Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – Insulating material
Reexamination Certificate
2000-09-28
2002-12-31
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
Insulating material
C257S706000, C257S714000, C257S723000, C257S724000
Reexamination Certificate
active
06501172
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power modules and especially to techniques for improving cooling performance of power modules.
2. Description of the Background Art
FIG. 34
is a schematic external view of a first conventional power module
101
P. In the power module
101
P, a copper base plate
9
P is disposed through a heat-conducting grease (not shown) over a radiating fin or heat sink
2
AP, and an insulating substrate
5
P is disposed on the base plate
9
P. On the insulating substrate
5
P, there are disposed a free-wheeling diode
1
AP (hereinafter also referred to as “diode”) and an insulated gate bipolar transistor
1
BP (hereinafter referred to as “IGBT”).
In the conventional power module
101
P, copper foils
6
P are placed on both main surfaces of the insulating substrate
5
P. The base plate
9
P and the copper foil
6
P are bonded together with solder, and the diode
1
AP and the IGBT
1
BT are soldered onto the copper foil
6
P. An electrode
3
P is provided through an insulating layer
4
P over the radiating fin
2
AP. Then, predetermined electrical connections are made by wires
7
P. The construction including the radiating fin
2
AP, the diode
1
AP, the IGBT
1
BP, and the like is housed in a case (not shown).
The electrode
3
P is connected to a bus bar or wiring
91
P which extends toward the outside of the case. Outside the case, a current transformer
92
P for current detection is attached to the bus bar
91
P. Further, a cylindrical capacitor
8
P for smoothing direct current is provided outside the case independently of the radiating fin
2
P and the like (the connection with the case is omitted in the figure).
FIG. 35
is a schematic external view of a second conventional power module
102
P. The power module
102
P has no base plate
9
P as above described, wherein the insulating substrate
5
P is disposed through a heat-conducting grease over the radiating fin
2
AP. The power module
102
P is in all other aspects identical to the above-mentioned power module
101
P.
FIG. 36
is a schematic external view of a third conventional power module
103
P. The power module
103
P is a so-called power transducer. In the power module
103
P, all the diodes
1
AP and IGBTs
1
BP are disposed on the insulating substrates
5
P. A heat sink
2
BP of the power module
103
P has through holes
2
BHP therethrough passing a cooling medium. The power module
103
P is in all other aspects identical to the above-mentioned power module
101
P.
The conventional power modules
101
P,
102
P, and
103
P have the following problems.
First is low temperature reliability during operation. More specifically, when the thermal expansion coefficient of the heat sink
2
AP or
2
BP differs from those of the diode(s)
1
AP and the IGBT(s)
1
BP, thermal stresses responsive to a temperature difference from the freezing point of solder will occur at the solder joints as above described. There is thus a problem of occurrence and progress of cracking at the solder joints through a heat cycle (or temperature cycle) in the use (or operation) of the power module
101
P,
102
P,
103
P and/or a heat cycle by repetitions of start and halt of the power module. Such cracking at the solder joints reduces the longevity of the power module.
To reduce the above thermal stresses, it is contemplated for example to increase solder thickness (e.g., 300 &mgr;m or more). However, such increased thickness of solder increases thermal resistance between the heat sink
2
AP or
2
BP and the diode(s)
1
AP and the like. This brings up another problem that the size of the heat sink
2
AP or
2
BP must be increased.
Further, in the conventional power modules
101
P,
102
P, and
103
P, the distribution of temperature in the insulating substrate(s)
5
P, the base plate
9
P, and the like due to heat generation in the diode(s)
1
AP and the like causes warps or winding in the insulating substrate(s)
5
P and the like. When the temperature difference is great, clearance is created between the radiating fin
2
AP,
2
BP and the base plate
9
P and the like. Thus, there is a problem of reduced heat transfer because the heat-conducting grease cannot completely fill in the space between the radiating fin
2
AP,
2
BP and the insulating substrate(s)
5
P or the base plate
9
P (due to the incoming air). Another problem is that the occurrence or progress of cracking at the solder joints, described above, may be encouraged. The formation of clearance thus results in deterioration in the reliability of the power module.
To prevent the formation of clearance, it is contemplated for example to make the temperature distribution uniform throughout the insulating substrate(s)
5
P and the like, or to increase the rigidity of the insulating substrate(s)
5
P and the like by increasing the thickness of the substrate(s)
5
P and the like. However, such increased thickness increases thermal resistance between the heat sink
2
AP,
2
BP and the insulating substrate(s)
5
P or the like. This brings up, as has been described, another problem that the size of the heat sink
2
AP,
2
BP must be increased.
Further, when the diode(s)
1
AP and the IGBT(s)
1
BP produce a large quantity of heat, the amount of current must be limited in order to ensure reliability since the characteristics of the elements vary with increasing temperature.
Secondly, each of the conventional power modules
101
P,
102
P, and
103
P as a whole is large in size since the current transformer
92
P and the cylindrical capacitor
8
P are provided independently outside the case for such a module. Besides, the current transformer
92
P has the property of becoming large when current to be measured has a large DC component, and also the current transformer
92
P makes measurements with errors (about 5%) due to its characteristics changes caused by heat generation.
Thirdly, in the power module
103
P, the distances from each of the power semiconductor devices, such as the diode
1
AP or the IGBT
1
BP, to the electrode
61
P connected to the low potential side of the power transducer and to the electrode
62
P connected to the high potential side vary according to where that power semiconductor device is located. This causes variations in the inductance of the wiring or wires
7
P from one power semiconductor device to another, thereby causing variations in output voltage.
SUMMARY OF THE INVENTION
A first aspect of the present invention is directed to a power module comprising: a heat sink; a first power semiconductor device disposed directly on the heat sink; and a capacitor disposed directly on the heat sink.
According to a second aspect of the present invention, in the power module of the first aspect, the heat sink has a plurality of surfaces; and the first power semiconductor device and the capacitor are disposed on different ones of the surfaces of the heat sink.
According to a third aspect of the present invention, in the power module of the first or second aspect, the heat sink has a passage of a cooling medium.
According to a fourth aspect of the present invention, in the power module of either of the first through third aspects, the heat sink has conductivity; and an electrode of the first power semiconductor device and an electrode of the capacitor are directly bonded to the heat sink.
According to a fifth aspect of the present invention, the power module of the fourth aspect further comprises: an insulating substrate disposed on the heat sink; and a second power semiconductor device disposed through the insulating substrate over the heat sink.
According to a sixth aspect of the present invention, the power module of the fourth aspect further comprises: another heat sink; and a second power semiconductor device disposed directly on the another heat sink.
According to a seventh aspect of the present invention, in the power module of the sixth aspect, the another heat sink has conductivity; and an electrode of the second power semiconductor device is directly bonded to the another heat sink. The power m
Fukada Masakazu
Nakajima Dai
Takanashi Ken
Clark Sheila V.
Mitsubishi Denki & Kabushiki Kaisha
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