Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-12-16
2004-04-06
Berhane, Adolf (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021160
Reexamination Certificate
active
06717827
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to an insulated switching power supply for supplying DC power from a bridge-rectified output-smoothed DC voltage source through a transformer and a switching element to a load, and more particularly, relates to techniques for improving an input power factor and for decreasing switching loss and steady-state loss of a switching power supply.
FIG. 5
shows a configuration of a conventional switching power supply that uses a typical choke input-type smoothing circuit to improve the power factor (refer to Japanese Patent Publication (Tokkai) No. 09-131055).
In
FIG. 5
, a reference number
1
designates an AC supply, reference numbers
2
and
4
designate capacitors, a reference number
3
designates a common mode reactor having two coils wound with the same polarity around the same core, a reference number
5
designates a bridge rectifying circuit, a reference number
6
designates a smoothing capacitor, a reference number
7
designates a transformer, a reference number
7
a
designates a primary winding, a reference number
7
b
designates a secondary winding, a reference number
8
designates a switching element, a reference number
9
designates a diode, a reference number
10
designates a smoothing capacitor, a reference number
11
designates a load, a reference number
12
designates a control circuit for controlling a switching operation of the switching element
8
, and a reference number
13
designates a choke coil.
Here, the capacitors
2
and
4
, together with the common mode reactor
3
, form a common mode noise filter. The noise filters which omit the capacitor
2
or capacitor
4
from the above described circuit configuration are well known to those skilled in the art. The noise filter which has any of the circuit configurations described above is called the “normal mode noise filter”. The normal mode noise filter removes the normal mode noise current which flows through the positive and negative output lines of the bridge rectifying circuit
5
. For simply forming a normal mode noise filter, a reactor having one single winding may be used in substitution for common made reactor
3
.
Although not illustrated, a common mode noise filter is formed by grounding each winding of the common mode reactor
3
via a capacitor. The common mode noise filter is used to remove the common mode noise current which flows between the positive and negative output lines of the bridge rectifying circuit
5
and the ground concurrent with the switching operation of switching element
8
.
In the abovementioned configuration, a choke input-type smoothing circuit comprised of the choke coil
13
and the smoothing capacitor
6
smoothes a full-wave rectified DC voltage output from the bridge rectifying circuit
5
. Then, through the switching operation of the switching element
8
, the voltage is supplied through the transformer
7
, the diode
9
, and the smoothing capacitor
10
to the load
11
as a nearly constant DC voltage. The control circuit
12
controls an ON/OFF duty cycle of the switching element
8
with pulse-width modulation (PWM) or other means so as to adjust the aforementioned DC voltage at a desired value.
A charging current is supplied to the smoothing capacitor
6
from the AC supply
1
through the noise filter, the bridge rectifying circuit
5
, and the choke coil
13
. According to an inductance value of the choke coil
13
, a peak value of the charging current is suppressed, and a conduction time is increased. Namely, the choke coil
13
smoothes the charging current flowing to the smoothing capacitor
6
, thereby improving the power factor (Japanese Patent Publication No. 9-131055).
However, since this switching power supply is used at a commercial switching frequency or twice that frequency, the choke coil
13
requires a large inductance of at least several mH, resulting in a large size and a heavy weight not suitable for practical applications.
Moreover, because of the large inductance, the number of coil windings increases. Accordingly, a voltage drop due to the resistance of the windings becomes larger and an intermediate DC voltage decreases. Therefore, an effective current flowing through the switching element increases, thereby causing problems such as an increased switching loss and decreased efficiency of the power supply.
As shown in
FIG. 6
, to achieve a power factor of nearly 1 and remove high frequency components from the input current, it is known that a switching power supply may use a method known as power factor correction (PFC) to convert an input current into an approximate sine wave.
In
FIG. 6
, a reference number
14
designates the second switching element, a reference number
15
designates a diode, a reference number
16
designates a current sense resistor, a reference number
17
designates an inductor, a reference number
18
designates the second control circuit, and other circuit elements are the same as in FIG.
5
. Here, a voltage of the smoothing capacitor
6
and a current value sensed by the current sense resistor
16
are input to the second control circuit
18
. The second switching element
14
is turned on and off based on these input signals.
In this example of the prior art, the inductor
17
, the second switching element
14
, the diode
15
, the smoothing capacitor
6
, the current sense resistor
16
, and the second control circuit
18
form a boost converter. The PWM control of the switching element
14
by the control circuit
18
makes the input current waveform sinusoidal to remove the high frequency components and improve the power factor.
The switching power supply shown in
FIG. 6
requires two control circuits
12
and
18
, and consequently has a complex circuit configuration and a high cost. Moreover, not many applications need to remove all of the high frequency components contained in the input current while maintaining a power factor of nearly 1. For most applications, since it is sufficient to suppress the high frequency components to a set value or below as specified by a specification or the like, this switching power supply is also wasteful in regard to functionality and a cost.
Therefore, an object of the present invention is to provide a switching power supply that improves the power factor by expanding the input current conduction angle over a wide range of the input voltage, removes the high frequency components of the input current to a level sufficient for a practical use, reduces the switching loss, and increases efficiency without increasing a size and a cost of a device.
Further objects and advantages of the invention will be apparent from the following description of the invention.
SUMMARY OF THE INVENTION
To solve the abovementioned problems, according to the first aspect of the invention, a switching power supply includes a noise filter; a bridge rectifying circuit connected to an AC power supply via the noise filter; a smoothing capacitor connected to an output side of the bridge rectifying circuit for smoothing a voltage output from the bridge rectifying circuit; a primary winding of a transformer and a switching element connected in series to one terminal of the smoothing capacitor; and a control circuit for controlling a switching operation of the switching element to adjust a DC voltage obtained by rectifying and smoothing the voltage on a secondary winding side of the transformer at a desired value based on a detected value of the DC voltage.
Further, the first reactor and the first diode are connected sequentially in series between one output terminal of the bridge rectifying circuit and one terminal of the smoothing capacitor. The second reactor and the second diode are connected in series between a node that mutually connects the first reactor and the first diode and a node that mutually connects the primary winding and the switching element.
According to the second aspect of the invention, a switching power supply includes a noise filter; a bridge rectifying circuit connected to an AC power
Berhane Adolf
Fuji Electric Co., Ltd
Kanesaka and Takeuchi
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