Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – Insulating material
Reexamination Certificate
2002-03-08
2004-05-25
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
Insulating material
C257S706000
Reexamination Certificate
active
06740968
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a power unit for driving a magnetron in which cooling of the power unit for driving a magnetron is improved in the technical field of a high frequency heating apparatus, such as a microwave oven, for dielectric heating using a magnetron. Furthermore, the present invention relates to the cooling configuration of a plurality of heat sinks assembled on a printed circuit board, wherein power semiconductor devices with non-insulation packages which are located at electric potential differences are mounted on respective said heat sinks.
Conventionally, as shown in
FIG. 4
, a switching power source used as a power unit for driving a magnetron is such that the semiconductor switching element
1
is attached to a heat radiating fin
2
, which is made of aluminum, by a screw
3
in order to prevent the elements from being damaged due to a temperature rise resulting from switching losses of the semiconductor switching element
1
, and heat that is generated due to switching losses are thermally transmitted to the heat radiating fin, whereby the heat is radiated from the heat radiating fin
2
. Further, in order to efficiently thermally conduct switching losses, which are generated by the semiconductor switching elements
1
, to the heat radiating fin
2
, the semiconductor switching elements
1
have a collector part exposed on the rear side thereof, a thermal conducting filler having good thermal conductivity is coated and filled between the rear side thereof and the heat radiating fin, and the collector part
4
is brought into contact with the heat radiating fin
2
, thereby having improved thermal conduction.
However, a high voltage-resisting semiconductor switching element for a power unit for driving a magnetron has been requested in line with high output of a high frequency heating apparatus. Since the yields of high voltage-resisting semiconductor switching elements are not sufficient and these are expensive, there is a problem in procurement thereof. Therefore, for a power unit for driving a magnetron, circuits that are composed of two versatile medium voltage-resisting semiconductor switching elements connected in series have been used as shown in FIG.
5
. That is, the commercially available power supply
5
is rectified to be a direct current by a rectification part
6
and is made into high frequency by a switching portion
9
composed of semiconductor switching elements
7
and
8
. The high frequency is boosted by a boosting transformer
10
, and is subjected to a voltage doubler rectifier by a high-voltage doubler rectifier circuit part
11
. Then, the same is provided to a magnetron
12
.
However, in a construction in which semiconductor switching elements are attached to a conventional heat radiating fin, although two semiconductor switching elements
7
and
8
are attached to one heat radiating fin
2
, in view of the circuit, a collector terminal part
13
of one semiconductor switching element
7
is connected to an emitter terminal part
14
of the other semiconductor switching element
8
, and these are to be held in the same potential. On the other hand, in order to efficiently thermally conduct the switching losses of the semiconductor switching elements
7
and
8
to the heat radiating fin, the collector terminal part
13
of the semiconductor switching element
7
and the collector part of the same potential are exposed on the rear side of the semiconductor switching elements, a thermal conducting filler of good thermal conductivity is coated thereon, and these are brought into contact with the heat radiating fin. However, where the two semiconductor switching elements having the collector parts thereof exposed on the rear side, where thermal conductivity is improved, are attached to one heat radiating fin as they are, the collector parts are made into the same potential, wherein the circuit shown in
FIG. 5
cannot be configured.
Therefore, as shown in
FIG. 6
, it was necessary that the heat radiating fin was divided into heat radiating fins
15
and
16
, wherein the semiconductor switching elements
19
and
20
having collector parts
17
and
18
exposed on the rear side thereof were, respectively, attached thereto by screws
21
and
22
, and two heat radiating fins were electrically insulated from each other, and the heat radiating fins were not held in the same potential. Or as shown in
FIG. 7
, in order to attach two semiconductor switching elements
24
and
25
to a single heat radiating fin
23
, a semiconductor switching element
24
having a larger switching loss had a collector part
26
exposed on the rear side thereof and the same was attached to the heat radiating fin
23
by a screw
27
while the other semiconductor switching element
25
was electrically insulated with a resin armoring the outside of the collector part on the rear side thereof and was attached to the heat radiating fin
23
with a screw
28
.
Thus, since in the former two heat radiating fins are required, and it is necessary that these two heat radiating fins are electrically insulated from each other, there is a problem in that a disadvantage is brought about with respect to the installation plane of the heat radiating fin in view of constituting a power unit for driving a magnetron.
Also, since one of the semiconductor switching elements has the collector part on the rear side thereof electrically insulated with an armoring resin although in the latter the semiconductor switching elements can be attached to a single heat radiating fin, it is difficult to conduct heat resulting from switching losses to the heat radiating fin in view of thermal conductivity, wherein it is necessary to make the heat radiating fin larger in order to secure a sufficient cooling effect or it is necessary to make large a cooling fan for cooling the heat radiating fin. Therefore, there is another problem in that a disadvantage occurs in view of the plane of installation of the heat radiating fin and cooling fan when constituting a power unit for driving a magnetron.
As another method, when attaching, to the heat radiating fin, one of the two semiconductor switching elements having the collector part thereof exposed on the rear side thereof, in which heat conductivity has been improved, a silicon sheet or mica plate having an insulative property and thermal conductivity is caused to intervene and is attached between the semiconductor switching element having the collector part exposed on the rear side thereof and the heat radiating fin
7
. However, since the thermal conductivity of the silicon sheet and mica plate is 1.0 through 1.5×10
−3
cal/cm.sec.K, and the thickness thereof is 0.3 through 1.0 mm, it is difficult to conduct heat, which is generated by the semiconductor switching elements, to the heat radiating fin, problems occur in that a necessity of making the heat radiating fin large arises in order to sufficiently cool the semiconductor switching elements, and a cooling fan for cooling the heat radiating fin is made large.
Further, in the prior art of such a heat sink unit for assembly on a printed circuit board, heat sinks at different potentials have been separately attached to a printed circuit board by screws or the like. Alternatively, an insulation sheet has been used for one of the semiconductor devices.
FIG. 11
is an assembly diagram showing a prior art heat sink unit for assembly on a printed circuit board in which heat sinks are separately attached to a printed circuit board.
FIG. 12
is a wiring print diagram of the printed circuit board.
In
FIG. 11
, a printed circuit board
1
is provided, thereon, with a first heat sink
103
to which a first power semiconductor device
102
is attached by a screw, and similarly with a second heat sink
105
which is separated therefrom by a predetermined insulation distance and to which a second power semiconductor device
104
is attached by a screw.
As shown in
FIG. 12
, each heat sink is attached to the printed circuit board
1
by a screw. The wiring pr
Matsukura Toyotsugu
Morikawa Hisashi
Sakai Shin-ichi
Clark Sheila V.
Matsushita Electric - Industrial Co., Ltd.
Pearne & Gordon LLP
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