Abstract:
There is provided by this invention a system for supply power utilizing a power supply having an adaptive feedforward circuit that uses a gating circuit to periodically apply a feedback signal to an integrator circuit in order to develop an optimal level of a scaled feedforward signal that is used to diminish perturbations of the output of the power supply due to ripple and transient voltages present at the DC bus that supplies power to the power supply. The gating circuit is synchronized to the periodic ripple in the DC bus voltage.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to switch mode power supplies, and more particularly, to switch mode power supplies suitable for plasma processing that utilize feedforward control to prevent ripple and transients in the power supply output.  
         [0003]     2. Brief Description of the Prior Art  
         [0004]     The ability of a regulated power supply to prevent ripple and transients at the input from being transferred to the output can be improved by taking a signal proportional to the input voltage and combining it with the output from the closed-loop feedback control circuit in a way that counteracts the effect of changes in the input voltage. U.S. Pat. No. 6,359,799 discloses a three-phase power supply that uses feedforward to reduce ripple in the output. The optimal amount of a feedforward signal to be combined with the feedback signal varies with the operating conditions, and fairly elaborate control schemes such as those disclosed in U.S. Pat. Nos. 5,541,833 and 5,711,843 have been devised to adaptively adjust feedforward signals in a variety of industrial processes including plasma processing.  
         [0005]     Feedforward techniques have been developed for use in pulse-width-modulated power supplies in which the voltage conversion ratio is determined by the switching duty cycle, such as those described in the publication by B. Arbetter, and D Maksimovic, “Feedforward pulse-width modulators for switching power converters,” IEEE Power Electronics Specialists Conference, June 1995, vol. 1, pp. 601-607. However, these techniques are not applicable to resonant power supplies. U.S. Pat. No. 6,049,473 utilizes a nonlinear variable-gain amplifier to adjust the small-signal gain of the feedforward signal path according to a pre-determined trajectory, but it lacks an adaptive feedforward scaling regulator that optimizes the amplitude of a feedforward signal based on measurements of the output of the power supply.  
         [0006]     U.S. Pat. Nos. 5,535,906 and 6,697,265 disclose frequency-controlled resonant DC power supply circuits that are suitable for use in plasma processing. In typical implementations, they receive power from a three-phase-rectified DC bus that lacks bulk energy storage capacitors. The DC bus voltage obtained from unfiltered three-phase bridge rectifiers changes rapidly near the cusps where diode commutation occurs. The bandwidth of typical control loops for these power supplies is insufficient to compensate for the rapid changes in the bus voltage that occur near the commutation cusps, and this produces ripple peaks in the output of the power supply that occur with a repetition rate of six times the line frequency. Some plasma processes such as self induced plasma copper processes require lower values of ripple in the DC power than these types of power supplies can provide. The conversion ratio of these power supplies depends on the operating conditions as well as the operating frequency, so if feedforward compensation were to be used, it would need to be adaptive in nature.  
         [0007]     It would be desirable if there were provided a simple and inexpensive adaptive feedforward circuit that minimizes perturbations in an output of a system that delivers power to a plasma process caused by periodic perturbations in a system input.  
       SUMMARY OF THE INVENTION  
       [0008]     There is provided by this invention a simple and inexpensive adaptive feedforward circuit that minimizes perturbations in an output of a system that delivers power to a plasma process caused by periodic perturbations in a system input. The preferred embodiment reduces output ripple in a power supply that receives power from a rectified three-phase DC bus by sending a combination of the output of a feedback regulator and a feedforward signal that is proportional to the AC component of the DC bus voltage. The feedforward signal is phased to the control input of a power supply to compensate for ripple and transients in the DC bus voltage. The amplitude of the feedforward signal is automatically adjusted by a feedforward scaling regulator to minimize the output ripple of the power supply.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a block diagram of a three-phase power supply with an adaptive feedforward circuit.  
         [0010]      FIG. 2  is a schematic diagram of an adaptive feedforward circuit.  
         [0011]      FIGS. 3-6  show waveforms illustrative of signals within the adaptive feedforward circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]      FIG. 1  shows a block diagram of a power supply system with an adaptive feedforward circuit. A three-phase power source  10  supplies power to a three-phase bridge rectifier  20  that produces a DC bus voltage between a pair of conductors  21  and  22 . A power supply  30  has a pair of DC input terminals  31  and  32  that are connected, respectively, to DC bus conductors  21  and  22 . The power supply delivers power from an output  33  to a load  43 . In general, power supply  30  could provide AC or DC power, depending on the application. An adaptive feedforward circuit  100  provides a scaled feedforward signal  129  that enhances the ability of a feedback regulator  60  to regulate the output of the power supply to achieve a desired voltage, current, or power level specified by a setpoint signal  76 . This feedforward signal  129  diminishes perturbations of the output of the power supply due to ripple and transient voltages present across the DC bus conductors.  
         [0013]     In operation, an output measurement circuit  50  measures a set of output parameters  53  such as the voltage, current and power delivered by power supply output  33  to load  43 , and provides a corresponding set of feedback signals  55 . Feedback regulator  60  has a feedback input  65  that receives a subset of feedback signal set  55  which includes some or all of the feedback signals. Adaptive feedforward circuit  100  has a feedforward scaling regulator  140  with a feedback input  145  that also receives a subset of feedback signal set  55 . A feedforward measurement circuit  110  has input terminals  111  and  112  that are connected, respectively, to DC bus conductors  21  and  22 , and it provides a feedforward measurement signal  113  to a feedforward input terminal  123  of a feedforward scaling amplifier  120 , and to a sync input  143  of feedforward scaling regulator  140 . Feedforward scaling amplifier  120  has a scaling input  128  that receives a scaling factor output signal  148  that is provided by feedforward scaling regulator  140 . A signal combiner  130  receives scaled feedforward signal  129  from feedforward scaling amplifier  120  at a first combiner input  139 , and also receives a feedback output signal  67  from feedback regulator  60  at a second combiner input  137 . Signal combiner  130  provides a combined regulation signal  134  that is connected to a control input  34  of power supply  30 .  
         [0014]      FIG. 2  shows a schematic diagram of adaptive feedforward circuit  100 . Operational amplifier U 1  and the resistors and capacitors surrounding it form a differential amplifier that measures the DC bus voltage and provides an output at junction J 1 . Operational amplifier U 2  with resistors R 4  and R 5  comprise an inverting amplifier with an input that is AC coupled to the output of the differential amplifier through capacitor C 5 . The output of the AC-coupled inverting amplifier provides a feedforward measurement signal  113  to a feedforward scaling amplifier  120 . Feedforward input  123  of the feedforward scaling amplifier  120  receives the feedforward measurement signal, and is connected to an input terminal X 1  of a multiplier integrated circuit U 3 .  
         [0015]     A sync input  143  of a feedforward scaling regulator  140  also receives feedforward measurement signal  113 , and is connected to a low-pass noise-rejection filter comprised of a resistor R 2  and a capacitor C 1 . The output of the low-pass filter appears across capacitor C 1 , and is connected to the inverting input of a comparator U 6 . The output of comparator U 6  is connected to node J 4  which provides a square-wave gating signal that is negative over a gating interval that is approximately centered on the cusps of the DC bus voltage waveform, which occur due to diode commutation in bridge rectifier  20  shown in  FIG. 1 , thereby synchronizing the gating interval to the periodic ripple in the DC bus voltage.  
         [0016]     Sync input terminal  145  of the feedforward scaling regulator  140  receives a subset of feedback signal set  55 . In the preferred embodiment, input  145  is connected to a signal that is proportional to the output power of power supply  30 . The output power signal is preferred because a voltage signal would be attenuated with loads that have low incremental AC impedance, and a current signal would be attenuated with loads that have high incremental AC impedance.  
         [0017]     An operational amplifier U 4  and resistors R 1  and R 3  form an inverting amplifier having an output that is connected to junction J 2 , and input that is AC coupled input terminal  145  through a capacitor C 4 . The voltage at junction J 2  is an amplified AC-coupled inverted power signal.  
         [0018]     The control input of an analog switch U 5  is connected to the gating comparator at junction J 4 . During the gating interval when the voltage at junction J 4  is negative, analog switch U 5  is turned off, and the AC-coupled inverted power signal at junction J 2  flows through resistor R 6 , appearing as a gated power signal at a junction J 3 . Junction J 3  is tied to ground when the gating signal at J 4  is positive.  
         [0019]     The gated power signal at junction J 3  is integrated by an inverting integrator circuit that is comprised of an operational amplifier U 7 , an integrating capacitor C 15 , a Zener Diode D 1  and a resistor R 9 . The integrator output provides scaling factor output signal  148 . An input terminal Y 1  of multiplier integrated circuit U 3  receives the scaling factor output signal from scaling input  128  of feedforward scaling amplifier  120 .  
         [0020]     The voltage at an output W of multiplier U 3  is equal to the product of the X 1  and Y 1  voltages divided by 10, and provides scaled feedforward signal  129 . Zener Diode D 1  limits the range of the integrator output voltage to prevent overdriving the Y 1  input of the multiplier.  
         [0021]     Signal combiner  130  consists of resistors R 10  and R 11  that are connected between combiner input terminals  137  and  139 . The combined regulation signal  134  is developed at the junction where R 10  and R 11  that are connected to each other. This simple signal combiner produces a linear combination of input signals  129  and  67 , but the signal combiner may be implemented to produce signal  134  according to any function of those input signals (e.g. multiplication) that is advantageous for a particular power supply.  
         [0022]      FIG. 3  shows circuit waveforms without feedforward (U 3  removed), and  FIG. 4  shows waveforms with feedforward (U 3  installed). In  FIGS. 3-6 , waveform V J1  illustrates the voltage at junction J 1 , waveform V OUT-AC  illustrates an AC-coupled power supply output voltage waveform, and V 139  illustrates the voltage at input  139  of signal combiner  130 . V JIA  indicates one of the cusps in the waveform of the measured DC bus voltage. The rms ripple in the DC output voltage of the power supply with the feedforward circuit is 23 percent of the rms ripple voltage without it, while the peak-peak ripple voltage with the feedforward circuit is 31 percent of the peak-peak ripple voltage without it. The output voltage of the power supply for these waveforms was 700V, and the output power was 20 kW.  
         [0023]     From  FIG. 3 , it can be seen that the control circuit is unable to track the rising edge of the DC bus voltage during the interval immediately following the cusps, and this produces a spike in the DC output voltage. The feedforward signal V 139  shown in  FIG. 4  falls rapidly during the time immediately following the cusp, and this compensates for the rapidly rising DC bus voltage.  
         [0024]      FIG. 5  shows circuit additional waveforms without feedforward (U 3  removed), and  FIG. 6  shows additional waveforms with feedforward (U 3  installed). In these two figures, waveform V 113  illustrates feedforward measurement signal  113 , V J3  illustrates the voltage at junction J 3 , and V J4  illustrates the voltage at junction J 4 . In  FIG. 5 , V J3  shows the gated power signal at J 3  when feedforward is disabled by removing U 3 . The average value of the AC-coupled power signal at J 2  is zero, and because the waveform is inverted, the voltage at J 2  will be negative during the positive spikes of the power supply DC output voltage. If the voltage at J 2  is gated by an interval around the cusps on the DC bus voltage, then the gated signal would have a negative average value. Consequently, the average value of the gated power signal at J 3  is negative, and when this voltage is integrated by U 7 , feedforward scaling signal  148  becomes positive. If too much feedforward compensation were applied, then the average value of the voltage at J 3  would be positive, and this would drive feedforward scaling signal  148  toward zero. The negative power pin of U 7  is tied to ground.  FIG. 6  shows the waveforms of  FIG. 5  when U 3  is installed and the adaptive feedforward circuit is operating.  
         [0025]     In addition to reducing the output ripple of a DC power supply, the adaptive feedforward circuit could also be applied to reduce the ripple in the envelope of RF power supplies that are powered from an unfiltered three-phase rectified DC bus. The adaptive feedforward circuit can be utilized in applications other than power supplies intended for plasma processing. In general, power supply  30  can be any type of controllable plant that operates a load  43 . The output measurements can correspond to any relevant output parameters of the plant.  
         [0026]     Although herein there is illustrated and described specific structure and details of operation of the invention, it is clearly understood that the same were merely for purposes of illustration and that changes and modifications may be readily made therein by those skilled in the art without departing from the spirit and the scope of this invention.