Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2003-05-08
2004-11-09
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S019000, C323S902000
Reexamination Certificate
active
06816392
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an overcurrent output protecting circuit and a constant-voltage switching power supply incorporating the same.
A constant-voltage switching power supply rarely generates unnecessary power as heat from a transistor since a switching transistor therein only repeats the ON-OFF switching operation. The constant-voltage switching power supply uses a compact and small-loss high-frequency transformer. Thus the constant-voltage switching power supply is advantageous in that the power loss of the entire circuit can be made small.
FIG. 4
is a circuit diagram showing an example of a related-art constant-voltage switching power supply.
A smoother
1
comprising a rectifier bridge DB
1
and a capacitor C
11
smoothes the AC voltage supplied from an AC power source Vin to convert the AC voltage to a DC voltage. The DC voltage is switched by a switching transistor (field-effect transistor) Q
1
and converted to a high-frequency pulse. The high-frequency pulse is transformed by a high-frequency transformer T
1
then converted to a DC voltage again by a high-frequency rectifier
2
and outputted across a Vout terminal and a GND terminal. In a case where there is a variation in the output voltage, the voltage comparator/detector
3
detects a variation in the voltage and notifies that to a duty ratio controller
4
via a photocoupler PC
1
. The duty ratio controller
4
changes the ON-OFF interval of the switching transistor Q
1
to control the duty ratio of the high-frequency pulse. The average voltage of the high-frequency pulse becomes the DC output voltage and the output voltage is controlled by the duty ratio. Control of the duty ratio of the high-frequency pulse is made so that, when the DC output voltage is higher, the ON duty will be smaller and when the DC output voltage is lower, the ON duty will be greater.
In the related-art constant-voltage switching power supply of this configuration, an AC voltage supplied from the AC power source Vin is converted to a DC voltage by the smoother
1
, and the DC voltage causes a current to flow through an activation resistor R
1
, thus elevating the gate voltage of the switching transistor Q
1
. This turns on the switching transistor Q
1
and generates a voltage on a first primary coil of the high-frequency transformer T
1
and a phase-inverted voltage corresponding to the number of turns on a second primary coil P
2
. The voltage generated in the second primary coil applies a positive feedback on the gate of the switching transistor Q
1
via a capacitor C
1
and a resistor R
2
. The base of the transistor Q
2
is charged by a coupler current of a photocoupler PC
1
which feeds back a variation in the DC output voltage and a current flowing through a Zener diode ZD
1
.
When a current flows through the first primary coil P
1
, a current attempts to flow through the secondary wiring S
1
of the high-frequency transformer T
1
. The diode D
1
blocks the current so that the corresponding energy is stored in the high-frequency transformer T
1
. When the base voltage of the transistor Q
1
reaches the ON voltage, the switching transistor Q
1
turns off, causing the energy to be transmitted from the secondary wiring S
1
. On the second primary coil P
2
is applied a reverse bias thus causing the base of the transistor Q
2
to be discharged. When all the energy stored in the high-frequency transformer T
1
is generated from the secondary wiring S
1
, the switching transistor Q
1
starts to turn on again with a counterelectromotive force.
A high-frequency pulse is generated by repeating the above operation. Then the transistor Q
2
is ON-OFF controlled by the coupler current of the photocoupler PC
1
which feeds back a variation in the DC output voltage. This causes ON-OFF control of the transistor Q
1
, which controls the duty ratio of the high-frequency pulse. Thus a voltage specified by a Zener diode ZD
2
is stably outputted at the DC output terminal Vout.
FIG. 5
shows the voltage waveform of the primary coils P
1
and P
2
of the high-frequency transformer T
1
and the voltage waveform across the base and the emitter Vbe of the transistor Q
2
in the related-art constant-voltage switching power supply.
FIG. 6
is a graph showing a relationship between a DC output current lout and the DC output voltage Vout of the related-art constant-voltage switching power supply.
Decreasing the load on the DC voltage output from infinity to zero increases the current flowing toward a load. This extends the ON duty of the switching transistor Q
1
(ON duty of a voltage generated in the first primary coil P
1
). When a certain length of ON duty is reached, the base of the transistor Q
2
is charged even in a case where the photocoupler PC
1
is not turned on, which decreases the DC output voltage (point designated by A in FIG.
6
). As the DC output voltage decreases, the ON duty voltage of the switching transistor Q
1
, that is, the ON-duty voltage of a voltage generated in the first primary coil P
1
decreases (waveform indicated by a chained line in FIG.
5
). This also causes the ON-duty voltage of a voltage generated in the second primary coil P
2
to decrease (waveform indicated by a chained line in FIG.
5
). The negative bias voltage (Vbe) of the transistor Q
2
decreases (waveform indicated by a chained line in FIG.
5
), which causes the transistor to tend to turn on.
As the DC output voltage further decreases, the voltage in the second primary coil P
2
decreases and drops below the Zener voltage of the Zener diode ZD
1
, which fails to turn on the transistor Q
2
. This prevents the transistor Q
2
from continuously driving the switching transistor Q
1
, causing the switching transistor Q
1
to start blocking oscillation (point designated by B in FIG.
6
). The ON duration of the blocking oscillation is determined by the time constant of the activation resistor R
1
and the capacitor C
2
, while the OFF duration is determined by a load. The heavier the load is, the shorter the blocking oscillation period becomes. With the DC voltage output shorted, the output current reaches its maximum (point designated by C in FIG.
6
).
In this way, the output current reaches its maximum with the DC voltage output shorted. In a case where the DC voltage output is shorted due to any abnormality or short mode fault in the load of apparatus mounting the constant-voltage switching power supply, an overcurrent could flow where the short has taken place.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an overcurrent output protecting circuit which can prevent an overcurrent from being outputted when the short-circuit is occurred in the DC voltage output side.
In order to achieve the above object, according to the invention, there is provided a constant-voltage switching power supply, comprising:
a smoother, which converts an AC voltage supplied from an AC power source into a first DC voltage;
a switching transistor, which converts the first DC voltage into a first cyclic pulse signal;
a transformer, which voltage-transforms the first cyclic pulse signal to obtain a second cyclic pulse signal;
a rectifier, which rectifies the second cyclic pulse signal to obtain a second DC voltage;
a detector, which detects a potential variation of the second DC voltage;
a duty ratio controller, which controls a duty ratio of the first cyclic pulse signal in accordance with the potential variation;
a duty ratio monitor, which judges whether the duty ratio is a predetermined ratio or more; and
a deactivator, which turns off the switching transistor in a case where the duty ratio monitor judges that the duty ratio is the predetermined ratio or more.
As the load connected to the output side of the constant-voltage switching power supply becomes heavier, the ON duty of the switching transistor accordingly increases. According to the above configuration, the switching transistor is turned off, so that the output of the first cyclic pulse signal to the transformer is halted in a case where the ON duty of
Seiko Epson Corporation
Vu Bao Q.
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