AC/DC converter and method of correcting power factor

The current application claims a foreign priority to the patent application of Taiwan No. 101145615 filed on Dec. 5, 2012.

1. Technical Field

The present invention relates generally to a converter, and more particularly to an AC/DC converter and method of correcting power factor.

2. Description of Related Art

Typically, an AC/DC converter is used to convert an alternate current (AC) into a direct current (DC). FIG. 1 shows a conventional AC/DC converter, which includes a rectifier circuit 300 and an output capacitor C. The rectifier circuit 300 converts an alternate current (AC) of an AC power supply into a direct circuit (DC), and the output capacitor C bridges over the outputs of the rectifier circuit 10 and are connected to a loading 400 in parallel. While the AC/DC converter is working, the phases of the input voltage and the input current of the AC are different, which leads to low power factor and poor total harmonic distortion. In addition, the output capacitor C is charged only when the direct current has higher voltage than the output capacitor C, hence the charging time of the output capacitor C is shortened. As a result, the conduction time of diodes in the rectifier circuit 300 is also shortened, which increases the peak value of the conduction current, distorts the waveform of the input current, and lowers the power factor. Low power factor not only wastes energy, but also poses unnecessary burdens for the power supply systems of the power company. Therefore, the AC/DC converter with power factor correction circuit is emerged.

Typically, a conventional power factor correction circuit for AC/DC converters has two types, which are active type and passive type. Active power factor correction circuits control the input current with active switch components, and this type of power factor correction circuits has several advantages, such as the power factor could excess 0.99, and the total harmonic distortion could be less than 10%. And moreover, active power factor correction circuit is compatible with wider range of input voltage, generates stable output voltage, and it's unaffected by the variation of output power. However, active power factor correction circuit has several main drawbacks too, such as higher cost due to additional active switch components required, high electromagnetic noises, and low durability. The industry is eager to overcome these drawbacks. Besides, in order to reduce the ripple of the output voltage of the loading 400 , the aforementioned output capacitor C has to adopt a high capacity electrolytic capacitor, but the electrolytic capacitor is easy to leak out the contained electrolyte due to being heated for a long period of time, which shortens life of circuit.

In view of the above, the primary objective of the present invention is to provide an AC/DC converter and a method of correcting power factor, which effectively increases the power factor, and suppresses voltage ripples provided to a loading.

The present invention provides an AC/DC converter for converting an alternate current (AC) of a power supply into a direct current (DC), and the DC is transmitted to a loading, which includes a rectifier circuit and an active power factor correction circuit, wherein the rectifier circuit is electrically connected to the power supply to convert the AC into the DC, which has a positive output and a negative output for outputting the DC; the active power factor correction circuit electrically connects the rectifier circuit to the loading, which is used to suppress voltage ripples provided to the loading, and includes a first diode, a second diode, a third diode, a first inductor, a second inductor, a first capacitor, a switch, a fourth diode, a third inductor, a second capacitor, and a third capacitor, wherein the first diode has an anode and a cathode, wherein the cathode of the first diode is connected to the positive output of the rectifier circuit; the second diode has an anode and a cathode, wherein the anode of the second diode is connected to the negative output of the rectifier circuit; the third diode has an anode and a cathode, wherein the anode of the third diode is connected to the anode of the first diode; the first inductor has two ends, one of which is connected to both the cathode of the first diode and the positive output of the rectifier circuit; the second inductor has an end connected to the cathode of the second diode and an opposite end connected to the anode of the first diode and the anode of the third diode; the first capacitor has an end connected to the first inductor and an opposite end connected to the second inductor, the anode of the first diode and the anode of the third diode; the switch has an end connected to both the first inductor and the first capacitor and an opposite end connected to both the negative output of the rectifier circuit and the anode of the second diode; the fourth diode has an anode and a cathode, wherein the anode of the fourth diode is connected to both the cathode of the second diode and the second inductor, and the cathode thereof is connected to the cathode of the third diode; the third inductor has an end connected to both the cathode of the third diode and the cathode of the fourth diode; the second capacitor has an end connected to both the cathode of the third diode and the cathode of the fourth diode and an opposite end connected to the third inductor; the third capacitor has an end connected to both the third inductor and the second capacitor and an opposite end connected to the cathode of the second diode, the second inductor and the anode of the fourth diode, wherein the third capacitor is connected to the loading in parallel.

According to the aforementioned concepts, the present invention further provides a method of correcting power factor with an active power factor correction circuit, wherein the active power factor correction circuit includes a first diode, a second diode, a third diode, a first inductor, a second inductor, a first capacitor, a switch, a fourth diode, a third inductor, a second capacitor, and a third capacitor, wherein the first diode has an anode and a cathode, wherein the cathode of the first diode is connected to the positive output of the rectifier circuit; the second diode has an anode and a cathode, wherein the anode of the second diode is connected to the negative output of the rectifier circuit; the third diode has an anode and a cathode, wherein the anode of the third diode is connected to the anode of the first diode; the first inductor has two ends, one of which is connected to both the cathode of the first diode and the positive output of the rectifier circuit; the second inductor has an end connected to the cathode of the second diode and an opposite end connected to the anode of the first diode and the anode of the third diode; the first capacitor has an end connected to the first inductor and an opposite end connected to the second inductor, the anode of the first diode and the anode of the third diode; the switch has an end connected to both the first inductor and the first capacitor and an opposite end connected to both the negative output of the rectifier circuit and the anode of the second diode; the fourth diode has an anode and a cathode, wherein the anode of the fourth diode is connected to both the cathode of the second diode and the second inductor, and the cathode thereof is connected to the cathode of the third diode; the third inductor has an end connected to both the cathode of the third diode and the cathode of the fourth diode; the second capacitor has an end connected to both the cathode of the third diode and the cathode of the fourth diode and an opposite end connected to the third inductor; the third capacitor has an end connected to both the third inductor and the second capacitor and an opposite end connected to the cathode of the second diode, the second inductor and the anode of the fourth diode, wherein the third capacitor is connected to the loading in parallel; the method includes the steps of:

A. Conduct the switch to let the DC power supply charge the first inductor with a DC, the first capacitor charge the second inductor to make the second inductor have a stored energy, and the third inductor, the second inductor and the third inductor provide energy to the loading;

B. Cut off the switch and conduct the first diode to let the first inductor provide energy to the first capacitor, and keep transmitting the stored energy of the second inductor to the third capacitor;

C. Conduct the fourth diode to let the second capacitor and the third inductor provide energy to the third capacitor to provide energy into the loading;

D. Keep providing energy from the second capacitor and the third inductor to the third capacitor to provide energy into the loading as the first diode is cut off after the first inductor finishing providing energy to the first capacitor.

With such design, it may effectively increase the power factor of the AC/DC converter, and suppress voltage ripples provided to the loading.

The present invention will be best understood by referring to the following detailed description of some illustrative embodiments in conjunction with the accompanying drawings, in which

FIG. 1 is a circuit diagram of the conventional AC/DC converter;

FIG. 2 is a circuit diagram of a preferred embodiment of the present invention;

FIG. 3 to FIG. 6 are the circuit diagrams of the preferred embodiment of the present invention, showing how the AC/DC converter works in different steps;

FIG. 7 is an oscillogram of the output voltage and the output current.

As shown in FIG. 2 , an AC/DC converter of the preferred embodiment of the present invention is used to convert an alternate current (AC) provided by a power supply 100 into a direct current (DC), and send the DC to a loading 200 . The AC/DC converter includes a rectifier circuit 10 and an active power factor correction circuit 20 .

The rectifier circuit 10 is electrically connected to the power supply 100 , to convert the AC of the power supply 100 into the DC, wherein the rectifier circuit 10 has a positive output 12 and a negative output 14 for sending out the DC.

The active power factor correction circuit 20 electrically connects the rectifier circuit 10 to the loading 200 to improve the power factor and suppress voltage ripples provided to the loading 200 . The active power factor correction circuit 20 include five diodes (a first diode D 1 , a second diode D 2 , to a fifth diode D 5 ), three inductors (a first inductor L 1 , a second inductor L 2 , and a third inductor L 3 ), three capacitors (a first capacitor C 1 , a second capacitor C 2 , and a third capacitor C 3 ), and a switch SW. A cathode of the first D 1 is connected to the positive output 12 of the rectifier circuit 10 . An anode of the second diode D 2 is connected to the negative output 14 of the rectifier circuit 10 . An anode of the third diode D 3 is connected to an anode of the first diode D 1 . An end of the first inductor L 1 is connected to the cathode of the first diode D 1 and the positive output 12 of the rectifier circuit 10 . An end of second inductor L 2 is connected to a cathode of the second diode D 2 , and the other end thereof is connected to the anode of the first diode D 1 and the anode of the third diode D 3 . The first capacitor C 1 is a non-polarity capacitor, and an end thereof is connected to the first inductor L 1 , and the other end thereof is connected to the second inductor L 2 , the anode of the first diode D 1 , and the anode of the third diode D 3 . An end of the switch SW is connected to the first inductor L 1 and the first capacitor C 1 , and another end thereof is connected to the negative output 14 of the rectifier circuit 10 and the anode of the second diode D 2 . An anode of the fourth diode D 4 is connected to the cathode of the second diode D 2 and the second inductor L 2 , and a cathode thereof is connected to the cathode of the third diode D 3 . An anode of the fifth diode D 5 is connected to the cathode of the third diode D 3 and a cathode of the fourth diode D 4 . An end of the third inductor L 3 is electrically connected to the cathode of the fourth diode D 4 . The second capacitor C 2 is a non-polarity capacitor, and an end thereof is connected to the cathode of the third diode D 3 and the cathode of the fourth diode D 4 , and the other end thereof is connected to the third inductor L 3 . The third capacitor C 3 is a non-polarity capacitor, and is connected to the loading 200 in parallel. An end of the third capacitor C 3 is connected to the third inductor L 3 and the second capacitor C 2 , and the other end thereof is connected to the cathode of the second diode D 2 , the second inductor L 2 , and the anode of the fourth diode D 4 .

Table 1 shows the specs of the capacitors C 1 -C 3 , the inductors L 1 -L 3 , an input voltage V in provide by the power supply 100 , an output voltage V out flows through the loading 200 , a switching frequency of the switch SW, and a resistance of the loading 200 :

With the aforementioned design and specifications, the active power factor correction circuit 20 may use a method to correct power factor after receiving the DC sent from the rectifier circuit 10 , which effectively increases the power factor and suppress voltage ripples provided to the loading 200 , wherein the method includes the following steps:

A. Conduct the switch SW to let the rectifier circuit 10 charge the first inductor L 1 with the DC, the first capacitor C 1 charge the second inductor L 2 to make the second inductor L 2 have a stored energy, and the third inductor L 3 , the second inductor L 2 and the third inductor L 3 provide energy to the loading 200 , as shown in FIG. 3 .

B. Cut off the switch SW and conduct the first diode D 1 to let the first inductor L 1 provide energy to the first capacitor C 1 , and keep transmitting the stored energy of the second inductor L 2 to the third capacitor C 3 through a resonant circuit formed by the second capacitor C 2 and the third inductor L 3 , as shown in FIG. 4 . As a result, the third capacitor C 3 provides energy into the loading 200 , and a voltage drop of the second capacitor C 2 gets reversed.

C. Please refer to FIG. 5 , when a voltage drop of the third inductor L 3 is higher than a voltage drop of the third capacitor C 3 , the fourth diode D 4 is conducted to let voltage drops of the second capacitor C 2 , the third inductor L 3 , and the third capacitor C 3 equal, and let the second capacitor C 2 and the third inductor L 3 provide energy to the third capacitor C 3 to provide energy into the loading 200 . In addition, the first inductor L 1 keeps providing energy to the first capacitor C 1 .

D. Keep providing energy from the second capacitor C 2 and the third inductor L 3 to the third capacitor C 3 to provide energy into the loading 200 as the first diode D 1 cuts off after the first inductor L 1 finishing providing energy to the first capacitor C 1 , as shown in FIG. 6 .

Every time the method executes from the step A to the step D, it is defined as one cycle. Therefore, while the AC/DC converter is working, the step A to the step D are proceeded repeatedly until the AC/DC converter stops working.

Please refer to FIG. 7 , the high-frequency ripple is merely about 0.9V when the output voltage is 12V in the present preferred embodiment. In other words, with the aforementioned design of structures and methods, the power factor of the conventional circuit could be increased, and furthermore, the output voltage ripple caused by every increased input energy (which is, when the switch SW is conducted) could be suppressed, due to the third inductor L 3 has a circuit capable of storing energy and filtering at the same time. Besides, by switching the active switch SW, and with the resonant between the third inductor L 3 and the second capacitor C 2 , the stored energy in voltage of the second capacitor C 2 could be converted into inductive current, and the polarity of the voltage drop of the second capacitor C 2 could be reversed, which changes the whole circuit structure of the fourth diode D 2 after it being conducted. In this way, the output voltage ripple caused by stopping providing energy (which is, when the switch SW cuts off) could be suppressed.

Furthermore, the fifth diode D 5 could effectively prevent the circuit around the loading 200 from having a back flow, which stables the whole circuit to increase the power factor of the AC/DC converter and suppress the ripples. Of course, the ripples could be suppressed in practice even without the fifth diode D 5 .

It must be pointed out that the embodiments described above are only some preferred embodiments of the present invention. All equivalent structures and methods which employ the concepts disclosed in this specification and the appended claims should fall within the scope of the present invention.