Abstract:
A power supply apparatus having a fuel cell and a rechargeable battery and a method of controlling power supplied by the apparatus to a load. A DC-DC converter is selectively supplied with power from the fuel cell or from the fuel cell and the battery based on an amount of a load current. The battery is recharged from an output of the DC-DC converter to increase an overall efficiency of the DC-DC converter and the fuel cell when the load current is low. The battery supplements power input to the DC-DC converter when the load current is high or when an output voltage of the fuel cell becomes unstable.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Korean Application No. 2005-33198, filed Apr. 21, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Aspects of the present invention relate to an apparatus for supplying power to a load by converting a voltage output from a fuel cell using a DC-DC converter, and a method of controlling the power supply apparatus, and more particularly, to a power supply apparatus for controlling connections between a rechargeable battery, a fuel cell and a DC-DC converter according to a current flowing through a load, and a method of controlling the same.  
         [0004]     2. Description of the Related Art  
         [0005]     A fuel cell is an electrochemical device which directly converts chemical energy of hydrogen and oxygen contained in a hydrocarbon series substance such as methanol, ethanol, or natural gas, into electrical energy. The energy conversion process of the fuel cell is very efficient and environmentally friendly, and over the last few decades various kinds of fuel cells have been suggested.  
         [0006]     The fuel cell is similar to a general chemical cell in terms of using an oxidation reaction and a deoxidation reaction. However, unlike the chemical cell, in which a cell reaction is performed in a closed system, the fuel cell continuously intakes reaction materials and continuously discharges reaction products.  
         [0007]     Power consumption of a load connected to a power supply apparatus varies. For example, when a cell-phone is connected to the power supply apparatus, the cell-phone consumes a very low power in an idle mode, but a high power during a phone-call, short message transmission, or data access. Since an output voltage of the power supply apparatus varies in response to changes of the power supplied to the load, the power supply apparatus includes a DC-DC converter to maintain a stable output voltage.  
         [0008]      FIG. 1  is a diagram illustrating a correlation between load current and output voltage of a fuel cell in a power supply apparatus. As described above, the power supplied to the load varies according to a state of the load, and the load current supplied by the power supply apparatus varies according to changes of the supply of power. As shown in  FIG. 1 , since the output voltage of the fuel cell decreases as the load current increases, the output voltage of the fuel cell is unstable.  
         [0009]      FIGS. 2A and 2B  are diagrams illustrating a correlation between load current and efficiency of a DC-DC converter in a power supply apparatus using a fuel cell, when a step up DC-DC converter is used to increase a DC voltage output by the fuel cell. As described above, the output voltage of the fuel cell and the efficiency of the DC-DC converter both decrease as the load current increases above a certain value of the load current.  
         [0010]      FIG. 2A  shows the correlation between the load current and the efficiency of the DC-DC converter at rated output voltages of 3.2V, 3.4V, 3.6V, and 3.8V of the fuel cell. As described above, the efficiency of the DC-DC converter decreases as the load current increases.  FIG. 2B  shows the correlation between the load current and the efficiency of the DC-DC converter at rated output voltages of 5V, 6V, and 7.4V of the fuel cell. In these cases, the efficiency of the DC-DC converter also decreases as the load current increases.  
         [0011]     Thus, when power is supplied to a load from a power supply apparatus using a conventional fuel cell, the efficiency of a DC-DC converter included in the power supply apparatus varies according to changes in the power consumed by the load, thereby causing reduced power supply efficiency.  
       SUMMARY OF THE INVENTION  
       [0012]     Aspects of the present invention provide a power supply apparatus for maintaining a high efficiency of a DC-DC converter by controlling connections between a rechargeable battery and a DC-DC converter being supplied by the fuel cell according to a current flowing to a load, and a method of controlling the same.  
         [0013]     According to an aspect of the present invention, there is provided a power supply apparatus comprising: a fuel cell; a rechargeable battery; a DC-DC converter converting a voltage input from the fuel cell or the fuel cell and the rechargeable battery to a voltage to be supplied to a load; a current measurement unit measuring a current which is output from the DC-DC converter and flowing to the load; and a controller controlling a connection between the rechargeable battery and an input and an output of the DC-DC converter and determining whether a voltage is input from the rechargeable battery to the DC-DC converter according to the measured load current, and whether the rechargeable battery is charged using power supplied by the DC-DC converter.  
         [0014]     The controller may connect the rechargeable battery to the output of the DC-DC converter to charge the rechargeable battery using the power supplied by the DC-DC converter if the measured load current is less than a first value.  
         [0015]     The controller may disconnect the rechargeable battery from the input and output of the DC-DC converter if the measured load current is greater than the first value and less than a second value.  
         [0016]     The controller may connect the rechargeable battery to the input of the DC-DC converter to input the voltage from the rechargeable battery to the DC-DC converter if the measured load current is greater than the second value.  
         [0017]     The second value may be set to maintain a predetermined efficiency of the DC-DC converter.  
         [0018]     The controller may comprise: a mode determinator determining a current state of the power supply apparatus to be a first mode if the measured load current is less than the first load current, to be a second mode if the measured load current is greater than the first load current and less than the second value, and to be a third mode if the measured load current is greater than the second value; and a switching controller connecting the rechargeable battery to the output of the DC-DC converter if the current state of the power supply apparatus is the first mode, disconnecting the rechargeable battery from the input and output of the DC-DC converter if the current state of the power supply apparatus is the second mode, and connecting the rechargeable battery to the input of the DC-DC converter if the current state of the power supply apparatus is the third mode.  
         [0019]     The power supply apparatus may further comprise a first voltage measurement unit measuring the output voltage of the rechargeable battery, and the controller may determine whether the rechargeable battery is fully charged using the output voltage of the rechargeable battery measured by the first voltage measurement unit and disconnect the rechargeable battery from the output of the DC-DC converter if the rechargeable battery is fully charged.  
         [0020]     The power supply apparatus may further comprise a second voltage measurement unit measuring the output voltage of the fuel cell, and the controller may determine whether the power of the fuel cell is stable using the output voltage of the fuel cell measured by the second voltage measurement unit, and connect the rechargeable battery to the input of the DC-DC converter if the power of the fuel cell is unstable.  
         [0021]     According to another aspect of the present invention, there is provided a method of controlling a power supply apparatus using a fuel cell, the method comprising: measuring a current which is output from a DC-DC converter and flows to a load; determining whether a voltage is input from the rechargeable battery to the DC-DC converter based on the measured load current, or whether the rechargeable battery is charged using power output by the DC-DC converter; and controlling a connection between the rechargeable battery and an input and an output of the DC-DC converter according to a result of the determining.  
         [0022]     In the determining of whether the voltage is input from the rechargeable battery to the DC-DC converter, if the measured load current is less than a first load current, the rechargeable battery may be charged using the power output by the DC-DC converter. In the controlling of the connection, the rechargeable battery may be connected to the output of the DC-DC converter.  
         [0023]     In the determining of whether the voltage is input from the rechargeable battery to the DC-DC converter, if the measured load current is greater than the first value, the voltage may be input from the rechargeable battery to the DC-DC converter. In the control of the connection, the rechargeable battery may be connected to the input of the DC-DC converter.  
         [0024]     In the determination, a current state of the power supply apparatus may be determined to be a first mode if the measured load current is less than the first value, to be a second mode if the measured load current is greater than the first f value and less than the second value, and to be a third mode if the measured load current is greater than the second value, and in the control of the connection, the rechargeable battery may be connected to the output of the DC-DC converter if the current state of the power supply apparatus is the first mode, the rechargeable battery may be disconnected from the input and output of the DC-DC converter if the current state of the power supply apparatus is the second mode, and the rechargeable battery may be connected to the input of the DC-DC converter if the current state of the power supply apparatus is the third mode.  
         [0025]     The method may further comprise: determining whether the rechargeable battery is fully charged by measuring the output voltage of the rechargeable battery, and if the rechargeable battery is fully charged, disconnecting the rechargeable battery from the output of the DC-DC converter.  
         [0026]     The method may further comprise: determining whether the power of the fuel cell is stable by measuring the output voltage of the fuel cell, and if the power of the fuel cell is unstable, connecting the rechargeable battery to the input of the DC-DC converter.  
         [0027]     According to another aspect of the present invention, there is provided a computer readable medium having recorded thereon a computer readable program for performing a method of controlling a power supply apparatus using a fuel cell.  
         [0028]     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
         [0030]      FIG. 1  is a diagram illustrating the correlation between load current and output voltage of a fuel cell in a power supply apparatus;  
         [0031]      FIGS. 2A and 2B  are diagrams illustrating the correlation between load current and the efficiency of a DC-DC converter in a power supply apparatus using a fuel cell;  
         [0032]      FIG. 3  is a block diagram of a power supply apparatus using a fuel cell according to an embodiment of the present invention;  
         [0033]      FIG. 4  is a diagram illustrating a method of dividing a state of the power supply apparatus into three modes based on the load current;  
         [0034]      FIG. 5  is a block diagram of another power supply apparatus using a fuel cell according to another embodiment of the present invention;  
         [0035]      FIG. 6  is a flowchart illustrating a method of controlling the power supply apparatus using a fuel cell according to the embodiment shown in  FIG. 3 ; and  
         [0036]      FIG. 7  is a flowchart illustrating a method of controlling the other power supply apparatus using a fuel cell according to the embodiment shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0037]     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.  
         [0038]      FIG. 3  is a block diagram of a power supply apparatus using a fuel cell according to an embodiment of the present invention. Referring to  FIG. 3 , the power supply apparatus includes a fuel cell  300 , a rechargeable battery (charge cell)  310 , a switching unit  320  including first, second and third switches  330 ,  340  and  350 , respectively, a DC-DC converter  360 , a controller  370 , and a current measurement unit  380 . An input of the DC-DC converter  360  is connected to the fuel cell  300  using the first switch  330 , and to the rechargeable battery  310  using the second switch  340 . Thus, a connection between the input of the DC-DC converter  360  and the fuel cell  300  is controlled by the first switch  330  and a connection between the DC-DC converter  360  and rechargeable battery  310  is controlled by the second switch  340 .  
         [0039]     The DC-DC converter  360  converts a DC voltage input from the fuel cell  300  and/or a DC voltage from the rechargeable battery  310  to a voltage to supply power to a load  390 . An output of the DC-DC converter  360  is connected to the load  390  to supply the converted DC voltage to the load  390 , and connected to the rechargeable battery  310  using the third switch  350  to supply a current output from the DC-DC converter  360  to the rechargeable battery when the third switch  350  is turned on. The current measurement unit  380  measures the current supplied from the DC-DC converter  360  to the load  390 .  
         [0040]     The controller  370  determines whether the voltages output from the fuel cell  300  and the rechargeable battery  310  are input to the DC-DC converter  360 , and whether the current output from the DC-DC converter  360  is input to the rechargeable battery  310 , by switching the switches  330 ,  340  and  350  based on the measured load current.  
         [0041]     Operation of the power supply apparatus shown in  FIG. 3  will now be described in detail with reference to a method of controlling the power supply apparatus illustrated in  FIG. 6 .  
         [0042]     In operation  600 , the current measurement unit  380  measures the current supplied from the DC-DC converter  360  to the load  390 . Since the load current measured by the current measurement unit  380  is proportional to the power consumed by the load  390 , the load current varies according to changes in the power consumption of the load  390 .  
         [0043]     In operation  610 , the controller  370  receives a value of the load current as measured by the current measurement unit  380  and determines a current state of the power supply apparatus to be a first mode, a second mode, or a third mode, based on the value of the measured load current.  FIG. 4  is a diagram illustrating a method of dividing the current state of the power supply apparatus into the three modes based on the load current. Referring to  FIG. 4 , the controller  370  may determine the current state of the power supply apparatus to be the first mode if the load current is less than a first current I 1 , to be the second mode if the load current is greater than the first current I 1  and less than a second current I 2 , and to be the third mode if the load current is greater than the second current I 2 .  
         [0044]     A method of setting the first and second current values I 1 , and I 2 , which are reference values to determine the modes, will now be described with reference to  FIG. 4 . When it is desired to maintain high efficiency and stability of the power supply apparatus and the fuel cell  300 , if the power output of the fuel cell  300  is small because the load current is small, performance and stability of the fuel cell  300  decrease. Thus, the first current value I 1  may be set to a value of a minimum current necessary to maintain the performance and stability of the fuel cell  300 , according to the characteristics of the fuel cell used. The second current value I 2  may be set at a value of the load current measured by the current measurement unit  380  when the output voltage of the fuel cell  300  is a minimum voltage to maintain the high efficiency required for the DC-DC converter  360 .  
         [0045]     Thus, the mode of the power supply apparatus is determined based on comparing the measured load current with the reference values I 1  and I 2 . Based on the measured load current, the controller  370  determines the mode of operation and generates and outputs signals for switching the switches  330 ,  340  and  350  included in the switching unit  320  according to the determined mode. In the first mode, in operation  620 , the controller  370  generates and outputs signals for turning the first and third switches  330  and  350  on and the second switch  340  off, in order to connect the fuel cell  300  to the input of the DC-DC converter  360  and connect the rechargeable battery  310  to the output of the DC-DC converter  360 . In the first mode, power output from the fuel cell  300  is supplied to the load  390  and the rechargeable battery  310  through the DC-DC converter  360 , thereby charging the rechargeable battery with power output from the DC-DC converter. Thus, in the first mode, the efficiency of the DC-DC converter  360  is maintained by increasing the current output by the DC-DC converter by charging the rechargeable battery  310  from the fuel cell  300 . Thus, a performance decrease of the power supply apparatus due to the low load current is prevented.  
         [0046]     In the second mode, in operation  630 , the controller  370  generates and outputs signals for turning the first switch  330  on and the second and third switches  340  and  350  off, in order to connect the fuel cell  300  to the input I of the DC-DC converter  360  and disconnect the rechargeable battery  310  from the input and output of the DC-DC converter  360 . In the second mode, since the efficiency of the DC-DC converter  360  is maintained, power is supplied to the load  390  from only the fuel cell  300  without using the rechargeable battery  310 .  
         [0047]     In the third mode, in operation  640 , the controller  370  generates and outputs signals for turning the first and second switches  330  and  340  on and the third switch  350  off, in order to connect the fuel cell  300  and the rechargeable battery  310  to the input of the DC-DC converter  360 . In this case, by supplying power from both the fuel cell  300  and the rechargeable battery  310  to the load  390 , even if the power supplied to the load  390  is high, a voltage drop of the fuel cell  300  is prevented.  
         [0048]      FIG. 5  is a block diagram of a power supply apparatus using a fuel cell according to another embodiment of the present invention. The power supply apparatus shown in  FIG. 5  includes the fuel cell  300 , the rechargeable battery  310 , the switching unit  320  including the three switches  330 ,  340  and  350 , the DC-DC converter  360 , a first voltage measurement unit  500 , a second voltage measurement unit  510 , a controller  520 , and the current measurement unit  380 . The operation of the power supply apparatus shown in  FIG. 5  will now be described in detail with reference to a method of controlling the power supply apparatus illustrated in  FIG. 7 .  
         [0049]     In operation  700 , the current measurement unit  380  measures the current supplied from the DC-DC converter  360  to the load  390 . In operation  710 , the controller  520  receives a value of the load current measured by the current measurement unit  380  and determines a current state of the power supply apparatus to be a first mode, a second mode, or a third mode, based on the value of the load current.  
         [0050]     In the second mode, in operation  720 , the controller  520  turns the first switch  330  on and the second and third switches  340  and  350  off, in order to supply power from only the fuel cell  300  to the load  390  via the DC-DC converter  360 . While the fuel cell  300  is supplying the power to the load  390 , in operation  730 , the first voltage measurement unit  500  measures the output voltage of the fuel cell  300 . In operation  740 , the controller  520  determines whether the power of the fuel cell  300  is stable based on the output voltage of the fuel cell  300  measured by the first voltage measurement unit  500 . As a result of the determination, if the power of the fuel cell  300  is unstable, in operation  750 , the controller  520  turns the first and second switches  330  and  340  on and the third switch  350  off, in order to connect both the fuel cell  300  and the rechargeable battery  310  to the input of the DC-DC converter  360 , thus switching to the third mode based on the measured output voltage of the fuel cell. Thus, even if the power of the fuel cell  300  is unstable, stable power is supplied to the load  390  from the fuel cell  300  and the rechargeable battery  310  together.  
         [0051]     In the first mode, in operation  760 , the controller  520  turns the first and third switches  330  and  350  on and the second switch  340  off, in order to connect the fuel cell  300  to the input of the DC-DC converter  360  and connect the rechargeable battery  310  to the output of the DC-DC converter  360 . While the rechargeable battery  310  is charged from the output from the DC-DC converter  360 , in operation  770 , the second voltage measurement unit  510  measures the output voltage of the rechargeable battery  310 . In operation  780 , the controller  520  determines whether the rechargeable battery  310  is fully charged based on the output voltage of the rechargeable battery  310  measured by the second voltage measurement unit  510 . As a result of the determination, if the rechargeable battery  310  is fully charged, in operation  720 , the controller  520  turns the first switch  330  on and the second and third switches  340  and  350  off, in order to stop recharging the battery  310 , thus switching from the first mode to the second mode.  
         [0052]     Some aspects of the embodiments of the present invention may be provided as computer programs and implemented in general-use digital computers that execute the programs using a computer readable recording medium. Examples of the computer readable recording medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical recording media (e.g., CD-ROMs, DVDs, etc.), and storage media such as carrier waves (e.g., transmission through the internet).  
         [0053]     As described above, in a power supply apparatus using a fuel cell and a method of controlling the power supply apparatus according to embodiments of the present invention, when power is supplied to a load by converting a voltage output from the fuel cell using a DC-DC converter, the efficiency of the DC-DC converter can be maintained even when the power supplied to the load changes, by controlling connections between the fuel cell, a rechargeable battery and an input and an output of the DC-DC converter based on at least one of the current flowing to the load, an output voltage of the fuel cell and an output voltage of the rechargeable battery, thereby supplying the power to the load for a long time with stable efficiency.  
         [0054]     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.