U.S. patent application number 09/535983 was filed with the patent office on 2002-02-07 for power control method and circuit, and power supply unit.
Invention is credited to Nakazawa, Shigeaki, Ozawa, Hidekiyo, Tanaka, Shigeo, Yano, Hidetoshi.
Application Number | 20020015316 09/535983 |
Document ID | / |
Family ID | 17147813 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015316 |
Kind Code |
A1 |
Nakazawa, Shigeaki ; et
al. |
February 7, 2002 |
Power control method and circuit, and power supply unit
Abstract
A power control circuit for controlling an output of a power
supply is provided with a setting section which variably sets a
maximum rated output based on input temperature information.
Inventors: |
Nakazawa, Shigeaki;
(Kawasaki-shi, JP) ; Yano, Hidetoshi;
(Kawasaki-shi, JP) ; Tanaka, Shigeo;
(Kawasaki-shi, JP) ; Ozawa, Hidekiyo;
(Kawasaki-shi, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Family ID: |
17147813 |
Appl. No.: |
09/535983 |
Filed: |
March 27, 2000 |
Current U.S.
Class: |
363/20 |
Current CPC
Class: |
H02M 3/33523
20130101 |
Class at
Publication: |
363/20 |
International
Class: |
H02M 003/335 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 1999 |
JP |
11-246389 |
Claims
WHAT IS CLAIMED IS:
1. A power control method for controlling an output of a power
supply comprising: a setting step variably setting a maximum rated
output based on a temperature.
2. The power control method as claimed in claim 1, wherein an input
to the power supply is an A.C. input or a D.C. input, and further
comprising: a converting step converting the A.C. or D.C. input to
a D.C. output different from said input.
3. The power control method as claimed in claim 1 or 2, wherein
said setting step variably sets a maximum rated current based on
the temperature.
4. A power control circuit for controlling an output of a power
supply, comprising: a setting section variably setting a maximum
rated output based on input temperature information.
5. The power control circuit as claimed in claim 4, wherein an
input to the power supply is an A.C. input or a D.C. input, and
further comprising: a control circuit converting the A.C. or D.C.
input to a D.C. output different from said input.
6. The power control circuit as claimed in claim 4, wherein said
setting section variably sets a maximum rated current based on the
input temperature information.
7. The power control circuit as claimed in claim 4, wherein said
setting section variably sets the maximum rated output based on the
input temperature information and reference information, said
reference information indicating a rated value of an output current
and/or an output voltage.
8. A power supply unit which converts an A.C. or D.C. input from a
power supply to a D.C. output different from said input,
comprising: a power control circuit variably setting a maximum
rated output based on input temperature information.
9. The power supply unit as claimed in claim 8, wherein said power
control circuit variably sets a maximum rated current based on the
input temperature information.
10. The power supply unit as claimed in claim 8, wherein said power
control circuit variably sets the maximum rated output based on the
input temperature information and reference information, said
reference information indicating a rated value of an output current
and/or an output voltage of the power control circuit.
11. The power supply unit as claimed in claim 8, 9 or 10, further
comprising: a temperature detector detecting a temperature and
inputting the input temperature information to said power control
circuit.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to power control
methods and circuits and power supply units, and more particularly
to a power control method and circuit and a power supply unit which
can set a rated output.
[0003] Electronic equipments such as portable or lap-top personal
computers are designed so that it is possible to use a power supply
unit such as an A.C. adapter as its power supply. The power supply
unit is designed to satisfy a specification as a power supply, so
that it is possible to supply a maximum power required by the
electronic equipment.
[0004] Generally, the power supply unit such as the A.C. adapter
supplies a predetermined output voltage and a predetermined output
current, which are referred to as an rated output. The power supply
unit guarantees that a serviceable life of the power supply unit, a
temperature rise of the power supply unit and the like are within
predetermined tolerable ranges even if the power supply unit is
continuously used at the rated output.
[0005] 2. Description of the Related Art
[0006] In the power supply unit such as the A.C. adapter,
restrictions are introduced with respect to the size, cost and the
like of the power supply unit, due to the need to guarantee
continuous operation at the rated output. When continuously
operating the power supply unit at the rated output, the
temperature rise of the power supply unit caused by the heat
generated from a power supply circuit within the power supply unit
in particular becomes a problem.
[0007] On the other hand, when the operation of the electronic
equipment such as the portable or lap-top personal computer is
studied, a continuous operation at the rated output actually does
not occur. For example, circuits within the portable or lap-top
personal computer are made up of various circuit elements, but not
all of these circuit elements operate constantly. The operation of
the portable or lap-top personal computer is dependent upon the
application program which is activated, and the circuits which
operate within the lap-top personal computer differ from time to
time depending on the application program which runs.
[0008] When the portable or lap-top personal computer is using its
communication function such as connection to the internet and
computer-to-computer communication, circuits related to a modem
which connects the portable or lap-top personal computer to the
telephone line operate, but the circuits related to the modem do
not operate when the communication function is not used. Similarly,
when the portable or lap-top personal computer is reading an
application program or is reading or writing data by executing a
program, circuits related to carrying out read and write with
respect to a hard disk drive operate, but the circuits related to
carrying out the read and write with respect to the hard disk drive
do not operate when the program is carrying out numerical
computations or is waiting for an input from an operator of the
portable or lap-top personal computer.
[0009] Accordingly, a power consumption of the portable or lap-top
personal computer changes every moment depending on the operation
of the program, and compared to a maximum power consumption, an
average power consumption of the portable or lap-top personal
computer is considerably low. The average power consumption is on
the order of approximately one-half the maximum power
consumption.
[0010] In order to reduce the size, weight and cost of the A.C.
adapter, for example, it is conceivable to define the rated power
under which the A.C. adapter can continuously operate depending on
the actual state of the power consumption of the portable or
lap-top personal computer, as the rated output according to the
specification of the A.C. adapter, so that it is possible to obtain
from the A.C. adapter a power greater than the rated output for an
extremely limited short time. However, when the A.C. adapter is
constructed so that it is possible to obtain the power greater than
the rated output, the time for which the power greater than the
rated output is used cannot be limited by the design of the
portable or lap-top personal computer which uses the A.C. adapter.
In other words, the power used by the portable or lap-top personal
computer depends greatly on the operating application program and
the manner in which the portable or lap-top personal computer is
used. As a result, it is virtually impossible to guarantee that the
time for which the power greater than the rated output is used will
be limited to an extremely short time. In addition, in a case where
the portable or lap-top personal computer uses an A.C. adapter of a
type different from the A.C. adapter which is originally designed
for use with the portable or lap-top personal computer, it is
impossible to limit the time for which the power greater than the
rated output is used to the extremely short time.
[0011] No problem occurs even if the A.C. adapter is used at the
rated output for a long period of time. However, when the A.C.
adapter is used at the power greater than the rated output for a
long period of time, the A.C. adapter abnormally generates heat due
to the heat generated from the power supply circuit within the A.C.
adapter. When the A.C. adapter abnormally generates heat, the A.C.
adapter may break down or, the A.C. adapter may operate erroneously
and cause the portable or laptop personal computer which uses this
A.C. adapter to operate erroneously or fail. In a worst case, the
A.C. adapter may cause fire due to extreme heat. Therefore, even
though it is possible to reduce the size, weight and cost of the
A.C. adapter by the conceivable method described above, it is
impossible to guarantee stable and sage operation of the A.C.
adapter. Furthermore, the conceivable method is not practical in
that the protection of the portable or lap-top personal computer
which uses the A.C. adapter becomes insufficient.
[0012] According to the conventional power control method, the
maximum power consumption of the electrical equipment which uses
the power supply unit is obtained, by taking the above described
problems into consideration, and the rated output of the power
supply unit is set so that the power supply circuit within the
power supply unit will not abnormally generate heat or break down
even if the electronic equipment operates continuously for a long
period of time at the maximum power consumption. This kind of power
supply unit uses a switching regulator.
[0013] However, when the power supply unit is designed by taking
into consideration the maximum power consumption of the electronic
equipment, there is a problem in that it is difficult to reduce the
size, weight and cost of the power supply unit. In addition, there
is another problem in that it is impossible to guarantee the stable
and safe operation of the power supply unit. Furthermore, there is
still another problem in that it is impossible to sufficiently
protect the electronic equipment which uses the power supply
unit.
SUMMARY OF THE INVENTION
[0014] Accordingly, it is a general object of the present invention
to provide a novel and useful power control method and circuit and
power supply unit, in which the problems described above are
eliminated.
[0015] Another and more specific object of the present invention to
provide a power control method and circuit and a power supply unit,
which can reduce the size, weight and cost of the power supply
unit, guarantee stable and sage operation of the power supply unit,
and sufficiently protect an electronic equipment which uses the
power supply unit.
[0016] Still another object of the present invention is to provide
a power control method for controlling an output of a power supply
comprising a setting step variably setting a maximum rated output
based on a temperature. According to the power control method of
the present invention, an overload state exceeding a rated output
is tolerated within a tolerable time, and a maximum tolerable rated
output is reduced if the overload state continues for a time
exceeding the tolerable time, so that an abnormal temperature rise
of a power supply circuit can be positively prevented using a
relatively simple construction.
[0017] An input to the power supply may be an A.C. input or a D.C.
input, and the power control method may further comprise a
converting step converting the A.C. or D.C. input to a D.C. output
different from said input. In this case, it is possible to realize
an A.C. adapter or a D.C. adapter which is compact and inexpensive,
and also guarantee stable and safe operation.
[0018] A further object of the present invention is to provide a
power control circuit for controlling an output of a power supply,
comprising a setting section variably setting a maximum rated
output based on input temperature information. According to the
power control circuit of the present invention, an overload state
exceeding a rated output is tolerated within a tolerable time, and
a maximum tolerable rated output is reduced if the overload state
continues for a time exceeding the tolerable time, so that an
abnormal temperature rise of a power supply circuit can be
positively prevented using a relatively simple construction.
[0019] Another object of the present invention is to provide a
power supply unit which converts an A.C. or D.C. input from a power
supply to a D.C. output different from said input, comprising a
power control circuit variably setting a maximum rated output based
on input temperature information. According to the power supply
unit of the present invention, an overload state exceeding a rated
output is tolerated within a tolerable time, and a maximum
tolerable rated output is reduced if the overload state continues
for a time exceeding the tolerable time, so that an abnormal
temperature rise of a power supply circuit can be positively
prevented using a relatively simple construction.
[0020] Other objects and further features of the present invention
may be apparent from the following detailed description when read
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a diagram showing a first embodiment of a power
supply unit according to the present invention;
[0022] FIG. 2 is a diagram showing an embodiment of the
construction of a power control circuit of the first
embodiment;
[0023] FIG. 3 is a diagram showing an output characteristic of an
A.C. adapter for a case where a reference voltage which determines
an output current of the A.C. adapter is fixed;
[0024] FIG. 4 is a diagram showing an output characteristic of the
A.C. adapter having a region of an overload state;
[0025] FIG. 5 is a diagram showing a resistance of a thermistor
which changes depending on a temperature;
[0026] FIG. 6 is a diagram showing a change in a reference voltage
which determines an output current of the A.C. adapter depending on
a temperature change;
[0027] FIG. 7 is a diagram showing voltages input to a PWM control
circuit when controlling an output voltage of the A.C. adapter;
[0028] FIG. 8 is a diagram showing voltage input to the PWM control
circuit when controlling an output current of the A.C. adapter;
[0029] FIG. 9 is a diagram showing an output characteristic of the
A.C. adapter in the first embodiment;
[0030] FIG. 10 is a diagram showing an embodiment of the
construction of a power control circuit in a second embodiment of
the power supply unit according to the present invention; and
[0031] FIG. 11 is a diagram showing a third embodiment of the power
supply unit according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] A description will be given of embodiment of a power control
method, a power control circuit and a power supply unit according
to the present invention, by referring to the drawings.
[0033] FIG. 1 is a diagram showing a first embodiment of a power
supply unit according to the present invention. This first
embodiment of the power supply unit employs a first embodiment of a
power control method according to the present invention and a first
embodiment of a power control circuit according to the present
invention. In this first embodiment, the present invention is
applied to an A.C. adapter which converts an A.C. input into a D.C.
output using a switching regulator.
[0034] In FIG. 1, an A.C. adapter includes a rectifying circuit 1
for rectifying a commercial A.C. input, a voltage converter circuit
2 for converting the A.C. input obtained via the rectifying circuit
1 into a D.C. output having a different voltage, a rectifying
circuit 3 for rectifying the D.C. output obtained via the voltage
converter circuit 2, an output control circuit 4 for controlling
the D.C. output obtained via the rectifying circuit 3, a coupler 5
for feeding back a state of the D.C. output obtained via the
rectifying circuit 3 to the voltage converter circuit 2, and a
temperature detector 6 for detecting a temperature within the A.C.
adapter.
[0035] The rectifying circuit 1 includes rectifying diodes D1
through D4, and a smoothing capacitor C1 for smoothing the
rectified A.C. input, which are connected as shown in FIG. 1. The
voltage converter circuit 2 includes a voltage converting
transformer T1, a transistor Tr1 for switching ON/OFF a current
flowing through the transformer T1, and a driving circuit 21 for
controlling the ON/OFF state of the transistor Tr1, which are
connected as shown in FIG. 1.
[0036] The rectifying circuit 3 includes a rectifying diode D5, and
a smoothing capacitor C2 for smoothing a D.C. output which is
obtained from the voltage converter circuit 2 and is rectified by
the rectifying diode D5, which are connected as shown in FIG. 1.
The output control circuit 4 includes a sense resistor RO for
detecting the D.C. output obtained via the rectifying circuit 3,
and a power control circuit 41 for controlling an output current
and an output voltage of the A.C. adapter, which are connected as
shown in FIG. 1. This power control circuit 41 forms a setting
section for variably setting a maximum rated output of the A.C.
adapter based on temperature information.
[0037] The coupler 5 is made up of a known circuit which is
provided to transmit a state of the output control on the secondary
side of the transformer T1 of the voltage converter circuit 2 to
the primary side of the transformer T1. This coupler 5 receives an
output of the power control circuit 41. In order to isolate the
primary side and the secondary side of the transformer T1, the
coupler 5 uses a photo-coupler, for example. As will be described
later, the temperature detector 6 obtains temperature information
by detecting the temperature within the A.C. adapter, and supplies
the temperature information to the power control circuit 41 within
the output control circuit 4. The temperature detector 6 simply
needs to be provided within the A.C. adapter, and the location of
the temperature detector 6 is not limited to a specific location
within the A.C. adapter. However, it is desirable to provide the
temperature detector 6 within the A.C. adapter at a location where
heat is generated when circuits operate, that is, in a vicinity of
the power control circuit 41.
[0038] FIG. 2 is a diagram showing an embodiment of the
construction of the power control circuit 41 within the output
control circuit 4 which is used in this first embodiment. The power
control circuit 41 includes a voltage amplifier AMP1, error
amplifiers ERA1 and ERA2, a triangular wave generator 42, a pulse
width modulation (PWM) control circuit 43, and terminals 44 through
48 which are connected as shown in FIG. 2. In this embodiment, the
power control circuit 41 is formed by a single semiconductor
integrated circuit device, that is, a single semiconductor chip.
This semiconductor chip may also include circuit elements of the
rectifying circuits 1 and 3 and/or the voltage converter circuit
2.
[0039] The terminals 46 and 47 are respectively connected to
terminals of the sense resistor RO, and to an inverting input
terminal and a non-inverting input terminal of the voltage
amplifier AMP1. The voltage amplifier AMP1 measures a voltage drop
caused by a current flowing through the sense resistor R0, and
outputs a voltage proportional to the current flowing through the
sense resistor R0. The error amplifier ERAL compares the output
voltage of the voltage amplifier AMP1 input to the non-inverting
input terminal and a reference voltage e3' input to the inverting
input terminal, and inputs an output error voltage to the PWM
control circuit 43. The reference voltage e3' input to the terminal
44 determines the output current of the A.C. adapter. This
reference voltage e3' is obtained by voltage-dividing a reference
voltage e3 from a reference voltage source e3 by a resistor R1 and
a thermistor Th1. On the other hand, the error amplifier ERA2
compares a voltage from the terminal 47 input to the inverting
input terminal and a reference voltage e2 from the terminal 45
input to the non-inverting input terminal, and inputs an output
error voltage to the PWM control circuit 43. The reference voltage
e2 input to the terminal 45 determines the output voltage of the
A.C. adapter, and is obtained from a reference voltage source
e2.
[0040] The PWM control circuit 43 has two non-inverting input
terminals respectively input with the output error voltages of the
error amplifiers ERAL and ERA2, and one inverting input terminal
input with a triangular wave voltage from the triangular wave
generator 42. The PWM control circuit 43 is a kind of voltage
comparator, and controls an ON-time of an output pulse width
depending on the input voltages. An output pulse voltage of the PWM
control circuit 43 is output from the terminal 48 and is input to
the coupler 5 shown in FIG. 1.
[0041] In this embodiment, the rated output of the A.C. adapter is
16 V/3.0 A, and the output voltage of 16 V is determined by the
reference voltage e2, while the output current of 3.0 A is
basically determined by the reference voltage e31. If it were
assumed for the sake of convenience that the reference voltage e3'
were fixed, the rated output current is limited to 3.0 A if the
load of the electronic equipment or the like connected to the A.C.
adapter assumes an overload state which requires an output current
exceeding 3.0 A, and the output voltage of the A.C. adapter would
decrease. Because the output voltage of the A.C. adapter decreases,
the abnormal heat generation from the A.C. adapter is prevented in
the overload state.
[0042] However, if the reference voltage e31 were fixed, a maximum
current consumable by the electronic equipment which uses the A.C.
adapter would not be able to exceed 3.0 A, since the rated output
of the A.C. adapter is 16 V/3.0 A. In the overload state exceeding
3.0 A, the output voltage of the A.C. adapter would decrease.
Accordingly, an output characteristic of the A.C. adapter becomes
as shown in FIG. 3 if the output current required by the load is
3.0 A or less. On the other hand, if the output current required by
the load exceeds 3.0 A, a region of the overload state indicated by
the hatching in FIG. 4 is generated. If the A.C. adapter were able
to continuously operate for more than a predetermined time in this
region of the overload state, the A.C. adapter would abnormally
generate heat. In FIGS. 3 and 4, the ordinate indicates the output
voltage of the A.C. adapter, and the abscissa indicates the output
current of the A.C. adapter.
[0043] On the other hand, according to this first embodiment, the
reference voltage e3' changes depending on the temperature within
the A.C. adapter. The resistance of the thermistor Th1 changes
depending on the temperature within the A.C. adapter. More
particularly, the resistance of the thermistor Th1 becomes large
when the temperature is low, and the resistance of the thermistor
Th1 becomes small when the temperature is high. As a result, the
reference voltage e31 which is obtained by voltage-dividing the
reference voltage e3 by the resistor R1 and the thermistor Th1
becomes high when the temperature is low and becomes low when the
temperature is high.
[0044] FIG. 5 is a diagram showing the resistance of the thermistor
Th1 which changes depending on the temperature. More particularly,
FIG. 5 shows the changes in a resistance ThR and an output voltage
Th0 of the thermistor Th1 with respect to the temperature
change.
[0045] In addition, FIG. 6 is a diagram showing a change in the
reference voltage e3' which determines the output current of the
A.C. adapter, depending on the temperature change. In FIG. 6, the
ordinate indicates the voltage, and the abscissa indicates the
temperature.
[0046] Therefore, according to this first embodiment, the error
amplifier ERAL outputs a low voltage if the voltage drop at the
sense resistor R0 is larger when compared with the reference
voltage e3', and outputs a high voltage if the voltage drop is
smaller when compared with the reference voltage e31. Moreover, the
error amplifier ERA2 outputs a low voltage if the voltage at the
terminal of the sense resistor R0 connected to the terminal 47 is
larger when compared with the reference voltage e2, and outputs a
high voltage when the voltage at the terminal of the sense resistor
RO connected to the terminal 47 is smaller when compared with the
reference voltage e2.
[0047] Therefore, when controlling the output voltage of the A.C.
adapter, output voltages ERAL and ERA2 of the error amplifiers ERAL
and ERA2 and an output triangular wave voltage of the triangular
wave generator 42 become as shown in FIG. 7(a), and an output
voltage PWM of the PWM control circuit 43 becomes a pulse voltage
as shown in FIG. 7(b). In addition, when controlling the output
current of the A.C. adapter, the output voltages ERA1 and ERA2 of
the error amplifiers ERA1 and ERA2 and the output triangular wave
voltage of the triangular wave generator 42 become as shown in FIG.
8(a), and the output voltage PWM of the PWM control circuit 43
becomes a pulse voltage as shown in FIG. 8(b). In FIGS. 7 and 8,
the ordinate indicates the amplitude of the voltage, and the
abscissa indicates the time.
[0048] Consequently, when the temperature within the A.C. adapter
is low, a maximum rated output current which is usable by exceeding
the rated output current of the A.C. adapter increases. On the
other hand, when the temperature within the A.C. adapter is high,
the reference voltage e3' decreases with the temperature rise, and
the output current of the A.C. adapter is limited based on the
reference voltage e3'. Hence, when the temperature within the A.C.
adapter is high, the maximum rated output current of the A.C.
adapter decreases.
[0049] As a result, an output characteristic of the A.C. adapter
becomes as shown in FIG. 9. In FIG. 9, the ordinate indicates the
output voltage of the A.C. adapter, and the abscissa indicates the
output current of the A.C. adapter. As described above, the rated
output of the A.C. adapter is set in this embodiment, so that the
rated output voltage is 16 V and the rated output current is 3.0 A.
FIG. 9 indicates that the continuous operation of the A.C. adapter
is guaranteed at this rated output, even when the temperature
within the A.C. adapter is 60.degree. C.
[0050] Furthermore, when the temperature within the A.C. adapter is
25.degree. C., even though the original rated output is 16 V/3.0 A,
this embodiment can tolerate a maximum rated current of up to 5.0
A, as the maximum rated output exceeding the rated output. When the
A.C. adapter is continuously used at the maximum rated current of
5.0 A for a predetermined time and the temperature within the A.C.
adapter rises 45.degree. C., the maximum rated current which is
tolerable with respect to the overload decreases to 4.0 A, so as to
operate the A.C. adapter in a direction so as to decrease the
temperature within the A.C. adapter. If the overload state of the
A.C. adapter is eliminated in this state, the temperature rise
within the A.C. adapter stops. On the other hand, if the overload
state is not eliminated but still continues at the point in time
when the temperature within the A.C. adapter rises to 45.degree.
C., the temperature within the A.C. adapter continues to rise, and
the maximum rated current is limited to 3.0 A when the temperature
within the A.C. adapter rises to 60.degree. C. This state where the
output current of the A.C. adapter is 3.0 A corresponds to the
rated output state, and for this reason, the temperature within the
A.C. adapter will not rise above 60.degree. C.
[0051] According to this embodiment, the maximum rated current is
variably set depending on the temperature within the A.C. adapter.
Hence, even if the overload state exceeding the rated current
continues within a tolerable time, the maximum rated current of the
A.C. adapter is maintained. On the other hand, if the overload
state continues exceeding the tolerable time, the maximum rated
current is automatically decreased, so that the temperature within
the A.C. adapter will not increase abnormally.
[0052] In this embodiment and each of the embodiments described
later, the thermistor Th1 is used as the temperature detector 6.
However, the temperature detector 6 is of course not limited to the
thermistor Th1, and any suitable element or device capable of
detecting the temperature may be used as the temperature detector
6. For example, a thermocouple which uses the Seebeck effect, may
be used as the temperature detector 6.
[0053] Next, a description will be given of a second embodiment of
the power supply unit according to the present invention. This
second embodiment of the power supply unit employs a second
embodiment of the power control method according to the present
invention and a second embodiment of the power control circuit
according to the present invention. In this second embodiment, the
present invention is also applied to an A.C. adapter which converts
an A.C. input into a D.C. output using a switching regulator.
[0054] The basic construction of this second embodiment of the
power supply unit is the same as that of the first embodiment shown
in FIG. 1, and a description and illustration thereof will be
omitted. In this second embodiment, a power control circuit 41-1
shown in FIG. 10 is used in place of the power control circuit 41
shown in FIG. 2. In FIG. 10, those parts which are the same as
those corresponding parts in FIG. 2 are designated by the same
reference numerals, and a description thereof will be omitted.
[0055] As shown in FIG. 10, the power control circuit 41-1 of this
second embodiment further includes a terminal 49, and uses a
3-input error amplifier ERA1-1 in place of the error amplifier ERAL
shown in FIG. 2. A reference voltage e1 is input to the terminal
49, and this reference voltage e1 is input to a second
non-inverting input terminal of the error amplifier ERA1-1. Hence,
in the error amplifier ERA1-1, the reference voltage e31 is used
for the comparison with the output voltage of the voltage amplifier
AMP1 when the reference voltage e3' is lower than the reference
voltage e1, and the reference voltage e1 is used for the comparison
with the output voltage of the voltage amplifier AMP1 when the
reference voltage e3' is higher than the reference voltage el. The
output voltage of the voltage amplifier AMP1 which generates a
voltage proportional to the current flowing through the sense
resistor R0 is input to the inverting input terminal of the error
amplifier ERA1-1. As a result, the output current dependent on the
reference voltages input to the non-inverting input terminals of
the error amplifier ERA1-1 becomes the output current of the A.C.
adapter.
[0056] Therefore, when the temperature within the A.C. adapter is
low, the maximum rated output current usable exceeding the rated
output current of the A.C. adapter becomes large, but it limited to
a current determined by the reference voltage e1 at a certain
temperature. On the other hand, when the temperature within the
A.C. adapter is high, the reference voltage e3' decreases with the
temperature rise, and the output voltage of the A.C. adapter is
limited based on the reference voltage e3', thereby making the
maximum rated output current of the A.C. adapter small.
Consequently, the output characteristic of the A.C. adapter becomes
as shown in FIG. 9, similarly to the first embodiment described
above.
[0057] Next, a description will be given of a third embodiment of
the power supply unit according to the present invention. This
third embodiment of the power supply unit employs a third
embodiment of the power control method according to the present
invention and a third embodiment of the power control circuit
according to the present invention. In this third embodiment, the
present invention is applied to a D.C. adapter which converts a
D.C. input to a D.C. output different from the D.C. input.
[0058] FIG. 11 is a diagram showing the third embodiment of the
power supply unit. In FIG. 11, those parts which are the same as
those corresponding parts in FIG. 1 are designated by the same
reference numerals, and a description thereof will be omitted. In
this third embodiment, the D.C. input is obtained from a D.C. power
supply 50 such as an automobile battery. This D.C. input is input
directly to the voltage converter circuit 2. Hence, a D.C. output
different from the D.C. input is obtained form the output control
circuit 4. In other words, this third embodiment basically has the
same construction as the first embodiment described above, except
that the rectifying circuit 1 shown in FIG. 1 is omitted in this
third embodiment.
[0059] Further, the present invention is not limited to these
embodiments, but various variations and modifications may be made
without departing from the scope of the present invention.
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