Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-06-01
2003-03-25
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S535000
Reexamination Certificate
active
06538492
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power supply, an electronic device using the same, and an output short-circuit protection method for the same.
2. Description of the Related Art
FIG. 7
shows the circuit diagram of a conventional power supply. In
FIG. 7
, a power supply
1
includes a direct-current power supply V
cc
, transistors Q
1
and Q
2
, an operational amplifier OP
1
, a Zener diode ZD, resistors R
1
, R
2
, R
3
, and R
4
, capacitors C
1
and C
2
, and an output terminal P
out
.
The direct-current power supply V
cc
is connected to the collector of the transistor Q
1
, which is a regulator element. The emitter of the transistor Q
1
is connected via the resistor R
s
to the output terminal P
out
. Two input terminals of the operational amplifier OP
1
are connected across the resistor R
s
, and the output of the operational amplifier OP
1
is connected to the base of the transistor Q
1
. The base of the transistor Q
1
is connected to the collector of the transistor Q
2
. The emitter of the transistor Q
2
is coupled to ground via the Zener diode ZD. The cathode of the Zener diode ZD is connected via the resistor R
1
to the direct-current power supply V
cc
. The resistor R
2
is connected between the base and the collector of the transistor Q
1
. The direct-current power supply V
cc
is connected in parallel with the capacitor C
1
. A capacitor is connected between P and ground and C
2
and it is also grounded via the resistors R
3
and R
4
are connected in parallel, as shown, to capacitor C
2
. The node between the resistors R
3
and R
4
is connected to the base of the transistor Q
2
.
In the power supply
1
having such a construction, the emitter of the transistor Q
2
is maintained at a reference voltage V
ref
by the resistor R
1
and the Zener diode ZD. The voltage of the output terminal P
out
is detected using the resistors R
3
and R
4
. The transistor Q
2
is controlled using the detected voltage. The transistor Q
2
controls the transistor Q
1
so that the voltage of the output terminal P
out
is maintained based on the reference voltage V
ref
. The capacitors C
1
and C
2
are smoothing capacitors. The resistor R
2
is a starting resistor for the transistor Q
1
. In addition, a load (not shown) is connected between the output terminal P
out
and ground.
An output short-circuit protection operation at the time when short-circuiting occurs at the output side is described. In this application, short-circuiting means not only complete short-circuiting but also a case in which a large amount of current flows through the load because the resistance of the load is decreased below a predetermined resistance. The output of the transistor Q
1
is connected in series with the resistor R
s
. The voltage drop across the resistor R
s
is generated in proportion to the magnitude of the output current and is input into the operational amplifier OP
1
. When a voltage drop of a predetermined value or above is input into the operational amplifier OP
1
, the output voltage is decreased. Therefore, a large amount of current flowing through the resistor R
s
due to the occurrence of short-circuiting at the output side causes the base voltage of the transistor Q
1
to decrease. When the base voltage of the transistor Q
1
is decreased, the transistor Q
1
is turned off, preventing the output current from flowing. That is, in the power supply
1
, the output short-circuit protection operation is realized using the resistor R
s
and the operational amplifier OP
1
.
FIG. 8
shows the circuit diagram of another conventional power supply. Components in
FIG. 8
that are equivalent to the corresponding components in
FIG. 7
have the same reference numerals and descriptions thereof are omitted.
In
FIG. 8
, a power supply
2
includes transistors Q
3
and Q
4
, resistors R
5
, R
6
, and R
7
, and capacitor C
3
instead of the resistor R
s
and the operational amplifier OP
1
in the power supply
1
shown in FIG.
7
. The direct-current power supply V
cc
is connected to ground via the resistor R
5
and the capacitor C
3
in this order. The node between the resistor R
5
and the capacitor C
3
is connected to the emitter of the transistor Q
3
. The output terminal P
out
is connected to ground via the resistors R
6
and R
7
in this order. The node between the resistors R
6
and R
7
is connected to the base of the transistor Q
3
. The collector of the transistor Q
3
is connected to the base of the transistor Q
4
. The collector of the transistor Q
4
is connected to the base of the transistor Q
1
, and the emitter of transistor Q
4
is grounded.
In a case in which the load is normal, the operation of the power supply
2
having such a construction is similar to that of the power supply
1
. The description of the operation of the power supply
2
in this case is therefore omitted.
When short-circuiting occurs at the output side of the power supply
2
, an output short-circuit protection operation is described. Since the resistors R
6
and R
7
are connected between the output terminal P
out
and the ground, the output voltage can be found at the node with the output terminal P
out
. When short-circuiting occurs at the output side, voltage drop occurs at the node between the resistors R
6
and R
7
, and the transistor Q
3
is turned on. When the transistor Q
3
is turned on, the current flows via the resistor R
5
and the transistor Q
3
into the base of the transistor Q
4
, and the transistor Q
4
is turned on. When the transistor Q
4
is turned on, the collector voltage of the transistor Q
4
is decreased. Accordingly, since the base voltage of the transistor Q
1
connected to the collector of the transistor Q
4
is also decreased, the transistor Q
1
is turned off. This prevents the output current from flowing. That is, in the power supply
2
, the output short-circuit protection operation is realized using the transistors Q
3
and Q
4
, the resistors R
5
, R
6
, and R
7
, and the capacitor C
3
.
When the power supply
2
is first turned on, since the base voltage of the transistor Q
3
is substantially 0V, the output short-circuit protection operation would be immediately activated. Therefore, a delay circuit including the resistor R
5
and the capacitor C
3
for preventing the transistor Q
3
from turning on when the power is first turned on is provided. Due to the operation of this delay circuit, a significant amount of time is required for the emitter voltage of the transistor Q
3
to increase. Since the transistor Q
3
is not turned on during this time, unnecessary output short-circuit protection operation is not caused at initial turn on of the power supply.
In the power supply
1
shown in
FIG. 7
, since the path through which the output current flows is connected in series with the resistor R
s
, even when the power supply is operated normally, electrical power is consumed at the resistor R
s
. This decreases the efficiency of the power supply
1
.
In the power supply
2
shown in
FIG. 8
, the delay circuit must be provided so that the output short-circuit protection operation is not activated due to the operation of the transistor Q
3
when the power is turned on. Provision of the delay circuit increases the circuit size, which prevents the power supply from being miniaturized and from being less expensive. Furthermore, in the power supply
2
, when the transistor Q
1
is turned off due to the output short-circuit protection operation, the voltage of the output terminal P
out
is 0V. Therefore, even though the short-circuit state at the output side is eliminated, the output short-circuit protection operation remains and is not restored to the normal state. Accordingly, once the output short-circuit protection operation is activated, the power must be turned on again in order to cause the power supply to restore the power supply to the normal state.
SUMMARY OF THE INVENTION
Accordingly, objects of the present invention are to solve the foregoing problems. There are provided a power supply which can perform an ou
Kunii Shingo
Sano Naoto
Cox Cassandra
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Wells Kenneth B.
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