A typical switching power supply is shown in FIG. 1. The supply comprises an AC/DC rectifier 1 and a DC/DC converter 2 in which an electrolytic capacitor C3 is connected as a filter for the bridge rectifier BD1.
FIG. 5 discloses a circuit structure in which the DC/DC converter 2 shown in FIG. 1 is a half-bridge converter. In accordance with the design structure shown in FIG. 5, a bridge rectifier BD1 is used to rectify the AC power VS1. A capacitor C3 is then used to filter the rectified power and generates a DC voltage VC3. The capacitor C1 and the capacitor C2 are connected to a common node to form a voltage divider. Therefore, the voltage at the common node between the two capacitors is VC3/2.
FIG. 6 is a time chart for the PWM signals, VHG and VLG, which are driving signals for the switches Q1 and Q2 shown in FIG. 5, respectively. Both PWM signals VHG and VLG are low (low voltage) for 0≦t≦t1, at which time the switches Q1 and Q2 are both turned off. Therefore, the output voltage VO is supplied by the capacitor C4.
When t1≦t≦t2, the PWM signal VLG is high (high voltage) and the PWM signal VHG is low (low voltage), at which time the switch Q1 is turned off and the switch Q2 is turned on. The current then flows through the capacitor C1, the primary winding P1 and the switch Q2 to the ground. In this situation, the transformer transfers the power from the primary winding P1 to the secondary winding S1 to supply power to the capacitor C4 and output a voltage VO.
When t2≦t≦t3, the switches Q1 and Q2 are once again both turned off, making the operation state of the circuit the same as that when 0≦t≦t1.
When t3≦t≦t4, the PWM signal VLG is low (low voltage) and the PWM signal VHG is high (high voltage), at which time the switch Q1 is turned on and the switch Q2 is turned off. The current then flows through the switch Q1, the primary winding P1 and the capacitor C2 to the ground. The transformer transfers the power from the primary winding P1 to the secondary winding S2 to supply power to the capacitor C4 and output a voltage VO.
The power switching cycle described above is performed repeatedly to supply power to a load while the output voltage VO is transferred to a feedback system 12. The feedback system 12 feeds a signal back to the high-frequency pulse signal control circuit 50 to modify the duty cycle of the PWM signals VHG and VLG. For example, if the power supplied to the load is insufficient when the output voltage is lower than a required value, the feedback signal lengthens the duty cycle of the PWM signals VHG and VLG to increase the conduction time of the switches Q1 and Q2. In effect, power is transferred from the primary winding to the secondary winding of the transformer T1 for a longer period of time; that is, the power supplied to the secondary winding is increased. The output voltage VO consequently increases to attain the required voltage. This means, however, that the power supplied to the load is overdriven when the output voltage is higher than the required value. In this situation, the duty cycle of the PWM signals VHG and VLG should be reduced.
Note that the input current Ipc in FIG. 5 is a pulse current as shown in the graph of FIG. 2. The power factor of the conventional switching power supply is significantly decreased (typically by about 50%) due to the distorted input current, causing the total harmonics distortion (hereinafter referred to as THD) to exceed 100% after the rectification performed by the AC/DC rectifier 1 shown in FIG. 1. As a result, the total harmonics is seriously distorted, the quality is poor, and worse yet, precious energy is wasted.
Thus, many countries have promulgated a number of harmonic current rules (e.g., EN-6100-3-2) which specify the current wave shape for power supplies that must be obeyed by manufacturers in order to improve the efficiency and quality of the power source being supplied.
As such, various designs of power factor correction circuits have been proposed by researchers in order to improve the power factor of the conventional switching power supply. Two examples of typical prior art are described in the following:
1. Inductor-Type Power Factor Correction Circuit
As shown in FIG. 3, the prior art discloses a design in which a low frequency large winding L1 is in series between a bridge rectifier BD1 and an electrolytic capacitor C1. The winding L1 and the capacitor C1 form a low pass filter to rectify the input current of a DC/DC converter 2. Such a design is similar in function to the ballast for correcting the power factor of a fluorescent lamp. However, the winding L1 is relatively large, has only a limited power factor improvement, and creates an abnormally high temperature during operation.
2. Active-Type Power Factor Correction Circuit
As shown in FIG. 4, the prior art discloses a design in which the AC/DC rectifier is redesigned to form a two-stage circuit with the DC/DC converter 2. Further, a complex control circuit 11 and a large switch element Q1 are added therein to improve the power factor. However, it is relatively complex in circuit design and is expensive to manufacture.
Many power factor correction circuits have been developed based on the basic concepts involved in the two examples of prior art mentioned above, all with similar drawbacks.