Abstract:
A controller for a power factor corrector (PFC), having a controllable switch and operable in a discontinuous conduction mode (DCM), and a method of controlling the controllable switch. In one embodiment, the controller includes: (1) a drive circuit adapted to drive the controllable switch and (2) a current mode control circuit adapted to sense a current of the PFC and develop a current control signal based thereon. The drive circuit reduces a duty cycle of the controllable switch based on the current control signal to reduce input current distortion in the PFC.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to power conversion and, more specifically, to a controller for a power factor corrector and a method of operating the controller. 
     BACKGROUND OF THE INVENTION 
     Power factor correction is a common requirement of many electronic systems today. To achieve a high power factor, switched-mode converters, such as boost converters, are often employed. Boost converters may be used to elevate an output voltage to a desired value greater than an input voltage. Boost converters can be operated in several different modes. In low to medium power applications, a discontinuous conduction mode (DCM) of operation is attractive because DCM operation uses a simple control circuit and does not need a complex snubber circuit. For higher power, power factor correction applications, a continuous conduction mode (CCM) of operation for the boost converter is normally used. However, CCM operation typically requires a more complex control circuit and a complex snubber circuit to operate satisfactorily. 
     Control of a conventional DCM boost converter employs essentially a constant duty cycle for the power switch of the boost converter. That is, the duty cycle of the power switch does not change significantly over a cycle of the AC input power. At “high line” where the input voltage is at its highest RMS value, the slow discharge of the current through the boost inductor around the peak of the AC input voltage produces an excessive peak amount of input current. This excessive peak amount of input current can generate a high amount of total harmonic distortion (THD) for the input current. 
     For example, a boost converter operating in DCM with an input voltage of 265 volts and an output voltage of 400 volts may have an input current THD of around 50 percent. This amount of THD is generally not acceptable. To reduce the THD of the input current, some boost converters are operated in a critical conduction mode. That is, the power switch of the boost converter is closed a fixed time when the boost inductor current reaches zero each control period. However, such operation will cause the power switch to experience an unusually wide range of switching frequencies, requiring a high level of input filtering to meet electromagnetic interference (EMI) standards. A boost converter operating in DCM may also move into CCM operation under certain situations. In CCM operation, special care should be taken to account for excessive currents that may damage components of the converter that were designed to operate primarily in DCM. 
     Accordingly, what is needed in the art is an improved way to control PFC converters operable primarily in DCM that enhances the performance thereof without the complexity and cost penalties associated with conventional CCM converters. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides a controller for a power factor corrector (PFC), operable in a discontinuous conduction mode (DCM) and having a controllable switch, and a method of controlling the controllable switch. In one embodiment, the controller includes: (1) a drive circuit adapted to drive the controllable switch and (2) a current mode control circuit adapted to sense a current of the PFC and develop a current control signal based thereon. The drive circuit reduces a duty cycle of the controllable switch based on the current control signal to reduce input current distortion in the PFC. 
     The present invention introduces, in one aspect, a controller that modifies a duty cycle of the controllable switch (power switch) of the PFC based on an input current thereof. By judiciously reducing the duty cycle of the controllable switch, preferably around a peak of the input voltage, the peak input current to the PFC can be reduced, thereby affording a significant reduction in the total harmonic distortion (THD) of the input current. This judicious limiting of the peak value of the input current also makes the PFC more robust if its operation should change from a discontinuous conduction mode (DCM) to a continuous conduction mode (CCM) wherein damage to components may be more probable. 
     In one embodiment of the present invention, the controller further includes a frequency modulation control circuit adapted to sense a rectified output voltage of the PFC and develop a voltage control signal based on the rectified output voltage. The drive circuit modifies a switching frequency of the controllable switch based on the voltage control signal. In a related embodiment, the frequency modulation circuit includes a voltage divider for sensing the rectified output voltage. Of course, other sensing schemes may be employed to sense the rectified output voltage. The principle of frequency modulation of the controllable switch is described in detail in U.S. patent application Ser. No. 09/324,074, entitled “Frequency Modulation Controller For Single-switch, Polyphase, DCM Boost Converter and Method of Operation Thereof” to Hengchun Mao, filed Jun. 1, 1999. The above-listed application is herein incorporated by reference as if reproduced herein in its entirety. 
     In one embodiment of the present invention, the current mode control circuit includes a current sensor adapted to sense the current of the PFC and a filter adapted to develop the current control signal based on the current of the PFC. The current sensor may be used to sense current from any pertinent point in the PFC. The filter then smooths the current control signal to an appropriate degree to obtain a high frequency averaged current signal commensurate with the requirements of the drive circuit. 
     In one embodiment of the present invention, the drive circuit includes: (1) a voltage compensation circuit adapted to receive a signal representing an output voltage of the PFC and develop an intermediate signal therefrom and (2) a modulator, coupled to the voltage compensation circuit, adapted to develop a drive signal for the controllable switch based on the intermediate signal. Of course, other ways of developing the drive signal are well within the broad scope of the present invention. 
     In a related embodiment of the present invention, the voltage compensation circuit includes an error amplifier adapted to compare the signal representing the output voltage to a reference signal and develop the intermediate signal therefrom. The error amplifier accurately determines any deviation between the output voltage and the reference voltage to provide an appropriate intermediate signal for output voltage correction purposes. 
     In another related embodiment, the modulator includes a duty cycle comparator adapted to receive a ramp signal and the intermediate signal and develop the drive signal therefrom. The comparator accurately compares the input values of the ramp signal and the intermediate signal to determine the duty cycle for the controllable switch. 
     In one embodiment of the present invention, the PFC employs a topology selected from the group consisting of a boost converter, a buck converter, a forward converter, a flyback converter, and a SEPIC converter. Of course, other topologies are well within the broad scope of the present invention. 
     In one embodiment of the present invention, the PFC is further operable in a continuous conduction mode (CCM). The PFC may enter CCM, for example, during a peak of the input line voltage. 
     In another embodiment of the present invention, the PFC includes an input EMI filter. Depending on the topology used, the input filter may include a filter inductor, a filter capacitor or both the filter inductor and the filter capacitor. Of course, the use of any input filter topology is well within the broad scope of the present invention. 
     The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a schematic diagram of an embodiment of a power factor corrector (PFC) constructed according to the principles of the present invention; and 
     FIG. 2 illustrates a schematic diagram of another embodiment of a PFC constructed according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, illustrated is a schematic diagram of an embodiment of a power factor corrector (PFC)  100  constructed according to the principles of the present invention. The PFC  100  has an input couplable to an AC source (not shown) having an input voltage Vin and supplying an input current Iin. The PFC  100  provides an output voltage Vout at an output thereof. The PFC  100  includes an electromagnetic interference (EMI) filter  105  and a rectifier  106  having first, second, third and fourth diodes D 1 , D 2 , D 3 , D 4 . The PFC  100  further includes a converter, such as a boost converter having a boost inductor Lb (with a boost inductor current I L ), a controllable switch S, a boost diode Db and an output filter capacitor Co. In the illustrated embodiment, the PFC  100  employs a boost converter topology wherein the output voltage Vout is greater than a rectified input voltage Vin. Of course, other topologies, including a buck converter, a forward converter, a flyback converter, and a SEPIC converter topology, are well within the broad scope of the present invention. The PFC  100  still further includes a controller  110 . The PFC  100  supplies the output voltage Vout to an output load R L . 
     The controller  110  includes a drive circuit  115  and a current mode control circuit  120 . The drive circuit  115  includes a voltage compensation circuit  125  and a modulator  130 . The voltage compensation circuit  125  includes an error amplifier A 1  having a feedback impedance Zf and an input impedance Zi. The error amplifier A 1  receives the output voltage Vout and a voltage reference signal Vref and provides an intermediate signal (compensation output voltage signal) Vc based on the output voltage Vout and the voltage reference signal Vref. The modulator  130  includes a duty cycle comparator C 1  and a summing junction Σ. The duty cycle comparator C 1  receives the intermediate signal Vc and a summing junction output signal Vsum and provides a switch control output signal Vs. The summing junction Σ receives a ramp signal Vrmp and a current control signal Ic from the current mode control circuit  120  and provides the summing junction output signal Vsum. The current mode control circuit  120  includes a current sensor Sc and a filter F 1 . The current sensor Sc senses a sense current Is from the PFC  100  and provides a sensed current signal Isens to the filter F 1  for smoothing. The filter F 1  then provides the current control signal Ic to the summing junction Σ. 
     The EMI filter  105  reduces electromagnetic interference generated by a ripple current component of an input current Iin to meet appropriate EMI standards. The rectifier  106 , arranged in a full-bridge topology, provides DC voltage to the boost inductor Lb. When the controllable switch S is closed, the boost inductor current I L  increases steadily, and the boost diode Db isolates the output voltage Vout from the closed controllable switch S and the boost inductor Lb. Under this condition, the output capacitor Co provides current to the output load R L . Opening the controllable switch S allows the boost inductor current I L  to flow through the boost diode Db to the output load R L  and the output capacitor Co. A boost in the output voltage Vout over the input voltage Vin is obtained when the controllable switch S is open. Normally, the PFC  100  operates in DCM wherein the boost inductor current I L  is not continuous. However, even if the PFC  100  enters continuous conduction mode (CCM) around the peak of the input line voltage, the current mode control circuit  120  can automatically limit the current such that the PFC  100  can operate reliably. 
     The drive circuit  115  and the current mode control circuit  120  work in concert to establish the duty cycle of the controllable switch S. The duty cycle of the controllable switch S may be defined as the normalized ratio of the portion of time that it is closed (e.g., a first portion D of the duty cycle) to a total time period representing the summation of the times that it is both open and closed (e.g., D+(1-D); wherein (1-D) represents a complementary portion of the duty cycle). The voltage compensation circuit  125  develops the intermediate signal Vc that is a function of both the output voltage Vout and the reference signal Vref. The voltage gain of the error amplifier A 1  is basically the ratio of the feedback impedance Zf to the input impedance Zi (i.e., Zf/Zi) In this embodiment, the reference signal Vref is an external DC voltage that establishes a nominal value of the output voltage Vout. 
     The duty cycle comparator C 1  of the modulator  130  compares the intermediate signal Vc to the summing junction output signal Vsum. The summing junction output signal Vsum is formed by a summation of the ramp signal Vrmp and the current control signal Ic. The ramp signal Vrmp is externally provided to the controller  110  in this embodiment. The controllable switch S is closed (for a first portion D of the duty cycle) when the summation of the ramp signal Vrmp and the current control signal Ic is less than the intermediate signal Vc. Alternately, the controllable switch S is open (for a complementary portion (1-D) of the duty cycle) when the summation of the ramp signal Vrmp and the current control signal Ic is greater than the intermediate signal Vc. 
     The frequency of the ramp signal Vrmp is typically 50 to 100 kilohertz and determines the overall nominal operating frequency of the controllable switch S. The frequency response of the current control signal Ic is much lower than the ramp signal Vrmp, since it may, in the illustrated embodiment, follow a rectified waveform of the input current Iin. The frequency response of the voltage compensation circuit  125  may also be much lower than the ramp signal vrmp, since the output filter capacitor Co attempts to maintain the output voltage Vout at its present value. 
     A small negative change in the output voltage Vout would produce a larger positive change in the intermediate signal Vc, which is coupled to an input of the modulator  130 . This positive change in the intermediate signal Vc requires that the summing junction output signal Vsum increase before the modulator  130  changes its output state. This increase causes the controllable switch to remain closed longer thereby increasing its duty cycle (i.e., a larger value of the first portion D of the duty cycle). An increased duty cycle increases the output voltage Vout until both the output voltage Vout and the duty cycle are again restored to their nominal values. 
     A primary contribution to the total harmonic distortion (THD) of the input current Iin would occur around the peak AC values of the input voltage Vin, where an uncorrected waveform of the input current Iin would tend to reach an excessively high peak. The current mode control circuit  120  reduces this excessive peaking of the input current Iin as would otherwise occur in a conventional PFC. In this embodiment, the sense current Is is substantially equal in magnitude to the boost inductor current IL. However, in other embodiments of the present invention, the sense current Is may be taken from currents flowing through the boost diode Db or the controllable switch S. 
     The sense current Is modifies the summing junction output signal Vsum to reduce the duty cycle of the controllable switch S during a peak of the input current Iin. This action limits the peak value of the input current Iin, thereby reducing the THD. In the illustrated embodiment, the THD may be reduced to about 5 percent, as compared to a THD of about 50 percent for a conventional PFC. Of course, other embodiments may reduce the THD by a greater or lesser amount. In either case, the operation of the PFC  100  may become more robust. Even if the PFC  100  moves into CCM operation, such as during a peak of the input line voltage, the peak value of the input current Iin would still be limited, thereby providing some protection against components being over-stressed. 
     Turning now to FIG. 2, illustrated is a schematic diagram of another embodiment of a PFC  200  constructed according to the principles of the present invention. The PFC  200  is couplable to an AC source (not shown) having an input voltage Vin and supplying an input current Iin. The PFC  200  provides an output voltage Vout at an output thereof. The PFC  200  includes an electromagnetic interference (EMI) filter  205  and a rectifier  206  having first, second, third and fourth diodes D 1 , D 2 , D 3 , D 4 . The PFC  200  further includes a converter, such as a boost converter, having a boost inductor Lb (with a boost inductor current I L ), a controllable switch S, a boost diode Db and an output filter capacitor Co. The PFC  200  still further includes a controller  210 . The PFC  200  supplies the output voltage Vout to an output load R L . 
     The controller  210  includes a drive circuit  215 , a current mode control circuit  220  and a frequency modulation control circuit  225 . The drive circuit  215  includes a voltage compensation circuit  230 , a modulator  235  and a ramp generator  236 . The voltage compensation circuit  230  includes an error amplifier A 1  having a feedback impedance Zf and an input impedance Zi. The error amplifier A 1  receives the output voltage Vout and a voltage reference signal Vref and provides an intermediate signal (compensation output voltage signal) Vc based on the output voltage Vout and the voltage reference signal Vref. The modulator  235  includes a duty cycle comparator C 1  and a summing junction Σ. The duty cycle comparator C 1  receives the intermediate signal Vc and a summing junction output signal Vsum and provides a switch control output signal Vs. The summing junction Σ receives a ramp signal Vrmp and a current control signal Ic from the current mode control circuit  220  and provides the summing junction output signal Vsum. The ramp generator  236  receives a voltage control signal Vcs from the frequency modulation control circuit  225  and determines a frequency of the ramp signal Vrmp based thereon. 
     The current mode control circuit  220  includes a current sensor Sc and a filter F 1 . The current sensor Sc senses a sense current Is from the PFC  200  and provides a sensed current signal Isens to the filter F 1  for smoothing. The filter F 1  then provides the current control signal Ic to the summing junction Σ. The frequency modulation control circuit  225  includes a rectified voltage monitoring circuit  226  having a coupling capacitor Cc and first and second resistors R 1 , R 2 . The frequency modulation control circuit  225  further includes a filter F 2 . The rectified voltage monitoring circuit  226  provides a sense voltage Vsens, which carries an AC portion of the rectified voltage, to the filter F 2  for smoothing. The filter F 2  provides the voltage control signal Vc to the ramp generator  236  to adjust the frequency of the ramp signal Vrmp. In the illustrated embodiment, the range of frequency adjustments may be about 10-20% of the frequency. Of course, other embodiments may be designed to have a greater or lesser frequency adjustment range. 
     The general operation of the PFC  200  employing the drive circuit  215  and the current mode control circuit  220  is analogous to the operation of the PFC  100  illustrated and described with respect to FIG. 1 above. Inclusion of the frequency modulation control circuit  225  incorporates an additional refinement over the PFC  100  of FIG.  1 . The PFC  100  may require an EMI filter  105  that is larger than desired due to high switching frequency current ripple in the input current Iin. Additionally, in some cases, the PFC  100  may also suffer from a sub-harmonic oscillation due to the use of current mode control alone. The operation of the PFC  200  may be improved with respect to the operation of the PFC  100  by the addition of the frequency modulation control circuit  225 , which utilizes a rectified waveform of the input voltage Vin to further modify the nominal operating frequency of the controllable switch S to spread out the harmonic spectrum of the input current and to avoid subharmonic oscillations. In the illustrated embodiment, the attenuation of the input filter may be reduced by about 10 dB. Of course, other ranges of reductions are possible and are well within the broad scope of the present invention. 
     The rectified voltage monitoring circuit  226  is basically a voltage divider that senses an AC ripple component of the rectified input voltage to provide the sense voltage Vsens. The voltage control signal Vcs is smoothed by the filter F 2  to provide the sense voltage Vsens, which is then used to modify the nominal operating frequency of the ramp generator  236 . The nominal operating frequency of the ramp generator  236  is modulated when the AC ripple component of the rectified input voltage is around both its peak and zero-crossing values. Of course, the operating frequency of the ramp generator  236  may be continuously modulated as required. Around the peak of the AC ripple, the operating frequency of the ramp generator  236  is increased, which decreases the average value of the input current Iin. Around the zero-crossing of the AC ripple, the operating frequency of the ramp generator  236  is decreased, which increases the average value of the input current Iin. 
     In this manner, the frequency modulation of the ramp generator  236  spreads the high-frequency spectrum of the input current Iin, further reducing its THD over the use of the current mode control circuit  220  alone. In the illustrated embodiment, a reduction in a dominant harmonic component of the input current Iin of typically 10 dB may be accomplished in the PFC  200  as compared to the PFC  100 , thereby appropriately reducing the size and cost of the EMI filter  205 . Of course, other embodiments may reduce the THD by a greater or lesser amount. The operation of the frequency modulation and the current feedback may be coordinated and adjusted to optimize the performance of the PFC  200  over a wide range of the input voltage Vin and the output load R L . 
     Portions of the controller  210  may be embodied in an integrated control circuit, such as the UC3842 manufactured by Unitrode Corporation of Merrimack, N.H. The UC3842 is an example of a low cost integrated circuit that may be employed to advantage in some embodiments of the present invention. Of course, other controllers may be employed and still be well within the broad scope of the present invention. 
     In summary, the present invention introduces a controller for use with a PFC that modifies a duty cycle of a drive signal employed by the PFC. The duty cycle is modified based on sensing a current of the PFC. This judicious modification of the duty cycle reduces the peak value of the input current to the PFC and thereby affords a significant reduction in the THD of the input current. This judicious limiting of the peak value of the input current also makes the PFC more robust if its operation should change from DCM to CCM, wherein damage to components may be more probable. The embodiments of the present invention discussed above produce an input current quality that is similar to CCM operation in a converter operating in DCM. 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.