Patent Publication Number: US-9851680-B2

Title: Power supply device and image forming apparatus including power supply device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a power supply device that includes two transformers and outputs voltages obtained by converting an AC voltage of a commercial power source into different voltages by the two transformers. 
     Description of the Related Art 
     In general, for an electronic apparatus, a power supply device with a two-converter configuration which outputs voltages of two systems: a first DC voltage which is necessary for an operation of a central processing unit (CPU) or an integrated circuit (IC) (application specific integrated circuit (ASIC) etc.) for controlling an operation of the electronic apparatus; and a second DC voltage which is necessary for an operation of a motor, a solenoid, or the like, is used. Such a power supply device adopts a configuration which includes two electromagnetic transformers (hereinafter, referred to as transformers) for outputting corresponding DC voltages. The first DC voltage is about 3 V to about 5 V, and the second DC voltage is about 24 V. Accordingly, the second DC voltage is higher than the first DC voltage. The power supply device converts a DC voltage which is obtained by rectifying and smoothing an AC voltage of a commercial power source into the first DC voltage with a first switching power source (hereinafter, referred to as a control-system power source) which includes a first transformer, and outputs the first DC voltage. Meanwhile, the power supply device converts a DC voltage which is obtained by rectifying and smoothing the AC voltage of the commercial power source into the second DC voltage with a second switching power source (hereinafter, referred to as a driving-system power source) which includes a second transformer, and outputs the second DC voltage. 
     As such a power supply device, a power supply device which achieves power saving by stopping a driving-system power source to reduce power consumed by a driving system when an electronic apparatus is in a standby mode (power-saving mode), which is an energy-saving state, has been known (see Japanese Patent Laid-Open No. 2006-311650). 
     With the above-mentioned power supply device having the two-converter configuration, when a power outage occurs or a power supply cable is pulled out, the AC voltage of the commercial power source is interrupted, and the first DC voltage from the control power source to the control system drops, which may cause an unintended operation (false operation) as the electronic apparatus. For example, in the case where the AC voltage of the commercial power source drops while a motor is rotating, it is desirable that at least rotation of the motor stops before functions of a controller stop. However, if power to the controller drops (or stops) before power to the motor stops, power supply to the motor continues in a state in which the controller cannot properly control the motor, and the motor continues unintended rotation. Accordingly, in the case where a power outage occurs or a power supply cable is pulled out, it is necessary for the power supply device with the two-converter configuration to stop the driving-system power source prior to the control-system power source so that an unintended state can be avoided. 
     However, with the power supply device described in Japanese Patent Laid-Open No. 2006-311650, the control-system power source may stop prior to the driving-system power source, as described above. Operation of the power supply device described in Japanese Patent Laid-Open No. 2006-311650 is controlled by a control IC, and voltage needs to be supplied to drive the control IC. The power supply device will be described specifically with reference to  FIG. 10 .  FIG. 10  illustrates a principal part of the power supply device. Reference numeral  101  denotes a control-system power source, and reference numeral  102  denotes a driving-system power source. Reference numeral  200  denotes a controller, and reference numeral  300  denotes a driving-system load. Reference numeral  103  denotes a smoothing capacitor for the control-system power source  101  and the driving-system power source  102 . Reference numeral  107  denotes a control IC for controlling the control-system power source  101 , and reference numeral  108  denotes a control IC for controlling the driving-system power source  102 . Furthermore, an auxiliary winding  109   c  which is wound in a direction opposite the winding direction of a main winding  109   p  of a transformer  109  of the control-system power source  101  (hereinafter, referred to as flyback coupling) is provided. A voltage Vcc 3 , which is obtained by rectifying and smoothing a pulse voltage induced by the auxiliary winding  109   c  with a diode  110  and a capacitor  111 , is used as a driving voltage of the control IC  107  and the control IC  108 . 
     A voltage Vdd which is induced by the auxiliary winding  109   c  is substantially represented by expression (1), where, in  FIG. 10 , an output voltage of the control-system power source  101  is represented by Vout 11 , a forward voltage of a diode  112  is represented by Vfd, the number of windings of a secondary winding  109   s  is represented by Nss, and the number of windings of the auxiliary winding  109   c  is represented by Ndd. 
     
       
         
           
             
               
                 
                   
                     V 
                     dd 
                   
                   ≈ 
                   
                     
                       ( 
                       
                         
                           V 
                           
                             out 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             11 
                           
                         
                         + 
                         
                           V 
                           fd 
                         
                       
                       ) 
                     
                     · 
                     
                       
                         N 
                         dd 
                       
                       
                         N 
                         ss 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The voltage Vdd is rectified and smoothed by the diode  110  and the capacitor  111 , and is supplied as a power supply voltage Vcc 3  of the control IC  107  and the control IC  108 . The control IC  107  and the control IC  108  control a switching operation based on the power supply voltage Vcc 3 . At this time, in the case where the forward voltage of the diode  110  is represented by Vfd 2 , the power supply voltage Vcc 3  is substantially represented by expression (2). 
     
       
         
           
             
               
                 
                   
                     V 
                     
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                       ⁢ 
                       
                           
                       
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                         ⁢ 
                         
                             
                         
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                         2 
                       
                     
                   
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                         ( 
                         
                           
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                       · 
                       
                         
                           N 
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                         ⁢ 
                         
                             
                         
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                         2 
                       
                     
                   
                 
               
               
                 
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                   2 
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     Accordingly, the rectified and smoothed power supply voltage Vcc 3  is substantially proportional to the output voltage Vout 11  of the control-system power source  101 . 
     With the power supply device described in Japanese Patent Laid-Open No. 2006-311650, in the case where a power outage occurs or a power supply cable is pulled out while the electronic apparatus is operating, the power supply voltage Vcc 3  of the control IC  108  does not drop before the output voltage Vout 11  of the control-system power source  101  drops, as represented by expression (2). Only after the output voltage Vout 11  of the control-system power source  101  drops, the power supply voltage Vcc 3  of the control IC  108  of the driving-system power source  102  also drops. However, even when the power supply voltage Vcc 3  drops, the control IC  108  does not quickly stop and is able to continue to operate. Therefore, the driving-system power source  102  maintains an output voltage Vout 22 . Accordingly, the output voltage Vout 11  of the control-system power source  101  first drops, power supply to a motor continues while the driving-system power source  102  is outputting the output voltage Vout 22  in a state in which the motor cannot be controlled properly, and the motor thus continues unintended rotation. Alternatively, an intended operation such as sudden motion of the motor may occur. 
     SUMMARY OF THE INVENTION 
     The present invention causes a driving-system power source to stop prior to a control-system power source in a case where an AC voltage of a commercial power source drops or stops while an electronic apparatus is operating so that a driving-system load does not perform an unintended operation. 
     A power supply device according to the present invention includes a first switching power source configured to include a first transformer to which a voltage obtained by rectifying and smoothing an input AC voltage is supplied and to output a first DC voltage by switching the first transformer; a first controller configured to control a switching operation of the first switching power source; a second switching power source configured to include a second transformer to which a voltage obtained by rectifying and smoothing the AC voltage is input and to output a second DC voltage by switching the second transformer; a second controller configured to control a switching operation of the second switching power source; a primary winding of the first transformer; an auxiliary winding which is wound in the same winding direction as the primary winding; and a voltage generator configured to be connected to the auxiliary winding and to generate a power supply voltage for driving the first controller and the second controller. When the AC voltage drops, the power supply device performs control such that the second controller stops prior to the first controller. 
     An image forming apparatus according to the present invention includes an image forming unit; and a power supply device configured to supply electric power to the image forming apparatus. The power supply device includes a first switching power source configured to include a first transformer to which a voltage obtained by rectifying and smoothing an input AC voltage is supplied and to output a first DC voltage by switching the first transformer, a first controller configured to control a switching operation of the first switching power source, a second switching power source configured to include a second transformer to which a voltage obtained by rectifying and smoothing the AC voltage is input and to output a second DC voltage by switching the second transformer, a second controller configured to control a switching operation of the second switching power source, a primary winding of the first transformer, an auxiliary winding which is wound in the same winding direction as the primary winding, and a voltage generator configured to be connected to the auxiliary winding and to generate a power supply voltage for driving the first controller and the second controller. When the AC voltage drops, the power supply device performs control such that the second controller stops prior to the first controller. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a configuration of a power supply device according to a first embodiment. 
         FIGS. 2A and 2B  illustrate operating waveforms in a case where a power outage of the power supply device according to the first embodiment occurs or a power supply cable is pulled out. 
         FIG. 3  is a circuit diagram illustrating a configuration of a power supply device according to a second embodiment. 
         FIGS. 4A and 4B  illustrate operating waveforms in a case where a power outage of the power supply device according to the second embodiment occurs or a power supply cable is pulled out. 
         FIG. 5  is a circuit diagram illustrating a configuration of a power supply device according to a third embodiment. 
         FIG. 6  illustrates operating waveforms in a case where a power outage of the power supply device according to the third embodiment occurs or a power supply cable is pulled out. 
         FIG. 7  is a circuit diagram illustrating a configuration of a power supply device according to a fourth embodiment. 
         FIG. 8  is a circuit diagram illustrating a configuration of a power supply device according to a fifth embodiment. 
         FIG. 9  is a diagram for explaining an application example of a power supply device. 
         FIG. 10  is a circuit diagram illustrating a configuration of a power supply device according to a related art. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to drawings. However, components described in the embodiments are merely exemplifications, and the scope of the present invention is not intended to be limited to the embodiments described below. 
     (Example of Apparatus to which Switching Power Source According to Embodiment is Applied) 
     A switching power source which will be described below in each of first to fifth embodiments may be adopted as, for example, a low-voltage power source of an image forming apparatus, that is, a power source for supplying power to a controller which controls an image forming operation and a driving unit such as a motor. First, a configuration of an image forming apparatus to which a switching power source according to each of the first to fifth embodiments is applied will be explained. A switching power source according to an embodiment may be applied not only to an image forming apparatus but also to any apparatus which includes a control-system load and a driving-system load. 
     [Configuration of Image Forming Apparatus] 
     As an example of an image forming apparatus, a schematic configuration of a laser beam printer as an example of an electrophotographic printer is illustrated in  FIG. 9 . A laser beam printer  500  includes a photosensitive drum  511  as an image carrying member on which an electrostatic latent image is formed. The laser beam printer  500  also includes a charging part  517  (charging unit) which uniformly charges the surface of the photosensitive drum  511 , and a developing part  512  (developing unit) which develops the electrostatic latent image formed on the photosensitive drum  511  using toner. A toner image which has been developed on the photosensitive drum  511  is transferred by a transfer part  518  (transfer unit) to a sheet (not illustrated in  FIG. 9 ) as a recording material which is supplied from a cassette  516 , and the toner image which has been transferred to the sheet is fixed by a fixing device  514  and discharged to a tray  515 . The photosensitive drum  511 , the charging part  517 , the developing part  512 , and the transfer part  518  serve as an image forming unit. Furthermore, the laser beam printer  500  includes a power supply device  550 , which will be explained below in the first to fifth embodiments. An image forming apparatus which may adopt the power supply device  550  according to the first to fifth embodiments is not limited to the configuration illustrated in  FIG. 9 , and for example, may be a color image forming apparatus which includes a plurality of image forming units capable of forming images of different colors. A color image forming apparatus may include, for example, a primary transfer part which transfers a toner image on the photosensitive drum  511  to an intermediate transfer belt, and a secondary transfer part which transfers the toner image on the intermediate transfer belt to a sheet. 
     The laser beam printer  500  includes a controller  520  which controls an image forming operation by the image forming unit and a conveyance operation for a sheet. For example, the power supply device  550  according to the first to fifth embodiments described below supplies power to the controller  520 . Furthermore, the power supply device  550  according to the first to fifth embodiments supplies power to a driving unit such as a motor for rotating the photosensitive drum  511  or for driving various rollers for conveying a sheet. 
     First Embodiment 
       FIG. 1  is a circuit diagram illustrating a configuration of a power supply device according to a first embodiment. This circuit has a two-converter configuration which supplies voltages of two systems, that is, a first DC voltage Vout 1  to a control-system load which requires a low voltage for an operation of a CPU, an ASIC, or the like, and a second DC voltage Vout 2  to a driving-system load which requires a high voltage for an operation of a motor, a solenoid, or the like. 
     In general, the first DC voltage Vout 1  is set to be lower than the second DC voltage Vout 2 . For example, setting is generally performed such that Vout 1  is set to DC 3.3 V and Vout 2  is set to DC 24 V or such that Vout 1  is set to DC 1.8 V and Vout 2  is set to DC 12 V. 
     Hereinafter, explanation will be provided by way of example in which Vout 1  is set to DC 3.3 V and Vout 2  is set to DC 24 V. However, the above values of DC voltage are merely an example, and necessary voltages may be selected for various loads to which the voltages are applied. 
     Furthermore, an image forming apparatus according to the first embodiment includes a circuit (not illustrated in  FIG. 1 ) configured to detect a drop of a commercial power source, and is able to detect a drop of an AC voltage caused by a power outage of the commercial power source or pulling out of a power supply cable. The circuit configured to detect a drop or stoppage of the AC voltage of the commercial power source may be, for example, a circuit (zero cross circuit) configured to detect a point at which the AC voltage crosses 0 V (referred to as a zero cross point). Alternatively, the circuit may be a circuit configured to detect the frequency of the AC voltage. The circuit configured to detect the frequency of the AC voltage is also used for controlling power applied to a fixing device in the image forming apparatus. The circuit configured to detect the frequency of the AC voltage is able to detect that a power outage occurs in the case where waveforms cannot be detected for a predetermined period of time. In the case where it is detected that a power outage has occurs, a controller for the CPU, the ASIC, or the like performs control such that predetermined termination processing is performed for the driving-system load for a motor and other circuits which are not illustrated in  FIG. 1 . 
     Referring to  FIG. 1 , reference numeral  20  denotes a first switching power source (hereinafter, referred to as a control-system power source) for supplying a DC voltage Vout 1  to a control-system load, and reference numeral  60  denotes a second switching power source (hereinafter, referred to as a driving-system power source) for supplying a DC voltage Vout 2  to a control-system load. Furthermore, reference numeral  70  denotes a load for the control-system power source  20 , and is a controller for a controller, an ASIC, and the like for controlling an operation of the apparatus. Reference numeral  80  denotes a load (driving-system load) for the driving-system power source  60 , and is a load for a motor, a solenoid, and the like for driving the apparatus. Furthermore, an auxiliary winding  24   b , a diode  25 , a capacitor  26 , and a Zener diode  30  form a voltage generation circuit configured to generate a power supply voltage of a control IC according to the first embodiment. 
     First, an operation of the control-system power source  20  will be described. When an AC voltage is applied from a commercial power source  10 , a voltage which is rectified by a rectifier  11  is charged to a capacitor  13 . When charging to the capacitor  13  starts and the voltage across the terminals of the capacitor  13  increases, a power supply voltage is supplied to a VH terminal of a control IC  22 , which is a first controller, via a starting resistor  21 , and the control IC  22  turns on a field-effect terminal (FET)  23  through an OUT terminal and starts a switching operation. A transformer  24  is wound by a secondary winding  24   s  and the auxiliary winding  24   b  as well as a primary winding  24   p . The secondary winding  24   s  is wound in a direction opposite the winding direction of the primary winding  24   p . The auxiliary winding  24   b  is wound in the same direction as the winding direction of the primary winding  24   p  (hereinafter, referred to as forward coupling). When the FET  23  is turned on, a current flows from the capacitor  13  to the primary winding  24   p  of the transformer  24 , and energy is stored by magnetic flux generated by the current. At this time, the voltage appearing at the secondary winding  24   s  is a voltage which allows an anode side of a diode  31  to be negative, and therefore no current flows. Furthermore, regarding the voltage appearing at the auxiliary winding  24   b , a current flows in the direction in which the capacitor  26  is charged through the diode  25 , and the voltage of the capacitor  26  increases. When the voltage of the capacitor  26  increases, the control IC  22  performs switching inside thereof such that the power supply voltage which has been supplied from the starting resistor  21  is changed to be supplied from the capacitor  26  which is connected to a VCC terminal. This switching processing is performed because a state in which the power supply voltage is consumed from the starting resistor  21  causes a large loss and the efficient is thus reduced. A voltage Vb which is induced by the auxiliary winding  24   b  is substantially represented by expression (3), where the voltage of the capacitor  13  is represented by Vh, the number of windings of the primary winding  24   p  is represented by Np, and the number of windings of the auxiliary winding  24   b  is represented by Nb. 
     
       
         
           
             
               
                 
                   
                     V 
                     b 
                   
                   ≈ 
                   
                     
                       V 
                       h 
                     
                     · 
                     
                       
                         N 
                         b 
                       
                       
                         N 
                         p 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The voltage Vb is rectified and smoothed by the diode  25  and the capacitor  26 , and is supplied as a power supply voltage Vcc 1  to the control IC  22 . Then, the control IC  22  continues to operate based on the power supply voltage Vcc 1 . The power supply voltage Vcc 1  is substantially represented by expression (4), where the forward voltage of the diode  25  is represented by Vf. 
     
       
         
           
             
               
                 
                   
                     V 
                     
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                     - 
                     
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                         h 
                       
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     Accordingly the rectified and smoothed power supply voltage Vcc 1  is substantially proportional to the voltage Vh of the capacitor  13 . 
     The current flowing through the primary winding  24   p  of the transformer  24  is converted into a voltage by a resistor  28 , and is supplied to an IS terminal of the control IC  22 . The control IC  22  turns off the FET  23  at a time when the voltage input to the IS terminal reaches the voltage input to the FB terminal. Then, the voltage of a drain-side terminal of the FET  23  of the primary winding  24   p  increases. Furthermore, as the voltage appearing at the secondary winding  24   s , a voltage which allows the anode side of the diode  31  to be positive appears, and the energy stored in the transformer  24  is discharged. Then, the current flows in the direction in which a capacitor  32  is discharged through the diode  31 , and the voltage of the capacitor  32  increases. In the first embodiment, a control IC which performs pulse-width modulation (PWM) control is adopted, and the control IC  22  turns on the FET  23  so that an operation at a switching frequency corresponding to the FB terminal voltage can be achieved. When the FET  23  is turned on, a current flows again via the primary winding  24   p  of the transformer  24 . The control IC  22  repeatedly turns on and off the FET  23  as described above, and the voltage of the capacitor  32  and the capacitor  26  gradually increases. The voltage of the capacitor  32  is equal to the output voltage Vout 1  of the control-system power source  20 . 
     A Ref terminal of a shunt regulator  35  is connected such that voltages obtained by dividing the output voltage Vout 1  of the control-system power source  20  by resistors  33  and  34  are input. Furthermore, a cathode terminal of the shunt regulator  35  is connected to a light-emitting diode  39  of a photocoupler via a resistor  38 , and a phototransistor  27  of the photocoupler is connected to the FB terminal of the control IC  22 . The voltage of the FB terminal of the control IC  22  varies according to the current discharged by the control IC  22  and flowing through the FB terminal and an operation of a secondary-side feedback circuit and the phototransistor  27 . When the output voltage of the control-system power source  20  drops, the output current of the shunt regulator  35  decreases. Therefore, the amount of light emission of the light-emitting diode  39  decreases, and the current flowing in the phototransistor  27  decreases. Therefore, a capacitor  29  is charged by the power source inside the control IC  22 , and the voltage of the FB terminal thus increases. At this time, the control IC  22  turns off the FET  23  at a point in time when the voltage of the IS terminal reaches the voltage of the FB terminal. Therefore, the increase in the voltage of the FB terminal lengthens the ON time of the FET  23 . In contrast, when the output voltage of the control-system power source  20  increases, the current flowing in the phototransistor  27  increases, electric charges at the capacitor  29  is discharged, and the voltage of the FB terminal thus drops. At this time, the control IC  22  turns off the FET  23  at a point in time when the voltage of the IS terminal reaches the voltage of the FB terminal. Therefore, the decrease in the voltage of the FB terminal shortens the ON time. As described above, the control IC  22  controls the ON time of the FET  23  such that a reference voltage Vref of the shunt regulator  35  is equal to voltages obtained by dividing the output voltage Vout 1  of the control-system power source  20  by the resistors  33  and  34 . Accordingly, a stable first DC voltage Vout 1  is output as an output voltage. 
     Next, an operation of the driving-system power source  60  will be described. For the driving-system power source  60  according to the first embodiment, a control IC  40  which is the same as that used in the control-system power source  20  is used. Therefore, regarding the same functions and operations as those of the control-system power source  20 , for example, reference signs and explanation for a configuration of a transformer, a photocoupler, and the like will be omitted. 
     Supply/stoppage of a voltage to a Vcc terminal of the control IC  40  of the driving-system power source  60  is controlled by the controller  70 , and operation/stoppage of the control IC  40  is controlled. When an operation start signal is output by the controller  70 , a current flows to a light-emitting diode of a photocoupler  43  via a resistor  42 . Then, a phototransistor of the photocoupler  43  is turned on, a base current is supplied to a transistor  45  via a resistor  44 , and the transistor  45  is turned on. Then, upon turning on of the transistor  45 , the voltage Vcc 1  of the control-system power source  20 , which is represented by expression (4), is supplied as a power supply voltage Vcc 2  of the control IC  40  via the Zener diode  30 . At this time, the power supply voltage Vcc 2  is substantially represented by expression (5), where the Zener voltage of the Zener diode  30  is represented by Vz. 
     
       
         
           
             
               
                 
                   
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     Accordingly, the rectified and smoothed voltage Vcc 2  is substantially proportional to the voltage Vh of the capacitor  13 . 
     As described above, there is a voltage difference Vz between the power supply voltage Vcc 1  of the control IC  22  and the power supply voltage Vcc 2  of the control IC  40 , as represented by expressions (4) and (5), and the first embodiment is characterized in that the power supply voltage Vcc 1  is higher than the power supply voltage Vcc 2 . When the voltage Vcc 2  is supplied, the control IC  40  starts a switching operation. Then, by an operation similar to the control-system power source  20 , a stable second DC voltage Vout 2  is output as an output voltage. 
     Next, transition of voltages, with the configuration according to the first embodiment, in the case where a steady state is shifted to a state in which a power outage occurs or a power supply cable is pulled out, will be described with reference to  FIGS. 2A and 2B . In order to make a comparison between effects in the first embodiment and the related art, transition of voltages in the case where the auxiliary winding  109   c  is configured to achieve flyback coupling with respect to the primary winding  109   p  of the transformer  109  in the circuit of the power supply source described with reference to  FIG. 10 , will also be described. 
     First, transition of voltage waveforms and an operation of the power supply device when an AC voltage drops by occurrence of a power outage or pulling out of the plug in the case where the load of the driving-system load  80  in a standby state of the electronic apparatus according to the first embodiment is relatively small, will be described with reference to  FIG. 2A . Before time t 0 , the voltage Vh of the capacitor  13  is stable in a state in which the AC voltage is supplied, and the control-system power source  20  and the driving-system power source  60  perform a normal operation. Therefore, the output voltages Vout 1  and Vout 2  change in a stable manner. For example, in the case where a power outage occurs at time t 0 , supply of the AC voltage from the commercial power source  10  stops, and therefore the voltage Vh of the capacitor  13  drops. At the same time, the power supply voltage Vcc 1  of the control IC  22  and the power supply voltage Vcc 2  of the control IC  40 , which are substantially proportional to the voltage Vh as expressed by expressions (4) and (5), also start to drop. A voltage difference which corresponds to the Zener voltage Vz of the Zener diode  30  occurs between the power supply voltages Vcc 1  and the Vcc 2 , as expressed by expression (5). Furthermore, during a period from time t 0  to time t 1 , the control IC  22  and the control IC  40  may continue to perform control. Therefore, the control IC  22  and the control IC  40  continue to perform a switching operation and output the output voltages Vout 1  and Vout 2 . The power supply voltage Vcc 2  reaches an operation stoppage voltage Vst of the control IC  40  at time t 1 . Therefore, the driving-system power source  60  first stops, and the output voltage Vout 2  starts to drop. Then, the power supply voltage Vcc 1  reaches an operation stoppage voltage Vst of the control IC  22  at time t 2 . Therefore, the control-system power source  20  stops, and the output voltage Vout 1  drops. 
     Next, transition of voltages in the case where a power outage occurs or a power supply cable is pulled out in a power supply device according to the related art will be described with reference to  FIG. 2B . Before time t 0 , the voltage of the capacitor  103  is stable in a state in which the AC voltage is supplied, as in  FIG. 2A , and the control-system power source  101  and the driving-system power source  102  perform a normal operation. Therefore, output voltages Vout 11  and Vout 22  change in a stable manner. For example, in the case where a power outage occurs at time t 0 , supply of the AC voltage from the commercial power source  10  stops, and the voltage of the capacitor  103  drops. Furthermore, during a period from time t 0  to time t 1 , the control IC  107  and the control IC  108  may continue to perform control. Therefore, the control IC  107  and the control IC  108  continue to perform an operation and output the output voltages Vout 11  and Vout 22 . During the period from time t 0  to time t 1 , the power supply voltage Vcc 3  of the control IC  107  and the control IC  108  is substantially proportional to the output voltage Vout 11 , as expressed by expression (2), and therefore the power supply voltage Vcc 3  does not drop, unlike  FIG. 2A . Then, the output voltage Vout 11  cannot be maintained with power stored at the capacitor  103  at time t 1 , the output voltage Vout 11  starts to drop, and the power supply voltage Vcc 3  also stars to drop based on the substantially proportional relationship with the output voltage Vout 11 . Since the electronic apparatus is in the standby mode, the driving-system load  300  does not operate, and does not consume much power. Meanwhile, the control-system power source  101  supplies power to the controller  200 , and therefore the output voltage Vout 11  of the control-system power source  101  drops prior to the driving-system power source  102 . When the power supply voltage of the controller  200  drops and the operation of the controller  200  stops at time t 2 , a driving signal of the driving-system power source  102  is not output. Therefore, the voltage Vcc 3  of the control IC  108  is not supplied, the driving-system power source  102  stops, and the output voltage Vout 22  drops. After time t 2 , the controller  200  is in an operation stoppage state, and the output voltage Vout 22  exhibits a high level to some degree. In this state, the driving-system load  300  is able to operate, and there is a possibility in which an unintended operation such sudden motion of a motor, a solenoid, or the like occurs, as described above. In contrast, as described with reference to  FIG. 2A , the power supply device according to the first embodiment is configured to allow the driving-system power source  60  to stop first, and when the operation of the controller  70  stops, a voltage supplied to the driving-system load  80  drops, and a motor or solenoid does not operate in an unintended manner. 
     As described above, in the case where a power outage occurs or a power supply cable is pulled out while the electronic apparatus is operating, the driving-system power source  60  may be caused to stop prior to the control-system power source  20 . As a voltage generator in the first embodiment, a circuit which adopts a Zener diode for providing a potential difference between the power supply voltages Vcc 1  and Vcc 2  is provided. However, the present invention is not limited to this. For example, as a different configuration which provides a potential difference, a circuit may be configured such that the power supply voltage Vcc 2  is generated from the power supply voltage Vcc 1  by using a series regulator or a shunt regulator. 
     Second Embodiment 
     Regarding the switching power sources illustrated in  FIG. 1  which are described in the first embodiment, supply of an AC voltage may be interrupted by a power outage or the like with a heavy load while the driving-system load  80  is operating. In this case, in order to drive the driving-system load  80 , the driving-system power source  60  continues to supply power for a while by using electric energy stored in the capacitor  13 . Then, the electric energy stored in the capacitor  13  is used by the driving-system power source  60 , and the time to stoppage of the control-system power source  20  is therefore shortened. A second embodiment is characterized in that even in the case where a power outage occurs when the driving-system power source  60  has a heavy load, the time to stoppage of the control-system power source  20  is equivalent to the case where the driving-system power source  60  has a light load. 
       FIG. 3  illustrates a power supply device according to the second embodiment. Features of the configuration similar to those of the power supply device described above with reference to  FIG. 1  will be referred to with the same reference signs, and explanation for those similar features will be omitted.  FIGS. 4A and 4B  illustrate operating waveforms which represent features of switching power sources according to the second embodiment. The configuration illustrated in  FIG. 3  differs from the first embodiment ( FIG. 1 ) in that a diode  12 , which serves as a backflow prevention unit according to the second embodiment, and a capacitor  41  for the driving-system power source  60  are arranged in a rectification and smoothing circuit. That is, the second embodiment is characterized in that individual rectification and smoothing capacitors are arranged for the control-system power source  20  and the driving-system power source  60  so that electric energy stored in the capacitor  13  of the control-system power source  20  does not flow back to the capacitor  41  of the driving-system power source  60 . 
     Referring to  FIG. 3 , when an AC voltage is applied from the commercial power source  10 , the voltage which is rectified by the rectifier  11  is charged to the capacitor  13  via the diode  12 . When the voltage across the terminals of the capacitor  13  increases, power is supplied to the VH terminal of the control IC  22  via the starting resistor  21 , and the control IC  22  turns on the FET  23  through the OUT terminal and starts a switching operation. Then, in accordance with an operation similar to the first embodiment, the control-system power source  20  outputs a stable first DC voltage Vout 1  as an output voltage. 
     Furthermore, when the AC voltage is applied from the commercial power source  10 , the capacitor  41  is charged by the voltage which is rectified by the rectifier  11 . When an operation start signal is output by the controller  70 , the power supply voltage Vcc 2  is supplied to the control IC  40 , and the control IC  40  starts a switching operation. Then, in accordance with an operation similar to the control-system power source  20 , the driving-system power source  60  outputs a stable second DC voltage Vout 2  as an output voltage. 
     Next, an operation according to the second embodiment will be described with reference to  FIGS. 4A and 4B . Explanation for parts similar to those in the first embodiment will be omitted. Transition of voltage waveforms and an operation of the power supply device in the case where a power outage occurs in the second embodiment will be described with reference to  FIG. 4A . Before time t 0 , the voltage Vh of the capacitor  13  which is represented by a solid line and the voltage Vh 2  of the capacitor  41  which is represented by a broken line are maintained stable, and the voltage Vh 2  is expressed by equation (6), where the forward voltage of the diode  12  is represented by Vf 12 .
 
 Vh 2= Vh+Vf 12  (6)
 
     When a power outage occurs at time t 0 , supply of electric charges from the commercial power source stops, and the voltage Vh of the capacitor  13  and the voltage Vh 2  of the capacitor  41  start to drop. In the case where the driving-system power source  60  has a heavy load, a large amount of electric energy stored in the capacitor  41  is discharged to the driving-system load  80 , and electric energy is not supplied from the commercial power source. Therefore, the voltage Vh 2  of the capacitor  41  sharply drops, as expressed by the broken line of  FIG. 3A . Meanwhile, electric energy does not flow back to the capacitor  41  by the diode  12 , and therefore the voltage Vh of the capacitor  13  drops gradually as expressed by the solid line, instead of dropping sharply. Then, in accordance with the drop of the voltage Vh, the voltage Vcc 1  of the control IC  22  and the voltage Vcc 2  of the control IC  40 , which are substantially proportional to the voltage Vh, as represented by expressions (4) and (5), start to drop. Furthermore, during the period from t 0  to t 1 , the control IC  22  and the control IC  40  are able to continue to perform control, and therefore continue to operate and output the output voltages Vout 1  and Vout 2 . When the voltage Vcc 2  reaches the operation stoppage voltage Vst of the control IC  40  at time t 1 , the driving-system power source  60  first stops, and the output voltage Vout 2  starts to drop. After that, when the voltage Vcc 1  reaches the operation stoppage voltage Vst of the control IC  22  at time t 2 , the control-system power source  20  stops, and the output voltage Vout 1  drops. In the second embodiment, with the diode  12  for backflow prevention, the gradient of decrease in the voltage Vh of the capacitor  13  is substantially constant irrespective of whether the driving-system power source  60  has a heavy load or a light load. Therefore, the time t 2 , which is the time from the occurrence of the power outage to stoppage of the control IC  22 , is substantially constant irrespective of the state of the driving-system load  80  of the driving-system power source  60 . 
     Next, in order to explain effects of the second embodiment, a comparison with operating waveforms in the case where the driving-system power source  60  has a heavy load in the configuration according to the first embodiment will be made with reference to  FIG. 4B . Before time t 0 , the voltage Vh of the capacitor  13  is maintained stable, as in the first embodiment, and the control-system power source  20  and the driving-system power source  60  perform a normal operation. Therefore, the output voltages Vout 1  and Vout 2  change in a stable manner. When a power outage occurs at time t 0 , in the case where the driving-system power source  60  has a heavy load, a large amount of electric energy stored in the capacitor  13  is discharged to the driving-system load  80 , and electric energy is not supplied from the commercial power source. Therefore, the voltage Vh drops sharply, as illustrated in  FIG. 4B . In accordance with the drop of the voltage Vh, the voltage Vcc 1  of the control IC  22  an the voltage Vcc 2  of the control IC  40 , which are substantially proportional to the voltage Vh, drop. When the voltage Vcc 2  reaches the operation stoppage voltage Vst of the control IC  40  at time t 1 , the driving-system power source  60  first stops, and the output voltage Vout 2  drops. Then, when the voltage Vcc 1  reaches the operation stoppage voltage Vst of the control IC  22  at time t 2 , the control-system power source  20  stops, and the output voltage Vout 1  drops. Accordingly, in the case where the driving-system power source  60  has a heavy load, the voltage Vh of the capacitor  13  drops sharply due to power supply to the driving-system load after the occurrence of the power outage. Therefore, time t 1 , which is a time from time t 0  at which the power outage occurs to the time at which the control IC  40  reaches the operation stoppage voltage, is shorter than the case illustrated in  FIG. 4A . Accordingly, time t 2 , at which the operation stoppage voltage Vst of the control IC  22  is reached and the output voltage Vout 11  drops, is also shortened. 
     As described above, according to the second embodiment, irrespective of the load state of the driving-system power source  60 , the time to completion of predetermined processing by the control-system power source  20  when a power outage occurs or a power supply cable is pulled out while the electronic apparatus is operating may be secured. Therefore, with the circuit configured to detect a drop of the commercial power source, which is not illustrated in  FIG. 3 , even while the driving-system load  80  of the driving-system power source  60  is operating, operation termination processing for the driving-system load  80  may be completed before functions of the controller  70  stop. 
     Third Embodiment 
       FIG. 5  illustrates a power supply device according to a third embodiment. Features of the configuration similar to those of the power supply devices described above with reference to  FIGS. 1 and 3  will be referred to with the same reference signs, and explanation for those similar features will be omitted.  FIG. 6  illustrates operating waveforms which represent features of switching power sources according to the third embodiment. 
     In the third embodiment, the control IC  22  of the control-system power source  20  and the control IC  40  of the driving-system power source  60  illustrated in  FIG. 5  have different operation stoppage voltages. Specifically, the operation stoppage voltage of the control IC  22 , which is represented by Vst 1 , is 6.5 V, and the operation stoppage voltage of the control IC  40 , which is represented by Vst 2 , is 9 V. As described above, the third embodiment is characterized by a combination of control ICs which have a relationship of Vst 1 &lt;Vst 2 . Furthermore, the third embodiment differs from the second embodiment ( FIG. 3 ) in that the Zener diode  30  is not provided in the third embodiment. Therefore, the voltage Vcc 1  supplied to the control IC  22  of the control-system power source  20  and the voltage Vcc 1  supplied to the control IC  40  of the driving-system power source  60  are substantially the same. 
     First, an operation of the control-system power source  20  will be described. When an AC voltage is applied from the commercial power source  10 , the voltage which is rectified by the rectifier  11  is charged to the capacitor  13  via the diode  12 . When the voltage across the terminals of the capacitor  13  increases, power is supplied to the VH terminal of the control IC  22  via the starting resistor  21 . The control IC  22  turns on the FET  23  through the OUT terminal, and starts a switching operation. Then, by an operation similar to the first embodiment, the control-system power source  20  outputs a stable first DC voltage Vout 1  as an output voltage. 
     Next, an operation of the driving-system power source  60  will be described. In the third embodiment, the driving-system power source  60  is a forward switching power source. When an AC voltage is applied from the sheet supply apparatus  10 , the capacitor  41  is charged by the voltage which is rectified by the rectifier  11 . When an operation start signal is output by the controller  70 , the power supply voltage Vcc 1  is supplied to the control IC  40 . The control IC  40  turns on an FET  46 , and starts a switching operation. A transformer  47  is wound by a secondary winding  47   s  as well as a primary winding  47   p . The secondary winding  47   s  is wound in the same direction as the winding direction of the primary winding  47   p . A pulse voltage which is generated by the secondary winding  47   s  of the transformer  47  is rectified and smoothed by a forward rectification and smoothing circuit which includes diodes  48  and  49 , a choke winding  50 , and a capacitor  51 , and a DC voltage Vout 2  is obtained. Voltages which are obtained by dividing the output voltage Vout 2  of the driving-system power source  60  by resistors  53  and  54  are connected to a Ref terminal of a shunt regulator  52  so that the output voltage Vout 2  exhibits a desired value. Then, a cathode terminal of the shunt regulator  52  is connected to a light-emitting diode  56  of a photocoupler via a resistor  55 , and a phototransistor  57  of the photocoupler is connected to the FB terminal of the control IC  40 . The voltage of the FB terminal of the control IC  40  varies according to the current at the FB terminal which is discharged by the control IC  40  and an operation of a secondary-side feedback circuit and the phototransistor  57 . The variations in the FB voltage serve as a main trigger for the switching duty of the FET  46  and a change in the switching frequency. Accordingly, control for a stable output voltage Vout 2  can be achieved. 
     Next, an operation according to the third embodiment will be described with reference to  FIG. 6 . Explanation for parts similar to those in the first embodiment will be omitted. First, transition of voltage waveforms and an operation of the power supply device in the case where a power outage occurs in the third embodiment will be described with reference to  FIG. 6 . Before time t 0 , the voltage Vh of the capacitor  13  which is represented by a solid line in  FIG. 6  is maintained stable, and the control-system power source  20  and the driving-system power source  60  perform a normal operation. Therefore, the output voltages Vout 1  and Vout 2  change in a stable manner. When a power outage occurs at time t 0 , supply of electric charges from the commercial power source stops, and the voltage Vh of the capacitor  13  drops. At the same time, the voltages Vcc 1  of the control IC  22  and the control IC  40 , which are substantially proportional to the voltage Vh, as represented by expression (4), start to drop. Furthermore, during the time from time t 0  to time t 1 , the control IC  22  and the control IC  40  are able to continue to perform control, and therefore continue to operate and output the output voltages Vout 1  and Vout 2 . When the voltage Vcc 1  reaches the operation stoppage voltage Vst 2 =9 V of the control IC  40  at time t 1 , the driving-system power source  60  stops first, and the output voltage Vout 2  starts to drop. Then, when the voltage Vcc 1  reaches the operation stoppage voltage Vst 1 =6.5 V of the control IC  22  at time t 2 , the control-system power source  20  stops, and the output voltage Vout 1  drops. 
     As described above, by adopting control ICs having different operation stoppage voltages, even if the Zener diode in the second embodiment is removed, the driving-system power source  60  may be stopped prior to the control-system power source  20  in the case where a power outage occurs or a power supply cable is pulled out while the electronic apparatus is operating. In the third embodiment, the control IC  22  and the control IC  40  are explained by way of example. However, the present invention is not limited to this. The same effects may be attained with a combination in which the operation stoppage voltage Vst 1  is lower than the operation stoppage voltage Vst 2 . 
     Fourth Embodiment 
       FIG. 7  illustrates a power supply device according to a fourth embodiment. Features of the configuration similar to those of the power supply devices described above with reference to  FIGS. 1 and 3  will be referred to with the same reference signs, and explanation for those similar features will be omitted. Referring to  FIG. 7 , the auxiliary winding  24   b , the diode  25 , the capacitor  26 , an auxiliary winding  24   c , a diode  58 , and a capacitor  59  form a voltage generator according to the fourth embodiment for generating power supply. The fourth embodiment differs from the second embodiment ( FIG. 3 ) in that the power supply voltage Vcc 2  of the control IC  40  of the driving-system power source  60  is supplied from the auxiliary winding  24   c , which is provided in the fourth embodiment, and the Zener diode  30  is removed. 
     First, an operation of the control-system power source  20  will be described. When an AC voltage is applied from the commercial power source  10 , the voltage which is rectified by the rectifier  11  is charged to the capacitor  13  via the diode  12 . When the voltage across the terminals of the capacitor  13  increases, power is supplied to the VH terminal of the control IC  22  via the starting resistor  21 . The control IC  22  turns on the FET  23  through the OUT terminal, and starts a switching operation. The transformer  24  is wound by the secondary winding  24   s , the auxiliary winding  24   b , and the auxiliary winding  24   c  as well as the primary winding  24   p . The secondary winding  24   s  is wound in a direction opposite the winding direction of the primary winding  24   p . The auxiliary winding  24   b  is wound in the same direction as the winding direction of the primary winding  24   p  (hereinafter, referred to as forward coupling). Furthermore, like the auxiliary winding  24   b , the auxiliary winding  24   c  achieves forward coupling with respect to the primary winding  24   p . When the FET  23  is turned on, a current flows from the capacitor  13  to the primary winding  24   p  of the transformer  24 , and energy is stored by magnetic flux generated by the current. At this time, the voltage appearing at the secondary winding  24   s  is a voltage which allows the anode side of the diode  31  to be negative, and therefore no current flows. Furthermore, regarding the voltage appearing at the auxiliary winding  24   b , a current flows in the direction in which the capacitor  26  is charged through the diode  25 , and the voltage of the capacitor  26  increases. Furthermore, regarding the voltage appearing at the auxiliary winding  24   c , a current flows in the direction in which the capacitor  59  is charged through the diode  58 , and the voltage of the capacitor  59  increases. 
     The voltage Vcc 1  of the Vcc terminal of the control IC  22  is represented by expression (4), which is described above in the first embodiment. The voltage Vc which is induced by the auxiliary winding  24   c  is substantially represented by expression (7), where the number of windings of the auxiliary winding  24   c  is represented by Nc. 
     
       
         
           
             
               
                 
                   
                     V 
                     c 
                   
                   ≈ 
                   
                     
                       V 
                       h 
                     
                     · 
                     
                       
                         N 
                         c 
                       
                       
                         N 
                         p 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     The voltage Vc is rectified and smoothed by the diode  58  and the capacitor  59 , and is supplied as the power supply voltage Vcc 2  to the control IC  40 . The voltage Vcc 2  is substantially represented by expression (8), where the forward voltage of the diode  58  is represented by Vf 58 . 
     
       
         
           
             
               
                 
                   
                     V 
                     
                       cc 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                   ≈ 
                   
                     
                       V 
                       c 
                     
                     - 
                     
                       V 
                       
                         f 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         58 
                       
                     
                   
                   ≈ 
                   
                     
                       
                         V 
                         h 
                       
                       · 
                       
                         
                           N 
                           c 
                         
                         
                           N 
                           p 
                         
                       
                     
                     - 
                     
                       V 
                       
                         f 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         58 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Therefore, like the voltage Vcc 1 , the rectified and smoothed voltage Vcc 2  is also substantially proportional to the voltage Vh of the capacitor  13 . Then, by an operation as in the second embodiment, the control-system power source  20  outputs a stable first DC voltage Vout 1  as an output voltage. 
     Next, an operation of the driving-system power source  60  will be described. The driving-system power source  60  according to the fourth embodiment uses the same control IC  40  as that in the control-system power source  20 . Therefore, the same functions and operations as those of the control-system power source  20  will not be referred to with reference signs, and explanation for those functions and operations will be omitted. The voltage supply state to the Vcc terminal of the control IC  40  of the driving-system power source  60  is controlled by the controller  70 , and operation/stoppage of the control IC  40  may thus be controlled. 
     When an operation start signal is output by the controller  70 , a current flows to a light-emitting diode of the photocoupler  43  via the resistor  42 . Then, the phototransistor is turned on, a base current to the transistor  45  is supplied via the resistor  44 , and the transistor  45  is turned on. The collector of the transistor  45  is connected to a line of the power supply voltage Vcc 2  which is represented by expression (8) of the control-system power source  20 , and when the power supply voltage Vcc 2  is supplied, the control IC  40  starts a switching operation. Then, by an operation similar to the control-system power source  20 , a stable second DC voltage Vout 2  is output as an output voltage. 
     The number of windings of the auxiliary winding  24   b  and the number of windings of the auxiliary winding  24   c  of the transformer  24  have a relationship represented by expression (9).
 
 Nb&gt;Nc   (9)
 
     Therefore, in accordance with expressions (4), (8), and (9), the power supply voltage Vcc 1  of the control IC  22  and the power supply voltage Vcc 2  of the control IC  40  may have the size relationship of Vcc 1 &gt;Vcc 2 . Operating waveforms in the case where a power outage occurs or a power supply cable is pulled out are similar to those described in the second embodiment with reference to  FIG. 4A . Therefore, explanation for the operating waveforms will be omitted. 
     As described above, in the case where a power outage occurs or a power supply cable is pulled out while the electronic apparatus is operating, the driving-system power source  60  may be stopped prior to the control-system power source  20 . Furthermore, by providing separate auxiliary windings, the influence of variations in the voltage caused by an operation of a different control IC is not received, and a more stable power supply line may be achieved. 
     Fifth Embodiment 
       FIG. 8  illustrates a power supply device according to a fifth embodiment. Features of the configuration similar to those of the power supply device described above with reference to  FIG. 1  and  FIGS. 2A and 2B  will be referred to with the same reference signs, and explanation for those similar features will be omitted. In the fifth embodiment, the case where the load capacity of the driving-system load  80  is large and two rectifiers are separately provided, will be described. 
     In the configuration illustrated in  FIG. 8 , in a rectification and smoothing circuit, a rectifier  56  is provided instead of the diode  12 , which is a backflow prevention unit. In the case of the circuit configuration illustrated in  FIG. 8 , the rectifier  11  and the rectifier  56  are separately provided. Thus, the potential on a low level side is different between the capacitor  13  and the capacitor  41  after rectification, and there is a concern in which there is a difference between the GND potential of the control IC  22  and the GND potential of the control IC  40 . Therefore, as illustrated in  FIG. 8 , a terminal that is opposite the terminal connected to the diode  58  of the auxiliary winding  24   c  is connected to the GND terminal of the control IC  40 . Accordingly, the pulse voltage output from the auxiliary winding  24   c  is based on the GND of the control IC  40 , and a stable starting voltage Vcc 2  may be supplied. 
     First, an operation of the control-system power source  20  will be described. When an AC voltage is applied from the commercial power source  10 , the voltage which is rectified by the rectifier  11  is charged to the capacitor  13 . When the voltage across the terminals of the capacitor  13  increases, power is supplied to the VH terminal of the control IC  22  via the starting resistor  21 . The control IC  22  turns on the FET  23  through the OUT terminal, and starts a switching operation. Then, by an operation similar to the fourth embodiment, the control-system power source  20  outputs a stable first DC voltage Vout 1  as an output voltage. 
     Next, an operation of the driving-system power source  60  will be described. When an AC voltage is applied from the commercial power source  10 , the capacitor  41  is charged by the voltage which is rectified by the rectifier  56 . When an operation start signal is output by the controller  70 , the power supply voltage Vcc 2  is supplied to the control IC  40 , and the control IC  40  starts a switching operation. Then, by an operation similar to the control-system power source  20 , the driving-system power source  60  outputs a stable second DC voltage Vout 2  as an output voltage. 
     Operating waveforms in the case where a power outage occurs or a power supply cable is pulled out are similar to those described above in the second embodiment with reference to  FIG. 4A . Therefore, explanation for the operating waveforms will be omitted. As described above, the driving-system power source  60  may be stopped prior to the control-system power source  20  in the case where a power outage occurs or a power supply cable is pulled out while the electronic apparatus is operating. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-132172 filed Jun. 30, 2015, which is hereby incorporated by reference herein in its entirety.