JP2019205247A - Inverter - Google Patents

Abstract

To provide a technique to combine cost and performance with respect to an inverter adopting a parallel connection of two kinds of different switching elements.SOLUTION: An inverter disclosed herein includes: a first switching element for power conversion made of silicon carbide; a second switching element for power conversion made of silicon; and a controller. The first switching element and the second switching element are connected in parallel. The controller drives the second switching element when the motor is locked, and drives the first switching element when the motor is not locked.SELECTED DRAWING: Figure 4

Description

  The technology disclosed in this specification relates to an inverter that converts DC power into AC power for driving a traveling motor.

  An electric vehicle, a fuel cell vehicle, and the like include an inverter that converts DC power of a power source into AC power for driving a running motor. The main component that performs power conversion with an inverter is a switching element. Patent Document 1 discloses an inverter in which two types of switching elements for power conversion are connected in parallel. The two types of switching elements are a unipolar switching element and a bipolar switching element. The inverter of Patent Document 1 increases the switching speed in the entire parallel circuit of two switching elements by switching the switching element to be turned on first according to the magnitude of the current flowing in the parallel circuit.

JP 2017-228912 A

  The present specification relates to an inverter that employs parallel connection of two different types of switching elements, and provides a technology that achieves both cost and performance.

  The inverter disclosed in this specification includes a first switching element for power conversion made of silicon carbide, a second switching element for power conversion made of silicon, and a controller. The first switching element and the second switching element are connected in parallel. The controller drives the second switching element when the motor is locked, and drives the first switching element when the motor is not locked.

  A switching element made of silicon carbide has a wider band gap than a switching element made of silicon, and has higher performance than a switching element made of silicon. In particular, the switching element made of silicon carbide has a smaller loss than the switching element made of silicon. On the other hand, the switching element made of silicon carbide is more expensive than the switching element made of silicon. In particular, when seeking high heat resistance (high heat capacity), not only the cost of the switching element itself increases, but also the cost increases due to the increase in size. On the other hand, the cost of a silicon switching element is lower than that of a silicon carbide switching element. On the other hand, when the motor is locked, only a specific phase among the three-phase outputs of the inverter continues to have a large current value, and the load concentrates on the switching element of that phase. Therefore, in the inverter disclosed in this specification, a silicon switching element (second switching element) is used when the motor is locked (high load state), and silicon carbide switching is used when the motor is not locked (normal load state). An element (first switching element) is used. By limiting the use conditions so that the silicon carbide switching element is not used at a special high load, the heat resistance requirement for the silicon carbide switching element can be lowered. As a result, high performance can be obtained during normal driving while suppressing costs. Note that the technology disclosed in this specification does not exclude further expansion of the conditions for using silicon switching elements (second switching elements).

  Details and further improvements of the technology disclosed in this specification will be described in the following “DETAILED DESCRIPTION”.

It is a block diagram of the electric power system of the electric vehicle containing the inverter of an Example. It is a block diagram of a controller. It is a waveform graph of an example of the output current at the time of a motor lock. It is a flowchart of the switching element selection process of a controller.

  An inverter according to an embodiment will be described with reference to the drawings. The inverter of an Example is mounted in the electric vehicle. FIG. 1 shows a block diagram of a power system of an electric vehicle 100 including an inverter 10. The electric vehicle 100 travels by driving the traveling motor 102 with the electric power of the battery 101. The output shaft 103 of the motor 102 is connected to the drive wheel 104 via a differential gear. The output voltage of the battery 101 exceeds 100 volts.

  The inverter 10 according to the embodiment converts the DC power of the battery 101 into AC power for driving the motor 102. The inverter 10 includes six switching circuits 7a-7f and a controller 6. The switching circuit 7 a includes a first switching element 2, a second switching element 3, and a diode 4. The first switching element 2 and the second switching element 3 are connected in parallel. The diode 4 is connected in antiparallel to the switching elements 2 and 3. The switching circuits 7b-7f have the same configuration as the switching circuit 7a. The switching circuits 7a and 7b are connected in series. Switching circuits 7c and 7d are also connected in series. Switching circuits 7e and 7f are also connected in series. Three sets of series connections are connected in parallel. The switching elements 2 and 3 of the switching circuits 7a to 7f are driven by the controller 6. When the controller 6 appropriately turns on or off the switching element 2 or the switching element 3 of the switching circuits 7a-7f, an alternating current is output from the midpoint of the series connection of the switching circuits. Switching elements 2, 3 are switching elements for power conversion.

  Each of the output three-phase alternating current is measured by the current sensor 5. The measured current value is fed back to the controller 6. A rotation speed sensor 105 is attached to the axle, and the rotation speed of the axle (rotation speed of the motor) is also fed back to the controller 6. The controller 6 receives a target output command (motor output command value) of the motor 102 from a host controller (not shown), and uses the measured values of the current sensor 5 and the rotation speed sensor 105 so as to realize the target output. To control.

  FIG. 2 shows the relationship between the internal structure of the controller 6 and the switching elements 2 and 3. The controller 6 includes a memory 11 that stores a program, a processor 12 that is a main body of the controller, and a gate driving circuit that converts a TTL level signal output from the processor 12 into a signal for driving the gates of the switching elements 2 and 3. 13 (gate driver). In FIG. 2, only one gate drive circuit 13 and switching elements 2 and 3 are shown, but the controller 6 includes six gate drive circuits 13, each of which is a switching circuit 7a-7f. Are connected to the respective switching elements 2 and 3. As described above, the controller 6 receives a motor output command value from a host controller (not shown). On the other hand, the controller 6 receives the measured values of the three-phase alternating current and the axle rotational speed from the current sensor 5 and the rotational speed sensor 105. The controller 6 generates a driving signal for the switching element 2 or the switching element 3 based on the motor output command value and the measured current value / rotation speed. The drive signal output from the controller 6 is a PWM signal (Pulse Width Modulation signal) that turns on or off the switching element 2 or the switching element 3 at an appropriate period. As will be described next, the controller 6 switches between the first switching element 2 and the second switching element 3 based on a predetermined condition, and drives one of them. Specifically, the processor 12 of the controller 6 determines which switching element to use, and transmits an element selection signal indicating the switching element to be driven to the gate drive circuit 13. The gate drive circuit 13 converts the PWM signal received from the processor 12 into a voltage suitable for gate drive and supplies it to the commanded switching element.

  The first switching element 2 is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) made of silicon carbide (SiC). The second switching element 3 is an IGBT (Insulated Gate Bipolar Transistor) made of silicon (Si). Silicon carbide has a wider band gap than silicon and is called a wide band gap semiconductor. Switching elements made of silicon carbide have the advantage of lower losses than switching elements made of silicon. On the other hand, a switching element made of silicon carbide is more expensive than a switching element made of silicon. As described above, the voltage handled by the inverter 10 is 100 volts or more, and when the output of the motor 102 is large, the load of the switching element is also large. If an attempt is made to increase the heat resistance of the switching element made of silicon carbide, not only the cost of the element itself increases, but also the cost increases due to an increase in size. The cost of silicon switching elements is not as high as that of silicon carbide switching elements, and the increase in cost due to increased heat resistance can be kept lower than that of silicon carbide switching elements. Therefore, the controller 6 drives the motor 102 using a silicon switching element (that is, the second switching element 3) when the load of the inverter 10 is high, and switches the silicon carbide switching element when the load is not high. (That is, the first switching element 2) is used. By properly using two different switching elements as described above, high performance (low loss) can be obtained during normal driving while suppressing costs.

  A typical case where the load is high is when the motor 102 is locked. The fact that the motor 102 is locked means a state in which the motor 102 does not rotate even though power is supplied to the motor 102. A typical motor lock state can occur when the accelerator is stepped on while the drive wheel is in contact with the wheel stop. FIG. 3 shows a waveform graph of the output current of the inverter 10 when the motor 102 is locked. The horizontal axis in FIG. 3 is time, and the vertical axis is the current value. A motor lock has occurred at time T1. Each of the three-phase alternating currents shows a sine wave until time T1, but after time T1, even if the switching element is driven, the rotor of the motor 102 does not rotate, so the output current of the inverter 10 is constant. As illustrated in FIG. 3, after the time T1, the output current of each phase of the three phases is maintained at the current value at the time T1. In the example of FIG. 3, the current value of the U phase (solid line) is the largest. The load of the switching element outputting the U phase is larger than that in the normal state. Whether or not the motor 102 is locked can be determined from the measured value of the current sensor 5 or the measured value of the rotation speed sensor 105.

  The controller 6 stops the first switching element 2 and drives the second switching element 3 when the motor 102 is locked. When the motor 102 is not locked, the controller 6 drives the first switching element 2 and stops the second switching element 3. The controller 6 also stops the first switching element 2 and uses the second switching element even when the output command value of the motor exceeds a predetermined output threshold value. A flowchart of the switching element selection process executed by the controller 6 is shown in FIG. The process of FIG. 4 is repeatedly executed at predetermined time intervals. The controller 6 selects the 2nd switching element 3 as a switching element to drive, when the output command value given from a high-order controller is larger than a predetermined output threshold value (step S2: NO, S5). On the other hand, when the output command value given from the host controller is lower than the output threshold, the controller compares the motor rotation speed with the rotation speed lower limit value (steps S2: YES, S3). A value close to zero is set as the rotation speed lower limit value. The controller 6 determines that the motor is in a locked state when the rotation speed of the motor 102 is below the rotation speed lower limit value. When the rotation speed of the motor 102 is below the rotation speed lower limit value, the controller 6 selects the second switching element as the switching element to be driven (step S3: NO, S5). On the other hand, when the rotational speed of the motor 102 exceeds the rotational speed lower limit value, the controller 6 selects the first switching element as the switching element to be driven (step S3: YES, S4).

  In the inverter 10 according to the embodiment, the first switching element 2 made of silicon carbide, which is expensive, is not used at a high load, so that the heat resistance performance requirement required for the first switching element 2 can be reduced. The lower the requirement for heat resistance, the smaller the element size. On the other hand, the loss can be suppressed by using the first switching element during normal traveling. In the inverter 10 of the embodiment, high performance (low loss) can be obtained during normal traveling while suppressing cost.

  Points to be noted regarding the technology described in the embodiments will be described. In the flowchart of FIG. 4, prior to the process of step S3, when the output command value to the motor 102 is lower than the predetermined output lower limit value, step S3 may be skipped and the process may proceed to step S4. When the output command value to the motor 102 is small, the load of the switching element is small even if the motor is locked, so the temperature of the first switching element 2 does not increase so much. In such a case, the first switching element 2 may be driven. In that case, furthermore, even if the output command value to the motor 102 is small, if the motor lock state continues beyond a predetermined period, the switching element to be driven is changed from the first switching element 2 to the second switching element. Switching to the switching element 3 is also suitable.

  Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or the drawings can achieve a plurality of objects at the same time, and has technical usefulness by achieving one of the objects.

2: First switching element 3: Second switching element 4: Diode 5: Current sensor 6: Controllers 7a-7f: Switching circuit 10: Inverter 11: Memory 12: Processor 13: Gate drive circuit 100: Electric vehicle 101: Battery 102 : Motor 103: Output shaft 104: Drive wheel 105: Speed sensor

Claims (1)

It is an inverter that converts DC power into AC power for driving a motor for traveling,
A first switching element for power conversion made of silicon carbide;
A second switching element for power conversion made of silicon, connected in parallel with the first switching element;
A controller that drives the second switching element when the motor is locked; and a controller that drives the first switching element when the motor is not locked;
Equipped with an inverter.

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