The present disclosure relates to a rotary electric machine driving device that drives and controls an alternating-current rotary electric machine.
In recent years, from the viewpoint of, for example, energy saving and reduction of environmental loads, hybrid vehicles and electric vehicles each provided with a rotary electric machine as a source of driving force have been gaining attention. Such a vehicle includes a direct-current power supply, such as a battery, which supplies power when the rotary electric machine functions as the source of driving force (motor), and stores generated power when the rotary electric machine serves as a source of power (generator). When the rotary electric machine functions as the source of driving force (motor), direct-current power supplied from the direct-current power supply is converted into alternating-current power by an inverter to drive the rotary electric machine. When the rotary electric machine functions as the generator, alternating-current power generated by the rotary electric machine is converted into direct-current power by the inverter to be stored as regenerated power in the direct-current power supply.
A smoothing capacitor for smoothing the direct-current power is provided between the direct-current power supply and the inverter to reduce fluctuation, such as pulsation, of the direct-current power. In general, the direct-current side of the inverter for driving the rotary electric machine as the source of driving force of a hybrid vehicle or an electric vehicle is at a high voltage of 200 [V] to 400 [V]. Hence, the smoothing capacitor is required to have a high voltage resistance performance against such a high voltage, and at the same time, the fluctuation due to the pulsation needs to be considered. In addition, when withstand voltages of switching elements constituting the inverter are taken into account, the smoothing capacitor is required to have a sufficient capacitance for reducing the pulse component. These requirements generally increase the cost of the smoothing capacitor, and increase the physical size thereof, thus requiring a large installation space. Furthermore, the inverter and the smoothing capacitor are often installed in an integrated manner or mutually close to each other, as parts of a rotary electric machine driving device. In particular, an in-vehicle rotary electric machine driving device is required to be light in weight and small in size from the viewpoint of weight and installation space, so that the inverter and the smoothing capacitor are desired to be light in weight and small in size.
For example, Japanese Patent Application Publication No. 2009-106046 (JP 2009-106046 A) describes a space-saving rotary electric machine driving device (power converter) including a cooling mechanism. In this rotary electric machine driving device, a power module including switching elements is disposed on a flat surface inside of a case having a heat radiating portion. A smoothing capacitor electrically connected to the power module is disposed adjacent to the power module on a flat surface formed one step lower than the flat surface on which the power module is disposed (refer to paragraphs 7 and 8, FIG. 1, etc.). The smoothing capacitor required to have a high withstand voltage and large capacity tends to have a large physical size. In JP 2009-106046 A, the heat radiating portion and the power module are disposed in accordance with the height of the smoothing capacitor such that the overall height of the rotary electric machine driving device is reduced, thereby saving space.
In this manner, space saving can be achieved to some extent by making improvements in the layout of, for example, the circuit portion (power module), the cooling mechanism, and the smoothing capacitor of the inverter. However, reduction of weight and size of the entire device has not been accomplished by reducing the physical size of the smoothing capacitor to be determined by the withstand voltage and the capacitance. If the capacity of the smoothing capacitor is reduced, the size can be reduced, but the pulse component described above cannot be sufficiently reduced, causing a possible degradation in effects of reducing deterioration of the direct-current power supply and the switching elements.
In general, the direct-current power supply includes a resistance component (R component) and an induction component (L component). Hence, an RLC circuit is formed in conjunction with the capacity component (C component) of the smoothing capacitor. This leads to the following: when a direct-current side voltage gain is obtained by dividing a system voltage that is the voltage on the direct-current side of the inverter by a system current as a current on the direct-current side of the inverter, and a direct-current side current gain is obtained by dividing a power supply current that is a current flowing through the direct-current power supply by the system current, the frequency characteristic of each of the direct-current side voltage gain and the direct-current side current gain has a resonance point. The value of each of the gains increases as a switching frequency increases from zero, reaches the maximum value (resonance point) at a resonance frequency, and decreases as the switching frequency increases from the resonance point serving as an inflection point. The pulsations of the system voltage and the power supply current occur in association with fluctuation in voltage and current associated with the switching of the switching elements of the inverter. Specifically, the pulsations of the system voltage and the power supply current occur corresponding to the switching frequency of the switching elements. For example, it is known that pulsations having a frequency component twice the switching frequency occur. In addition, the pulsations also have harmonic components (in particular, a sixth-order harmonic component and harmonic components of integer multiple orders thereof) generated corresponding to the rotating speed of the rotary electric machine, so that both pulsations need to be considered. The amounts (amplitudes) of the pulsations increase corresponding to the value of the direct-current side voltage gain as an indicator and the value of the direct-current side current gain as another indicator. Hence, to keep the pulsations of the system voltage and the power supply current low, it is preferable to prevent the values of the direct-current side voltage gain and the direct-current side current from excessively increasing. In particular, when the capacity of the smoothing capacitor is reduced to reduce the size thereof, sharp resonance peaks occur. Hence, the switching frequency is preferably kept from coming close to the resonance frequency to keep the values of the respective gains at low levels.