WO2006095529A1

WO2006095529A1 - Voltage conversion device, motor drive device, and method of controlling voltage converter

Info

Publication number: WO2006095529A1
Application number: PCT/JP2006/302312
Authority: WO
Inventors: Takahiko Hirasawa
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2005-03-04
Filing date: 2006-02-03
Publication date: 2006-09-14
Also published as: JP2006246653A

Abstract

When prescribed abnormality occurs, a control unit suspends a boost converter (S20). When an abnormal state is eliminated (YES at S40), the control unit checks whether or not a voltage VH of a high-voltage side (an inverter side) of the boost converter is lower than a withstand voltage of a part of equipment connected to a low-voltage side (a battery side) of the boost converter (S60). The control unit stops issuance of an instruction to suspend the boost converter after voltage VH becomes lower than the withstand voltage of the part (S70).

Description

DESCRIPTION

Voltage Conversion Device, Motor Drive Device, and

Method of Controlling Voltage Converter

Technical Field

The present invention relates to a voltage conversion device, a motor drive device, and a method of controlling a voltage converter, and more particularly to a voltage conversion device, a motor drive device, and a method of controlling a voltage converter capable of protecting equipment connected to a low voltage side of the voltage converter against overvoltage. Background Art

Iapanese Patent Laying-Open No. 2004-274945 discloses a motor drive device preventing application of overvoltage to equipment connected to a primary side (a DC power supply side) of a voltage converter that performs voltage conversion when a DC power supply fails. The motor drive device includes a DC power supply, a boost converter, inverters IA, IB, a DC/DC converter, a system relay, and a control unit. The boost converter is connected to the DC power supply through the system relay. Inverters IA, EB are connected in parallel to the boost converter, so as to drive AC motors MA, MB respectively. The DC/DC converter is connected between the system relay and the boost converter.

In the motor drive device, when failure of the DC power supply is detected, the control unit controls inverters IA, IB such that AC motors MA, MB output zero output torque. Thereafter, the control unit outputs a signal instructing turn-off of the system relay to the system relay, thereby turning off the same. Then, the control unit switches control of the boost converter to voltage-down-conversion control. According to the motor drive device, application of overvoltage to the DC/DC converter connected to the primary side (the DC power supply side) of the boost converter can be prevented. When abnormality which may lead to malfunction or damage of the boost converter occurs, generally, the control unit stops an operation of the boost converter and holds data within the control unit. Upon recovering from an abnormal state, the control unit starts the operation of the boost converter, cancels holding of the data, and starts control of the boost converter based on data newly taken in from the outside.

As the control unit operates in a prescribed control cycle, the control unit controls the boost converter during one control cycle immediately after the start of operation of the boost converter at a voltage conversion rate that has been held during the abnormal state (a boost rate or a voltage-down-conversion rate). If a voltage on a secondary side (inverter side) of the boost converter is raised as a result of regenerative operation of the motor in the abnormal state, overvoltage may be supplied from the secondary side to the primary side of the boost converter.

For example, it is assumed here that the abnormal state occurs while the voltage- down-conversion rate of the boost converter is set to R and the voltage on the secondary side of the boost converter is raised during the abnormal state. Here, as the control unit starts control of the boost converter with the setting of the voltage-down- conversion rate to R immediately after the start of operation of the boost converter, the primary side is supplied with the voltage obtained by down-converting the raised voltage on the secondary side at voltage-down-conversion rate R. If this supplied voltage is higher than a withstand voltage of a part of equipment (such as a DC/DC converter) connected to the primary side of the boost converter, the equipment is damaged.

Japanese Patent Laying-Open No. 2004-274945 described above discloses neither a problem of the overvoltage that could occur immediately after the start of the operation of the boost converter nor a measure to solve the problem. In addition, the technique disclosed in Japanese Patent Laying-Open No. 2004-274945 described above cannot solve such a problem. Disclosure of the Invention

The present invention was made to solve the above-described problems, and an object of the present invention is to provide a voltage conversion device preventing overvoltage that could occur on a low- voltage side of a voltage converter when the voltage converter starts a voltage-down-converting operation from a suspended state.

Another object of the present invention is to provide a motor drive device preventing overvoltage that could occur on a low- voltage side of a voltage converter when the voltage converter starts a voltage-down-converting operation from a suspended state.

Yet another object of the present invention is to provide a method of controlling a voltage converter preventing overvoltage that could occur on a low- voltage side of a voltage converter when the voltage converter starts a voltage-down-converting operation from a suspended state.

According to the present invention, a voltage conversion device includes a voltage converter down-converting a DC voltage and supplying the resultant voltage to an electric load, and a control unit outputting an instruction to suspend the voltage converter to the voltage converter when prescribed abnormality occurs and stopping issuance of the instruction when it is confirmed that the DC voltage is lower than a prescribed voltage level after the prescribed abnormality is eliminated.

In the voltage conversion device according to the present invention, whether the DC voltage before voltage-down-conversion (on the high-voltage side of the voltage converter) is lower than a prescribed voltage level is checked at the time of start of the voltage-down-converting operation of the voltage converter after the prescribed abnormality is eliminated. When it is confirmed that the DC voltage is lower than the prescribed voltage level, issuance of the instruction to suspend the voltage converter, that has been output to the voltage converter, is stopped. Accordingly, when the voltage converter starts the voltage-down-converting operation, a voltage equal to or higher than the prescribed voltage level is not supplied to the electric load that receives the down-converted voltage from the voltage converter.

Therefore, according to the voltage conversion device of the present invention, the prescribed voltage level is set to an appropriate value, so that application of the overvoltage to the electric load that receives the down-converted voltage from the voltage converter can be prevented.

Preferably, the prescribed voltage level is determined based on a withstand voltage of a part of the electric load.

In the voltage conversion device, the prescribed voltage level is determined based on the withstand voltage of the part of the electric load that receives the down- converted voltage from the voltage converter. Therefore, when the voltage converter starts the voltage-down-converting operation, a voltage equal to or higher than the withstand voltage of the part is not supplied to the electric load.

Therefore, according to the voltage conversion device, application of the overvoltage equal to or higher than the withstand voltage of the part to the electric load that receives the down-converted voltage from the voltage converter can be prevented.

Preferably, the voltage conversion device further includes a discharge circuit lowering the DC voltage when the DC voltage is equal to or higher than the prescribed voltage level after the prescribed abnormality is eliminated.

In the voltage conversion device, when the DC voltage before voltage-down- conversion (on the high- voltage side of the voltage converter) is equal to or higher than the prescribed voltage level after prescribed abnormality is eliminated, the DC voltage before voltage-down-conversion is lowered by means of the discharge circuit.

Accordingly, the DC voltage before voltage-down-conversion can be supplied to the electric load after it is made lower than the withstand voltage of the part of the electric load that receives the down-converted voltage from the voltage converter.

Therefore, according to the voltage conversion device, protection of the electric load that receives the down-converted voltage from the voltage converter against the overvoltage can be ensured.

In addition, according to the present invention, a motor drive device includes: a DC power supply; a drive device driving a motor; a voltage converter down-converting a DC voltage from the drive device and supplying the resultant voltage to a side of the DC power supply; an electric load connected between the DC power supply and the voltage converter; and a control unit outputting an instruction to suspend the voltage converter to the voltage converter when prescribed abnormality occurs and stopping issuance of the instruction when it is confirmed that the DC voltage from the drive device is lower than a prescribed voltage level after the prescribed abnormality is eliminated.

In the motor drive device according to the present invention, whether or not the DC voltage on the high- voltage side (on a side of the drive device) of the voltage converter is lower than the prescribed voltage level is checked at the time of start of the voltage-down-converting operation of the voltage converter after the prescribed abnormality is eliminated during regenerative operation of the motor. When it is confirmed that the DC voltage is lower than the prescribed voltage level, issuance of the instruction to suspend the voltage converter that has been output to the voltage converter is stopped. Accordingly, when the voltage converter starts the voltage- down-converting operation, a voltage equal to or higher than the prescribed voltage level is not supplied to the electric load connected to the low-voltage side (the DC power supply side) of the voltage converter.

Therefore, according to the motor drive device of the present invention, the prescribed voltage level is set to an appropriate value, so that application of the overvoltage to the electric load connected to the low-voltage side (the DC power supply side) of the voltage converter can be prevented.

Preferably, the prescribed voltage level is determined based on a withstand voltage of a part of the electric load. In the motor drive device, the prescribed voltage level is determined based on the withstand voltage of the part of the electric load. Therefore, when the voltage converter starts the voltage-down-converting operation, a voltage equal to or higher than the withstand voltage of the part is not supplied to the electric load connected to the low- voltage side (the DC power supply side) of the voltage converter.

Therefore, according to the motor drive device, application of the overvoltage equal to or higher than the withstand voltage of the part to the electric load connected to the low- voltage side (the DC power supply side) of the voltage converter can be prevented.

Preferably, the motor drive device further includes a discharge circuit connected between the drive device and the voltage converter.

In the motor drive device, when the DC voltage on the high- voltage side (the side of the drive device) of the voltage converter is equal to or higher than the prescribed voltage level after prescribed abnormality is eliminated, the DC voltage is lowered by means of the discharge circuit. Accordingly, the DC voltage on the high- voltage side (the side of the drive device) of the voltage converter can be supplied to the electric load after it is made lower than the withstand voltage of the part of the electric load connected to the low- voltage side (a side of a power supply unit) of the voltage converter.

Therefore, according to the motor drive device, protection of the electric load connected to the low-voltage side (the side of the power supply unit) of the voltage converter against the overvoltage can be ensured.

Preferably, the voltage converter further attains a boost function to boost a DC voltage from the DC power supply and supplying the resultant voltage to the drive device.

In addition, according to the present invention, a method of controlling a voltage converter down-converting a DC voltage and supplying the resultant voltage to an electric load includes: a first step to output an instruction to suspend the voltage converter to the voltage converter when prescribed abnormality occurs; a second step to determine whether or not the prescribed abnormality has been eliminated; a third step to determine whether or not the DC voltage is lower than a prescribed voltage level when it is determined at the second step that the prescribed abnormality has been eliminated; and a fourth step to stop issuance of the instruction when it is determined at the third step that the DC voltage is lower than the prescribed voltage level.

According to the method of controlling the voltage converter of the present invention, if it is determined at the second step that the prescribed abnormality has been eliminated, whether or not the DC voltage before voltage-down-conversion (on the high-voltage side of the voltage converter) is lower than the prescribed voltage level is determined at the third step. If it is determined that the DC voltage before voltage- down-conversion is lower than the prescribed voltage level, issuance of the instruction to suspend the voltage converter that has been output to the voltage converter is stopped at the fourth step. Accordingly, when the voltage converter starts the voltage- down-converting operation, a voltage equal to or higher than the prescribed voltage level is not supplied to the electric load that receives the down-converted voltage from the voltage converter.

Therefore, according to the method of controlling the voltage converter of the present invention, the prescribed voltage level is set to an appropriate value, so that application of the overvoltage to the electric load that receives the down-converted voltage from the voltage converter can be prevented.

Preferably, the prescribed voltage level is determined based on a withstand voltage of a part of the electric load. In the method of controlling the voltage converter, the prescribed voltage level is determined based on the withstand voltage of the part of the electric load that receives the down-converted voltage from the voltage converter. Therefore, when the voltage converter starts the voltage-down-converting operation, a voltage equal to or higher than the withstand voltage of the part is not supplied to the electric load. Therefore, according to the method of controlling the voltage converter, application of the overvoltage equal to or higher than the withstand voltage of the part to the electric load that receives the down-converted voltage from the voltage converter can be prevented. As described above, according to the present invention, when the voltage converter starts the voltage-down-converting operation after the prescribed abnormality is eliminated, the DC voltage before voltage-down-conversion (on the high-voltage side of the voltage converter) is referred to. After it is confirmed that the DC voltage before voltage-down-conversion is lower than the prescribed voltage level (such as the withstand voltage of the part of the electric load connected to the low- voltage side of the voltage converter), issuance of the instruction to suspend the voltage converter that has been output to the voltage converter is stopped. Therefore, application of the overvoltage to the electric load that receives the down-converted voltage from the voltage converter can be prevented. Brief Description of the Drawings

Fig. 1 is a schematic block diagram of a motor drive device according to an embodiment of the present invention.

Fig. 2 is a functional block diagram with regard to control of a boost converter in a control unit shown in Fig. 1.

Fig. 3 is a timing chart for illustrating a concept of abnormality processing by the control unit shown in Fig. 2.

Fig. 4 is a flowchart with regard to abnormality processing by the control unit shown in Fig. 2. Best Modes for Carrying Out the Invention

An embodiment of the present invention will be described hereinafter in detail with reference to the drawings. The same or corresponding elements have the same reference characters allotted. Therefore, detailed description thereof will not be repeated. Fig. 1 is a schematic block diagram of a motor drive device according to an embodiment of the present invention. Referring to Fig. 1, a motor drive device 100 includes a battery B, a boost converter 10, an inverter 20, a control unit 30, a DC/DC converter 40, an auxiliary machinery battery 50, a resistor element 60, a system relay SR, capacitors Cl, C2, voltage sensors 72, 74, 76, power supply lines PLl, PL2, and a ground line SL.

A motor-generator MG driven by motor drive device 100 is an AC motor and implemented, for example, by a three-phase AC synchronous motor-generator. Motor- generator MG generates drive torque with a three-phase AC voltage received from inverter 20. In addition, motor-generator MG generates a three-phase AC voltage in the regenerative operation, and outputs the same to inverter 20.

Battery B serving as the DC power supply is a battery that can be charged or discharged, and implemented, for example, by a secondary battery such as a nickel hydrogen battery, a lithium ion battery or the like. Battery B outputs the generated DC voltage to power supply line PLl . In addition, battery B is charged with the DC voltage generated by motor-generator MG and down-converted by boost converter 10 after rectification by inverter 20.

Boost converter 10 includes a reactor L, power transistors Ql, Q2, and diodes Dl, D2. Reactor L has one end connected to power supply line PLl and the other end connected to a connection point of power transistors Ql, Q2. Power transistors Ql,

Q2 are implemented, for example, by an IGBT (Insulated Gate Bipolar Transistor).

Power transistors Ql, Q2 are connected in series between power supply line PL2 and ground line SL, and receive a signal PWC from control unit 30 at the base. Diodes Dl, D2 are connected between the collector and the emitter of power transistors Ql, Q2 respectively, so that a current flows from the emitter side to the collector side.

Inverter 20 includes a U-phase arm 22, a V-phase arm 24 and a W-phase arm 26.

U-phase arm 22, V-phase arm 24 and W-phase arm 26 are connected in parallel between power supply line PL2 and ground line SL. U-phase arm 22 includes power transistors Q3, Q4 connected in series, V-phase arm 24 includes power transistors Q5, Q6 connected in series, and W-phase arm 26 includes power transistors Q7, Q8 connected in series. Each of power transistors Q3 to Q8 is implemented, for example, by an

IGBT. Diodes D3 to D8 are connected between the collector and the emitter of power transistors Q3 to Q8 respectively, so that a current flows from the emitter side to the collector side. A connection point of the power transistors in the arm of each phase is connected to an end of a coil opposite to a neutral point of the coil of each phase of motor-generator MG. System relay SR is connected between battery B and power supply line PLl, ground line SL, and turned on and off in response to a signal SE from control unit 30. System relay SR electrically connects battery B to power supply line PLl and ground line SL when it is turned on in response to signal SE, and electrically disconnects battery B from power supply line PLl and ground line SL when it is turned off in response to the same. Capacitor Cl is connected between power supply line PLl and ground line SL, and smoothes voltage fluctuation between power supply line PLl and ground line SL.

DC/DC converter 40 is connected between power supply line PLl and ground line SL, and down-converts the voltage from power supply line PLl and outputs the resultant voltage to auxiliary machinery battery 50. Auxiliary machinery battery 50 serves as a power supply for auxiliary machinery of motor drive device 100, and supplies a voltage for auxiliary machinery (hereinafter, referred to as auxiliary machinery voltage) to control unit 30 or other not-shown auxiliary machinery.

Capacitor C2 is connected between power supply line PL2 and ground line SL, and smoothes voltage fluctuation between power supply line PL2 and ground line SL. Resistor element 60 is connected between power supply line PL2 and ground line SL in parallel to capacitor C2. When the voltage level of power supply line PL2 is not controlled, resistor element 60 attains a function as a discharge circuit that releases charges on power supply line PL2 to ground line SL so as to down-convert the voltage of power supply line PL2.

Boost converter 10 boosts the DC voltage supplied from battery B through power supply line PLl and outputs the resultant voltage to power supply line PL2. More specifically, based on signal PWC from control unit 30, boost converter 10 accumulates a current that flows in accordance with a switching operation of power transistor Q2 in reactor L as magnetic energy, so as to boost the DC voltage from battery B, and outputs the boosted voltage to power supply line PL2 through diode Dl in synchronization with timing of turn-off of power transistor Q2. In addition, based on signal PWC from control unit 30, boost converter 10 down-converts the DC voltage supplied from inverter 20 through power supply line PL2 to the voltage level of battery B, and outputs the down-converted voltage to battery B and DC/DC converter 40.

Moreover, boost converter 10 stops the switching operation of power transistors Ql, Q2 based on a signal SDOWN from control unit 30. Specifically, power transistors Ql, Q2 are turned off in response to signal SDOWN at H (logic high) level. On the other hand, while power transistors Ql, Q2 receive signal SDOWN at L (logic low) level, they perform the switching operation in response to signal PWC from control unit 30. Inverter 20 converts the DC voltage supplied from power supply line PL2 to a three-phase AC voltage based on a signal PWM from control unit 30, to drive motor- generator MG. Accordingly, motor-generator MG is driven so as to generate torque designated by a torque instruction value TR. In addition, inverter 20 converts the three-phase AC voltage generated by motor-generator MG upon receiving external rotation force to the DC voltage based on signal PWM from control unit 30, and outputs the resultant DC voltage to power supply line PL2.

Voltage sensor 72 detects a voltage between power supply line PLl and ground line SL, that is, a voltage VL on the primary side (low-voltage side) of boost converter 10, and outputs detected voltage VL to control unit 30. Voltage sensor 74 detects a voltage between power supply line PL2 and ground line SL, that is, a voltage VH on the secondary side (high- voltage side) of boost converter 10, and outputs detected voltage VH to control unit 30. Voltage sensor 76 detects an auxiliary machinery voltage VA of auxiliary machinery battery 50, and outputs detected auxiliary machinery voltage VA to control unit 30.

Control unit 30 generates signal PWC for driving boost converter 10 based on torque instruction value TR and revolution number MRN of motor-generator MG received from the outside, voltage VL from voltage sensor 72 and voltage VH from voltage sensor 74, and outputs generated signal PWC to boost converter 10.

In addition, control unit 30 detects an abnormal state such as lowering in auxiliary machinery voltage VA, overvoltage of voltages VL, VH, and overheat of boost converter 10. Moreover, control unit 30 generates signal SDOWN instructing stop of the switching operation of power transistors Ql, Q2 in boost converter 10. When control unit 30 detects the abnormal state as above, control unit 30 outputs signal SDOWN at H level to boost converter 10.

Further, control unit 30 generates signal PWM for driving motor-generator MG based on voltage VH from voltage sensor 74 as well as on a motor current and torque instruction value TR of motor-generator MG, and outputs generated signal PWM to inverter 20.

Fig. 2 is a functional block diagram with regard to control of boost converter 10 in control unit 30 shown in Fig. 1. Referring to Fig. 2, control unit 30 includes a voltage instruction operation unit 112, a duty ratio operation unit 114, a PWM signal conversion unit 1 16, and an abnormality processing unit 118. Voltage instruction operation unit 112 receives torque instruction value TR and motor revolution number MRN of motor-generator MG from the outside. When motor-generator MG is driven to achieve power running, voltage instruction operation unit 112 calculates an optimal value (a target value) for voltage VH based on torque instruction value TR and motor revolution number MRN, and outputs the calculated target value for voltage VH to duty ratio operation unit 1 14.

In addition, when motor-generator MG is driven to perform regenerative operation, voltage instruction operation unit 112 calculates an optimal value (a target value) for voltage VL based on torque instruction value TR and motor revolution number MRN, and outputs the calculated target value for voltage VL to duty ratio operation unit 114.

Moreover, voltage instruction operation unit 112 receives a signal HOLD from abnormality processing unit 118. When signal HOLD is at H level, voltage instruction operation unit 112 no longer takes in torque instruction value TR and motor revolution number MRN from the outside, but holds values immediately before signal HOLD attains to H level.

Duty ratio operation unit 114 receives voltages VL, VH from voltage sensors 72, 74 respectively, and receives the target value for voltage VL or the target value for voltage VH from voltage instruction operation unit 112. Then, duty ratio operation unit 114 calculates a duty ratio for setting voltage VH to that target value while motor- generator MG is driven to achieve power running, and outputs the calculated duty ratio to PWM signal conversion unit 116.

Moreover, duty ratio operation unit 114 calculates a duty ratio for setting voltage VL to that target value while motor-generator MG is driven to perform regenerative operation, and outputs the calculated duty ratio to PWM signal conversion unit 116.

In addition, duty ratio operation unit 114 receives signal HOLD from abnormality processing unit 118. When signal HOLD is at H level, duty ratio operation unit 114 no longer takes in voltages VL, VH from the outside, but holds values immediately before signal HOLD attains to H level.

PWM signal conversion unit 116 generates signal PWC for turning on/off power transistors Ql, Q2 of boost converter 10 based on the duty ratio received from duty ratio operation unit 114, and outputs generated signal PWC to power transistors Ql, Q2 of boost converter 10.

Since accumulation of power in reactor L becomes greater by increasing on-duty of power transistor Q2 in a lower arm of boost converter 10, the boost rate of boost converter 10 becomes greater. On the other hand, since an amount of discharge through power transistor Ql becomes greater by increasing on-duty of power transistor Ql in an upper arm, the voltage-down-conversion rate of boost converter 10 becomes greater.

Abnormality processing unit 118 receives auxiliary machinery voltage VA from voltage sensor 76, receives voltages VL, VH from voltage sensors 72, 74 respectively, and receives a temperature T of boost converter 10 detected by a not-shown temperature sensor from the same. When abnormality processing unit 118 detects an abnormal state such as lowering in auxiliary machinery voltage VA, overvoltage of voltages VL, VH, and abnormal increase in temperature T, abnormality processing unit 118 sets signal SDOWN that instructs stop of the switching operation of power transistors Ql, Q2 to H level and outputs the signal to boost converter 10.

When abnormality processing unit 118 detects the abnormal state as above, abnormality processing unit 118 outputs signal HOLD at H level prohibiting take-in of data from the outside and instructing holding of a data value to voltage instruction operation unit 112 and duty ratio operation unit 114.

When the abnormal state as above is eliminated, abnormality processing unit 118 sets signal HOLD to L level. While motor-generator MG is driven to perform regenerative operation, however, abnormality processing unit 118 does not immediately set signal SDOWN to L level. Instead, abnormality processing unit 118 sets signal SDOWN to L level only after it is confirmed that voltage VH from voltage sensor 74 becomes lower than the withstand voltage of the part of DC/DC converter 40 or system relay SR. In other words, abnormality processing unit 118 stops issuance of the instruction to stop the switching operation of power transistors Ql, Q2 only after it is confirmed that voltage VH on the secondary side (high- voltage side) of boost converter 10 is lower than the withstand voltage of the part of the equipment connected to the primary side (low-voltage side) of boost converter 10 (hereinafter, such equipment is also referred to as "low-voltage side equipment").

Fig. 3 is a timing chart for illustrating a concept of abnormality processing by control unit 30 shown in Fig. 2. Referring to Fig. 3, when an abnormality flag indicating an abnormal state such as lowering in auxiliary machinery voltage VA attains to H level at time tl, signal SDOWN that instructs suspension of boost converter 10 is set to H level. In response, boost converter 10 stops its operation. Here, voltage VH on the secondary side (high- voltage side) of boost converter 10 is denoted as Vl, and the voltage-down-conversion rate of boost converter 10 is denoted as Rl. Control unit 30 holds the voltage-down-conversion rate at Rl while the abnormal flag is at H level.

It is assumed that voltage VH is raised from Vl to V2 at time t2 during the abnormal state, as a result of regeneration by motor-generator MG. At time t3, the abnormality is eliminated, and the abnormality flag is set to L level.

As control unit 30 operates in a prescribed control cycle, control unit 30 performs an operation using voltage-down-conversion rate Rl that has been held in the abnormal state, during a time period Δt corresponding to one control cycle immediately after the start of operation of boost converter 10. On the other hand, voltage VH on the secondary side (high-voltage side) of boost converter 10 is raised to V2 after time t2.

If signal SDOWN is set to L level at t3 at which the abnormality flag is set to L level as a result of elimination of abnormality as with the case of conventional signal SDOWN, boost converter 10 down-converts voltage VH that was once raised, at voltage-down-conversion rate Rl smaller than an actual input/output voltage ratio during time period Δt from time t3 to t4. Therefore, an unexpectedly high voltage is supplied to the primary side (low-voltage side) of boost converter 10. If the supplied voltage is higher than the withstand voltage of the part of the low-voltage side equipment arranged on the primary side (low- voltage side) of boost converter 10, damage of the low- voltage side equipment is caused. In contrast, according to the embodiment of the present invention, signal

SDOWN is not immediately set to L level at t3 at which the abnormality flag is set to L level as a result of elimination of the abnormality. When voltage VH is lowered to a voltage V3 corresponding to the withstand voltage of the part of the low- voltage side equipment as a result of discharge by resistor element 60 at time t5, control unit 30 sets signal SDOWN to L level and starts a voltage-down-converting operation of boost converter 10. Therefore, the voltage exceeding the withstand voltage of the part of the low-voltage side equipment is not supplied to the low-voltage side equipment. Fig. 4 is a flowchart with regard to abnormality processing by control unit 30 shown in Fig. 2. Referring to Fig. 4, abnormality processing unit 118 of control unit 30 determines whether or not the abnormal state such as lowering in auxiliary machinery voltage VA, overvoltage of voltages VL, VH, and abnormal increase in temperature T has occurred, based on auxiliary machinery voltage VA from voltage sensor 76, voltage VL from voltage sensor 72, voltage VH from voltage sensor 74, and temperature T of boost converter 10 (step SlO). If it is determined that no abnormality has occurred (NO at step SlO), abnormality processing unit 118 ends a series of processing.

On the other hand, if it is determined at step SlO that abnormality has occurred (YES at step SlO), abnormality processing unit 118 outputs signal SDOWN at H level to boost converter 10, so as to suspend boost converter 10 (step S20). Abnormality processing unit 118 outputs signal HOLD at H level to voltage instruction operation unit 112 and duty ratio operation unit 114. Then, voltage instruction operation unit 112 and duty ratio operation unit 114 no longer take in torque instruction value TR, motor revolution number MRN and voltages VL, VH from the outside, but hold the values immediately before occurrence of the abnormality (step S30).

Thereafter, abnormality processing unit 118 determines whether or not the abnormality that had occurred has been eliminated (step S40). If abnormality processing unit 1 18 determines that the abnormality has not been eliminated (NO at step S40), the process proceeds to step S20. On the other hand, if it is determined at step S40 that the abnormality has been eliminated (YES at step S40), abnormality processing unit 1 18 sets signal HOLD to L level. Then, voltage instruction operation unit 112 starts to take in torque instruction value TR and motor revolution number MRN from the outside, and duty ratio operation unit 114 starts to take in voltages VL, VH from the outside (step S 50).

Thereafter, abnormality processing unit 118 determines whether or not voltage VH from voltage sensor 74 is lower than the withstand voltage of the part of the low- voltage side equipment connected to the primary side (low- voltage side) of boost converter 10 (step S60). If it is determined that voltage VH is not lower than the withstand voltage of the part of the low-voltage side equipment (NO at step S60), abnormality processing unit 118 waits until voltage VH becomes lower than the withstand voltage of the part of the low- voltage side equipment as a result of discharge by resistor element 60. If it is determined that voltage VH is lower than the withstand voltage of the part of the low- voltage side equipment (YES at step S60), abnormality processing unit 118 sets signal SDOWN to L level and stops issuance of the instruction to suspend boost converter 10 (step S70).

As described above, according to the present embodiment, when the voltage- down-converting operation of boost converter 10 is started after the abnormality is eliminated, voltage VH on the secondary side (high- voltage side) of boost converter 10 is referred to. After it is confirmed that voltage VH is lower than the withstand voltage of the part of the low- voltage side equipment connected to the primary side (low- voltage side) of boost converter 10, signal SDOWN is set to L level and issuance of the instruction to suspend boost converter 10 is stopped. Therefore, application of the overvoltage to DC/DC converter 40 can be prevented.

In the embodiment above, DC/DC converter 40 or system relay SR is connected to the primary side (low- voltage side) of boost converter 10 as the low- voltage side equipment. If an inverter for an air-conditioner or other equipment is further connected to the primary side of the boost converter, however, a withstand voltage of a part of such equipment may be taken into consideration.

In addition, in the embodiment above, voltages VL, VH are detected by voltage sensors 72, 74 respectively, however, voltages VL, VH may be calculated by using a current sensor and a resistor, instead of the voltage sensor. Further, in the embodiment above, battery B is employed as the secondary battery that can be charged and discharged, however, the battery may be a fuel cell. The fuel cell refers to a DC power generating cell that obtains electric energy from chemical reaction energy generated by chemical reaction between a fuel such as hydrogen and an oxidizer.

In the embodiment above, boost converter 10 corresponds to the "voltage converter" in the present invention, and the low-voltage side equipment corresponds to the "electric load" in the present invention. Boost converter 10 and control unit 30 correspond to the "voltage conversion device" in the present invention. In addition, battery B corresponds to the "DC power supply" in the present invention, and inverter 20 corresponds to the "drive device" in the present invention.

It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

Claims

1. A voltage conversion device, comprising: a voltage converter down-converting a DC voltage and supplying resultant voltage to an electric load; and a control unit outputting an instruction to suspend said voltage converter to said voltage converter when prescribed abnormality occurs and stopping issuance of said instruction when it is confirmed that said DC voltage is lower than a prescribed voltage level after said prescribed abnormality is eliminated.

2. The voltage conversion device according to claim 1, wherein said prescribed voltage level is determined based on a withstand voltage of a part of said electric load.

3. The voltage conversion device according to claim 1, further comprising a discharge circuit lowering said DC voltage when said DC voltage is equal to or higher than said prescribed voltage level after said prescribed abnormality is eliminated.

4. A motor drive device comprising: a DC power supply; a drive device driving a motor; a voltage converter down-converting a DC voltage from said drive device and supplying resultant voltage to a side of said DC power supply; an electric load connected between said DC power supply and said voltage converter; and a control unit outputting an instruction to suspend said voltage converter to said voltage converter when prescribed abnormality occurs and stopping issuance of said instruction when it is confirmed that said DC voltage from said drive device is lower than a prescribed voltage level after said prescribed abnormality is eliminated.

5. The motor drive device according to claim 4, wherein said prescribed voltage level is determined based on a withstand voltage of a part of said electric load.

6. The motor drive device according to claim 4, further comprising a discharge circuit connected between said drive device and said voltage converter.

7. The motor drive device according to claim 4, wherein said voltage converter further attains a boost function to boost a DC voltage from said DC power supply and supplying resultant voltage to said drive device.

8. A method of controlling a voltage converter down-converting a DC voltage and supplying resultant voltage to an electric load, comprising: a first step to output an instruction to suspend said voltage converter to said voltage converter when prescribed abnormality occurs; a second step to determine whether said prescribed abnormality has been eliminated; a third step to determine whether said DC voltage is lower than a prescribed voltage level when it is determined at said second step that said prescribed abnormality has been eliminated; and a fourth step to stop issuance of said instruction when it is determined at said third step that said DC voltage is lower than said prescribed voltage level.

9. The method of controlling a voltage converter according to claim 8, wherein said prescribed voltage level is determined based on a withstand voltage of a part of said electric load.