Abstract:
A power factor corrector control device accomplishes control of power factor correction with a simpler and cheaper circuit, overcomes the drawback of higher current harmonics occurred in the prior art, and also accommodates mains voltage distortion. This power factor corrector control device uses a built-in circuit to discriminate the mains frequency, and generates a pure sinusoidal signal having the same frequency with the mains frequency. The product of a feed-forward signal and an output error signal is exploited to get a constant by using a sample-and-hold circuit to determine the amplitude of a reference current signal, hence preventing ripples of the feed-forward signal and the output error signal from generating distortion of the reference current signal after circuit operation. Moreover, a division approximate circuit is used to accomplish simple feed-forward control so as to apply to various different mains voltage levels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a power factor corrector control device for accommodating mains voltage distortion and achieving high power factor and low harmonic and, more particularly, to a power factor corrector control device capable of allowing the input current to be in phase with the mains voltage and also keeping a pure sinusoidal waveform to achieve high power factor and low harmonic when the input mains voltage has distortion and contains voltage harmonics.  
         [0003]     2. Description of Related Art  
         [0004]     Because the existing electric appliances will produce an input current with high harmonics to an input electric power terminal (mains supply terminal) to deteriorate the quality of electric power, a power factor corrector is thus required for power factor correction and harmonics suppression. Its primary function is to compensate the phase difference between the current generated by an electric appliance and the voltage and to suppress the current harmonics generated by the electric appliance so as to prevent the quality of electric power from being affected. In general, electric power companies prefer connecting a simple resistive load to a power stage circuit than the generation of a high-harmonic current because a high-harmonic current may easily open a circuit breaker to cause disorder of a voltage regulating circuit. A power factor corrector can generally be divided into a power stage and a control stage.  FIG. 1  shows the architecture of an electric appliance having a power factor corrector  32 , wherein a rectifying circuit  30  converts an input AC mains supply into a DC power source, and a load  34  represents other circuit parts of this electric appliance. Common topologies of the power stage  322  of the power factor corrector  32  include the boost type, the buck type, the flyback type, and so on. Among these architectures, the boost type is most often used in the power factor corrector  32  because it can make use of a single stage circuit to achieve high power factor and lower harmonic. The control stage  324  usually makes use of a feedback output voltage signal, an input current signal, or an input voltage signal to determine a gate signal for driving a power switching component of the power stage. Through high-frequency switching, the input current is forced to follow a reference current signal determined by the waveform of the AC mains voltage, thereby achieving the object of power factor correction.  
         [0005]     Today, the UC3854 (or other IC of similar type, e.g., UC3852) is used in the power stage topology of most power factor correctors for control. A control circuit  26  of the UC3854 is shown in  FIG. 2 . The control circuit  26  comprises three parts: a current mode controller  266 , a voltage feedback control stage  264 , and a feed-forward control stage  262 . The voltage feedback control stage  264  uses an error amplifier EA to compare an output voltage V dc  and a reference voltage V ref  for getting an output error signal v e , and then multiplies the output error signal v e  by a sinusoidal signal of the input AC mains voltage to get a reference current signal i ref . The current mode controller  266  adjusts the duty cycle of a gate control signal V g  of a power switching component Q based on the above reference current signal i ref  and the input current signal, thereby forcing the input current to follow the reference current signal i ref . Because this reference current signal i ref  is determined by the input AC mains voltage, the input current will follow the AC mains voltage. In the control circuit  26 , in order for the power factor corrector of the control circuit  26  to apply to various different mains voltage levels without control of an adjustment knob, the feed-forward control stage  262  makes use of an RC circuit to get a root-mean-squared value of the input AC mains voltage. The output error signal v e  is then divided by this value squared to adjust the amplitude of the reference current signal so that the output voltage and the input power can be stably controlled in the designed range to have little variation due to change of the input voltage.  
         [0006]     In the control circuit  26  of the UC3854, the reference current circuit iref can be represented by:  
               i   ref     =         v   e       v   rms   2       ×          v   line                    (   1   )             
 
 where v e  is the output error signal, v 2   rms  is the mean-squared value of the mains voltage, and v line  is the mains voltage. In (1), because both the output error signal v e  and the mean-squared value of the mains voltage v 2   rms  have ripples with twice the mains frequency, harmonic signals not of the same mains frequency will be got after multiplication of these two signals. Therefore, the reference current signal i ref  will have harmonics even the mains supply is a pure sinusoidal signal. In existent electric power systems, because feed-in of recyclable energies is more and more common, the waveform of the mains voltage often contains harmonics and thus is not a pure sinusoidal signal. Therefore, the reference current signal i ref  will no longer be a pure sinusoidal signal, and the current inputted to the power factor corrector will certainly contain harmonics. 
 
         [0007]     The architecture of a conventional power factor corrector using the UC3854 series for control has the following drawbacks: 
        1. Ripples of the output signal and the feed-forward signal will cause distortion of the reference current signal so that the input current will contain harmonics.     2. When the input mains supply contains harmonics, the input current will no longer keep a pure sinusoidal waveform and will have high-harmonic components as the input mains supply.     3. The processing circuit of the feed-forward signal is more complex. It must contain a multiplier circuit and a divider circuit. The manufacturing cost and design complexity will be increased.     4. Because the reference current signal is got after the mains supply is rectified by a rectifier, distortion of the reference signal will occur at zero-cross points.        
 
         [0012]     Accordingly, the present invention proposes a power factor corrector control device allowing a power factor corrector to achieve high power factor and low harmonic even when the mains supply has distortion.  
       SUMMARY OF THE INVENTION  
       [0013]     An object of the present invention is to provide a power factor corrector control device, which can improve the phase difference problem between the voltage and current at the input terminal of a conventional power factor corrector, and can avoid distortion of a reference current signal of the input current of a conventional power factor corrector caused by ripples of the output voltage and the feed-forward signal and distortion of the input mains voltage so that the input current of the conventional power factor corrector will have harmonics. Therefore, the present invention can improve the drawback of deterioration of the quality of electric power caused by a conventional power factor corrector.  
         [0014]     Another object of the present invention is to provide a power factor corrector control device, which can discriminate the mains frequency to produce a reference pure sinusoidal signal with the accurate frequency so that the power factor corrector can apply to various different mains frequencies. Moreover, a feed-forward control circuit is used to accomplish feed-forward control of the power factor corrector control device of the present invention so that the power factor corrector can apply to various different mains voltage levels without control of an adjustment knob.  
         [0015]     In order to achieve the above objects, the present invention provides a power factor corrector control device, which comprises a voltage feedback control circuit connected to a load end for receiving a feedback voltage signal and outputting a reference current signal after internal processing, and a current feedback control circuit connected to the voltage feedback control circuit and an input terminal of the system circuit for receiving the reference current signal and an input current signal to produce a gate signal for controlling switching of a power switch. Through high-frequency switching of the power switch, the input current is forcedly controlled. The voltage feedback control circuit comprises a sine-wave generating circuit for producing a pure sinusoidal signal to determine the waveform of the reference current signal, and a sample-and-hold circuit (SAH), which samples the product for determining the amplitude of the reference current signal of an output error signal and the feed-forward signal once at the initial stage of a mains period and keeps this sampled value during this mains period. Through the self-generated pure sinusoidal signal and the constant amplitude in a mains period, a reference current signal which is a pure sinusoidal wave in a mains period is generated. Matched with a well-designed current mode controller in the current feedback control circuit, current harmonics generated at the input terminal of the power factor corrector will be reduced to almost none.  
         [0016]     In order to achieve the above objects, the present invention also provides a sine-wave generating circuit, which comprises a zero-cross detector for detecting zero-cross points of the input mains voltage, a frequency detector for discriminating the mains frequency, and a sine-wave generator for generating a pure sinusoidal signal. Reference pure sinusoidal signals with different frequencies will thus be generated according to different mains frequencies. The present invention further makes use of an RC circuit in the feed-forward circuit to get the input mains voltage for outputting a feed-forward signal v rms , which is sent to a division approximate circuit connected to the RC circuit. This division approximate circuit is used to output an approximate inverse value 1/v rms  of the feed-forward signal v rms . Therefore, when the input voltage is changed, this feed-forward signal can be used to adjust the amplitude of the reference current signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which:  
         [0018]      FIG. 1  is an architecture diagram of a conventional power factor correction circuit;  
         [0019]      FIG. 2  is an architecture diagram of a conventional power factor corrector control device using UC3854 as the controller;  
         [0020]      FIG. 3  is a circuit block diagram of a power factor corrector control device for accommodating mains voltage distortion and achieving high power factor and low harmonic of the present invention;  
         [0021]      FIG. 4  is a diagram illustrating a division approximate circuit of the present invention; and  
         [0022]      FIG. 5  is a waveform diagram showing the operation of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     The present invention proposes a power factor corrector control device for accommodating mains voltage distortion and achieving high power factor and low harmonic. The present invention is connected to an power input terminal of a power stage circuit for achieving high power factor and low harmonic of the power stage circuit. Its primary function is to get the voltage signal and current signal of the mains supply inputted by the power stage circuit. With also an output voltage signal, an accurate gate control signal of a power switch can be determined. Through high-frequency switching of a power switch, the input current is forced to follow a reference current signal so that the input current of the power stage circuit will have the same phase as the input voltage and also keep a pure sinusoidal waveform. The present invention adopts a control way that can avoid distortion of the reference current signal to accomplish control of the power factor corrector, thereby improving the situation that the current contains harmonics to affect the quality of electric power.  
         [0024]     shown in  FIG. 3 , the present invention provides a power factor corrector control device  10  for accommodating mains voltage distortion and achieving high power factor and low harmonic. The power factor corrector control device  10  comprises a feed-forward control circuit  11 , a voltage feedback control circuit  13 , and a current feedback control circuit  15 . The feed-forward control circuit  11  comprises an RC circuit  111  capable of measuring a root-mean-squared signal v rms  of the mains voltage and a division approximate circuit  113  for performing inverse operation of the root-mean-squared signal v rms . The division approximate circuit  113  can get an inverse signal 1/v rms  of the root-mean-squared value of the mains voltage. When the root-mean-squared signal v rms  fed back to the mains voltage is the feed-forward signal used to cancel out the influence to the output voltage v dc  caused by the variation of the mains voltage, variation of the input voltage only influences the output voltage (i.e., the amplitude of the reference current signal i ref,con ) but doesn&#39;t influence the waveform of the reference current (since this part is the unit pure sinusoidal waveform built in the controller), it is only necessary to divide the output error signal v e  by the feed-forward signal v rms . For IC fabrication, however, the fabrication cost of a divider will much larger than that of a multiplier. Therefore, the present invention makes use of the division approximate circuit to get the inverse 1/v rms  of the feed-forward signal, and then multiplies the output error signal v e  by this value to obtain the same result of dividing the output error signal v e  by the feed-forward signal v rms . The voltage feedback control circuit  13  has a sine-wave generating circuit  131 , which comprises a zero-cross detector  1311  for detecting zero-cross points of the mains voltage signal, a frequency detector  1313  for discriminating the frequency (e.g., 50 or 60Hz) of the mains voltage signal, and a sine-wave generator  1315  for generating a pure sinusoidal signal i sin  having the same frequency and phase with the mains voltage signal based on signals sent out by the zero-cross detector  1311  and the frequency detector  1313 . The zero-cross detector  1311  and the frequency detector  1313  constitute a mains supply signal detection circuit used to detect zero-cross points and the frequency of the mains voltage signal for outputting a zero-cross detection signal S 1311  and a frequency detection signal S 1313 .  
         [0025]     The pure sinusoidal signal i sin  will determine the waveform of the reference current signal i ref,con . An error amplifier EA whose function is to amplify the error between the output voltage v dc  and a reference voltage v ref  is also provided. This error is called the output error signal v e . This output error signal v e  multiplied by the inverse 1/v rms  of the feed-forward signal will be used to determine the amplitude of the reference current signal i ref,con . In order to avoid distortion of the reference current signal i ref,con  caused by ripples of the output voltage and the feed-forward voltage, the present invention makes use of a sample-and-hold circuit (SAH)  133  to get a trigger signal outputted by the voltage feedback control circuit  13  to sample a mains period once and then hold the sampled value during the mains period. The product V k,con  of the output error signal v e  and the inverse 1/ rms  of the feed-forward signal for determining the amplitude of the reference current signal i ref,con  will thus keep constant in a mains period. This value V k,con  is an amplitude adjustment signal. Moreover, a multiplier  135  connected to the sine-wave generating circuit  131  and the SAH  133  is used to receive the pure sinusoidal signal i sin  and the amplitude adjustment signal V k,con  and then output the reference current signal i ref,con  after multiplication operation.  
         [0026]     In the present invention, the reference current signal i ref,con  can be expressed as follows:  
               i     ref   .   con       =       V     k   ,   con       ×          sin   ⁢           ⁢   ω   ⁢           ⁢   t                    (   2   )             
 
 wherein the amplitude adjustment signal V k,con  is the product of the output error signal v e  and the inverse 1/v rms  of the feed-forward signal sampled by the SAH  133  (V k,con  is a constant in a mains period), sin ωt is the pure sinusoidal signal i sin  generated by the sine-wave generating circuit  131  and having the same phase and frequency as the mains voltage, and ω is the frequency of the mains voltage. In the control circuit  10  of the present invention, the reference current signal i ref,con  won&#39;t be affected by other signals to have distortion, and will keep a pure sinusoidal waveform in each mains period. 
 
         [0027]     The current feedback control circuit  15  has a current mode controller  151 , which is connected to the power stage circuit and the voltage feedback control circuit and used for getting a mains current signal I line  and the reference current signal i ref,con  and controlling switching of a power switch component of the power stage circuit to adjust the duty cycle of a gate control signal V g  of the power switch component. Through controlling the gate control signal V g  of the power switch component to switch the power switch component in a high frequency, the mains current signal I line  is forced to follow the waveform of the reference current signal i ref,con  so as to accomplish the object of controlling the mains current signal I line . Because the reference current signal i ref,con  is a pure sinusoidal signal having the same phase and frequency as the input mains voltage signal, the current feedback control circuit  15  will make the mains current signal I line  a pure sinusoidal signal having the same phase and frequency as the input mains voltage signal v line . Therefore, the power factor corrector control device of the present invention can easily achieve high power factor and low harmonic.  
         [0028]     The principle of the division approximate circuit  113  is shown in  FIG. 4 . Because the curve of the inverse 1/v rms  of the feed-forward signal between points V 1x (110) and 2V 1x (220V) is close to a dotted approximate line shown in  FIG. 4 , only a subtractor and a multiplier are required for accomplishing the function of division operation by means of straight-line approximation. In  FIG. 4 , V 1x  and 2V 1x  are the inverses of the root-mean-squared value v rms  of the mains voltage signal obtained by the RC circuit  111  when the mains voltage signal is at 110V and 220V, respectively, and V x  and V y  are the intersects of the approximate line designed based on V 1x , 2V 1x , V 1y , and 2V 1y  with the x- and y-axes, respectively. In straight-line approximation, the inverse 1/v rms  of the feed-forward signal can be expressed as K(C-v rms ), wherein C is a constant designed by the user or directly defined in IC design. Besides, K can also be incorporated into design of the operating point of the output error signal. V e /v rms  can thus have the same effect as v e ×K(C-v rms ), as shown in the following equation: 
 
 v   e   /v   rms   =v   e   ×K ( C−v   rms )=( Kv   e )×( C - v   rms )  (3) 
 
 wherein v e  is the output error signal, v rms  is the root-mean-squared value of the input mains voltage signal, and K and C are constants designed by the user. Kv e  is the output error signal of a newly designed operating point based on the K value. Therefore, the K and C values can be designed according to user&#39;s requirements. 
 
         [0029]     Please refer to  FIG. 5  as well as  FIG. 3 . The zero-cross detector  1311  will send out the zero-cross detection signal S 1311  or S 1311 ′ at each zero-cross point of the mains voltage signal v line  or v line ′. The frequency detection signal  1313  will send out the frequency detection signal S 1313  only when receiving the mains voltage signal of 60 Hz (or close to 60 Hz). The sine-wave generator  1315  starts to send out a pure sinusoidal signal i sin  or i sin ′ at the second half (the positive or negative half cycle of the mains voltage signal) of the mains period after finishing the determination of the frequency of the mains voltage signal. Moreover, a sample-and-hold activation signal S 1310  or S 1310 ′ of the SAH  133  is controlled by the zero-cross detection signal S 1311  or S 1311 ′. In the positive half-cycle of each mains period, the sample-and-hold activation signal S 1310  or s 1310 ′ is activated, and the sampled signal is held constant in each mains period. In  FIG. 5 , the SAH  133  samples the amplitude adjustment signal V k,con  or V k,con ′. The waveforms shown in  FIG. 5  only illustrate the actions of each important circuit and may be different from real waveforms.  
         [0030]     To sum up, the power factor corrector control device of the present invention has the following characteristics: 
        1. Influence of ripples of the output signal and feed-forward signal to the reference current signal can be eliminated to make the input current a pure sinusoidal signal having no harmonic.     2. The input current can still keep pure sinusoidal even the input mains supply contains harmonics. In other words, the input current contains no harmonics at any situation.     3. The processing circuit of the feed-forward signal is simplified, and a divider required by conventional feed-forward processing is replaced with a division approximate circuit to lower the IC fabrication cost and design complexity.     4. The self-generated sinusoidal signal of the sine-wave generating circuit can avoid distortion at zero-cross points.     5. The present invention applies to power factor corrector circuits of various different circuit architectures.        
 
         [0036]     Although the present invention has been described with reference to the preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and other will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.