Abstract:
A power factor control circuit for an AC to DC power converter includes an inductor receiving AC rectified power. The charging time of the inductor is controlled by a switching circuit based on a comparison between a DC bus voltage and a fixed reference voltage. The circuit operates without an AC rectified line sensing network, and without a current-sensing resistor connected to the source of the MOSFET switch.

Description:
This application claims the benefit of U.S. Provisional Application Serial No. 60/142,949 filed Jul. 12, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power factor correction for AC to DC power converters, and more specifically, to AC to DC power converters having power factor correction circuitry utilizing a minimal component count and minimal IC pin count without loss of performance. 
     2. Brief Description of the Related Art 
     In most AC to DC power converters, it is convenient to have the circuit act as a pure resistor to the AC input line voltage. To achieve this, active power factor correction (PFC) can be implemented which, for an AC input line voltage, produces an AC input line current. 
     It also is important to produce a sinusoidal input current which has a low total harmonic distortion (THD). THD and power factor (PF) represent performance measurements of how well the PFC circuit works. A power factor (PF) of 1.0 represents the highest achievable, and a THD lower than about 15% is acceptable in practice. 
     A typical solution for providing active power factor correction is shown in circuit  2  of FIG.  1 . Circuit  2  has a boost-type converter topology and a PFC IC  4  such as the Motorola 34262. The resulting circuit requires a voltage divider network (resistors  6  and  8  and capacitor  10 ) for sensing the AC rectified line input. Additionally, a secondary winding on the boost inductor  12  detects the zero-crossing of the inductor current. Also, a current sensing resistor  14  in the source of the boost switch  16  shapes the peak inductor current and detects an over-current condition. A voltage-divider network (resistors  18  and  20 ) senses and regulates a constant DC bus voltage and detects an over-voltage condition due to load transients. A compensation capacitor  22  is required for a stable loop response. 
     Accordingly, the need exists in the prior art for implementation of a simpler active power factor correction (PFC) circuit having fewer components. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the deficiencies of the prior art by providing a new control method that results in a minimal component count, minimal IC pin count, and the same performance as standard PFC ICs available on the market. 
     The power factor control circuit of the present invention includes an inductor for receiving AC rectified power and a switch for charging/discharging the inductor. A switching circuit connected to the inductor controls the on-time of the switch, and thereby the charging time of the inductor, by comparing a DC bus voltage to a fixed reference voltage. The switching circuit also controls the off-time of the switch, and thereby the discharging time of the inductor, by turning the switch off until the inductor current discharges to zero, as detected by the switching circuit, such that the off-time of the switch varies as a function of the peak inductor current during each switching cycle. Preferably the switch is a MOSFET, and the inductor includes a secondary winding which is used by the switching circuit to determine the inductor current. 
     Advantageously, the MOSFET operates without a current-sensing resistor connected in series with the source of the MOSFET. Further, the on-time of the switch is modulated as a function of the off-time of the switch to achieve lower total harmonic distortion. In addition, the current in the inductor follows the sinusoidal voltage of the AC rectified power as the switching circuit is turned on and off at a much higher frequency than the line frequency of the AC rectified power, thereby eliminating the need to sense the rectified AC line input voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a prior art power factor control circuit of an AC to DC power converter. 
     FIG. 2 is a circuit diagram showing a power factor control circuit according to the present invention. 
     FIG. 3 is a circuit diagram showing an AC to DC power converter incorporating a power factor control circuit according to the present invention. 
     FIG. 4 is a timing diagram for the circuit of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 2, the power factor correction circuit  30  of the present invention is shown. Circuit  30  includes IC  32 . A secondary winding on the boost inductor  34  detects the zero-crossing of the inductor current. Unlike the prior art circuit shown in FIG. 1, in the circuit of the present invention, no current sensing resistor is required in series with the source of MOSFET  36 . A voltage-divider network (resistors  38  and  40 ) senses and regulates a constant DC bus voltage and detects an over-voltage condition due to load transients. A compensation capacitor  22  provides a stable loop response. 
     The invention will be described in further detail with reference to FIG. 3, which shows the circuitry within IC  32 , wherein like elements are identified by like reference numerals. The corresponding timing diagram for the invention is shown in FIG.  4 . The circuit of the present invention is classified as running in critical continuous mode, in which the inductor current discharges to zero during each switching cycle. The functionality of the circuit relies on the fact that there is no need to sense the rectified AC line input voltage because it is already sinusoidal. Therefore, the current in inductor  34  will naturally follow the sinusoidal voltage envelope as the boost MOSFET  36  is turned on and off at a much higher frequency (&gt;10 kHZ) than the input line frequency (˜50-60 Hz). 
     The circuit of the present invention compares the DC bus voltage to a fixed reference voltage (Vref) to determine the charging time of the boost inductor  34  (or on-time of the boost switch  36 ). The circuit then turns off the boost switch  36  until the inductor current discharges to zero, as detected by the secondary winding  35  on the boost inductor  34 . 
     The on-time is controlled by the DC bus and the off-time changes as a function of how high the peak inductor charges each switching cycle. The result is a system where the switching frequency is free-running and constantly changing from a higher frequency near the zero-crossings of AC input line voltage, to a lower frequency at the peaks. 
     A further improvement to the circuit, to achieve a low total harmonic distortion (THD), involves dynamically modulating the on-time as a function of the off-time. All of these functions are described in more detail in the following text. 
     When the circuit is first enabled (ENABLE signal goes logic “high”) the Q output of latch  58  is low, both inputs of the AND gate  60  are high, and the boost MOSFET  36  is turned on. The boost inductor  37  is shorted to ground and begins charging (see Timing Diagram, FIG.  4 ). 
     The inductor current charges up until the sawtooth voltage (VSAW), resulting from capacitor  62  being charged by the current mirror comprised of transistors  64  and  66 , reaches the output voltage (VDC′) from the DC bus feedback circuitry. Once this occurs, the set input S of latch  58  goes high causing the Q output to go “high” and the boost MOSFET  54  to turn off. The Q output of latch  58  also discharges capacitor  62  through OR gate  68  and MOSFET  70 , and the Q output of latch  58  forces the reset input R of latch  72  “low”, therefore freeing latch  72 . 
     When the boost MOSFET  36  turns off, the secondary winding output  35  of the boost inductor  34  goes “high,” causing the output of comparator  74  to go “high,” as well as the S input of latch  72 . During this “off” time, the inductor current discharges into the DC bus capacitor  76  through diode  78  and the modulation capacitor  80  charges up through current source  82 . 
     When the boost inductor current discharges to zero, secondary winding output  56  goes “low”, causing the output of NOR gate  84  to go “high,” and therefore the reset input R of latch  58  goes “high” and the boost MOSFET  36  turns on again, and the boost inductor  37  charges again. The transition of secondary winding output  35  to “low” also turns MOSFET  86  off, therefore turning the current source  82  off as well. 
     The voltage on capacitor  80  then remains constant for the duration of the on time. This voltage is converted to a current through OPAMP  88 , transistor  90 , and variable resistor  92 , and defines the charging current for capacitor  62 . As the off-time varies for each switching cycle, so does the voltage on capacitor  80 , and therefore the rate at which capacitor  62  charges. By adjusting the modulation gain with resistor  92 , the amount of modulation of the on-time as a function of the off-time can be controlled. The longer the off-time, the higher capacitor  80  charges, the higher the current charging capacitor  62 , the faster capacitor  62  reaches the VDC threshold, and the shorter the on-time of boost MOSFET  54 . 
     Inversely, the shorter the off-time, the longer the on-time. This modulation effect changes dynamically over each cycle of the low-frequency AC line input voltage, with the on-time being slightly longer at the zero-crossings than at the peaks. Compared to a fixed on-time over the entire cycle, the modulated solution results in a “flatter” envelope with less cross-over distortion in the line current which gives lower total harmonic distortion (THD). 
     The voltage on capacitor  80  is discharged to zero at the beginning of each offtime with a pulse generator (PGEN 1 )  94  and MOSFET  96 . OPAMP  98  and biasing resistors  100  and  102  and capacitor  104  determine the gain and speed of the feedback loop for the DC bus regulation. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention is to be limited not by the specific disclosure herein, but only by the appended claims.