POWER CONVERSION SYSTEM

A power conversion system includes a converter, a current sensor, and a motor ECU. The converter is configured to boost a voltage by operating at a set carrier frequency. The current sensor detects a current flowing through the converter. The motor ECU executes control for protecting the converter. The motor ECU estimates a temperature rise amount of the converter according to at least one of the carrier frequency and a voltage ratio before and after boosting the converter, and a detected value of the current sensor, and executes control for suppressing the current flowing through the converter in a case where an integrated value of the temperature rise amount reaches a threshold.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2021-029773 filed on Feb. 26, 2021, incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a power conversion system, and more particularly to a power conversion system including a converter.

2. Description of Related Art

Japanese Unexamined Patent Application Publication No. 2011-049032 (JP 2011-049032 A) discloses a control system capable of protecting a converter. This control system includes a battery, a buck-boost converter circuit, a current sensor, and a control unit. The current sensor detects a current flowing through the buck-boost converter circuit. The control unit compares a squared integrated value of a detected value of the current sensor with a threshold for protecting the buck-boost converter circuit. When the integrated value is equal to or smaller than the threshold, the control unit performs a continuous control such that a battery temperature rises. On the other hand, when the integrated value is larger than the threshold, the control unit stops the control such that the battery temperature ceases to increase to protect the buck-boost converter circuit from overheating.

SUMMARY

The current flowing through the converter includes a ripple component in addition to a DC component. Even if the DC component is kept at the same magnitude, a temperature rise amount of the converter becomes larger as the ripple component becomes larger. Lots of current sensors may not be able to accurately detect the ripple component depending on their degree of detection accuracy. Therefore, in a case where the temperature rise amount of the converter is estimated according to the detected value of the current sensor for the purpose of protecting the converter from overheating, it is also likely that the temperature rise amount of the converter is estimated under the assumed condition that the ripple component is always at the maximum level with respect to the detected value of the current sensor. However, if protecting the converter from overheating is performed based on such an estimated temperature rise amount, the converter will be overprotected. The disclosure in JP 2011-049032 A does not particularly consider this problem.

The present disclosure provides a power conversion system including a converter, in which the converter is appropriately protected from overheating according to a detected value of a current sensor that detects a current flowing through the converter.

The power conversion system according to an aspect of the present disclosure includes a converter, a current sensor, and a control device. The converter boosts a voltage by operating at a set carrier frequency. The current sensor detects a current flowing through the converter. The control device executes control for protecting the converter. The control device estimates a temperature rise amount of the converter according to at least one of the carrier frequency and a voltage ratio before and after boosting the converter, and a detected value of the current sensor, and executes control for suppressing the current flowing through the converter in a case where an integrated value of the temperature rise amount reaches a threshold.

In the above configuration, the temperature rise amount of the converter is estimated based on at least one of the voltage ratio and the carrier frequency of the converter, both of which affect a ripple component, as well as the detected value of the current sensor. At least one of the voltage ratio and the carrier frequency of the converter is taken into account, thus protecting the converter from overheating may not be performed under the assumed condition that the ripple component is at the maximum level. Consequently, the converter can be adequately protected from overheating.

In the aspect, the power conversion system may further include a storage unit configured to store a predetermined correlation between the temperature rise amount, the detected value of the current sensor, and at least one of the voltage ratio and the carrier frequency. The control device may estimate the temperature rise amount according to the detected value of the current sensor, using the predetermined correlation and at least one of the voltage ratio and the carrier frequency.

In the above configuration, the temperature rise amount of the converter is estimated based on the correlation prepared in advance. Consequently, the power conversion system can have a streamlined configuration while the converter can be adequately protected from overheating.

In the aspect, the control device may set, in a case where the integrated value reaches the threshold, the carrier frequency to be higher than a carrier frequency immediately before the integrated value reaches the threshold.

In the above configuration, the carrier frequency of the converter is set to be higher, thus a ripple amplitude of the current flowing through the converter is reduced. Consequently, the temperature rise amount of the converter is reduced, and thus the converter can be protected from overheating.

In the aspect, the converter may be electrically connected between a power storage device and a load device. The control device may control the load device such that electric power input to and output from the power storage device is respectively limited to a charging upper limit and a discharging upper limit of the power storage device. The control device may also set, in a case where the integrated value reaches the threshold, the charging upper limit and the discharging upper limit to be lower than a charging upper limit and a discharging upper limit immediately before the integrated value reaches the threshold.

Accordingly, the current flowing through the converter is suppressed, and thus the temperature rise amount of the converter is reduced. Consequently, the converter can be protected from overheating.

With the aspect of the present disclosure, it is possible to provide the power conversion system including the converter, in which the converter is appropriately protected from overheating according to the detected value of the current sensor that detects the current flowing through the converter.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, the present embodiment will be described referring to drawings. In the drawings, the same or equivalent components will have the same reference signs assigned, and descriptions thereof will be omitted. In the following embodiment, a configuration of a vehicle shown as an example in which the power conversion system is adopted will be described, but the power conversion system of the present disclosure may be adopted in any other applications, not limited to the vehicle.

FIG. 1is a diagram illustrating an overall configuration of the vehicle in which the power conversion system according to the present embodiment is adopted. In the present embodiment, a case where a vehicle10is an electric vehicle will be described as one example, but the vehicle10may be a hybrid vehicle further equipped with an internal combustion engine or may be a fuel cell vehicle further equipped with a fuel cell.

The vehicle10includes a battery pack1, a power control unit (PCU)2, a motor generator (MG)3, and a vehicle electronic control unit (ECU)50.

The battery pack1includes a battery11, a voltage sensor12, a current sensor13, a temperature sensor14, a system main relay (SMR)15, and a battery ECU16.

The battery11is a power storage device that can be charged and discharged. The battery11is a rechargeable secondary battery, for example, a lithium-ion battery, a nickel-metal hydride battery, or a lead storage battery. Instead of the battery11, a power storage device configured by a power storage element such as an electric double layer capacitor may be used. The battery11supplies the PCU2with electric power for generating a driving force of wheels (not shown) of the vehicle10. Further, the battery11is configured to store the electric power generated by the MG3(described later).

The voltage sensor12detects a voltage Vb of the battery11. The current sensor13detects a current Ib input to and output from the battery11. The temperature sensor14detects a temperature Tb of the battery11. Each sensor outputs its detected value to the battery ECU16.

The SMR15is provided between the battery11and a converter21(described later). The SMR15is turned on and off according to a command from the battery ECU16.

The battery ECU16includes a processor such as a central processing unit (CPU), and a memory such as a read only memory (ROM) or a random access memory (RAM).

The battery ECU16monitors a state of the battery11and controls the SMR15based on, for example, a signal received from each sensor, as well as a program and a map stored in the memory. As an example, the battery ECU16calculates a state-of-charge (SOC) of the battery11based on, for example, the current Ib, the voltage Vb, and the temperature Tb of the battery11, and the program and the map stored in the memory. The battery ECU16transmits the calculated SOC to a vehicle ECU50(described later).

The PCU2includes a positive electrode line PL1, a negative electrode line NL, a capacitor C1, a converter21, a positive electrode line PL2, a capacitor C0, voltage sensors22and24, an inverter23, and a motor ECU4.

The positive electrode line PL1electrically connects a positive electrode of the battery11and a high potential end of the converter21(described later). The negative electrode line NL electrically connects a negative electrode of the battery11and a low potential end of the converter21. A voltage VL is a voltage between the positive electrode line PL1and the negative electrode line NL.

The capacitor C1is connected between the positive electrode line PL1and the negative electrode line NL. The capacitor C1smooths the voltage between the positive electrode line PL1and the negative electrode line NL.

The voltage sensor24detects the voltage VL, which is a voltage across the capacitor C1, and outputs a detected value to the motor ECU4.

The converter21is a boost chopper circuit and includes a reactor L1, a current sensor210, switching elements Q1and Q2, and diodes D1and D2.

The reactor L1is electrically connected between the positive electrode of the battery11and an intermediate point (connection node) between the switching element Q1and the switching element Q2.

The current sensor210detects a current IL flowing through the reactor L1and outputs a detected value to the motor ECU4. The current sensor210cannot accurately detect a ripple component of the current IL. In the present embodiment, the current sensor210outputs a value corresponding to the average of the maximum peak value and the minimum peak value of the ripple component of the current IL, as a detected value.

The switching elements Q1and Q2are connected in series between the positive electrode line PL2and the negative electrode line NL. The switching elements Q1and Q2are respectively switched (turned on/off) according to driving signals S1and S2from the motor ECU4. The switching elements Q1and Q2are, for example, insulated gate bipolar transistors (IGBTs) or metal oxide semiconductor field effect transistors (MOSFETs).

The diodes D1and D2are respectively connected to the switching elements Q1and Q2in antiparallel.

The converter21is controlled by the motor ECU4(described later) such that the switching elements Q1and Q2are switched. The converter21is configured to boost the voltage VL to output a boosted voltage VH as operated at the set carrier frequency.

In the converter21, a voltage ratio (VL/VH) between the voltage VH and the voltage VL (i.e., the voltage ratio before and after boosting) is controlled by an on-period ratio (duty ratio) of the switching elements Q1and Q2to a switching cycle (carrier cycle) of the converter21. Hereinafter, the voltage ratio (VL/VH) is referred to as a “step-up ratio”. The details of the control executed by the converter21will be described later.

The positive electrode line PL2electrically connects the high potential end of the converter21and a high potential end of an inverter23(described later). The negative electrode line NL electrically connects the low potential end of the converter21and a low potential end of the inverter23(described later).

The capacitor C0is connected between the positive electrode line PL2and the negative electrode line NL to smooth the voltage therebetween.

The voltage sensor22detects the voltage VH, which is a voltage across the capacitor C0, and outputs a detected value to the motor ECU4.

The inverter23includes a U-phase arm231, a V-phase arm232, and a W-phase arm233. The U-phase arm231includes switching elements Q3and Q4, and diodes D3and D4, which are respectively connected to the switching elements Q3and Q4in antiparallel. The V-phase arm232includes switching elements Q5and Q6, and diodes D5and D6, which are respectively connected to the switching elements Q5and Q6in antiparallel. The W-phase arm233includes switching elements Q7and Q8, and diodes D7and D8, which are respectively connected to the switching elements Q7and Q8in antiparallel.

The switching elements Q3to Q8are respectively switched (turned on/off) according to driving signals S3to S8from the motor ECU4.

The inverter23converts DC power output from the converter21into AC power by switching the switching elements Q3to Q8, and outputs the converted AC power to MG3. On the other hand, when regenerative braking of the vehicle10is performed, the inverter23converts AC power generated by the MG3into DC power and outputs the DC power to the converter21. The DC power output to the converter21is stepped down according to the step-up ratio (VL/VH) of the converter21and then stored in the battery11.

The MG3is shown as one example of the load device, which is a three-phase permanent magnet synchronous motor. In the MG3, ends of three coils, i.e., U-phase, V-phase, and W-phase coils, are connected to a neutral point. The other ends of the U-phase, V-phase, and W-phase coils are respectively connected to intermediate points of the U-phase arm231, the V-phase arm232, and the W-phase arm233. An output torque of the MG3is transmitted to a drive wheel through a power transmission gear (neither of them is shown), whereby the vehicle10travels. Further, the MG3generates electric power by a rotational force of the drive wheel during the regenerative braking of the vehicle10.

Similar to the battery ECU16, the motor ECU4includes a processor (not shown) such as a CPU, and a memory5configured by, for example, a ROM and a RAM. The motor ECU4is configured to establish communication with the vehicle ECU50(described later), so as to exchange various data and signals with each other.

The motor ECU4controls the converter21and the inverter23with pulse width modulation (PWM) based on the signals received from each sensor, as well as the program and the map stored in the memory5. The motor ECU4sets, for example, a carrier frequency for PWM control of the converter21, and controls the voltage VH boosted by the converter21.

The vehicle ECU50is a higher-level ECU that controls the entire vehicle10based on signals output from various sensors of the vehicle10. The vehicle ECU50controls, for example, a charging upper limit Win and a discharging upper limit Wout of the battery11based on the SOC of the battery11transmitted from the battery ECU16. The vehicle ECU50controls a torque of the MG3such that the input power and the output power of the battery11are respectively limited to the charging upper limit Win and the discharging upper limit Wout.

The current flowing through the converter21includes a ripple component in addition to a DC component. The ripple component is generated due to switching operations by the switching elements Q1and Q2. Even if the DC component of the current flowing through the converter21is kept at the same magnitude, a temperature rise amount of the converter21becomes larger as the ripple component becomes larger. The current sensor210cannot accurately detect the ripple component. Therefore, in a case where the temperature rise amount of the converter21is estimated according to the detected value of the current sensor210for the purpose of protecting the converter21from overheating, it is also likely that the temperature rise amount of the converter21is estimated under the assumed condition that the ripple component is always at the maximum level with respect to the detected value of the current sensor210. However, if control of protecting the converter21from overheating is executed based on such an estimated temperature rise amount, the converter21will be overprotected.

In consideration of the problems stated above, the inventors focused on the fact that an amplitude of the ripple component (hereinafter, also referred to as a “ripple amplitude”) of the current flowing through the converter21varies according to the step-up ratio (VL/VH) of the converter21and the carrier frequency of the PWM control of the converter21.

Therefore, the motor ECU4according to the present embodiment estimates the temperature rise amount of the converter21according to the step-up ratio (VL/VH) of the converter21, the carrier frequency of the converter21, and the detected value of the current flowing through the converter21. Therefore, it is not necessary to overestimate the temperature rise amount under the assumed condition that the ripple component is always at the maximum level with respect to the detected value obtained by the current sensor210. When an integrated value of the temperature rise amount estimated as described above reaches a threshold, the current flowing through the converter21is controlled to be suppressed. Hereinafter, the control is also referred to as “converter current suppression control”. Specific examples of the converter current suppression control will be described later.

Hereinafter, the control of the motor ECU4according to the present embodiment will be described in more detail. The current IL is employed as one example of the current flowing through the converter21in the following description.

It is known that a ripple amplitude ILpp (peak-peak value) of the current IL is a function of the voltage VL and voltage VH of the converter21with a carrier frequency fc of the converter21as shown in the following equation (1).

The equation (1) is modified as follows with respect to the step-up ratio (VL/VH=k).

Therefore, as denoted by line300inFIG. 2, when the step-up ratio (VL/VH=k) is 0.5, the ripple amplitude ILpp reaches the maximum level with respect to the step-up ratio (VL/VH). Further, in the converter21, since the voltage VH after boosting is equal to or larger than the voltage VL before boosting, (VL≤VH), 0<k≤1 is established for the step-up ratio VL/VH (=k). In a case where the step-up ratio is 1, the ripple amplitude ILpp is at the minimum level (value 0).

As can be seen from the equations (1) and (2), the ripple amplitude ILpp decreases as the carrier frequency fc increases. In a case where the carrier frequency fc is set between a lower limit fcmin and an upper limit fcmax, as denoted by line305inFIG. 3, the ripple amplitude ILpp is at the maximum level with respect to the carrier frequency fc when the carrier frequency fc is the lower limit fcmin. On the other hand, when the carrier frequency fc reaches the upper limit fcmax, the ripple amplitude ILpp reaches the minimum level with respect to the carrier frequency fc.

As described above, the ripple amplitude ILpp is at the maximum level under the conditions regarding the step-up ratio (VL/VH) and the carrier frequency fc, that the step-up ratio (VL/VH) is 0.5 and the carrier frequency fc reaches fcmin. Hereinafter, this condition is also referred to as a “maximized ripple amplitude condition”.

On the other hand, the ripple amplitude ILpp is at the minimum level (that is, 0) with respect to the step-up ratio (VL/VH) under the condition that the step-up ratio (VL/VH) is 1. Hereinafter, this condition is also referred to as a “minimized ripple amplitude condition”.

The temperature rise amount of the converter21is related to an amount of heat generated in the reactor L1. This amount of generated heat is related to the square of the current IL. The current IL is composed of a DC component and a ripple component. Therefore, even in a case where the DC component of the current IL is kept at the same value, the amount of generated heat increases as the ripple amplitude ILpp representing the ripple component of the current IL increases, and thus the temperature rise amount also increases. As described above, the temperature rise amount depends on the ripple amplitude ILpp.

The ripple amplitude ILpp is a function of the step-up ratio (VL/VH) of the converter21and the carrier frequency fc of the converter21as shown in the equation (2). Therefore, the temperature rise amount of the converter21depends on the step-up ratio (VL/VH) of the converter21and the carrier frequency fc of the converter21.

Referring toFIG. 4, details of a temperature rise of the converter21depending on the step-up ratio of the converter21and the carrier frequency of the converter21will be described.

FIG. 4is a diagram illustrating a temporal transition of a temperature TC of the converter21according to the step-up ratio (VL/VH) and the carrier frequency (fc). InFIG. 4, a horizontal axis represents a time t elapsed from a time when the temperature TC is an initial temperature T0, and a vertical axis represents the temperature TC of the converter21.

Referring toFIG. 4, dashed-dotted lines402,410, and420denote the temporal transition of the temperature TC under the maximized ripple amplitude condition in a case where the current IL is Ia, Ib, or Ic, respectively. Since the temperature rise amount of the converter21increases as the ripple amplitude ILpp increases, an amount of the ripple amplitude ILpp contributing to the temperature rise amount of the converter21reaches a maximum under the maximized ripple amplitude condition.

Meanwhile, solid lines405,415, and425denote the temporal transition of the temperature TC under the minimized ripple amplitude condition in a case where the current IL is Ia, Ib, or Ic, respectively. Since the temperature rise amount of the converter21decreases as the ripple amplitude ILpp decreases, an amount of the ripple amplitude ILpp contributing to the temperature rise amount of the converter21reaches a minimum under the minimized ripple amplitude condition

A threshold temperature TTH is appropriately predetermined by, for example, experiments in order to protect the converter21from overheating. The threshold temperature TTH is determined based on, for example, the amount of heat generated in the reactor L1as well as specific heat capacities and operating temperature limits of the components constituting the converter21.

A time tTH1, tTH2, or tTH3is a time taken for the temperature TC to reach the threshold temperature TH from the predetermined initial temperature T0under the maximized ripple amplitude condition in a case where the current IL is kept at Ia, Ib, or Ic, respectively. Hereinafter, the time required for the temperature TC to reach the threshold temperature TH from the predetermined initial temperature T0is also referred to as a “threshold arrival time”. The threshold arrival time is also a time required for the temperature rise amount from the initial temperature T0of the converter21to reach ΔTTH. Further, the initial temperature T0is determined in advance, for example.

A time tTH1′, tTH2′, or tTH3′ is a threshold arrival time under the minimized ripple amplitude condition in a case where the current IL is kept at Ia, Ib, or Ic, respectively.

When the temperature TC of the converter21reaches the threshold temperature TH, the converter current suppression control is executed in order to prevent the converter21from being overheated due to the heat generated by the reactor L1.

As one example of the converter current suppression control, control is executed to set the charging upper limit Win and the discharging upper limit Wout of the battery11to be lower than those immediately before the threshold arrival time elapses. For example, in a case where the discharging upper limit Wout is set to be lower, and the electric power discharged from the battery11to acquire the torque of MG3according to a torque command value is equal to or larger than the discharging upper limit set to be lower, the motor ECU4controls the inverter23such that the torque of the MG3is limited in order to restrict the electric power discharged from the battery11. Consequently, the electric power supplied to the converter21is limited as compared with the electric power before the threshold arrival time elapses, thus the current IL flowing through the converter21is suppressed. Therefore, the converter21is prevented from being overheated.

As shown in the drawings, even in a case where the detected value of the current IL is kept at the same value, the threshold arrival time varies depending on the step-up ratio (VL/VH) and the carrier frequency fc. For example, referring to lines402and405, even when the detected value of the current IL is kept at the same Ia, the step-up ratio (VL/VH) in a case denoted by line405is 1 in which the ripple amplitude ILpp reaches the minimum level (seeFIG. 2), and the step-up ratio (VL/VH) in a case denoted by line402is 0.5 in which the ripple amplitude ILpp reaches the maximum level (seeFIG. 2). The carrier frequency fc in a case denoted by line405is larger than the carrier frequency fc in a case denoted by line402(FIG. 3).

Therefore, the ripple amplitude ILpp in a case denoted by line405is smaller than the ripple amplitude ILpp in a case denoted by line402. The smaller the ripple component, the smaller the temperature rise amount of the converter21, thus the temperature rise amount of the converter21per unit time in a case denoted by line405is smaller than the temperature rise amount in a case denoted by line402. Therefore, tTH1′ as the threshold arrival time in a case denoted by line405is longer than tTH1as the threshold arrival time in a case denoted by line402.

As described above, the threshold arrival time varies depending on the ripple amplitude ILpp (specifically, the step-up ratio and the carrier frequency fc). Meanwhile, the current sensor210cannot accurately detect the ripple component of the current IL.

If the temperature rise amount of the converter21is estimated under the assumed conditions that the ripple component of the current IL is not accurately detected and the maximized ripple amplitude condition is always satisfied, it leads to the excessive protection of the converter21.

For example, in a case where the conditions stated above do not accurately reflect the actual ripple amplitude ILpp, the converter current suppression control may be executed at an unnecessarily early timing. In particular, the unnecessarily early timing indicates that the control is executed when the temperature TC of the converter21has not actually risen to the threshold temperature TH and the control does not need to be executed yet.

As the converter current suppression control, for example, when the charging upper limit Win and the discharging upper limit Wout of the battery11are set to be lower than those immediately before the threshold arrival time elapses, the torque of the MG3is likely to be restricted after the threshold arrival time has elapsed. In this case, traveling performance of the vehicle10deteriorates. It is preferable that the control be executed as late as possible to the extent that the converter21is protected from overheating.

In the present embodiment, the control (converter current suppression control) of protecting the converter21from overheating is executed in a situation in which the step-up ratio (VL/VH) and the carrier frequency fc are taken into consideration, unlike a case where the converter21is protected under the assumed condition that the maximized ripple amplitude condition is always satisfied.

Referring toFIG. 5, a difference in the threshold arrival time, between a case where the ripple amplitude ILpp reaches the maximum level and a case where the ripple amplitude ILpp is at the minimum level, will be described in more detail.

FIG. 5is a diagram illustrating maps500,505respectively showing a correlation between the detected value of the current IL and the threshold arrival time. In upper and lower rows ofFIG. 5, a vertical axis represents the detected value of the current IL, and a horizontal axis represents the threshold arrival time shown inFIG. 4. The maps500,505are predetermined by, for example, experiments, and stored in advance in the memory5(seeFIG. 1) of the motor ECU4.

Referring to the upper row ofFIG. 5, the map500shows the threshold arrival time when the detected value of the current IL keeps being obtained under the maximized ripple amplitude condition. The map500also represents the threshold arrival times tTH1, tTH2, and tTH3shown in relation to lines402,410, and420(seeFIG. 4), respectively.

Meanwhile, referring to the lower row ofFIG. 5, the map505shows the threshold arrival time when the detected value of the current IL keeps being obtained under the minimized ripple amplitude condition. The map505also represents the threshold arrival times tTH1′, tTH2′, and tTH3′ shown in relation to lines405,415, and425(seeFIG. 4), respectively.

As described above, the maps500,505respectively define the correlation between the detected value of the current IL and the threshold arrival time, according to the step-up ratio (VL/VH) and the carrier frequency fc of the converter21. Additionally, the memory5further stores a plurality of other maps (not shown) that predefine the correlation according to other combinations of the step-up ratio (VL/VH) and carrier frequency fc.

It is possible to simplify the configuration for the control of protecting the converter from overheating by using the maps prepared in advance by, for example, experiments.

The motor ECU4selects a map corresponding to the step-up ratio (VL/VH) and the carrier frequency fc of the converter21, from the maps500,505, as well as other maps, stored in the memory5. The motor ECU4selects the map500, for example, in a case where the step-up ratio (VL/VH) is 0.5 and the carrier frequency fc is fcmin. Further, the motor ECU4selects the map505, for example, in a case where the step-up ratio (VL/VH) is 1 and the carrier frequency fc is fcmax.

The motor ECU4acquires the threshold arrival time tTH corresponding to the detected value of the current IL, based on the detected value and the selected map. For example, in a case where the detected value is IL1and the map505is selected, the threshold arrival time tTH in a case where the detected value keeps being obtained is tTH1′. The threshold arrival time tTH acquired according to the map thus selected is used for estimating the temperature rise amount of the converter21as described below.

A method for estimating the temperature rise amount of the converter21according to the acquired threshold arrival time tTH will be described referring toFIG. 6.

FIG. 6is a diagram illustrating a table600showing a correlation between the detected value of the current IL and the temperature rise amount ΔTC of the converter21. In particular, a table600stores the correlation in a case where the step-up ratio (VL/VH) is VLa/VHa and the carrier frequency fc is fca. The table600is stored in advance in the memory5of the motor ECU4. The table600includes columns605,610, and615. The column605represents the detected value of the current IL.

The column610represents the threshold arrival time tTH corresponding to the detected value. The threshold arrival time is a threshold arrival time in a case where the detected value keeps being obtained. As illustrated referring toFIG. 5, the threshold arrival time tTH is a function of the step-up ratio (VL/VH) and the carrier frequency fc, because it varies depending on those factors.

The column610represents the threshold arrival time tTH in a case where the step-up ratio (VL/VH) is VLa/VHa and the carrier frequency fc is fca. For example, in a case where the detected value of the current IL is IL1, the threshold arrival time tTH in a case where the detected value keeps being obtained is tTH11. In a case where (VLa/VHa) is 0.5 and fca is fcmin (maximized ripple amplitude condition), tTH11is tTH1(seeFIG. 4). Further, in a case where (VLa/VHa) is 1 (minimized ripple amplitude condition), tTH11is tTH1′ (seeFIG. 4).

The column615represents the temperature rise amount ΔTC of the converter21according to the threshold arrival time tTH corresponding to the current IL. The temperature rise amount is the temperature rise amount of the converter21over a sampling cycle from a sampling timing of the detected value of the current sensor210to the next sampling timing. Since the temperature rise amount ΔTC is calculated based on the threshold arrival time tTH as described below, it is a function of the step-up ratio (VL/VH) and the carrier frequency fc as in the threshold arrival time tTH. The column615represents the temperature rise amount ΔTC estimated in a case where the step-up ratio (VL/VH) is VLa/VHa and the carrier frequency fc is fca.

The detected value of the current sensor210is acquired by the motor ECU4in a sampling cycle TS. The temperature TC of the converter21rises due to heat generated by the reactor L1over the sampling cycle TS from such a sampling timing to the next sampling timing. The correlation between the sampling cycle TS and the temperature rise amount ΔTC of the converter21over the sampling cycle TS is the same as the correlation between the threshold arrival time tTH in a case where a certain detected value of the current IL keeps being obtained, and ΔTTH (seeFIG. 4) as the total temperature rise amount of the converter21when the threshold value arrival time has elapsed.

In the example shown inFIG. 6, in a case where the detected value of the current IL is IL1, the threshold arrival time tTH is tTH11when IL1keeps being obtained as the detected value. A temperature rise rate indicating how much the temperature TC has increased from the initial temperature T0with respect to the threshold temperature TTH (seeFIG. 4for both) is expressed as a percentage.

For example, when the temperature TC has not yet risen from the initial temperature T0, the temperature rise rate of the converter21is 0%. When the temperature TC has risen to an average temperature of the initial temperature T0and the threshold temperature TTH, the temperature rise rate is 50%. When the temperature TC has risen to the threshold temperature TTH, the temperature rise rate is 100%.

Therefore, how much the temperature TC has risen for each sampling cycle TS is also expressed in a percentage. For example, in a case where IL1keeps being obtained as the detected value of the current IL, the temperature rise rate over the time interval of tTH11is 100%. Therefore, in a case where the detected value of the current IL is IL1, the temperature rise rate over the sampling cycle TS from the sampling timing of the detected value to the next sampling timing is estimated to be (TS/tTH11)×100(%).

Therefore, the temperature rise amount over the sampling cycle TS is estimated to be (TS/tTH11)×100×ΔTTH, based on the total temperature rise amount (ΔTTH) (seeFIG. 4) of the converter21from the time when the temperature TC is the initial temperature T0to the time when the threshold arrival time tTH has elapsed, and the temperature rise rate. Even in a case where the detected value of the current IL is the other value (for example, IL2or IL3), the temperature rise amount ΔTC over the sampling cycle TS is similarly estimated (see the column615).

In addition to the table600, the memory5also includes tables such as tables620,630for calculating the temperature rise amount ΔTC in a case where other combinations are taken as the step-up ratio (VL/VH) and the carrier frequency fc.

For example, the table620is a table for calculating the temperature rise amount ΔTC over the sampling cycle TS when the step-up ratio (VL/VH) is VLb/VHb and the carrier frequency fc is fcb. The table630is a table for calculating the temperature rise amount ΔTC over the sampling cycle TS when the step-up ratio (VL/VH) is VLc/VHc and the carrier frequency fc is fcc.

The motor ECU4estimates the temperature rise amount ΔTC over the sampling cycle TS from the sampling timing to the next sampling timing according to the detected value of the current sensor210at the sampling timing. The temperature rise amount ΔTC is different from the temperature rise amount estimated under the assumption that the maximized ripple amplitude condition is always satisfied, and reflects the actual step-up ratio (VL/VH) and the carrier frequency fc, of the converter21, at the sampling timing.

It is estimated that the temperature TC of the converter21reaches the threshold temperature TH at a timing when an integrated value of the temperature rise amount ΔTC of the converter21reaches ΔTTH (seeFIG. 4) as the threshold, thus the converter current suppression control is executed. The integrated value is the integrated value of the temperature rise amount ΔTC integrated from the time when the temperature TC is the initial temperature T0.

Since the converter current suppression control is executed as stated above, the timing at which the current (current IL) flowing through the converter21is suppressed is not unnecessarily advanced, unlike the timing calculated under the maximized ripple amplitude condition. In other words, it is possible to delay the timing at which the converter current suppression control is executed (timing at which the traveling performance of the vehicle10deteriorates) compared to the timing calculated under the condition, within a range in which the converter21is protected from overheating.

FIG. 7is a functional block diagram of the motor ECU4. The motor ECU4includes a data selection unit702, a temperature rise amount estimation unit705, a threshold determination unit710, a converter current suppression unit715, a carrier frequency determination unit717, a carrier wave generation unit720, and a driving signal generation unit725.

The data selection unit702receives the voltage VH and the voltage VL, respectively output from the voltage sensor22and the voltage sensor24. Further, the data selection unit702receives the carrier frequency fc output from the carrier frequency determination unit717. The data selection unit702selects data corresponding to the step-up ratio (VL/VH) calculated from the voltage VL and the voltage VH, and corresponding to the carrier frequency fc, from among temperature rise amount estimation data701stored in the memory5, according to the step-up ratio and the carrier frequency.

The temperature rise amount estimation data701is data that defines a predetermined correlation between the detected value of the current IL, the step-up ratio (VL/VH) and the carrier frequency fc, and the temperature rise amount ΔTC of the converter21. In particular, the temperature rise amount estimation data701is composed of a plurality of maps (including, for example, the maps500,505shown inFIG. 5) and a plurality of tables (for example, including the tables600,620, and630shown inFIG. 6).

For example, in a case where the step-up ratio is 0.5 and the carrier frequency fc is fcmin, the data selection unit702selects the map500(seeFIG. 5) corresponding to a combination of the step-up ratio and the carrier frequency. In this case, the data selection unit702selects the table (seeFIG. 6) corresponding to the combination of the step-up ratio and the carrier frequency. The map and the table, selected by the data selection unit702, are output to the temperature rise amount estimation unit705.

Using the data (map and table) selected by the data selection unit702, the temperature rise amount estimation unit705estimates the temperature rise amount ΔTC of the converter21over the sampling cycle TS (seeFIG. 6) from the sampling timing of the detected value to the next sampling timing, according to the detected value of the current IL. The estimated temperature rise amount ΔTC is output to the threshold determination unit710.

The threshold determination unit710determines whether or not the integrated value ΔTCS of the temperature rise amount ΔTC is equal to or larger than ΔTTH (seeFIG. 4) as the threshold. When the integrated value ΔTCS is ΔTTH or larger, the threshold determination unit710outputs a request to the converter current suppression unit715so as to execute the converter current suppression control.

Upon receiving the request, the converter current suppression unit715executes control for suppressing the current IL in order to protect the converter21from overheating. The current IL is suppressed based on the charging upper limit Win and the discharging upper limit Wout transmitted from the vehicle ECU50as described below.

Upon receiving the request from the threshold determination unit710, the converter current suppression unit715outputs a request to the vehicle ECU50to set the charging upper limit Win and the discharging upper limit Wout to be lower than those immediately before the threshold arrival time elapses.

Upon receiving the request, the vehicle ECU50determines the charging upper limit Win and the discharging upper limit Wout, which are smaller, after the threshold arrival time has elapsed. These upper limit values are determined based on the information indicating a state of the battery11such as the SOC and the temperature Tb of the battery11, transmitted from the battery ECU16. The vehicle ECU50transmits, to the converter current suppression unit715, the charging upper limit Win and the discharging upper limit Wout after the threshold arrival time has elapsed.

Upon receiving the charging upper limit Win and the discharging upper limit Wout, which are smaller, after the threshold arrival time has elapsed, from the vehicle ECU50, the converter current suppression unit715generates a voltage command value for the inverter23according to these upper limit values, and outputs the generated voltage command value to the driving signal generation unit725.

The driving signal generation unit725compares the voltage command value with a carrier wave CWI generated by the carrier wave generation unit720. The carrier wave CWI is used for PWM control of the inverter23, and is generated based on a carrier frequency (not shown) for the inverter23.

Then, the driving signal generation unit725generates PWM signals of which logical states change according to the comparison result, as the driving signals S3to S8. The driving signal generation unit725outputs the generated driving signals S3to S8to the switching elements Q3to Q8(seeFIG. 1) of the inverter23, respectively.

The driving signal generation unit725also compares a carrier wave CWC generated by the carrier wave generation unit720based on the carrier frequency fc for the converter21with a command value of the voltage VH. PWM signals of which logical states change based on the comparison result are generated as driving signals S1and S2. The switching elements Q1and Q2(seeFIG. 1) of the converter21are driven according to the driving signals S1and S2.

As described above, the inverter23is controlled according to the charging upper limit Win and the discharging upper limit Wout. Consequently, a regenerative torque of the MG3is limited when the vehicle10is braked, and a power running torque of the MG3is restricted when the vehicle10is running. Therefore, the electric power supplied to the positive electrode lines PL1and PL2and the negative electrode line NL (seeFIG. 1) is also limited, thus the current IL is suppressed. Accordingly, the converter21is protected from overheating.

FIG. 8is a diagram illustrating one example of a process executed by the motor ECU4. This flowchart is executed at predetermined intervals. Each sensor value is sampled for each cycle.

The motor ECU4acquires the detected value of the voltage VL from the voltage sensor24(S105), and acquires the detected value of the voltage VH from the voltage sensor22(S110). The motor ECU4selects the temperature rise amount estimation data701(seeFIG. 7) according to the carrier frequency fc and the step-up ratio (VH/VL) (S115).

The motor ECU4acquires the detected value of the current IL from the current sensor210(S120). Using the carrier frequency fc, the step-up ratio (VH/VL), and the selected temperature rise amount estimation data701, the motor ECU4estimates the temperature rise amount ΔTC of the converter21over the sampling cycle TS (seeFIG. 6) from the sampling timing of the detected value to the next sampling timing, according to the detected value of the current IL (S125).

The motor ECU4determines whether the integrated value ΔTCS of the temperature rise amount ΔTC (seeFIG. 7) is equal to or larger than ΔTTH (seeFIG. 4) as the threshold (S130). In a case where the integrated value ΔTCS of the temperature rise amount ΔTC is ΔTTH or larger (YES in S130), the motor ECU4proceeds to step S135. If otherwise (NO in S130), the motor ECU4returns the process to step S105.

In step S135, the motor ECU4executes the control for suppressing the current flowing through the converter21as the control for protecting the converter21. In particular, the motor ECU4outputs the request to the vehicle ECU50to set the charging upper limit Win and the discharging upper limit Wout, of the battery11, to be lower than those immediately before the threshold arrival time elapses. Accordingly, the charging upper limit Win and the discharging upper limit Wout become smaller, and thus the current IL is suppressed. The motor ECU4then returns the process.

FIG. 9is a diagram illustrating a timing at which the converter current suppression control is executed in the present embodiment. InFIG. 9, as in a case shown inFIG. 5, a vertical axis represents the detected value of the current IL, and a horizontal axis represents the threshold arrival time tTH. The maps500,505ofFIG. 5are also shown.

A motor ECU of a comparative example estimates the temperature rise amount of the converter over each sampling cycle using the map500under the assumption that the maximized ripple amplitude condition is always satisfied. On the other hand, the motor ECU4of the present embodiment estimates the temperature rise amount ΔTC of the converter21over each sampling cycle using the map505considering the step-up ratio (VL/VH) and the carrier frequency fc, related to the ripple amplitude ILpp.

In the comparative example (map500), the threshold arrival time tTH is tTHamin in a case where the detected value of the current IL is kept at ILa. tTHamin is the threshold arrival time under the maximized ripple amplitude condition. Under this circumstance, it is assumed that the amount of the ripple amplitude ILpp contributing to the temperature rise amount of the converter21is at the maximum level. Therefore, tTHamin is the smallest value that the threshold arrival time tTH can represent to the extent that the step-up ratio (VL/VH) and the carrier frequency fc can change. Consequently, the converter current suppression control may be executed at an unnecessarily early timing, and the traveling performance of the vehicle10may be declined at an unnecessarily early timing.

On the other hand, in the present embodiment, even in a case where the detected value of the current IL is kept at ILa, the threshold arrival time tTHa may vary depending on the step-up ratio (VL/VH) and the carrier frequency fc at the sampling timing at which the voltage VL and the voltage VH are detected.

In the present embodiment, the step-up ratio (VL/VH) may not be 0.5 (seeFIG. 2) and the carrier frequency fc may not be fcmin (seeFIG. 3) until the integrated value ΔTCS of the temperature rise amount ΔTC of the converter21reaches the threshold (ΔTTH inFIG. 4). Further, the step-up ratio (VL/VH) and the carrier frequency fc may change for each sampling timing. Therefore, in the present embodiment, the threshold arrival time tTHa falls within a range of tTHamin<tTHa<tTHamax (described later) according to the step-up ratio (VL/VH) and the carrier frequency fc at the sampling timing of the current IL (within a range denoted by a white arrow in the drawing).

Therefore, in the present embodiment, the converter current suppression control is not executed at an unnecessarily early timing, unlike the comparative example in which the threshold arrival time tTH is tTHamin. Accordingly, the traveling performance of the vehicle10is not declined at an unnecessarily early timing. In the present embodiment, it is possible to suppress a decrease in drivability within a range in which the converter21is protected from overheating.

tTHamax is the threshold arrival time under the minimized ripple amplitude condition. Under this condition, since the ripple amplitude ILpp is 0, the amount of the ripple amplitude ILpp contributing to the temperature rise amount of the converter21is at the minimum level (0). Therefore, tTHamax is the largest value that the threshold arrival time tTH can represent to the extent that the step-up ratio (VL/VH) and the carrier frequency fc can change.

Modified Example

A modified example of the embodiment will be described with reference toFIG. 10.FIG. 10is a functional block diagram of the motor ECU4in the modified example of the present embodiment.

In the embodiment stated above, in a case where the integrated value ΔTCS of the temperature rise amount ΔTC reaches the threshold, the converter current suppression unit715outputs the request to the vehicle ECU50to limit the charging upper limit Win and the discharging upper limit Wout.

On the other hand, in the modified example of the embodiment, the converter current suppression unit715outputs a command to the carrier frequency determination unit717such that the carrier frequency fc of the converter21is set to be higher than that immediately before the integrated value ΔTCS reaches the threshold, in the same circumstance. Consequently, the ripple amplitude ILpp becomes smaller (seeFIG. 2), thus the ripple component of the current IL decreases. Therefore, the current IL is suppressed by the reduction of the ripple component.

In a case where the carrier frequency fc is increased, the carrier frequency determination unit717determines the increased carrier frequency fc when receiving the command stated above. The carrier frequency determination unit717outputs the increased carrier frequency fc to the data selection unit702and the carrier wave generation unit720.

The carrier wave generation unit720generates the carrier wave CWC for the PWM control of the converter21based on the increased carrier frequency fc. The driving signal generation unit725generates the driving signals S1and S2(seeFIG. 1) for executing the PWM control of the converter21according to the carrier wave CWC after being increased. The converter21is driven according to a duty ratio based on the driving signal.

Consequently, the ripple amplitude ILpp of the current IL flowing through the converter21becomes smaller than the amplitude before the carrier frequency fc is increased. Therefore, the current IL is suppressed by the reduction of the ripple amplitude ILpp. Since the amount of heat generated in the reactor L1is reduced, the temperature rise amount of the converter21is reduced. Therefore, the converter21is protected from overheating. As stated above, in a case where the integrated value ΔTCS of the temperature rise amount ΔTC reaches the threshold, the converter current suppression unit715may increase the carrier frequency fc of the converter21as compared with the carrier frequency immediately before the integrated value ΔTCS reaches the threshold.

Other Modified Example

In the embodiment stated above, the motor ECU4estimates the temperature rise amount ΔTC of the converter21according to the step-up ratio (VL/VH) and the carrier frequency fc of the converter21. On the other hand, the motor ECU4may estimate the temperature rise amount ΔTC of the converter21according to any one of the step-up ratio (VL/VH) and the carrier frequency fc of the converter21.

In such a case, the temperature rise amount estimation data701defines the predetermined correlation between the detected value of the current IL and the temperature rise amount ΔTC of the converter21, according to any one of the step-up ratio (VL/VH) and the carrier frequency fc, of the converter21. For example, the temperature rise amount estimation data701defines the correlation for each of the step-up ratio (VL/VH) and the carrier frequency fc of the converter21.

The motor ECU4selects data corresponding to any one of the step-up ratio (VL/VH) and the carrier frequency fc of the converter21, from among the temperature rise amount estimation data701. Using the selected data corresponding to any one of the step-up ratio (VL/VH) and the carrier frequency fc, the motor ECU4estimates the temperature rise amount ΔTC of the converter21over the sampling cycle TS from the sampling timing of the detected value to the next sampling timing, according to the detected value of the current IL.

In the embodiment stated above, the current IL is employed as the current flowing through the converter21, but the present disclosure is not limited thereto. For example, the current Ib flowing through the battery11may be employed instead of the current IL. In this case, the current sensor210may not be provided, and the motor ECU4estimates the temperature rise amount ΔTC of the converter21according to the detected value of the current sensor13transmitted via the battery ECU16and the vehicle ECU50.

In the embodiment stated above, the motor ECU4includes the memory5, but the memory5may be provided as a component separately from the motor ECU4.

In the embodiment and its modified examples, the motor ECU4and the vehicle ECU50correspond to the exemplified “control device” of the present disclosure. Further, the PCU2and the vehicle ECU50correspond to the exemplified “power conversion system” of the present disclosure.

The embodiments disclosed are to be considered as illustrative and not restrictive. The scope of the present disclosure is defined by the terms of the claims, rather than the description stated above, and includes any modifications within the scope and meanings equivalent to the terms of the claims.