24b and 24c. 
     The control circuit 22 receives the current signal representing the current in the line 17a from sensing means comprising the current transformer 26a and a burden resistor 32. The burden resistor 32 is in turn coupled to first summing means comprising a summing junction 34. The summing junction 34 sums the current signal with a reference sine wave developed by sine wave generating means comprising a synchronizing circuit 36 and an amplitude controlled sine wave reference generator 38. The synchronizing circuit 36 receives the current signal from the burden resistor 32 and a feedback signal from the output of the sine wave reference generator 38 and controls the phase of the reference sine wave so that it is in phase with the fundamental frequency component of the AC power on the line 17a. 
     The first summing means develops an error signal V e  representing the difference between the current signal and the reference sine wave. The error signal V e  is coupled to an error amplifier 40 in a pulse width modulator circuit 42. The error amplifier is in turn coupled to second summing means comprising a summing junction 44 which receives a second input from a constant amplitude triangle wave generator 46. The summing junction 44 sums the amplified error signal V e  with the constant amplitude triangle wave from the generator 46 to derive a modified error signal V e1 . 
     The output of the second summing means is coupled to a comparator 48 which compares the modified error signal V e1  with a reference signal to derive a PWM error signal on a line 50. 
     It can be seen that the circuitry represented by the blocks 40,44,46,48 together comprise means for deriving a pulse width modulated signal from the error signal V e . 
     Further, the circuitry comprising the elements 34-38 comprises means for developing a pulse width modulated wave on the line 50 having a duty cycle dependent generally upon the AC line current, and more specifically upon the error between the sensed AC line current in the line 17a and the reference sine wave. 
     The line 50 is coupled to logic and drive circuits 52 which are a part of the switching circuit 24a. The logic and drive circuits receive a synchronizing signal from an output of the synchronizing circuit 36, as described in greater detail hereinafter. 
     The logic and drive circuits 52 operate the switches Q A ,Q B  in accordance with the PWM error signal on the line 50 so that appropriate harmonic current compensation is provided via the switching circuit 24a to the power line 17a. 
     Referring now to FIG. 4, there is illustrated in greater detail the circuitry represented by the blocks 36 and 38 in FIG. 3. The synchronizing circuit 36 includes a waveshaping circuit 58 which converts the signal developed across the burden resistor 32 into a square wave which is coupled to a first input of a phase comparator 60. An output of the phase comparator 60 is connected to a low pass filter 62 which is in turn coupled to a voltage controlled oscillator, or VCO 64 in the sine wave reference generator 38. The output of the VCO 64 is coupled via a squaring amplifier 66 in the synchronizing circuit 36 to a second input of the phase comparator 60. The circuits 60,62,64,66 together comprise a phase-locked loop or PLL 68 having a first output which is coupled to a variable-gain amplifier 70 in the sine wave reference generator 38. The variable-gain amplifier varies the magnitude of the alternating current signal from the VCO 64 in accordance with the output current from the rectifier and filter circuit 18 as detected by the current sensor 28. The variable-gain amplifier 70 affords control over output current by varying the magnitude of the reference sine wave coupled to the second input of the first summing means 34. 
     The signal from the synchronizing circuit 36 to the logic and drive circuits 52 is obtained at a second output of the PLL 68 on a line 72 coupled to the squaring amplifier 66. As seen in FIG. 5, the synchronizing signal on the line 72 and the PWM error signal on the line 50 are coupled to noninverting inputs of a first AND gate 74. 
     A second AND gate 66 includes an inverting input coupled to the second output of the PLL 68 over the line 72 and an inverting input coupled to the PWM waveform on the line 50. The outputs of the AND gate 74 and 76 are coupled to first and second noninverting inputs of an OR gate 78. The OR gate 78 includes an output which is coupled by a base drive circuit 80 to the base or control electrode of the power switch Q A . The output of the OR gate 78 is further coupled by means of an inverter 82 and a base drive circuit 84 to the base or control electrode of the power switch Q B . 
     The gates shown in FIG. 5 synchronize the operation of the switches Q A ,Q B  so that these switches are operated in appropriate fashion to develop the necessary compensating power. 
     Referring now to FIG. 6, the waveform A illustrates the desired line current in the power line 17a. The waveform B illustrates the current in the power line 17a which results from operation of the rectifier circuit 18 in the absence of compensation by the switching circuit 24a. 
     The waveform C illustrates the resultant current which is added to the uncompensated line current shown as waveform B to remove the undesired harmonics on the line 17a. A fragment of the waveform C is enlarged in waveform D to illustrate a portion of the current which is added to the line current on the line 17a. The portion of the compensating current shown as waveform D is produced by operation of the switch Q A  as shown by waveform E. In general, the switch Q A  is operated so that the area under the pulse representing the operation of this switch as shown in waveform E equals the area under the pulse shown in waveform D. This ensures that the switching circuit 24a develops the appropriate amount of compensating current to compensate for the harmonics in the line current on the line 17a. 
     It can be seen that the circuits 40-52 and the transistors Q A  and Q B  together comprise means for deriving compensating current from the error signal V e  while the transformer 30 comprises means for combining the compensating current with the AC current to reduce the harmonics therein. 
     In the embodiment just described, circuits duplicative of that shown in FIG. 3 are required to operate the remaining switching circuits 24b,24c. The parts count of the overall system can be reduced by using the circuit shown in FIG. 7 whereby certain components are shared by the controls for the switching circuits 24a-24c. 
     More specifically, the synchronizing circuit 36 and the sine wave reference generator 38 are coupled to N-1 phase shifters 80 a ,80 b  . . . 80 N-1 , where N is the number of phases of the AC power. As before, the sine wave reference generator 38 is also coupled to the first summing junction 34 for developing the error signal V e  which is coupled to the error amplifier 40. The phase shifters 80 a  -80 N-1  are in turn coupled to summing junctions 84 a  -84 N-1  which receive current signals from current transformers 86 a  -86 N-1  and burden resistors 88 a  -88 N-1 . The phase shifters 80 shift the phase of the reference signal from the generator 38 by an integer multiple of 2π/N electrical degrees with respect to the phase of the signal from the reference generator 38. In other words, the reference sine wave generating means comprising the circuits 36 and 38 and the phase shifters 80 together generate a succession of N reference sine waves each displaced 2 π/N electrical degrees with respect to successive reference sine waves. For example, if the AC power is three-phase, the phase shifter 80 a  develops a sine wave reference which is displaced 120° with respect to the signal from the generator 38 while the phase shifter 80 b  develops a signal which is shifted in phase 240° with respect to the output of the reference generator 38. 
     The summing junctions 84 are in turn coupled to error amplifiers 90 a  -90 N-1  which are identical to the error amplifier 40. These error amplifiers are coupled to duplicate circuits each of which is identical to the circuits 44,46,48,52 described in connection with FIG. 3. 
     As seen in FIG. 7, the synchronizing circuit 36 and the sine wave reference generator 38 can thereby be shared to produce the signals for operating the switches in the remaining switching circuits. 
     In either of the foregoing embodiments, harmonics in the AC power lines are reduced to a significant degree so that the size of the filter 16 can be reduced or the filter eliminated entirely.