Abstract:
A power supply device of the present invention includes a switching element and a switch that operate so that when a primary DC voltage source of the device is in a voltage drop state, the power of a secondary auxiliary voltage source thereof is mixed with the power of the DC voltage source in a balanced manner and supplied to a load. This structure can reduce the maxim amount of power storage necessary for the auxiliary voltage source and thus the number of power storage elements necessary for the auxiliary voltage source. Thus, a smaller power supply device can be provided.

Description:
This Application is a U.S. National Phase Application of PCT International Application PCT/JP07/064333. 
     Technical Field 
     The present invention relates to a power supply device in which voltage fluctuations of a direct-current (DC) voltage source thereof is compensated for. 
     Background Art 
     In order to protect global environment, idling stop, electrically-driven power steering, and electrically-driven turbo systems have been developed in recent years from the viewpoint of improving fuel efficiency particularly in vehicles. When one of a starter, steering motor, and turbo motor is operated in these systems, a large current on the order of 100 A is consumed, thus causing a voltage drop in the DC voltage source made of batteries. A large voltage drop hinders proper operation of such a load to which the DC voltage source supplies power. 
     A power supply device is devised as a method of preventing the influence of such temporary voltage fluctuations of the DC voltage source on the load. For instance, in this power supply device, an auxiliary voltage source is connected in series with the DC voltage source, and the power from the auxiliary voltage source is supplied to the load via a DC/DC converter, at voltage drop. The structure proposed in Patent Document 1 can be applied to the structure of such a power supply device. In the circuit structure of Patent Document 1, in order to supply power to both of a 14-V load and a 42-V load, two types of power supplies, i.e. a DC voltage source and an auxiliary voltage source, are connected in series, and a DC/DC converter transfers power between both voltage sources. When this circuit structure is used to prevent the influence of temporary voltage fluctuations of the DC voltage source on the load as described above, the circuit structure is as shown in  FIG. 4 . 
     With reference to  FIG. 4 , auxiliary voltage source  103  is connected in series with DC voltage source  101  made of batteries. Usable for auxiliary voltage source  103  is a power storage element made of a high-capacity electric double-layer capacitor, a secondary battery, or the like. To load  105 , DC voltage source  101  is coupled via diode  107 . Auxiliary voltage source  103  is also coupled to the load via two-way DC/DC converter  109 . Diode  107  works to block the output of two-way DC/DC converter  109  from flowing back into DC voltage source  101 . 
     The detailed structure of two-way converter  109  is as follows. First switching element  111  and second switching element  113  are connected in series with one end of auxiliary voltage source  103 . The other end of second switching element  113  is connected to the negative terminal of DC voltage source  101 . One end of inductance element  115  is connected to the junction point between first switching element  111  and second switching element  113 . The other end of inductance element  115  is connected to load  105 . 
     First switching element  111  and second switching element  113  are controlled by control circuit  117  so that one of the first and second switching elements is alternately switched on. Control circuit  117  also controls auxiliary voltage source selector switch  119  that switches charging into auxiliary voltage source  103  and discharging therefrom in response to a signal from the external electronic control unit (hereinafter abbreviated as the external ECU, not shown) of a vehicle. Connected to the discharge side terminal of auxiliary voltage source selector switch  119  is first error detection amplifier  121  that outputs the difference between a voltage of load  105  and a predetermined voltage to be supplied to load  105 . On the other hand, second error detection amplifier  123  that outputs the difference between a voltage of auxiliary voltage source  103  and a predetermined voltage to which auxiliary voltage source  103  is to be charged is connected to the charge side terminal of auxiliary voltage source selector switch  119 . 
     Next, a description is provided of the operation of such a power supply device. First, when the ignition switch (not shown) of the vehicle is turned on, the external ECU transmits a charging signal to control circuit  117  so that auxiliary voltage source  103  is charged. In response to this signal, control circuit  117  switches auxiliary voltage source selector switch  119  to the charge side. As a result, two-way DC/DC converter  109  starts to charge the power of DC voltage source  101  to auxiliary voltage source  103 . When the charging makes voltage VC of auxiliary voltage source  103  equal to the predetermined voltage of second error detection amplifier  123 , the converter operates to keep the charged voltage. 
     Next, assume that the above-mentioned system that consumes large current operates, as load  105 . At this time, a discharging signal is transmitted to control circuit  117  from the external ECU. Then, control circuit  117  switches auxiliary voltage source selector switch  119  to the discharge side. As a result, two-way DC/DC converter  109  outputs a voltage to load  105  so that the voltage is equal to the predetermined voltage of first error detection amplifier  121 . With this structure, even when a large current consumption changes voltage VB of DC voltage source  101  from a normal voltage state to a voltage drop state, voltage VL of load  105  is kept substantially equal to the voltage in the normal voltage state. Thus, load  105  can keep normal operation. At this time, VL&gt;VB, and thus diode  107  blocks the power of two-way DC/DC converter  109  from flowing back into DC voltage source  101 . 
     Next, after the completion of the large current consumption, voltage VB of DC voltage source  101  is returned to the normal voltage state. At this time, the external ECU transmits a charging signal to control circuit  117 . In response to this signal, control circuit  117  switches auxiliary voltage source selector switch  119  to the charge side so that the power supplied from auxiliary voltage source  103  to load  105  during the large current consumption is recharged to auxiliary voltage source  103 . Thus, auxiliary voltage source  103  is fully charged. 
     Repeating such operations allows the supply of stable voltage to load  105  and thus the stable operation of load  105 , even at large current consumption. 
     Such a conventional power supply device is capable of supplying a stable voltage to a load even when the voltage of DC voltage source  101  fluctuates. However, there is a problem that a large number of high-capacity power storage elements are required to supply power to load  105  in a voltage drop state of DC voltage source  101 . This problem is described with reference to  FIG. 5 . 
       FIG. 5  is a diagram showing a change of voltage V 1  with time at the junction point between first switching element  111  and second switching element  113  in two-way DC/DC converter  109 . The abscissa axis shows time t; the ordinate axis shows voltage V 1 . In  FIG. 5 , when voltage V 1  is equal to VB+VC, first switching element  111  is switched on. When voltage V 1  is equal to 0, second switching element  113  is switched on. Voltage VL is supplied as a voltage obtained by smoothing voltage V 1  using inductance element  115 . Therefore, when the ratio of switching on first switching element  111  in one on/off cycle (hereinafter referred to as an on/off ratio) is indicated as D, output voltage VL of two-way DC/DC converter  109  is given by the following equation:
   VL=D× ( VB+VC )+(1− D )×0 =D× ( VB+VC )  (1) 
Voltage VL required by load  105  is a fixed value. Thus, the above equation shows that, in order to provide a necessary voltage when voltage VB of DC voltage source  101  drops, on/off ratio D needs to be increased. As a result, the time period during which power is supplied from auxiliary voltage source  103  to load  105  is increased. This necessitates a larger number of high-capacity power storage elements. For this reason, in the conventional structure, a large number of power storage elements increase the size of the power supply device.
 
[Patent Document 1] Japanese Patent Unexamined Publication No. 2002-218667
 
     SUMMARY OF THE INVENTION 
     A power supply device includes a second switching element coupled between an inductance element and the positive terminal of a DC voltage source. With this structure, voltage V 1  at the junction point between a first switching element and the second switching element is VB instead of 0, and thus on/off ratio D can be reduced. As a result, the number of power storage elements necessary for an auxiliary voltage source thereof can be reduced. In this power supply device, when the second switching element is switched on in a voltage drop state of the DC voltage source, voltage VB, which is dropped but not equal to 0, is fed into a two-way DC/DC converter. This structure can reduce the number of power storage elements necessary for the auxiliary voltage source and provide a smaller power supply device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block circuit diagram of a power supply device in accordance with a first exemplary embodiment of the present invention. 
         FIG. 2  is a timing chart showing an operation of the power supply device in accordance with the first exemplary embodiment of the present invention. 
         FIG. 3  is a block circuit diagram of a power supply device in accordance with a second exemplary embodiment of the present invention. 
         FIG. 4  is a block circuit diagram of a conventional power supply device. 
         FIG. 5  is a timing chart showing an operation of the conventional power supply device. 
     
    
    
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           1  Direct-current (DC) voltage source 
           3  Auxiliary voltage source 
           5  Load 
           9  Inductance element 
           11  First switching element 
           13  Second switching element 
           15  Control circuit 
           17  Selector switch 
           19  Auxiliary power source selector switch 
           31  Switch 
           33  Third switching element 
       
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, a description is provided of exemplary embodiments of the present invention, with reference to the accompanying drawings. Herein, a description is provided of a structure in which a voltage conversion is performed by a two-way DC/DC converter on the power from a direct current (DC) voltage source and an auxiliary voltage source and the power is supplied to a load, when the voltage of the DC voltage source is dropped by such an operation of driving the starter of a vehicle. 
     First Exemplary Embodiment 
       FIG. 1  is a block circuit diagram of a power supply device in accordance with the first exemplary embodiment of the present invention.  FIG. 2  is a timing chart, i.e. a diagram showing a change of voltage V 1  with time in the power supply device in accordance with the first exemplary embodiment of the present invention. 
     With reference to  FIG. 1 , auxiliary voltage source  3  is connected in series with DC voltage source  1  made of batteries. Used for auxiliary voltage source  3  is a power storage element made of a high-capacity electric double-layer capacitor particularly excellent in rapid charge/discharge characteristics. Load  5  that consumes power is coupled to the junction point between DC voltage source  1  and auxiliary voltage source  3 , via diode  7 . To one end of load  5 , inductance element  9  is connected. Inductance element  9  works to smooth the power to be supplied to load  5 . 
     First switching element  11  is connected to the other end of inductance element  9  and to one end (the positive side in  FIG. 1 ) of auxiliary voltage source  3 . First switching element  11  intermittently applies the total voltage of DC voltage source  1  and auxiliary voltage source  3  to inductance element  9  by repeating on/off operation. On the other hand, second switching element  13  is coupled to the other end of inductance element  9  and to the other end (negative side in  FIG. 1 ) of auxiliary voltage source  3  via selector switch  17 . Second switching element  13  intermittently applies the voltage of DC voltage source  1  to inductance element  9  by repeating on/off operation. First switching element  11  and second switching element  13  repeat on/off operation such that one of the first and second switching elements is alternately switched on. 
     An on/off ratio between first switching element  11  and second switching element  13  of D is controlled by control circuit  15 . Thus, the power to be supplied to load  5  can be controlled. 
     Also connected to the above structure is selector switch  17  that switches the coupling of second switching element  13  to DC voltage source  1  so that the second switching element is coupled to either positive terminal or negative terminal of DC voltage source  1 . Control circuit  15  controls the switching of selector switch  17 . Selector switch  17  is switched to the negative terminal side during charging of auxiliary voltage source  3 , and to the positive terminal side during discharging from auxiliary voltage source  3 . 
     Control circuit  15  also controls auxiliary voltage source selector switch  19  that switches charging into auxiliary voltage source  3  and discharging therefrom, in response to a signal from the external ECU (not shown). Connected to the discharge side terminal of auxiliary voltage source selector switch  19  is first error detection amplifier  23  that outputs the difference between a voltage of load  5  and first predetermined voltage  21  to be supplied to load  5 . Connected to the charge side terminal of auxiliary voltage source selector switch  19  is second error detection amplifier  27  that outputs the difference between a voltage of auxiliary voltage source  3  and second predetermined voltage  25  to which auxiliary voltage source  3  is to be charged. 
     In this manner, inductance element  9 , first switching element  11 , second switching element  13 , control circuit  15 , auxiliary voltage source selector switch  19 , first error detection amplifier  23 , and second error detection amplifier  27  constitute two-way DC/DC converter  29 . 
     Next, a description is provided of the operation of such a power supply device. First, when the ignition switch (not shown) of the vehicle is turned on, the external ECU transmits a charging signal to control circuit  15  so that auxiliary voltage source  3  is charged. In response to this signal, control circuit  15  switches selector switch  17  and auxiliary voltage source selector switch  19  to the corresponding charge sides. As a result, two-way DC/DC converter  29  operates as an inverting DC/DC converter in which voltage VB of DC voltage source  1  is inverted with respect to the positive terminal of DC voltage source  1  so that auxiliary voltage source  3  connected in series with DC voltage source  1  is charged. Thus, two-way DC/DC converter  29  starts to charge auxiliary voltage source  3  using the power of DC voltage source  1 . When the charging makes voltage VC of auxiliary voltage source  3  equal to second predetermined voltage  25  of second error detection amplifier  27 , the converter operates to keep the charged voltage. The operations up to this step are the same as those in the conventional structure. 
     Next, assume that the starter or another system that consumes large current operates, as load  5 . At this time, a discharging signal is transmitted to control circuit  15  from the external ECU. Then, control circuit  15  switches selector switch  17  and auxiliary voltage source selector switch  19  to the corresponding discharge sides. As a result, two-way DC/DC converter  29  outputs a voltage to load  5  so that the voltage is equal to second predetermined voltage  21  of first error detection amplifier  23 . With this structure, even when a large current consumption changes voltage VB of DC voltage source  1  from a normal voltage state to a voltage drop state, voltage VL of load  5  is kept substantially equal to the voltage in the normal voltage state. Thus, load  5  can keep normal operation. At this time, VL&gt;VB, and thus diode  7  blocks the power of two-way DC/DC converter  29  from flowing back into DC voltage source  1 . 
     Herein, the normal voltage refers to a voltage range covering fluctuations approximately 10% of the rated output voltage of DC voltage source  1 . More specifically, when the rated output voltage of DC voltage source  1  is 12V, voltages equal to or higher than approximately 11V are referred to as a normal voltage state, and voltages lower than approximately 11V are referred to as a voltage drop state. In the normal voltage state, abnormal voltage seldom causes malfunction of the load. In the voltage drop state, abnormal voltage can cause malfunction of the load. 
       FIG. 2  is a diagram showing a change of voltage V 1  with time at the junction point between first switching element  11  and second switching element  13  in two-way DC/DC converter  29 , that is, a timing chart thereof. The abscissa axis shows time t; the ordinate axis shows voltage V 1 . In  FIG. 2 , when voltage V 1  is equal to VB+VC, first switching element  11  is switched on. When voltage V 1  is equal to VB, second switching element  13  is switched on. In the conventional structure, as obvious from the circuit diagram of  FIG. 4 , second switching element  113  is fixedly connected to the negative terminal of DC voltage source  101 . Thus, when second switching element  113  is switched on, voltage V 1 =0. On the other hand, in the first exemplary embodiment, second switching element  13  is coupled to the positive terminal of DC voltage source  1  via selector switch  17  at discharging. Thus, voltage V 1 =VB. Voltage VL is a value obtained by smoothing voltage V 1  using inductance element  9 . Therefore, when the on/off ratio is indicated as D 1 , output voltage VL of two-way DC/DC converter  29  is given by the following equation:
   VL=D 1×( VB+VC )+(1− D 1)× VB=VB+D 1 ×VC   (2) 
Voltage VL required by load  5  is a fixed value. Thus, the comparison between the following two diagrams shows that the time period (the width of the rectangular pulses in  FIG. 2 ) during which first switching element  11  is switched on to supply the required voltage is shorter than that of the conventional structure in  FIG. 5 . This comparison is shown by equations as follows.
 
     According to Equation (1), on/off ratio D of the conventional structure is given by the following equation:
 
 D=VL /( VB+VC )  (3)
 
     On the other hand, according to Equation (2), on/off ratio D 1  of the first exemplary embodiment is given by the following equation:
 
 D 1=( VL−VB )/ VC   (4)
 
     Therefore, when the difference between on/off ratio D and on/off ratio D 1  is indicated as ΔD, and Equation (4) is subtracted from Equation (3), the following equation is obtained.
 
Δ D=VL /( VB+VC )−( VL−VB )/ VC=VB ( VB+VC−VL )/( VC ( VB+VC ))  (5)
 
     In Equation (5), because VB+VC&gt;VL according to  FIG. 2 , VB+VC−VL&gt;0. The other terms are also positive. Thus, difference in on/off ratio ΔD=(D−D 1 )&gt;0. This result shows on/off ratio D 1  of the first exemplary is smaller. The on/off ratio is a ratio of the time period during which the total voltage of DC voltage source  1  and auxiliary voltage source  3  is supplied. Thus, at a smaller off/off ratio, smaller power is supplied from auxiliary voltage source  3 . Therefore, the power supply device of the first exemplary embodiment requires a smaller number of power storage elements in auxiliary voltage source  3 , and can be made smaller in size than the conventional structure. More specifically, when VB=9V, VC=5V, and VL=12V, for instance, on/off ratio D=0.875 and on/off ratio D 1 =0.6 according to Equations (3) and (4). Thus, on/off ratio D 1  of the first exemplary embodiment is approximately 30% smaller than on/off ratio D of the conventional structure. As a result, as described above, the auxiliary voltage source of the first exemplary embodiment can be made approximately 30% smaller in size than that of the conventional structure. 
     In this manner, power is supplied from DC voltage source  1  even when second switching element  13  is switched on. This structure can reduce the power to be supplied from auxiliary voltage source  3 . As a result, this structure can reduce the number of power storage elements and thus the size of the power supply device. 
     Next, after the completion of the large current consumption, voltage VB of DC voltage source  1  is returned to the normal voltage state. At this time, the external ECU transmits a charging signal to control circuit  15 . In response to this signal, control circuit  15  switches selector switch  17  and auxiliary voltage source selector switch  19  to the corresponding charge sides so that the power supplied from auxiliary voltage source  3  to load  5  during the large current consumption is recharged to auxiliary voltage source  3 . Thus, auxiliary voltage source  3  is fully charged. 
     In this manner, even when DC voltage source  1  operates to intermittently repeat the normal voltage state and the voltage drop state, the above charge/discharge operations repeated by two-way DC/DC converter  29  and auxiliary voltage source  3  allow the supply of stable voltage to load  5  and the stable operation of load  5 . 
     The above operations are summarized. First, when DC voltage source  1  is in the normal voltage state, DC voltage source  1  supplies power directly to load  5 . At this time, control circuit  15  switches selector switch  17  to the negative terminal side of DC voltage source  1  and switches auxiliary voltage source selector switch  19  to the side of second error detection amplifier  27 . Thereby, the power of DC voltage source  1  is charged to auxiliary voltage source  3  through inductance element  9 , first switching element  11 , and second switching element  13 . Next, when DC voltage source  1  is brought into the voltage drop state by the large current consumption of the starter or the like, control circuit  15  switches selector switch  17  to the positive terminal side of DC voltage source  1 . Thereby, power is supplied to load  5 . With these operations, even when DC voltage source  1  intermittently repeats the normal voltage state and the voltage drop state, load  5  can keep the stable operation. 
     The above structure and operations can reduce the number of power storage elements in the auxiliary voltage source and thus the size of the power supply device. 
     In the first exemplary embodiment, two-way DC/DC converter  29  is used to charge auxiliary voltage source  3 . However, auxiliary voltage source  3  can be charged by another means. For instance, a buck DC/DC converter having a simpler structure than two-way DC/DC converter  29  can be used. Specifically, in  FIG. 1 , selector switch  17 , auxiliary voltage source selector switch  19 , and second error detection amplifier  27  are eliminated, and second switching element  13  is connected between first switching element  11  and the negative electrode (negative side in  FIG. 1 ) of auxiliary voltage source  3 . Also in this case, when second switching element  13  is switched on, voltage V 1 =VB. This structure can reduce on/off ratio D 1  and the number of power storage elements, and thus the size of the power supply device. Because the buck DC/DC converter is a one-way DC/DC converter, second switching element  13  can be made of a rectifier. 
     Second Exemplary Embodiment 
       FIG. 3  is a block circuit diagram of a power supply device in accordance with the second exemplary embodiment of the present invention. In  FIG. 3 , elements similar to those in  FIG. 1  have the same reference marks, and the detailed descriptions of these elements are omitted. 
     The differences in structure between  FIG. 3  and  FIG. 1  are listed as follows: 
     (1) Second switching element  13  is made of a rectifier (diode). Thus, control circuit  15  controls the power to be supplied to load  5  by changing on/off ratio D 2  of first switching element  11 . 
     (2) Selector switch  17  is eliminated, and switch  31  is provided so that the switch is connected in series with second switching element  13  and switched on when power is supplied to load  5 . The on/off control of switch  31  is made by control circuit  15 . 
     (3) Third switching element  33  is provided so that the switching element is connected between first switching element  11  and the negative terminal of DC voltage source  1 , repeats switching on/off alternately with first switching element  11 , and is normally switched off when power is supplied to load  5 . 
     The structure other than the above is identical with that of the first exemplary embodiment. 
     Next, a description is provided of the operation of the power supply device structured as above. 
     First, when the ignition switch (not shown) of a vehicle is turned on, the external ECU transmits a charging signal to control circuit  15  so that auxiliary voltage source  3  is charged. In response to this signal, control circuit  15  switches auxiliary voltage source selector switch  19  to the charge side, and turns off switch  31 . Thereafter, control circuit  15  controls first switching element  11  and third switching element  33  so that one of the first and third switching elements is alternately switched on. Thereby, in a manner similar to the first exemplary embodiment, two-way DC/DC converter  29  starts to charge auxiliary voltage source  3  using the power of DC voltage source  1 . When the charging makes voltage VC of auxiliary voltage source  3  equal to second predetermined voltage  25  of second error detection amplifier  27 , the converter operates to keep the charged voltage. 
     Next, assume that the starter or another system that consumes large current operates, as load  5 . At this time, a discharging signal is transmitted to control circuit  15  from the external ECU. Then, control circuit  15  switches auxiliary voltage source selector switch  19  to the discharge side, and turns on switch  31 . Further, third switching element  33  is normally switched off. Thereafter, control circuit  15  makes on/off control on first switching element  11  only. Thus, second switching element  13  made of a rectifier is switched off when first switching element  11  is switched on, and second switching element  13  is switched on when first switching element  11  is switched off. As a result, two-way DC/DC converter  29  outputs a voltage to load  5  so that the voltage is equal to second predetermined voltage  21  of first error detection amplifier  23 . Therefore, even when a large current consumption changes voltage VB of DC voltage source  1  from a normal voltage state to a voltage drop state, voltage VL of load  5  is kept substantially equal to the voltage in the normal voltage state. Thus, load  5  can keep normal operation. At this time, VL&gt;VB, and thus diode  7  blocks the power of two-way DC/DC converter  29  from flowing back into DC voltage source  1 . 
     At this time, a change of voltage V 1  with time at the junction point between first switching element  11  and second switching element  13  in two-way DC/DC converter  29  is exactly the same as that shown in  FIG. 2 . Thus, off/off ratio D 2  in the second exemplary embodiment is smaller than conventional on/off ratio D. Therefore, the power supply device of the second exemplary embodiment also requires a smaller number of power storage elements in auxiliary voltage source  3 , and can be made smaller in size than the conventional structure. 
     Also in the second exemplary embodiment, the auxiliary voltage source can be made approximately 30% smaller in size than that of the conventional structure, under the same condition of values in the first exemplary embodiment. 
     In this manner, also in the second exemplary embodiment, power is supplied from the DC voltage source when second switching element  13  is switched on. This structure can reduce the power to be supplied from auxiliary voltage source  3 . As a result, this structure can reduce the number of power storage elements and thus the size of the power supply device. 
     Next, after the completion of the large current consumption, voltage VB of DC voltage source  1  is returned to the normal voltage state. At this time, the external ECU transmits a charging signal to control circuit  15 . In response to this signal, control circuit  15  switches auxiliary voltage source selector switch  19  to the charge side and turns off switch  31  so that the power supplied from auxiliary voltage source  3  to load  5  during the large current consumption is recharged to auxiliary voltage source  3 . Thus, auxiliary voltage source  3  is fully charged. 
     In this manner, even when DC voltage source  1  operates to intermittently repeat the normal voltage state and the voltage drop state, repeating the above charge/discharge operations allows the supply of stable voltage to load  5  and the stable operation of load  5 . 
     The above operations are summarized. First, when DC voltage source  1  is in the normal voltage state, DC voltage source  1  supplies power directly to load  5 . At the same time, control circuit  15  turns off switch  31 , switches auxiliary voltage source selector switch  19  to the side of second error detection amplifier  27 , and switches on/off first switching element  11  and third switching element  33  so that one of the first and third switching elements is alternately turned on. Thereby, the power of DC voltage source  1  is charged to auxiliary voltage source  3 . Next, when DC voltage source  1  is brought into the voltage drop state by the large current consumption of load  5 , control circuit  15  turns on switch  31 , and switches on/off first switching element  11  while keeping third switching element  33  normally off. Thereby, power is supplied to load  5 . With these operations, even when DC voltage source  1  intermittently repeats the normal voltage state and the voltage drop state, load  5  can keep the stable operation. 
     In comparison with the first exemplary embodiment, the second exemplary embodiment is structured so that second switching element  13  made of a rectifier, and switch  31  are added, and selector switch  17  is eliminated. Selector switch  17  for use in the first exemplary embodiment is an externally-controllable three-terminal switch. Such a switch formed by combination of semiconductors has a complicated structure. Thus, the second exemplary embodiment, which does not require selector switch  17 , can implement a power supply device with a simpler structure. 
     The above structure and operations can reduce the number of power storage elements in the auxiliary voltage source and thus the size of the power supply device. 
     In each of the first and second exemplary embodiments, an electric double-layer capacitor is used as a power storage element of auxiliary voltage source  3 . Instead, a high-capacity capacitor, such as an electrochemical capacitor, or a secondary battery can be used. However, from the viewpoints of rapid charge/discharge characteristics and reliability, a high-capacity capacitor is more preferable than a secondary battery. 
     INDUSTRIAL APPLICABILITY 
     A power supply device of the present invention is capable of supplying a voltage to a load from not only the auxiliary voltage source but also the DC voltage source, even at a voltage drop of the DC voltage source. Thus the present invention is useful as a small power supply device or the like that allows the load to keep stable operation even with a smaller number of power storage elements in the auxiliary voltage source.