Fuel cell system

Abstract

The present invention provides a fuel cell system capable of appropriately measuring the AC impedance of a fuel cell. A fuel cell system is characterized in that a control unit performs on / off control of n-phase switches, causes the n-phase switches to operate at different phases respectively, and causes the n-phase switches to operate at the same duty ratio respectively. When it is determined that a specific condition is satisfied, the n-phase switches are caused to operate at different duty ratios respectively, and the AC impedance of the fuel cell is measured based on the current waveform and voltage waveform of the fuel cell.

Description

Technical Field

[0001] The present disclosure relates to a fuel cell system. Background Art

[0002] Regarding converters provided in systems mounted and used in vehicles such as fuel cell electric vehicles (FCEVs), various studies have been conducted. In DC / DC converters used in various electronic devices and the like, a circuit composed of a reactor, a switch, a diode, a capacitor, etc. is often used. The DC / DC converter controls the increase and decrease of the current flowing through the reactor according to the on / off signal of the switch.

[0003] For example, in Patent Document 1, a fuel cell system that measures the AC impedance of a fuel cell stack with high precision is disclosed.

[0004] Moreover, in Patent Document 2, a fuel cell system mounted on a vehicle that can measure the impedance of a fuel cell with high precision without being affected by load fluctuations of auxiliary machines is disclosed.

[0005] Furthermore, in Patent Document 3, an impedance measurement method that calculates impedance with high precision in a short time using a simple device and structure is disclosed.

[0006] In addition, in Patent Document 4, a fuel cell system that uses a bidirectional buck-boost converter provided with a dead time to apply AC for measuring the impedance of a fuel cell to the fuel cell with high precision is disclosed.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-098134

[0010] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2014-232681

[0011] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2013-145692

[0012] Patent Document 4: Japanese Unexamined Patent Application Publication No. 2014-235781

[0013] As one of the indicators for controlling the operating state of a fuel cell to an optimal state, the AC impedance of the fuel cell is used. A converter controls the step-up / step-down of the output voltage of the fuel cell based on the switching operation of a switching element (switch). It is known that there is a region where the response performance deteriorates with respect to the change in the duty ratio in the converter. When measuring the AC impedance of the fuel cell in such a response performance deterioration region, the overlapping accuracy of the high-frequency signal to the fuel cell by the converter deteriorates, and thus there is a problem that the AC impedance measurement accuracy significantly deteriorates.

[0014] Aiming at miniaturization of the converter, the introduction of a magnetic coupling converter is considered as an option. The magnetic coupling converter includes a magnetic coupling reactor that magnetically couples a plurality of coils on the same core. In the magnetic coupling converter, in order to alleviate the magnetic saturation of the core of the reactor and reduce the output current ripple, the magnetically coupled coils are usually switched alternately at the same duty ratio and at equal intervals of phase difference. The magnetic coupling reactor operates in a discontinuous mode that includes a time when the current flowing in the coil becomes zero at low output of the fuel cell.

[0015] The present inventors newly found that during the operation in the discontinuous mode where the current is discontinuous, due to the negative current from other coils generated by the mutual inductance, which is a phenomenon peculiar to the magnetic coupling reactor, it becomes a suspected on-state of the switch, and there is a "dead zone" in which the control of the switch from off to on does not smoothly switch and the output current value of the fuel cell does not change even when the duty ratio is changed. In the dead zone, even if the duty ratio is changed and an AC signal for AC impedance measurement is applied, the output current value of the fuel cell hardly changes as described above, and thus the AC impedance of the fuel cell cannot be appropriately measured. This corresponds to the response performance deterioration region of the above-mentioned Patent Document 1, but since the period when the AC impedance is not measured or the period when the AC impedance cannot be measured frequently occurs, it may not be possible to control the operating state of the fuel cell to an optimal state. Summary of the Invention

[0016] The present disclosure has been completed in view of the above actual situation, and its main object is to provide a fuel cell system capable of appropriately measuring the AC impedance of a fuel cell.

[0017] The fuel cell system of the present disclosure is a fuel cell system. Among them, the fuel cell system includes a fuel cell and a converter. The converter performs at least one selected from the group consisting of boosting and bucking the output voltage of the fuel cell. The converter includes a reactor in which n-phase coils are magnetically coupled to each other, n-phase switches respectively connected to the coils, and a control unit, where n is an integer of 2 or more. The control unit performs on / off control of the n-phase switches. The control unit monitors the current values of the coils. The control unit measures the AC impedance of the fuel cell based on the current waveform and voltage waveform of the fuel cell. The control unit operates the n-phase switches at different phases respectively and operates the n-phase switches at the same duty ratio respectively. When the control unit determines that the following condition 1 is satisfied, the control unit operates the n-phase switches at different duty ratios. Condition 1 is the following operating condition: when the n-phase switches are operated at different phases respectively, the current flowing in the coils is in discontinuous mode, and when the current value flowing in one-phase of the coils is maintained at zero, the switch connected to at least one other-phase of the coils is switched from on to off.

[0018] In the fuel cell system of the present disclosure, it is possible that the control unit operates the n-phase switches at a phase difference of (360 / n)°.

[0019] In the fuel cell system of the present disclosure, it is possible that the control unit sets the offset current value y, which is added to or subtracted from the command current values of the magnetically coupled n-phase coils so that the target current values of the magnetically coupled n-phase coils are different, to 25 to 75% of the dead zone current interval. The control unit unevenly distributes the target current values in such a way that the target current values obtained by adding or subtracting the offset current value y from the command current values are different for each coil, and operates the n-phase switches at different duty ratios respectively.

[0020] In the fuel cell system of the present disclosure, it is possible that the control unit pre-stores a current distribution difference map derived from the relationship between the input voltage and output voltage of the converter and the current values flowing in the coils of the reactor. The control unit unevenly distributes the target current values in such a way that the target current values are different for each coil based on the current distribution difference map, and operates the switches at different duty ratios respectively.

[0021] In the fuel cell system of the present disclosure, it is possible that the control unit stores the operating conditions of the converter equivalent to the condition 1 in advance as a data set, and the control unit compares the current operating conditions of the converter with this data set to determine whether the condition 1 is satisfied.

[0022] In the fuel cell system of the present disclosure, it is possible that when n = 2, and when the switches of two phases that are mutually magnetically coupled operate with a phase difference of 180°, the current flowing in the coil is in a discontinuous mode, the duty ratios of the switches of the two phases are less than 50% (D < 0.5), and the following formula (A) or formula (B) is satisfied, it is determined that the condition 1 is satisfied. Formula (A): D < {(1 / 2)(L - M)(V H - V L )} / (LV L + MV L - MV H ), formula (B): D < (1 / 2){1 - (V L / V H )}, in formula (A) and formula (B), L is the self-inductance of the reactor, M is the mutual inductance of the reactor, V H is the output voltage of the converter, V L is the input voltage of the converter, and D is the duty ratio (-).

[0023] In the fuel cell system of the present disclosure, it is possible that the control unit confirms whether there is a requirement for measuring the AC impedance of the fuel cell. When the control unit confirms that there is a requirement for measuring the AC impedance of the fuel cell, it is determined that the condition 1 is satisfied. When the control unit determines that the condition 1 is satisfied, the switches of n phases operate with different duty ratios to measure the AC impedance of the fuel cell.

[0024] Advantages of the Invention

[0025] According to the fuel cell system of the present disclosure, the AC impedance of the fuel cell can be appropriately measured. Brief Description of the Drawings

[0026] Figure 1 FIG. is an example of a circuit structure of a fuel cell system having a boost converter and peripheral components.

[0027] Figure 2 FIG. is a graph showing the current change of the average current value (single-phase average current value) flowing in each coil of the reactor when the input voltage Vfc is 200V, the output voltage Vh is 350V, and the boost ratio is kept constant and the duty ratio is slowly increased while driving the mutually magnetically coupled two-phase coils.

[0028] Figure 3 It is a diagram showing the current waveform when sweeping the DC load current in the range of 5 - 25 A by overlapping sine waves of 270 Hz with amplitudes of ±3A in each phase for the coils of two phases with mutually magnetic coupling driving.

[0029] Figure 4 It is a diagram showing the current waveforms of the coils of two phases with magnetic coupling in the region where dead zones are generated.

[0030] Figure 5 It is a diagram showing an example of a two - phase magnetic - coupling boost circuit for the U - phase and V - phase.

[0031] Figure 6 It is a diagram showing an example of the current waveform when the current flowing through the coil of the reactor in the boost converter is in the continuous mode.

[0032] Figure 7 It is a diagram showing an example of the current waveform when the current flowing through the coil of the reactor in the boost converter is in the discontinuous mode.

[0033] Figure 8 It is a flowchart showing an example of the control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter with a two - phase magnetic - coupling reactor.

[0034] Figure 9 It is a flowchart showing another example of the control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter with a two - phase magnetic - coupling reactor.

[0035] Figure 10 It shows the current behavior when the target current value is evenly distributed and flows in such a way that the average current values flowing through the coils of two phases with magnetic coupling are the same, and then the average current value rises from 5 A to 25 A and then decreases.

[0036] Figure 11 It is Figure 10 a partial enlarged view of

[0037] Figure 12 It shows the current behavior when the target current value is unevenly distributed and flows with a 2A current offset in such a way that the average current values flowing through the coils of two phases with magnetic coupling are different, and then the average current value rises from 5 A to 25 A and then decreases.

[0038] Figure 13 It is Figure 12 a partial enlarged view of

[0039] Figure 14It is a flowchart showing another example of control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter having a two-phase magnetically coupled reactor.

[0040] Figure 15 It is a diagram showing the dead zone generation area in the discontinuous mode.

[0041] Figure 16 It is a flowchart showing another example of control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter having a two-phase magnetically coupled reactor. Detailed Description

[0042] The fuel cell system of the present disclosure is a fuel cell system including a fuel cell and a converter. The converter performs at least one selected from the group consisting of boosting and bucking the output voltage of the fuel cell. The converter includes a reactor in which n-phase coils are magnetically coupled to each other, n-phase switches respectively connected to the coils, and a control unit, where n is an integer of 2 or more. The control unit performs on / off control of the n-phase switches, monitors the current values of the coils, measures the AC impedance of the fuel cell based on the current waveform and voltage waveform of the fuel cell, causes the n-phase switches to operate at different phases respectively, and causes the n-phase switches to operate at the same duty ratio respectively. When it is determined that the following condition 1 is satisfied, the control unit causes the n-phase switches to operate at different duty ratios respectively.

[0043] Condition 1 is the following operating condition: when the n-phase switches operate at different phases respectively, the current flowing in the coils is in the discontinuous mode, and when the current value flowing in one-phase of the coils is maintained at zero, the switch connected to at least one other-phase of the coils switches from on to off.

[0044] In a magnetically coupled converter, a dead zone may occur where the output current value of the fuel cell does not change even when the duty ratio of the switch changes. In this case, the AC impedance of the fuel cell cannot be measured appropriately. In the present disclosure, the dead zone, which is a phenomenon peculiar to the magnetically coupled reactor, is controlled to have a minimum generation frequency to measure the AC impedance of the fuel cell. As a specific control, under the condition of dead zone generation, the average current values of the coils flowing in the magnetically coupled phases are made different, thereby avoiding the dead zone and appropriately measuring the AC impedance of the fuel cell. By making the current values flowing in the coils of the magnetically coupled phases different, at least one phase is out of the dead zone range. As a result, the phase out of the dead zone can cause current fluctuations, and thus the AC impedance of the fuel cell can be measured appropriately, and the operating state of the fuel cell can be controlled optimally.

[0045] Figure 1 This is a diagram showing an example of the circuit configuration of a fuel cell system having a boost converter and peripheral components.

[0046] Figure 1 The fuel cell system shown is mounted on, for example, a vehicle and is connected to the drive motor of the vehicle, which is an external load 50, via a converter. Also, although not shown, a storage battery may be provided in parallel with the fuel cell 10 and the boost converter 20. The output power of the fuel cell 10 is boosted by the boost converter 20 and then converted from DC to AC by the converter and supplied to the motor.

[0047] The boost converter 20 includes six-phase boost circuits connected in parallel with each other. In Figure 1 this, a structure including a six-phase boost circuit is shown, but the number of phases is not particularly limited.

[0048] The boost circuit includes a reactor 21, a current sensor 22, a switch 23, a diode 24, and a capacitor 25. The boost circuit may also include an input voltage sensor and an output voltage sensor. In the six-phase boost circuit, every two phases can share the core of one reactor 21 and are magnetically coupled to each other. In each boost circuit, when the switch 23 is turned on, the current flowing in the reactor 21 increases, and when the switch 23 is turned off, the current flowing in the reactor 21 decreases and remains zero when the current reaches zero. The current sensor 22 acquires the current value flowing in the reactor 21.

[0049] A control unit (not shown) controls the boost ratio in the converter 20 and the output current value from the fuel cell 10 by controlling the on / off of the switch 23.

[0050] The output power of the fuel cell 10 varies greatly according to the requirements of the vehicle (such as speed, acceleration, load, and road gradient), and accordingly, the output current also varies greatly. When the output current of the fuel cell 10 is large, if this current flows into one boost circuit, the heat generation increases and the power conversion efficiency of the boost converter 20 decreases. Also, when only a small current flows in a boost circuit that can withstand a large current, the energy loss also increases and the power conversion efficiency of the boost converter 20 decreases.

[0051] Therefore, the boost converter 20 includes a multi-phase boost circuit ( Figure 1In the illustrated example (six phases), the boost converter 20 switches the number of phases for driving according to the output current value of the fuel cell 10. For example, when the output current value of the fuel cell 10 is 0 to 150 A, it is driven with two phases; when it is 150 to 300 A, it is driven with four phases; and when it is 300 to 600 A, it is driven with six phases. Since the efficiency of the boost circuit varies depending on the flowing current, by changing the number of driving phases, it is possible to operate with the optimal efficiency in each current range.

[0052] The fuel cell system includes a fuel cell and a converter.

[0053] The fuel cell may have only a single cell or may be a fuel cell stack in which a plurality of single cells are stacked.

[0054] The converter performs at least one selected from the group consisting of boosting and bucking the output voltage of the fuel cell. The converter may be a boost converter, a buck converter, or a buck-boost converter.

[0055] The converter may also be a DC / DC converter.

[0056] The converter includes a reactor in which n (n is an integer of 2 or more) phase coils are magnetically coupled to each other, n phase switches respectively connected to the coils, and a control unit. The converter may also include a diode, a current sensor, a voltage sensor, an optocoupler, and a capacitor, etc.

[0057] The reactor has a coil and a core.

[0058] On the core, n (n is an integer of 2 or more) phase coils may be wound. As long as n is 2 or more, the upper limit is not particularly limited and may be 10 or less, 5 or less, 4 or less, or 3 or less.

[0059] The core and coil of the reactor may also be the core and coil used in a conventionally known converter.

[0060] In the present disclosure, a reactor having a core with a single independent coil wound thereon is called a non-magnetically coupled reactor. In the present disclosure, a converter equipped with a non-magnetically coupled reactor is called a non-magnetically coupled converter. In the present disclosure, a reactor having a core with two or more independent coils wound thereon is called a magnetically coupled reactor. In the present disclosure, a converter equipped with a magnetically coupled reactor is called a magnetically coupled converter.

[0061] In the present disclosure, an independent coil means a coil having one or more spiral portions and two terminal portions.

[0062] The switch (switching element) is composed of a transistor and a protection diode. The transistor is an npn-type transistor, and for example, it can be an IGBT (Insulated Gate Bipolar Transistor) and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), etc.

[0063] The diode can also be the diode used in a conventionally known converter.

[0064] The current sensor only needs to be able to obtain the current value flowing in the coil of the reactor (sometimes referred to as the reactor current value), and is not particularly limited, and a conventionally known ammeter, etc. can be used. And in the present disclosure, the reactor average current refers to the average current flowing in the coil during the switching period of the duty ratio control.

[0065] The voltage sensor only needs to be able to obtain the input voltage output from the fuel cell and input to the converter, and is not particularly limited, and a conventionally known voltmeter, etc. can be used.

[0066] The control unit can also be an electronic control unit (ECU: Electronic Control Unit), etc. The ECU is configured to include a CPU (Central Processing Unit), a memory, and an input / output buffer.

[0067] The control unit detects the current value flowing in the coils of the n phases of the reactor based on the signal from the current sensor, and monitors the current value of the coils.

[0068] The control unit performs on / off control of the switches of the n phases. The control unit can operate the magnetically coupled switches of the n phases at a constant frequency.

[0069] The control unit can perform on / off control of the switches by periodically switching the on command and off command given to the switches, and control the duty ratio of the switches of the n phases. Thereby, the output current value from the fuel cell can be controlled.

[0070] In the boost converter, the operation of the switch that accumulates and releases electric energy relative to the reactor is repeated to perform boosting. The duty ratio specifies the proportion of the accumulation period during which the switching element is turned on and electric energy is accumulated in the reactor during the switching period of the boosting operation. Let the switching period of the boosting operation in the boost converter be T, and let the period during which the switching element is turned on be T ON and the period during which it is turned off be T OFF When, the duty ratio D is expressed as D(-) = T ON / T. For convenience, the duty ratio is sometimes expressed as a percentage (%). In this case, D(%) = T ON / T × 100.

[0071] In the present disclosure, the period of the switch (switching period) refers to the period from the moment when the switch changes from off to on until the moment when the switch changes from off to on again.

[0072] The control unit can operate the n-phase switches with a phase difference of (360 / n)°.

[0073] When the switch is 2-phase and the 2-phase switches operate with different phases, the control unit can operate the 2-phase switches with a phase difference of 30 to 180°. From the perspective of improving power conversion efficiency, they can operate with a phase difference of 180°, that is, in antiphase. In the case of operation in antiphase, a phase error can be generated within the range of -5° to +5°.

[0074] The control unit operates the n-phase switches with different phases and operates the n-phase switches with the same duty ratio.

[0075] When the control unit determines that the following Condition 1 is satisfied, it operates the n-phase switches with different duty ratios. That is, the average current values flowing in the coils of each phase are made different from each other.

[0076] Condition 1 is the following operating condition: when the n-phase switches are operated with different phases, the current flowing in the coil is in the discontinuous mode, and when the current value flowing in one phase of the coil remains zero, the switch connected to at least one other phase of the coil changes from on to off.

[0077] In the case of a magnetic coupling reactor with 3 or more phases, the operating condition that the current value flowing in one phase of the coil is zero and the switch connected to other coils changes from on to off can be the operating condition that at least any one of the switches in the remaining phases changes from on to off.

[0078] When driving the mutually magnetically coupled n-phases with a phase difference of (360° / n), Condition 1 can be the discontinuous mode and when the duty ratio is less than (100 - 100 / n)%.

[0079] The control unit can pre-store the operating conditions of the converter corresponding to Condition 1 as a data group, and compare the current operating conditions of the converter with this data group to determine whether Condition 1 is satisfied. Thereby, it is possible to avoid entering the dead zone and improve the accuracy of the AC impedance measurement of the fuel cell.

[0080] [Explanation of dead zone]

[0081] The "dead zone" refers to the region where even if the PWM duty ratio of the converter is increased or decreased, the average current value flowing through the coil of the reactor hardly changes. Due to the negative current generated by the mutual inductance of magnetic coupling, it appears in a part of the discontinuous mode region where the duty ratio is 50% or less in a two-phase magnetically coupled single-phase boost (buck) circuit.

[0082] Figure 2 It is a graph showing the current change of the average current value (single-phase average current value) flowing through each coil of the reactor when the input voltage Vfc is 200V, the output voltage Vh is 350V, the boost ratio is kept constant, and the duty ratio is slowly increased by driving the two-phase coils with mutual magnetic coupling. In Figure 2 it, the self-inductance of the coil is 96.4 μH, the mutual inductance of the coil is 62.7 μH, and the driving frequency of the switch is set to 20 kHz.

[0083] According to the state equation (described later) for calculating the average current value flowing through the coil of the reactor, it should be as Figure 2 shown by the dotted line, that is, the average current value flowing through the coil of the reactor increases monotonically as the duty ratio increases. However, in reality, as shown by the solid line, it shows the characteristic that the current increases step by step. There is a "dead zone" in the part surrounded by the dotted line where the average current value flowing through the coil of the reactor does not increase even if the duty ratio is increased. When the average current value flowing through the coil of the reactor is in the above-mentioned dead zone, even if the duty ratio is changed, the average current value flowing through the coil of the reactor, that is, the output current value of the fuel cell, does not change, and the AC impedance of the fuel cell cannot be measured properly.

[0084] Figure 3 It is a graph showing the current waveform when driving the two-phase coils with mutual magnetic coupling and overlapping sine waves with amplitudes of ±3A and a frequency of 270 Hz in each phase to sweep the DC load current in the range of 5 - 25A. In Figure 3 it, similar to Figure 2 the input voltage Vfc is 200V, the output voltage Vh is 350V, the boost ratio is kept constant, the self-inductance of the coil is 96.4 μH, the mutual inductance of the coil is 62.7 μH, and the driving frequency of the switch is set to 20 kHz. In Figure 3 this case, it can be considered that the relationship between the duty ratio and the average current value flowing through the coil of the reactor follows the Figure 2 dotted line shown. Therefore, the duty ratio amplitude is adjusted along the dotted line. However, as described above, in reality, the change of the current value is step-like as shown by the Figure 2 solid line. Therefore, in the Figure 2 dead zone shown by the dotted line, the current does not change even if the duty ratio is increased or decreased. As Figure 3As shown by the dotted line, there are regions in the current where sine waves cannot overlap, and within these regions, it is impossible to measure the AC impedance of the fuel cell. It should be noted that, in Figure 3 When a sudden change in the DC load occurs within the specific region indicated by the dotted line, the output response of the fuel cell may be delayed, and the load on components such as the battery may increase.

[0085] Taking into account Figure 3 , when the control unit increases or decreases the duty ratio of the n-phase switches in such a way that the amplitude of the current value of the coil becomes the normal amplitude, if the measured amplitude of the current value of the coil is less than the expected value, it is determined that Condition 1 is satisfied. When the amplitude of the current value of the coil is less than the expected value, it is equivalent to a state where sine waves do not overlap, and it can be determined that the system has entered the dead zone.

[0086] Figure 4 FIG. is a diagram showing the current waveforms of the two-phase coils representing magnetic coupling within the region where the dead zone occurs. In Figure 4 , the current of the U-phase is denoted as L1, and the current of the V-phase is denoted as L2.

[0087] During the period from time t0 to t1, the switch of the V-phase is turned on, and the L2 current slowly increases. On the other hand, the switch of the U-phase is turned off, and the L1 current remains at 0. At time t1, the switch of the V-phase is switched from on to off, causing the L2 current of the V-phase to decrease. Here, the switch of the U-phase remains off, but after time t1, due to the interaction of the V-phase, the L1 current flowing in the U-phase alternately decreases and increases.

[0088] Here, for example, at time t2, although the switch of the U-phase is turned on when the L1 current of the U-phase is increasing, the current is already increasing, so the signal indicating that the switch is turned on is not recognized. Then, at time t3, it becomes the timing when the L1 current of the U-phase decreases, and for the first time, the signal indicating that the switch of the U-phase is turned on is recognized, and then the L1 current increases until the switch of the U-phase is turned off. The timing when the signal indicating that the switch is turned on is not recognized and the timing when the signal indicating that the switch is turned on is recognized alternate, so as Figure 2 such, the dead zone recurs.

[0089] It should be noted that, as described above, when the current value of one phase of the magnetically coupled coil is 0 and the switch of the other phase is switched from on to off, a current increase or decrease occurs in one phase of the coil, and as a result, the dead zone appears.

[0090] Therefore, under operating conditions such as Condition 1, where the current flowing in the coil is in a discontinuous mode when the n-phase switches are driven with different phases respectively, and when the current value of one phase of the magnetically coupled coil remains 0 and the switch of the other phase is switched from on to off, it can be considered that the dead zone is generated.

[0091] In the case of two-phase magnetic coupling, the phase difference for driving the two phases can be 180°. Dead zones are generated under the following operating conditions: when the phase difference for driving the two phases is 180 degrees, if it is in discontinuous mode and the duty ratio is less than 50%, then when the average current value of the coil of one phase in magnetic coupling is maintained at 0, the switch of the other phase changes from on to off.

[0092] Figure 5 It is a diagram showing an example of a two-phase magnetic coupling boost circuit for the U phase and the V phase. In Figure 5 V L represents the input voltage (voltage before boosting), I U represents the U-phase current, I V represents the V-phase current, V H represents the output voltage (voltage after boosting), D represents a diode, S represents a switch, M represents a mutual inductor, L represents a self-inductor, and r represents an internal resistance.

[0093] Equation (1) is the state equation of the current I vector. By solving this state equation, the Figure 2 current curve indicated by the dashed line in

[0094] Because of the negative current generated by the mutual inductance of magnetic coupling represented by Figure 5 and M in Equation (1), dead zones appear in a part of the discontinuous mode region where the switches of the two phases are off, that is, driven with a 180° phase difference and a duty ratio less than 50% in a two-phase magnetic coupling unidirectional boost (buck) circuit, as shown by the shaded part in Table 1. The mechanism is as follows: as obtained by solving Equation (1) over time under the conditions of Table 1, when the current flowing in one coil is 0 A and the current flowing in the other coil is positive in the state where the switches of the two phases are off, a counter electromotive voltage is generated in the 0 A side coil by the mutual inductance, generating a negative current. Then, since the negative current increases and the electromotive force of the coil disappears and immediately starts to decrease, during the period until it returns to 0 A, the switch becomes a pseudo-on state as shown by the shaded part in Table 1. Therefore, regardless of whether the negative current side switch is on or off, it has no effect on the current waveform, resulting in a control dead zone.

[0095] It should be noted that, for example, in the case of a three-phase magnetic coupling boost circuit of the U phase, V phase, and W phase, the dead zone generation conditions are assumed to be when all three-phase switches are off and I U > 0, I V ≤ 0, I W ≤ 0, the case of I U > 0, I V > 0, IW Cases where it is ≤ 0, etc.

[0096] Not only in the boost circuit, but also in the buck circuit and the buck-boost circuit, there is a problem of current control dead zone as in the boost circuit. When the dead zone occurs, in-phase driving is performed, thereby being able to avoid or suppress the generation of the dead zone. It should be noted that for a bidirectional circuit that does not have a discontinuous mode, it can be considered that the dead zone does not exist.

[0097]

Mathematical formula 1

[0098]

[0099]

Table 1

[0100]

[0101] [Explanation of continuous mode and discontinuous mode]

[0102] Figure 6 It is a diagram showing an example of the current waveform in the case where the current flowing in the coil of the reactor of the boost converter is in the continuous mode.

[0103] Figure 7 It is a diagram showing an example of the current waveform in the case where the current flowing in the coil of the reactor of the boost converter is in the discontinuous mode.

[0104] As Figure 6 shown, the current flowing in the coil of the reactor of the boost converter (reactor current) becomes a triangular wave along with the switching operation, and the central value of the triangular wave is the reactor average current (hereinafter referred to as the average current). Here, when the duty ratio is reduced and the average current decreases, the lowest point of the triangular wave reaches 0 A. When the average current is further decreased from here, since the boost converter is a unidirectional circuit, as Figure 7 shown, a period during which the reactor current becomes zero starts to occur. Thus, the operation having a period during which the current flowing in the coil of the reactor becomes zero in the switching cycle of the duty ratio control of the converter is called the discontinuous mode, and the operation not having a period during which the current flowing in the coil of the reactor becomes zero in the switching cycle of the duty ratio control is called the continuous mode.

[0105] <Mode A>

[0106] In the case of a two-phase magnetically coupled reactor, during the operation of the fuel cell system, when suddenly entering or approaching the region where the dead zone equivalent to Condition 1 is generated, when driving the two mutually magnetically coupled coils, the two phases are controlled under unequal current command values to avoid simultaneously entering Figure 2The shown dead zone is such that at least one phase is out of the dead zone, maintaining a controllable state and enabling sine wave overlap.

[0107] The region where the dead zone corresponding to Condition 1 is generated can be when it is in the discontinuous mode and the duty ratio is less than 50% in the case of a two-phase magnetically coupled reactor. Therefore, for example, when the required output of the fuel cell decreases, at the timing when it is in the discontinuous mode and the duty ratio decreases from 50% or more to less than 50%, it can transfer from a state where the average current values flowing in the mutually magnetically coupled two-phase coils are the same (the state where the target current values are the same) to a state where the average current values flowing in the mutually magnetically coupled two-phase coils are different (the state where the target current values are different).

[0108] Figure 8 It is a flowchart showing an example of the control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter having a two-phase magnetically coupled reactor.

[0109] Figure 8 It is an example of the aforementioned Mode A. The control unit operates the two-phase switches at different phases and the same duty ratio during normal times. Then, the control unit determines whether Condition 1 is satisfied. When the control unit satisfies Condition 1, it changes the operation of the two-phase switches from the operation at different phases and the same duty ratio to the operation at different phases and different duty ratios, and measures the AC impedance of the fuel cell. On the other hand, when Condition 1 is not satisfied, the control unit operates the two-phase switches at different phases and the same duty ratio, and measures the AC impedance of the fuel cell.

[0110] The control unit can set the offset current value y added to or subtracted from the command current value of each of the magnetically coupled n-phase coils so that the target current values of the magnetically coupled n-phase coils are different, and set it to 25 - 75% of the dead zone current interval.

[0111] The control unit can unevenly distribute the target current values so that the target current values obtained by adding or subtracting the offset current value y from the command current value are different for each coil, and operate the switches at different duty ratios.

[0112] The control unit can pre-store a current distribution difference map derived from the relationship between the input voltage and output voltage of the converter and the current value flowing in the coil of the reactor, and unevenly distribute the target current values so that the target current values are different for each coil based on this current distribution difference map, and operate the switches at different duty ratios.

[0113] When applying current to the phases of a driving-phase distributed fuel cell, if the total input current to the converter is set to I [A] and the number of driving phases is set to n (n is, for example, 2, 4, or 6), the current command value for the coils of each phase is I / n [A].

[0114] Based on the current command value (I / n [A]) for the coils of each phase, the same offset value (y [A]) is set among the magnetic coupling reactors of the UV2 phase, the magnetic coupling reactors of the WX2 phase, and the magnetic coupling reactors of the YZ2 phase.

[0115] In the magnetic coupling reactor of the UV2 phase, the offset value (y [A]) can be added to the current command value (I / n [A]) in the U phase to make (I / n + y [A]) flow, and the offset value (y [A]) can be subtracted from the current command value (I / n [A]) in the V phase to make (I / n - y [A]) flow, thereby achieving unequal current distribution. Unequal current distribution can be achieved in the magnetic coupling reactors of the WX2 phase and the YZ2 phase in the same way as in the magnetic coupling reactor of the UV2 phase.

[0116] The value of the above-mentioned offset value y can be, for example, 1 to 3.

[0117] As Figure 2 When moving as such, the interval of the current at which the dead zone appears can be obtained in advance, and the value of y can be selected in such a way that the interval at which the dead zone appears does not coincide with 2y.

[0118] It should be noted that when the discontinuous mode corresponding to Condition 1 is not satisfied and the duty ratio is less than 50%, the average current values flowing in the coils of the two magnetically coupled phases can be made the same. By doing so as described above, the pulsating current generated in the converter can be reduced, heat generation can be suppressed, and the efficiency in the converter can be maintained at a relatively high level.

[0119] <Mode B>

[0120] In the example of the above-mentioned Mode A, when entering the region where the dead zone corresponding to Condition 1 is generated, it is switched in such a way that the average current values flowing in the coils of each magnetically coupled phase are different. However, it can also be switched when the AC impedance of the fuel cell is obtained again.

[0121] As described above, usually the AC impedance is obtained at each predetermined timing during the operation of the fuel cell system. When the average current values flowing in the reactors of the magnetically coupled phases are made different, as described above, the pulsating current becomes larger and the heat generation also becomes larger. Therefore, the average current values flowing in the coils of each magnetically coupled phase can be made the same during normal operation.

[0122] By limiting the timing that makes the average current value different only when obtaining the AC impedance, the pulsating current generated in the converter can be reduced, heat generation can be suppressed, and the efficiency in the converter can be maintained relatively high.

[0123] It should be noted that in the case where it enters the dead zone area when measuring the AC impedance and the AC impedance measurement cannot be performed, the current can be offset and the AC impedance can be measured. And when it is impossible to get out of the dead zone even if the current is offset, the offset amount can be changed.

[0124] Figure 9 It is a flowchart showing another example of the control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter having a two-phase magnetic coupling reactor.

[0125] Figure 9 It is an example of the aforementioned mode B. The control unit operates the two-phase switches with different phases and the same duty ratio respectively. Then, the control unit determines whether condition 1 is satisfied. When the control unit determines that condition 1 is satisfied, it determines whether there is a requirement for measuring the AC impedance of the fuel cell. When the control unit determines that there is a requirement for measuring the AC impedance of the fuel cell, it changes the operation of the two-phase switches from the operation with different phases and the same duty ratio to the operation with different phases and different duty ratios, and measures the AC impedance of the fuel cell. On the other hand, when the control unit determines that there is no requirement for measuring the AC impedance of the fuel cell, it operates the two-phase switches with different phases and the same duty ratio.

[0126] Therefore, the control unit can drive the two-phase switches with different duty ratios only when condition 1 is satisfied and the AC impedance of the fuel cell is measured, and drive the two-phase switches with the same duty ratio in other cases. It should be noted that the control unit can also determine whether there is a requirement for measuring the AC impedance of the fuel cell, and when it determines that there is a requirement for measuring the AC impedance of the fuel cell, it determines whether condition 1 is satisfied.

[0127] <Mode C>

[0128] On the other hand, since the state with different average current values and the state with the same average current value are frequently switched, the control may become complicated. Therefore, the average current value can also be always different. Although the efficiency decreases, the complication of the control caused by the current switching can be suppressed.

[0129] <Mode D>

[0130] In order to equalize the load of the switches and the reactors, the upper and lower relationship of the offset value y of the paired phases can be changed every time the ignition switch of the vehicle is turned on (IG-ON) or every constant time.

[0131] Figure 10 It shows the current behavior when the target current value is equally distributed and flows in such a way that the average current values flowing in the magnetically coupled two-phase coils are the same, and then the average current value rises from 5 A to 25 A and then decreases. Figure 11 is Figure 10 a partial enlarged view of

[0132] Figure 12 It shows the current behavior when the current is offset by 2 A in such a way that the average current values flowing in the magnetically coupled two-phase coils are different, and the target current value is unequally distributed and flows, and then the average current value rises from 5 A to 25 A and then decreases. Figure 13 is Figure 12 a partial enlarged view of

[0133] In Figure 10 and Figure 11 where the current is equally distributed in the two-phase coils, although a sine wave command value is given, a sine wave is not obtained as the current value.

[0134] On the other hand, in Figure 12 and Figure 13 where the current is unequally distributed in the two-phase coils, the timing of the dead zone generation in the current waveforms of the U-phase and V-phase is staggered. At the timing surrounded by the dotted line in Figure 12 with respect to the command value overlapping the sine wave, a sine wave is not obtained in the U-phase entering the dead zone, but a current value in the shape of a sine wave is obtained in the V-phase that is not in the dead zone.

[0135] And, in Figure 12 and Figure 13 even when observing the sine wave signal intensity, which is an index of whether the sine wave is stably carried or not, it can be seen that the sine wave does not drop to near 0 and a constant amount of sine wave is carried. It should be noted that the sine waves overlap alternately at 270 Hz and 20 Hz.

[0136] Therefore, as condition 1, when the n-phase switches operate at different phases and the n-phase switches operate at the same duty ratio and the current waveform does not overlap the sine wave, it can be determined that the dead zone is entered. That is, by determining whether the current waveform overlaps the sine wave when the n-phase switches operate at different phases and the n-phase switches operate at the same duty ratio, it is possible to determine whether the dead zone is entered.

[0137] Figure 14 It is a flowchart showing another example of the control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter having a two-phase magnetically coupled reactor.

[0138] Figure 14 This is a modified example of the aforementioned mode A. The control unit operates the two-phase switches with different phases and the same duty ratio. Then, the control unit determines whether condition 1 is satisfied. When the current waveform does not overlap with the sine wave, the control unit sets it as satisfying condition 1, changes the operation of the two-phase switches from the operation with different phases and the same duty ratio to the operation with different phases and different duty ratios, and measures the AC impedance of the fuel cell. On the other hand, when the current waveform overlaps with the sine wave, the control unit determines that condition 1 is not satisfied, operates the two-phase switches with different phases and the same duty ratio, and measures the AC impedance of the fuel cell.

[0139] Whether the current waveform overlaps with the sine wave can be determined by the following method. When increasing or decreasing the duty ratio so that the amplitude of the current value of the coil becomes the amplitude of the normal current value (target current amplitude value A), if the amplitude of the measured (actual) current value of the coil is less than the expected value (target current amplitude value A), it can be determined that the current waveform does not overlap with the sine wave.

[0140] Figure 15 This is a diagram showing the dead zone generation area in the discontinuous mode.

[0141] The present researchers found that in a two-phase (per two-phase) magnetically coupled boost converter with L > M, the area surrounded by slashes as the dead zone generation area satisfies condition 1 and generates a dead zone.

[0142] Note that L is the self-inductance of the reactor, and M is the mutual inductance, which are inherent values determined by the physical property values of the reactor. V L is the input voltage (voltage before boosting) of the boost converter, and V H is the output voltage (voltage after boosting) of the boost converter.

[0143] In the case of two-phase magnetic coupling (n = 2), it is only necessary that the phases for driving the two phases are different, and the phase difference can also be 180°. The following operating conditions are obtained: When the phases for driving the two phases are different, in the discontinuous mode and when the duty ratio is less than 50% (D < 0.5), when the average current value of the coil in one phase of the magnetic coupling is maintained at 0, the switch in the other phase switches from on to off.

[0144] From the perspective of accurately determining the operating conditions for dead zone generation, in the case of two-phase magnetic coupling (n = 2), the control unit can determine that Figure 15The conditions for the dead zone generation region shown are that "when the switches of two phases with mutual magnetic coupling operate with a phase difference of 180°, the current flowing in the coil is in a discontinuous mode, the duty ratio (D) of the switches of the two phases is less than 50% (D < 0.5), and the following formula (A) or formula (B) is satisfied", and it is determined that condition 1 is satisfied.

[0145] Formula (A): D < {(1 / 2)(L - M)(V H - V L )} / (LV L + MV L - MV H )

[0146] Formula (B): D < (1 / 2){1 - (V L / V H )}

[0147] [In formula (A) and formula (B), L is the self-inductance of the reactor, M is the mutual inductance of the reactor, V H is the output voltage of the converter, V L is the input voltage of the converter, and D is the duty ratio.]

[0148] In the case of two-phase (per two-phase) magnetic coupling, "when the switches of two phases with mutual magnetic coupling operate with a phase difference of 180°, the current flowing in the coil is in a discontinuous mode, and the duty ratio (D) of the switches of the two phases is less than 50%" becomes one of the conditions in condition 1.

[0149] On the other hand, when n-phase (per n-phase) magnetic coupling is performed and the n phases with mutual magnetic coupling are driven with a phase difference of (360° / n) respectively, condition 1 replaces the condition of "when the switches of two phases with mutual magnetic coupling operate with a phase difference of 180°, the current flowing in the coil is in a discontinuous mode, and the duty ratio (D) of the switches of the two phases is less than 50%", and becomes "when the switches of n phases with mutual magnetic coupling operate with a phase difference of (360° / n) respectively, the current flowing in the coil is in a discontinuous mode, and the duty ratio of the switches of the n phases is less than (100 - 100 / n) %".

[0150] (Specific example)

[0151] In the case of two-phase magnetic coupling (n = 2), the control unit can determine that condition 1 is satisfied when it is determined that the above "when the switches of two phases with mutual magnetic coupling operate with a phase difference of 180°, the current flowing in the coil is in a discontinuous mode, the duty ratio (D) of the switches of the two phases is less than 50% (D < 0.5), and formula (A) or formula (B) is satisfied".

[0152] Figure 16This is a flowchart showing another example of control during the measurement of the AC impedance of a fuel cell in a fuel cell system that includes a converter with a two-phase magnetically coupled reactor.

[0153] The control unit operates the two-phase switches with a 180° phase difference and the same duty ratio respectively and determines whether there is a requirement for measuring the AC impedance of the fuel cell. When the control unit determines that there is no requirement for measuring the AC impedance of the fuel cell, it ends the control. On the other hand, when the control unit determines that there is a requirement for measuring the AC impedance of the fuel cell, it monitors the current value of the coil. As condition 1, it determines whether "when the two-phase switches that are magnetically coupled to each other operate with a 180° phase difference, the current flowing in the coil (the phase being driven) is in the discontinuous mode, and the duty ratio (D) of the two-phase switches is less than 50% (D < 0.5), and equation (A) or equation (B) is satisfied". When the control unit determines that condition 1 is not met, it operates the two-phase switches with a 180° phase difference and the same duty ratio respectively to measure the AC impedance of the fuel cell. On the other hand, when the control unit determines that condition 1 is met, it changes the operation of the two-phase switches from operating with a 180° phase difference and the same duty ratio to operating with a 180° phase difference and different duty ratios to measure the AC impedance of the fuel cell.

[0154] As Figure 15 shown, the "region where dead zone is generated" is when "the current flowing in the coil is in the discontinuous mode, and the duty ratio (D) of the two-phase switches is less than 50% (D < 0.5), and equation (A) or equation (B) is satisfied". Therefore, it is also possible to change the operation of the two-phase switches from operating with a 180° phase difference and the same duty ratio to operating with a 180° phase difference and different duty ratios at the timing when the operating conditions transfer to the region where dead zone is generated as shown in Figure 15 shown.

[0155] The means for determining whether condition 1 for dead zone generation is met is not limited to the above. For example, it is also possible to pre-define a range corresponding to the dead zone based on the input voltage, output voltage, and duty ratio, and when it indicates that the input voltage and duty ratio enter or approach the range corresponding to the dead zone at a predetermined output voltage, it is determined that condition 1 is met.

[0156] As described above, by pre-defining a range corresponding to the dead zone and changing the operation of the two-phase switches from operating with a 180° phase difference and the same duty ratio to operating with a 180° phase difference and different duty ratios when entering this range, it is possible to quickly obtain the AC impedance when obtaining the AC impedance of the fuel cell.

[0157] In Figure 16In [Case 1], as Condition 1, it is set that "when the switches of two phases that are magnetically coupled to each other operate with a phase difference of 180°, the current flowing in the coil is in the discontinuous mode, the duty ratios of the switches of the two phases are less than 50%, and Equation (A) or Equation (B) is satisfied". However, the condition of "when Equation (A) or Equation (B) is satisfied" can also be removed.

[0158] That is, as Condition 1, it can also be set that "when the switches of two phases that are magnetically coupled to each other operate with a phase difference of 180°, the current flowing in the coil is in the discontinuous mode, and the duty ratios of the switches of the two phases are less than 50%". In this case, for example, when the required output of the fuel cell decreases, at the timing of changing from the discontinuous mode of the current flowing in the coil and the duty ratio of the switches of the two phases being 50% or more to the discontinuous mode of the current flowing in the coil and the duty ratios of the switches of the two phases being less than 50%, the operation of the switches of the two phases can be changed from the operation with a phase difference of 180° and the same duty ratio to the operation with a phase difference of 180° and different duty ratios.

[0159] As Figure 15 shown, the area removed from the range of changing from the same duty ratio to different duty ratios according to Equation (A) and Equation (B) is not large. Therefore, it can be considered that even if the condition of "when Equation (A) or Equation (B) is satisfied" is removed, the impact on increasing the battery life is limited. On the other hand, by removing the condition of "when Equation (A) or Equation (B) is satisfied", the control can be simplified and the AC impedance of the fuel cell can be measured appropriately.

[0160] It should be noted that as a modified example, as Condition 1, it can also be set that "when the switches of two phases that are magnetically coupled to each other operate with a phase difference of 180°, the current flowing in the coil is in the discontinuous mode, the duty ratios of the switches of the two phases are less than 50%, and Equation (A) is satisfied" or "when the switches of two phases that are magnetically coupled to each other operate with a phase difference of 180°, the current flowing in the coil is in the discontinuous mode, the duty ratios of the switches of the two phases are less than 50%, and Equation (B) is satisfied".

[0161] [Measurement of AC Impedance]

[0162] The control unit measures the AC impedance of the fuel cell based on the current waveform and voltage waveform of the fuel cell.

[0163] The control unit can confirm whether there is a requirement for measuring the AC impedance of the fuel cell. When it is confirmed that there is a requirement for measuring the AC impedance of the fuel cell, it determines whether Condition 1 is satisfied. When it is determined that Condition 1 is satisfied, the switches of the n phases operate with different duty ratios, and the AC impedance of the fuel cell is measured.

[0164] When it is determined that Condition 1 is satisfied, the control unit may also confirm whether there is a requirement for measuring the AC impedance of the fuel cell. When it is confirmed that there is a requirement for measuring the AC impedance of the fuel cell, the switches of the n phases are operated with different duty ratios respectively to measure the AC impedance of the fuel cell.

[0165] In order to grasp the state of the electrolyte membrane of the fuel cell and the state of gas supply, the control unit measures the AC impedance of the fuel cell at a predetermined frequency during the operation of the fuel cell.

[0166] The control unit switches while periodically increasing and decreasing the PWM duty ratio of the converter, obtains the output voltage and output current values of the fuel cell when a load current including a certain frequency component is applied for more than one wavelength as time-series waveform data, performs a discrete Fourier transform on the waveform data, and divides the discrete Fourier transform result of the voltage signal by the discrete Fourier transform result of the current signal, thereby calculating the AC impedance of the fuel cell.

[0167] In the low load region called the discontinuous mode, when the duty ratio is increased, the average current value flowing in the coil of the reactor also increases.

[0168] In order to obtain the above-mentioned AC impedance of the fuel cell, for example, the duty ratio can be controlled in such a way that a sine wave with an amplitude of the output current value of about ±3 A is obtained.

[0169] It should be noted that the output voltage value of the fuel cell can be either the voltage of the entire fuel cell stack or the voltage of each single cell. If the voltage value of the entire fuel cell stack is used, the AC impedance of the entire fuel cell stack can be obtained. If the voltage value of each single cell is used, the AC impedance of each single cell can be obtained.

[0170] Moreover, when the voltage is obtained for every multiple single cells (for example, every two single cells, every four single cells, etc.), the AC impedance of any single cell block can be obtained.

[0171] In the case where the area of a single cell is about several hundred cm 2 or so, the components above 200 Hz of the AC impedance obtained during power generation mainly represent the proton transfer resistance and contact resistance of the electrolyte membrane, and the components of several tens of Hz represent the gas diffusion resistance.

[0172] It should be noted that the method for measuring the AC impedance of the fuel cell in the present disclosure is not particularly limited, and a known method can be applied. For example, it can be the same method as the method described in Japanese Patent Laid-Open No. 2008-098134.

[0173] Description of Reference Numerals

[0174] 10: Fuel cell

[0175] 20: Boost converter

[0176] 21: Reactor

[0177] 22: Current sensor

[0178] 23: Switch

[0179] 24: Diode

[0180] 25: Capacitor

[0181] 50: External load

Claims

1. A fuel cell system, characterized in that, The fuel cell system includes a fuel cell and a converter, and the converter performs at least one selected from the group consisting of boosting and bucking the output voltage of the fuel cell. The converter includes a reactor in which n-phase coils are magnetically coupled to each other, n-phase switches respectively connected to the coils, and a control unit, where n is an integer of 2 or more. The control unit performs on / off control of the n-phase switches. The control unit monitors the current value of the coils. The control unit measures the AC impedance of the fuel cell based on the current waveform and voltage waveform of the fuel cell. The control unit determines whether there is a dead zone where, when the n-phase switches are operated with different phases and the same duty ratio, even if the duty ratio is changed and an AC signal for AC impedance measurement is applied, the average current value flowing in the coils of the reactor, i.e., the output current value of the fuel cell, does not change. When the operating condition of condition 1 is satisfied, i.e., the current flowing in the coils is in the discontinuous mode, and when the current value flowing in one-phase of the coils is maintained at zero and the switch connected to at least one other phase of the coils is switched from on to off, the control unit changes the operation of the n-phase switches from the operation with different phases and the same duty ratio to the operation with different phases and different duty ratios. The control unit sets the offset current value y, which is added to or subtracted from the command current values of the magnetically coupled n-phase coils so that the target current values of the magnetically coupled n-phase coils are different, to 25% to 75% of the dead zone current interval. The control unit unevenly distributes the target current value so that the target current value obtained by adding or subtracting the offset current value y from the command current value is different for each coil, and operates the switches with different duty ratios so that at least one phase is out of the dead zone.

2. The fuel cell system according to claim 1, wherein, The control unit operates the n-phase switches with a phase difference of (360 / n)°.

3. The fuel cell system according to claim 1, wherein, The control unit prestores a current distribution difference map derived based on the relationship between the input voltage and output voltage of the converter and the current value flowing in the coils of the reactor. Based on the current distribution difference map, the control unit unevenly distributes the target current value so that the target current value is different for each coil, and operates the switches with different duty ratios.

4. The fuel cell system according to any one of claims 1 to 3, wherein, The control unit prestores the operating condition of the converter corresponding to condition 1 as a data group. The control unit compares the current operating condition of the converter with this data group to determine whether condition 1 is satisfied.

5. The fuel cell system according to any one of claims 1 to 3, wherein, When n = 2, and when the current flowing in the coils is in the discontinuous mode when the magnetically coupled two-phase switches are operated with a phase difference of 180°, and the duty ratios of the two-phase switches are less than 50%, i.e., D < 0.5, and the following formula (A) or formula (B) is satisfied, the control unit determines that condition 1 is satisfied. Formula (A): D < { (1 / 2) (L - M) (V H - V L )} / (LV L + MV L - MV H ), Formula (B): D < (1 / 2) {1 - (V L / V H )}, In formula (A) and formula (B), L is the self-inductance of the reactor, M is the mutual inductance of the reactor, V H is the output voltage of the converter, V L is the input voltage of the converter, and D is the duty cycle.

6. The fuel cell system according to any one of claims 1 to 3, wherein, The control unit confirms whether there is a requirement for measuring the AC impedance of the fuel cell. When the control unit confirms that there is a requirement for measuring the AC impedance of the fuel cell, it determines that the condition 1 is satisfied. When the control unit determines that the condition 1 is satisfied, it operates the switches of the n phases with different duty ratios respectively to measure the AC impedance of the fuel cell.

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