Power source apparatus having first and second switching power source units

A power source apparatus includes first and second switching power source units. The input of the second switching power source unit is connected to the output of the first switching power source unit. A large capacity capacitor is connected across the output of the first switching power source unit. When a motor connected across the output of the second switching power source unit is started, even if the motor enters a state in which a sudden peak current is needed, electric power is supplied from the large capacity capacitor, thereby preventing large current flow at the input side of the power source apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a switching power source apparatus for controlling 
an output voltage to a constant voltage by switching an input power source 
and, more particularly, to a switching power source apparatus which is 
used for a load such as to temporarily request a large peak current for a 
stationary output current. 
1. Related Background Art 
FIG. 5 is a block diagram showing a construction of a conventional 
switching power source apparatus of a step-down chopper type. In the 
diagram, reference numerals 501 and 502 denote input terminals. A DC power 
source Vin is connected between the input terminals 501 and 502 and a DC 
voltage is supplied. Reference numerals 503 and 504 denote output 
terminals. A load 505 is connected between the output terminals 503 and 
504. Q5 denotes a switching transistor in which a collector is connected 
to one input terminal 501, an emitter is connected to one terminal of a 
choke coil L5 and a cathode of a diode D5, and a base is connected to a 
control circuit 506. 
Another terminal of the choke coil L5 is connected to one terminal of a 
current detecting resistor R5 and a current detecting circuit 507. Another 
terminal of the resistor R5 is connected to the plus side of a smoothing 
capacitor C5, the current detecting circuit 507, a voltage detecting 
circuit 508, and one output terminal 503. An output of the current 
detecting circuit 507 connected between the current detecting resistor R5 
and an output of the voltage detecting circuit 508 connected to one output 
terminal 503 are connected to the control circuit 506. The other input 
terminal 502, the other output terminal 504, an anode side of the diode 
D5, and a cathode side of the smoothing capacitor C5 are commonly 
connected to the ground. 
The operation of the switching power source apparatus of the step-down 
chopper type of such a construction will now be described. 
When the DC power source Vin is supplied from the input terminals 501 and 
502, a PWM (pulse width modulation) control is executed by the control 
circuit 506 on the basis of outputs of the current detecting circuit 507 
and voltage detecting circuit 508 and a pulse which is pulse width 
modulated is supplied to the base of the switching transistor Q5. When the 
pulse is supplied to the base of the switching transistor Q5, the 
switching transistor Q5 is turned on/off. For a period of time during 
which the switching transistor Q5 is ON, energy is supplied to the choke 
coil L5, smoothing capacitor C5, and load 505. Since current flows, the 
energy is accumulated in the choke coil L5. In this instance, the diode D5 
is OFF. 
When the switching transistor Q5 is turned off, the energy accumulated in 
the choke coil L5 is supplied to the load 505 through the diode D5. The 
current detecting circuit 507 detects the current flowing in the current 
detecting resistor R5 and generates a signal to the control circuit 506. 
The voltage detecting circuit 508 detects the voltage (output voltage) at 
one output terminal 503 and generates a signal to the control circuit 506. 
The control circuit 506 changes a duty ratio of the pulse which is 
supplied to the base of the switching transistor Q5 so that an output 
voltage is made constant by a signal from the voltage detecting circuit 
508. Further, when an overvoltage occurs in the output or an overcurrent 
flows in the apparatus by a signal from the current detecting circuit 507, 
the control circuit 506 stops the operation of the switching transistor 
Q5, thereby keeping the output voltage constant and protecting the 
apparatus from the overcurrent and overvoltage. 
A set value to protect against the overcurrent has to be set to a slightly 
large value so as to correspond to a peak output current. 
It is now assumed that the load 505 in FIG. 5 is, for example, a motor 
which is used for a paper feed in a copying apparatus or the like such 
that a sudden rotation is performed after a stop period or a sudden 
rotation and a stop are repeatedly carried out or, even if the motor is 
not a paper feed motor, when the motor is suddenly rotated as in a case 
where a rotational speed suddenly rises from a constant rotating state or 
the like, a very large peak current is requested for a stationary current 
(current when the motor is stopped or when it is rotated at a constant 
speed). For example, it is now assumed that a current as shown in an 
output current Iout in FIG. 6A is requested in the load (when the motor 
load is activated from a stop state, an output current suddenly increases 
and a peak current flows). 
Input/output power in FIG. 5 will now be considered. Since the output power 
suddenly increases, the input power also obviously suddenly increases. In 
this instance, a peak current such as input current Iin in FIG. 6B also 
flows to the input side of the switching power source apparatus. 
Although an explanation using the drawings is omitted, even in case of a 
power source apparatus (forward converter, flyback converter, etc.) in 
which a commercially available AC power source is inputted and AC is 
rectified and smoothed and switched through a transformer, thereby 
supplying electric power to the secondary side, when it is used for the 
load as mentioned above, a peak current flows to the input side. 
In the conventional switching power source apparatus, when a sudden peak 
current is requested in the load (for example, at the time of an 
activation of the motor load or the like), since a large current also 
flows to the input side of the switching power source apparatus, as an 
input power source, a transformer, and circuit elements of the switching 
power source apparatus, those having large capacities have to be used so 
that they can cope with the peak current. There is a problem such that the 
output voltage drops when a temporary abnormality of the input power 
source (voltage drop, power supply interruption, or the like) occurs. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a power source apparatus in 
which the foregoing drawbacks are eliminated. 
Another object of the invention is to provide a switching power source 
apparatus which can reduce capacities of an input power source and each 
element and can obtain a stable output even if a temporary abnormality of 
the input power source occurs. 
The above and other objects and features of the present invention will 
become apparent from the following detailed description and the appended 
claims with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the invention will now be described hereinbelow with 
reference to FIGS. 1, 2A, 2B, 2C, 3 and 4. 
First Embodiment 
The first embodiment of the invention will be first explained with 
reference to FIGS. 1, 2A, 2B and 2C. FIG. 1 is a block diagram showing a 
construction of a switching power source apparatus of a step-up/down type 
chopper according to the first embodiment of the invention. In the 
diagram, reference numeral 1 denotes a first switching power source unit 
and 2 indicates a second switching power source unit. The first switching 
power source unit 1 has input terminals 3 and 4 and output terminals 5 and 
6. The DC power source Vin is connected between the input terminals 3 and 
4 for supplying DC voltage. A large capacity power accumulating and 
smoothing capacitor C1 is connected between the output terminals 5 and 6. 
The first switching power source unit 1 has a switching transistor Q1 in 
which a collector is connected to one input terminal 3, an emitter is 
connected to one terminal of a choke coil L1 and a cathode of a diode D1, 
and a base is connected to a control circuit 7. 
Another terminal of the choke coil L1 is connected to one terminal of a 
current detecting resistor R1 and a current detecting circuit 8. Another 
terminal of the resistor R1 is connected to a plus side of the power 
accumulating and smoothing capacitor C1, the current detecting circuit 8, 
a voltage detecting circuit 9, and the output terminal 5. An output of the 
current detecting circuit 8 connected across the current detecting 
resistor R1 and an output of the voltage detecting circuit 9 connected to 
the output terminals 5 and 6 are connected to the control circuit 7. The 5 
other input terminal 4, the other output terminal 6, an anode side of the 
diode D1, and a cathode side of the power accumulating and smoothing 
capacitor C1 are commonly connected to ground. 
Input terminals of the second switching power source unit 2 are connected 
to the output terminals 5 and 6 of the first switching power source unit 
1. The second switching power source unit 2 has output terminals 10 and 
11. A load 12 is connected between the output terminals 10 and 11. 
Now, assuming that a positive polarity side of the power accumulating and 
smoothing capacitor C1 is set to the terminal 5 side and a negative 
polarity side is set to the terminal 6 side, the circuit of FIG. 1 is 
constructed by: the first switching power source unit 1 in which the 
terminals 3 and 4 are set to inputs and the terminals 5 and 6 are set to 
outputs; the second switching power source unit 2 in which the terminals 5 
and 6 are set to inputs and the terminals 10 and 11 are set to outputs; 
and the power accumulating and smoothing capacitor C1. As a whole, a 
switching power source apparatus of a step-up/down type chopper is formed. 
A positive polarity side of the power accumulating and smoothing capacitor 
C1 is connected to a collector of a switching transistor Q2 through a 
choke coil L2 and is also connected to an anode side of a diode D2. A 
cathode side of the diode D2 is connected to a smoothing capacitor C2, the 
output terminal 10, and the load 12. An emitter of the switching 
transistor Q2 is connected to ground through a current detecting resistor 
R2. A current detecting circuit 15 is connected across the current 
detecting resistor R2 and an output signal from current detecting circuit 
15 is inputted to a control circuit 14. A voltage detecting circuit 16 is 
connected between the output terminals 10 and 11 and its output is 
inputted to the control circuit 14. The input terminal 4, the output 
terminal 11, the anode side of the diode D1, the minus side of the 
smoothing capacitor C2, the minus side of the power accumulating and 
smoothing capacitor C1, and one end of the current detecting resistor R2 
are commonly connected to ground. 
The operation of the switching power source apparatus according to the 
embodiment of the above construction will now be described. When a DC 
power source is supplied from the input terminals 3 and 4, on the basis of 
detection values of the current detecting circuit 8 and voltage detecting 
circuit 9, the switching transistor Q1 is turned on/off by the control 
circuit 7 so that the output current and output voltage in the first 
switching power source unit 1 are set to certain predetermined values. For 
a period of time during which the switching transistor Q1 is ON, energy is 
supplied to the choke coil L1, power accumulating and smoothing capacitor 
C1, and second switching power source unit 2. Since current flows, energy 
is accumulated in the choke coil L1. At this time, the diode D1 is OFF. 
When the switching transistor Q1 is turned off, energy accumulated in the 
choke coil L1 is supplied to the second switching power source unit 2 
through the diode D1. 
The current detecting circuit 8 detects the current flowing in the resistor 
R1 and generates a detection signal to the control circuit 7. The voltage 
detecting circuit 9 detects the voltage (output voltage of the first 
switching power source unit 1) at the output terminal 5 and generates a 
detection signal to the control circuit 7. On the basis of the signals 
from the current detecting circuit 8 and voltage detecting circuit 9, the 
control circuit 7 changes a duty ratio of a driving pulse which is 
supplied to the base of the switching transistor Q1 so that the current 
flowing in the resistor R1 and the voltage between the output terminals 5 
and 6 are set to certain predetermined values, namely, the output of the 
first switching power source unit 1 is set to a constant voltage and 
constant current output, thereby on/off controlling the switching 
transistor Q1. A protection to stop the operation of the switching 
transistor Q1 is performed against an overcurrent and an overvoltage in 
the first switching power source unit 1. 
A set value to protect from the overcurrent is set to a slightly low value, 
thereby suppressing peak current flow in the input current of the first 
switching power source unit 1. The DC power derived between the output 
terminals 5 and 6 as mentioned above is supplied to the second switching 
power source unit 2. When DC power is supplied to the second switching 
power source unit 2, on the basis of the detection values of the current 
detecting circuit 15 and voltage detecting circuit 16, the switching 
transistor Q2 is turned on/off by the control circuit 14 so that the 
output voltage is set to a predetermined value. For a period of time 
during which the switching transistor Q2 is ON, current flows in the choke 
coil L2. In this instance, since current flows, energy is accumulated in 
the choke coil L2. 
When the switching transistor Q2 is turned off, the energy accumulated in 
choke coil L2 passes through the diode D2 and is supplied to the load 12 
connected to the output terminals 10 and 11. In this instance, the voltage 
between the output terminals 10 and 11 is set to a voltage in which a 
counter electromotive force of the choke coil L2 is added to the voltage 
between the output terminals 5 and 6 (output voltage of the first 
switching power source unit 1). The current detecting circuit 15 detects 
the current flowing in the current detecting resistor R2 and generates a 
detection signal to the control circuit 14. The voltage detecting circuit 
16 detects the voltage (output voltage) between the output terminals 10 
and 11 and generates a detection signal which is provided to the control 
circuit 14. On the basis of the signals from the current detecting circuit 
15 and voltage detecting circuit 16, the control circuit 14 on/off 
controls the switching transistor Q2 so that the voltage between the 
output terminals 10 and 11 is set to a predetermined value. 
Namely, the second switching power source unit 2 constant voltage controls 
the output voltage. When the overcurrent and overvoltage in the second 
switching power source unit 2 are detected by the current detecting 
circuit 15 and voltage detecting circuit 16, by stopping the operation of 
the switching transistor Q2 or by another method, the circuit is 
protected. 
It is now assumed that the load 12 in FIG. 1 is a load such as a motor 
which is used for a paper feed in a copying apparatus or the like such 
that a sudden rotation is performed after a stop period or a sudden 
rotation and a stop are repeatedly carried out or, even if the motor is 
not a paper feed motor, when the motor is suddenly rotated as in a case 
where a rotational speed suddenly rises from a constant rotating state or 
the like, a very large peak current is requested for a stationary current 
(current when the motor is stopped or when it is rotated at a constant 
speed). It is a feature of the invention that when a peak current is 
requested, an electric power necessary for the load 12 is supplied from 
the power accumulating and smoothing capacitor C1 instead of the input 
power source from the input terminals 3 and 4. 
The operation in this instance will now be described. For example, it is 
assumed that a current as shown in Iout in FIG. 2A is requested for the 
load 12 (when the motor load is activated from a stop state, the output 
current suddenly increases and the peak current flows). At this time, in 
the second switching power source unit 2, since the output power suddenly 
increases, the input electric power also obviously suddenly increases. 
However, since the output of the first switching power source unit 1 as an 
input unit of the second switching power source unit 2 is always 
controlled to a constant voltage and a constant current, when the load 12 
requests a current such that the input unit of the second switching power 
source unit 2 is set to a certain power or more, the electric power which 
was suddenly increased cannot be supplemented by only the output of the 
first switching power source 1. In such a case, a necessary electric power 
is also supplied from the power accumulating and smoothing capacitor C1 to 
the input unit of the second switching power source unit 2. 
As mentioned above, when a sudden increase in current is requested for the 
load 12, by supplying the electric power to the second switching power 
source 2 from the power accumulating and smoothing capacitor C1 connected 
to the output unit of the first switching power source unit 1 and the 
input unit of the second switching power source unit 2, an output current 
(Ir1) of the first switching power source unit 1 is suppressed to a slight 
increase and the output of the first switching power source unit 1 can 
always maintaining a constant voltage and a constant current. Therefore, 
in the first switching power source unit 1, even when the load current 
(Iout) suddenly increases, the electric power which is supplied from the 
input terminals 3 and 4 doesn't suddenly increase. Namely, even when the 
load current (Iout) suddenly increases, the input current (Iin) is 
suppressed to a slight increase as compared with the increase in load 
current. 
Therefore, there is no need to set the input power source to a large 
capacity in accordance with the peak value of the load current (Iout) and 
it is possible to cope with a temporary sudden increase in current of the 
load while maintaining a small capacity of the input power source. Even 
when a temporary abnormality of the input power source (voltage drop, 
power supply interruption, or the like) occurs, by supplying the electric 
power from the power accumulating and smoothing capacitor C1 to the second 
switching power source unit 2, a stable output is derived for the load 12. 
Second Embodiment 
The second embodiment of the invention will now be described with reference 
to FIG. 3. FIG. 3 is a block diagram showing a construction of a switching 
power source apparatus according to the second embodiment of the 
invention. In the diagram, substantially the same portions as those in 
FIG. 1 regarding the foregoing first embodiment are designated by the same 
reference numerals. 
The embodiment relates to a circuit in which a first switching power source 
unit 1' is constructed by a switching power source of a step-up type 
chopper and a second switching power source 2' is constructed by a 
switching power source of a step-down type chopper. Now, assuming that a 
positive polarity side of the power accumulating and smoothing capacitor 
C1 is set to the output terminal 5 and a negative polarity side is set to 
the output terminal 6, the circuit is constructed by: the first switching 
power source unit 1' in which the input terminals 3 and 4 are set to 
inputs and the output terminals 5 and 6 are set to outputs; and the second 
switching power source unit 2' in which the output terminals 5 and 6 are 
set to inputs and the output terminals 10 and 11 are set to outputs. As a 
whole, a switching power source of a step-up/down type chopper is formed. 
A construction of the first switching power source unit 1' will now be 
described. Reference numerals 3 and 4 denote the input terminals and the 
DC power source Vin is connected thereto. The input terminal 3 is 
connected to a collector of a switching transistor Q1 through the choke 
coil L1 and to an anode side of a diode D1'. A cathode side of the diode 
D1' is connected to the current detecting resistor R1. Another end of the 
current detecting resistor R1 is connected to the output terminal 5, 
namely, the power accumulating and smoothing capacitor C1. The current 
detecting circuit 8 is connected across the current detecting resistor R1 
and its output signal is inputted to the control circuit 7. The voltage 
detecting circuit 9 is connected between the output terminals 5 and 6, 
namely, across the power accumulating and smoothing capacitor C1 and its 
output is inputted to the control circuit 7. 
Since a circuit construction of the second switching power source unit 2' 
is almost the same as the foregoing conventional construction of FIG. 5, 
its detailed description is omitted. The second switching power source 
unit 2' differs from the circuit construction of the conventional one with 
respect to a point that a power source to be inputted is not a DC power 
source but is an output of the first switching power source unit 1'. The 
input terminal 4, output terminal 11, an anode side of a diode D2', the 
cathode side of the smoothing capacitor C2, the cathode side of the power 
accumulating and smoothing capacitor C1, and the collector of the 
switching transistor Q1' are commonly connected to ground. 
The operation of the switching power source apparatus according to the 
embodiment will now be described. When the DC power source is supplied 
from the input terminals 3 and 4, on the basis of the detection values of 
the current detecting circuit 8 and voltage detecting circuit 9, the 
switching transistor Q1' is turned on/off by the control circuit 7 so that 
the output current (current flowing in the resistor R1) and an output 
voltage (voltage between the output terminals 5 and 6) are set to certain 
predetermined values in the first switching power unit 1'. For a period of 
time during which the switching transistor Q1' is ON, the current flows in 
the choke coil L1. In this instance, since the current flows, energy is 
accumulated in the choke coil L1. 
When the switching transistor Q1' is subsequently turned off, the energy 
accumulated in the choke coil L1 passes through the diode D1' and is 
supplied to the load (second switching power source unit 2') connected to 
the output terminals 5 and 6. In this instance, the voltage between the 
output terminals 5 and 6 of the first switching power source unit 1' is 
set to a voltage in which a counter electromotive force of the choke coil 
L1 is added to the input voltage from the DC power source connected 
between the input terminals 3 and 4. The current detecting circuit 8 
detects the current flowing in the current detecting resistor R1 and 
generates a detection signal to the control circuit 7. The voltage 
detecting circuit 9 detects the voltage (output voltage) between the 
output terminals 10 and 11 and generates a detection signal to the control 
circuit 7. 
On the basis of the signals from the current detecting circuit 7 and 
voltage detecting circuit 9, the control circuit 7 changes a duty ratio of 
a driving pulse which is supplied to a base of the switching transistor 
Q1' so that the voltage between the output terminals 5 and 6 is set to a 
certain predetermined value and the current flowing in the current 
detecting resistor R1 is set to a certain predetermined value, namely, the 
output of the first switching power source unit 1' is set to a constant 
voltage and a constant current output, thereby on/off controlling the 
switching transistor Q1'. When the current flowing in the current 
detecting resistor R1 and the voltage between the output terminals 5 and 6 
are set to certain predetermined values or more, by stopping the output of 
the control circuit 7 or by another method, the circuit is protected. A 
set value for protection against overcurrent is set to a slightly low 
value, thereby suppressing peak current flow in the input current of the 
first switching power source unit 1'. 
The operation of the second switching power source unit 2' will now be 
described. When the electric power is supplied from the first switching 
power source unit 1' to the output terminals 5 and 6 as input terminals of 
the second switching power source unit 2', a PWM control is executed by 
the control circuit 14 on the basis of the outputs of the current 
detecting circuit 15 and voltage detecting circuit 16. The pulse is 
supplied to a base of the switching transistor Q2'. When the pulse is 
supplied to the base of the switching transistor Q2', the switching 
transistor Q2' is turned on/off. For a period of time during which the 
switching transistor Q2' is ON, energy is supplied to the choke coil L2, 
smoothing capacitor C2, and load 12. Since current flows, energy is 
accumulated in the choke coil L2. In this instance, the diode D2' is OFF. 
Subsequently, when the switching transistor Q2' is turned off, the energy 
accumulated in the choke coil L2 is supplied to the load 12 through the 
diode D2'. The current detecting circuit 15 detects the current flowing in 
the current detecting resistor R2' and generates a detection signal to the 
control circuit 14. The voltage detecting circuit 16 detects the voltage 
(output voltage) at the output terminal 10 and generates a detection 
signal to the control circuit 14. The control circuit 14 changes a duty 
ratio to turn on/off the switching transistor Q2' so that the output 
voltage is set to be constant by the signal from the voltage detecting 
circuit 16, thereby keeping the output voltage constant. When the 
overcurrent and overvoltage in the second switching power source unit 2' 
are detected by the current detecting circuit 15 and voltage detecting 
circuit 16, by stopping the operation of the switching transistor Q2' or 
by another method, the circuit is protected. 
It is now assumed that the load 12 in FIG. 3 is a load such as a motor 
which is used for a paper feed in a copying apparatus or the like such 
that a sudden rotation is performed after a stop period or a sudden 
rotation and a stop are repeatedly carried out or, even if the motor is 
not a paper feed motor, when the motor is suddenly rotated as in a case 
where a rotational speed suddenly rises from a constant rotating state or 
the like, a very large peak current is requested for a stationary current 
(current when the motor is stopped or when it is rotated at a constant 
speed). It is a feature of the invention that when peak current is 
requested, electric power necessary for the load is supplied from the 
power accumulating and smoothing capacitor C1 instead of the input power 
source from the input terminals 3 and 4. 
The operation in this instance will now be described. For example, it is 
assumed that a current as shown in Iout in FIG. 2A is requested for the 
load (when the motor load is activated from a stop state, the output 
current suddenly increases and the peak current flows). At this time, in 
the second switching power source unit 2', since the output power suddenly 
increases, the input electric power also obviously suddenly increases. 
However, since the output of the first switching power source unit 1' as 
an input unit of the second switching power source unit 2' is always 
controlled to a constant voltage and a constant current, when the load 
requests a current such that the input unit of the second switching power 
source unit 2' is set to a certain power or more, the electric power which 
was suddenly increased cannot be supplemented by only the output of the 
first switching power source unit 1'. In such a case, a necessary electric 
power is also supplied from the power accumulating and smoothing capacitor 
C1 to the input unit of the second switching power source unit 2'. 
As mentioned above, when a sudden increase in current is requested for the 
load, by supplying the electric power to the second switching power source 
2' from the power accumulating and smoothing capacitor C1 connected to the 
output unit of the first switching power source unit 1' and the input unit 
of the second switching power source unit 2', an output current of the 
first switching power source unit 1' is suppressed to a slight increase 
and the output of the first switching power source unit 1' can always 
maintaining a constant voltage and a constant current. Therefore, in the 
first switching power source unit 1', even when the load current suddenly 
increases, the electric power which is supplied from the input terminals 3 
and 4 doesn't suddenly increase. Namely, even when the load current 
suddenly increases, the input current is merely increased as compared with 
the increase in load current. 
Therefore, there is no need to set the input power source to a large 
capacity in accordance with the peak value of the load current and it is 
possible to cope with a temporary sudden increase in current of the load 
while maintaining a small capacity of the input power source. Even when a 
temporary abnormality of the input power source (voltage drop, power 
supply interruption, or the like) occurs, by supplying the electric power 
from the power accumulating and smoothing capacitor C1 to the second 
switching power source unit 2', stable output is derived for the load 12. 
Third Embodiment 
The third embodiment of the invention will now be described with reference 
to FIG. 4. The embodiment relates to a case where the input power source 
is set to a commercially available AC power source and a circuit between 
the input unit and the output unit has to be insulated. 
FIG. 4 is a block diagram showing a construction of a switching power 
source apparatus according to the third embodiment of the invention. In 
the diagram, substantially the same portions as those in FIGS. 1 and 3 
mentioned above are designated by the same reference numerals. 
In the embodiment, a first switching power source unit 1" is constructed by 
a general forward type switching power source. C1 denotes the power 
accumulating and smoothing capacitor and it is connected to the output 
terminals 5 and 6 of the first switching power source unit 1". 
A circuit construction of the first switching power source unit 1" will now 
be described. A commercially available AC power source is connected to the 
input terminals 3 and 4. The input terminals 3 and 4 are connected to a 
diode bridge D32. An output of the diode bridge D32 is connected across a 
smoothing capacitor C33. A cathode side of the smoothing capacitor C33 is 
connected to an emitter of a switching transistor Q1" and is also 
connected to the ground. An anode side of the smoothing capacitor C33 is 
connected to a collector of the switching transistor Q1" through a primary 
side of a transformer T31. A base of the switching transistor Q1" is 
connected to the control circuit 7. One end of a secondary side of the 
transformer T31 is connected to the choke coil L1 through a diode D33 and 
another end is connected to the choke coil L1 through the diode D1. 
Another end of the choke coil L1 is connected to the output terminal 5 of 
the first switching power source unit 1" through the resistor R1 and to 
the anode side of the power accumulating and smoothing capacitor C1. 
The cathode side of the power accumulating and smoothing capacitor C1 is 
connected to the output terminal 6, the anode side of the diode D1, and 
one end of the secondary side of the transformer T31. The current 
detecting circuit 8 is connected across the resistor R1. An output of the 
current detecting circuit 8 is connected to the control circuit 11 through 
a photocoupler 37. The voltage detecting circuit 9 is connected to the 
anode side of the power accumulating and smoothing capacitor C1 and to the 
output terminal 5. An output of the voltage detecting circuit 9 is 
connected to the control circuit 11 through a photocoupler 38. A second 
switching power source unit 2" is constructed by a power source of a 
step-down type chopper. Outputs (both of the output terminals 5 and 6) of 
the first switching power source unit 1" are supplied as inputs of the 
second switching power source unit 2". The output terminals 10 and 11 are 
set to the outputs of the second switching power source unit 2". The load 
12 is connected to the output terminals 10 and 11. 
A description of a circuit construction of the second switching power 
source unit 2" is omitted because it is the same as that of the second 
switching power source unit 2' in FIG. 3 according to the foregoing second 
embodiment. 
The operation of the switching power source apparatus according to the 
embodiment will now be described. 
First, the first switching power source unit 1" will be described. When a 
commercially available AC power source is supplied from the input 
terminals 3 and 4, AC is rectified by the diode bridge D32 and is smoothed 
by the smoothing capacitor C1. On the basis of the detection values of the 
current detecting circuit 8 and voltage detecting circuit 9, the switching 
transistor Q1" is turned on/off by the control circuit 7 so that an output 
current (current flowing in the resistor R1) and an output voltage 
(voltage between the output terminals 5 and 6) are set to certain 
predetermined values in the first switching power source unit 1". For a 
period of time during which the switching transistor Q1" is ON, a current 
flows in the primary side of the transformer T31, a current also flows in 
the secondary side through the diode D33, choke coil L1, and resistor R1, 
and an electric power is accumulated in the power accumulating and 
smoothing capacitor C1. Electric power is supplied to the second switching 
power source unit 2" connected to the output terminals 5 and 6. In this 
instance, since current flows, energy is accumulated in the choke coil L1. 
When the switching transistor Q1" is subsequently turned off, energy 
accumulated in the choke coil L1 is supplied through the diode D1 to the 
load (second switching power source unit 2") connected to the output 
terminals 5 and 6. The current detecting circuit 8 detects current flowing 
in the resistor R1 and generates a detection signal which is provided to 
the control circuit 7 through the photocoupler 37. The voltage detecting 
circuit 9 detects the voltage (output voltage) between the output 
terminals 5 and 6 and generates a detection signal which is provided to 
the control circuit 7 through the photocoupler 38. On the basis of the 
detection signals, the control circuit 7 changes a duty ratio of a driving 
pulse which is supplied to the base of the switching transistor Q1" so 
that the voltage between the output terminals 5 and 6 is set to a constant 
value and current flowing in the resistor R1 is set to the constant value, 
namely, the output of the first switching power source unit 1" is set to a 
constant voltage and constant current is output, thereby on/off 
controlling the switching transistor Q1". When the current flowing in the 
resistor R1 and the voltage between the output terminals 5 and 6 are set 
to the constant values or more, by stopping the output of the control 
circuit 7 or another method, the circuit is protected. The set value for 
protection against overcurrent is set to a slightly low value, thereby 
suppressing peak current flow into the input current of the first 
switching power source unit 1". 
Since the operation of the second switching power source unit 2" is the 
same as that of the second embodiment, its description is omitted. 
In a manner similar to the first and second embodiments, for example, it is 
now assumed that the load requests current as shown in Iout in FIG. 2A 
(when the motor load is activated from a stop state, the output current 
suddenly increases and the peak current flows). In this instance, in the 
second switching power source unit 2", since an output electric power 
suddenly increases, an input electric power also obviously suddenly 
increases. However, since the output of the first switching power source 
unit 1" as an input unit of the second switching power source unit 2" is 
always controlled to the constant voltage and constant current, when the 
load requests a current such that the input unit of the second switching 
power source unit 2" is set to a certain electric power or more, the 
electric power which was suddenly increased cannot be supplemented by only 
the output of the first switching power source unit 1". In such a case, a 
necessary electric power is also supplied to the input unit of the second 
switching power source unit 2" from the power accumulating and smoothing 
capacitor C1. So long as a range where the peak current that is requested 
from the load side can be supplemented by the electric power accumulated 
in the power accumulating and smoothing capacitor C1, the power is not 
suddenly increased for the output current of the first switching power 
source unit 1" and can be always controlled to the constant current. 
As mentioned above, when the load 12 requests a sudden current increase, by 
supplying electric power to the second switching power source unit 2" from 
the power accumulating and smoothing capacitor C1 connected to the output 
unit of the first switching power source unit 1" and the input unit of the 
second switching power source unit 2", output current of the first 
switching power source unit 1" can be suppressed to a slight increase. The 
output of the first switching power source unit 1" can always maintain the 
constant voltage and constant current. Therefore, in the first switching 
power source unit 1", even when the load current suddenly increases, the 
electric power which is supplied from the input terminals 3 and 4 doesn't 
suddenly increase. 
Namely, even when the load current suddenly increases, the input current is 
merely slightly increased as compared with the increase in load current. 
Therefore, since a transformer T31 and devices are used for the first 
switching power source unit 1", there is no need to use the transformer 
and devices each having a large capacity in accordance with the peak value 
of the load current. It is possible to cope with a temporary sudden 
increase in current of the load by a small capacity. Even if a temporary 
abnormality of input power source (voltage drop, power supply 
interruption, or the like) occurs, by supplying the electric power from 
the power accumulating and smoothing capacitor C1 to the second switching 
power source unit 2", a stable output is derived in the load 12. 
The first switching power source unit 1" is not limited to the forward 
converter, a similar effect is obtained even if a flyback converter or a 
chopper regulator in which a commercially available transformer and a 
rectifying circuit are provided in an are used. 
As described in detail above, by providing the power accumulating large 
capacity electrolytic capacitor in which electric power is always 
accumulated and by supplying an electric power from the power accumulating 
electrolytic capacitor to the load when the load current suddenly 
increases, the sudden increase in input current can be suppressed. 
Therefore, in case of a switching power source apparatus of a DC power 
source input type, a power source of a small capacity can be used as a 
power source which is used for an input. In case of a switching power 
source apparatus of a commercially available AC power source input type, 
capacities of the transformer and each device can be reduced. Even for a 
temporary abnormality of the input power source (voltage drop or output 
stop), a stable output is obtained in the load. 
The present invention is not limited to the foregoing embodiments, many 
modifications and variations are possible within the spirit and scope of 
the appended claims of the invention.