Switching power source apparatus

Abstract: A power source apparatus is constructed by a first switching power source unit; a second switching power source unit for receiving an output of the first switching power source unit as an input; and a capacitor of a large capacity provided for the output of the first switching power source unit. When the driving of a motor is started, even if the motor enters a state in which a sudden peak current is needed, an electric power is supplied from the capacitor of a large capacity, thereby preventing that a large current flows in an input side of the power source apparatus.

Description: BACKGROUND OF THE INVENTION 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. 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 was 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, an energy is supplied to the choke coil L5, smoothing capacitor C5, and load 505. Since the 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 or the like, 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 load like 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 powers 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 like an 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 the AC is rectified and smoothed and switched through a transformer, thereby supplying an 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 or the like. EP-A-0 503 715 discloses an apparatus comprising the features of the preamble of claim 1. SUMMARY OF THE INVENTION The invention is defined by the features of claim 1. Preferred embodiments are defined in the dependent claims. 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. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a construction of a switching power source apparatus according to the first embodiment of the invention; Figs. 2A, 2B and 2C are block diagrams showing current waveforms of respective portions of the switching power source apparatus; Fig. 3 is a block diagram showing a construction of a switching power source apparatus according to the second embodiment of the invention; Fig. 4 is a block diagram showing a construction of a switching power source apparatus according to the third embodiment of the invention; Fig. 5 is a block diagram showing a construction of a conventional switching power source apparatus; and Figs. 6A and 6B are diagrams showing current waveforms of respective portions of the conventional switching power source apparatus. 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 and a DC voltage is supplied. A power accumulating and smoothing capacitor C1 of a large capacity 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 one 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 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 the 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 the ground through a current detecting resistor R2. A current detecting circuit 15 is connected across the current detecting resistor R2 and an output signal 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 the 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, an energy is supplied to the choke coil L1, power accumulating and smoothing capacitor C1, and second switching power source unit 2. Since the current flows, the energy is accumulated into the choke coil L1. At this time, the diode D1 is OFF. When the switching transistor Q1 is turned off, the 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 or the like 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 that a peak current flows 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 from the output terminals 5 and 6 to the second switching power source unit 2. When the 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, the current flows in the choke coil L2. In this instance, since the current flows, the energy is accumulated in the choke coil L2. When the switching transistor Q2 is turned off, the energy accumulated in the 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 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 like 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 supplied also 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 keep 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 keeping 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, the 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 the 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, the energy is accumulated into 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 D' 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 the overcurrent is set to a slightly low value, thereby suppressing that a peak current flows 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, the energy is supplied to the choke coil L2, smoothing capacitor C2, and load 12. Since the current flows, the 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 like 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 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 lout 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 supplied also 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 keep 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 keeping 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', the 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, an 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. An electric power is supplied to the second switching power source unit 2" connected to the output terminals 5 and 6. In this instance, since the current flows, the 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 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 the current flowing in the resistor R1 and generates a detection signal 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 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 the constant value and the 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 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 the overcurrent is set to a slightly low value, thereby suppressing that a peak current flows 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 the 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 the 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", the 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 keep 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, as a transformer T31 and devices which 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", the stable output is derived in the load 12. The first switching power source unit 1" is not limited to the forward converter but a similar effect is obtained even if it is a flyback converter or a chopper regulator in which a commercially available transformer and a rectifying circuit are provided in an input. As described in detail above, by providing the power accumulating electrolytic capacitor of a large capacity in which the 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 but many modifications and variations are possible within the scope of the appended claims of the invention. A power source apparatus is constructed by a first switching power source unit; a second switching power source unit for receiving an output of the first switching power source unit as an input; and a capacitor of a large capacity provided for the output of the first switching power source unit. When the driving of a motor is started, even if the motor enters a state in which a sudden peak current is needed, an electric power is supplied from the capacitor of a large capacity, thereby preventing that a large current flows in an input side of the power source apparatus.

Claims: A power source apparatus comprising: a first switching power source unit (1) for outputting a constant voltage and a constant current by switching an input power source (Vin), a second switching power source unit (2) for receiving an output of said first switching power source unit (1) as an input power source and outputting a constant voltage to a load (12), and smoothing and accumulating means (C1), connected to the output of said first switching power source unit (1), for smoothing the output of said first switching power source unit (1) in a stationary state and for supplying an electric power to said second switching power source unit (2) when a current flowing in said load (12) increases, characterized in that said first switching power source unit (1) includes first current detection means (8) for detecting the output current, and a first overcurrent protection circuit (7) for performing protection against overcurrent when the current detected by said first current detection means (8) causes the overcurrent, said second switching power source unit (2) includes second current detection means (15) for detecting the output current, and a second overcurrent protection circuit (14) for performing protection against overcurrent when the current detected by said second current detection means (15) causes the overcurrent, and a setting value for the protection against the overcurrent in said first overcurrent protection circuit (7) is set to be lower than a setting value for the protection against the overcurrent in said second overcurrent protection circuit (14). An apparatus according to claim 1, characterized in that said second switching power source unit (2) supplies a current to a motor (12), and said smoothing and accumulating means (C1) supplies the current to the input of said second switching power source unit (2) without increasing the current supply from said first switching power source unit (1) equal to or more than a predetermined amount, when the motor (12) starts rotating. An apparatus according to claim 1, characterized in that said first and second overcurrent protection circuits (7, 14) respectively stop operations of switching elements (Q1, Q2) of said first and second switching power source units (1, 2) when the overcurrents respectively flow. An apparatus according to claim 1, characterized in that each of said first and second switching power source units (1, 2) is a switching power source of a chopper type having a choke coil (L1, L2).

Inventors: MINE RYUTA, MINE, RYUTA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: H02M3
IPC codes: H02M3, H02J1