Power conversion device, motor drive unit, and electric power steering device

A power conversion device may include a first inverter to which one end of each phase winding of a motor is coupled, a second inverter to which the other end of each phase winding is coupled, and a switch circuit having at least one of a first switch element that switches between connection and disconnection of the first inverter to and from a ground, a first protection circuit being coupled in parallel to the first switch element, and a second switch element that switches between connection and disconnection of the second inverter to and from the ground, a second protection circuit being coupled in parallel to the second switch element.

TECHNICAL FIELD

The present disclosure relates to power conversion devices for converting power from a power supply into power that is to be supplied to an electric motor, motor drive units, and electric power steering devices.

BACKGROUND

Electric motors (hereinafter simply referred to as “motors”) such as brushless DC motors and AC synchronous motors are typically driven by three phase currents. A complicated control technique, such as vector control, is needed to accurately control the waveforms of the three phase currents. Such a control technique requires complicated mathematical calculation and is therefore performed using a digital computation circuit, such as a microcontroller (microcomputer). The vector control technique is utilized in the fields of applications in which the load on a motor varies significantly, such as washing machines, motorized bicycles, electric scooters, electric power steering devices, electric cars, and industrial equipment. Meanwhile, other motor control techniques, such as pulse width modulation (PWM), are employed for motors that have a relatively low output.

In the field of vehicle-mounted devices, an automotive electronic control unit (ECU) is used in a vehicle. The ECU includes a microcontroller, a power supply, an input/output circuit, an A/D converter, a load drive circuit, and a read only memory (ROM), etc. An electronic control system is constructed using the ECU as a main component. For example, the ECU processes a signal from a sensor to control an actuator, such as a motor. More specifically, the ECU controls an inverter in a power conversion device while monitoring the rotational speed or torque of a motor. The power conversion device converts drive power that is to be supplied to the motor, under the control of the ECU.

A mechanically and electronically integrated motor in which a motor, a power conversion device, and an ECU are integrated together has in recent years been developed. In particular, in the field of vehicle-mounted devices, high quality needs to be ensured for safety. Therefore, a fault-tolerant design is employed in order to allow the motor system to continue a safe operation even if some part of the motor system fails. As an example of such a fault-tolerant design, a single motor may be provided with two power conversion devices. As another example, the ECU may be provided with a backup microcontroller in addition to a main microcontroller.

For example, Japanese Laid-Open Patent Publication No. 2014-192950 describes a power conversion device for converting power that is to be supplied to a three-phase motor, the device including a control unit and two inverters. The two inverters are each coupled to a power supply and a ground (hereinafter referred to as a “GND”). One of the two inverters is coupled to one end of each of the three phase windings of the motor, and the other inverter is coupled to the other end of each of the three phase windings. Each inverter includes a bridge circuit that includes three legs each including a high-side switching element and a low-side switching element. The control unit, when detecting a failure in a switching element in the two inverters, switches the control of the motor from control under normal conditions to control under abnormal conditions. As used herein, the term “abnormal conditions” mainly means that a switching element has failed. The term “control under normal conditions” means control that is performed when all the switching elements are operating normally. The term “control under abnormal conditions” means control that is performed in the event of a failure in a switching element.

The control under normal conditions has a control mode in which one of the two inverters is used to drive a motor with a neutral point being formed in the other inverter, and another control mode in which a motor is driven by turning on and off the switching elements of the two inverters. The power conversion device of Japanese Laid-Open Patent Publication No. 2014-192950 can switch between these modes according to the rotational speed and output torque, etc., of a motor.

In the control under abnormal conditions, a neutral point for the windings is formed by turning on and off switching elements according to a predetermined rule in one of the two inverters that includes a switching element that has failed (hereinafter referred to as a “failed inverter”). According to the rule, for example, in the event of an open-circuit failure in which a high-side switching element is always off, the three high-side switching elements other than the failed switching element are turned off, and the three low-side switching elements are turned on, in the bridge circuit of the failed inverter. In this case, the neutral point is formed on the low side. In the event of a short-circuit failure in which a high-side switching element is always on, the three high-side switching elements other than the failed switching element are turned on, and the three low-side switching elements are turned off, in the bridge circuit of the failed inverter. In this case, the neutral point is formed on the high side. In the power conversion device of Japanese Laid-Open Patent Publication No. 2014-192950, the neutral point for the three phase windings is formed in a failed inverter under abnormal conditions. Even in the event of a failure in a switching element, the motor can continue to be driven using one of the inverters that is operating normally.

SUMMARY

In the above conventional technique, there has been demand for further improvement in current control.

An embodiment of the present disclosure provides a power conversion device that can perform suitable current control.

An example power conversion device according to the present disclosure for converting power from a power supply into power that is to be supplied to a motor having n phase windings (n is an integer of three or more), includes a first inverter to which one end of each phase winding of the motor is coupled, a second inverter to which the other end of each phase winding is coupled, and a switch circuit having at least one of a first switch element that switches between connection and disconnection of the first inverter to and from a ground, a first protection circuit being coupled in parallel to the first switch element, and a second switch element that switches between connection and disconnection of the second inverter to and from the ground, a second protection circuit being coupled in parallel to the second switch element.

According to the illustrative embodiment of the present disclosure, the inverters can be connected and disconnected to and from at least one of the power supply and the GND by the switch elements coupled in parallel to the respective protection circuits. As a result, provided are a power conversion device that can perform suitable current control, a motor drive unit having the power conversion device, and an electric power steering device having the motor drive unit.

DETAILED DESCRIPTION

Before describing embodiments of the present disclosure, the present inventor's findings that are the basis of the present disclosure will be described.

In the power conversion device of Japanese Laid-Open Patent Publication No. 2014-192950, the two inverters are each always connected to the power supply and the GND. This configuration does not allow the power supply and the failed inverter to be disconnected from each other. The present inventors have found the problem that even when a neutral point is formed in a failed inverter under abnormal conditions, a current flows from the power supply into the failed inverter. As a result, a power loss occurs in the failed inverter.

As with the power supply, a failed inverter cannot be disconnected from the GND. The present inventors have found the problem that even when a neutral point is formed in a failed inverter under abnormal conditions, a current supplied to each phase winding through an inverter that is operating normally is not returned to that source inverter, and flows to the GND through the failed inverter. In other words, a closed loop of a drive current cannot be formed. It is desirable that a current supplied to each phase winding through an inverter that is operating normally should flow to the GND through that source inverter.

Embodiments of a power conversion device, motor drive unit, and electric power steering device according to the present disclosure will now be described in detail with reference to the accompanying drawings. To avoid unnecessarily obscuring the present disclosure, well-known features may not be described or substantially the same elements may not be redundantly described, for example. This is also for ease of understanding the present disclosure.

Embodiments of the present disclosure are herein described using, as an example, a power conversion device that converts power that is to be supplied to a three-phase motor having three phase (U-phase, V-phase, and W-phase) windings. Note that the present disclosure encompasses a power conversion device that converts power that is to be supplied to an n-phase motor having n phase windings (n is an integer of four or more), such as four phase windings or five phase windings.

First Embodiment

FIG. 1schematically shows a circuit configuration of a power conversion device100according to this embodiment.

The power conversion device100includes two switch circuits110, a first inverter120, and a second inverter130. The power conversion device100can convert power that is to be supplied to various motors. A motor200is, for example, a three-phase AC motor.

The motor200includes a U-phase winding M1, a V-phase winding M2, and a W-phase winding M3, and is coupled to the first inverter120and the second inverter130. Specifically, the first inverter120is coupled to one end of each phase winding of the motor200, and the second inverter130is coupled to the other end of each phase winding. As used herein, the terms “couple” and “connect” with respect to parts (components) mainly means an electrical coupling and connection between the parts. The first inverter120has terminals U_L, V_L, and W_L which correspond to the respective phases, and the second inverter130has terminals U_R, V_R, and W_R which correspond to the respective phases.

The terminal U_L of the first inverter120is coupled to one end of the U-phase winding M1, the terminal V_L is coupled to one end of the V-phase winding M2, and the terminal W_L is coupled to one end of the W-phase winding M3. As with the first inverter120, the terminal U_R of the second inverter130is coupled to the other end of the U-phase winding M1, the terminal V_R is coupled to the other end of the V-phase winding M2, and the terminal W_R is coupled to the other end of the W-phase winding M3. Such coupling with a motor is different from the so-called star and delta couplings.

The two switch circuits110have first to fourth switch elements111,112,113, and114. Of the two switch circuits110, one that includes the first and second switch elements111and112and is closer to the GND is hereinafter referred to as a “GND-side switch circuit,” and one that includes the third and fourth switch elements113and114and is closer to the power supply is hereinafter referred to as a “power supply-side switch circuit.” In other words, the GND-side switch circuit has the first and second switch elements111and112, and the power supply-side switch circuit has the third and fourth switch elements113and114.

In the power conversion device100, the first inverter120and the second inverter130can be electrically connected to the power supply101and the GND by the two switch circuits110.

Specifically, the first switch element111switches between connection and disconnection of the first inverter120to and from the GND. The second switch element112switches between connection and disconnection of the second inverter130to and from the GND. The third switch element113switches between connection and disconnection of the power supply101to and from the first inverter120. The fourth switch element114switches between connection and disconnection of the power supply101to and from the second inverter130.

The first to fourth switch elements111,112,113, and114may be turned on and off under the control of, for example, a microcontroller or dedicated driver. The first to fourth switch elements111,112,113, and114can block a current in the opposite directions. The first to fourth switch elements111,112,113, and114may, for example, be a semiconductor switch such as a thyristor or analog switch IC, a mechanical relay, etc. A combination of a diode and an insulated-gate bipolar transistor (IGBT), etc., may be used. Note that the switch elements of the present disclosure include semiconductor switches such as a field-effect transistor in which a parasitic diode is formed (typically a MOSFET). An example circuit configuration in which MOSFETs are used as switch elements is described in another embodiment. The first to fourth switch elements111,112,113, and114may be herein denoted by SWs111,112,113, and114, respectively.

A protection circuit is electrically connected in parallel to each SW in the two switch circuits110. Specifically, a protection circuit PC1is coupled in parallel to the SW111, a protection circuit PC2is coupled in parallel to the SW112, a protection circuit PC3is coupled in parallel to the SW113, and a protection circuit PC4is coupled in parallel to the SW114. For example, the protection circuits PC1, PC2, PC3, and PC4are diodes111D,112D,113D, and114D, respectively.

The present disclosure is not limited to the examples shown in the drawings. The number of switch elements that are used is determined as appropriate, taking into account design and specifications, etc. In particular, in the field of vehicle-mounted devices, high quality needs to be ensured for safety, and therefore, the power supply-side switch circuit and the GND-side switch circuit may include a plurality of switch elements for each inverter.

FIG. 2schematically shows another circuit configuration of the power conversion device100of this embodiment.

The power supply-side switch circuit110may further have a fifth switch element115and a sixth switch element116for reverse connection protection. The fifth and sixth switch elements115and116are typically a MOSFET semiconductor switch having a parasitic diode. The fifth switch element115is coupled in series to the SW113, and is disposed such that a forward current flows through the parasitic diode in a direction toward the first inverter120. The sixth switch element116is coupled in series to the SW114, and is disposed such that a forward current flows through the parasitic diode in a direction toward the second inverter130. Even if the power supply101is coupled in the reverse direction, a reverse current can be blocked by the two FETs for reverse connection protection.

Also in the example configuration ofFIG. 2, a diode is coupled as a protection circuit in parallel to an SW in the power supply-side switch circuit110. For example, a diode113D may be coupled in parallel to the SW113, and a diode114D may be coupled in parallel to the SW114.

The power supply101generates a predetermined power supply voltage. The power supply101may, for example, be a DC power supply. Note that the power supply101may be an AC/DC converter or DC/DC converter, or alternatively, a battery (electric battery).

The power supply101may be a single power supply that is shared by the first and second inverters120and130. Alternatively, a first power supply for the first inverter120and a second power supply for the second inverter130may be provided.

A coil102is provided between the power supply101and the power supply-side switch circuit110. The coil102functions as a noise filter to perform smoothing so that high-frequency noise contained in a voltage waveform supplied to each inverter or high-frequency noise occurring in each inverter does not flow into the power supply101. A capacitor or capacitors103are coupled to power supply terminals of the inverters. The capacitor103is a so-called bypass capacitor, and prevents or reduces voltage ripple. The capacitor103is, for example, an electrolytic capacitor. The capacities and number of capacitors103that are used are determined as appropriate, taking into account design and specifications, etc.

The first inverter120(may also be referred to as a “bridge circuit L”) includes a bridge circuit including three legs. Each leg has a low-side switching element and a high-side switching element. Switching elements121L,122L, and123L shown inFIG. 1are a low-side switching element, and switching elements121H,122H, and123H shown inFIG. 1are a high-side switching element. The switching elements may, for example, be a FET or IGBT. In the description that follows, it is, for example, assumed that the switching elements are a FET, and may be denoted by FETs. For example, the switching elements121L,122L, and123L are denoted by FETs121L,122L, and123L.

The first inverter120includes three shunt resistors121R,122R, and123R as a current sensor for detecting currents flowing through the U-phase, V-phase, and W-phase windings, respectively (seeFIG. 5). The current sensor150includes a current detection circuit (not shown) for detecting a current flowing through each shunt resistor. For example, the shunt resistors121R,122R, and123R are each coupled between the corresponding one of the three low-side switching elements included in the three legs of the first inverter120, and the GND. Specifically, the shunt resistor121R is electrically connected between the FET121L and the SW111, the shunt resistor122R is electrically connected between the FET122L and the SW111, and the shunt resistor123R is electrically connected between the FET123L and the SW111. The shunt resistors have a resistance value of, for example, about 0.5-1.0 mΩ.

As with the first inverter120, the second inverter130(may be denoted by a “bridge circuit R”) includes a bridge circuit including three legs. FETs131L,132L, and133L shown inFIG. 1are a low-side switching element, and FETs131H,132H, and133H are a high-side switching element. The second inverter130also includes three shunt resistors131R,132R, and133R. The shunt resistors are coupled between the three low-side switching elements included in the three legs, and the GND. Each of the FETs included in the first and second inverters120and130may be controlled by, for example, a microcontroller or dedicated driver.

In the example configuration ofFIG. 1, a shunt resistor is provided in each leg of each inverter. Note that the first and second inverters120and130can include six or less shunt resistors. For example, the six or less shunt resistors can be coupled between the six or less low-side switching elements of the six legs of the first and second inverters120and130, and the GND. In the case where this configuration is extended to an n-phase motor, the first and second inverters120and130can include 2n or less shunt resistors. For example, the 2n or less shunt resistors can be coupled between the 2n or less low-side switching elements of the 2n legs of the first and second inverters120and130, and the GND.

FIGS. 3 and 4schematically show other circuit configurations of the power conversion device100of this embodiment.

As shown inFIG. 3, three shunt resistors can be disposed between the legs of the first or second inverter120or130and the windings M1, M2, and M3. For example, shunt resistors121R,122R, and123R may be disposed between the first inverter120and one end of the respective windings M1, M2, and M3. Alternatively, for example, although not shown, shunt resistors121R and122R may be disposed between the first inverter120and one end of the respective windings M1and M2, and a shunt resistor123R may be disposed between the second inverter130and the other end of the winding M3. In such a configuration, it is sufficient to dispose three shunt resistors for the U-, V-, and W-phases, and at least two shunt resistors are provided.

As shown inFIG. 4, for example, a single shunt resistor may be provided in each inverter and shared by the phase windings. A single shunt resistor may, for example, be electrically connected between a low-side node N1(coupling point of the legs) of the first inverter120, and the SW111, and another single shunt resistor may, for example, be electrically connected between a low-side node N2of the second inverter130, and the SW112.

As with the low side, a single shunt resistor is, for example, electrically connected between a high-side node N3of the first inverter120, and the SW113, and another single shunt resistor is, for example, electrically connected between a high-side node N4of the second inverter130, and the SW114. Thus, the number of shunt resistors that are used, and the arrangement of the shunt resistors, are determined as appropriate, taking into account manufacturing cost, design, specifications, etc.

FIG. 5schematically shows a typical block configuration of a motor drive unit400that includes the power conversion device100.

The motor drive unit400includes the power conversion device100, the motor200, and a control circuit300.

The control circuit300includes, for example, a power supply circuit310, an angle sensor320, an input circuit330, a microcontroller340, a drive circuit350, and a ROM360. The control circuit300is coupled to the power conversion device100, and controls the power conversion device100to drive the motor200.

Specifically, the control circuit300controls the rotor such that the rotor takes a desired position, rotational speed, and current, etc., and can achieve closed-loop control. Note that the control circuit300may include a torque sensor instead of the angle sensor. In this case, the control circuit300can control the rotor such that the rotor takes a desired motor torque.

The power supply circuit310generates a DC voltage (e.g., 3 V or 5 V) used for the circuit blocks. The angle sensor320is, for example, a resolver or Hall IC. Alternatively, the angle sensor320may be implemented by a combination of a magnetic reluctance (MR) sensor having an MR element, and a sensor magnet. The angle sensor320detects the angle of rotation of the rotor of the motor200(hereinafter referred to as a “rotation signal”), and outputs the rotation signal to the microcontroller340. The input circuit330receives a motor current value (hereinafter referred to as an “actual current value”) detected by the current sensor150, and if necessary, converts the level of the actual current value into an input level of the microcontroller340, and outputs the resultant actual current value to the microcontroller340.

The microcontroller340controls the switching operation (turning-on or turning-off) of each FET in the first and second inverters120and130of the power conversion device100. The microcontroller340calculates a desired current value on the basis of the actual current value and the rotor rotation signal, etc., to generate a PWM signal, and outputs the PWM signal to the drive circuit350. The microcontroller340can also control the on/off operation of each SW in the two switch circuits110of the power conversion device100.

The drive circuit350is typically a gate driver. The drive circuit350generates control signals (gate control signals) for controlling the switching operations of the respective FETs in the first and second inverters120and130, on the basis of the PWM signal, and outputs the control signals to the gates of the respective FETs. The drive circuit350also generates control signals for controlling the on/off operations of the respective SWs in the two switch circuits110according to an instruction from the microcontroller340. Note that the microcontroller340may control each SW in the two switch circuits110. Note that the microcontroller340may also function as the drive circuit350. In this case, the control circuit300may not include the drive circuit350.

The ROM360is, for example, a writable memory (e.g., a PROM), rewritable memory (e.g., a flash memory), or read-only memory. The ROM360stores a control program including instructions to cause the microcontroller340to control the power conversion device100. For example, the control program is temporarily loaded to a RAM (not shown) during booting.

The power conversion device100performs control under normal conditions and control under abnormal conditions. The control circuit300(mainly the microcontroller340) can switch the control of the power conversion device100from the control under normal conditions to the control under abnormal conditions. The on/off-state of each SW in the two switch circuits110is determined on the basis of a pattern of a failed FET or FETs (hereinafter also referred to as a “failure pattern”). The on/off-state of each FET in a failed inverter is also determined.

(1. Control Under Normal Conditions)

Firstly, a specific example method for controlling the power conversion device100under normal conditions will be described. As described above, the term “normal conditions” means that none of the FETs in the first and second inverters120and130has failed, and none of the SWs of the two switch circuits110has failed.

Under normal conditions, the control circuit300turns on all the SWs111,112,113, and114of the two switch circuits110. As a result, the power supply101and the first inverter120are electrically connected together, and the power supply101and the second inverter130are electrically connected together. In addition, the first inverter120and the GND are electrically connected together, and the second inverter130and the GND are electrically connected together. In this connection state, the control circuit300performs three-phase conduction control using both of the first and second inverters120and130to drive the motor200.

FIG. 6shows example current waveforms (sine waves) that are obtained by plotting values of currents flowing through the U-phase, V-phase, and W-phase windings of the motor200when the power conversion device100is controlled by the three-phase conduction control. The horizontal axis represents motor electrical angles (deg), and the vertical axis represents current values (A). In the current waveforms ofFIG. 6, current values are plotted every electrical angle of 30°. Ipkrepresents the greatest current value (peak current value) of each phase.

Table 1 shows the values of currents flowing through the terminals of each inverter every predetermined electrical angle of the sine waves ofFIG. 6. Specifically, Table 1 shows the values of currents flowing through the terminals U_L, V_L, and W_L of the first inverter120(the bridge circuit L) every electrical angle of 30°, and the values of currents flowing through the terminals U_R, V_R, and W_R of the second inverter130(the bridge circuit R) every electrical angle of 30°. Here, a positive current direction with respect to the bridge circuit L is defined as a direction in which a current flows from a terminal of the bridge circuit L to a terminal of the bridge circuit R. This definition applies to current directions shown inFIG. 6. A positive current direction with respect to the bridge circuit R is defined as a direction in which a current flows from a terminal of the bridge circuit R to a terminal of the bridge circuit L. Therefore, there is a phase difference of 180° between the current in the bridge circuit L and the current in the bridge circuit R. In Table 1, the magnitude of a current value I1is [(3)1/2/2]*Ipk, and the magnitude of a current value I2is Ipk/2.

At an electrical angle of 0°, a current does not flow through the U-phase winding M1. A current having a magnitude of I1flows through the V-phase winding M2from the bridge circuit R to the bridge circuit L, and a current having a magnitude of I1flows through the W-phase winding M3from the bridge circuit L to the bridge circuit R.

At an electrical angle of 30°, a current having a magnitude of I2flows through the U-phase winding M1from the bridge circuit L to the bridge circuit R, a current having a magnitude of Ipkflows through the V-phase winding M2from the bridge circuit R to the bridge circuit L, and a current having a magnitude of I2flows through the W-phase winding M3from the bridge circuit L to the bridge circuit R.

At an electrical angle of 60°, a current having a magnitude of I1flows through the U-phase winding M1from the bridge circuit L to the bridge circuit R, and a current having a magnitude of I1flows through the V-phase winding M2from the bridge circuit R to the bridge circuit L. A current does not flow through the W-phase winding M3.

At an electrical angle of 90°, a current having a magnitude of Ipkflows through the U-phase winding M1from the bridge circuit L to the bridge circuit R, a current having a magnitude of I2flows through the V-phase winding M2from the bridge circuit R to the bridge circuit L, and a current having a magnitude of I2flows through the W-phase winding M3from the bridge circuit R to the bridge circuit L.

At an electrical angle of 120°, a current having a magnitude of I1flows through the U-phase winding M1from the bridge circuit L to the bridge circuit R, and a current having a magnitude of I1flows through the W-phase winding M3from the bridge circuit R to the bridge circuit L. A current does not flow through the V-phase winding M2.

At an electrical angle of 150°, a current having a magnitude of I2flows through the U-phase winding M1from the bridge circuit L to the bridge circuit R, a current having a magnitude of I2flows through the V-phase winding M2from the bridge circuit L to the bridge circuit R, and a current having a magnitude of Ipkflows through the W-phase winding M3from the bridge circuit R to the bridge circuit L.

At an electrical angle of 180°, a current does not flow through the U-phase winding M1. A current having a magnitude of I1flows through the V-phase winding M2from the bridge circuit L to the bridge circuit R, and a current having a magnitude of I1flows through the W-phase winding M3from the bridge circuit R to the bridge circuit L.

At an electrical angle of 210°, a current having a magnitude of I2flows through the U-phase winding M1from the bridge circuit R to the bridge circuit L, a current having a magnitude of Ipkflows through the V-phase winding M2from the bridge circuit L to the bridge circuit R, and a current having a magnitude of I2flows through the W-phase winding M3from the bridge circuit R to the bridge circuit L.

At an electrical angle of 240°, a current having a magnitude of I1flows through the U-phase winding M1from the bridge circuit R to the bridge circuit L, and a current having a magnitude of I1flows through the V-phase winding M2from the bridge circuit L to the bridge circuit R. A current does not flow through the W-phase winding M3.

At an electrical angle of 270°, a current having a magnitude of Ipkflows through the U-phase winding M1from the bridge circuit R to the bridge circuit L, a current having a magnitude of I2flows through the V-phase winding M2from the bridge circuit L to the bridge circuit R, and a current having a magnitude of I2flows through the W-phase winding M3from the bridge circuit L to the bridge circuit R.

At an electrical angle of 300°, a current having a magnitude of I1flows through the U-phase winding M1from the bridge circuit R to the bridge circuit L, and a current having a magnitude of I1flows through the W-phase winding M3from the bridge circuit L to the bridge circuit R. A current does not flow through the V-phase winding M2.

At an electrical angle of 330°, a current having a magnitude of I2flows through the U-phase winding M1from the bridge circuit R to the bridge circuit L, a current having a magnitude of I2flows through the V-phase winding M2from the bridge circuit R to the bridge circuit L, and a current having a magnitude of Ipkflows through the W-phase winding M3from the bridge circuit L to the bridge circuit R.

In a typical Y-connected motor, the sum of currents flowing through the three phase windings is “0” at any electrical angle, where the directions of currents are taken into account. Note that, in the three-phase conduction control of the circuit configuration of the present disclosure, currents flowing through the three phase windings are separately controlled, and therefore, typically, a zero-phase current may flow. As a result, a control error may occur due to the influence of the zero-phase current. It should be noted that the sum of currents flowing through the three phase windings is not exactly “0” at any electrical angle. For example, the control circuit300can control the switching operations of the FETs of the bridge circuits L and R by PWM control such that the current waveforms ofFIG. 6are obtained.

(2. Control Under Abnormal Conditions)

As described above, the term “abnormal conditions” mainly means that a FET(s) has failed. Failures of a FET are roughly divided into an “open-circuit failure” and a “short-circuit failure.” The “open-circuit failure” with respect to a FET means that there is an open circuit between the source and drain of the FET (in other words, a resistance rds between the source and drain has a high impedance). The “short-circuit failure” with respect to a FET means that there is a short circuit between the source and drain of the FET.

The “open-circuit failure” with respect to a switching element SW means that the SW is always off (blocked), and is not turned on (i.e., is not put into a conductive state). The “short-circuit failure” with respect to a switching element SW means that the SW is always on, and is not turned off.

Referring back toFIG. 1, it is considered that, during the operation of the power conversion device100, a random failure occurs in which one of the 16 FETs randomly fails. The present disclosure is mainly directed to a method for controlling the power conversion device100when a random failure has occurred. Note that the present disclosure is also directed to a method for controlling the power conversion device100when multiple FETs have failed together, etc. Such a multi-failure means that, for example, a failure occurs in the high-side and low-side switching elements of one leg simultaneously.

When the power conversion device100is used for a long period of time, a random failure is likely to occur. Note that the random failure is different from the manufacture failure that may occur during manufacture. When even one of the FETs of the two inverters fails, the normal three-phase conduction control can be no longer carried out.

A failure may be detected as follows, for example. The drive circuit350monitors the drain-source voltage Vds of a FET, and compares Vds with a predetermined threshold voltage, in order to detect a failure in the FET. The threshold voltage is set in the drive circuit350by, for example, data communication with an external IC (not shown), and an external part. The drive circuit350is coupled to a port of the microcontroller340, and sends a failure detection signal to the microcontroller340. For example, the drive circuit350, when detecting a failure in a FET, asserts the failure detection signal. The microcontroller340, when receiving an asserted failure detection signal, reads internal data from the drive circuit350, and determines which of the FETs has failed.

Alternatively, a failure may be detected as follows, for example. The microcontroller340can detect a failure in a FET on the basis of a difference between an actual current value of the motor and a desired current value. Note that the failure detection is not limited to these techniques, and may be performed using a wide variety of known techniques related to the failure detection.

A failure in each SW of the switch circuits110can, for example, be detected by the microcontroller340monitoring a current flowing through the SW.

The microcontroller340, when receiving an asserted failure detection signal, switches the control of the power conversion device100from the control under normal conditions to the control under abnormal conditions. For example, a timing at which the control of the power conversion device100is switched from the control under normal conditions to the control under abnormal conditions is about 10-30 msec after the assertion of a failure detection signal.

The failure of the power conversion device100includes various failure patterns. Failure patterns will now be classified, and the control under abnormal conditions of the power conversion device100will now be described in detail for each pattern. In this embodiment, of the two inverters, the first inverter120is assumed to be a failed inverter, and the second inverter130is assumed to be operating normally.

The control under abnormal conditions will be described that is performed in the event of an open-circuit failure in at least one of the three high-side switching elements in the bridge circuit of the first inverter120.

It is assumed that, of the high-side switching elements (the FETs121H,122H, and123H) of the first inverter120, an open-circuit failure has occurred in the FET121H. Note that, in the event of an open-circuit failure in the FET122H or123H, the power conversion device100can also be controlled by a control method described below.

In the event of an open-circuit failure in the FET121H, the control circuit300puts the SWs111,112,113, and114of the two switch circuits110and the FETs122H,123H,121L,122L, and123L of the first inverter120into a first state. In the first state, the SWs111and113are off and the SWs112and114are on in the two switch circuits110. In addition, the FETs122H and123H other than the failed FET121H (the high-side switching elements other than the failed FET121H) are off and the FETs121L,122L, and123L are on in the first inverter120.

In the first state, the first inverter120is electrically disconnected from the power supply101and the GND, and the second inverter130is electrically connected to the power supply101and the GND. In other words, when the first inverter120is not operating normally, the SW113breaks the connection between the power supply101and the first inverter120, and the SW111breaks the connection between the first inverter120and the GND. In addition, all the three low-side switching elements are turned on so that the low-side node N1functions as a neutral point for the windings. As used herein, the term “a neutral point is formed” means that a certain node functions as the neutral point. The power conversion device100drives the motor200using a neutral point that is formed on the low side of the first inverter120, and the second inverter130.

As described above, in the three-phase conduction control, a zero-phase current may flow in the circuit. Here, it is assumed that, unlike the present disclosure, none of the two switch circuits110has a protection circuit. It is also assumed that the control circuit300switches the control from the control under normal conditions to the control under abnormal conditions. For example, the FETs121L,122L, and123L of the first inverter120are turned on so that a neutral point is formed on the low side. In this situation, if the SWs111and113are turned off, the node N1at which one end of each of the windings M1, M2, and M3is coupled to each other, i.e., the neutral point on the low side, is insulated from the power supply101and the GND. Therefore, the current path of the zero-phase current that has existed so far is suddenly cut or removed. As a result, an overvoltage occurs, which may induce a failure in an electronic component, such as a FET, in the circuit.

In the power conversion device100of this embodiment, a protection circuit is coupled in parallel to each of SWs111,112,113, and114. Even when the SWs111and113are turned off, the neutral point in the first inverter120is not insulated from the power supply101or the GND. Therefore, the path of the zero-phase current can be ensured by the protection circuits. The zero-phase current can flow through the diode111D of the protection circuit PC1or the diode113D of the protection circuit PC3. In other words, when a neutral point is formed in one of the inverters, the zero-phase current remaining in the circuit can be caused to flow out through a protection circuit. As a result, a failure in an electronic component in the circuit can be effectively prevented or reduced.

After turning on the FETs121L,122L, and123L, the control circuit300may turn off the SWs111and113at the time when some portion of the zero-phase current has flown out through a protection circuit and therefore the zero-phase current is small. For example, the control circuit300firstly turns on the FETs121L,122L, and123L. The control circuit300may monitor the zero-phase current, and turn off the SWs111and113after having confirmed that the zero-phase current is less than a predetermined value. This control can more reliably prevent or reduce a failure in the SWs111and113and each FET.

FIG. 7schematically shows flows of currents in the power conversion device100that occur when the SWs of the two switch circuits110and the FETs of the first inverter120are in the first state.FIG. 8shows example current waveforms that are obtained by plotting values of currents flowing through the U-phase, V-phase, and W-phase windings in the motor200when the power conversion device100is controlled in the first state.FIG. 7shows flows of currents that occur at a motor electrical angle of, for example, 270°. The three solid lines represent currents flowing from the power supply101to the motor200. In the state shown inFIG. 7, the FETs131H,132L, and133L are on and the FETs131L,132H, and133H are off in the second inverter130. A current flowing through the FET131H of the second inverter130flows through the winding M1and the FET121L of the first inverter120to the neutral point. A portion of the current flows through the FET122L to the winding M2, and the remaining portion of the current flows through the FET123L to the winding M3. The currents flowing through the windings M2and M3flow through the SW112for the second inverter130to the GND.

FIG. 7shows a zero-phase current I0that may flow through the diode111D of the protection circuit PC1when the control is switched from the control under normal conditions to the control under abnormal conditions. When the zero-phase current has a negative value, a forward current can flow through the diode111D.

Table 2 shows example values of currents flowing through terminals of the second inverter130every predetermined electrical angle of the current waveforms ofFIG. 8. Specifically, Table 2 shows examples values of currents flowing through the terminals U_R, V_R, and W_R of the second inverter130(the bridge circuit R) every electrical angle of 30°. The definitions of the directions of currents are as described above. Note that, according to the definitions of the current directions, the sign (positivity or negativity) of each current value shown inFIG. 8is opposite to that shown in Table 2 (phase difference: 180°).

For example, at an electrical angle of 30°, a current having a magnitude of I2flows through the U-phase winding M1from the bridge circuit L to the bridge circuit R, a current having a magnitude of Ipkflows through the V-phase winding M2from the bridge circuit R to the bridge circuit L, and a current having a magnitude of I2flows through the W-phase winding M3from the bridge circuit L to the bridge circuit R. At an electrical angle of 60°, a current having a magnitude of I1flows through the U-phase winding M1from the bridge circuit L to the bridge circuit R, and a current having a magnitude of I1flows through the V-phase winding M2from the bridge circuit R to the bridge circuit L. A current does not flow through the W-phase winding M3. The sum of a current(s) flowing into a neutral point and a current(s) flowing out of the neutral point is “0” at any electrical angle. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained.

As can be seen from Tables 1 and 2, motor currents flowing through the motor200at any electrical angle are the same between the control under normal conditions and the control under abnormal conditions. Therefore, compared to the control under normal conditions, the motor assistive torque is not reduced in the control under abnormal conditions.

The power supply101is not electrically connected to the first inverter120, and therefore, a current does not flow from the power supply101into the first inverter120. In addition, the first inverter120is not electrically connected to the GND, and therefore, a current flowing through the neutral point does not flow to the GND. As a result, a power loss can be prevented or reduced, and suitable current control can be achieved by the formation of a closed loop of a drive current.

In the event of an open-circuit failure in a high-side switching element (the FET121H), the state of the SWs of the two switch circuits110and the FETs of the first inverter120is not limited to the first state. For example, the control circuit300may put these SWs and FETs into a second state. In the second state, the SW113is on and the SW111is off, and the SWs112and114are on, in the two switch circuits110. In addition, the FETs122H and123H other than the failed FET121H are off, and the FETs121L,122L, and123L are on, in the first inverter120. The first state is different from the second state in whether or not the SW113is on. A reason why the SW113may be on is that, in the event of an open-circuit failure in the FET121H, if the FETs122H and123H are controlled to be off, all the high-side switching elements are put into the open state, and therefore, in this case, even when the SW113is on, a current does not flow from the power supply101to the first inverter120. Thus, in the event of an open-circuit failure, the SW113may be either on or off.

The control under abnormal conditions will be described that is performed in the event of a short-circuit failure in one of the three high-side switching elements in the bridge circuit of the first inverter120.

It is assumed that a short-circuit failure has occurred in the FET121H of the high-side switching elements (the FETs121H,122H, and123H) of the first inverter120. Note that, in the event of a short-circuit failure in the FET122H or123H, the power conversion device100can also be controlled using a control method described below.

In the event of a short-circuit failure in the FET121H, the control circuit300puts the SWs111,112,113, and114of the two switch circuits110and the FETs122H,123H,121L,122L, and123L of the first inverter120into the first state. Note that, in the event of a short-circuit failure, if the SW113is turned on, a current flows from the power supply101into the short-circuited FET121H. Therefore, the control in the second state is forbidden.

As in the event of an open-circuit failure, all the three low-side switching elements are turned on so that a neutral point for the windings is formed at the low-side node N1. The power conversion device100drives the motor200using the neutral point on the low side of the first inverter120, and the second inverter130. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained. For example, in the first state in the event of a short-circuit failure, the flows of currents flowing in the power conversion device100at an electrical angle of 270° are as shown inFIG. 7. In addition, the values of currents flowing through the windings every predetermined motor electrical angle are as shown in Table 2.

In this control, the power supply101is not electrically connected to the first inverter120, and therefore, a current does not flow from the power supply101into the first inverter120. In addition, the first inverter120is not electrically connected to the GND, and therefore, a current flowing through the neutral point does not flow to the GND.

The control under abnormal conditions will be described that is performed in the event of an open-circuit failure in one of the three low-side switching elements in the bridge circuit of the first inverter120.

It is assumed that, of the low-side switching elements (the FETs121L,122L, and123L) of the first inverter120, an open-circuit failure has occurred in the FET121L. Note that, in the event of an open-circuit failure in the FET122L or123L, the power conversion device100can also be controlled by a control method described below.

In the event of an open-circuit failure in the FET121L, the control circuit300puts the SWs111,112,113, and114of the two switch circuits110and the FETs121H,122H,123H,122L, and123L of the first inverter120into a third state. In the third state, the SWs111and113are off and the SWs112and114are on in the two switch circuits110. In addition, the FETs122L and123L other than the failed FET121L (the low-side switching elements other than the failed FET121L) are off, and the FETs121H,122H, and123H are on, in the first inverter120.

In the third state, the first inverter120is electrically disconnected from the power supply101and the GND, and the second inverter130is electrically connected to the power supply101and the GND. In addition, all the three high-side switching elements of the first inverter120are on, and therefore, a neutral point for the windings is formed at the high-side node N3.

FIG. 9schematically shows flows of currents in the power conversion device100that occur when the SWs of the two switch circuits110and the FETs of the first inverter120are in the third state.FIG. 9shows flows of currents at a motor electrical angle of, for example, 270°. The three solid lines represent currents flowing from the power supply101to the motor200.

In the state shown inFIG. 9, the FETs131H,132L, and133L are on and the FETs131L,132H, and133H are off in the second inverter130. A current flowing through the FET131H of the second inverter130flows through the winding M1and the FET121H of the first inverter120to the neutral point. A portion of the current flows through the FET122H to the winding M2, and the remaining current flows through the FET123H to the winding M3. The currents flowing through the windings M2and M3flow through the SW112for the second inverter130to the GND.

FIG. 9shows a zero-phase current I0that may flow through the diode113D of the protection circuit PC3when the control is switched from the control under normal conditions to the control under abnormal conditions. When the zero-phase current has a positive value, a forward current may flow through the diode113D.

The power conversion device100drives the motor200using the neutral point formed on the high side of the first inverter120, and the second inverter130. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained.

In this control, the power supply101is not electrically connected to the first inverter120, and therefore, a current does not flow from the power supply101into the neutral point of the first inverter120. In addition, the first inverter120is not electrically connected to the GND, and therefore, a current does not flow from the first inverter120to the GND.

In the event of an open-circuit failure in the low-side switching element (the FET121L), the state of the SWs of the two switch circuits110and the FETs of the first inverter120is not limited to the third state. For example, the control circuit300may put these SWs and FETs into a fourth state. In the fourth state, the SW113is off and the SW111is on, and the SWs112and114are on, in the two switch circuits110. In addition, the FETs122L and123L other than the failed FET121L are off, and the FETs121H,122H, and123H are on, in the first inverter120. The third state is different from the fourth state in whether or not the SW111is on. A reason why the SW111may be on is that, in the event of an open-circuit failure in the FET121L, if the FETs122L and123L are controlled to be off, all the low-side switching elements are put into the open state, and therefore, in this case, even when the SW111is on, a current does not flow to the GND. Thus, in the event of an open-circuit failure, the SW111may be either on or off.

The control under abnormal conditions will be described that is performed in the event of a short-circuit failure in one of the three low-side switching elements in the bridge circuit of the first inverter120.

It is assumed that, of the low-side switching elements (the FETs121L,122L, and123L) of the first inverter120, a short-circuit failure has occurred in the FET121L. Note that, in the event of a short-circuit failure in the FET122L or123L, the power conversion device100can also be controlled by a control method described below.

In the event of a short-circuit failure in the FET121L, the control circuit300puts the SWs111,112,113, and114of the two switch circuits110and the FETs121H,122H,123H,122L, and123L of the first inverter120into the third state as in the event of an open-circuit failure. Note that, in the event of a short-circuit failure, if the SW111is turned on, a current flows from the short-circuited FET121L into the GND. Therefore, the control in the fourth state is forbidden.

FIG. 10schematically shows flows of currents in the power conversion device100that occur when the SWs of the two switch circuits110and the FETs of the first inverter120are in the third state.FIG. 10shows flows of currents at a motor electrical angle of, for example, 270°. The three solid lines represent currents flowing from the power supply101to the motor200, and a long dashed line represents a current flowing through the FET121L.

In the state shown inFIG. 10, the FETs131H,132L, and133L are on and the FETs131L,132H, and133H are off in the second inverter130. A current flowing through the FET131H of the second inverter130flows through the winding M1and the FET121H of the first inverter120to the neutral point. A portion of the current flows through the FET122H to the winding M2, and the remaining current flows through the FET123H to the winding M3. The currents flowing through the windings M2and M3flow through the SW112for the second inverter130to the GND. In addition, a freewheeling current flows through the freewheeling diode of the FET131L in a direction toward the winding M1of the motor200. Furthermore, in the event of a short-circuit failure, a current flows from the short-circuited FET121L to the low-side node N1unlike the event of an open-circuit failure. A portion of the current flows through the freewheeling diode of the FET122L to the winding M2, and the remaining current flows through the freewheeling diode of the FET123L to the winding M3. The currents flowing through the windings M2and M3flow through the SW112to the GND.

For example, the values of currents flowing through the windings every predetermined motor electrical angle are as shown in Table 2.

The power conversion device100drives the motor200using the neutral point formed on the high side of the first inverter120, and the second inverter130. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained.

In this control, the power supply101is not electrically connected to the first inverter120, and therefore, a current does not flow from the power supply101into the neutral point of the first inverter120. In addition, the first inverter120is not electrically connected to the GND, and therefore, a current does not flow from the first inverter120to the GND.

The control under abnormal conditions will be described that is performed in the event of an open-circuit failure in the SW113of the power supply-side switch circuit110.

It is assumed that an open-circuit failure has occurred in the SW113of the power supply-side switch circuit110. In this case, the control circuit300puts the SWs111,112, and114of the two switch circuits110and the FETs121H,122H,123H,121L,122L, and123L of the first inverter120into a fifth state. In the fifth state, the SW111is off and the SWs112and114are on in the two switch circuits110. In addition, the FETs121L,122L, and123L are on and the FETs121H,122H, and123H are off in the first inverter120.

In the fifth state, the first inverter120is electrically disconnected from the power supply101and the GND, and the second inverter130is electrically connected to the power supply101and the GND, since the SW113is in the open state. In addition, all the three low-side switching elements of the first inverter120are on, and therefore, a neutral point for the windings is formed at the low-side node N1.

FIG. 11schematically shows flows of currents in the power conversion device100that occur when the SWs of the two switch circuits110and the FETs of the first inverter120are in the fifth state.FIG. 11shows flows of currents at a motor electrical angle of, for example, 270°. The three solid lines represent currents flowing from the power supply101to the motor200.

In the state shown inFIG. 11, the FETs131H,132L, and133L are on and the FETs131L,132H, and133H are off in the second inverter130. A current flowing through the FET131H of the second inverter130flows through the winding M1and the FET121L of the first inverter120to the neutral point. A portion of the current flows through the FET122L to the winding M2, and the remaining current flows through the FET123L to the winding M3. The currents flowing through the windings M2and M3flow through the SW112for the second inverter130to the GND. For example, the values of currents flowing through the windings every predetermined motor electrical angle are as shown in Table 2.

The power conversion device100drives the motor200using the neutral point formed on the low side of the first inverter120, and the second inverter130. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained. For example, the values of currents flowing through the windings every predetermined motor electrical angle are as shown in Table 2.

In the event of an open-circuit failure in the SW113, a neutral point may be formed on either the low side or the high side. The control circuit300can put the SWs111,112, and114of the two switch circuits110and the FETs121H,122H,123H,121L,122L, and123L of the first inverter120into a sixth state. In the sixth state, the SWs112and114are on in the two switch circuits110. In addition, the FETs121L,122L, and123L are off and the FETs121H,122H, and123H are on in the first inverter120. The SW111of the GND-side switch circuit110may be either on or off.

In the sixth state, the first inverter120is electrically disconnected from the power supply101and the GND, and the second inverter130is electrically connected to the power supply101and the GND, since the SW113is in the open state. In addition, all the three low-side switching elements of the first inverter120are on, and therefore, a neutral point for the windings is formed at the high-side node N3.

FIG. 12schematically shows flows of currents in the power conversion device100that occur when the SWs of the two switch circuits110and the FETs of the first inverter120are in the sixth state.FIG. 12shows flows of currents at a motor electrical angle of, for example, 270°. The three solid lines represent currents flowing from the power supply101to the motor200.

The power conversion device100drives the motor200using the neutral point formed on the high side of the first inverter120, and the second inverter130. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained.

In this control, the power supply101is not electrically connected to the first inverter120, and therefore, a current does not flow from the power supply101into the neutral point of the first inverter120. In addition, irrespective of whether the SW111is on or off, all the low-side switching elements are off, and therefore, a current does not flow from the first inverter120to the GND.

In the event of an open-circuit failure in the SW113, the state of the SWs of the two switch circuits110and the FETs of the first inverter120is not limited to the fifth or sixth state. For example, the control circuit300may put these SWs and FETs into a seventh state. In the seventh state, the SW111is off and the SWs112and114are on in the two switch circuits110. In addition, the FETs121L,122L, and123L are on and at least one of the FETs121H,122H, and123H is on in the first inverter120. The seventh state is different from the fifth state in that at least one of the high-side switching elements is on.

For example, when one FET of the three high-side switching elements is on, a current does not flow through that FET due to the freewheeling diodes of the other two FETs at certain motor electrical angles. For example, in the fifth state of the FETs shown inFIG. 11, when the motor electrical angle is 270°, then if the FET121H is on and the other FETs122H and123H are off, a current does not flow on the high side. When the motor electrical angle is 180°-360° in Table 2, a current does not flow on the high side. Meanwhile, in the fifth state of the FETs shown inFIG. 11, when the motor electrical angle is 0°-120° in Table 2, then if the FET121H is on and the other FETs122H and123H are off, a freewheeling current flows through the freewheeling diode of the FET122H to the FET121H. When the motor electrical angle is 60°-180° in Table 2, a freewheeling current flows through the freewheeling diode of the FET123H to the FET121H. Note that because an open-circuit failure has occurred in the SW113, a current does not flow from the power supply101to the high-side node N3. Thus, if at least one of the high-side switching elements is turned on, a current may be shunted, i.e., currents may flow in a more distributed manner, when the motor electrical angle is within a certain range, resulting in a reduction in heat influence.

If all the high-side switching elements are turned on, two neutral points are formed on the low and high sides. Note that because an open-circuit failure has occurred in the SW113, a current does not flow from the power supply101to the neutral point on the high-side node. A current may be shunted using the two neutral points, i.e., currents may flow in a more distributed manner, resulting in a reduction in heat influence on the inverter.

The control under abnormal conditions will be described that is performed in the event of a short-circuit failure in the SW113of the power supply-side switch circuit110.

It is assumed that a short-circuit failure has occurred in the SW113of the power supply-side switch circuit110. In this case, the control circuit300puts the SWs111,112, and114of the two switch circuits110and the FETs121H,122H,123H,121L,122L, and123L of the first inverter120into the fifth state. Note that if at least one of the high-side switching elements is turned on, a current flows through the SW113to the on-state high-side switching element. Therefore, the control in the seventh state is forbidden.

As in the event of an open-circuit failure, all the three low-side switching elements are turned on so that a neutral point for the windings is formed at the low-side node N1. The power conversion device100drives the motor200using the neutral point formed on the low side of the first inverter120, and the second inverter130. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained. For example, in the fifth state in the event of a short-circuit failure, the flows of currents in the power conversion device100at an electrical angle of 270° are as shown inFIG. 11. The values of currents flowing through the windings every predetermined motor electrical angle are as shown in Table 2.

In this control, all the high-side switching elements are off, and therefore, a current does not flow from the power supply101into the first inverter120, irrespective of the occurrence of a short circuit in the SW113. In addition, the first inverter120is not electrically connected to the GND, and therefore a current flowing the neutral point does not flow to the GND.

The control under abnormal conditions will be described that is performed in the event of an open-circuit failure in the SW111of the GND-side switch circuit110.

It is assumed that an open-circuit failure has occurred in the SW111of the GND-side switch circuit110. In this case, the control circuit300puts the SWs112,113, and114of the two switch circuits110and the FETs121H,122H,123H,121L,122L, and123L of the first inverter120into an eighth state. In the eighth state, the SW113is off and the SWs112and114are on in the two switch circuits110. In addition, the FETs121L,122L, and123L are off and the FETs121H,122H, and123H are on in the first inverter120.

In the eighth state, the first inverter120is electrically disconnected from the power supply101and the GND, and the second inverter130is electrically connected to the power supply101and the GND, since the SW111is in the open state. In addition, all the three high-side switching elements of the first inverter120are on, and therefore, a neutral point for the windings is formed at the high-side node N3.

FIG. 13schematically shows flows of currents in the power conversion device100that occur when the SWs of the two switch circuits110and the FETs of the first inverter120are in the eighth state.FIG. 13shows flows of currents at a motor electrical angle of, for example, 270°. The three solid lines represent currents flowing from the power supply101to the motor200, and a dashed line represents a freewheeling current that flows back to the winding M1of the motor200.

In the state shown inFIG. 13, the FETs131H,132L, and133L are on and the FETs131L,132H, and133H are off in the second inverter130. A current flowing through the FET131H of the second inverter130flows through the winding M1and the FET121H of the first inverter120to the neutral point. A portion of the current flows through the FET122H to the winding M2, and the remaining current flows through the FET123H to the winding M3. The currents flowing through the windings M2and M3flow through the SW112for the second inverter130to the GND. In addition, a freewheeling current flows through the freewheeling diode of the FET131L in a direction toward the winding M1of the motor200.

The power conversion device100drives the motor200using the neutral point formed on the high side of the first inverter120, and the second inverter130. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained. For example, the values of currents flowing through the windings every predetermined motor electrical angle are as shown in Table 2.

In this control, the power supply101is not electrically connected to the first inverter120, and therefore, a current does not flow from the power supply101into the neutral point of the first inverter120. In addition, the failed SW111is in the open state, and therefore, a current does not flow from the first inverter120to the GND.

In the event of an open-circuit failure in the SW111, the state of the SWs of the two switch circuits110and the FETs of the first inverter120is not limited to the eighth state. For example, the control circuit300may put these SWs and FETs into a ninth state. In the ninth state, the SW113is off and the SWs112and114are on in the two switch circuits110. In addition, at least one of the FETs121L,122L, and123L is on and the FETs121H,122H, and123H are on in the first inverter120. The ninth state is different from the eighth state in that at least one of the low-side switching elements is on.

For example, when one FET of the three low-side switching elements is on, a current does not flow through that FET due to the freewheeling diodes of the other two FETs at certain motor electrical angles. For example, in the eighth state of the FETs shown inFIG. 13, when the motor electrical angle is 270°, then if the FET121L is on and the other FETs122L and123L are off, a freewheeling current flows through the FET121L to the freewheeling diodes of the FETs122L and123L. When the motor electrical angle is 180°-360° in Table 2, a current flows on the low side. Note that because an open-circuit failure has occurred in the SW111, a current does not flow from the neutral point on the low side to the GND. Thus, if at least one of the low-side switching elements is on, a current may be shunted, i.e., currents may flow in a more distributed manner, when the motor electrical angle is within a certain range, resulting in a reduction in heat influence.

If all the low-side switching elements are turned on, two neutral points are formed on the low and high sides. Note that because an open-circuit failure has occurred in the SW111, a current does not flow the neutral point on the low side to the GND. A current may be shunted using the two neutral points, i.e., currents may flow in a more distributed manner, resulting in a reduction in heat influence on the inverter.

The control under abnormal conditions will be described that is performed in the event of a short-circuit failure in the SW111of the GND-side switch circuit110.

It is assumed that a short-circuit failure has occurred in the SW111of the GND-side switch circuit110. In this case, the control circuit300puts the SWs112,113, and114of the two switch circuits110and the FETs121H,122H,123H,121L,122L, and123L of the first inverter120into the eighth state. Note that if at least one of the low-side switching elements is turned on, a current flows through the SW111to the GND. Therefore, the control in the ninth state is forbidden.

As in the event of an open-circuit failure, all the three high-side switching elements are turned on so that a neutral point for the windings is formed at the high-side node N3. The power conversion device100drives the motor200using the neutral point formed on the high side of the first inverter120, and the second inverter130. The control circuit300can control the switching operations of the FETs of the bridge circuit R by PWM control such that, for example, the current waveforms ofFIG. 8are obtained. For example, in the eighth state in the event of a short-circuit failure, the flows of currents in the power conversion device100at an electrical angle of 270° are as shown inFIG. 13. The values of currents flowing through the windings every predetermined motor electrical angle are as shown in Table 2.

In this control, a current does not flow from the power supply101into the neutral point of the first inverter120. In addition, all the low-side switching elements are turned off, and therefore, a current does not flow from the first inverter120to the GND, irrespective of the occurrence of a short circuit in the SW111.

According to this embodiment, in the control under abnormal conditions, a power loss can be prevented or reduced, and a closed loop of a drive current can be formed to achieve suitable current control. In addition, the protection circuit can prevent or reduce a breakage of an SW or a FET due to a zero-phase current.

In the present disclosure, the control under normal conditions is not limited to the above three-phase conduction control for separately controlling currents flowing through three phase windings. As in Japanese Laid-Open Patent Publication No. 2014-192950, the control may be conduction control for driving a motor using one of two inverters with a neutral point being formed in the other inverter. For example, the control circuit300can switch between the two controls according to the rotational speed of the motor200. According to the present disclosure, a zero-phase current that may occur during this switching can be effectively reduced. In particular, if the conduction controls are switched as appropriate, the control performance may be improved, and losses such as a copper loss and a power loss in a FET may be reduced.

The on/off-states of switching elements that form a neutral point in an inverter are not limited to the above first to ninth states. Some example sets of on/off-states of switching elements that form a neutral point in an inverter will now be described.

FIGS. 14A-14Cshow other sets of on/off-states of switching elements that form a neutral point in the first inverter120.

The formation of a neutral point means that three nodes N11, N12, and N13at which the legs of the bridge circuit of the inverter120are coupled to the respective phase windings are caused to have equal potentials. As shown inFIG. 14A, the three nodes N11, N12, and N13can be caused to have equal potentials by turning on all the switching elements in the first inverter120. As a result, a neutral point is formed in the first inverter120.

In the conduction state in the control under normal conditions, the motor200can be driven by turning on and off the switching elements of the second inverter130with a neutral point being formed in the first inverter120by the above technique. In addition, for example, in the event of a short-circuit failure in the FET121H of the first inverter120, the motor200can be driven using the same technique as under normal conditions.

As shown inFIG. 14B, in the bridge circuit of the first inverter120, the three nodes N11, N12, and N13can be caused to have equal potentials by turning on the FETs121H,122H,122L, and123L and turning off the FET121L. The FET123H is either on or off. For example, this pattern is applicable when the first inverter120is operating normally, or when an open or short-circuit failure has occurred in the FET123H.

As shown inFIG. 14C, the three nodes N11, N12, and N13can be caused to have equal potentials by turning on the FETs121L,122L, and123L in the bridge circuit of the first inverter120. The FETs121H,122H, and123H are each either on or off. For example, this pattern is applicable when the first inverter120is operating normally, or when an open or short-circuit failure has occurred in the FET123H.

Variations of the circuit configuration of the power conversion device100will be described with reference toFIGS. 15A-15D.

In this embodiment, the two switch circuits110of the power conversion device include the SWs111,112,113, and114. However, the present disclosure is not limited to this. The two switch circuits110may include at least one of the SWs111,112,113, and114. For example, the two switch circuits110can include the SWs111and113of the SWs111,112,113, and114.

FIG. 15Ashows a circuit configuration of a power conversion device100A that includes the power supply-side switch circuit including the SWs113and114of the two switch circuits110. In this variation, for example, in the event of a short-circuit failure in the FET121H, if the SW113is turned off, a current can be prevented from flowing from the power supply101to the FET121H. In other words, when the first inverter120is not operating normally, the SW113breaks the connection between the power supply101and the first inverter120. In addition, the FETs121L,122L, and123L are turned off and the FETs122H and123H are turned on so that a neutral point is formed at the high-side node N3.

FIG. 15Bshows a circuit configuration of a power conversion device100A that includes the GND-side switch circuit including the SWs111and112of the two switch circuits110. In this variation, for example, in the event of a short-circuit failure in the FET121L, if the SW111is turned off, a current can be prevented from flowing through the FET121L to the GND. In other words, when the first inverter120is not operating normally, the SW111breaks the connection between the first inverter120and the GND. In addition, the FETs121H,122H, and123H are turned off and the FETs122L and123L are turned on so that a neutral point is formed at the low-side node N1.

FIG. 15Cshows a circuit configuration of a power conversion device100A that includes a switch circuit110including only the SW113of the above switch elements. In this variation, for example, in the event of a short-circuit failure in the FET121H, if the SW113is turned off, a current can be prevented from flowing from the power supply101to the FET121H. In addition, the FETs121L,122L, and123L are turned off and the FETs122H and123H are turned on so that a neutral point is formed at the high-side node N3.

FIG. 15Dshows a circuit configuration of a power conversion device100A that includes a switch circuit110including only the SW111of the above switch elements. In this variation, for example, in the event of a short-circuit failure in the FET121L, if the SW111is turned off, a current can be prevented from flowing from the FET121L to the GND. In addition, the FETs121H,122H, and123H are turned off and the FETs122L and123L are turned on so that a neutral point is formed at the low-side node N1.

Note that the two switch circuits110may have only either the SW112or the SW114of the above switch elements. Alternatively, the two switch circuits110may include any combination of the above switch elements, i.e., one or more selected from the SWs111,112,113, and114.

Second Embodiment

The protection circuit of the present disclosure may have a resistance element, an RC circuit including a resistance element and a capacitor, or a combination thereof, instead of a diode. Alternatively, the protection circuit may be a snubber circuit having a resistance element, a capacitor, and a diode, etc. These can be used to prevent or reduce an overvoltage in a switching element and thereby protect the switching element, and in addition, cause a zero-phase current to flow out of an inverter circuit, whereby a breakage of an electronic component can be prevented or reduced. Several variations of the protection circuit will now be mainly described.

FIGS. 16A-16Cschematically show circuit configurations of a power conversion device100B including a resistance element as a protection circuit.

As shown inFIG. 16A, the protection circuits PC1, PC2, PC3, and PC4may be resistance elements111R,112R,113R, and114R. The resistance values of the resistance elements are, for example, about 40 kΩ, which is sufficiently greater than the values of the on-resistances of the SWs. In the control under normal conditions, the SWs111,112,113, and114in the two switch circuits110are on. In this case, a weak current may flow through each of the resistance elements111R,112R,113R, and114R, that depends on the ratio of that resistant element and the on-resistance of the corresponding SW. It is considered that power losses in the resistance elements111R,112R,113R, and114R are considerably low. If a resistance element having a greater resistance value is selected, a power loss caused by a current flowing through a resistance element under normal conditions can be prevented or reduced. In addition, if a resistance element is selected as a protection circuit, the protection circuit can be implemented at relatively low cost, as with a diode.

The protection circuit of the present disclosure can include a plurality of resistance elements coupled together in series or in parallel. The number of resistance elements is not limited to two, and may be three or more. For example, as shown inFIG. 16B, the protection circuit PC3can include two resistance elements coupled together in series. For example, as shown inFIG. 16C, the protection circuit PC3can include two resistance elements113R coupled together in parallel. The protection circuits PC1, PC2, and PC4can also each include two resistance elements coupled together in series or in parallel, as with the protection circuit PC3. In the present disclosure, at least one of the four protection circuits PC1, PC2, PC3, and PC4may include a plurality of resistance elements coupled together in series or in parallel.

FIG. 17schematically shows a circuit configuration of a power conversion device100B including an RC circuit as a protection circuit.

The protection circuit of the present disclosure can include an RC circuit.FIG. 17shows an example circuit configuration in which four protection circuits PC1, PC2, PC3, and PC4include an RC circuit. Note that at least one of the four protection circuits PC1, PC2, PC3, and PC4may include an RC circuit. The capacitance of the capacitor is determined, as appropriate, according to the magnitude of a zero-phase current flowing through the protection circuit. The protection circuit may be an RC snubber circuit, or an RCD snubber circuit further including a diode, for example.

If an RC circuit is used as the protection circuit, spike-like high voltage noise that would occur when an SW is turned off can be effectively prevented or reduced. The RC circuit can also effectively prevent or reduce harmonic current components (current components other than the fundamental wave) that would occur when each FET is turned on or off. Furthermore, a breakage of an SW itself, or a breakage of electronic components around the SW, can be prevented or reduced, and the influence of electromagnetic noise on electronic components can be minimized.

FIGS. 18A, 18B, and 19schematically show variations of the circuit configuration of the power conversion device100B.

As shown inFIG. 18A, the protection circuit PC3can, for example, be implemented by a diode113D and a resistance element113R coupled in parallel thereto. As shown inFIG. 18B, the protection circuit PC3can, for example, be implemented by a diode113D and an RC circuit coupled in parallel thereto. The protection circuits PC1, PC2, and PC4may have a configuration similar to that of the protection circuit PC3.

FIG. 19shows an example circuit configuration in which a diode and an RC circuit coexist as protection circuits. The protection circuits do not necessarily need to be of the same type. As shown inFIG. 19, a diode and an RC circuit may coexist. Alternatively, a resistance element, a diode, and an RC circuit may coexist.

Third Embodiment

A power conversion device100C according to a third embodiment includes a FET that has a parasitic diode, as a switching element in the two switch circuits110. Differences between the third embodiment and the first and second embodiments will now be mainly described, and features common to these embodiments will not be described.

FIG. 20schematically shows a circuit configuration of a power conversion device100C of this embodiment.

The power conversion device100C includes FETs111,112,113, and114as switch elements111,112,113, and114. The FETs are typically a MOSFET. It is herein assumed that a protection circuit coupled in parallel to a switching element includes a freewheeling diode of the FET.

The FETs113and114in the power supply-side switch circuit110are disposed such that a forward current flows through parasitic diodes113D and114D, respectively, in a direction toward the power supply101. The FETs111and112in the GND-side switch circuit110are disposed such that a forward current flows through parasitic diodes111D and112D, respectively, in a direction toward an inverter.

For example, in the event of a failure in the first inverter120, a zero-phase current flows through the parasitic diode111D of the FET111or the parasitic diode113D of the FET113. Thus, if an FET is used as a switching element, the parasitic diode can function as a protection circuit.

FIG. 21schematically shows a circuit configuration of a power conversion device100C that further includes two FETs for reverse connection protection.

As described in the first embodiment, the power conversion device100C can include two FETs115and116for reverse connection protection. The FETs113and115are disposed such that the directions of the parasitic diodes thereof are opposite to each other. The FETs114and116are disposed such that the directions of the parasitic diodes thereof are opposite to each other. The FETs115and116are always on, and therefore, a zero-phase current can flow through the parasitic diode113D or114D.

FIG. 22schematically shows another circuit configuration of the power conversion device100C of this embodiment.

As shown inFIG. 22, a resistance element111R can be coupled in parallel to the FET111, for example. For example, it is assumed that an open-circuit failure has occurred in all the FETs121H,122H, and123H of the first inverter120. In this case, a zero-phase current cannot flow through the parasitic diode of the FET111, and can flow through the resistance element111R. In other words, a zero-phase current can be caused to flow out.

In the power conversion device of the present disclosure, a switching element coupled to a protection circuit, and a FET having a parasitic diode, may coexist.

Fourth Embodiment

Vehicles such as automobiles are typically equipped with an electric power steering device. The electric power steering device generates an assistive torque that is added to the steering torque of a steering system that is generated by a driver turning a steering wheel. The assistive torque is generated by an assistive torque mechanism, and can reduce a driver's burden of turning a steering wheel. For example, the assistive torque mechanism includes a steering torque sensor, an ECU, a motor, and a deceleration mechanism. The steering torque sensor detects a steering torque in the steering system. The ECU generates a drive signal on the basis of a detection signal from the steering torque sensor. The motor generates an assistive torque depending on the steering torque on the basis of the motor drive signal. The assistive torque is transferred through the deceleration mechanism to the steering system.

The motor drive unit400of the present disclosure may be used in the electric power steering device.

FIG. 23schematically shows a typical configuration of an electric power steering device500according to this embodiment. The electric power steering device500includes a steering system520and an assistive torque mechanism540.

The steering system520includes, for example, a steering wheel521, a steering shaft522(also called a “steering column”), universal couplings523A and523B, a rotating shaft524(also called a “pinion shaft” or “input shaft”), a rack and pinion mechanism525, a rack shaft526, left and right ball joints552A and552B, tie rods527A and527B, knuckles528A and528B, and left and right steerable wheels (e.g., left and right front wheels)529A and529B. The steering wheel521is linked through the steering shaft522and the universal couplings523A and523B to the rotating shaft524. The rotating shaft524is linked through the rack and pinion mechanism525to the rack shaft526. The rack and pinion mechanism525has a pinion531provided on the rotating shaft524, and a rack532provided on the rack shaft526. A right end of the rack shaft526is linked to the right steerable wheel529A through the ball joint552A, the tie rod527A, and the knuckle528A in this order with the ball joint552A being closest to the right end of the rack shaft526. As with the right side, a left end of the rack shaft526is linked to the left steerable wheel529B through the ball joint552B, the tie rod527B, and the knuckle528B in this order with the ball joint552B being closest to the left end of the rack shaft526. Here, the right and left sides correspond to the right and left sides, respectively, of a driver sitting on a seat.

In the steering system520, a steering torque is generated by a driver turning the steering wheel521, and is transmitted through the rack and pinion mechanism525to the left and right steerable wheels529A and529B. As a result, the driver can control the left and right steerable wheels529A and529B.

The assistive torque mechanism540includes, for example, a steering torque sensor541, an ECU542, a motor543, a deceleration mechanism544, and a power conversion device545. The assistive torque mechanism540applies an assistive torque to the steering system520including from the steering wheel521to the left and right steerable wheels529A and529B. Note that the assistive torque may also be called an “additional torque.”

As the ECU542, the control circuit300of this embodiment can be used. As the power conversion device545, the power conversion device100of this embodiment can be used. The motor543is equivalent to the motor200of this embodiment. As a mechanically and electronically integrated motor including the ECU542, the motor543, and the power conversion device545, the motor drive unit400of this embodiment may be used.

The steering torque sensor541detects a steering torque that is applied to the steering system520using the steering wheel521. The ECU542generates a drive signal for driving the motor543on the basis of a detection signal (hereinafter referred to as a “torque signal”) from the steering torque sensor541. The motor543generates an assistive torque depending on the steering torque on the basis of the drive signal. The assistive torque is transmitted through the deceleration mechanism544to the rotating shaft524of the steering system520. The deceleration mechanism544is, for example, a worm gear mechanism. The assistive torque is further transmitted from the rotating shaft524to the rack and pinion mechanism525.

The electric power steering device500may be categorized into the pinion assist type, rack assist type, column assist type, etc., according to a portion of the steering system520to which the assistive torque is added.FIG. 23illustrates the electric power steering device500of the pinion assist type. Note that the electric power steering device500may be of the rack assist type, column assist type, etc.

In addition to the torque signal, a vehicle speed signal may be input to the ECU542, for example. A piece of external equipment560may, for example, be a vehicle speed sensor. Alternatively, the external equipment560may, for example, be another ECU that can communicate with the ECU542over an in-vehicle network, such as a controller area network (CAN). The microcontroller of the ECU542can perform vector control or PWM control on the motor543on the basis of the torque signal and the vehicle speed signal, etc.

The ECU542determines a desired current value on the basis of at least the torque signal. The ECU542may determine the desired current value, taking into account the vehicle speed signal detected by the vehicle speed sensor, and in addition, a rotor rotation signal detected by an angle sensor. The ECU542can control a drive signal, i.e. a drive current, for the motor543such that an actual current value detected by a current sensor (not shown) is equal to the desired current value.

The electric power steering device500can control the left and right steerable wheels529A and529B through the rack shaft526using a composite torque obtained by adding the assistive torque of the motor543to a driver's steering torque. In particular, if the motor drive unit400of the present disclosure is applied to the above mechanically and electronically integrated motor, an electric power steering device including a motor drive unit is provided in which the quality of parts can be improved, and suitable current control can be performed under both normal and abnormal conditions.