Active power factor correction circuit

Details Active power factor correction circuit Active power factor correction circuit

2001-12-27

2003-05-27

Riley, Shawn (Department: 2838)

Electricity: power supply or regulation systems

For reactive power control

Using converter

C363S089000

Reexamination Certificate

active

06570366

ABSTRACT:

BACKGROUND OF THE INVENTION
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 Therefore, passive PFC circuits have been replaced by active PFC circuits.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.
SUMMARY OF THE INVENTION
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 t

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