Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2002-01-29
2003-11-11
Patel, Rajnikant B. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C363S089000
Reexamination Certificate
active
06646848
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply apparatus for supplying a stabilized DC voltage to industrial and consumer electronic apparatuses. More particularly, the present invention relates to an overcurrent protection circuit of a switching power supply apparatus. The overcurrent protection circuit prevents overcurrent flowing through the switching power supply apparatus itself and apparatuses connected to the input and output of the switching power supply apparatus in an overload condition.
In recent years, electronic apparatuses are made more inexpensive, compact, efficient and energy saving. Accordingly, switching power supply apparatuses for these electronic apparatuses are strongly demanded to have higher output stability and to be more compact and efficient. At the same time, switching power supply apparatuses being high in safety are demanded in the field of electronic apparatuses. Even when an electronic circuit serving as a load causes an abnormality and its input impedance lowers, an overcurrent protection circuit of a switching power supply apparatus satisfying the above-mentioned demands is required to appropriately restrict the current flowing through the electronic circuit serving as the load and to maintain the electronic circuit in a safe condition.
A conventional overcurrent protection circuit of a switching power supply apparatus will be described below referring to an accompanying drawing, FIG.
12
.
FIG. 12
shows a conventional overcurrent protection circuit for a step-down type switching power supply apparatus.
In
FIG. 12
, an input DC power source
201
is formed of a circuit for rectifying and smoothing a commercial power source or a battery. This input DC power source
201
is connected across input terminals
202
a
and
202
b
. A current transformer
203
has a primary winding
203
a
and a secondary winding
203
b
. One terminal of the primary winding
203
a
is connected to one (
202
a
) of the input terminals
202
a
and
202
b
. The other terminal of the primary winding
203
a
of the current transformer is connected to one terminal of a switching device
204
. The other terminal of the switching device
204
is connected to the cathode of a rectifying diode
205
. Furthermore, the other terminal of the switching device
204
is connected to one terminal of an inductance device
206
. The switching device
204
connected in this way is configured so as to repeat ON/OFF operation. The anode of the rectifying diode
205
is connected to the other input terminal
202
b.
As shown in
FIG. 12
, the inductance device
206
and a smoothing capacitor
207
are connected in series, thereby forming a series element. This series element is connected across the rectifying diode
205
, thereby forming a smoothing circuit. This smoothing circuit averages a rectangular wave voltage generating across the rectifying diode
205
and obtains a DC voltage.
A voltage averaged by the smoothing capacitor
207
is output across the output terminals
208
a
and
208
b
of the conventional overcurrent protection circuit of the switching power supply apparatus shown in
FIG. 12. A
load
209
is connected across the output terminals
208
a
and
208
b
, and consumes the power from the overcurrent protection circuit of the switching power supply apparatus.
A control circuit
210
detects the voltage across the output terminals
208
a
and
208
b
, and outputs a control signal for controlling the ON/OFF ratio of the switching device
204
so that a stabilized voltage is output. A first resistor
211
is connected in parallel with the secondary winding
203
b
of the current transformer
203
. In the OFF period of the switching device
204
, an exciting current flows through the secondary winding
203
b
of the current transformer
203
, whereby the exciting energy of the current transformer
203
is consumed.
When the switching device
204
is ON, the current flowing through the primary winding
203
a
of the current transformer
203
is converted into a current corresponding to the winding ratio of the current transformer
203
. The converted current flows through a second resistor
213
via a diode
212
. Hence, a voltage Vs proportional to the current flowing through the primary winding
203
a
of the current transformer
203
generates across the second resistor
213
.
The voltage Vs generating across the second resistor
213
is compared with the predetermined reference voltage of a reference power source
214
by a comparator
215
. When the voltage Vs reaches the reference voltage, the switching device
204
is turned OFF via the control circuit
210
. In other words, in the overcurrent protection circuit of the switching power supply apparatus shown in
FIG. 12
, the current flowing through the switching device
204
is detected in real time. The switching device
204
is controlled so that the instantaneous value of the current does not exceed a certain value. In this overcurrent protection circuit, the current flowing through the switching element
204
, an object to be detected, passes through the inductance device
206
and becomes an output current. As a result, the operation for controlling the switching element
204
becomes an operation for restricting the output current.
In the overcurrent protection circuit of the switching power supply apparatus configured as described above, an output current Iout is the average value Iav of a current flowing through the inductance device
206
. Furthermore, the peak value of a current flowing through the switching device
204
, in other words, the peak value of a current flowing through the inductance device
206
is restricted in real time. The fluctuation width &Dgr;I of the current flowing through the inductance device
206
is a function of an input voltage Vin and an output voltage Vout, and is given by the following equation (1). In Equation (1), D designates a duty ratio, that is, the ON/OFF ratio of the switching device
204
, Ts designates a switching cycle, and Lf designates the inductance value of the inductance device
206
.
Δ
I
=
V
out
(
1
-
D
)
T
s
L
f
(
1
)
Accordingly, the relationship between the peak value Ip of the current flowing through the inductance device
206
and the average value Iav of the current flowing through the inductance device
206
is represented by the following equation (2).
I
p
=
I
av
+
Δ
I
2
=
I
av
+
V
out
(
1
-
D
)
T
s
2
L
f
(
2
)
FIG. 13
is a graph showing current waveforms during the operation of the conventional overcurrent protection circuit. Even when the output current is made constant, the peak voltage differs depending on the input voltage. Hence, in the configuration of the conventional overcurrent protection circuit, control is carried out so that the peak value Ip of the current flowing through the inductance device
206
becomes constant. As a result, the output current Iout has a characteristic of changing together with the fluctuations in the output voltage Vout and the input voltage Vin.
FIG. 14
is a waveform graph showing an overcurrent drooping characteristic in the conventional overcurrent protection circuit. When the output voltage Vout lowers as shown in
FIG. 14
, the output current Iout increases abruptly. In particular, when the inductance value Lf of the inductance device
206
is small, the fluctuation width &Dgr;I of the current flowing through the inductance device
206
becomes large. The difference between the peak value Ip and the average value Iav of the current increases. As a result, the drooping characteristic becomes worse in this case, and the output current Iout increases. As the output current Iout increases in this way, the currents flowing through the switching device
204
and the rectifying diode
205
become larger. For this reason, devices having a large breakdown resistance are required to be used for the switching device
204
and the rectifying diode
205
of the conventional overcurrent protection circuit. This raises problems
Kuranuki Masaaki
Yoshida Koji
Akin Gump Strauss Hauer & Feld L.L.P.
Matsushita Electric - Industrial Co., Ltd.
Patel Rajnikant B.
LandOfFree
Switching power supply apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Switching power supply apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Switching power supply apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3182492