JP4990573B2 - Fuel cell system - Google Patents

Description

  The present invention relates to a fuel cell system that supplies electric power generated by a fuel cell to a load.

  At present, secondary batteries such as lithium ion batteries and nickel metal hydride batteries are widely used as power sources in portable electronic devices such as personal computers and electric machines such as electric tools. However, when such devices are operated using a secondary battery, sufficient time for continuous operation of the device cannot be obtained due to the limitation of the battery capacity. For example, if a portable personal computer is operated with a secondary battery, the time during which power can be continuously supplied is, for example, about 4 hours.

  On the other hand, in recent years, fuel cells that can supply power continuously for a long time have attracted attention. For example, when power is supplied to a personal computer, a fuel cell that can supply power continuously for 20 to 40 hours is considered.

  A fuel cell has a structure in which a single cell in which an electrolyte layer is sandwiched between a fuel electrode (−) and an air electrode (+) is laminated. Fuel is supplied to the fuel electrode and air is supplied to the air electrode. Electric power is generated by reacting. For example, hydrogen or methanol is used as the fuel. Since the output voltage of such a fuel cell does not necessarily match the power supply voltage for operation required by the load device, a DC-DC converter that converts the output voltage of the fuel cell into the power supply voltage for operation of the load device is used. .

  As such a DC-DC converter, a DC-DC of a so-called switching power supply system in which a desired operation power supply voltage is obtained by controlling an on / off duty ratio of a switching element for turning on / off an output voltage of a fuel cell. DC converters are widely used. In such a DC-DC converter, for example, the output voltage of the DC-DC converter is monitored by a control circuit using a microcomputer, and the output voltage of the DC-DC converter is set to a desired power supply voltage for operation by software processing of the microcomputer. A device that controls the on / off duty ratio of the switching element so as to match the above is known.

  By the way, for example, when the load is abruptly reduced because the load device is suddenly disconnected from the DC-DC converter, there is no place for power and the output voltage of the DC-DC converter rapidly increases. Then, in the DC-DC converter that controls the output voltage by the control circuit using the microcomputer, the processing time required for the software processing of the microcomputer is not in time for such a sudden increase in the output voltage. Or applied to the internal circuit of the DC-DC converter, which may cause damage.

  Further, the fuel cell has a property that the output voltage decreases when the output current is increased, and the output voltage increases when the output current is decreased. Fuel cells also have the property that power generation efficiency varies depending on the output voltage. Therefore, in recent years, there has been known a technique in which the current drawn from the fuel cell to the DC-DC converter is adjusted so that the output voltage of the fuel cell is kept constant at a voltage at which high power generation efficiency can be obtained.

  As described above, in the DC-DC converter in which the output voltage of the fuel cell, that is, the input voltage of the DC-DC converter is controlled to be constant, for example, the output voltage of the DC-DC converter is reduced due to, for example, a drastically light load. Even if the voltage rises, the control operation for lowering the output voltage is not performed to maintain the current drawn from the fuel cell regardless of the output voltage of the DC-DC converter, so the output voltage of the DC-DC converter rises. As a result, the risk of damaging the load circuit and the internal circuit of the DC-DC converter is further increased by the overvoltage.

  Therefore, by providing an overvoltage protection circuit in parallel with the load device, when the output voltage rises, by bypassing the output current of the DC-DC converter to the overvoltage protection circuit, the load device and the DC-DC converter are detected from the overvoltage. There is known a fuel cell system that protects the internal circuit (see, for example, Patent Document 1).

In addition, a comparator that compares the output voltage of the DC-DC converter and the reference voltage is provided, and when the output voltage of the DC-DC converter exceeds the reference voltage, the fuel supply to the fuel cell is stopped by the output signal of the comparator. A fuel cell system has also been proposed that suppresses the occurrence of overvoltage by stopping the operation of the DC-DC converter (see, for example, Patent Document 2).
JP-T-2006-501798 JP 2005-56764 A

  However, when the overvoltage protection circuit is provided as in Patent Document 1 and the output current of the DC-DC converter is bypassed to the overvoltage protection circuit, the output power of the fuel cell is consumed by the overvoltage protection circuit. The overvoltage protection circuit needs to have a power rating equal to or higher than the output power of the fuel cell. For example, when a 20 W fuel cell is used, an overvoltage protection circuit having a power rating of 20 W or more is necessary. Thus, in order to construct an overvoltage protection circuit with a large power rating, it is necessary to use components with a large power rating as components of the overvoltage protection circuit, such as transistors and resistors. However, since parts having a large power rating have large external dimensions, the use of such an overvoltage protection circuit has the disadvantage that the circuit becomes large.

  Further, as in Patent Document 2, the output voltage of the DC-DC converter is monitored using a comparator, and when the output voltage of the DC-DC converter exceeds a certain reference voltage, a fuel cell is detected by an ON signal of the comparator. In the technology that stops the fuel supply to the fuel cell, if the load suddenly decreases, the output power of the fuel cell does not decrease immediately even if the fuel supply is stopped. There is a disadvantage that the output voltage of the DC-DC converter increases. In addition, when the output voltage of the DC-DC converter is monitored using a comparator and the operation of the DC-DC converter is stopped when the output voltage of the DC-DC converter exceeds a certain reference voltage, the DC-DC converter When the DC converter is stopped and the output voltage falls below the reference voltage, the comparator is turned off and the DC-DC converter starts to operate again. Therefore, the DC-DC converter repeats the operation and the stop to stably perform the DC- There is a disadvantage that the DC converter cannot be stopped and the occurrence of overvoltage cannot be sufficiently suppressed.

  The present invention has been made in view of such circumstances, and provides a fuel cell system capable of reducing the occurrence of overvoltage when a sudden change in load occurs while suppressing an increase in circuit size. For the purpose.

  The fuel cell system according to the present invention includes a fuel cell that generates electric power based on fuel and oxygen, a connection terminal for connecting a load, and an output voltage of the fuel cell that is converted into a predetermined voltage to perform the connection. When the output voltage of the voltage converter to the terminal and the output voltage of the voltage converter are equal to or higher than a preset voltage, an output voltage reduction process is performed to generate a control signal for reducing the output voltage of the voltage converter. When the output voltage of the first control unit and the voltage converter is detected and the detected voltage becomes equal to or higher than the set voltage, the operation of the voltage converter is stopped, and the output voltage drop by the first control unit A second control unit that operates the voltage converter in accordance with the control signal after the process is executed.

  According to this configuration, electric power is generated by the fuel cell based on the fuel and oxygen. Further, the output voltage of the fuel cell is converted into a predetermined voltage by the voltage converter and output to the connection terminal for connecting the load. When the output voltage of the voltage converter becomes equal to or higher than a preset voltage, the second control unit promptly reduces the output voltage of the voltage converter by the control signal generated by the first control unit. Since the operation of the voltage converter is stopped, the occurrence of overvoltage can be reduced even when the output voltage of the voltage converter suddenly increases due to a sudden change in the load. In addition, even if the output voltage of the voltage converter decreases, the second control unit does not immediately start the operation of the voltage converter, but after the control signal for reducing the output voltage of the voltage converter is generated by the first control unit. Since the voltage converter is operated according to the control signal, the operation state and the stop state of the voltage converter may be repeated as in the case where the output voltage of the voltage converter decreases and the operation of the voltage converter starts immediately. Reduced. Furthermore, since an overvoltage protection circuit that bypasses the current is not used in order to reduce the occurrence of overvoltage, it is not necessary to use a component having a large power rating and therefore a large external dimension, and the size of the circuit can be suppressed.

  In addition, it is preferable to further include a storage element that is charged by the output voltage of the voltage converter and outputs the charged power to the connection terminal. According to this configuration, when the load current flowing through the load decreases, the surplus power is stored in the storage element, and when the load current increases, the power stored in the storage element is supplied to the load. As a result, the output voltage of the voltage converter is stabilized.

  The second control unit compares the output voltage of the voltage converter with the set voltage, and stops the operation of the voltage converter when the output voltage of the voltage converter becomes equal to or higher than the set voltage. It is preferable to include a comparator that outputs a stop signal and a holding circuit that holds the stop signal for a time longer than the processing time for the output voltage reduction process.

  According to this configuration, the output voltage of the voltage converter is compared with the set voltage by the comparator, and when the output voltage of the voltage converter exceeds the set voltage, the operation of the voltage converter is performed according to the output signal of the comparator. Since the operation is stopped, the operation of the voltage converter can be quickly stopped when the output voltage of the voltage converter becomes equal to or higher than the set voltage. In addition, the stop signal for stopping the operation of the voltage converter of the comparator is held by the holding circuit for a time longer than the processing time for the output voltage reduction processing, so that the output voltage of the voltage converter immediately decreases. As in the case where the operation of the voltage converter is started, the repetition of the operation state and the stop state of the voltage converter is reduced.

  The first control unit preferably determines that the output voltage of the voltage converter is equal to or higher than a preset voltage when the stop signal is output from the comparator.

  According to this configuration, the first control unit executes the output voltage reduction process by determining that the output voltage of the voltage converter is equal to or higher than the set voltage when the stop signal is output from the comparator. There is no need to provide a separate voltage detection unit in addition to the comparator, and the circuit can be simplified.

  The voltage converter includes a switching element that turns on and off the output voltage of the fuel cell according to a duty ratio of the control signal, and outputs to the connection terminal according to a decrease in the duty ratio of the control signal. It is preferable that the voltage is decreased and the first control unit decreases the duty ratio of the control signal as the output voltage reduction process.

  According to this configuration, when the voltage converter is configured by a so-called switching power supply type power supply circuit in which the output voltage to the connection terminal decreases in accordance with a decrease in the on-duty ratio of the switching element, the first control unit The output voltage of the voltage converter can be reduced by reducing the duty ratio of the control signal for turning on / off the switching element as the output voltage reduction process.

  In the output voltage reduction process, the first controller preferably further reduces at least one of the amount of fuel and oxygen supplied to the fuel cell. According to this configuration, when reducing the output voltage of the voltage converter, by reducing at least one of the amount of fuel and oxygen supplied to the fuel cell, the power generated by the fuel cell is reduced, and surplus power is reduced. It is possible to reduce the reduction in power generation efficiency in the fuel cell.

  In the fuel cell system having such a configuration, when the output voltage of the voltage converter becomes equal to or higher than a preset setting voltage, the output voltage of the voltage converter is reduced by the control signal generated by the first control unit. Since the operation of the voltage converter is promptly stopped by the second control unit, the occurrence of overvoltage can be reduced even when the output voltage of the voltage converter suddenly increases due to a sudden change in the load. In addition, even if the output voltage of the voltage converter decreases, the second control unit does not immediately start the operation of the voltage converter, but after the control signal for reducing the output voltage of the voltage converter is generated by the first control unit. Since the voltage converter is operated according to the control signal, the operation state and the stop state of the voltage converter may be repeated as in the case where the output voltage of the voltage converter decreases and the operation of the voltage converter starts immediately. Reduced. Furthermore, since an overvoltage protection circuit that bypasses the current is not used in order to reduce the occurrence of overvoltage, it is not necessary to use a component having a large power rating and therefore a large external dimension, and the size of the circuit can be suppressed.

  Embodiments according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an example of the configuration of a fuel cell system according to an embodiment of the present invention. A fuel cell system 1 shown in FIG. 1 is a fuel cell system that supplies electric power to a load 2, and includes a connection terminal 3 for connecting the load 2, and supply of fuel and oxygen to generate electric energy by a chemical reaction. The generated fuel cell 4 is connected to the output side of the fuel cell 4, and the built-in switching element Q1 is turned on and off to boost the output voltage Vt of the fuel cell 4, and as a voltage Vout via the connection terminal 3 A voltage converter 5 supplied to the load 2, a voltage detection unit 7 configured to detect an output voltage Vout of the voltage converter 5, for example, an AD converter, and a switching element Q1 based on the voltage Vout detected by the voltage detection unit 7 A control signal S1 for controlling the on / off duty ratio of the output signal to the AND gate 9, and a voltage controller. It has connected the secondary battery 6 (electric storage device) between the connection point and the ground over motor 5 and the connection terminal 3.

  Furthermore, the reference voltage source 10 configured using, for example, a constant voltage circuit that outputs the set voltage Vref, the output voltage Vout of the voltage converter 5 and the set voltage Vref output from the reference voltage source 10 are compared, and the output voltage A comparator 11 (comparator) that outputs a low level signal to turn off the switching element Q1 and stop the operation of the voltage converter 5 when Vout becomes equal to or higher than the set voltage Vref, and an output signal of the comparator 11 in advance. The holding circuit 12 that holds the set time tw and outputs the control signal S2 to the control unit 8 and the AND gate 9 and the signal S3 obtained by the logical product of the control signal S1 and the control signal S2 And an AND gate 9 for outputting to the gate. In this case, the reference voltage source 10, the comparator 11, and the holding circuit 12 correspond to an example of a second control unit.

  The set voltage Vref of the reference voltage source 10 is set to an upper limit value of a rated voltage range in which the load 2 and the secondary battery 6 are not destroyed by overvoltage, for example.

  The voltage converter 5 is a so-called switching power supply type DC-DC converter. In the voltage converter 5, the output terminal of the fuel cell 4 is connected to one end of the coil L, the anode of the diode D1 is connected to the other end of the coil L, and the cathode of the diode D1 is connected to the ground via the capacitor C2. . The connection point between the fuel cell 4 and the coil L is connected to the ground via the capacitor C1, and the connection point between the coil L and the diode D1 is configured using a transistor such as an FET (Field Effect Transistor). The switching element Q1 is connected to the ground. In the voltage converter 5, the output voltage Vout increases when the on-duty ratio of the switching element Q1 increases, and the output voltage Vout decreases when the on-duty ratio of the switching element Q1 decreases. The voltage converter 5 is not limited to a DC-DC converter, and may be a DC-AC converter.

  The load 2 is a load that consumes electric power supplied from the fuel cell system 1 such as a main body of a personal computer, for example. The fuel cell 4 generates electric power from methanol fuel supplied from a fuel supply device (not shown) and oxygen contained in the air. The fuel cell 4 has, for example, a stack structure in which a plurality of fuel cells are stacked in series. This fuel cell is, for example, a direct methanol fuel cell having a power generation voltage of about 0.4 V per cell, and 10 fuel cells are connected in series to form a stack. Therefore, the output voltage of the fuel cell 4 is 4V, for example.

  Note that the fuel cell 4 is not limited to a plurality of fuel cells stacked in series, and may be configured by one fuel cell. Further, as the fuel cell, various fuel cells such as an active DMFC (Direct Methanol Fuel Cell), a passive DMFC, a DDFC (Direct DME Fuel Cell), and an RMFC (Reformed Methanol Fuel Cell) can be used. .

  The secondary battery 6 is configured by connecting, for example, two lithium ion secondary batteries in series, and the output voltage is about 6V to 8V. The secondary battery 6 is not limited to a lithium ion secondary battery, and various secondary batteries such as a nickel hydride secondary battery and a nickel cadmium secondary battery can be used. Further, instead of the secondary battery 6, a capacitor such as an electric double layer capacitor may be used as the power storage element.

  The control unit 8 includes, for example, a CPU (Central Processing Unit) that executes predetermined arithmetic processing, a ROM (Read Only Memory) that stores a predetermined control program, and a RAM (Random Access Memory) that temporarily stores data. And these peripheral circuits and the like. And the control part 8 controls the duty ratio of control signal S1 based on the voltage Vout detected by the voltage detection part 7 by running the control program memorize | stored in ROM.

  Further, the control unit 8 stops the operation of the voltage converter 5 when the control signal S2 output from the holding circuit 12 becomes low level, that is, the output voltage Vout becomes equal to or higher than the set voltage Vref, and outputs the output voltage Vout. When the output signal of the comparator 11 is set to a low level in order to decrease the on-duty ratio, the on-duty ratio of the control signal S1 is reduced by executing a program stored in advance in the ROM, and the fuel supply device (not shown) An output voltage lowering process is performed in which the amount of methanol fuel supplied to the battery 4 is decreased to reduce the power generation amount of the fuel cell 4.

  The fuel cell system 1 includes an oxygen amount adjusting device that adjusts the amount of oxygen supplied to the fuel cell 4, and the control unit 8 uses an oxygen amount instead of decreasing the fuel supply amount in the output voltage reduction process. The amount of oxygen supplied to the fuel cell 4 by the adjusting device may be reduced to reduce the power generation amount of the fuel cell 4, or both the fuel supply amount and the oxygen supply amount may be reduced. .

  Next, the operation of the fuel cell system 1 configured as described above will be described. The relationship between the duty ratio D of the switching element Q1 in the fuel cell system 1 and the boost ratio Va of the voltage converter 5 is expressed by the following equations (1) to (3).

D = Fon / (Fon + Foff) (1)
D = (1−Vt / Vout) (2)
Va = Vout / Vt
= 1 / (1-D) (3)
However, Fon is the time when the switching element Q1 is on, Foff is the time when the switching element Q1 is off, Vt is the input voltage of the voltage converter 5 (output voltage of the fuel cell 4), and Vout is the output of the voltage converter 5. The voltage is shown.

  Here, the output voltage Vout of the voltage converter 5 is equal to the output voltage of the secondary battery 6 in a normal operation state, and is about 6V to 8V. Further, since the power generation efficiency of the fuel cell 4 changes according to the output voltage Vt, the voltage Vt is set in advance so that the power generation efficiency of the fuel cell 4 is good.

  Then, based on the voltage Vt set in this way, for example, 4 V, by the control unit 8, the voltage Vout detected by the voltage detection unit 7 becomes an output voltage of the secondary battery 6, for example, 8 V, for example ( 2), the duty ratio D is calculated, and the control signal S1 having the duty ratio D is applied to the gate of the switching element Q1 via the AND gate 9. As a result, the switching element Q1 is turned on and off at the duty ratio D to 4V. The voltage Vt is boosted to a voltage Vout of 8V. The voltage Vout thus obtained is applied to the secondary battery 6 and is applied to the load 2 via the connection terminal 3.

  In this way, the electric power output from the fuel cell 4 is supplied to the load 2 at a predetermined voltage. Here, when the load current Ir flowing through the load 2 fluctuates, the power generation amount of the fuel cell 4 cannot be changed rapidly according to the change of the load current Ir, and thus appears as a fluctuation of the output voltage Vt of the fuel cell 4. For example, when the load current Ir decreases, the output voltage Vt of the fuel cell 4 increases, and the voltage Vout applied to the load 2 increases from Equation (2). On the other hand, for example, when the load current Ir increases, the output voltage Vt of the fuel cell 4 decreases and the voltage Vout applied to the load 2 decreases from the equation (2). Become stable.

  Therefore, in the fuel cell system 1, the secondary battery 6 absorbs fluctuations in the load current Ir of the load 2, that is, fluctuations in power consumption, and stabilizes the voltage Vout. Specifically, when the power output from the voltage converter 5 remains even if power is supplied to the load 2, the secondary battery 6 is charged with the surplus power. On the other hand, when the power supplied to the load 2 is not enough for the power output from the fuel cell 4, the secondary battery 6 discharges the insufficient power and the fuel cell 4 outputs from the connection terminal 3 to the load 2. Power that is a combination of the power and the power discharged from the secondary battery 6 is supplied. Thus, the secondary battery 6 plays a role of absorbing a sudden change in the power supplied to the load 2.

  Next, the operation of the fuel cell system 1 when the fluctuation of the load current Ir of the load 2 decreases beyond the fluctuation amount that can be absorbed by the secondary battery 6 will be described. FIG. 2 is a timing chart for explaining the operation of the fuel cell system 1. First, at the timing t1 indicating the normal operation state in FIG. 2, the control unit 8 outputs the control signal S1 having the duty ratio D to the AND gate 9 so as to increase the power generation efficiency of the fuel cell 4. To the gate of the switching element Q1. Then, switching element Q1 is turned on with duty ratio D.

  FIG. 3 is an explanatory diagram for explaining the operation of the fuel cell 4 under conditions where the supply amounts of fuel and oxygen are constant. The graph G1 shows the relationship between the output current (horizontal axis) and the output power (vertical axis) of the fuel cell 4, and the graph G2 shows the output current (horizontal axis) and the output voltage Vt (vertical axis) of the fuel cell 4. Shows the relationship. As shown in the graph G1, when the output current of the fuel cell 4 is Ai, the output power is maximum at Aw. As shown in the graph G2, the output voltage Vt of the fuel cell 4 at which the output current is Ai is Av. Therefore, at the timing t1, the control unit 8 sets the duty ratio D so that the voltage Vt in Expression (2) is Av and the voltage Vout is, for example, 8V. In this case, the fuel cell 4 is operating at the operating point P1 shown in FIG.

  Next, at time t2, when the load current Ir suddenly decreases beyond the amount of fluctuation that can be absorbed by the secondary battery 6, for example, the unillustrated power switch of the load 2 is turned off and the load current Ir suddenly increases. When it becomes zero, all output currents of the voltage converter 5 including the load current Ir that has been supplied to the load 2 so far flow into the secondary battery 6 as the current Ibat, and the voltage that is the terminal voltage of the secondary battery 6 Vout rises rapidly.

  When the voltage Vout reaches the set voltage Vref, the output signal of the comparator 11 is set to a low level, and the control circuit S2 is held at a low level for a time tw by the holding circuit 12 to the control unit 8 and the AND gate 9. Is output. The time tw is set to be equal to or longer than the processing time tc of the output voltage reduction process by the control unit 8.

  Then, the output signal S3 of the AND gate 9 is forcibly set to the low level regardless of the control signal S1 and is output to the gate of the switching element Q1, the switching element Q1 is turned off, and the operation of the voltage converter 5 is stopped. To do. In this case, the time from when the voltage Vout reaches the set voltage Vref until the operation of the voltage converter 5 is stopped is only the operation delay time of the comparator 11, the holding circuit 12, and the AND gate 9, so that it is extremely low, for example, about 10 nsec or less. It will be a short time.

  Next, when the switching element Q1 is turned off and the operation of the voltage converter 5 is stopped at timing t3, the fuel cell 4 and the secondary battery 6 are connected via the coil L and the diode D1. At this time, if the open circuit voltage of the fuel cell 4 is higher than the terminal voltage of the secondary battery 6, the step-up ratio Va is 1 if the voltage drop due to the diode D1 is ignored. In addition, the open circuit voltage of the fuel cell 4 is considerably higher than the normal operating voltage, while the output current of the fuel cell 4 is decreased as the output voltage is increased, resulting in an extremely small current value. Conversely, if the open circuit voltage of the fuel cell 4 is lower than the terminal voltage of the secondary battery 6, no current flows through the diode D1.

  As a result, when the fluctuation of the load current Ir of the load 2 decreases beyond the fluctuation amount that can be absorbed by the secondary battery 6, even if the voltage Vout suddenly rises, the voltage Vout is, for example, the load 2 and when the secondary battery 6 reaches the set voltage Vref set to the upper limit value of the rated voltage range that is not destroyed by the overvoltage, the comparator 11, the holding circuit 12, and the AND gate 9 cause an extremely low value of, for example, 10 nsec or less. Since the switching element Q1 is turned off in a short time and the voltage Vout is lowered to the output voltage value of the secondary battery 6, the occurrence of overvoltage when the load suddenly changes is reduced, and the load 2 and the secondary battery 6 are Damage due to overvoltage can be suppressed.

  In this case, for example, when the overvoltage is detected by the voltage detection unit 7 and the switching element Q1 is turned off by the CPU program execution process by the control unit 8, a time of about several milliseconds is required for the program execution process. There is a possibility that an overvoltage is generated during the processing time and the load 2 and the secondary battery 6 are damaged. However, in the fuel cell system 1, since the switching element Q1 can be turned off in an extremely short time, for example, 10 nsec or less, by the comparator 11, the holding circuit 12, and the AND gate 9, the load 2 and the secondary battery 6 can be prevented from overvoltage. The certainty of protecting can be improved.

  Here, for example, the open circuit voltage of the fuel cell 4 is set to about 8 V to 10 V, which is higher than the terminal voltage of the secondary battery 6, so that the step-up ratio of the voltage converter 5 is 1. Then, the output voltage Vt of the fuel cell 4 becomes Bv (for example, 6V to 8V) equal to the terminal voltage of the secondary battery 6. Then, as shown in FIG. 3, the operating point of the fuel cell 4 moves to P2, and from the power generation characteristics of the fuel cell 4, the output current of the fuel cell 4 becomes a current value Bi that is almost zero.

  On the other hand, when the control unit 8 receives the low-level control signal S2 from the holding circuit 12, the control unit 8 executes an output voltage reduction process by executing a program by the CPU. Then, after the processing time tc required for the output voltage reduction processing by the control unit 8 from the falling edge of the control signal S2, for example, about several milliseconds, the control unit 8 decreases the duty ratio D of the control signal S1. In this case, the duty ratio D is set by the control unit 8 so that the output current of the voltage converter 5 becomes a current value in a range that can be absorbed by the secondary battery 6.

  In this case, since the time tw during which the control signal S2 is held at the low level by the holding circuit 12 is set to be longer than the processing time tc, the switching element Q1 is in the OFF state during the output voltage reduction processing time by the control unit 8. As a result, the voltage converter 5 starts operating before the control unit 8 reduces the duty ratio D of the control signal S1, and the voltage Vout rises to cause an overvoltage, which damages the load 2 and the secondary battery 6. Is suppressed.

  Next, at timing t4 when the processing time tc has elapsed, the output signal of the comparator 11 is output as it is to the control unit 8 and the AND gate 9 as the control signal S2. Then, since the voltage Vout has already dropped to a voltage less than the set voltage Vref at the timing t4, the control signal S2 is output from the holding circuit 12 to the control unit 8 and the AND gate 9 at a high level.

  Then, the control signal S1 output by the AND gate 9 with the duty ratio D decreased by the control unit 8 is output to the base of the switching element Q1, and the switching element Q1 starts on / off operation with a small on-duty ratio. Therefore, the output current of the voltage converter 5, that is, the current Ibat flowing through the secondary battery 6 is suppressed to a small current value that can be absorbed by the secondary battery 6. As a result, the voltage Vout is stabilized by being the output voltage of the secondary battery 6. .

  Further, when the voltage converter 5 starts operating at the timing t4, the output current of the fuel cell 4 increases from Bi to Ci, and the output voltage Vt of the fuel cell 4 decreases from Bv to Cv. The point moves to P3. At this time, the duty ratio D is set by the control unit 8 so that the current Ci becomes, for example, ½ of the current Ai.

  Here, when the operating point of the fuel cell 4 changes from P1 to P3, as shown in FIG. 3, the output power also decreases to Cw which is about ½ of Aw. Then, since the fuel supply amount to the fuel cell 4 is the same at the operating point P3, the power charge that can be taken out from the fuel cell 4 with the same fuel supply amount is about ½, and the power generation efficiency of the fuel cell 4 is also about 1 / Decrease to 2.

  Therefore, at the timing t5, the supply amount of the fuel supplied from the fuel supply device (not shown) to the fuel cell 4 is reduced by the control unit 8 to improve the power generation efficiency. FIG. 4 is an explanatory diagram for explaining the operation of the fuel cell 4 when the fuel supply amount is changed. In order to simplify the explanation, the fuel supply amount is expressed in three stages of “Large”, “Medium”, and “Small”. The wavy line indicates the case where the fuel supply amount is “small”.

  The graphs G1 and G2 of the fuel supply amount “large” indicated by bold lines are the same as the graphs G1 and G2 shown in FIG. The fuel supply amount “medium” graphs G3 and G4 indicated by thin lines indicate the output current (horizontal axis) and the output power (vertical axis) of the fuel cell 4 when the fuel supply amount is decreased as compared with the graphs G1 and G2. And the relationship between the output current (horizontal axis) of the fuel cell 4 and the output voltage Vt (vertical axis). The fuel supply amount “small” graphs G5 and G6 indicated by the broken line indicate the output current (horizontal axis) and output power (vertical axis) of the fuel cell 4 when the fuel supply amount is further decreased than the graphs G3 and G4. And the relationship between the output current (horizontal axis) of the fuel cell 4 and the output voltage Vt (vertical axis).

  As shown in FIG. 4, when the fuel supply amount decreases, the fuel cell 4 also decreases the output current at which the output power is maximized, that is, the power generation efficiency is maximized. In FIG. 4, at the operating point P3, the maximum power generation efficiency is obtained in the graph G5 of the fuel supply amount “small”. Therefore, when the fuel supply amount to the fuel cell 4 is decreased by the control unit 8 and the fuel supply amount becomes “small” at the timing t6, the power generation efficiency of the fuel cell 4 is reduced at the operating point P3 as shown in the graph G5. Thus, the reduction in power generation efficiency of the fuel cell 4 due to the reduction in the occurrence of overvoltage can be reduced.

  The case where the load suddenly changes and the voltage Vout increases is, for example, when the fuel cell system 1 is connected to a portable personal computer, and the user suddenly removes the fuel cell system 1 from the portable personal computer. A case where the load 2 is removed from the fuel cell system 1 as in the case of the case is assumed.

  Further, when viewed from the voltage converter 5, the secondary battery 6 is also a load. Then, at timing t2, for example, when the connection wiring connecting the secondary battery 6 to the voltage converter 5 is disconnected, or when the secondary battery 6 is detachable like a battery pack, for example, When the secondary battery 6 is disconnected from the circuit by removing the secondary battery 6 or the like, the current Ibat that has flown into the secondary battery 6 until then flows to the load 2, the load current Ir increases, and the voltage Vout increases. . Even in the case of such a load change, the processing at the timings t3 to t6 described above can reduce the occurrence of overvoltage when the load suddenly changes, and can suppress the load 2 from being damaged by the overvoltage. The reduction in power generation efficiency of the fuel cell 4 due to the reduction in the occurrence of overvoltage can be reduced.

  When the secondary battery 6 is configured as a battery pack, an overvoltage protection circuit that disconnects the secondary battery 6 from the circuit when an overvoltage is applied may be incorporated in the battery pack. Even when such an overvoltage protection circuit for the battery pack is activated, the secondary battery 6 may be disconnected from the circuit, resulting in a load fluctuation. As a result, an overvoltage may occur, but the overvoltage protection circuit for the battery pack uses the set voltage Vref. If the voltage is lower than the working voltage, the voltage Vout is lowered before the overvoltage protection circuit works, and the possibility of overvoltage can be reduced.

  Moreover, although the example which improves the electric power generation efficiency of the fuel cell 4 by reducing the fuel supply amount to the fuel cell 4 by the control part 8 was shown, instead of reducing the fuel supply amount, oxygen to the fuel cell 4 is shown. The power generation efficiency may be improved by reducing the power generation amount of the fuel cell 4 by decreasing the supply amount or by reducing both the fuel supply amount and the oxygen supply amount.

  The fuel cell system according to the present invention is particularly useful when a fuel cell is connected to a portable small electronic device such as a mobile phone, a personal digital assistant (PDA), a portable personal computer, a video camera or the like. The present invention can also be applied when a fuel cell such as an electric wheelchair, an electric scooter, or a portable power source is mounted.

It is a block diagram which shows an example of a structure of the fuel cell system which concerns on one Embodiment of this invention. 2 is a timing chart for explaining the operation of the fuel cell system shown in FIG. 1. It is explanatory drawing for demonstrating operation | movement of the fuel cell in the conditions with the supply amount of a fuel and oxygen constant. It is explanatory drawing for demonstrating operation | movement of a fuel cell when changing the supply_amount | feed_rate of fuel.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel cell system 2 Load 3 Connection terminal 4 Fuel cell 5 Voltage converter 6 Secondary battery 7 Voltage detection part 8 Control part 9 And gate 10 Reference voltage source 11 Comparator 12 Holding circuit C1, C2 Capacitor D1 Diode L Coil Q1 Switching element D Duty ratio Vref Setting voltage tc Processing time

Claims (6)

A fuel cell that generates electricity based on fuel and oxygen;
A connection terminal for connecting a load;
A voltage converter that converts the output voltage of the fuel cell into a predetermined voltage and outputs the voltage to the connection terminal;
A first control unit that executes an output voltage lowering process for generating a control signal for lowering the output voltage of the voltage converter when the output voltage of the voltage converter is equal to or higher than a preset setting voltage;
When the output voltage of the voltage converter is detected and the detected voltage becomes equal to or higher than the set voltage, the operation of the voltage converter is stopped, and the voltage is reduced after the output voltage reduction process is performed by the first control unit. A fuel cell system comprising: a second control unit that operates a converter in response to the control signal. The fuel cell system according to claim 1, further comprising a power storage element that is charged by an output voltage of the voltage converter and outputs the charged power to the connection terminal. The second controller is
A comparator that compares the output voltage of the voltage converter with the set voltage, and outputs a stop signal to stop the operation of the voltage converter when the output voltage of the voltage converter becomes equal to or higher than the set voltage;
The fuel cell system according to claim 1, further comprising: a holding circuit that holds the stop signal for a time equal to or longer than a processing time for the output voltage reduction process. The said 1st control part determines with the output voltage of the said voltage converter becoming more than the preset setting voltage, when the said stop signal is output from the said comparator. Fuel cell system. The voltage converter includes a switching element that turns on and off the output voltage of the fuel cell according to a duty ratio of the control signal, and outputs an output voltage to the connection terminal according to a decrease in the duty ratio of the control signal. Lower,
The fuel cell system according to any one of claims 1 to 4, wherein the first control unit reduces a duty ratio of the control signal as the output voltage reduction process. The said 1st control part further reduces at least one among the supply amount of the fuel and oxygen to the said fuel cell in the said output voltage fall process. The fuel cell system described in 1.

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