GB2301950A

GB2301950A - AC generator for vehicle

Info

Publication number: GB2301950A
Application number: GB9611865A
Authority: GB
Inventors: Makoto Taniguchi; Atsushi Umeda; Shin Kusase
Original assignee: NipponDenso Co Ltd
Current assignee: Denso Corp
Priority date: 1995-06-06
Filing date: 1996-06-06
Publication date: 1996-12-18
Also published as: GB9611865D0; JPH08336268A

Abstract

A vehicular AC generator is provided with a three-phase full-wave rectifier (19) held on a wall portion of a generator housing (1) other than the rear end wall of the housing. The rectifier comprises SiC-MOS transistors (19a-19f) attached to the circumferential wall (1a) or an end wall (1f in Fig. 13) on the side of a pulley of the generator housing and is isolated from radiant heat emitted by an exhaust pipe of an engine.

Description

AC GENERATOR FOR VEHICLE The present invention relates to an AC generator provided with a rectifier fixed to the housing thereof.

In a conventional vehicular AC generator, a rectifier is fixed to the housing of the AC generator because the electrically conductive housing having a large heat capacity and a high heat conductivity can be utilized as a heat sink and a grounding member (chassis ground), the short distances between the armature windings of the generator and the rectifier are advantageous to reducing loss and wiring work.

The rectifier is fixed to an end wall remote from a pulley, i.e., rear end wall. The rectifier fixed to this end wall is subject to the exhaust gases of the engine, particularly, the effect of radiant heat. Usually, the rear end wall of the generator housing is provided with a cooling air inlet opening.

When the rectifier is fixed to the rear end wall, the area of the cooling air inlet opening is reduced.

and a cooling fan is used resulting in undesirable fan noise.

With those problems in view, the assignee of the present patent application proposed a rectifier holding method that fixes a rectifier to a near end wall of the housing of a generator near a pulley (front wall) remote from the exhaust pipe of an engine in JP-A No. 5-351444, and a rectifier holding method that fixes a rectifier to an end wall of the housing of a generator remote from a pulley and disposes a radiation shield cover and a cooling fan between the rectifier and the exhaus pipe in JP-A No.

5-56604.

An invention disclosed in JP-A No. 4-138030 employs MOS power transistors as semiconductor rectifying devices for a rectifier. Usually, a MOS power transistor of this kind employs a vertical MOS power transistor construction using an N type silicon substrate as one of the principal electrodes of the MOS power transistor, and an N+ type region formed in the surface of a P-type well region formed in the surface of a chip as the other principal electrode to secure voltage withstand ability and to reduce on-state resistance.

The fcrmer rectifier holding method entails a problem that the large rectifier obstructs work for belting the pulley.

The latter rectifier holding method entails a problem that the area of the cooling air inlet opening formed ifl the rear end wall of the housing is reduced, the rectifier needs large cooling fins that obstruct the cooling performance of the cooling fan for cooling the armature windings and the field windin gs so reducing the cooling efficiency of the cooling fan, and needs a radiation shield cover.

The rectifier may be fixed to the circumferential wall of the housing. However, where as the rectifier fixed to the rear end wall is cooled with cooling air currents of a low temperature which have just been blown into the generator, the rectifier fixed to the circumferential wall of the housing cannot be exposed to a sufficient amount of cooling air currents of a sufficiently low temperature, and the rectifier fixed to the circumferential wall of the housing reduces air-cooling function remarkably. Consequently, the rectifier needs a very large cooling fins to maintain the temperature of the rectifier below the maximum allowable temperature of about 1600C and, if the rectifier is fixed to the circumferential wall of the generator, the radial dimension of the generator increases greatly.

Viewed from one aspect the invention providcs an AC generator for a vehicle including a housing disposed near an engine, armature windings and a rectifier placed on a circumferential wall of said housing and having a plurality of SiC-MOS transistors, wherein each of said SiC-MOS transistors comprises a single crystal SiC substrate having a semiconductor region of one conduction type on which a inversion channel is formed, a source region and a drain region of opposite conduction type, and said source region and said drain region are electrically connected by said inversion channel.

Viewed from another aspect the invention provides an AC generator for a vehicle including a housing disposed near an enginc, armature windings and a rectifier placed on a an end wall of the housing on the side of a pulley and having a plurality of SiC-MOS transistors, whercin each of said SiC-MOS transistors comprises a single crystal SiC substrate having a semiconductor region of one conduction type on which an inversion channel is formcd, a source region and a drain region of opposite conduction type, and said source region and said drain region are electrically connected by said inversion channel.

Various other aspects and features of the invention are sct out in the appended claims.

The present invention addresses the foregoing problems and it is therefore an object of at least preferred embodiments of the present invention to provide a vehicular AC generator that reduces the likelihood of the ovcrheating of a rectifier, reduces difficulty in belting a pulley, avoids enlarging the size of the gcnerator, and increases the flow of cooling air currents through the rear end wall thercof.

Because a large amount of magnetic energy is stored in the three-phase armature windings and the field windings of a vehicular AC generator, the breakdown voltage of the semiconductor power devices of the three-phase full-wave rectifier must be twenty times the battery voltage, i.e., the rectified output voltage of the three-phase full-wave rectifier, or above, for example, 100 V or above, to cope with troubles due to the instantaneous discharge of the large stored magnetic energy. Recent increased vehicular electrical loads, such as a heater for a defroster, require an output current of 100 A or above. A MOS power transistor having such a high breakdown voltage and a high current capacity would not reduce power loss caused by a diode and hence it makes no sense to use a MOS power transistor having a complex structure instead of a diode.

In an inverter having N channel MOS power transistors (for example, as shown in Fig. 3), the drain electrode of a high-side MOS power transistor and the source electrode of a low-side MOS power transistor are connected to the output of one phase of a three-phase AC generator, the drain electrode of the low-side MOS power transistor is connected to the low-potential terminal of a battery and the source electrode of the high-side MOS power transistor is connected to the high-potential terminal of the battery. In principle, the MOS power transistors - have a source-connected parasitic diode between the P type well region and the source electrode and a drain-connected parasitic diode between the P type well region and the drain electrode.

Usually, the P type well region of one conduction type (for example, a region 103 in Fig. 4) in which an inversion channel is formed, and a source region of the opposite conduction type connected by the inversion channel (for example, a region 106 in Fig. 4) or a drain region (for example, a region 104 in Fig. 4) are connected to apply a potential to the P type well region. When the inverter is used as one phase of the three-phase full-wave rectifier, the drain-connected parasitic diode connecting the P type well region (for example, region 103 in Fig. 4) and the drain electrode (for example, region 104 in Fig. 4) must be short-circuited.

Nevertheless, the conventional MOS power transistor needs to increase its breakdown voltage by short-circuiting the P type well region and the N type region formed in its surface and by extending a PN junction depletion layer formed between the P type well region and an N type epitaxial dielectric layer toward the N type epitaxial layer. When the vehicular AC generator comprises the MOS power transistors of the conventional structure, the Ne type substrate and the Ne type region must unavoidably be used as a source region and a drain region, respectively, which, however, entails the series connection of a large source parasitic resistance of the N type dielectric layer across a virtual source contact and the source electrode.

The drain saturation current Idsat of the MOS transistor is expressed, when threshold voltage Vt is neglected for simplicity, by: Idsat = k(Vg - Vs')2 = K(AVgs - IDSST rS) 2 ....... (1) where K is proportional constant, AVgs is gate-source voltage (Vg - Vs), Vg is gate voltage, Vs' = Vs + Idsat.Rs is the potential of the virtual source contact.

Drain saturation current (a maximum current for a predetermined gate voltage) Idsat translates into a drop of the gate voltage Vg by Idsat-Rs. Change in threshold voltage Vt due to the effect of the substrate is neglected.

Suppose that the gate voltage is +20 V, the source (battery) potential is +12 V, the current is 100 A and the source parasitic resistance Rs is 0.05 Q. Then, an actual source potential Vs is 17 V and the channel current is as low as 9/64 of that with Rs = 0; that is, a small increase in the source parasitic resistance Rs causes a great reduction in the channel current. Such a current reducing effect, i.e., the effect of increase in channel resistance, will be called a source resistance feedback effect hereinafter.

Expression (1) applies to a drain current saturation region. Similarly, drain nonsaturation current decreases in a nonsaturation region due to increase in Rs. Increase in channel resistance signifies reduction in drain current. Since increase in the source parasitic resistance Rs brings about power loss by itself and power loss due to increase in channel resistance, so that increase in the source parasitic resistance Rs causes a large power loss and heat generation.

The present invention recognises that loss and heat generation in a vehicular AC generator can significantly be reduced if the breakdown field strength of the dielectric layer can be enhanced, found from the results of analysis that presently existing silicon MOS power transistors are unsuitable for use on a vehicular AC generator, the marked reduction of the resistance of the dielectric layer is strongly advantageous to achieve ~ three-phase full-wave rectifier comprising MOS power transistors, the marked reduction of the thickness and the marked increase of the impurity concentration of the dielectric layer are strongly advantageous, and the marked reduction of the thickness of the dielectric layer and the marked increase of the impurity concentration of the same can be achieved through the marked enhancement of the breakdown field strength of the dielectric layer.

The breakdown field strength of SiC is about 400 V/pm, which is about 13 times that of Si (about 30 V/ m). Since the breakdown field strength of SiC is far greater than that of Si, power loss of the vehicular AC generator caused by the component devices can remarkably be reduced when MOS power transistors fabricated in SiC are employed in the vehicular AC generator.

Suppose, by way of example, that SiC-MOS power transistors are employed as the MOS transistors of Fig. 3 to support 300 V.

Suppose that 300 V is supported only by an N type dielectric layer for simplicity and the breakdown field strength of SiC is 400 V/lSm. Then, the necessary thickness of the N type dielectric layer 105 is about a um and the N type dielectric layer must have an impurity concentration cf 2x106 cm-3 and a resistivity of about 1.25 Q cm. Thus, the resistance of the N type dielectric layer of the SiC-MOS power transistor is as low as 1/20 of that of the N type dielectric layer of the Si-MOS power transistor. Naturally, the impurity concentration of the N type dielectric layer can be further reduced in relation with the impurity concentration of the P type well region.Thus, the use of the SiC-MOS power transistors (FETs) in accordance with the present invention in the vehicular AC generator reduces ohmic power loss caused by the dielectric layer, i.e., the source parasitic resistance Ks, greatly and reduces the channel resistance by the reduction of the source resistance feedback effect. The synergistic effect of the reduction of the ohmic power loss and that of the channel resistance makes the power loss far less than those of a vehicular AC generator employing Si-MOS power transistors and a diode type three-phase full-wave rectifier. The vehicular AC generator in accordance with the present invention can very easily be cooled.

At least preferred embodiments of the present invention provide a compact AC generator for a vehicle provided with a rectifier capable of reducing loss markedly, as compared with a conventional rectifier and of being easily cooled, having high heat resistance and attached to the housing of the vehicular AC generator, not requiring a large space for installation and capable of being installed in a hot environment.

One preferred embodiment of the invention provides an AC generator for a vehicle including a housing and a rectifier placed on a circumferential wall of the housing and having a plurality of SiC-MOS transistors, each of the SiC-MOS transistors comprises a single crystal SiC substrate having a semiconductor region of one conduction type on which an inversion channel is formed, a source region and a drain region of opposite conduction type, and the source region and the drain region are electrically connected by the inversion channel.

Another preferred embodiment of the invention provides an AC generator for a vehicie including a housing and a rectifier placed on an end wall of the housing on the side of a pulley and having a plurality of SiC-MOS transistors, each of the SiC-MOS transistors comprises a single crystal SiC substrate having a semiconductor region of one conduction type on which an inversion channel is formed, a source region and a drain region of opposite conduction type, and the source region and the drain region are electrically connected by the inversion channel.

Preferably the SiC-MOS transistors are fixedly disposed in a recess formed in the wall of the housing.

Further preferably, the rectifier comprises: a high-side switch circuit having a half of the SiC-MOS transistors interconnecting the armature windings and a high-level DC output terminal; and a low-side switch circuit having another half of the SiC-MOS transistors interconnecting the armature and a low-level DC output terminal; and a main electrode of each of the SiC-MOS transistors of the low-side switch circuit is fixed directly to the housing of the generator and a main electrode of each of the SiC-MOS transistors of the high-side switch is fixed through an insulating layer to the housing of the generator.

Since the three-phase full-wave rectifier employing SiC MOS transistors causes very small power loss and has very high heat resistance as compared with the existing vehicular three-phase full-wave rectifier, the cooling structures including a heat sink and cooling fins may be simple and small, and cooling conditions including the flow rate and the temperature of cooling air for the three-phase AC generator of at least preferred embodiments of the present invention may be far less severe than those for the conventional vehicular AC generator.

In other words, the rectifier need not be cooled by low-temperature cooling air at a large flow rate, the cooling air blown into the generator need not come into contact first with the rectifier, i.e., before the cooling air is heated by the armature windings and the field windings, and the rectifier need not necessarily be placed on the rear wall of the housing of the generator.

As mentioned above, the rectifier employing the SiC-MOS power transistors as its semiconductor rectifying devices is placed on the front end wall of the housing of the generator on the side of the pulley.

As mentioned in connection with the effect of the arrangement in accordance with the first aspect of the present invention, the three-phase full wave rectifier (rectifier) employing the SiC-MOS transistors as its semiconductor rectifying devices, as compared with the existing vehicular three-phase full-wave rectifier, cause low loss and has high heat resistance, the cooling structure including a heat sink and cooling fins can be simplified and constructed in small size accordingly, and conditions including the flow rate and the temperature of cooling air for the rectifier of the present invention may be far relaxed as compared with those for the existing vehicular three-phase full-wave rectifier.

Accordingly, the rectifier may be placed on the front wall of the housing of the generator on the side of the pulley.

Consequently, an enlarged opening can be formed in the rear end wall of the housing to improve the effect of cooling the armature windings and field windings, while preventing the overheating of the rectifier. Since the cooling structure of the rectifier including a heat sink and cooling fins has a simple, compact construction, the work for belting the pulley is no obstructed even if the rectifier is placed on the front end wall of the housing on the side of the pulley.

In preferred embodiments the rectifier may be fixedly disposed in a recess formed in the circumferential wall of the housing or the front end wall of the housing on the side of the pulley. Therefore, the rectifier can rigidly be held in place, the rectifier need not be contained in a strong case for protection, and the rectifier can further be miniaturized, securing its durability.

It is further preferred that the MOS transistors serving as the high-side switches of the rectifier are fixed through an insulating film to the housing of the generator, and the MOS transistors serving as the low-side switches are fixed directly to the housing of the generator.

Therefore, heat transfer from the rectifier to the housing of the generator is improved, and the cooling structure and the fixing structure can be further simplified.

The invention will now be described ,by way of example with reference to the accompanying drawings, throughout which like parts arc referred to by like references, and in which: Fig. 1 is a sectional view of an AC generator according to a first embodiment according to the present invention; Fig. 2 is a circuit diagram of the AC generator shown in Fig. 1; Fig. 3 is a circuit diagram of an equivalent circuit of an inverter for one phase of a three-phase full-wave rectifier shown in Fig. 1; Fig. 4 is an enlarged fragmentary sectional view of a MOS power transistors serving as a high-side switch for the three-phase full-wave rectifier shown in Fig. 1; Fig. 5 is an enlarged fragmentary sectional view a MOS power transistors serving as a low-side switch for the three-phase full-wave rectifier shown in Fig. 1;; Fig. 6 is a graph showing the voltage-current characteristics of a PN junction diode formed in Si; Fig. 7 is a graph showing the voltage-current characteristics of a MOS power transistor formed in Si; Fig. 8 is a graph showing the voltage-current characteristics of a MOS power transistor formed in SiC; Fig. 9 is a graph showing the relation between the breakdown voltage and the channel resistance of MOS power transistors having the voltage-current characteristics shown in Figs. 7 and 8; Fig. 10 is a graph showing the output currents and the efficiencies of a vehicular AC generator employing a three-phase full-wave rectifier comprising Si-MOS power transistors and a vehicular AC generator employing a three-phase full-wave rectifier comprising SiC-MOS power transistors as a function of the rotating speed of the vehicular AC generators; ; Fig. 11 is an enlarged longitudinal sectional view of a portion of a vehicular AC generator around a rectifier; Fig. 12 is a plan view taken in along lines indicated by arrows XII-XII in Fig. 11; Fig. 13 is a longitudinal sectional view of a vehicular AC generator according to a second embodiment of the present invention; Fig. 14 is a front view of the vehicular AC generator of Fig. 13 taken from the side of a pulley; and Fig. 15 is a longitudinal sectional view of a vehicular AC generator according to a third embodiment of the present invention.

First Embodiment.

The general construction of a vehicular AC generator, i.e., alternator, in a first embodiment according to the present invention to be driven by an engine of a vehicle will be described with reference to Fig. 1.

A housing of the generator is composed of a drive frame 1 and a rear frame 2, which are made of aluminum by die casting and joined directly together with a plurality of stud bolts, not shown. A stator core 3 is fixed to the inner circumference of the frame 1, and three-phase armature windings 5 are wound on the stator core 3. A shaft 9 is supported for rotation in bearings 13 and 14 fixedly fitted in the frames 1 and 2, respectively.

A rotor core 6 is fixed to the shaft 9 so as to be surrounded by the stator core 3, and a field winding 10 is wound on the rotor core 6. Cooling fans 11 and 12 are disposed on the opposite end surfaces of the rotor core 6. A voltage regulator 18 is attached to the outside surface of the rear end wall of the rear frame 2.

A recess 1b is formed in the circumferential wall la of the drive frame 1 so as to extend axially forward from the rear end surface of the drive frame 1. A rectifier (referred to also as three-phase full-wave rectifier) 19 is placed in the recess lb, and the rectifier 19 is sealed fixedly in the recess 1b with a resin lc by a potting method. The construction and the state of attachment of the rectifier 19 will be described in detail later. Openings W are formed in the end walls of the drive frame 1 and the rear frame 2 to take cooling air inside, and openings W' are formed in the circumferential walls of the drive frame 1 and the rear frame 2 to discharge cooling air outside.

The circuit configuration of the vehicular AC generator in the first embodiment will be described with reference to Fig.

2. The rectifier 19 is of a three-phase full-wave rectifying type comprising enhancement mode n-channel MOS transistors (power transistors) 19a to 19f formed in single-crystal SiC. The high-side transistors 19a to 19c are connected across the output terminals of the phases of the three-phase armature windings 5, respectively, and the high-potential terminal of a battery 21.

The low-side transistors 19d to l9f are connected across the output terminals of the phases of the three-phase armature windings 5, respectively, and a low-potential terminal of the battery 21.

The voltage regulator 18 is connected through brushes and a slip ring to the field winding 10. The phase output terminals of the phases of the three-phase armature windings are connected to apply voltages generated in the phases to the voltage regulator 18. The voltage regulator 18 controls gate voltages to be applied to the gate electrodes of the MOS power transistors 19a to 19f according to input signals given thereto.

A voltage control operation to be tarried out by the voltage regulator 18 will briefly described. When the engine rotates the rotor core 6, the voltage regulator 18 detects the battery voltage and switches on and off the field current flowing through the field winding 10 so that the voltage of the battery 21 remains constant. Then, three-phase AC voltages are generated in the three-phase armature windings 5, the three-phase full-wave rectifier 19 rectifies the three-phase AC voltages in a full-wave rectification mode, and DC currents provided by the three-phase full-wave rectifier 19 is supplied to the battery 21 for charging and to the electric loads of the vehicle. The cooling fans 11 and 12 are rotated to cool the field winding 10 the three-phase armature windings 5 and the voltage regulator 18.

The operation of the voltage regulator 18 for switching the MOS power transistors 19a to l9f of the three-phase full-wave rectifier 19 will be described hereinafter.

The voltage regulator 18 detects the phase voltages Vu, Vv and Vw, i.e., output voltages of the three-phase armature windings 5, selects a line-to-line voltage greater than the terminal voltage of the battery 21 from the line-to-line voltages Vu-Vv, Vv-Vw and Vw-Vu, and turns on one of the high-side MOS power transistors l9a to 19c and one of the low-side MOS power transistors l9d to l9f to apply the selected line-to-line voltage to the battery 21. Consequently, the selected three-phase armature winding supplies a charging current to the battery 21.

The voltage regulator 18, similarly to an ordinary regulator, measures the terminal voltage of the battery 21, compares the measured terminal voltage with a predetermined reference voltage, and interrupts the exciting current according to the result of comparison to maintain the terminal voltage of the battery 21 at a desired level.

The configuration of the three-phase full-wave rectifier comprising the SiC-MOS power transistors (referred to also as SiC-MOS transistors) will be described in further detail with reference to Figs. 3, 4 and 5. Fig. 3 is a circuit diagram of an inverter showing one of the phases of the MOS power transistor type three-phase full-wave rectifier in the first embodiment and Figs. 4 and 5 are sectional views showing the sectional construction of the MOS power transistors 19a to 19f.

Referring to Fig. 3 showing an inverter comprising n-channel MOS power transistors, the drain electrode D of a high-side MOS power transistor 101 and the source electrode S of a low-side MOS power transistor 102 are connected to the output terminal of one of the phases of the three-phase armature windings 5, the drain electrode D of the low-side MOS power transistor 102 is connected to the low-potential terminal of the battery 21, and the source electrode S of the high-side MOS power transistor 101 is connected to the high-potential terminal of the battery 21. The direction of the charging current for charging the battery 21 and that of the migration of electrons are opposite to each other. Carrier chargers are injected into the channel through the source electrode S during charging.

A source-connected parasitic diode Ds and a drain-connected parasitic diode Dd are formed between a P type well region 103, i.e., a region directly underlying the gate electrode 110, and a source electrode S and between the P type well region 103 and a drain electrode D, respectively, of each of the MOS power transistors 101 and 102 as shown in Fig. 3. The P type well region 103 and the drain electrode D of the MOS power transistor 101 are short-circuited to give a potential to the P type well region 103. Thus, the source-connected parasitic diode Ds stops the reverse current from the battery 21.

The P type well region 103 and the source electrode S of the MOS power transistor 102 are connected through a current limiting resistor r to give a potential to the P type well region 103.

The sectional structure of the MOS power transistors 101 serving as the high-side switches 19a, l9b and 19c will be described with reference to Fig. 4 by way of example.

An N type dielectric layer 105 is formed by an epitaxial growth process on an N+ type substrate 106 of SiC, the P type well region 103 is formed in the surface on the surface of the N type dielectric layer 105 by an epitaxial growth process, and an N+ type region 104 is formed in the surface of the P type well region 103 by nitrogen ion implantation. The surface of a workpiece thus formed is coated with a resist film or an insulating film having an opening corresponding to a region in the surface of the workpiece in which to form a trench 108, and the workpiece is subjected to a known RIE process, i.e., a dry etching process to form the trench 108.Then, a gate insulating film 109, i.e., a silicon dioxide film, is formed over the surface of the trench 108 by a thermal oxidation process, and a gate electrode 110 of doped polycrystalline silicon on the trench 108. Then, a metal electrode 111 is connected to the surfaces of the N+ type region (drain electrode) and the surface 104 of the P type well region, and a metal electrode 112 is connected to the N+type substrate (source electrode) 106 to complete a MOS power transistor.

The sectional structure of the MOS power transistors 102 serving as the low-side switches 19d, 19e and 19f will be described with reference to Fig. 5 by way of example. The P type well region 103 and the source electrode S of the MOS power transistor 102 are connected through a resistor r of a high resistance instead of short circuiting the drain electrode D and the P type well region 103 as shown in Fig. 4. The substrate 106 shown in Fig. 4 serves as the source electrode S in each of the high-side switches 19a, l9b and 19c, and N+ surface region 104 shown in Fig. 5 serves as the source electrode S in each of the low-side switches 19d, 19e and l9f.

Accordingly, a depletion layer extends mainly into the N type dielectric layer 105 to support a high voltage, for example, +300 V, when the high voltage is applied across the source electrode 106 and the drain electrode 111 while the MOS power transistor 101 is in the off state. Consequently, the N type dielectric layer 105 translates into a source feedback resistance Rs and, as mentioned above, the resistance of the N type dielectric layer 105 and an increase in channel resistance cause power loss. However, the thickness of the N type dielectric layer 105 is smaller and the impurity concentration is larger than those of the MOS power transistor formed in Si, because the MOS power transistor in this embodiment is formed in SiC.

That is, since the breakdown field strength of SiC is 400 V/pm, the N type dielectric layer 105 has a thickness of about 4 Mm, an impurity concentration of 2x1016 cm-3 and a resistivity of about 1.25 Q-cm. Therefore, the resistance of the N type dielectric layer 107 of the SiC-MOS power transistor is as small as 1/20 of that of the N type dielectric layer 107 of the Si-MOS power transistor.

Figs. 6 to 8 show the voltage-current characteristics of an Si diode, an Si-MOS power transistor and an SiC-MOS transistor of the same chip size and the same design rule, respectively, determined through experiments. The breakdown voltages of those devices are 250 V. As is obvious from Figs. 6 to 8, power loss caused by the three-phase full-wave rectifier 19 in the first embodiment when output current is 75 A is lower than that caused by the conventional three-phase full-wave rectifier by 90% or above.

Fig. 9 shows calculated on-state resistivity of MOS power transistors for different required breakdown voltages. On-state resistivity is the sum of channel resistance and the resistance of the N type dielectric layer. Although channel resistance is dependent on many factors, as is obvious from Fig. 8, the resistance of the N type dielectric layer 105 is dominant in a high breakdown voltage range.

Supposing that increase in channel resistance due to the feedback effect resulting from increase in the source parasitic resistance Rs is ignored, channel resistance changes scarcely even if breakdown voltage increases, while the resistance of the N type dielectric layer 105 increases with the increase of breakdown voltage, maintaining positive correlation between the resistance of the N type dielectric layer 105 and breakdown voltage.Therefore, whereas the on-state resistivity of the Si MOS power transistor increases in proportion to breakdown voltage in a range beyond the vicinity of a breakdown voltage of 25 V, increase in the N type dielectric layer 105 of the SiC-MOS power transistor is substantially negligible in a breakdown voltage range below 250 V and the on-state resistivity starts increasing gradually after breakdown voltage increases beyond 250 V.

Fig. 10 shows the characteristics of a vehicular AC generator incorporating a three-phase full-wave rectifier 19 comprising SiC-MOS power transistors (Example), and those of a vehicular AC generator incorporating a three-phase full-wave rectifier comprising Si-MOS power transistors of the same chip size as that of the SiC-MOS power transistors (Comparative example). The three-phase full-wave rectifier 19 was placed on the outside surface of the rear frame 2 to compare the Example and the Comparative example under the same conditions. The output current and the rectification efficiency of Example (twelve poles, 5000 rpm) were higher than those of Comparative example by about 105 and about 3 to about 5%, respectively, because rectification loss caused by Example was substantially negligible.

The relation between breakdown voltage and resistance in the Si-MOS power transistor and the SiC-MOS power transistor will be described hereinafter.

The MOS power transistors 19a to 19f of the embodiment are formed in 6H-SiC and design breakdown voltage for those MOS power transistors is 250 V. The results of analysis (Fig. 9) of the resistances of the three-phase full-wave rectifier 19 comprising the 6H-SiC-MOS power transistors l9a to 19f for a vehicular AC generator and the three-phase full-wave rectifier 19 comprising the Si-MOS power transistors will be explained theoretically on an assumption that the channel resistance increasing effect of the feedback effect of the source parasitic resistance Rs is negligible, and the 6H-SiC-MOS power transistors and the Si-MOS power transistors are of the vertical structure shown in Figs. 4 and 5, and have the same chip area.

The resistance R of a transistor is the sum of the channel resistance rc and the resistance rb of the N+ type dielectric layer 105. When the channel resistance rc and the resistance rb are expressed by: rc = (L/W) {(1/ps) E 5 E o} 1- Tox/(Vg - Vt) ........ (2) rb = 4Vb.(1/ ).#s##o#Ec#-A ......... (3) The resistance of the SiC-MOS power transistor was about 1/15 of that of the Si-MOS transistor. The Si-MOS power transistor was 3X105 V/cm in breakdown field strength Ec, 11.8 in dielectric constant ES, 1 mm in area A, 1100 cm2/(VS) in bulk electron mobility , 1 Mm in channel length L, 222 am in channel width W and 500 cm-/(V-S) in electron channel mobility ps. The SiC-MOS power transistor was 3x106 V/cm in breakdown field strength Ec, 10.0 in dielectric constant Es, 1 mm2 in area A, 370 cm2/(VS) in electron bulk mobility , 1 tim in channel length L, 222 tim in channel width W and 100 cm2/(V-S) in electron channel mobility. In Expressions (2) and (3), Vb is breakdown voltage.

It was known from Expressions (2) and (3) that the resistance of the SiC-MOS power transistor is smaller than that of the Si-MOS power transistor when the breakdown voltage is 50 V or above. Since the calculation was carried out for a case where the substrate is used as the drain, the resistance of the Si-MOS power transistor must increase greatly due to increase in channel resistance caused by the feedback effect of the source parasitic resistance Rs when the substrate is used as the source.

Therefore, it is certainly expected that the resistance of the SiC-MOS power transistor is lower than that of the Si-MOS transistor when the breakdown voltage is 100 V or above even if the design rule is changed somewhat.

The construction and the disposition of the rectifier 19 in this embodiment will be described in further detail with reference to Figs. 11 and 12. Fig. 11 is a sectional view in a plane perpendicular to the circumferential direction of the three-phase full-wave rectifier 19, and Fig. 12 is a plan view of the three-phase full-wave rectifier of Fig. 11.

The rectifier 19 has a low-side base plate 190 formed from an aluminum plate and closely in contact with a base plate support surface ld formed in a recess lb. A screw le is screwed through a through hole, not shown, formed in a circumferential wall la in a threaded hole, not shown, formed in the low-side base plate 190 to fasten the low-side base plate 190 to the circumferential wall la. A high-side base plate 193 formed from an aluminum plate is attached adhesively to an electrically insulating heat-resistant resin film 192 (Fig. 11) attached to the low-side base plate 190.

The substrates (indicated at 106 in Fig. 5), i.e., SiC chips, of the low-side switches 19d, 19e and 19f are soldered to the low-side base plate 190. The substrates (indicated at 106 in Fig. 4), i.e., SiC chips, of the high-side switches l9a, l9b and 19c are soldered to the high-side base plate 193. Gate electrodes 108 (Fig. 5) and source electrodes 111 (Fig. 5), i.e., contact electrodes, are exposed on the surfaces of the low-side switches 19d, 19e and 19f. Bus bars 197U, 197V and 197W are bonded to the source electrodes 111, respectively, by, for example, an ultrasonic bonding method. Heat-conductive three bars 198 serving as control electrode lines are bonded to the gate electrodes 108, respectively by, for.example, an ultrasonic bonding method, and the bars 198 are connected to the voltage regulator 18.

Gate electrodes 108 (Fig. 4) and drain electrodes 111 (Fig. 4), i.e., contact electrodes, are exposed on the surface of the high-side switches 19a, 19b and 19c. The bus bars 197U, 197V and 197W are bonded to the drain electrodes 111, respectively, by, for example, an ultrasonic bonding method.

Three heat-conductive bars 199 serving as control electrode lines are bonded to the gate electrodes 108, respectively, by, for example, an ultrasonic bonding method. The bars 199 are connected to the voltage regulator 18. A b-terminal 194, i.e., a copper or aluminum bolt, is fixed to the back surface of the high-side base plate 193. After thus assembling the components, the chips are sealed by, for example, a resin molding method to complete the rectifier 19.

After placing the rectifier 19 in the recess lb and fastening the same to the circumferential wall la with the screw la, a resin 191 is poured into the recess 1b by a potting process to incorporate the rectifier 19 into the drive frame 1. The three input bus bars 197U, 197V and 197W are fastened to the terminals 51U, 51V and 51W of the three-phase armature windings 5 with fastening members 52U, 52V and 52W, respectively. As shown in Fig. 11, the B-terminal 194 projects radially outside through an opening formed near the rear end of the drive frame 1.

In this embodiment, since the three-phase full-wave rectifier 19 generates remarkably reduced heat and SiC has a high heat resistance, the rectifier 19 can satisfactorily be cooled only through heat transfer from the same to the frames 1 and 2 even if the rectifier 19 is not cooled directly by low-temperature cooling air blown into the frames 1 and 2, and the temperature of the solder bonding together the semiconductor rectifying devices and the metal base plates can be suppressed below the fatigue limit temperature of 200 C of the solder even if the rectifier 19 is placed in the recess ffib of the drive frame 1.

Consequently, the three-phase full-wave rectifier 19 need not be fixed to the rear end wall of the rear frame 2, the three-phase full-wave rectifier 19 is not affected by radiant heat emitted by the exhaust pipe, not shown, any heat shielding structure need not be interposed between the three-phase full-wave rectifier 19 and the exhaust pipe, and the construction and processes are simplified.

The vehicular AC generator in this embodiment solves the problem in the conventional vehicular AC generator that the three-phase full-wave rectifier 19 fixed to the rear end wall of the rear frame 2 and provided with large cooling fins obstructs the flow of cooling air through the cooling air inlet openings formed in the rear end wall, and the three-phase armature windings 5 and the field coil 10 can satisfactorily be cooled.

Since the three-phase full-wave rectifier 19 is placed in the circumferential wall la of the drive frame 1, the rectifier 19 can satisfactorily be protected from mechanical damages. The base plate 190 is electrically connected for grounding through the drive frame 1 to the body of the vehicle.

Second Embodiment.

A second embodiment of the present invention will be described with reference to Figs. 13 and 14, in which parts like or corresponding to those of the first embodiment are designated by the same reference characters to facilitate understanding.

A vehicular AC generator in the second embodiment is characterized by a rectifier voltage regulator 290 comprising the same rectifier 19 as that shown in Figs. 11 and 12 and an Si bipolar monolithic IC chip equivalent in function to the voltage regulator 18 and soldered to a heat-conductive base plate 190, and sealed in a resin by molding. The rectifier voltage regulator 290 is fixed to the front end wall (end wall on the side of a pulley) lf of a drive frame 1.

Circular walls lg projects forward from the front end wall lf of the drive frame 1, and the rectifier voltage regulator 290 is placed in a partial annular recess lm defined by the circular walls 102. The heat-conductive base plate 190 has a shape conforming to that of the partial annular recess lm. The rectifier voltage regulator 290 is fastened with a screw, not shown, to the outside surface of the front end wall If in a manner similar to that shown in Fig. 12 and is sealed in a potting resin lc similarly to the rectifier in the first embodiment.

In the second embodiment, SiC-MOS transistors serving as low-loss semiconductor rectifying devices rectify AC currents, and SiC-MOS transistors are employed for switching field current.

Therefore, rectifier voltage regulator 290 has a cooling structure far smaller than the conventional one. Consequently, work for winding belts 201 around a pulley 200 is not obstructed at all even if the rectifier voltage regulator 290 is fixed to the front end wall if of the drive frame 1.

In the first and the second embodiment, it is preferable to introduce more cooling air through cooling air inlet openings formed in the drive frame 1 into the vehicular AC generator than through cooling air inlet openings formed in the rear frame 2, because, in the conventional vehicular AC generator, the rectifier needs unavoidably to be attached to the rear end wall of the rear frame 2, cooling air must be introduced at a high flow rate through the cooling air inlet openings formed in the rear end wall of the rear frame to cool the rectifier and the cooling air heated by the exhaust pipe and such has an inferior cooling effect.On the contrary, in the first and the second embodiment, cooling air need not be introduced through the cooling air inlet openings formed in the rear end wall into the vehicular AC generator at a high flow rate, and cooling air can be introduced through the cooling air inlet openings formed in the front end wall into the vehicular AC generator at a high flow rate for the further enhanced cooling effect.

Third Embodiment.

A third embodiment of the present invention will be described with reference to Fig. 15, in which parts like or corresponding to those of the first embodiment are designated by the same reference characters to facilitate understanding.

The third embodiment differs in the formation of a recess Ib from the first embodiment.

In the third embodiment, a rectifier 19 is shifted forward, a B-terminal 194 is projected forward through a hole ln formed in the circumferential wall la of a drive frame 1, and then bus bars 197U, 197V and 197W are connected to terminals 51U, 51V and 51W, respectively. Then, the rectifier 19 is sealed in a resin ic by molding. An outer circumferential wall 300 defining the recess Ib is a forward extension 2b of the circumferential wall 2a of a rear frame 2. After sealing the rectifier 19 with the resin lc by potting, the drive frame 1 and the rear frame 2 are joined together to conplete the vehicular AC generator. The B-terminal 194 of the third embodiment may be projected radially outside, and the B-terminal 194 of the first embodiment may be projected like the B-ter:ttnal i94 of the third embodiment.

In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the broader scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention in this document is to be regarded in an iLlustrative, rather than restrictive, sense.

Claims

1. An AC generator for a vehicle including a housing disposed near an engine, armature windings and a rectifier placed on a circuferential wall of said housing and having a plurality of SiC-MOS transistors, wherein each of said SiC-MOS transistors comprises a single crystal SiC substrate having a semiconductor region of one conduction type on which an inversion channel is formed, a source region and a drain region of opposite conduction type, and said source region and said drain region are electrically connected by said inversion channel.

2. An AC generator for a vehicle including a housing disposed near an engine, armature windings and a rectifier placed on an end wall of the housing on the side of a pulley and having a plurality of SiC-MOS transistors, wherein each of said SiC-MOS transistors comprises a single crystal SiC substrate having a semiconductor region of one conduction type on which an inversion channel is formed, a source region and a drain region of opposite conduction type, and said source region and said drain region are electrically connected by said inversion channel.

3. An AC generator according to claim 1, wherein said SiC-MOS transistors are fixedly disposed in a recess formed in said wall of said housing.

4. An AC generator according to one of claims 1 and 2, wherein said rectifier comprises: a high-side switch circuit having a half of said SiC-MOS transistors interconnecting said armature windings and a high-level DC output terminal; and a low-side switch circuit having another half of said SiC-MOS transistors interconnecting said armature and a low-level DC output terminal; and wherein a main electrode of each of said SiC-MOS transistors of said low-side switch circuit is fixed directly to the housing of the generator and a main electrode of each of said SiC-MOS transistors of said high-side switch is fixed through an insulating layer to the housing of the generator.

5. An AC generator for a vehicle according to one of claim 1 and claim 2, wherein each of said SiC-MOS transistors has an N type dielectric layer which has a thickness of about 4 tim, an impurity concentration of 2x1016 cm-3 and a resistivity of about 1.25 Q-cm.

6. An AC generator for a vehicle comprising a rectificr having a plurality of SiC MOS transistors.

7. An AC generator of a vehicle substantially as hereinbefore described with reference to Figures 1 to 12 of the accompanying drawings.

8. An AC generator of a vehicle substantially as hereinbefore described with reference to Figures 13 and 14 of the accompanying drawings.

9. An AC generator of a vehicle substantially as hereinbefore described with reference to Figure 15 of the accompanying drawings.