Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-03-29
2001-09-25
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021060
Reexamination Certificate
active
06295214
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switching power supply units outputting a DC current and a DC voltage.
2. Description of the Related Art
FIG. 5
shows an example of a prior art main circuit structure of a switching power supply unit. The switching power supply unit shown in
FIG. 5
is a forward converter in which a transformer
1
is disposed. A primary coil
2
of the transformer
1
is electrically connected to a primary-side circuit having a main switching element
3
(an N-channel MOS-FET in the example shown in
FIG. 5
) and a capacitor
4
, and a secondary coil
5
is electrically connected to an output-side circuit (a secondary-side circuit)
10
having a rectifying diode
6
, a commutating diode
7
, an inductor
8
, and a capacitor
9
.
In addition, the gate of the main switching element
3
is electrically connected to a switching control circuit
11
controlling the on-off switching operation of the main switching element
3
. Furthermore, an output voltage detection unit
11
a
and an output current detection unit
12
, which will be described below, are also provided as shown.
There is provided a structure in which a DC current and a DC voltage are inputted to the primary-side circuit via input terminals
13
a
and
13
b
. As already known, the inputted DC current and DC voltage are converted into an AC current and an AC voltage by the on-off switching operation of the main switching element
3
, and then, the AC current and AC voltage are outputted to the output-side circuit
10
from the secondary coil
5
of the transformer
1
.
In the output-side circuit
10
, the AC current and AC voltage are rectified and smoothed by using a choke-input rectifying method, and the rectified and smoothed DC current and DC voltage are outputted as an output current Iout and an output voltage Vout of the switching power supply unit to a load
15
via output terminals
14
a
and
14
b.
The output voltage detection unit
11
a
shown in
FIG. 5
directly detects the output voltage Vout of the switching power supply unit outputted to the load
15
from the output-side circuit
10
, and then outputs a voltage corresponding to the output voltage Vout to the switching control circuit
11
. The switching control circuit
11
controls the on-off switching operation of the main switching element
3
so that the output voltage Vout is outputted with a specified voltage value in a stabilized manner based on the applied voltage.
As shown in
FIG. 5
, the output current detection circuit
12
comprises a current transformer
16
, a rectifying diode
17
, a resistor
18
, and a capacitor
19
. The current transformer
16
is disposed on a current-flowing path through which the same current as a drain current of the main switching element
3
flows. The drain current of the main switching element
3
is converted into a voltage and is amplified by the current transformer
16
and the resistor
18
. The voltage is rectified by the rectifying diode
17
and then is charged in the capacitor
19
.
The value of the drain current of the main switching element
3
periodically changes. A current value at peak (hereinafter referred to as a peak current value) during a period as a cycle in the drain current of the main switching element
3
is a current value corresponding to the output current Iout of the switching power supply unit. Since the charging voltage value of the capacitor
19
is approximately equivalent to a voltage value corresponding to the peak current value of the drain current of the main switching element
3
, the charging voltage of the capacitor
19
is also a voltage corresponding to the output current Iout of the switching power supply unit. The output current detection circuit
12
outputs the charging voltage of the capacitor
19
as a voltage corresponding to the output current Iout.
In the example shown in
FIG. 5
, the output voltage of the output current detection circuit
12
is applied to the switching control circuit
11
. When the switching control circuit
11
detects that the output current Iout is in an overcurrent state due to an abnormal condition of the load
15
or the like based on the output voltage of the output current detection circuit
12
, it controls the switching of the main switching element
3
in such a manner that the output current Iout decreases so as to eliminate the overcurrent state, by which the switching power supply unit is protected.
In the example shown in
FIG. 5
, the output current detection circuit
12
, as described above, has a circuit structure of detecting and outputting the voltage corresponding to the output current Iout by using the current transformer
16
. However, since the current transformer
16
is an expensive and large component, cost reduction and miniaturization of the switching power supply unit cannot be achieved.
Thus, an output current detection circuit in which a resistor is disposed as an alternative to the current transformer
16
has been provided. However, in this case, there is a large amount of conduction loss of the resistor, which leads to a problem in that the circuit efficiency of the switching power supply unit deteriorates.
Regarding this case, the reason for causing such a problem will be described as follows. In the aforementioned output current detection circuit having the resistor as an alternative to the current transformer
16
, a diode is used for rectification. When the voltage applied to the resistor is approximately equal to the forward voltage drop of the diode, the output voltage outputted from the output current detection circuit changes according to changes in temperature due to a negative influence of dropping of the forward voltage of the diode, regardless of changes in the output current Iout, which is a problem of temperature drift.
In order to prevent such a problem of temperature drift, there is provided a structure in which a resistance value of the resistor is set to be large so that the voltage applied to the resistor is significantly larger (for example, 5 to 6 V) than the forward voltage of the drop of the diode. As a result, as described above, the conduction loss in the resistor significantly increases, and this leads to the problem that the circuit efficiency of the switching power supply unit deteriorates.
SUMMARY OF THE INVENTION
The present invention is presented to solve the above problems. It is an object of the present invention to provide a switching power supply unit having an output current detection circuit capable of providing cost reduction and miniaturization of the switching power supply unit and preventing the deterioration of circuit efficiency.
To this end, the present invention provides the following arrangement to solve the above problems. According to a first aspect of the present invention, there is provided a switching power supply unit which rectifies and smoothes an AC current and an AC voltage obtained from the on-off switching operation of a main switching element to output a DC current and a DC voltage. This switching power supply unit has an output current detection circuit including a current detection resistor disposed in an intermittent-current flowing path through which a current intermittently flows in sync with the on-off switching operation of the main switching element, a synchronous rectifying element comprising a switching element for rectifying a current flowing through the current detection resistor, and a capacitor for charging the current rectified by the synchronous rectifying element, in which the current flowing through the current detection resistor is charged in the capacitor by a rectifying operation of the synchronous rectifying element, and the charging voltage of the capacitor is outputted as a voltage corresponding to the output current of the switching power supply unit. This arrangement can solve the above-described problems.
According to a second aspect of the present invention, the structure of the first aspect of the invention described above being provided, an ov
Matsumoto Tadahiko
Nagai Jun
Nishiyama Takayoshi
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Patel Rajnikant B.
Wong Peter S.
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