Abstract:
Active power factor correction (PFC) circuits are used to minimize unwanted harmonic distortion in applications where AC electrical power is rectified to produce DC power needed for operating electrical equipment. A variable amplitude regulator (VAR) is a PFC control interface which is simpler to implement than conventional circuits, and offers a wider dynamic operating range. The VAR functions as a resistor scaling network using a two-stage RC filter to maintain the DC output voltage constant for various load conditions and to maintain the rectified current in phase with the sinusoidal circuit flow in an AC power line, through both slow and rapid changes in the load coupled to the direct current output. This control interface offers excellent performance characteristics and requires only a few components for a useful implementation.

Description:
RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of co-pending application Ser. No. 10/199,646 filed on Jul. 7, 2062. 
     
    
     
       BACKGROUND  
         [0002]    Power Factor Correction (PFC) circuits are used to minimize unwanted disturbances in AC power lines, and to provide a constant DC output voltage under all load conditions. The AC line disturbances are caused by normal operation of DC powered electrical equipment, and are exhibited as phase shift of the AC input current and distortion of the current waveform. The PFC minimizes the distortion and corrects the phase shift. Existing PFC control circuits are complex, difficult and time consuming to implement, and have a limited dynamic range. By incorporating a power factor correction circuit between the alternating current supply and the direct current supply connected to the load, however, harmonic distortion in the AC power line is reduced; and the operational characteristics of some electrical equipment is improved. It is desirable to provide an improved PFC control circuit which is simple, has a wide dynamic range and requires minimal expertise to implement using a variable Amplitude Regulator (VAR) to accomplish this by using simple resistive scaling, instead of complex multiply and divide circuit functions, to produce the PFC control signal.  
         SUMMARY OF THE INVENTION  
         [0003]    It is an object of this invention to provide an improved power factor correction (PFC) system.  
           [0004]    It is another object of this invention to provide an improved variable amplitude regulator (VAR) signal interface in a switch-mode PFC system.  
           [0005]    It is an additional object of this invention to provide an improved analog variable amplitude voltage regulator for use in a power factor correction system.  
           [0006]    It is a further object of this invention to provide an improved variable amplitude voltage regulator (VAR) for use in a power factor correction system in which the VAR interface functions as a resistor scaling network utilizing at least one variable resistor for responding to a wide dynamic range of load variations.  
           [0007]    In accordance with a preferred embodiment of the invention, a variable amplitude voltage regulator (VAR) utilized in a power factor correction system operates as a resistor scaling network. The network consists of at least one variable resistor. A source of rectified alternating current input voltage (ACR) is coupled to the resistor scaling network. The output of a voltage error differential amplifier (VES) is coupled through a filter to a digital signal processor (DSP) which converts the VES voltage into a proportional duty ratio (DR) signal. The signal then controls the resistance value of the variable resistor such that the scaling network produces a demand level control signal (DLS) for the power factor correction circuit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    [0008]FIG. 1 is a schematic diagram of a power factor correction circuit incorporating a preferred embodiment of the invention;  
         [0009]    [0009]FIG. 2 illustrates waveforms useful in understanding the operation of the system shown in FIG. 1;  
         [0010]    [0010]FIG. 3 is a simplified schematic diagram of a resistor scaling network useful in explaining the operation of the preferred embodiments of the invention; and  
         [0011]    [0011]FIG. 4 is a detailed schematic diagram of an embodiment of the invention; and  
         [0012]    [0012]FIG. 5 is a detailed schematic diagram of another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Reference now should be made to the drawings, in which the same reference numbers are used throughout the different figures to designate the same or similar components. FIG. 1 is a schematic diagram of a switch-mode boost converter of a typical configuration, which is used for a power factor correction circuit. In the circuit of FIG. 1, the location and functional interconnections of a variable amplitude regulator interface (VAR) are illustrated.  
         [0014]    In the circuit shown in FIG. 1, standard alternating current (AC) utility power is connected across the input terminals  10 , also designated as ACL-NTL. Although a specific implementation of a switch-mode boost converter is illustrated in FIG. 1, other implementations may be used. The alternating current input voltage is filtered by a common mode choke consisting of a pair of independent windings  14 ,  16  and a pair of capacitors  18  and  20  interconnected in a conventional  7  manner across the AC input line  10 . This filtered alternating current voltage is applied to a full wave bridge rectifier  22 . The rectifier  22  output is connected to a switch-mode boost stage consisting of an inductor  28 , a capacitor  29 , a transistor (MOSFET)  24  and a resistor  26 . The capacitor  29  is connected across the DC terminals of the rectifier  22 ; and the positive terminal of the rectifier  22  is connected through the inductor  28  and an additional diode  25  to the positive DC output load terminal  27 . The negative or return terminal of the rectifier  22  is coupled to the lower side of the resistor  26 , shown in FIG. 1. A large electrolytic capacitor  50  is placed across the DC output terminals to smooth the output voltage and current.  
         [0015]    In order to achieve the desired sinusoidal current flow in the AC power line (see FIG. 2, top set of waveforms), it is necessary to regulate the current flow through the power inductor  28  in the switch-mode boost stage. At any instant in time, the magnitude of the current in the inductor  28  is equal to the absolute value of the alternating line current. This is indicated by the designation Iac in FIG. 1. As is apparent from FIG. 1, this same current flows through the transistor  24  and the resistor  26 , which are connected in series with the inductor  28  across the terminals of the rectifier  22 . Regulation of this current through the inductor  28  and the resistor  26  is controlled by a pulse width modulation circuit (PWM)  30 , used to adjust the on/off duty ratio of the MOSFET transistor  24 .  
         [0016]    The functional characteristics of pulse width modulation circuits, such as the circuit  30 , are well understood and a detailed description of the operation of such a circuit is not considered necessary here. The PWM circuit  30  shown in FIG. 1 uses an average current-mode control method to regulate the current through the inductor  28 , as a function of the Iac feedback current from the resistor  26  and of a demand level signal (DLS)  32  provided by a PFC control circuit. The output of the PWM circuit  30  connects through a driver  34  to the gate of the MOS FET  24  to modulate its on/off duty ratio in order to control current flow through the inductor.  
         [0017]    Required inputs to the PFC control circuit are the AC rectified voltage signal (ACR) and the voltage error signal (VES). The ACR is generated through the resistor divider  36  and  38 , and the VES is the output of the differential amplifier  44  on the lead  45 . The PFC circuit causes the current waveform in the power inductor  28  to be congruent and in phase with the AC voltage waveform. It also responds to changes in load by adjusting the amplitude of the AC input current. In this new PFC circuit implementation which is illustrated in FIG. 1, the control functions needed for power factor corrections are provided by the VAR interface.  
         [0018]    The VAR interface causes the waveform of the current in the power inductor  28  to be congruent and in phase with the rectified AC input voltage (ACR), as indicated in FIG. 2. At the same time, the Iac RMS value must be regulated to maintain the DC output voltage for various changing load conditions. Load variations are detected by a voltage error differential amplifier  44 , which produces a voltage error signal VES on the line  45 . This signal is produced by comparing the output DC voltage on the load terminal  27 , as it appears across a resistor divider consisting of the a resistor  46  and the resistor  48  coupled to one input of the amplifier  44  against a fixed reference provided across the Zener diode  52  connected to the other input of the amplifier  44 . The filter capacitor  50  also is connected across the DC output, as is readily apparent from FIG. 1.  
         [0019]    Whenever changes occur in the direct current load connected across the positive direct current terminal  27  and the negative or return (RTN) terminal shown in FIG. 1, a corresponding change or variation occurs in the output of the amplifier  44  in the VES signal applied over the line  45 . The VES control signal on the line  45  is supplied as one of two inputs to the variable amplitude regulator interface circuit (VAR)  42 . The other input is the rectified AC voltage (ACR) obtained from a voltage divider consisting of the resistors  36  and  38  connected across the output terminals  18  of the rectifier  22 . The two signals are combined to produce the control signal DLS for operating  19  the pulse width modulator  30 , which in turn controls the conductivity of the FET transistor  24  for regulating the current flow through the power inductor  28 , as described above.  
         [0020]    As mentioned previously, in a power factor correction (PFC) application, it is desirable to change the RMS value of the current, but not the wave shape, to prevent harmonic distortion of the alternating input current applied at the terminal  10 . Consequently, whenever the load connected to the terminal  27  changes, the system must both regulate the wave shape of the incoming alternating current signal on the terminals  10 , as well as the changes in the DC load current, without much delay. By employing the VAR interface  42 , which can respond to both slow changes of the DC load as well as stepped changes or rapid changes, the switch-mode modulator  30 , which controls the operation of the transistor  24 , is allowed to run at frequencies as low as 25 Khz, in contrast to systems of the prior art which typically had a 80 Khz lower limit.  
         [0021]    The VAR interface produces a single output, the demand level signal (DLS), which is used as a control input by the PWM in the power stage of the PFC. This single output serves two functions, correction of the AC current waveform and phase, and adjustment of the AC current amplitude in response to changes in load.  
         [0022]    There are two inputs to the VAR interface, the AC rectified voltage (ACR), and the voltage error signal (VES). The ACR is the output of the bridge rectifier  22  through the resistor divider  36  and  38 , and is used to control the waveform and phase of the AC input current. The VES is the output of the differential amplifier  44 , and is used to control the average value of the AC input current.  
         [0023]    The VAR interface design is based on a simple concept using a variable resistor ratio to control the amplitude of the signal (acrx) derived from the rectified AC input voltage (ACR). This control arrangement is illustrated in a simplified circuit diagram shown in FIG. 3. The relationship between the ACR and the DLS is defined by the following equation  
       DLS   =       (     R2     R1   +   R2       )     ×     (     1   +     R4   R3       )     ×   ACR                           
 
         [0024]    In a typical application, the VES is connected to the terminal  45  to control the variable resistor  64 . A resistor  80  and a capacitor  82  form a low pass filter to block AC line frequency ripple, from causing harmonic distortion. A resistor divider consisting of a resistor  62  (R 1 ) and a variable resistor  64  (R 2 ) produce a reduced ACR voltage which is connected to the (+) input of a differential amplifier  68 . The amplifier  68  is used as an impedance buffer and a fixed gain stage as determined by the values of resistors  70  (R 3 ) and  72  (R 4 ). The output of the amplifier  68  connects to the terminal  32 , the demand level signal (DLS). Based on the circuit configuration shown in FIG. 3, it is obvious that a change in the resistance of variable resistor  64  (R 2 ) produces a proportional change in the demand level signal (DLS). The voltage error signal  45  (VES) controls the value of the variable resistor  64  (R 2 ), which in turn controls the input of the amplifier  68 . The output of the amplifier  68  is the demand level signal (DLS) on terminal or line  32 . The circuit of FIG. 3 also includes a feedback resistor  72  and a filter capacitor  74 , interconnected in a conventional manner.  
         [0025]    In the simplified circuit of FIG. 3, it is apparent that the VAR interface which is illustrated produces a demand level signal (DLS) on the line  32 , which satisfies both of the desired control functions of regulating the RMS value and maintaining the sinusoidal waveform in the current of the inductor  28 . This is accomplished by combining the two input signals ACR and VES on the terminals  40  and  45 , respectively. Because the value of the resistor  64  may be varied, the value of the DLS output on the line  32  is variable. By dynamically controlling the resistivity of the resistor R 2 , the variation in the DLS signal on the line  32  effectively may be utilized to control the PWM  30  of FIG. 1 to maintain the output voltage steady as the load changes, and to keep the input current in phase and congruent with the AC line voltage applied across the terminals  10 .  
         [0026]    Reference now should be made to FIG. 4, which is a detailed schematic diagram of the VAR circuit  42  of a preferred embodiment of the invention. The circuit of FIG. 4 is the specific implementation of an actual configuration of the resistor scaling network described generally in conjunction with FIG. 3.  
         [0027]    In the circuit of FIG. 4, the ACR signal is applied on the terminal  40 ; and the VES signal is applied on the lead  45 , as in the case of the circuit of FIG. 3. The input return signal is shown at the bottom of FIG. 4 as RTN. The output differential amplifier  68  is illustrated providing the DLS output on the line  32 , and is shown with the feedback resistor  72  connected in series with the resistor  70  to provide gain to the differential amplifier  68 . The input obtained from the scaling circuit is supplied to the +terminal of the amplifier  68 .  
         [0028]    As shown in FIG. 4, the ACR signal on terminal  40  connects through the resistor  62  to the drains of three identical JFETs  92 ,  94 , and  96 . These JFET devices are connected in parallel with each other and function as a voltage controlled variable resistance. Many different field effect transistor (FET) types are available and can be used; however, the VAR circuit schematic illustrated in FIG. 4 is designed to incorporate a type J175. The voltage error signal (VES) applied at terminal  45  provides the control for regulating the resistance of the three JFETs,  92 ,  94  and  96 . The frequency response of the VAR interface is tailored to accommodate three basic requirements: low harmonic distortion, fast reaction to sudden changes in AC input voltage, and fast reaction to sudden changes in DC output (load) current.  
         [0029]    In order to achieve low harmonic distortion and a power factor of 0.99 or better, it is necessary to insert a low pass filter between the VES terminal  45  and the gates of the three JFET devices  92 ,  94  and  96 . This filter is necessary to block the AC line frequency ripple, which is superimposed on the voltage error signal (VES). The AC line frequency ripple occurs in PFC circuits because the AC input current is cyclical and the DC current is nearly constant during steady state operation. In this implementation, the filter consists of a network of seven components, namely resistors  78 ,  80  and  86 ; capacitors  82  and  88 ; and diodes  84  and  85 . The selection of each component is based on specific functional requirements. Resistors  78  and  86 , and capacitor  88  are chosen to form a high frequency attenuator which reduces switching noise in the VAR interface. Resistor  80  and capacitor  82  form a low pass filter to block the AC line frequency ripple to achieve low harmonic distortion.  
         [0030]    The gates of the transistors  92 ,  94  and  96  are connected in common to a filter network which includes resistors  78  and  86  and a capacitor  88  connected between the VES input terminal  45  and RTN. A resistor  80 , having a high value of resistance (typically on the order of 100 k Ohm), in conjunction with a capacitor  82 , operates as an input filter having an RC time constant which preferably is 100 ms or longer than the time constant provided by the filter including the resistor  86  and the capacitor  88 . This time constant assures a very constant gate control voltage on the gates of the JFETS  92 ,  94  and  96  for steady state or slow variations of the control signal VES on the terminal  45 .  
         [0031]    To provide a faster response during step load or rapid load changes, a pair of opposite conductivity diodes  84  and.  85  are connected in parallel to bypass the resistor  80 . The forward voltage drop of these diodes is approximately 0.6 Volts; so that VES level changes on the terminal  45  of 0.6 Volts or greater are propagated through the resistor  78  and the diodes  84  and  85  to the gates of JFETs  92 ,  94  and  96 , with a time constant of 2 ms or less, since the resistor  80  essentially is out of the circuit for such greater magnitude step load changes. Small perturbations (less than +/−0.5 V) are attenuated by the large value of resistor  80  and capacitor  82 . The acrx signal is connected to the (+) input of amplifier  68 . This amplifier is configured as a voltage follower with gain, and provides the demand level signal  32  (DLS), which is a low impedance output. The amplifier gain is set by the values of the resistors  70  and  72 , which can be selected to meet specific DLS output requirements.  
         [0032]    By providing the two different time constants through the filter circuit at the gates of the transistors  92 ,  94  and  96  with different RC combinations, the system is allowed to accommodate a slow response for steady state and slow variations in the DC load, as well as a fast response for step load changes using the VAR interface circuit. It is important to note that the DLS output on the terminal  32  is congruent with the rectified AC line voltage (ACR) and that there is very little phase shift between the signals, as illustrated in the idealized waveforms of FIG. 2. These characteristics are significant because the DLS output is the control reference for the PWM  30 , which in turn regulates the AC line input current by controlling the on/off duty ratio of the transistor  24 , as described previously in conjunction with FIG. 1.  
         [0033]    Phase shift and waveform irregularities contribute to harmonic distortion and reduced power factor, as is well known. By utilizing the dynamic control response of the circuit of FIG. 4, a simple and accurate analog control circuit is provided for utilization in a power factor correction application.  
         [0034]    [0034]FIG. 5 illustrates another variation of the system employing a bi-polar transistor and a digital signal processing (DSP) circuit in place of the JFET devices  92 , 94  and  96  of FIG. 4 to perform the variable resistor function. The use of bi-polar transistors permits the implementation of a single adjustable duty ratio (DR) signal capable of controlling a single phase input or a two-phase or three-phase system. The control transistor is connected in parallel with a relatively large resistor and the transistor is switched on and off in varying amounts (varying duty cycle) to control the average resistance of the variable resistor consisting of the parallel-connected transistor and resistor. This implementation is effective for single-phase or multiple phase (such as three-phase) operation.  
         [0035]    In FIG. 5, the ACR input  40  and the VES input  45  to the VAR circuit represented in FIG. 5 are the same as shown in FIG. 4. The same low pass filter circuit of FIG. 4, connected between the VES terminal  45  and the gates of the JFET devices also is employed in FIG. 5 between the VES terminal  45  and the input to a digital signal processor (DSP) circuit  102 . This processor circuit essentially comprises a saw tooth generator and a comparator to provide a pulse width modulation of squarewave output pulses on the output terminal thereof. These output pulses are supplied through a coupling resistor  10  to the base of a bi-polar NPN transistor  112 . A typical circuit which may be used for the DSP circuit  102  is a UNITRODE No. UCC3889. Other commercially available DSP circuits, however, could be used as well as the UNITRODE circuit to provide the same pulse width modulated output.  
         [0036]    The output applied through the coupling resistor  110  to the transistor  112  has a proportional duty ration (DR) at a fixed frequency above the audible range. A filter comprising a resistor  104  and capacitor  106  is coupled between the VCC input to the DSP circuit  104  and the RC/CT input terminal. Another coupling resistor  108  connects the output of the low pass filter to the comparator input of the DSP circuit. The operation of the DSP circuit essentially converts the analog VES input at the output of the low pass filter into a digital signal with a varying duty cycle. This signal is used to switch on and off the transistor  112 , the collector emitter path of which is connected in parallel with a relatively high value resistor  98  between the DLS output terminal  32  and the RTN line.  
         [0037]    The output of the variable resistor comprising the transistor  112  and the fixed high value resistor  98  is filtered by another low pass filter comprising the resistor  114 , capacitor  116 , and resistor  66 . This filter is used since there is a short delay between the DLS and ACR signals as a result of their low pass filter coupled to the VES terminal  45  required to attenuate the modulation switching frequency. Since the ACR and DLS signals are line frequency related (50/60 Hz), it is reasonable to use an RC averaging filter for a 62 KHz modulated signal.  
         [0038]    For the circuit design and component selection of the circuit shown in FIG. 5, DLS (the signal appearing on terminal  32 ) is defined by the following equation:  
           DLS =[( R   112 )/( R   112 + R   98 )]×(1− DR )× ACR    
         [0039]    The foregoing description of the preferred embodiment of the invention is to be considered as illustrative and not limiting. Various changes and modifications will occur to those skilled in the art for performing substantially the same function, in substantially the same way, to achieve substantially the same result, without departing from the true scope of the invention as defined in the appended claims.