Regulated power source circuit including an overcurrent...

Details Regulated power source circuit including an overcurrent... Regulated power source circuit including an overcurrent...

2001-02-28

2002-08-06

Sterrett, Jeffrey (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S901000

Reexamination Certificate

active

06429631

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to regulated power supply circuits and regulated power supply devices, and in particular, overcurrent protection mechanisms.
BACKGROUND OF THE INVENTION
We will first discuss conventional regulated power supply circuits in reference to
FIG. 11
which is a circuit block diagram representing a dropper-type regulated power supply circuit
51
. In the regulated power supply circuit
51
, a PNP power transistor TR
1
hereinafter, transistor TR
1
acting as an output control element drops an input voltage Vin to produce an output voltage Vo. During normal operation, an error amplifier
53
compares a partial voltage of the output voltage Vo detected by resistors R
1
and R
2
to a reference voltage supplied from a constant voltage circuit
54
. The error amplifier
53
adjusts the base current of a transistor TR
5
according to an error obtained from the comparison, which in turn adjusts the base current of a transistor TR
1
via a transistor TR
2
to keep the output voltage Vo at a constant value.
However, an overcurrent flowing through the collector of the transistor TR
1
reduces the output voltage Vo. In contrast, under a normal condition, an overcurrent detection circuit
52
, upon detection of such an overcurrent, directly varies the base potential of the transistor TR
1
for the purpose of restraining the current flow.
When a load R
3
is short-circuited, the output voltage V
0
of the transistor TR
1
falls to GND level, and so does the potential at the contact between the resistors R
1
and R
2
. Therefore, in the short circuit protection circuit
56
, the base potential of the transistor TR
3
falls. This turns on the transistor TR
3
and thus produces a current flow to the resistor R
4
, which then elevates the base potential of, and thus turns on, the transistor TR
4
. As a result, the current flow from the constant current circuit
55
to the base of the transistor TR
5
branches off to be coupled to the collector of the transistor TR
4
. This reduces the base current of the transistor TR
5
and causes the current flow to the base of the transistor TR
2
to branch off to the emitter, then the base, of the transistor TR
3
, and further pass through the diode D
1
and the resistor R
2
. Accompanying great fall in the base current of the transistor TR
2
causes a great fall in the base current of the transistor TR
1
, limiting the output current Io of the transistor TR
1
.
When the output short circuit is incomplete as described above, however, a problem develops such that great loss occurs at the transistor TR
1
. To explain this phenomenon, reference is made to
FIG. 12
showing overcurrent limiting characteristics representing a relationship between the output voltage Vo and output current Io of a typical regulated power supply circuit.
A complete short circuit, denoted by point A in
FIG. 12
, is a condition in which a resistor with a small impedance is used to short-circuit the output terminals to completely short-circuit the output. At point A, a loss of (Vin−Vo)×Io occurs in the transistor TR
1
; however, the output current Io is limited to a small value, and the loss is relatively small.
In contrast, an incomplete short circuit, denoted by point B in
FIG. 12
, is a condition in which a resistor with a greater impedance than in the case of a complete short circuit is used to short-circuit the output terminals. At point B, the output current Io is not limited, and the loss of (Vin−Vo)×Io grows large. If the device is in such a condition for an extended period, various inconveniences will entail including breakdown of the device and heating of the printed wire board.
Although the foregoing description clearly predicts the importance of curbing the loss in the output control element when the short circuit is incomplete and an overcurrent is detected, attempt to be compatible with every kind of overcurrent results in restraining the indispensable overcurrent that occurs inevitably during a startup in which the output voltage Vo rises to an operating level. This renders it to difficult to start up the device.
SUMMARY OF THE INVENTION
The present invention has an objective to provide regulated power supply circuits and regulated power supply devices such that no loss develops in the output control element when overcurrent flows, except during a startup.
A regulated power supply circuit in accordance with the present invention, in order to achieve the above objective, includes:
an output control element;
an overcurrent detection circuit for detecting an overcurrent flowing through the output control element;
a cut-off circuit for cutting off an output of the output control element when the overcurrent detection circuit detects the overcurrent; and
a cut-off operation prohibition circuit for prohibiting the cut-off circuit from performing the cut-off operation from an outset to an end of a startup of the regulated power supply circuit.
According to the invention, when the overcurrent detection circuit detects an overcurrent flowing through the output control element, the cut-off circuit operates to cut off the output of the output control element. This way, if an overcurrent is caused by a short circuit, whether complete or incomplete, no current is allowed to run through the output control element, thus developing no loss in the output control element. However, the output voltage will not build up provided that the output of the output control element is cut off in response to an overcurrent that occurs inevitably during a startup. Therefore, the cut-off operation prohibition circuit is adapted to prohibit the cut-off circuit from performing the cut-off operation from the outset to the end of the startup.
Hence, a regulated power supply circuit can be provided which, even if an overcurrent is detected, develops no loss in the output control element during non-startup time.
A regulated power supply device in accordance with the present invention, fabricated by sealing an integrated regulated power supply circuit in a single package, in order to achieve the above objective, is such that the regulated power supply circuit includes:
an output control element;
an overcurrent detection circuit for detecting an overcurrent flowing through the output control element;
a cut-off circuit for cutting off an output of the output control element when the overcurrent detection circuit detects the overcurrent; and
a cut-off operation prohibition circuit for prohibiting the cut-off circuit from performing the cut-off operation from an outset to an end of a startup of the regulated power supply circuit.
According to the invention, the regulated power supply circuit is integrated and sealed in a single package and is therefore can be readily mounted on a printed board.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.


REFERENCES:
patent: 4809122 (1989-02-01), Fitzner
patent: 4831483 (1989-05-01), Matsumura et al.
patent: 5666044 (1997-09-01), Tuozzolo
patent: 5942881 (1999-08-01), Oxada et al.
patent: 6038520 (1994-10-01), None

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