CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 2009-2739 filed in the Korean Intellectual Property Office on Jan. 13, 2009, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Aspects of the present invention relate to a fuel cell system. 
         [0004]    2. Description of the Related Art 
         [0005]    A fuel cell system is a power system that directly converts energy of a chemical reaction of hydrogen in a hydrocarbon-based material, such as methanol, ethanol, and natural gas, with oxygen into electrical energy. As a clean energy source that can replace fossil energy, the fuel cell has an advantage of producing various ranges of outputs because of a stack structure formed by stacking unit cells, and such a fuel cell has been spotlighted as a small-sized and portable power source since energy density thereof is 4 to 10 times higher than that of a small lithium battery. 
         [0006]    However, each unit cell has a different operation state due to deviations during a manufacturing process or uneven dispersion of location, pressure, and temperature of each unit cell in the fuel cell stack. Even though each unit cell has a similar operation state, performance of each unit cell deteriorates differently as the fuel cell system is continuously driven. Further, when performance of a given unit cell is severely decreased during use of the fuel cell stack, the power generation amount of the fuel cell stack is decreased while causing serial performance deterioration of adjacent unit cells so that a life-span of the fuel cell stack is reduced. Therefore, it is important to measure a performance difference between unit cells. 
         [0007]    Meanwhile, in order to reduce the production cost of the fuel cell stack, the price of the fuel cell as well as the price of other devices should be decreased. For the cost reduction, a device to measure a performance difference of unit cells can be realized by an integrated circuit, such as an application-specific integrated circuit (ASIC). The ASIC generally requires 4-5V for operation. Conventionally, an operation voltage of the ASIC is supplied from a fuel cell stack; however, the ASIC may not be sufficiently driven by the voltage of the fuel cell stack. For example, a lithium ion battery has a unit cell voltage of about 3V, and when measuring a voltage of 10 unit cells, the voltage of the unit cells becomes greater than 30V so that the ASIC can be sufficiently driven. The configuration of driving the ASIC with the lithium ion battery is disclosed in Japanese Patent Laid-Open Publication No. 2003-70179. However, a voltage of a unit cell of the fuel cell is 0 to 0.8V so the ASIC cannot be sufficiently driven. 
         [0008]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY OF THE INVENTION 
       [0009]    Aspects of the present invention provide a fuel cell system having a reduced cost of operation by measuring a performance difference of unit cells via an application-specific integrated circuit (ASIC) and operating the ASIC with a low voltage from a separately included power source supply device. 
         [0010]    A fuel cell system according to aspects of the present invention includes a fuel cell stack including a plurality of unit cells, an ASIC, and a constant voltage generator. According to aspects of the present invention, the ASIC is supplied with a power source voltage and a reference voltage from corresponding unit cells among the plurality of unit cells through connections established between the unit cells and the ASIC, measures a voltage of each unit cell, and discharges a unit cell when the measured voltage of the unit cell is higher than a predetermined reference value. According to aspects of the present invention, the constant voltage generator supplies a constant voltage to the ASIC that is higher than the power source voltage by a floating voltage. According to aspects of the present invention, a voltage difference between the constant voltage and the reference voltage is sufficient to operate at least the AISC. According to aspects of the present invention, the ASIC includes a power source voltage terminal to which the power source voltage is input, a constant voltage terminal to which the constant voltage is input, a first reference voltage terminal to which the reference voltage is input, and a plurality of cell voltage input terminals respectively connected to a plurality of anode terminals and a plurality of cathode terminals of the corresponding unit cells. According to aspects of the present invention, the power source voltage may be the highest voltage among voltages of the plurality of anode terminals of the unit cells, and the reference voltage is the lowest voltage among voltages of the plurality of cathode terminals of the unit cells. 
         [0011]    According to aspects of the present invention, the constant voltage generator includes an input terminal to which a voltage to generate the floating voltage is supplied, an output terminal connected to the constant voltage terminal, and a second reference voltage terminal connected to the power source voltage terminal of the ASIC. According to aspects of the present invention, the constant voltage generator includes a transistor having a collector terminal connected to the input terminal and an emitter terminal connected to the output terminal, a Zener diode having a cathode terminal connected to a base terminal of the transistor and an anode terminal connected to the second reference voltage terminal, and a resistor connected between the collector terminal of the transistor and the cathode terminal of the Zener diode. According to aspects of the present invention, the floating voltage is a voltage obtained by subtracting a threshold voltage of the transistor from a breakdown voltage of the Zener diode. According to aspects of the present invention, the fuel cell system further includes a voltage supply unit to supply the voltage to the constant voltage generator to generate the floating voltage. According to aspects of the present invention, the fuel cell system further includes a plurality of resistors respectively connected between the anode terminal and the cathode terminal of each of the plurality of unit cells and the power source voltage terminal, the first reference voltage terminal, and the plurality of cell voltage input terminals. According to aspects of the present invention, the fuel cell system further includes a current blocking element between one of the resistors and the power source voltage terminal. According to aspects of the present invention, the current blocking element includes an operation amplifier having an input terminal connected to the resistor and an output terminal connected to the power source voltage terminal. 
         [0012]    As described above, according to the aspects of the present invention, cost can be reduced by realizing a device for measuring a performance difference of unit cells as an ASIC and the ASIC can be normally driven with a low voltage by separately including a power source supply device for driving the ASIC. 
         [0013]    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 
         [0014]    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: 
           [0015]      FIG. 1  shows a fuel cell system according to an exemplary embodiment of the present invention; 
           [0016]      FIG. 2  shows a first constant voltage generator  40  of  FIG. 1 ; and 
           [0017]      FIG. 3  shows a fuel cell system according to another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]    Reference will now be made in detail to exemplary embodiments according to aspects of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The exemplary embodiments are described below in order to explain the present invention by referring to the figures. 
         [0019]    Throughout this specification and the claims that follow, when it is described that an element is “coupled” to another element, the element may be “directly coupled” to the other element or “electrically coupled” to the other element through a third element. In addition, unless explicitly described to the contrary, the word “comprise” and variations, such as “comprises” or “comprising,” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
         [0020]    Hereinafter, a fuel cell system according to an exemplary embodiment of the present invention will be described.  FIG. 1  shows a fuel cell system according to an exemplary embodiment of the present invention. Referring to  FIG. 1 , the fuel cell system of the present exemplary embodiment includes a fuel cell stack  10 , a plurality of resistors R 1  to R 22 , first to third application-specific integrated circuits (ASICs)  20 ,  30 , and  40 , first to third constant voltage generators  50 ,  60 , and  70 , and a voltage supply unit  80 . 
         [0021]    The fuel cell stack  10  includes a plurality of unit cells C 1  to C 21  connected in series between a first voltage terminal (+) and a second voltage terminal (−). According to aspects of the present invention, the fuel cell stack  10  exemplarily includes  21  unit cells, of which groups of  7  unit cells are respectively connected to first to third ASICs  20 ,  30 , and  40 . However, aspects of the present invention are not limited thereto, and the fuel cell stack  10  may include a different number of unit cells and ASICs. The number of ASICs is determined according to the number of unit cells, the number of input pins, the number of output pins, and the processing speed of the ASIC. For example, the fuel cell stack  10  may include only one ASIC if such ASIC can appropriately control  21  unit cells. 
         [0022]    An anode terminal and a cathode terminal of each of the plurality of unit cells C 1  to C 21  are connected to first sides of the corresponding protection resistors R 1  to R 22 . Second sides of the plurality of protection resistors R 1  to R 8  are respectively connected to a first power voltage terminal PVT_ 1  of the first ASIC  20 , cell voltage input terminals S 1  to S 6 , and a first reference voltage terminal RVT_ 1 . Second sides of the plurality of protection resistors R 9  to R 15  are respectively connected to a second power voltage terminal PVT_ 2  of the second ASIC  30 , cell voltage input terminals S 11  to S 15 , and a second reference voltage terminal RVT_ 2 . Second sides of the plurality of protection resistors R 16  to R 22  are respectively connected to a third power voltage terminal PVT_ 3  of the third ASIC  40 , cell voltage input terminals S 21  to S 25 , and a third reference voltage terminal RVT_ 3 . The plurality of protection resistors R 1  to R 22  protect transmission of surge voltages from the plurality of unit cells C 1  to C 21  to the first to third ASICs  20 ,  30 , and  40 . 
         [0023]    The first ASIC  20  includes a first constant voltage terminal PT_ 1  to which a first constant voltage VOUT_ 1  is input, a first power source voltage terminal PVT_ 1  to which a first power source voltage is input, a first reference voltage terminal RVT_ 1  to which a first reference voltage is input, and the plurality of cell voltage input terminals S 1  to S 6  that are connected to nodes of the plurality of unit cells C 1  to C 7  through the plurality of resistors R 2  to R 7 . The first constant voltage VOUT_ 1  is a voltage input from the first constant voltage generator  50 , the first power source voltage is a voltage supplied from the anode terminal of the unit cell C 1  through the resistor R 1 , and the first reference voltage is a voltage input from the cathode terminal of the unit cell C 7  through the resistor R 8 . The first ASIC  20  measures a voltage difference between the first power source voltage terminal PVT_ 1  and the cell voltage input terminal S 1  so as to measure a voltage of the unit cell Cl. Further, the first ASIC  20  may measure a voltage difference between adjacent cell voltage input terminals to measure voltage of respective unit cells; for example, the first ASIC  20  may measure a voltage difference between the cell voltage input terminals S 1  and S 2  to determine a voltage of the unit cell C 2 . Or, the first ASIC  20  may measure a voltage difference between a cell voltage input terminal and the first power source voltage terminal PVT_ 1  to determine voltage of respective unit cells; for example, the first ASIC  20  may measure a voltage difference between the cell voltage input terminal S 2  and the first power source voltage terminal PVT_ 1  to determine a voltage of the unit cell C 2  if the voltage of the unit cell C 1  is known. The first ASIC  20  may perform such measurements in a similar way so as to determine voltages for the unit cells C 1  to C 7 . 
         [0024]    It should be noted that the power source voltage maybe the highest voltage among voltages of the plurality of anode terminals, and the reference voltage may be the lowest voltage among voltages of the plurality of cathode terminals, i.e., the first power source voltage input to the first power source voltage terminal PVT_ 1  is the highest voltage among the voltages input to the first power source voltage terminal PVT_ 1 , the plurality of cell voltage input terminals S 1  to S 6 , and the first reference voltage terminal RVT_ 1 ; and, the first reference voltage input to the first reference voltage terminal RVT_ 1  is the lowest among the voltages input to the first power source voltage terminal PVT_ 1 , the plurality of cell voltage input terminals S 1  to S 6 , and the first reference voltage terminal RVT_ 1 . 
         [0025]    The second ASIC  30  includes a second constant voltage terminal PT_ 2  to which a second constant voltage VOUT_ 2  is input, a second power source voltage terminal PVT_ 2  to which a second power source voltage is input, a second reference voltage terminal RVT_ 2  to which a second reference voltage is input, and a plurality of cell voltage input terminals S 11  to S 15  connected to nodes of the plurality of unit cells C 8  to C 14  through the plurality of resistors R 10  to R 14 . The second constant voltage VOUT_ 2  is a voltage input from the second constant voltage generator  60 , the second power source voltage is a voltage supplied from the cathode terminal of the unit cell C 8  through the resistor R 9 , i.e., supplied from the anode of the unit cell C 9 , and the second reference voltage is a voltage from a cathode of the unit cell C 14  that is input through the resistor R 15 , i.e., supplied from the anode of the unit cell C 15 . Here, the second power source voltage terminal PVT_ 2  is connected to the first reference voltage terminal RVT_ 1  of the first ASIC  20 , and the second reference voltage terminal RVT_ 2  is connected to the third power source voltage terminal PVT_ 3  of the third ASIC  40 . The second ASIC  30  measures a voltage difference between the second power source voltage terminal PVT_ 2  and the cell voltage input terminal S 11  so as to measure a voltage of the unit cell C 9 . 
         [0026]    The third ASIC  40  includes a third constant voltage terminal PT_ 3  to which a third constant voltage VOUT_ 3  is input, a third power source voltage terminal PVT_ 3  to which a third power source voltage is input, a third reference voltage terminal RVT_ 3  to which a third reference voltage is input, and the plurality of cell voltage input terminals S 21  to S 25  connected to nodes of the plurality of unit cells C 17  to C 21  through the plurality of resistors R 17  to R 21 . The third constant voltage VOUT_ 3  is a voltage input from the third constant voltage generator  70 , the third power source voltage is a voltage supplied from the anode terminal of the unit cell C 16  through the resistor R 16 , and the third reference voltage is a voltage from a cathode of the unit cell C 21  that is input through the resistor R 22 . The third ASIC  40  measures a voltage difference between the third power source voltage terminal PVT_ 3  and the cell voltage input terminal S 21  so as to measure a voltage of the unit cell C 16 . 
         [0027]    Here, the first ASIC  20  is supplied with a voltage obtained by subtracting the first reference voltage from the first constant voltage VOUT_ 1 . In further detail, the first reference voltage supplied to the first ASIC  20  is a cathode voltage of the unit cell C 7  input to the first reference voltage terminal RVT_ 1 , and the first power source voltage is an anode voltage of the unit cell C 1 . The first constant voltage VOUT_ 1  is a voltage that is higher than the first power source voltage by a floating voltage of a predetermined level. For example, if it is assumed that the anode voltage of the unit cell C 1  is 5V, the cathode voltage of the unit cell C 7  is 3.6V, and the floating voltage supplies the first constant voltage VOUT_ 1  that is higher than the first power source voltage by 3.5V, then the first constant voltage VOUT_ 1  is 8.5V and the first reference voltage is 3.6V, and therefore 4.9V is supplied to the first ASIC  20 . The second ASIC  30  and the third ASIC  40  are similar to the first ASIC  20 . That is, the second ASIC  30  is supplied with a voltage that corresponds to a voltage difference between the second constant voltage VOUT_ 2  from the second constant voltage generator  60  and the second reference voltage, and the third ASIC  40  is supplied with a voltage that corresponds to a voltage difference between the third constant voltage VOUT_ 3  from the third constant voltage generator  70  and the third reference voltage. 
         [0028]    The first constant voltage generator  50  receives a voltage VI from the voltage supply unit  80  and applies the first constant voltage VOUT_ 1  to the first ASIC  20 . Here, an input voltage VI terminal of the first constant voltage generator  50  is connected to an output of the voltage supply unit  80 , and a first output voltage VOUT_ 1  terminal is connected to the first constant voltage terminal PT_ 1 . In addition, a first reference voltage VGND_ 1  terminal of the first constant voltage generator  50  is connected to the first power source voltage terminal PVT_ 1 . 
         [0029]    The second constant voltage generator  60  receives the voltage VI from the voltage supply unit  80  and applies a second constant voltage VOUT_ 2  to the second ASIC  30 . Here, an input voltage VI terminal of the second constant voltage generator  60  is connected to the output of the voltage supply unit  80 , and a second output voltage VOUT_ 2  terminal is connected to the second constant voltage terminal PT_ 2 . In addition, a second reference voltage VGND_ 2  terminal of the second constant voltage generator  60  is connected to the second power source voltage terminal PVT_ 2 . Moreover, the second reference voltage VGND_ 2  terminal of the second constant voltage generator  60  is also connected to the first reference voltage terminal RVT 1  of the first ASIC  20 . 
         [0030]    The third constant voltage generator  70  receives the voltage VI from the voltage supply unit  80  and applies a third constant voltage VOUT_ 3  to the third ASIC  40 . Here, an input voltage VI terminal of the third constant voltage generator  70  is connected to the output of the voltage supply unit  80 , and a third output voltage VOUT_ 3  terminal is connected to the third constant voltage terminal PT_ 3 . In addition, a third reference voltage VGND_ 3  of the third constant voltage generator  70  is connected to the third power source voltage terminal PVT_ 3 . Moreover, the third reference voltage VGND_ 3  of the third constant voltage generator  70  is also connected to the second reference voltage terminal RVT_ 2  of the second ASIC  30 . Configurations of the first to third constant voltage generators  50 ,  60 , and  70  will be described in further detail later with reference to  FIG. 2 . 
         [0031]    The voltage supply unit  80  supplies voltages for the first to third constant voltage generators  50 ,  60 , and  70  to generate the first to third constant voltages VOUT_ 1  to VOUT_ 3 , respectively. In further detail, the voltage supplied by the voltage supply unit  80  may be sufficiently higher than the maximum voltage of the fuel cell stack  10  by a predetermined-level floating voltage. In this exemplary embodiment of the present invention, the output voltage of the voltage supply unit  80  is set to  40 V. However, aspects of the present invention are not limited thereto. 
         [0032]      FIG. 2  shows the first constant voltage generator  50  of  FIG. 1 . The second and third constant voltage generators  60  and  70  are the same as the first constant voltage generator  50  in configuration and operation. The first to third constant voltage generators  50 ,  60 , and  70  according to the exemplary embodiment of the present invention can use a regulated power source ASIC, and a simple circuit thereof will be described with reference to  FIG. 2 . 
         [0033]    Referring to  FIG. 2 , the first constant voltage generator  50  includes a transistor T 1 , a resistor R 23 , and a Zener diode Z 1 . The transistor T 1  is an NPN-type bipolar junction transistor, and a base terminal thereof is connected to a cathode terminal of the Zener diode Z 1 . In addition, a collector terminal of the transistor T 1  is connected to an input voltage VI terminal, and an emitter terminal is connected to the first output voltage VOUT_ 1  terminal. The resistor R 23  is connected between the input terminal VI and the cathode terminal of the Zener diode Z 1 . An anode terminal of the Zener diode Z 1  is connected to the first reference voltage VGND_ 1  terminal. 
         [0034]    The first constant voltage generator  50  divides a voltage VI received from the voltage supply unit  80  by using the resistor R 23  and the Zener diode Z 1 , and supplies a clamping voltage of the Zener diode Z 1  to the base terminal of the transistor T 1 . The Zener diode Z 1  is turned on when a voltage difference between the cathode electrode and the anode electrode of the Zener diode Z 1  is greater than a breakdown voltage, and in this case, the voltage difference is maintained at the breakdown voltage. In this exemplary embodiment of the present invention, the Zener diode Z 1  is turned on so as to maintain a constant voltage of the cathode electrode of the Zener diode Z 1 , and the voltage maintained at the cathode electrode is referred to as a clamping voltage. The transistor T 1  is turned on by the clamping voltage, and a predetermined constant voltage is output therethrough. The predetermined constant voltage is a voltage obtained by subtracting a threshold voltage of the transistor T 1  from the clamping voltage. Therefore, the floating voltage of the first constant voltage generator  50  according to this exemplary embodiment of the present invention is a voltage that is obtained by subtracting the threshold voltage of the transistor T 1  from the breakdown voltage of the Zener diode Z 1 . However, in the above-described configuration, a current path may be formed from the voltage VI terminal to the first reference voltage VGND_ 1  terminal. In such case, current is as weak as 1 mA to 10 mA, but a voltage measurement error may occur when a current flowing to the fuel cell stack  10  is less than 1 A. Therefore, inflow of current generated from the first constant voltage generator  50  to the fuel cell stack  10  can be prevented by providing a current blocking element according to aspects of the present invention. 
         [0035]      FIG. 3  shows a fuel cell system according to an exemplary embodiment of the present invention. Like elements of the configuration of  FIG. 1  are assigned like reference numerals. Referring to  FIG. 3 , the fuel cell system according to this exemplary embodiment of the present invention includes a fuel cell stack  10 , a plurality of protection resistors R 1  to R 22 , first to third ASICs  20 ,  30 , and  40 , first to third constant voltage generators  50 ,  60 , and  70 , a voltage supply unit  80 , and first to third current blocking elements AMP 1  to AMP 3 . Configuration and operation of the fuel cell stack  10 , the first to third ASICs  20 ,  30 , and  40 , the first to third constant voltage generators  50 ,  60 , and  70 , and the voltage supply unit  80  are the same as those described with reference to  FIG. 1 , and thus they will not be further described. 
         [0036]    This exemplary embodiment is different from the previous exemplary embodiment in that the fuel cell system further includes first to third current blocking elements AMP 1  to AMP 3 . An input terminal of the first current blocking element AMP 1  is connected to the resistor R 1 , and an output terminal of the first current blocking element AMP 1  is connected to the first reference voltage VGND_ 1  terminal of the first constant voltage generator  50 . An input terminal of the second current blocking element AMP 2  is connected to the resistor R 8 , and an output terminal of the second current blocking element AMP 2  is connected to the second reference voltage VGND_ 2  terminal of the second constant voltage generator  60 . Further, the output terminal of the second current blocking element AMP 2  is connected to the second power source voltage terminal PVT_ 2 . An input terminal of the third current blocking element AMP 3  is connected to the resistor R 15 , and an output terminal of the third current blocking element AMP 3  is connected to the third reference voltage VGND_ 3  terminal of the third constant voltage generator  70 . Further, the output terminal of the third current blocking element AMP 3  is connected to the third power source voltage terminal PVT_ 3 . In addition, the first to third current blocking elements AMP 1  to AMP 3  are driven by a voltage VI output from the voltage supply unit  80 . 
         [0037]    Here, the first to third current blocking elements AMP 1  to AMP 3  may be formed of an operation amplifier. The operation amplifier generally includes transistors T 2  and T 3  that are connected in series between the voltage VI terminal and the second power source voltage terminal (−) at an output terminal thereof. The transistors T 2  and T 3  may be NMOS transistors, and may be weakly turned on when a drain terminal potential is higher than a source terminal potential. Since a potential of the first reference voltage VGND_ 1  terminal is higher than a potential of the second power source voltage terminal (−), i.e., the ground voltage (GND), the transistor T 3  is weakly turned on. Then, a current path is formed from the first reference voltage VGND_ 1  terminal to the second power source voltage terminal (−) through the transistor T 3  so that inflow of the current flowing to the first reference voltage VGND_ 1  terminal to the fuel cell stack  10  can be prevented. Here, an output terminal and an inversed input terminal (−) of the operation amplifier are connected to each other, and the input terminals of the first to third current blocking elements AMP 1  to AMP 3  correspond to the non-inversed input terminal (+) of the operation amplifier. 
         [0038]    As described above, the fuel cell system according to aspects of the present invention additionally receives power source voltages of the first to third ASICs  20 ,  30 , and  40  from the first to third constant voltage generators  50 ,  60 , and  70 . Therefore, when the voltages of the unit cells C 1  to C 7  are low, the first ASIC  20  can be normally operated. The second and third ASICs  30  and  40  can be normally operated since they can similarly receive a sufficient voltage from the second and third constant voltage generators  60  and  70 . 
         [0039]    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.
A fuel cell system, and a method of operating the fuel cell system to measure a performance difference of unit cells via an application-specific integrated circuit (ASIC) and to drive the ASIC with a low voltage from a separately included power source supply device.