Abstract:
A power-factor-improving circuit and method for an offline converter block the DC component and obtain the AC component of an input voltage of the offline converter. The AC component is superpositioned onto a DC bias signal to generate a dimming signal for the offline converter to adjust an output current of the offline converter. The offline converter has a high power factor due to the dimming signal with the AC component of the input voltage. In addition, the average of the dimming signal is determined by the DC bias signal, hence the output current can be precisely controlled according to the DC bias signal.

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to a power-factor-improving circuit and method, especially for an offline converter. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  is a diagram depicting a power supply  10  driving light emitting diodes (LEDs). The power supply  10  includes a bridge rectifier  14  for rectifying an AC voltage VAC and an offline converter  16  for providing an output current Io to an LED  12 . Conventionally, the offline converter  16  adjusts the output current Io according to a dimming signal Sdim. As shown in  FIG. 2 , the output current Io is proportional to the voltage level of the dimming signal Sdim. The average of the output current Io determines the brightness of the LED  12 , and thus the brightness of the LED  12  can be adjusted by changing the dimming signal Sdim. 
     However, as shown by waveforms  20  and  22  in  FIG. 4 , the input voltage VIN to the offline converter  16  has a periodic waveform, and thus the AC information of the input voltage VIN is necessary for the offline converter  16  to have a high power factor. As shown in  FIG. 3 , a conventional power-factor-improving method uses serially connected resistors R 1  and R 2  to divide the input voltage VIN for generating the dimming signal Sdim for the offline converter  16 , which thus contains the AC component of the input voltage VIN, to improve the power factor of the offline converter  16 . This approach, however, causes the average of the dimming signal Sdim to vary with the DC component of the input voltage VIN, so the average of the output current Io cannot be accurately controlled under the influence of the DC component of the input voltage VIN.  FIG. 4  shows waveforms of the dimming signal Sdim under different input voltages VIN. When the input voltage VIN is higher, as shown by waveform  20 , the input voltage VIN has a greater DC component VHavg, so the obtained dimming signal Sdim has a greater average Vavg 1 , as shown by waveform  24 , thereby making the offline converter  16  provide a greater output current Io. When the input voltage VIN is lower, as shown by waveform  22 , the DC component VLavg of the input voltage VIN is smaller, so the obtained dimming signal Sdim has a smaller average Vavg 2 , as shown by waveform  26 , so the offline converter  16  provides a smaller output current Io. 
     Therefore, it is desired a circuit and method for improving the power factor and accurately controlling the output current of an offline converter. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a power-factor-improving circuit and method for an offline converter. 
     Another objective of the present invention is to provide a high power factor offline converter for accurately controlling the output current thereof. 
     According to the present invention, a power-factor-improving circuit for an offline converter includes a voltage detector and a DC bias circuit. The voltage detector blocks the DC component of the input voltage of the offline converter and detects the AC component of the input voltage to obtain a detection signal which is proportional to the AC component. The DC bias circuit provides a DC bias signal to superpose on the detection signal to generate a dimming signal for the offline converter to adjust the output current of the offline converter. 
     According to the present invention, a power-factor-improving method for an offline converter includes blocking the DC component of an input voltage of the offline converter to obtain a detection signal which is proportional to the AC component of the input voltage, superposing the detection signal onto a DC bias signal to generate a dimming signal, and determining the output current of the offline converter according to the dimming signal. 
     According to the present invention, an offline converter includes a controller that determines the output current of the offline converter according to a dimming signal, and a power-factor-improving circuit that provides the dimming signal. The power-factor-improving circuit includes a voltage detector and a DC bias circuit. The voltage detector blocks the DC component of the input voltage of the offline converter and obtains a detection signal which is proportional to the AC component of the input voltage. The DC bias circuit provides a DC bias signal to superpose onto the detection signal to generate the dimming signal. 
     Since the dimming signal provided by the power-factor-improving circuit contains the AC component of the input voltage of the offline converter, it makes the offline converter achieve a high power factor. In addition, since the average of the dimming signal is determined by the DC bias signal, the average of the output current of the offline converter can be accurately controlled through controlling the DC bias signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a conventional LED driver; 
         FIG. 2  shows waveforms of the dimming signal and the output current of  FIG. 1 ; 
         FIG. 3  shows a conventional method for improving the power factor of an offline converter; 
         FIG. 4  shows waveforms of the input voltage and the dimming signal of  FIG. 3 ; 
         FIG. 5  shows a first embodiment of a power-factor-improving circuit according to the present invention; 
         FIG. 6  shows waveforms of the input voltage and the dimming signal of  FIG. 5 ; 
         FIG. 7  shows a second embodiment of the power-factor-improving circuit according to the present invention; 
         FIG. 8  is an embodiment of the controller of  FIG. 7 ; 
         FIG. 9  shows a waveform of the dimming signal of  FIG. 8 ; and 
         FIG. 10  shows an embodiment of a power-factor-improving circuit applied in an offline isolation converter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 5  shows a power supply  10  including a bridge rectifier  14  and an offline converter  16  as shown in  FIG. 1 . However, this power supply  10  further includes a power-factor-improving circuit  30  for providing the dimming signal Sdim for the offline converter  16  according to the input voltage VIN of the offline converter  16 . The power-factor-improving circuit  30  includes a voltage detector  32  and a DC bias circuit  34 . The voltage detector  32  includes resistors R 1  and R 2  connected in series between the voltage input terminal VIN and ground GND, and a decoupling capacitor C 1  connected in series to the resistor R 1  between the voltage input terminal VIN and the resistor R 2 . The decoupling capacitor C 1  is so configured to block the DC component of the input voltage VIN. The resistors R 1  and R 2  divide the AC component of the input voltage VIN to generate a detection signal Sac which is superposed onto a DC bias signal Vbias′ provided by the DC bias circuit  34  to generate the dimming signal Sdim. The DC bias circuit  34  includes a voltage source  35  to provide the DC bias signal Vbias′. Since the DC component of the input voltage VIN is blocked by the decoupling capacitor C 1 , the DC level of the detection signal Sac is zero. Therefore, after the detection signal Sac is superposed onto the DC bias signal Vbias′ to generate the dimming signal Sdim, the dimming signal Sdim has a DC level (i.e. an average) equal to the DC bias signal Vbias′. In other words, the DC bias signal Vbias′ determines the average of the output current Io of the offline converter  16 , so it is feasible to accurately control the average of the output current. To by controlling the DC bias signal Vbias′. Additionally, since the AC component of the dimming signal Sdim is determined by the detection signal Sac, and the detection signal Sac is proportional to the AC component of the input voltage VIN, the dimming signal Sdim contains the AC component of the input voltage VIN, so it enables the offline converter  16  to achieve a high power factor. In this embodiment, the decoupling capacitor C 1  is used to block the DC component of the input voltage VIN. In other embodiments, other devices or circuits capable of blocking the DC component of the input voltage VIN may be used instead of the decoupling capacitor C 1 . 
     For readers&#39; easy understanding,  FIG. 6  shows a waveform of the dimming signal Sdim of  FIG. 5  under different input voltages VIN. When the input voltage VIN is higher, as shown by waveform  20  in  FIG. 6 , after the power-factor-improving circuit  30  blocks the DC component VHavg of the input voltage VIN, the AC component of the input voltage VIN is divided and superposed onto the DC bias signal Vbias′ to generate the dimming signal Sdim as shown by waveform  36  in  FIG. 6 . When the input voltage VIN is lower, as shown by waveform  22  in  FIG. 6 , after the power-factor-improving circuit  30  blocks the DC component VLavg of the input voltage VIN, the AC component of the input voltage VIN is divided and superposed onto the DC bias signal Vbias′ to generate the dimming signal Sdim as shown by waveform  38  in  FIG. 6 . As can be learned from the waveforms  36  and  38  in  FIG. 6 , whether the input voltage VIN is high or low, the DC level of the dimming signal Sdim is equal to the DC bias signal Vbias′, and the AC component of the dimming signal Sdim is proportional to the AC component of the input voltage VIN. 
     The offline converter  16  of  FIG. 5  may be an offline converter of one of various types, including boost, buck, non-isolation and isolation ones, and the power-factor-improving circuit  30  may be integrated into the offline converter  16 . 
       FIG. 7  shows a second embodiment of the power-factor-improving circuit  30  according to the present invention, in which the offline converter  16  is an offline non-isolation buck converter, and the power-factor-improving circuit  30  is integrated into the offline converter  16 . In the offline converter  16  of  FIG. 7 , the voltage input terminal  40  of the offline converter  16  receives the input voltage VIN, a power switch Q 1  and a diode D 1  are connected in series between the voltage input terminal  40  and ground terminal GND, an inductor L 1  is connected between the power switch Q 1  and the output terminal  46  of the offline converter  16 , a current sensor  44  including a resistor Rs is connected in series to the inductor L 1  for sensing the current IL of the inductor L 1  to generate a current sense signal Vcs, a power-factor-improving circuit  30  detects the input voltage VIN at the voltage input terminal  40  of the offline converter  16  to generate the dimming signal Sdim, and a controller  42  determines a control signal Vg according to the dimming signal Sdim and the current sense signal Vcs for switching the power switch Q 1 . 
     The power-factor-improving circuit  30  of  FIG. 7  includes a voltage detector  32 , a DC bias circuit  34 , a resistor R 5 , and capacitors C 2  and C 3 . The voltage detector  32  includes a decoupling capacitor C 1  for blocking the DC component of the input voltage VIN and resistors R 1  and R 2  for dividing the AC component of the input voltage VIN to generate the detection signal Sac. The DC bias circuit  34  includes resistors R 3  and R 4  and a Zener diode ZD. The resistors R 3  and R 4  are used as a current limit resistor for limiting the current of the Zener diode ZD. The Zener diode ZD has its cathode connected to the output terminal  48  of the DC bias circuit  34 , and also connected to the voltage input terminal  40  of the offline converter  16  through the resistors R 3  and R 4 . When the input voltage VIN is higher than the breakdown voltage of the Zener diode ZD, the Zener diode ZD becomes reverse biased, and the voltage at its cathode remains constant, so the DC bias circuit  34  will provide a stable DC bias signal Vbias′. The resistors R 5  and R 2  establish a voltage divider to divide the DC bias signal Vbias′ for generating the DC bias signal Vbias to superpose onto the detection signal Sac to generate the dimming signal Sdim. The capacitors C 2  and C 3  are used to filter off unexpected surges. 
       FIG. 8  is an embodiment of the controller  42  of  FIG. 7 , which includes a driver  50 , a flip-flop  52 , a comparator  54 , and an amplifier  56 . The amplifier  56  linearly amplifies the dimming signal Sdim to generate a dimming signal Sdim_a. The comparator  54  compares the dimming signal Sdim_a to the current sense signal Vcs to generate a comparison signal Sr. The flip-flop  52  has a set terminal S to receive a clock signal Clk and a reset terminal R to receive the comparison signal Sr, and thus generates a signal Q for the driver  50  to generate the control signal Vg. In other embodiments, the amplifier  56  may be omitted or replaced by another device, such as a buffer. 
     In the embodiment of  FIG. 8 , the amplifier  56  has a gain of 1/K and its power-source terminals receive voltages VCC and VSS, respectively. When the amplified dimming signal Sdim_a has its maximum greater than the voltage VCC or has its minimum lower than the voltage VSS, misoperation can happen. Thus, the waveform of the dimming signal Sdim_a has to stay between the voltages VCC and VSS. In other words, the dimming signal Sdim must remain within the range between an upper limit Vmax=VCC×K and a lower limit Vmin=VSS×K, as shown in  FIG. 9 . In  FIG. 9 , there are three areas between the dimming signal Sdim and the DC bias signal Vbias, namely, the areas A 1  and A 2  where the DC bias signal Vbias is higher than the dimming signal Sdim, and the area A 3  where the DC bias signal Vbias is lower than the dimming signal Sdim. Since the average of the dimming signal Sdim is equal to the DC bias signal Vbias, A 3 =A 1 +A 2 . 
     The power-factor-improving circuit  30  according to the present invention may be useful to an offline isolation converter. In  FIG. 10 , the offline converter  16  is an offline isolation converter, which has a power-factor-improving circuit  30  for detecting the input voltage VIN to generate a dimming signal Sdim. The power-factor-improving circuit  30  of  FIG. 10 , similar to the circuit of  FIG. 7 , has a voltage detector  32 , a DC bias circuit  34 , a resistor R 5 , and capacitors C 2  and C 3 . The offline converter  16  has a transformer  50  including a primary coil L 1 , a secondary coil L 2 , and an auxiliary coil L 3 . The primary coil L 1  and the power switch Q 1  are connected in series between the voltage input terminal  40  and ground terminal GND. The secondary coil L 2  is connected to the output terminal  46 . The auxiliary coil L 3  is used to sense the voltage Vo of the secondary coil L 2  to generate a voltage VL 3 . Serially connected resistors R 6  and R 7  are connected to the auxiliary coil L 3 , for dividing the voltage VL 3  to generate a voltage Vfb for the controller  42 , enabling the feedback network path to achieve constant current and constant voltage. The current sensor  44  includes a resistor Rs connected in series to the power switch Q 1 , for sensing the current Iq 1  of the power switch Q 1  to generate the current sense signal Vcs. The controller  42  determines the control signal Vg according to the dimming signal Sdim and the current sense signal Vcs for switching the power switch Q 1  and in turn controlling the output current Io at the output terminal  46 . The controller  42  of  FIG. 10  is conceptually similar to the circuit of  FIG. 8 , and people skilled in the art would derive the circuit of controller  42  from the circuit of  FIG. 8 . 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.