Active power factor correction circuit

1. Field of the Invention

The present invention relates to a power factor correction circuit (hereinafter referred to as a PFC circuit), more particularly to an active power factor correction circuit.

2. Description of the Related Art

Generally, there are two types of PFC circuits, active and passive. However, passive PFC circuits have been eliminated from the competition gradually, due to the large layout area of the passive PFC circuit and a power factor below 0.95, with the harmonic waves thereof being large. Therefore, passive PFC circuits have been replaced by active PFC circuits. As shown in FIG. 1 a , a conventional active PFC circuit 10 comprises a bridge rectifier that consists of four diodes D 1 , D 2 , D 3 and D 4 , and a voltage source Vs, an inductor L 1 , a diode D 5 , a capacitor C 1 and a switching device S 1 . By switching the switching device in high frequency appropriately, the active PFC circuit attains a high power factor.

The operation of the conventional active PFC circuit, as shown in FIG. 1 a , is described as follows. When the voltage at node A 1 is positive, the voltage at node B 1 is negative, and the switching device S 1 is off, the main current I flows to the capacitor C 1 (or load) through the diode D 1 , the inductor L 1 , and diode D 5 , and then flows back to the voltage source Vs through the diode D 4 , as shown in FIG. 1 b . When the switching device S 1 is on, the main current I flows from node A 1 to node B 1 though diode D 1 , the inductor L 1 , the switching device S 1 , and the diode D 4 , as shown in FIG. 1 c . A power factor of more than 0.99 can be achieved by the above method.

Although the power factor can be increased to more than 0.99 by switching the switching device in high frequency, the switching device inevitably generates power dissipation (or loss) when it turns on and turns off, thus degrading the efficiency of the PFC circuit and wasting energy. Also, the temperature raised due to the operation of the switching device will damage the elements in the PFC circuit.

Consequently, a PFC circuit with a snubber circuit is disclosed to reduce the power loss due to the operation of the switching device, as shown in FIG. 2 a . The PFC circuit 20 comprises a bridge rectifier coupled to a voltage source Vs, an inductor L 2 , a diode D 26 , a capacitor C 2 , a switching device S 2 , and a snubber circuit 210 connected in parallel with diode D 26 and the switching device S 2 . As shown in FIG. 2 a , the bridge rectifier consists of four diodes D 21 , D 22 , D 23 and D 24 . The snubber circuit 210 consists of an inductor Lr 2 , a diode D 28 , and a switching device Sa 2 connected in series, two diodes D 25 and D 27 , and a capacitor Cr 2 . The switching device S 2 can carry out zero-voltage switching operation to avoid power dissipation, in conjunction with the snubber circuit 210 .

The operation of the conventional active PFC circuit 20 , as shown in FIG. 2 a , is described as follows. When the voltage at node A 2 is positive and the voltage at node B 2 is negative, and the switching device S 2 is off, the main current I 2 flows to the capacitor C 2 (or load) through the diode D 21 , the inductor L 2 and diode D 26 , and then flows back to the voltage source Vs though the diode D 24 , as shown in FIG. 2 b.

Referring to FIG. 2 c , when both switching devices S 2 and Sa 2 are off, the main current I 2 flows as described in FIG. 2 b . The current Io equals the main current I 2 , therefore the current Ir is zero. Before the switching device S 2 turns on, the switching device Sa 2 must turn on first. When the switching device Sa 2 turns on, a voltage across the inductor Lr 2 equals the voltage on the capacitor C 2 . Consequently, the current on the inductor Lr 2 increases from zero slowly. When the current Ir equals to the main current I 2 , based on Kirchoff's Law, the current Io becomes zero. Namely, the diode D 26 is off. At this time, the capacitor Cs 2 and inductor Lr 2 start to resonate. Until the voltage on the capacitor Cs 2 decreases to zero, the switching device S 2 can then be turned on, so the switching device S 2 has no power loss during the switching period.

As shown in FIG. 2 d , the main current I 2 flows from node A 2 to node B 2 through the diode D 21 , the inductor L 2 , the switching device S 2 , and diode D 24 . By the above operation, the switching device S 2 dissipates no power during its switching period, due to zero-voltage switching operation, and a high power factor is also obtained.

After the switching device S 2 turns on, the energy stroed in the inductor Lr 2 charges the capacitor Cr 2 through the diode D 25 when the switching device Sa 2 turns off, as shown in FIG. 2 d . When the current Ir decreases to zero, the diode D 25 turns off. Consequently, the switching device Sa 2 is soft-switched off and the diode D 25 is soft-switched on and off.

However, the main current (I or I 2 ) of the PFC circuit (without or with a snubber circuit), must flow through at least three power electronic devices. Namely, as shown in FIG. 1 b , the main current I of the PFC circuit 10 flows though diode D 1 , D 5 and D 4 when the switching device S 1 is off. As shown in FIG. 1 c , the main current I of the PFC circuit 10 flows through diode D 1 , the switching device S 1 and diode D 4 when the switching device S 1 is on. Further, the main current I 2 of the PFC circuit 20 , as shown in FIG. 2b, flows though diode D 21 , D 26 and D 24 when the switching device S 2 is off. As shown in FIG. 2 c , the main current I 2 of the PFC circuit 20 flows through diode D 21 , the switching device S 2 and diode D 24 when the switching device S 2 is on. The more power electronic elements the main current (I or I 2 ) flows through, the more power dissipation is generated, therefore resulting in poor efficiency in energy transformation.

It is therefore an object of the present invention to provide an active PFC circuit, making the main current only flow through two power electronic elements using two switching devices, thereby reducing power consumption and improving the efficiency of the PFC circuit.

The other object of the present invention is to provide a soft-switched active PFC circuit, wherein the main current not only flows through two electric elements, but also avoids power loss due to switching, thereby improving the efficiency.

The present invention achieves the above-indicated objects by providing an active PFC circuit for improving the efficiency of a voltage source with first and second terminals, comprising the following structure.

An inductor having an terminal coupled to the first terminal of the voltage source.

A first rectifying device having an anode coupled to the other terminal of the inductor, and a cathode.

A second rectifying device having a cathode coupled to the cathode of the first rectifying device, and an anode;

A first switching device having a first terminal coupled to the anode of the first rectifying device, and a second terminal.

A second switching device, having a first terminal coupled to the anode of the second rectifying device and the second terminal of the voltage source, and a second terminal.

A capacitor having two terminals coupled to the cathode of the second rectifying device and the second terminal of the second switching device respectively.

Further, the present invention also provides a soft-switched active PFC circuit for improving the efficiency of a voltage source with first and second terminals, comprising a first module and a second module. The structure and function of the first module are identical to the PFC circuit described above according to the present invention.

The main object of the second module is to make the first and second switching devices S 31 and S 32 carry out the operation of zero-voltage switching. The second module (or auxiliary circuit) comprises the following structure.

A third rectifying device having an anode coupled to the first terminal of the second switching device, and a cathode.

A fourth rectifying device having an anode and cathode coupled to the anode of the first rectifying device and the cathode of the third rectifying device respectively.

A fifth rectifying device having a cathode coupled to the cathode of the second rectifying device, and an anode.

A sixth rectifying device having a cathode coupled to the anode of the fifth rectifying, and anode.

An auxiliary capacitor having two terminals coupled to the cathode of the third rectifying device and anode of the fifth rectifying device respectively.

An auxiliary inductor having two terminal coupled to the cathode of the third rectifying device and the anode of the sixth rectifying device respectively.

An auxiliary switching device having a first terminal and a second terminal coupled to the anode of the sixth rectifying device and the second terminal of the second switching device respectively, and an enable terminal En 4 .

It is noted that the auxiliary switching device turns on before either the first switching device or the second switching device turn on.

The following detailed description, given by way of example and not intended to limit the invention solely to the embodiment described herein, will best be understood in conjunction with the accompanying drawings in which:

FIG. 1 a is a circuit diagram illustrating a conventional active PFC circuit.

FIG. 1 b and FIG. 1 c are diagrams illustrating operation of the conventional active PFC circuit.

FIG. 2 a is a circuit diagram illustrating a conventional active PFC circuit with a snubber circuit.

FIGS. 2 b - 2 d are diagrams illustrating operations of the conventional active PFC circuit with a snubber circuit.

FIG. 3 a is a circuit diagram illustrating an active PFC circuit of the present invention.

FIGS. 3 b to 3 e are diagrams illustrating operations of the active PFC circuit of the present invention.

FIG. 4 a is a circuit diagram illustrating an active power factor correction circuit with an auxiliary circuit of the present invention.

FIGS. 4 b to 4 f are diagrams illustrating operation of the active power factor correction circuit with an auxiliary circuit of the present invention.

FIG. 5 is a wave diagram illustrating the switching operation of switching devices and the key wave forms of the active power factor correction circuit with an auxiliary circuit of the present invention As shown in FIG. 4 a.

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment)

Referring to FIG. 3 a , according to this invention, the active PFC (power factor correction) circuit comprises: an inductor L 3 , a first rectifying device D 31 , a second rectifying device D 32 , a first switching device S 31 , a second switching device S 32 , and a capacitor C 3 . For example, the first and second rectifying devices D 31 and D 32 are diodes in this embodiment.

The first and second rectifying devices (D 31 and D 32 ) and the first and second switching devices (S 31 and S 32 ) are sequentially connected in loop. The cathode of the first and second rectifying devices D 31 and D 32 are connected. One terminal of the capacitor C 3 is connected to the cathodes of the first and second rectifying devices (D 31 and D 32 ); the other terminal is connected to the connection node of the first and second switching devices (S 31 and S 32 ). One terminal of the inductor L 3 is connected to the anode of the first rectifying device S 31 ; the other terminal of the inductor L 3 is coupled to one terminal of a voltage source Vs. The other terminal of the voltage source Vs is coupled to the anode of the second rectifying device D 32 .

For example, the first and second switching devices (S 31 and S 32 ) are MOSFET or IGBT having parasitic diodes (Ds 1 and Ds 2 ) and parasitic capacitors (Cs 1 and Cs 2 ), in this embodiment. Turning the first and second switching devices S 31 and S 32 on or off depends on the enable signals applied to the gates (or enable terminals En 1 and En 2 ) of the first and second switching devices S 31 and S 32 .

The operation of the active PFC circuit 30 of the present invention will be described below in reference to FIG. 3 b and FIG. 3 c . When the voltages at nodes A 3 and B 3 of the voltage source Vs are positive and negative respectively, and the first switching device S 31 is off, the main current I 3 flows from node A 3 to node B 3 , through the inductor L 3 , the first rectifying device D 31 , the capacitor C 3 , and the parasitic diode Ds 2 when S 32 turns off (or both the parasitic diode Ds 2 and the second switching device S 32 when S 32 turns on), as depicted in FIG. 3 b.

Referring to FIG. 3 c , when the first switching device S 31 turns on, the main current I 3 flows from node A 3 to node B 3 , through the inductor L 3 , the first switching device S 31 , and the parasitic diode Ds 2 (or both the parasitic diode Ds 2 and the second switching device S 32 ) directly.

In the same way, when the voltages at nodes A 3 and B 3 of the voltage source Vs are negative and positive respectively, and the second switching device S 32 is off, the main current I 3 flows from node B 3 to node A 3 , through the second rectifying device D 32 , the capacitor C 3 , the parasitic diode Ds 1 when the first switching device S 31 turns off (or both the parasitic diode Ds 1 and the first switching device S 31 when S 31 turns on), and the inductor L 3 , as depicted in FIG. 3 d.

Subsequently, when the second switching device S 32 turns on, the main current I 3 flows from node B 3 to node A 3 , through the second switching device S 32 , the parasitical diode Ds 1 (or both the parasitic diode Ds 1 and the first switching device S 31 ), and the inductor L 3 directly, as depicted in FIG. 3 e.

The first switching device S 31 and second switching device S 32 can turn on at the same or different times according to the control of the enable signals.

The main current I 3 only flows through the first rectifying device D 31 and the parasitical diode Ds 2 of the second switching device S 32 , as shown in FIG. 3 b and only flows through the first switching device S 31 and second switching device S 32 as shown in FIG. 3 c . Because the main current I 3 only flows through two power electronic devices (shown in FIGS. 3 b ˜ 3 e ), thus the active PFC circuit of the present invention can attain the object of correcting power factor and dissipate less power than the conventional PFC circuit.

(Second Embodiment)

FIG. 4 a shows a soft-switched active PFC circuit 40 according to an embodiment of the present invention. For brevity, the elements in FIG. 4 a ˜ 4 d the same as or similar with the elements in FIG. 3 a ˜ 3 e are depicted in the same numerals or notations.

Referring to FIG. 4 a , the soft-switched active PFC circuit 40 comprises a first module 41 and a second module 42 . The structure and function of the first module 41 are identical to the PFC circuit 30 described in the first embodiment, thus omitting its description in this embodiment. The main object of the second module 42 is to make the first and second switching devices S 31 and S 32 carry out the operation of zero-voltage switching.

The second module 42 comprises a third rectifying device D 43 , a fourth rectifying device D 44 , a fifth rectifying device D 45 and a sixth rectifying devices D 46 , an auxiliary capacitor Cr 4 , an auxiliary inductor Lr 4 and an auxiliary switching device Sr 4 . For example, the third to sixth rectifying devices D 43 ˜D 46 are diodes, and the auxiliary switching device Sr 4 is MOSFET or IGBT with a parasitic capacitor Cs 4 and a parasitic diode Ds 4 in this embodiment. Turning on or off the auxiliary switching devices Sr 4 depends on the enable signal applied to the gate (or enable terminals En 4 ) of the auxiliary switching devices Sr 4 . The auxiliary switching device Sr 4 , the sixth rectifying device D 46 and the fifth rectifying device D 45 , connected in series, are connected in parallel with the capacitor C 3 , as depicted in FIG. 4 a . The cathode of the third rectifying device D 43 connects one terminal of the auxiliary capacitor Cr 4 , the anode of the third rectifying device D 43 connects the anode of the second rectifying device D 32 , and the other terminal of the auxiliary capacitor Cr 4 connects the cathode of the sixth rectifying device D 46 (the anode of the fifth rectifying device D 45 ). The cathode of the fourth rectifying device D 44 connects one terminal of the auxiliary inductor Lr 4 , the anode of the fourth rectifying device D 44 connects the anode of the first rectifying device D 31 , and the other terminal of the auxiliary inductor Lr 4 connects the anode of the sixth rectifying device D 46 .

It is noted that the auxiliary switching device Sr 4 turns on before the first switching device S 31 and the second switching device S 32 , and the inductance of the auxiliary inductor Lr 4 is much smaller than that of the inductor L 3 .

The operation of the soft-switched active PFC circuit 40 according to an embodiment of the present invention is described in detail hereinafter in reference with FIGS. 4 a ˜ 4 f and FIG. 5 .

Before time t 1 , the first, second and auxiliary switching devices (S 31 , S 32 and Sr 4 ) are off. Provided the voltages at nodes A 4 and B 4 of the voltage source Vs are positive and negative respectively, the main current Is flows from node A 4 to node B 4 , through the inductor L 3 , the first rectifying device D 31 , the capacitor C 3 (load), and the parasitic diode Ds 2 of the second switching device S 32 , as depicted in FIG. 4 b.

At time t 1 , the auxiliary switching device Sr 4 turns on before the first switching device S 31 so as to achieve soft-switched operation and avoid switching power loss caused by the first switching device S 31 . Referring to FIG. 4 c and FIG. 5, after the auxiliary switching device Sr 4 has turned on, the voltage across the auxiliary inductor Lr 4 is approximately equal to that across the capacitor C 3 , because the first rectifying device D 31 is on. Consequently, the current flow Isr 4 through the auxiliary inductor L 4 increases from zero.

At time t 2 , the current Isr 4 is increased to equal the main current Is, and according to Kirchoff's Law, no current flows through the first rectifying device D 31 , namely, the first rectifying device D 31 is off at this time.

Furthermore, between times t 2 and t 3 , the parasitic capacitor Cs 1 of the first switching device D 31 starts to resonate with the auxiliary inductor Lr 4 , such that the current Isr 4 increases continuously until the voltage (V 31 ) across the parasitical capacitor Cs 1 of the first switching device S 31 discharges to zero.

Then, at time t 3 , the first switching device S 31 turns on with no switching power loss since there is no voltage across the parasitic capacitor Cs 1 of the first switching device S 31 .

After the switching device S 31 has turned on at time t 3 , the main current Is flows back to voltage source Vs through the inductor L 3 , the first switching device S 31 and the parasitic diode Ds 2 of the second switching device S 32 , as shown in FIG. 4 d.

Also referring to FIG. 4 d , at time t 4 , the auxiliary switching device Sr 4 turns off. The current Isr 4 flowing in the auxiliary inductor Lr 4 will charge the auxiliary capacitor Cr 4 through the sixth rectifying device D 46 until the current Isr 4 decreases to zero, then the sixth rectifying device D 46 turns off. Consequently, the auxiliary capacitor Cr 4 serves as a switching snubber of the auxiliary switching device Sr 4 . The auxiliary capacitor Cr 4 also discharges to release energy by turning on the fifth rectifying device D 45 to release energy, when the voltage across the auxiliary capacitor Cr 4 is beyond that of the capacitor C 3 .

The soft-switched operation of the auxiliary switching device Sr 4 is described as follows in reference with FIGS. 4 e and 4 f . As shown in FIG. 4 e , when the auxiliary switching device Sr 4 turns on at time t 1 , the current Isr 4 flows back to the voltage source Vs through the fourth rectifying device D 44 , the auxiliary inductor Lr 4 and the auxiliary switching device Sr 4 . However, when the auxiliary switching device Sr 4 turns off at time t 4 , the current Isr 4 in the auxiliary inductor Lr 4 can not instantaneously change, thereby flowing to the auxiliary capacitor Cr 4 through the sixth rectifying device D 46 . Further, the current Isr 4 charges the auxiliary capacitor Cr 4 until the energy on the auxiliary inductor Lr 4 decreases to zero, namely, the Isr 4 is zero. Then, the sixth rectifying device D 46 turns off. It is clear that when the auxiliary switching device Sr 4 carries out the switching operation, no current flows through it. Therefore, the auxiliary switching device Sr 4 is soft-switched off. Also, the sixth rectifying device D 46 is soft-switched on and off.

According to the above description, the first switching device S 31 can turn on with zero voltage across it, and the auxiliary switching device Sr 4 is soft-switched off. Therefore, the soft-switched active PFC circuit 40 can improve the power factor via switching operation and reduce the power dissipation (loss).

Similarly, the operation of the active PFC circuit 40 of the present invention, when the voltages at nodes A 4 and B 4 of the voltage source Vs are negative and positive, is analogous to the above description so as to be omitted.

Turning on or off the first, second and auxiliary switching devices S 31 , S 32 and Sr 4 depends on the signal applied to the enable terminals En 1 , En 2 and En 4 (or gates) of those switching devices respectively. It is noted that the auxiliary switching device Sr 4 must turn on first, before the first or second switching device (S 31 or S 32 ) turns on. After the first or second switching device (S 31 or S 32 ) turns on, the auxiliary switching device Sr 4 turns off immediately or keeps turning on for a delayed time then turning off.

Moreover, as shown in FIG. 4 b , the main current Is only flow through two power electronic devices, the first rectifying device D 31 and the parasitical diode Ds 2 of the second switching device S 32 . Similarly, as shown in FIG. 4 d , the main current Is also flows through only two power electronic devices, the first switching device S 31 and the parasitical diode Ds 2 of the second switching device S 32 . In view of this, the main current of the present invention flows through fewer elements than the conventional power factor correction circuit as shown in FIG. 2 a , such that power loss is decreased, thereby increasing efficiency.