JP7528906B2 - Fuel Cell Systems - Google Patents

Description

This disclosure relates to a fuel cell system.

Various researches have been conducted on converters provided in systems used on vehicles such as fuel cell electric vehicles (FCEVs). DC/DC converters used in various electronic devices often use circuits composed of reactors, switches, diodes, capacitors, etc. The DC/DC converter controls the increase or decrease of the current flowing through the reactor by the ON/OFF signal of the switch.
For example, Patent Document 1 discloses a fuel cell system that accurately measures the AC impedance of a fuel cell stack.

Patent document 2 discloses a fuel cell system mounted on a vehicle that can measure the impedance of the fuel cell with high accuracy without being affected by load fluctuations of auxiliary equipment.

Patent document 3 discloses an impedance measurement method that uses a simple device and configuration to calculate impedance with high accuracy in a short time.

Patent Document 4 also discloses a fuel cell system that uses a bidirectional buck-boost converter with dead time to accurately apply an alternating current to the fuel cell in order to measure the impedance of the fuel cell.

JP 2008-098134 A JP 2014-232681 A JP 2013-145692 A JP 2014-235781 A

The AC impedance of a fuel cell is used as one of the indices for optimally controlling the operating state of the fuel cell. The converter controls the voltage of the fuel cell by increasing or decreasing the voltage through the switching operation of a switching element (switch). It is known that the converter has a region where its response performance to the duty ratio fluctuation decreases. If the AC impedance of a fuel cell is measured in such a region where the response performance decreases, the accuracy of the converter's superposition of the high-frequency signal to the fuel cell decreases, resulting in a significant decrease in the accuracy of the AC impedance measurement.
Aiming to miniaturize the converter, the introduction of a magnetically coupled converter equipped with a magnetically coupled reactor, in which multiple coils are magnetically coupled on the same core, is one option. In a magnetically coupled converter, in order to mitigate the magnetic saturation of the reactor core and reduce the output current ripple, it is common to alternately switch the magnetically coupled coils with the same duty ratio and equal phase differences. When the fuel cell is at low output, the magnetically coupled reactor operates in discontinuous mode, which includes times when the current flowing through the coils becomes zero.
The present researchers have newly discovered that there exists a "dead zone" in which, during operation in a discontinuous mode in which the current is intermittent, a pseudo-switch-on state occurs due to a negative current from another coil caused by mutual inductance, which is a phenomenon specific to magnetic coupling reactors, and the control of the switch does not change from off to on smoothly, and the output current value of the fuel cell does not change even if the duty ratio is changed. In the dead zone, even if an AC signal for measuring the AC impedance is applied by changing the duty ratio, the output current value of the fuel cell hardly changes as described above, so that the AC impedance of the fuel cell cannot be measured appropriately. This corresponds to the response performance reduction region in the above-mentioned Patent Document 1, and there is a risk that the operating state of the fuel cell cannot be optimally controlled due to frequent occurrence of time periods in which the AC impedance is not measured or cannot be measured.

This disclosure was made in consideration of the above-mentioned circumstances, and its main objective is to provide a fuel cell system that can appropriately measure the AC impedance of a fuel cell.

The fuel cell system of the present disclosure is a fuel cell system,
The fuel cell system includes a fuel cell and a converter that performs at least one operation selected from the group consisting of stepping up and stepping down an output voltage of the fuel cell;
the converter includes a reactor having n-phase coils (n is an integer of 2 or more) magnetically coupled to each other, n-phase switches connected to the coils, and a control unit;
The control unit performs ON/OFF control of the switches of n phases,
The control unit monitors a current value of the coil,
the control unit measures an AC impedance of the fuel cell from a current waveform and a voltage waveform of the fuel cell;
the control unit operates the switches of n phases at different phases and operates the switches of n phases at the same duty ratio;
When it is determined that the following condition 1 is satisfied, the control unit operates the switches of n phases with different duty ratios.
Condition 1: An operating condition in which, when the n-phase switches are operated at different phases, the current flowing through the coil is in discontinuous mode and the current value flowing through the coil of one phase is maintained at zero, the switch connected to the coil of at least one other phase switches from ON to OFF.

In the fuel cell system of the present disclosure, the control unit may operate the n-phase switches with a phase difference of (360/n)°.

In the fuel cell system of the present disclosure, the control unit sets an offset current value y to be added to or subtracted from a 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, the offset current value y being 25 to 75% of a dead band current interval;
The control unit may unequally distribute the target current value obtained by adding or subtracting the offset current value y to the command current value so that the target current value is different for each coil, and operate the n-phase switches with different duty ratios.

In the fuel cell system of the present disclosure, the control unit stores in advance a current distribution difference map derived from a relationship between an inlet voltage and an outlet voltage of the converter and a current value flowing through a coil of the reactor,
The control unit may unevenly distribute the target current value so that the target current value differs for each of the coils based on the current distribution difference map, and operate the switches with different duty ratios.

In the fuel cell system of the present disclosure, the control unit stores in advance the operating conditions of the converter corresponding to the condition 1 as a data group,
The control unit may check the current operating conditions of the converter against the data group to determine whether or not the condition 1 is satisfied.

In the fuel cell system of the present disclosure, the control unit may determine that condition 1 is satisfied when n=2, and when the two-phase switches that are magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil is in a discontinuous mode, and the duty ratio of the two-phase switches is less than 50% (D<0.5), and either one of the following formula (A) or formula (B) is satisfied:
Formula (A): D<{(1/2)(LM)(V _H -V _L )}/(LV _L +MV _L -MV _H )
Formula (B): D<(1/2) {1-(V _L /V _H )}
[In the formulas (A) and (B), L is the self-inductance of the reactor, M is the mutual inductance of the reactor, _VH is the output voltage of the converter, _VL is the input voltage of the converter, and D is the duty ratio (-).]

In the fuel cell system of the present disclosure, the control unit checks whether or not there is a request to measure the AC impedance of the fuel cell,
When the control unit confirms that there is a request to measure the AC impedance of the fuel cell, the control unit determines whether or not the condition 1 is satisfied;
When it is determined that the condition 1 is satisfied, the control unit may operate the switches of n phases with different duty ratios, and measure the AC impedance of the fuel cell.

The fuel cell system disclosed herein allows the AC impedance of the fuel cell to be measured appropriately.

FIG. 1 is a diagram showing an example of a circuit configuration of a fuel cell system having a boost converter and peripheral components. FIG. 2 is a diagram showing the change in the average current value (single-phase average current value) flowing through each coil of the reactor when the input voltage Vfc is 200 V, the output voltage Vh is 350 V, the step-up ratio is constant, two-phase coils that are magnetically coupled to each other are driven, and the duty ratio is gradually increased. FIG. 3 shows the current waveforms obtained when two magnetically coupled phase coils are driven, a 270 Hz sine wave with an amplitude of ±3 A is superimposed on each phase, and the DC load current is swept in the range of 5 to 25 A. FIG. 4 is a diagram showing the current waveforms of the magnetically coupled coils of two phases in a region where a dead zone occurs. FIG. 5 is a diagram showing an example of a two-phase magnetically coupled boost circuit having a U phase and a V phase. FIG. 6 is a diagram showing an example of a current waveform when a current flowing through a coil of a reactor of a boost converter is in a continuous mode. FIG. 7 is a diagram showing an example of a current waveform when a current flowing through a coil of a reactor of a boost converter is in a discontinuous mode. FIG. 8 is a flowchart showing an example of control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter equipped with a two-phase magnetic coupling reactor. FIG. 9 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 equipped with a two-phase magnetic coupling reactor. FIG. 10 shows the current behavior when the target current value is equally distributed so that the average current value flowing through the magnetically coupled coils of two phases is the same, and the average current value is increased from 5 A to 25 A and then decreased. FIG. 11 is a partially enlarged view of FIG. FIG. 12 shows the current behavior when the current is offset by 2 A so that the average current values flowing through the magnetically coupled coils of two phases are different, the target current value is unevenly distributed, and the average current value is increased from 5 A to 25 A and then decreased. FIG. 13 is a partially enlarged view of FIG. FIG. 14 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 equipped with a two-phase magnetic coupling reactor. FIG. 15 is a diagram showing a region where a dead zone occurs in the discontinuous mode. FIG. 16 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 equipped with a two-phase magnetic coupling reactor.

The fuel cell system of the present disclosure is a fuel cell system,
The fuel cell system includes a fuel cell and a converter that performs at least one operation selected from the group consisting of stepping up and stepping down an output voltage of the fuel cell;
the converter includes a reactor having n-phase coils (n is an integer of 2 or more) magnetically coupled to each other, n-phase switches connected to the coils, and a control unit;
The control unit performs ON/OFF control of the switches of n phases,
The control unit monitors a current value of the coil,
the control unit measures an AC impedance of the fuel cell from a current waveform and a voltage waveform of the fuel cell;
the control unit operates the switches of n phases at different phases and operates the switches of n phases at the same duty ratio;
When it is determined that the following condition 1 is satisfied, the control unit operates the switches of n phases with different duty ratios.
Condition 1: An operating condition in which, when the n-phase switches are operated at different phases, the current flowing through the coil is in discontinuous mode and the current value flowing through the coil of one phase is maintained at zero, the switch connected to the coil of at least one other phase switches from ON to OFF.

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 is changed, and in such a case, the AC impedance of the fuel cell cannot be measured appropriately. In this disclosure, the dead zone, which is a phenomenon specific to magnetically coupled reactors, is controlled to occur with a minimum frequency, and the AC impedance of the fuel cell is measured. As a specific control, under conditions where the dead zone occurs, the average current value of the coils flowing through each of the magnetically coupled phases is made different, thereby avoiding the dead zone and appropriately measuring the AC impedance of the fuel cell. By making the current value flowing through the coils of the magnetically coupled phases different, at least one phase falls outside the range of the dead zone, and the phase outside the dead zone can fluctuate its current, and as a result, the AC impedance of the fuel cell can be measured appropriately, and the operating state of the fuel cell can be optimally controlled.

FIG. 1 is a diagram showing an example of a circuit configuration of a fuel cell system having a boost converter and peripheral components.
1 is mounted on, for example, a vehicle, and is connected to a drive motor of the vehicle via an inverter as an external load 50. Although not shown, a battery may also 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 further converted from direct current to alternating current by the inverter and supplied to the motor.
The boost converter 20 includes six-phase boost circuits connected in parallel to one another. Although Fig. 1 illustrates a configuration including six phases of boost circuits, the number of phases is not particularly limited.
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 include an input voltage sensor and an output voltage sensor. Of the six-phase boost circuits, two phases each share the core of one reactor 21 and can be magnetically coupled to each other. In each boost circuit, when the switch 23 is turned ON, the current flowing through the reactor 21 increases, and when the switch 23 is turned OFF, the current flowing through the reactor 21 decreases, and when the current reaches zero, it is maintained at zero. The current sensor 22 acquires the value of the current flowing through the reactor 21.
A control unit (not shown) controls the ON/OFF control of the switch 23 to control the boost ratio in the converter 20 and the output current value from the fuel cell 10 .
The output power of the fuel cell 10 varies greatly depending on the requirements of the vehicle (speed, acceleration, load, road gradient, etc.), and the output current also varies greatly accordingly. When the output current of the fuel cell 10 is large, if that current is passed through one boost circuit, heat generation increases and the power conversion efficiency of the boost converter 20 decreases. Also, if only a small current is passed through a boost circuit that can withstand a large current, energy loss increases and the power conversion efficiency of the boost converter 20 decreases. Therefore, the boost converter 20 is provided with a multi-phase boost circuit (six phases in the example shown in FIG. 1), and the boost converter 20 switches the number of phases to be driven 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. The efficiency of the boost circuit varies depending on the current flowing through it, and by changing the number of driving phases, it can be operated at the optimum efficiency in each current range.

The fuel cell system includes a fuel cell and a converter.

The fuel cell may have only one unit cell, or it may be a fuel cell stack made up of multiple unit cells.

The converter performs at least one of stepping up and stepping down the output voltage of the fuel cell, and may be a step-up converter, a step-down converter, or a step-up/step-down converter.
The converter may be a DC/DC converter.
The converter includes a reactor having n-phase coils (n is an integer of 2 or more) magnetically coupled to each other, n-phase switches connected to the coils, and a control unit. The converter may include a diode, a current sensor, a voltage sensor, a photocoupler, a capacitor, etc.

The reactor includes a coil and a core.
The core may have n-phase coils wound thereon (n is an integer of 2 or more). As long as n is 2 or more, there is no upper limit to n, and it may be 10 or less, 5 or less, 4 or less, or 3 or less.
The core and coil of the reactor may be the same as those used in conventionally known converters.
In this disclosure, a reactor having a core around which one independent coil is wound is referred to as a non-magnetically coupled reactor. In this disclosure, a converter including a non-magnetically coupled reactor is referred to as a non-magnetically coupled converter. In this disclosure, a reactor having a core around which two or more independent coils are wound is referred to as a magnetically coupled reactor. In this disclosure, a converter including a magnetically coupled reactor is referred to as a magnetically coupled converter.
In this disclosure, a separate coil refers to a coil with one or more turns and two terminals.

The switch (switching element) is composed of a transistor and a protective diode. The transistor is an npn type transistor, and may be, for example, an insulated gate bipolar transistor (IGBT) or a metal oxide semiconductor field effect transistor (MOSFET).
The diode may be any diode used in a conventionally known converter.

The current sensor is not particularly limited as long as it can obtain the value of the current flowing through the reactor coil (sometimes called the reactor current value), and a conventionally known ammeter or the like can be used. In addition, in this disclosure, the reactor average current means the average current flowing through the coil during the switching period of the duty ratio control.

The voltage sensor is not particularly limited as long as it can acquire the input voltage output from the fuel cell and input to the converter, and a conventionally known voltmeter or the like can be used.

The control unit may be an electronic control unit (ECU) or the like. The ECU includes a central processing unit (CPU), a memory, and an input/output buffer.

The control unit detects the current value flowing through the n-phase coil of the reactor using a signal from the current sensor and monitors the coil current value.

The control unit performs ON/OFF control of the n-phase switches. The control unit may operate the magnetically coupled n-phase switches at a constant frequency.
The control unit may periodically switch between an ON command and an OFF command to control the ON/OFF of the switch, thereby controlling the duty ratio of the n-phase switch, thereby controlling the output current value from the fuel cell.
In a boost converter, the voltage is boosted by repeating a switching operation that stores and releases electrical energy in a reactor. The duty ratio determines the proportion of the storage period during which the switching element is open and electrical energy is stored in the reactor during the switching period of the boost operation. If the switching period of the boost operation in the boost converter is T, the period during which the switching element is turned on is T _ON , and the period during which the switching element is turned off is T _OFF , the duty ratio D is expressed as D(-)=T _ON /T. For convenience, the duty ratio may be expressed as a percentage (%). In this case, D(%)=T _ON /T×100.
In this disclosure, a switching period (switching cycle) means the period from when a switch is switched from off to on to when the switch is switched from off to on again.
The control unit may operate the n-phase switches with a phase difference of (360/n)°.
When the control unit has two phases and operates the two phase switches with different phases, the control unit may operate the two phase switches with a phase difference of 30 to 180 degrees, or may operate the two phase switches with a phase difference of 180 degrees, i.e., in opposite phases, from the viewpoint of improving power conversion efficiency. In the case of operation in opposite phases, a phase error in the range of -5° to +5° may occur.

The control unit operates each of the n-phase switches at different phases and with the same duty ratio.

When the control unit determines that the following condition 1 is satisfied, it operates the n-phase switches with different duty ratios. In other words, it makes the average current values flowing through the coils of each phase different from each other.

Condition 1: An operating condition in which, when the n-phase switches are operated at different phases, the current flowing through the coil is in discontinuous mode and the current value flowing through the coil of one phase is maintained at zero, the switch connected to the coil of at least one other phase switches from ON to OFF.
In the case of a magnetic coupling reactor having three or more phases, the operating condition under which the current value flowing through the coil of one phase is zero and the switch connected to the other coils is switched from ON to OFF may be an operating condition under which the switch of at least any one of the remaining phases is switched from ON to OFF.
When n phases magnetically coupled to each other are driven with a phase difference of (360°/n), condition 1 may be a discontinuous mode and a duty ratio of less than (100−100/n)%.
The control unit may store in advance operating conditions of the converter corresponding to condition 1 as a data group, and compare the current operating conditions of the converter with the data group to determine whether or not the condition 1 is satisfied. This makes it possible to avoid entering the dead zone, improving the accuracy of the AC impedance measurement of the fuel cell.

[Deadband explanation]
The "dead zone" refers to the region where the average current value flowing through the reactor coil hardly changes even if the PWM duty ratio of the converter is increased or decreased. Due to the negative current caused by the mutual inductance of the 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 unidirectional step-up (step-down) circuit.
2 is a diagram showing the change in the average current value (single-phase average current value) flowing through each coil of the reactor when the duty ratio is gradually increased by driving two-phase coils that are magnetically coupled to each other, with the input voltage Vfc being 200 V, the output voltage Vh being 350 V, and the step-up ratio being constant. In FIG. 2, the self-inductance of the coils is 96.4 μH, the mutual inductance of the coils is 62.7 μH, and the drive frequency of the switch is 20 kHz.
According to the state equation (described later) for calculating the average current flowing through the reactor coil, as the duty ratio increases, the average current flowing through the reactor coil should increase monotonically, as shown by the dashed line in Figure 2, but in reality, as shown by the solid line, the current increases in a stepped manner, and there is a "dead zone" in the area surrounded by the dotted line where the average current flowing through the reactor coil does not increase even if the duty ratio is increased. If the average current flowing through the reactor coil is in the dead zone, the average current flowing through the reactor coil, i.e., the output current value of the fuel cell, does not change even if the duty ratio is changed, and the AC impedance of the fuel cell cannot be measured appropriately.

Fig. 3 shows the current waveform when two magnetically coupled coils are driven, a 270 Hz sine wave with an amplitude of ±3 A is superimposed on each phase, and the DC load current is swept in the range of 5 to 25 A. In Fig. 3, as in Fig. 2, the input voltage Vfc is 200 V, the output voltage Vh is 350 V, the step-up ratio is constant, the self-inductance of the coil is 96.4 μH, the mutual inductance of the coil is 62.7 μH, and the drive frequency of the switch is 20 kHz. In the case of Fig. 3, the relationship between the duty ratio and the average current value flowing through the coil of the reactor is considered to follow the dashed line shown in Fig. 2, so the duty ratio is oscillated along the dashed line. However, as described above, in reality, the change in the current value is stepped as shown by the solid line in Fig. 2, so the current does not change even if the duty ratio is increased or decreased in the dead zone shown by the dotted line in Fig. 2, and there is a region where a sine wave cannot be superimposed on the current, as shown by the dotted line in Fig. 3, and in that region the AC impedance of the fuel cell cannot be measured. Note that if a sudden change in the DC load occurs in the specific region shown by the dotted line in Fig. 3, the output response of the fuel cell may be delayed, resulting in an increase in the load on the battery, etc.
3, the control unit may determine that condition 1 is satisfied when the amplitude of the measured coil current value is smaller than an expected value when the duty ratio of the n-phase switch is increased or decreased so that the amplitude of the coil current value becomes a normal amplitude. When the amplitude of the coil current value is smaller than the expected value, this corresponds to a state in which a sine wave is not superimposed, and it can be determined that the dead zone has been entered.

4 is a diagram showing the current waveforms of the magnetically coupled coils of two phases in a region where a dead zone occurs, in which the U-phase current is denoted as L1 and the V-phase current is denoted as L2.
Between time t0 and t1, the V-phase switch is ON, and the L2 current gradually increases. Meanwhile, the U-phase switch is OFF, and the L1 current is maintained at 0. At time t1, the V-phase switch is switched from ON to OFF, causing the V-phase L2 current to begin to decrease. Here, the U-phase switch remains OFF, but after time t1, the L1 current flowing through the U-phase due to V-phase interaction alternates between decreasing and increasing.
Here, even if the U-phase switch is turned ON when the U-phase L1 current is increasing, for example at time t2, the switch ON signal is not recognized because the current is already increasing. After that, the U-phase switch ON is recognized only when the U-phase L1 current decreases at time t3, and thereafter the L1 current increases until the U-phase switch is turned OFF. Timings when the switch ON signal is not recognized and timing when it is recognized alternate, so that a dead zone appears repeatedly, as shown in FIG. 2.
As mentioned above, when the current value of the coil of one of the magnetically coupled phases is zero and the switch of the other phase is switched from ON to OFF, the current in the coil of one of the phases increases or decreases, resulting in a dead zone.
Therefore, as per condition 1, when the n-phase switches are driven at different phases, while the current flowing through the coils is in discontinuous mode and the current value of the magnetically coupled coil of one phase is maintained at 0, it can be said that a dead zone occurs under operating conditions in which the switch of the other phase switches from ON to OFF.
In the case of two-phase magnetic coupling, the phase difference for driving the two phases may be 180°. When the phase difference for driving the two phases is 180°, if the discontinuous mode is used and the duty ratio is less than 50%, a dead zone occurs under operating conditions in which the switch of one magnetically coupled phase switches from ON to OFF while the average current value of the coil of the other phase is maintained at 0.

Fig. 5 is a diagram showing an example of a two-phase magnetically coupled boost circuit of U phase and V phase. In Fig. 5, _VL is an input voltage (voltage before boosting), _IU is a U-phase current, _IV is a V-phase current, _VH is an output voltage (voltage after boosting), D is a diode, S is a switch, M is a mutual inductance, L is a self-inductance, and r is an internal resistance.
Equation (1) is the state equation of the current I vector. By solving this state equation, the current curve shown by the dashed line in Fig. 2 can be obtained. The voltage V vector in equation (1) is expressed as shown in Table 1. Table 1 shows the voltage V vector in equation (1) and the conditions for generating the dead zone (shaded area).
Due to the negative current caused by the mutual inductance of the magnetic coupling represented by M in Fig. 5 and formula (1), in a two-phase magnetically coupled unidirectional step-up (step-down) circuit, as shown in the shaded area in Table 1, a dead zone appears in a part of the discontinuous mode region where the switches of both phases are off, that is, the duty ratio is less than 50% when driven with a 180° phase difference. The mechanism is as shown by solving formula (1) over time under the conditions in Table 1, and when the switches of both phases are off and the current flowing through one coil is 0 A and the current flowing through the other coil is positive, an electromotive voltage is generated by mutual inductance in the coil on the 0 A side, generating a negative current. After that, the electromotive force of the coil disappears as the negative current increases, and it soon starts to decrease, but during the period until it returns to 0 A, the switch is in a pseudo ON state as shown in the shaded area in Table 1, so whether the switch on the negative current side is ON or OFF, it cannot affect the current waveform, resulting in a control dead zone.
For example, in the case of a three-phase magnetically coupled boost circuit having U, V, and W phases, the conditions under which the dead zone occurs are assumed to be when all three phase switches are OFF and _IU > 0, _IV ≦ 0, _IW ≦ 0, or when _IU > 0, _IV > 0, _IW ≦ 0.
Not only boost circuits, but also step-down circuits and step-up/step-down circuits have the same current control dead zone problem as boost circuits, and the occurrence of the dead zone can be avoided or suppressed by driving in phase when the dead zone occurs. Note that it is considered that the dead zone does not exist in bidirectional circuits that do not have discontinuous mode.

[Explanation of continuous mode and discontinuous mode]
FIG. 6 is a diagram showing an example of a current waveform when a current flowing through a coil of a reactor of a boost converter is in a continuous mode.
FIG. 7 is a diagram showing an example of a current waveform when a current flowing through a coil of a reactor of a boost converter is in a discontinuous mode.
As shown in FIG. 6, the current (reactor current) flowing through the coil of the reactor of the boost converter becomes a triangular wave with the switching operation, and the median value of the triangular wave is the reactor average current (hereinafter referred to as the average current). Here, if the duty is reduced to lower the average current, the lowest point of the triangular wave reaches 0 A. If the average current is further reduced from this point, a period in which the reactor current is zero begins to occur, as shown in FIG. 7, since the boost converter is a one-way circuit. An operation in which the current flowing through the reactor coil is zero during the switching period of the duty ratio control of the converter is called a discontinuous mode, and an operation in which the current flowing through the reactor coil does not have a period in which the current flowing through the reactor coil is zero during the switching period of the duty ratio control is called a continuous mode.

<Pattern A>
In the case of a two-phase magnetically coupled reactor, when the fuel cell system is operating and enters or approaches a region in which a dead zone corresponding to condition 1 occurs, the two phases that are magnetically coupled to each other are controlled based on unequal current command values when driving the coils of two phases so that the two phases do not simultaneously enter the dead zone as shown in Figure 2. This allows at least one phase to leave the dead zone, maintain a controllable state, and enable sine wave superposition.
In the case of a two-phase magnetically coupled reactor, the region in which the dead zone corresponding to condition 1 occurs may be when the mode is discontinuous and the duty ratio is less than 50%. Therefore, for example, at the timing when the required output to the fuel cell decreases and the mode is discontinuous and the duty ratio is reduced from 50% or more to less than 50%, the state may transition from one in which the average current values flowing through the coils of the two phases magnetically coupled to each other are the same (the target current values are the same) to one in which the average current values flowing through the coils of the two phases magnetically coupled to each other are different (the target current values are different).

FIG. 8 is a flowchart showing an example of control when measuring the AC impedance of a fuel cell in a fuel cell system including a converter equipped with a two-phase magnetic coupling reactor.
8 is an example of the aforementioned pattern A. The control unit normally operates the two-phase switches with different phases and the same duty ratio. The control unit then determines whether or not condition 1 is met. If condition 1 is met, the control unit changes the operation of the two-phase switches from different phases and the same duty ratio to different phases and different duty ratios, and measures the AC impedance of the fuel cell. On the other hand, if condition 1 is not met, the control unit operates the two-phase switches with different phases and the same duty ratio, and measures the AC impedance of the fuel cell.

The control unit may set an offset current value y to be added to or subtracted from the command current value of each of the magnetically coupled n-phase coils so that the target current value of each of the magnetically coupled n-phase coils is different, to be 25 to 75% of the dead band current interval.
The control unit may unequally distribute the target current value so that the target current value obtained by adding or subtracting an offset current value y to or from the command current value is different for each of the coils, and operate the switches with different duty ratios.
The control unit may pre-store a current distribution difference map derived from the relationship between the inlet voltage and outlet voltage of the converter and the current value flowing through the coil of the reactor, and may unevenly distribute the target current value based on the current distribution difference map so that the target current value is different for each coil, thereby operating the switches with different duty ratios.

When distributing the fuel cell current to the driven phases, if the total input current to the converter is I [A] and the number of driven phases is n (n is, for example, 2, 4, or 6), then the current command value to the coil of each phase is I/n [A].
Based on the current command value (I/n [A]) of the coil of each phase, the same offset value (y [A]) is set between the UV two-phase magnetic coupling reactor, the WX two-phase magnetic coupling reactor, and the YZ two-phase magnetic coupling reactor.
In a UV two-phase magnetic coupling reactor, unequal current distribution may be achieved by adding an offset value (y [A]) to a current command value (I/n [A]) to flow a current (I/n+y [A]) in the U phase, and subtracting an offset value (y [A]) from the current command value (I/n [A]) to flow a current (I/n-y [A]) in the V phase. Similarly to the UV two-phase magnetic coupling reactor, unequal current distribution may also be achieved in a WX two-phase magnetic coupling reactor and a YZ two-phase magnetic coupling reactor.
The offset value y may be, for example, 1 to 3.
When moving as shown in FIG. 2, the current interval at which the dead zone appears is obtained in advance, and the value of y can be selected so that the interval at which the dead zone appears does not coincide with 2y.
In addition, when the discontinuous mode corresponds to condition 1 and the condition when the duty ratio is less than 50% is not satisfied, the average current values flowing through the two magnetically coupled coils may be made to match. By doing so, it is possible to reduce the ripple current generated in the converter, suppress heat generation, and maintain high efficiency in the converter.

<Pattern B>
In the example of pattern A described above, when entering an area in which a dead zone corresponding to condition 1 occurs, the average current values flowing through the coils of each phase that are magnetically coupled to each other are switched to be different, but the switching may also be performed when obtaining the AC impedance of the fuel cell.
As described above, generally, AC impedance is acquired at a predetermined timing during operation of the fuel cell system. If the average current values flowing through the reactors of the mutually magnetically coupled phases are made different, the ripple current becomes large as described above, and the heat generation also becomes large, so that the average current values flowing through the coils of the mutually magnetically coupled phases may be made the same during normal operation.
By limiting the difference in the average current value to only when acquiring the AC impedance, it is possible to reduce the ripple current generated in the converter, suppress heat generation, and maintain high efficiency in the converter.
If the AC impedance measurement is not possible because the AC impedance falls into a dead zone when attempting to measure the AC impedance, the current may be offset to measure the AC impedance. If the current cannot escape from the dead zone even after offsetting the current, the offset amount may be changed.

FIG. 9 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 equipped with a two-phase magnetic coupling reactor.
9 is an example of the aforementioned pattern B. The control unit operates the two-phase switches with different phases and the same duty ratio. Thereafter, the control unit determines whether or not condition 1 is satisfied. When the control unit determines that condition 1 is satisfied, it determines whether or not there is a request for measuring the AC impedance of the fuel cell. When the control unit determines that there is a request for measuring the AC impedance of the fuel cell, it changes the operation of the two-phase switches from different phases and the same duty ratio to different phases and different duty ratios, and performs AC impedance measurement of the fuel cell. On the other hand, when the control unit determines that there is no request for measuring the AC impedance of the fuel cell, it operates the two-phase switches with different phases and the same duty ratio.
Therefore, the control unit may switch-drive the two phases with different duty ratios only when condition 1 is satisfied and AC impedance measurement of the fuel cell is to be performed, and may switch-drive the two phases with the same duty ratio in other cases. Note that the control unit may determine whether or not there is a request for AC impedance measurement of the fuel cell, and when it determines that there is a request for AC impedance measurement of the fuel cell, may determine whether or not condition 1 is satisfied.

<Pattern C>
On the other hand, frequent switching between a state in which the average current value is different and a state in which the average current value is the same may complicate control. Therefore, it is acceptable to always keep the average current value different. Although the efficiency will decrease, it is possible to suppress the complication of control caused by current switching.

<Pattern D>
In order to equalize the load on the switches and reactors, the up-down relationship of the offset values y of the paired phases may be reversed every time the vehicle ignition switch is turned on (IG-ON) or at regular intervals.

Fig. 10 shows the current behavior when a target current value is equally distributed so that the average current value flowing through the magnetically coupled coils of two phases is the same, and the average current value is increased from 5 A to 25 A and then decreased. Fig. 11 is a partially enlarged view of Fig. 10.
Fig. 12 shows the current behavior when the current is offset by 2 A so that the average current values flowing through the magnetically coupled coils of two phases are different, the target current value is unevenly distributed, and the average current value is increased from 5 A to 25 A and then decreased.
In Figs. 10 and 11, in which the current is equally distributed to the coils of two phases, a sine wave is not obtained as the current value, even though a sine wave command value is given.
On the other hand, in Figures 12 and 13, in which the current is unequally distributed to the coils of two phases, the timing at which the dead zones occur in the current waveforms of the U phase and V phase are shifted, and at the timing surrounded by dotted lines in Figure 12, no sine wave is obtained in the U phase, which is within the dead zone, in response to the command value for superimposing a sine wave, but a sine waveform-shaped current value is obtained in the V phase, which is not in the dead zone.
12 and 13, the sine wave signal strength, which is an index of whether a sine wave is properly carried or not, does not drop to near 0, and it can be seen that a certain amount of sine wave is carried. Note that the sine wave is superimposed alternately at 270 Hz and 20 Hz.
Therefore, as condition 1, when n-phase switches are operated with different phases and the same duty ratio, if a sine wave is not superimposed on the current waveform, it can be determined that the dead zone has been entered. In other words, when n-phase switches are operated with different phases and the same duty ratio, it can be determined whether the dead zone has been entered by determining whether a sine wave is superimposed on the current waveform.

FIG. 14 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 equipped with a two-phase magnetic coupling reactor.
14 is a modified example of the above-mentioned pattern A. The control unit operates the two-phase switches with different phases and the same duty ratio. The control unit then determines whether or not condition 1 is satisfied. When the current waveform does not have a sine wave superimposed thereon, the control unit determines that condition 1 is satisfied, changes the operation of the two-phase switches from different phases and the same duty ratio to different phases and different duty ratios, and measures the AC impedance of the fuel cell. On the other hand, when the current waveform has a sine wave superimposed thereon, the control unit determines that condition 1 is not satisfied, and operates the two-phase switches with different phases and the same duty ratio to measure the AC impedance of the fuel cell.
Whether or not the current waveform has a sine wave superimposed thereon may be determined by the following method: When the duty ratio is increased or decreased so that the amplitude of the coil current value becomes the amplitude of the normal current value (target current amplitude value A), if the amplitude of the measured (actual) coil current value is smaller than the expected value (target current amplitude value A), it can be determined that the current waveform does not have a sine wave superimposed thereon.

FIG. 15 is a diagram showing a region where a dead zone occurs in the discontinuous mode.
The present researchers discovered that in a boost converter in which two phases are magnetically coupled and L>M, condition 1 is satisfied and a dead zone occurs in the region enclosed by diagonal lines as the dead zone occurrence region.
In addition, L is the self-inductance of the reactor, and M is the mutual inductance, each of which is a unique value determined by the physical property value of the reactor. _VL is the inlet voltage of the boost converter (voltage before boosting), and _VH is the outlet voltage of the boost converter (voltage after boosting).
In the case of two-phase magnetic coupling (n=2), the phases driving the two phases need only be different, and the phase difference may be 180°. When the phases driving the two phases are different, if the mode is discontinuous and the duty ratio is less than 50% (D<0.5), when the average current value of the coil of one of the magnetically coupled phases is maintained at 0, the operating condition is such that the switch of the other phase is switched from ON to OFF.
From the viewpoint of accurately determining the operating conditions under which a dead zone occurs, in the case of two-phase magnetic coupling (n=2), the control unit may determine that condition 1 is satisfied if it is determined that the conditions for the dead zone occurrence region shown in FIG. 15 are met, that is, "when two-phase switches that are magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil is in discontinuous mode, and the duty ratio (D) of the two-phase switches is less than 50% (D<0.5), and either one of the following formula (A) or formula (B) is satisfied."
Formula (A): D<{(1/2)(LM)(V _H -V _L )}/(LV _L +MV _L -MV _H )
Formula (B): D<(1/2) {1-(V _L /V _H )}
[In the formulas (A) and (B), L is the self-inductance of the reactor, M is the mutual inductance of the reactor, _VH is the output voltage of the converter, _VL is the input voltage of the converter, and D is the duty ratio.]
When two phases are magnetically coupled, one of the conditions 1 is that "when the switches of the two phases that are magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil is in discontinuous mode, and the duty ratio (D) of the switches of the two phases is less than 50%."
On the other hand, when n phases are magnetically coupled and the magnetically coupled n phases are driven with a phase difference of (360°/n), condition 1 is changed from "when two magnetically coupled switches are operated with a phase difference of 180°, the current flowing through the coil is in discontinuous mode, and the duty ratio (D) of the two-phase switches is less than 50%" to "when n magnetically coupled switches are operated with a phase difference of (360°/n), the current flowing through the coil is in discontinuous mode, and the duty ratio of the n-phase switches is less than (100-100/n)%".

(Concrete example)
In the case of two-phase magnetic coupling (n=2), the control unit may determine that condition 1 is satisfied if it determines that "when two-phase switches that are magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil is in discontinuous mode, and the duty ratio (D) of the two-phase switches is less than 50% (D<0.5), and either formula (A) or formula (B) is satisfied" as described above.
FIG. 16 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 equipped with a two-phase magnetic coupling reactor.
The control unit operates the two-phase switches with a phase difference of 180° and the same duty ratio to determine whether or not there is a request to measure the AC impedance of the fuel cell. When the control unit determines that there is no request to measure the AC impedance of the fuel cell, it ends the control. On the other hand, when the control unit determines that there is a request to measure the AC impedance of the fuel cell, it monitors the current value of the coil and determines whether or not, as condition 1, "when the two-phase switches magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil (the driving phase) is in discontinuous mode, and the duty ratio (D) of the two-phase switches is less than 50% (D<0.5), and either one of formula (A) or formula (B) is satisfied." When the control unit determines that condition 1 is not satisfied, it operates the two-phase switches with a phase difference of 180° and the same duty ratio to measure the AC impedance of the fuel cell. On the other hand, when the control unit determines that condition 1 is satisfied, it changes the operation of the two-phase switches from an operation with a phase difference of 180° and the same duty ratio to an operation with a phase difference of 180° and different duty ratios, and measures the AC impedance of the fuel cell.

As shown in FIG. 15 , the “region in which the dead zone occurs” is when “the current flowing through the coil is in discontinuous mode, and the duty ratio (D) of the two-phase switch is less than 50% (D<0.5), and either formula (A) or formula (B) is satisfied.” Therefore, for example, at the timing when the operating conditions shift to the region in which the dead zone occurs shown in FIG. 15 , the operation of the two-phase switches may be changed from an operation with a phase difference of 180° and the same duty ratio to an operation with a phase difference of 180° and different duty ratios.
The means for determining whether or not condition 1 for the occurrence of a dead zone is satisfied is not limited to the above. For example, a range corresponding to the dead zone may be defined in accordance with the input voltage, output voltage, and duty ratio, and it may be determined that condition 1 is satisfied when the input voltage and duty ratio at a specified output voltage indicate that they have entered or approached the range corresponding to the dead zone.
As described above, by defining in advance the range that corresponds to the dead zone, and changing the operation of the two-phase switches from a phase difference of 180° and the same duty ratio to a phase difference of 180° and different duty ratios when that range is entered, the AC impedance of the fuel cell can be quickly obtained when desired.

In FIG. 16 , condition 1 is set as a case where “when two-phase switches that are magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil is in discontinuous mode, and the duty ratio of the two-phase switches is less than 50%, and either one of formula (A) or formula (B) is satisfied,” but the condition “when either one of formula (A) or formula (B) is satisfied” may be excluded.
That is, condition 1 may be "when two-phase switches magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil is in discontinuous mode and the duty ratio of the two-phase switches is less than 50%." In this case, for example, when the required output to the fuel cell decreases and the current flowing through the coil is in discontinuous mode and the duty ratio of the two-phase switches transitions from 50% or more to a state where the current flowing through the coil is in discontinuous mode and the duty ratio of the two-phase switches is less than 50%, the operation of the two-phase switches may be changed from an operation with a phase difference of 180° and the same duty ratio to an operation with a phase difference of 180° and different duty ratios.
As shown in Fig. 15, the area excluded from the range in which the same duty ratio is changed to a different duty ratio by formula (A) and formula (B) is not large. Therefore, even if the condition "when either formula (A) or formula (B) is satisfied" is excluded, it is considered that the effect on the increase in the battery life is limited. On the other hand, by excluding the condition "when either formula (A) or formula (B) is satisfied", the control is simplified and the AC impedance of the fuel cell can be appropriately measured.
As a modified example, condition 1 may be "when two-phase switches magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil is in discontinuous mode, the duty ratio of the two-phase switches is less than 50%, and formula (A) is satisfied" or "when two-phase switches magnetically coupled to each other are operated with a phase difference of 180°, the current flowing through the coil is in discontinuous mode, the duty ratio of the two-phase switches is less than 50%, and formula (B) is satisfied."

[AC impedance measurement]
The control unit measures the AC impedance of the fuel cell from the current waveform and voltage waveform of the fuel cell.
The control unit may check whether or not there is a request to measure the AC impedance of the fuel cell, and if it checks that there is a request to measure the AC impedance of the fuel cell, it may determine whether or not condition 1 is satisfied, and if it determines that condition 1 is satisfied, it may operate the n-phase switches with different duty ratios to measure the AC impedance of the fuel cell.
When the control unit determines that condition 1 is satisfied, it checks whether there is a request to measure the AC impedance of the fuel cell, and if it checks that there is a request to measure the AC impedance of the fuel cell, it may operate the n-phase switches with different duty ratios to measure the AC impedance of the fuel cell.
The control unit measures the AC impedance of the fuel cell at a predetermined frequency while the fuel cell is in operation in order to grasp the state of the electrolyte membrane of the fuel cell and the state of the gas supply.
The control unit acquires the output voltage and output current values of the fuel cell as time-series waveform data of one wavelength or more when a load current containing a certain frequency component is applied by switching the converter while periodically increasing and decreasing the PWM duty ratio, performs a discrete Fourier transform on the waveform data, and calculates the AC impedance of the fuel cell by dividing the discrete Fourier transform result of the voltage signal by the discrete Fourier transform result of the current signal.
In a low load region called discontinuous mode, when the duty ratio is increased, the average current value flowing through the reactor coil also increases.
In order to obtain the AC impedance of the fuel cell, for example, the duty ratio may be controlled so that the amplitude of the output current value becomes a sine wave of about ±3A.
The output voltage value of the fuel cell may be 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, and if the voltage value of each single cell is used, the AC impedance of each single cell can be obtained.
Furthermore, by obtaining the voltage for each of a plurality of unit cells (for example, every two unit cells, every four unit cells, etc.), the AC impedance for each arbitrary unit cell block can be obtained.
When the surface area of a single cell is several hundred ^cm2 , the components of 200 Hz or higher in 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.
The method for measuring the AC impedance of the fuel cell in the present disclosure is not particularly limited, and any known method can be applied, and may be the same as the method described in, for example, JP-A-2008-098134.

10: fuel cell 20: boost converter 21: reactor 22: current sensor 23: switch 24: diode 25: capacitor 50: external load

Claims (7)

1. A fuel cell system comprising:
The fuel cell system includes a fuel cell and a converter that performs at least one operation selected from the group consisting of stepping up and stepping down an output voltage of the fuel cell;
the converter includes a reactor having n-phase coils (n is an integer of 2 or more) magnetically coupled to each other, n-phase switches connected to the coils, and a control unit;
The control unit performs ON/OFF control of the switches of n phases,
The control unit monitors a current value of the coil,
the control unit measures an AC impedance of the fuel cell from a current waveform and a voltage waveform of the fuel cell;
the control unit operates the switches of n phases with different phases and operates the switches of n phases with the same duty ratio;
A fuel cell system, characterized in that the control unit operates the switches of n phases with different duty ratios when it is determined that the following condition 1 is satisfied:
Condition 1: An operating condition in which, when the n-phase switches are operated at different phases, the current flowing through the coil is in discontinuous mode and the current value flowing through the coil of one phase is maintained at zero, the switch connected to the coil of at least one other phase switches from ON to OFF. The fuel cell system according to claim 1, wherein the control unit operates the n-phase switches with a phase difference of (360/n)°. the control unit sets an offset current value y to be added to or subtracted from a 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, the offset current value y being 25 to 75% of a dead band current interval;
3. The fuel cell system according to claim 1, wherein the control unit unevenly distributes the target current value obtained by adding or subtracting the offset current value y to the command current value so that the target current value differs for each of the coils, and operates the switches with different duty ratios. the control unit stores in advance a current distribution difference map derived from a relationship between an inlet voltage and an outlet voltage of the converter and a current value flowing through a coil of the reactor;
4. The fuel cell system according to claim 3, wherein the control unit unevenly distributes the target current value so that the target current value differs for each of the coils based on the current distribution difference map, and operates the switches with different duty ratios. The control unit stores in advance an operating condition of the converter corresponding to the condition 1 as a data group,
5. The fuel cell system according to claim 1, wherein the control unit checks the current operating conditions of the converter against the data group to determine whether or not the condition 1 is satisfied. The fuel cell system according to any one of claims 1 to 5, wherein the control unit determines that condition 1 is satisfied when n = 2, and when the switches of two phases that are magnetically coupled to each other are operated with a phase difference of 180°, a current flowing through the coil is in a discontinuous mode, and a duty ratio of the switches of the two phases is less than 50% (D < 0.5), and either one of the following formula (A) or formula (B) is satisfied:
Formula (A): D<{(1/2)(LM)(V _H -V _L )}/(LV _L +MV _L -MV _H )
Formula (B): D<(1/2) {1-(V _L /V _H )}
[In the formulas (A) and (B), L is the self-inductance of the reactor, M is the mutual inductance of the reactor, _VH is the output voltage of the converter, _VL is the input voltage of the converter, and D is the duty ratio (-).] The control unit checks whether or not there is a request to measure the AC impedance of the fuel cell;
When the control unit confirms that there is a request to measure the AC impedance of the fuel cell, the control unit determines whether or not the condition 1 is satisfied;
7. The fuel cell system according to claim 1, wherein when it is determined that condition 1 is satisfied, the control unit operates the n-phase switches with different duty ratios and measures the AC impedance of the fuel cell.

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