Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2003-01-31
2004-12-21
Sherry, Michael (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S021070, C363S021150, C363S098000
Reexamination Certificate
active
06834002
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a power factor correction circuit and more particularly, to a power factor correction circuit for improving a power factor of a switching power supply designed in bridge topologies in order to comply with the requirements of Class A or Class D as stipulated in harmonic current rules IEC-1000-3-2.
BACKGROUND OF THE INVENTION
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 C
3
is connected as a filter for the bridge rectifier BD
1
.
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 BD
1
is used to rectify the AC power V
S1
. A capacitor C
3
is then used to filter the rectified power and generates a DC voltage V
C3
. The capacitor C
1
and the capacitor C
2
are connected in a common node to form a voltage divider. Therefore, the voltage in the common node between the two capacitors is V
C3
/2.
FIG. 7
is a time chart for the PWM signals, V
HG
and V
LG
, which are driving signals for the switch Q
1
and the switch Q
2
shown in
FIG. 5
, respectively. Both PWM signals V
HG
and V
LG
are low (low voltage) for 0≦t≦t
1
. At this time, the switch Q
1
and the switch Q
2
are both turned off. Therefore, the output voltage V
O
is supplied by the capacitor C
4
.
When t
1
≦t≦t
2
, the PWM signal V
LG
is high (high voltage) and the PWM signal V
HG
is low (low voltage). At this time, the switch Q
1
is turned off and the switch Q
2
is turned on. The current flows through the capacitor C
1
, the primary winding P
1
and the switch Q
2
to the ground. Under this situation, the transformer transfers the power from the primary winding P
1
to the secondary winding S
1
to supply power to the capacitor C
4
and output a voltage V
O
.
When t
2
≦t≦t
3
, the switch Q
1
and the switch Q
2
are both turned off. At this time, the operation state of the circuit is same as the operation state of the circuit at O≦t≦t
1
.
When t
3
≦t≦t
4
, the PWM signal V
LG
is low (low voltage) and the PWM signal V
HG
is high (high voltage). At this time, the switch Q
1
is turned on and the switch Q
2
is turned off. Therefore, the current flows through the switch Q
1
, the primary winding P
1
and the capacitor C
2
to the ground, The transformer transfers the power from the primary winding P
1
to the secondary winding S
2
to supply power to the capacitor C
4
and output a voltage V
O
.
The power switching cycle described above is then performed repeatedly to supply power to a loading. On the other hand, the output voltage V
O
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 V
HG
and V
LG
. For example, if the power supplied to the load is insufficient when the output voltage is lower than a required value, the feedback signal enlarges the duty cycle of the PWM signals V
HG
and V
LG
to increase the conduction time of the switch Q
1
and the switch Q
2
. Therefore, the time for transferring power from the primary winding to the secondary winding of the transformer T
1
is increased. In other words, the power supplied to the secondary winding is increased. The output voltage V
O
is therefore also increased. Finally, the output voltage V
O
again attains 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 V
HG
and V
LG
should be reduced.
Note that the input current I
pc
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 (e.g., approximately 50%) due to the distorted input current, and the total harmonics distortion (hereinafter referred as THD) is even higher than 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, even worse, precious energy is wasted.
Thus, many countries have promulgated a number of harmonic current rules (e.g., IEC-1000-3-2) which specify the current wave shape of the power supply for manufacturers to obey 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 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 L
1
is in series between a bridge rectifier BD
1
and an electrolytic capacitor C
1
. The winding L
1
and the capacitor C
1
form a low pass filter to rectify the input current of a DC/DC converter
2
. Such design is similar in function to the ballast for correcting the power factor of a fluorescent lamp. However, winding L
1
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 Q
1
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, and with similar drawbacks.
SUMMARY OF THE INVENTION
In accordance with the foregoing description, there are many drawbacks in the conventional power factor correction circuit. For example, the circuit structure depicted in
FIG. 3
is relatively large, while the circuit structure depicted in
FIG. 4
is relatively complex in circuit design and is expensive to manufacture.
Therefore, the main purpose of the present invention is to provide a power factor correction circuit with a high power factor.
Another purpose of the present invention is to provide a power factor correction circuit to solve the problems existing in the prior art.
A further purpose of the present invention is to provide a switching power supply structure that is small and economical to manufacture. It is an object of the present invention to provide a power factor correction circuit comprising a series connection of a bridge rectifier and a first capacitor. The first capacitor, a winding and a first switching device are connected in series. The first switching device is the low-side switching device in a bridge converter and is connected with a first anti-parallel diode. The first switching device, a second switching device and a second capacitor are also connected in series. The second switching device is the high-side switching device in the bridge converter and is connected with a second anti-parallel
5
diode. The second capacitor acts as a boost capacitor in the PFC circuit and provides the DC operating voltage for the bridge converter. The winding can be one additional winding of the main transformer in the bridge converter or an independent inductor. Further, the power factor of the off-line switching power supply is increased to above 0.9 by appropriately selecting the value of the first capacitor and the winding in order to comply with the requirements of Class A or Class D as stipulated in harmonic current rules IEC-1000-3-2. Furthermore, the inserted PPC circuit does not affect the normal operation of the bridge converter.
REFERENCES:
patent: 5408403 (1995-04-01), Nerone et al.
patent: 5594635 (1997-01-01), Gegne
Entrust Power Co., Ltd.
Laxton Gary L.
Sherry Michael
LandOfFree
Power factor correction circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Power factor correction circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Power factor correction circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3306262