Power converter for reducing a difference between reference potentials of semiconductor devices that are simultaneously turned on and off

A power converter is provided with semiconductor devices, a capacitor, a positive bus bar, and a negative bus bar. The negative bus bar includes a negative side body and a plurality of negative side branches. The negative side branches include an interposed negative side branch interposed between two positive side branches connected to the upper arm semiconductor devices that belong to the same semiconductor device group as the lower arm semiconductor devices connected to the negative side branches, and include an end negative side branch that is not interposed between two positive side branches. The self-inductance of the end negative side branch is smaller than the self-inductance of the interposed negative side branch.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the U.S. national phase of International Application No. PCT/JP2016/082866 filed Nov. 4, 2016 which designated the U.S. and claims priority to Japanese Patent Application No. 2015-254463 filed Dec. 25, 2015, the description of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a power converter including a plurality of semiconductor devices, a smoothing capacitor, and positive and negative bus bars electrically connecting them.

BACKGROUND ART

Some known power converters such as inverters and DC-DC converters include semiconductor devices such as IGBTs, a smoothing capacitor, and positive and negative bus bars electrically connecting them (refer to PTL 1 below).

The positive bus bar includes a positive side body connected to the capacitor and a plurality of positive side branches extending from the positive side body and connected to semiconductor devices. The negative bus bar includes a negative side body connected to the capacitor and a plurality of negative side branches extending from the negative side body and connected to semiconductor devices. The positive side branches and the negative side branches are arranged alternately. Therefore, the positive side branches and the negative side branches are close to each other, which reduces the parasitic inductances of the branches.

Power converters that have been recently developed can simultaneously turn on and off a plurality of parallel-connected semiconductor devices so as to produce a high output current as a whole even when only a low current flows through the individual semiconductor devices.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

However, the inventors have found through their detailed study that the following problem occurs if positive side branches and negative side branches are arranged alternately and if a plurality of semiconductor devices is simultaneously turned on and off.

Specifically, mutual and self-inductances are parasitic in each negative side branch. If positive side branches and negative side branches are arranged alternately and if a plurality of semiconductor devices connected to the branches are simultaneously turned on and off, effective inductances, i.e., the sum of mutual and self-inductances, can differ greatly between the negative side branches, as described later. Therefore, induced electromotive forces generated due to the effective inductances when current flows can differ greatly between the negative side branches. Thus, the electrical potentials (that is, reference potentials) of the reference electrodes such as emitters can differ greatly between the plurality of semiconductor devices that are electrically connected to the negative side branches and simultaneously turned on and off. Therefore, as described later, high voltages may be locally applied to the control terminals of some semiconductor devices.

An object of the present disclosure is to provide a power converter capable of reducing the difference between the reference potentials of a plurality of semiconductor devices that is simultaneously turned on and off.

A first aspect of the present disclosure is a power converter including: semiconductor devices including a plurality of upper arm semiconductor devices and a plurality of lower arm semiconductor devices connected in series;

freewheeling diodes connected in inverse parallel with respective semiconductor devices;

a capacitor that smooths a DC voltage;

a positive bus bar having a positive side body electrically connected to the capacitor and a plurality of positive side branches extending from the positive side body and electrically connected to the upper arm semiconductor devices; and

a negative bus bar having a negative side body electrically connected to the capacitor and a plurality of negative side branches extending from the negative side body and electrically connected to the lower arm semiconductor devices, wherein

the positive side branches and the negative side branches are arranged alternately,

two or more of the upper arm semiconductor devices that are simultaneously turned on and off and two or more of the lower arm semiconductor devices that are connected in series with the upper arm semiconductor devices and simultaneously turned on and off constitute a semiconductor device group,

the negative side branches include an interposed negative side branch interposed between two of the positive side branches connected to the upper arm semiconductor devices that belong to the same semiconductor device group as the lower arm semiconductor devices connected to the negative side branches, and include an end negative side branch that is not interposed between two positive side branches, and a self-inductance of the end negative side branch is smaller than a self-inductance of the interposed negative side branch.

Effect of the Invention

In the power converter, the positive side branches and the negative side branches are arranged alternately, and a plurality of semiconductor devices that are connected to these branches and simultaneously turned on and off constitutes a semiconductor device group. The self-inductance of the end negative side branch of the plurality of negative side branches is smaller than that of the interposed negative side branch.

Therefore, the difference between the reference potentials of the plurality of semiconductor devices that is simultaneously turned on and off can be reduced. Specifically, since the end negative side branch is not interposed between two positive side branches, as described later, the end negative side branch is likely to have a larger mutual inductance than the interposed negative side branch interposed between two positive side branches. In the present aspect, the end negative side branch that is likely to have a large mutual inductance is configured to have a smaller self-inductance than the interposed negative side branch. Therefore, the difference between the effective inductances (i.e., the sum of mutual and self-inductances) of the interposed negative side branch and the end negative side branch can be reduced. Therefore, induced electromotive forces generated due to the effective inductances when current flows do not differ greatly between the interposed negative side branch and the end negative side branch. Thus, the difference between the reference potential of the semiconductor device electrically connected to the interposed negative side branch and the reference potential of the semiconductor device electrically connected to the end negative side branch can be reduced. In other words, the difference between the reference potentials of the plurality of semiconductor devices that is connected to these negative side branches and simultaneously turned on and off can be reduced. Therefore, it is possible to prevent the defect of local application of high voltages to the control terminals of some semiconductor devices.

As described above, the above aspect can provide a power converter capable of reducing the difference between the reference potentials of a plurality of semiconductor devices that is simultaneously turned on and off.

DESCRIPTION OF EMBODIMENTS

The power converters can be in-vehicle power converters to be mounted on vehicles such as electric cars and hybrid vehicles.

First Embodiment

An embodiment of the power converter will be described with reference toFIGS. 1 to 11. As illustrated inFIGS. 1 and 2, the power converter1according to the present embodiment includes an upper arm semiconductor device2u, a lower arm semiconductor device2d, a capacitor3, a positive bus bar4, and a negative bus bar5.

As illustrated inFIG. 7, the upper arm semiconductor device2uand the lower arm semiconductor device2dare connected in series. A plurality of upper arm semiconductor devices2uand a plurality of lower arm semiconductor devices2dare provided. A freewheeling diode7is connected in inverse parallel with each of the upper arm semiconductor devices2uand the lower arm semiconductor devices2d.

The capacitor3smooths a DC voltage of a DC power source8.

As illustrated inFIGS. 2 and 3, the positive bus bar4includes a positive side body40and a plurality of positive side branches41. The positive side body40is electrically connected to the capacitor3. Each of the positive side branches41extends from the positive side body40to be electrically connected to the corresponding one of the upper arm semiconductor devices2u.

The negative bus bar5includes a negative side body50and a plurality of negative side branches51. The negative side body50is electrically connected to the capacitor3. Each of the negative side branches51extends from the negative side body50to be electrically connected to the corresponding one of the lower arm semiconductor devices2d.

As illustrated inFIG. 2, the positive side branches41and the negative side branches51are arranged alternately. As illustrated inFIGS. 2 and 7, a plurality of upper arm semiconductor devices2uthat is simultaneously turned on and off and a plurality of lower arm semiconductor devices2dthat is connected in series with the upper arm semiconductor devices2uand simultaneously turned on and off constitute a semiconductor device group20, and a plurality of semiconductor device groups20are formed.

As illustrated inFIG. 2, the negative side branches51include an interposed negative side branch51iinterposed between two positive side branches41connected to the upper arm semiconductor devices2uthat belong to the same semiconductor device group20as the lower arm semiconductor devices2dconnected to the negative side branches51, and include an end negative side branch51ethat is not interposed between two positive side branches.

The self-inductance of the end negative side branch51eis set to be smaller than the self-inductance of the interposed negative side branch51i.

The power converter1according to the present embodiment is an in-vehicle power converter to be mounted on a vehicle such as an electric car and a hybrid vehicle.

As illustrated inFIG. 7, the power converter1is connected to the DC power source8and a three-phase AC motor81. The power converter1is configured to turn on and off the individual semiconductor devices2to convert DC power supplied from the DC power source8into AC power. The power converter1then drives the three-phase AC motor81using the obtained AC power to operate the vehicle.

One upper arm semiconductor device2uand one lower arm semiconductor device2dare sealed in a body60(refer toFIG. 3) of a semiconductor module6. As described above, in the present embodiment, a plurality of semiconductor devices2are simultaneously turned on and off. Consequently, the power converter1produces a high output current as a whole even when only a low current flows through the individual semiconductor devices2.

The upper arm semiconductor devices2uare electrically connected to the positive side branches41(refer toFIG. 2) of the positive bus bar4. The lower arm semiconductor devices2dare electrically connected to the negative side branches51of the negative bus bar5.

As described above, in the present embodiment, the plurality of upper arm semiconductor devices2uthat are simultaneously turned on and off and the plurality of lower arm semiconductor devices2dthat are connected in series with the upper arm semiconductor devices2uand simultaneously turned on and off constitute the semiconductor device group20. Semiconductor device groups20include a U-phase semiconductor device group20U, a V-phase semiconductor device group20V, and a W-phase semiconductor device group20W.

Next, the reason why the mutual inductance of the interposed negative side branch51iis smaller than that of the end negative side branch51ewill be described. Suppose all the semiconductor devices2(2uand2d) constituting the semiconductor device group20are off as illustrated inFIG. 8, and then the upper arm semiconductor devices2uare turned on as illustrated inFIG. 9. As illustrated inFIG. 8, while the upper arm semiconductor devices2uare off, freewheeling current i flows through the freewheeling diodes7connected in inverse parallel with the lower arm semiconductor devices2dby the action of the three-phase AC motor81that is an inductive load. Freewheeling current i flows through the negative side branches51. After that, the upper arm semiconductor devices2uare turned on as illustrated inFIG. 9, and current i flowing through the upper arm semiconductor devices2ugradually increases. Specifically, current i flowing through the positive side branches41gradually increases. Meanwhile, since the freewheeling diodes7of the lower arm semiconductor devices2dare reversely biased, freewheeling current i gradually decreases.

FIG. 10is a diagram illustrating temporal fluctuations of currents i flowing through the positive side branches41and the negative side branches51. As illustrated in this figure, the upper arm semiconductor devices2uare turned on at time t1, and current i flowing through the positive side branches41gradually increases. Meanwhile, current i (namely, freewheeling current) flowing through the negative side branches51gradually decreases. At time t2, current i flowing through the negative side branches51reaches zero. After that, until time t3, a recovery current flows through the freewheeling diodes7, and the recovery current flows into the negative side branches51.

As illustrated inFIG. 10, the temporal fluctuation rate di/dt of current i flowing through the positive side branches41and the temporal fluctuation rate di/dt of current i flowing through the negative side branches51are opposite to each other. Therefore, the interposed negative side branch51i(refer toFIG. 2) interposed between two positive side branches41is more easily magnetically coupled to the positive side branches41than the end negative side branch51ethat is only adjacent to a single positive side branch41is. This is why the mutual inductance of the interposed negative side branch51iis smaller than that of the end negative side branch51e.

Next, the structure of the negative bus bar5will be described. As illustrated inFIG. 6, the negative bus bar includes the negative side body50and the plurality of negative side branches51extending from the negative side body50. The negative side body50is provided with comb teeth501. The negative side branches51extend from the comb teeth501. The end negative side branch51eis thicker than the interposed negative side branch51i. The end negative side branch51ecan be formed, for example, by welding a metal plate thicker than the interposed negative side branch51ito the comb tooth501. The interposed negative side branch51iincludes the same plate-like member as the negative side body50. Specifically, the negative side body50and the interposed negative side branch51iare formed by bending a single plate-like member.

As illustrated inFIGS. 2 and 3, the structure of the positive bus bar4is similar to that of the negative bus bar5. The positive bus bar4includes the plate-like positive side body40and the plurality of positive side branches41extending from the positive side body40. In the present embodiment, all the positive side branches41are equal in thickness.

As illustrated inFIGS. 2 and 3, the positive side body40and the negative side body50overlap each other. An extending direction (hereinafter also referred to as the Y direction) in which the negative side branches51extend from the negative side body50coincides with the direction in which the positive side branches41extend from the positive side body40. The positive side branches41and the negative side branches51are arrayed in an array direction (hereinafter also referred to as the X direction) orthogonal to the Y direction. As illustrated inFIG. 3, the positive side branches41and the negative side branches51overlap each other when viewed in the X direction.

The positive bus bar4and the negative bus bar5are connected to the capacitor3. The capacitor3includes a capacitor element30and a sealing member31that seals the capacitor element30.

As illustrated inFIG. 3, the semiconductor module6includes the body60that seals the semiconductor devices2, power terminals61projecting from the body60, and control terminals62. The power terminals61include a positive terminal61pconnected to the positive bus bar4, a negative terminal61nconnected to the negative bus bar5, and an AC terminal61aconnected to the three-phase AC motor81(refer toFIG. 7). The control terminals62are connected to a control circuit board18. The on/off operation of the semiconductor devices2is controlled by the control circuit board18.

As illustrated inFIG. 5, in the present embodiment, a plurality of semiconductor modules6and a plurality of cooling pipes11that cool the semiconductor modules6are alternately stacked to constitute a stacked body10. Two adjacent cooling pipes11are coupled by two coupling pipes15. The coupling pipes15are provided at the Y-directional opposite ends of the cooling pipes11.

An end cooling pipe11aof the plurality of cooling pipes11located at one end in the array direction of the positive side branches41and the negative side branches (namely, X direction: refer toFIG. 2) is equipped with an inlet pipe13for introducing a refrigerant12and an outlet pipe14for discharging the refrigerant12. Once the refrigerant12is introduced through the inlet pipe13, the refrigerant12passes through the coupling pipes15and flows through all the cooling pipes11to be discharged through the outlet pipe14. The semiconductor modules6are configured to be cooled in this manner.

A pressurizing member16(e.g., plate spring) is arranged between a first wall171of a case17and the stacked body10. The pressurizing member16presses the stacked body10against a second wall172of the case17. Consequently, contact pressure between the cooling pipes11and the semiconductor modules6is secured, and the stacked body10is fixed inside the case17.

Below is the reason why a great difference between the effective inductances of the end negative side branch51eand the interposed negative side branch51ican cause a great difference between the reference potentials of the lower arm semiconductor devices2dconnected to the negative side branches51eand51i.FIG. 21is a partial circuit of a conventional power converter. As illustrated in this figure, conventionally, the effective inductance Leof the end negative side branch51eis larger than the effective inductance Liof the interposed negative side branch51i. Specifically, since the mutual inductance of the end negative side branch51eis larger than that of the interposed negative side branch51iwhereas the self-inductances of the negative side branches51eand51iare almost the same, the effective inductance of the end negative side branch51eis larger than that of the interposed negative side branch51i. Gate terminals of the two lower arm semiconductor devices2deand2diare connected on the control circuit board18. The control circuit board18is provided with a line181connecting emitter terminals Eeand Eiof the two lower arm semiconductor devices2deand2di. A power circuit180is provided between the line181and the gate terminals.

Suppose the two lower arm semiconductor devices2deand2diare simultaneously turned on. Since the effective inductance Leof the end negative side branch51eis large as described above, a relatively large induced electromotive force Ve(=Ledi/dt) is generated by the effective inductance Leat the time that current i starts to flow. In contrast, since the effective inductance Liof the interposed negative side branch51iis small, a small induced electromotive force Vi(=Lidi/dt) is generated. Therefore, the electric potential of the emitter terminal Eeconnected to the end negative side branch51eis higher than the electric potential of the emitter terminal Eiconnected to the interposed negative side branch51i.

As described above, when the electrical potentials (that is, reference potentials) of the emitter terminals (that is, reference electrodes) of the lower arm semiconductor device2deconnected to the end negative side branch51eand the lower arm semiconductor device2diconnected to the interposed negative side branch51iare greatly different, a high voltage may be locally applied between the emitter and gate terminals of the semiconductor device2di.

In contrast, the difference between the effective inductances L of the two negative side branches51eand51ican be reduced if the self-inductance of the end negative side branch51eis reduced as in the present embodiment. Therefore, it is possible to prevent the lower arm semiconductor device2deconnected to the end negative side branch51eand the lower arm semiconductor device2diconnected to the interposed negative side branch51ifrom differing greatly in reference potential.

In a case where only a single semiconductor device2is incorporated in a single semiconductor module6(refer toFIG. 23), a great difference between the effective inductances L of the negative side branches51eand51ihardly causes a great difference between the reference potentials of the two upper arm semiconductor devices2ueand2ui. However, in a case where one upper arm semiconductor device2uand one lower arm semiconductor device2dare incorporated in a single semiconductor module6as in the present embodiment, a great difference between the effective inductances L of the negative side branches51eand51ican cause a great difference between the reference potentials of the two upper arm semiconductor devices2ueand2ui. The reason is as follows. As illustrated inFIG. 23, in a case where only a single semiconductor device2is incorporated in a single semiconductor module6, the emitter terminals of the upper arm semiconductor devices2uand the collector terminals of the lower arm semiconductor devices2dare connected by an AC bus bar19. The AC bus bar19includes an upper arm191, a lower arm192, and a single connector190connecting these arms. The upper arm191connects the plurality of upper arm semiconductor devices2u. The lower arm192connects the plurality of lower arm semiconductor devices2d.

Suppose different induced electromotive forces Veand Viare generated in a certain moment respectively at the two negative side branches51eand51idue to the difference between the effective inductances L (Leand Li) of the negative side branches51eand51i. At this time, since a forward voltage is applied to the freewheeling diodes7, the electrical potentials of the lower arm semiconductor devices2deand2diare approximately Veand Vi, respectively. However, since the two lower arm semiconductor devices2deand2diare connected to the two upper arm semiconductor devices2ueand2uivia the single connector190, the electrical potentials (namely, reference potentials) of the emitters of the two upper arm semiconductor devices2ueand2uiare the median value Vmof Veand Vi, which means that the two reference potentials are almost equal.

In contrast, in a case where one upper arm semiconductor device2uand one lower arm semiconductor device2dare incorporated in a single semiconductor module6as illustrated inFIG. 22, a great difference between the effective inductances L of the two negative side branches51eand51ican cause a great difference between the reference potentials of the two upper arm semiconductor devices2ueand2ui. Specifically, in this case, in each semiconductor module6, the upper arm semiconductor device2uand the lower arm semiconductor device2dare individually connected by a conductive member69inside the semiconductor module6. In other words, the two lower arm semiconductor devices2deand2diare separately connected to the upper arm semiconductor devices2ueand2ui. Therefore, if different induced electromotive forces Veand Viare generated at the negative side branches51eand51idue to the difference between the effective inductances L (Leand Li), the freewheeling diodes7conduct, and the electrical potentials of the emitters of the two upper arm semiconductor devices2ueand2uihave different values Veand Vi.

Therefore, in a case where one upper arm semiconductor device2uand one lower arm semiconductor device2dare incorporated in a single semiconductor module6as in the present embodiment, a reduction in the difference between the effective inductances L (Leand Li) of the negative side branches51eand51ican reduce the difference between the induced electromotive forces Veand Vigenerated at the negative side branches51eand51ias illustrated inFIG. 11. Thus, the difference between the reference potentials Veand Viof the two upper arm semiconductor devices2ueand2uican be reduced.

Next, the effects of the present embodiment will be described. In the present embodiment, as illustrated inFIG. 2, the positive side branches41and the negative side branches51are arranged alternately, and a plurality of semiconductor devices2that is connected to these branches and simultaneously turned on and off constitutes the semiconductor device group20. The self-inductance of the end negative side branch51eof the plurality of negative side branches51is smaller than that of the interposed negative side branch51i.

Therefore, the difference between the reference potentials of the plurality of lower arm semiconductor devices2dthat is simultaneously turned on and off can be reduced. Specifically, since the end negative side branch51eis not interposed between two positive side branches41, as described above, the end negative side branch51eis likely to have a larger mutual inductance than the interposed negative side branch51iinterposed between two positive side branches41. In the present embodiment, the end negative side branch51ethat is likely to have a large mutual inductance is configured to have a smaller self-inductance than the interposed negative side branch51ihas. Therefore, the difference between the effective inductances L (i.e., the sum of mutual and self-inductances) of the interposed negative side branch51iand the end negative side branch51ecan be reduced. Therefore, as illustrated inFIG. 11, induced electromotive forces generated due to the effective inductances L when current flows do not differ greatly between the interposed negative side branch51iand the end negative side branch51e. Thus, the difference between the reference potential of the lower arm semiconductor device2diconnected to the interposed negative side branch51iand the reference potential of the lower arm semiconductor device2deconnected to the end negative side branch51ecan be reduced. In other words, the difference between the reference potentials of the plurality of lower arm semiconductor devices2diand2dethat are connected to these negative side branches51iand51eand simultaneously turned on and off can be reduced. Therefore, it is possible to prevent the problem of local application of high voltages to the control terminals of some lower arm semiconductor devices2dfrom occurring.

In the present embodiment, as illustrated inFIG. 1, the semiconductor modules6and the cooling pipes11are stacked to constitute the stacked body10.

In this case, the positive side branches41connected to the positive terminals61pof the semiconductor modules6and the negative side branches51connected to the negative terminals61nare alternately arrayed. This configuration can form the interposed negative side branch51isandwiched between two positive side branches41connected to the upper arm semiconductor devices2uthat belong to the same semiconductor device group20and having a relatively small mutual inductance, and form the end negative side branch51ethat is not sandwiched between two positive side branches41and having a relatively large mutual inductance. Therefore, in the present embodiment, reducing the self-inductance of the end negative side branch51eto reduce the difference between the effective parasitic inductances L of the two negative side branches51eand51ibrings about a significant effect.

In the present embodiment, as illustrated inFIGS. 2 and 11, one upper arm semiconductor device2uand one lower arm semiconductor device2dare incorporated in the single semiconductor module6.

In this case, as described above, a great difference between the effective parasitic inductances L of the two negative side branches51eand51ican cause a great difference between the reference potentials of the two upper arm semiconductor devices2ueand2ui(refer toFIG. 22). In the present embodiment, however, since the difference between the effective parasitic inductances L of the two negative side branches51eand51ican be reduced, the difference between the reference potentials of the two upper arm semiconductor devices2ueand2uican be reduced as illustrated inFIG. 11.

In the present embodiment, as illustrated inFIG. 2, the X-directional thickness of the end negative side branch51eis larger than the X-directional thickness of the interposed negative side branch51i. Consequently, the self-inductance of the end negative side branch51eis reduced.

Therefore, the self-inductance of the end negative side branch51ecan be reliably reduced, and the difference between the effective inductances of the two types of negative side branches51iand51ecan be reliably reduced.

As described above, the present embodiment can provide a power converter capable of reducing the difference between the reference potentials of a plurality of semiconductor devices that are simultaneously turned on and off.

In the present embodiment, IGBTs are used as the semiconductor devices2. However, the present invention is not limited to this example, but MOSFETs or bipolar transistors may be used, for example. In a case where MOSFETs are used as the semiconductor devices2, the electrical potentials of source electrodes are regarded as reference potentials. In a case where bipolar transistors are used, the electrical potentials of emitter electrodes are regarded as reference potentials. SiC or GaN can also be used as a semiconductor material.

In the present embodiment, a single semiconductor device group20includes two upper arm semiconductor devices2uthat are simultaneously turned on and off and two lower arm semiconductor devices2dthat are simultaneously turned on and off. However, the present invention is not limited to this example. Specifically, a single semiconductor device group20may include three or more upper arm semiconductor devices2uthat are simultaneously turned on and off and three or more lower arm semiconductor devices2dthat are simultaneously turned on and off.

In the drawings for the following embodiments, reference signs identical to those used in the first embodiment represent components or the like similar to those of the first embodiment, unless otherwise specified.

Second Embodiment

In the present embodiment, a shape of a negative bus bar5is changed. As illustrated inFIG. 12, in the present embodiment, the Y-directional length of the end negative side branch51eis shorter than the Y-directional length of an interposed negative side branch51i. Consequently, the length of the current path of the end negative side branch51eis reduced, and the end negative side branch51ehas a smaller self-inductance than the interposed negative side branch51i.

In contrast to the configuration of the first embodiment, the above configuration eliminates the need to form a thick end negative side branch51e, and thus can facilitate the manufacture of the negative bus bar5.

Other configurations and effects of the present embodiment are similar to those of the first embodiment.

Third Embodiment

In the present embodiment, a shape of a negative bus bar5is changed. As illustrated inFIGS. 13 and 14, in the present embodiment, a through hole58is formed in a interposed negative side branch51i. Consequently, a region580having a locally high current density is formed at the interposed negative side branch51ito increase the self-inductance of the interposed negative side branch51i. As a result, an end negative side branch51ehas a smaller self-inductance than the interposed negative side branch51i.

In contrast to the configuration of the first embodiment, the above configuration eliminates the need to form a thick end negative side branch51e, and thus can facilitate the manufacture of the negative bus bar5.

Other configurations and effects of the present embodiment are similar to those of the first embodiment.

Fourth Embodiment

In the present embodiment, a shape of a negative bus bar5is changed. As illustrated inFIG. 15, in the present embodiment, a recess59is formed in an interposed negative side branch51i. Consequently, a region590having a locally high current density is formed at the interposed negative side branch51ito increase the self-inductance of the interposed negative side branch51i. As a result, an end negative side branch51ehas a relatively smaller self-inductance than the interposed negative side branch51i.

In contrast to the configuration of the first embodiment, the above configuration eliminates the need to form a thick end negative side branch51e, and thus can facilitate the manufacture of the negative bus bar5.

Other configurations and effects of the present embodiment are similar to those of the first embodiment.

Fifth Embodiment

In the present embodiment, a structure of a negative bus bar5is changed. As illustrated inFIG. 16, in the present embodiment, an end negative side branch51eincludes a conductive material different from the material for the interposed negative side branch51i. More specifically, in the present embodiment, the end negative side branch51eincludes a material having a smaller electrical resistivity than the material for the interposed negative side branch51i. As a result, an end negative side branch51ehas a smaller self-inductance than the interposed negative side branch51i.

Other configurations and effects of the present embodiment are similar to those of the first embodiment.

Sixth Embodiment

In the present embodiment, a shape of a negative bus bar5is changed. As illustrated inFIG. 17, in the present embodiment, an end negative side branch51eis longer than an interposed negative side branch51iin an orthogonal direction (hereinafter also referred to as the Z direction) orthogonal to both the X and Y directions. As a result, the end negative side branch51ehas a smaller self-inductance than the interposed negative side branch51i.

In contrast to the configuration of the first embodiment, the above configuration eliminates the need to form a thick end negative side branch51e, and thus can facilitate the manufacture of the negative bus bar5.

Other configurations and effects of the present embodiment are similar to those of the first embodiment.

Seventh Embodiment

In the present embodiment, a shape of a positive bus bar4is changed. As illustrated inFIG. 18, positive side branches41of the positive bus bar4include an interposed positive side branch41iinterposed between two negative side branches51connected to lower arm semiconductor devices2dthat belong to the same semiconductor device group20as upper arm semiconductor devices2uconnected to the positive side branches41, and include an end positive side branch41ethat is not interposed between the two negative side branches51. In the present embodiment, an end positive side branch41eis thicker than an interposed positive side branch41i. As a result, the end positive side branch41ehas a smaller self-inductance than the interposed positive side branch41i.

Since the interposed positive side branch41iis interposed between the two negative side branches51, the mutual inductance of the interposed positive side branch41iis relatively small. Since the end positive side branch41eis not interposed between the two negative side branches51, the mutual inductance of the end positive side branch41eis relatively large. Therefore, by reducing the self-inductance of the end positive side branch41ehaving a relatively large mutual inductance, the difference between the effective inductances (i.e., the sums of mutual and self-inductances) of the interposed positive side branch41iand the end positive side branch41ecan be reduced. Thus, the semiconductor devices2can be exposed to equal surges. Therefore, it is possible to prevent the problem of local application of high surges to some lower arm semiconductor devices2and resultant reduction in the lifetime of the semiconductor devices2.

Other configurations and effects of the present embodiment are similar to those of the first embodiment.

In the present embodiment, a thick end positive side branch41eis formed so that the end positive side branch41ehas a smaller self-inductance than the interposed positive side branch41i. However, the present invention is not limited to this example. Specifically, the end positive side branch41emay have a shorter Y-directional length than the interposed positive side branch41i, a through hole or recess may be formed in the interposed positive side branch41i, or the end positive side branch41emay include a material having a smaller electrical resistivity than the material for the interposed positive side branch41i. The end positive side branch41emay be longer than the interposed positive side branch41iin the Z direction.

Eighth Embodiment

In the present embodiment, the number of semiconductor device groups20is changed. As illustrated inFIG. 19, the present embodiment includes only a single semiconductor device group20. The semiconductor device group20constitutes a boosting circuit. A reactor88is connected to the semiconductor device group20. In the present embodiment, lower arm semiconductor devices2dare turned on and off, and the voltage of a DC power source8is boosted using the reactor88. The boosted voltage is then smoothed by the capacitor3.

In the present embodiment, a difference generated between the effective parasitic inductances L of a plurality of negative side branches51can cause a great difference between the reference potentials of the lower arm semiconductor devices2das in the first embodiment. Therefore, in the present embodiment, the difference between the effective inductances L of the plurality of negative side branches51is reduced. Specifically, in the present embodiment, positive side branches41and the negative side branches51are arranged alternately as illustrated inFIG. 20. The negative side branches51include an interposed negative side branch51iinterposed between two positive side branches41and an end negative side branch51ethat is not interposed between two positive side branches41. The thickness of the end negative side branch51eis larger than the thickness of the interposed negative side branch51i. Consequently, the self-inductance of the end negative side branch51eis reduced, and the difference between the effective parasitic inductances of the two negative side branches51eand51iis reduced. As a result, the difference between the reference potentials of the lower arm semiconductor devices2delectrically connected to the negative side branches51eand51iis reduced.

Other configurations and effects of the present embodiment are similar to those of the first embodiment.

The present invention is not limited to the above embodiments, but the embodiments can be combined with one another. For example, the end negative side branch51emay be thicker than the interposed negative side branch51iin the X direction, and the end positive side branch41emay be longer than the interposed positive side branch41iin the Z direction.

While the present disclosure has been described with reference to examples, it is to be understood that the present disclosure is not limited to the examples and structures. The present disclosure covers various modifications and equivalent variations. In addition to various combinations and configurations, other combinations and configurations including one, or more or fewer elements thereof are also within the spirit and scope of the present disclosure.