Load actuating semiconductor circuit having a thermally resistive member

An intelligence power device (IPD), having a temperature-sensitive and heat-damage protecting function, is used as a semiconductor relay which controls load current supplied to an electric load. A target sensitive temperature T.sub.SD of IPD is determined according to an allowable load current value, using an equation T.sub.SD =I.sup.2 .times.R.sub.ON .times.R.sub.TEMP +T.sub.ROOM, where I represents an allowable load current value, R.sub.ON represents an on-resistance value of a power MOSFET, R.sub.TEMP represents a thermal resistance value of an entire radiation system of IPD, and T.sub.ROOM represents an ambient temperature of IPD. When the load current exceeds the allowable load current value, IPD stops the load current so that the load circuit can be prevented from being subjected to excessively large current.

BACKGROUND OF THE INVENTION 
This invention relates to a load actuating circuit incorporating a 
semiconductor circuit which actuates an electric load. 
In conventional automotive vehicles, semiconductor relays (e.g., power 
MOSFET) are widely used to actuate various electric loads including a 
lighting system and a motor driving system. In such a conventional load 
actuating circuit, a fuse is generally inserted in series with the power 
MOSFET to prevent the load circuit from being subjected to excessively 
large current due to short circuit of the wire harness or dead short in 
the load. 
However, recent trends of automotive vehicles are compactness and light 
weight. Thus, it is preferable to eliminate the fuse for realizing 
downsizing and weight reduction. To realize this, a load circuit, 
comprising a load and associated wire harness, needs to be protected 
against excessive current without using any fuse. For example, a 
current-detecting resistance may be connected to the power MOSFET which 
actuates the load. This current-detecting resistance causes a voltage 
reduction when current flows across the load. Thus, an actual load current 
value is detected from the voltage reduction caused by the 
current-detecting resistance. When the load current is increased 
excessively, the load current is stopped or reduced adequately so as not 
to exceed a predetermined value (refer to Unexamined Japanese Patent 
Application No. 8-47168 published in 1996). 
However, according to the above-described arrangement, there is a necessity 
of specially providing the current-detecting resistance. Furthermore, a 
control circuit is additionally provided to control the power MOSFET in 
accordance with the detected load current so that the load circuit is 
protected against excessive current. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve the above-described 
problems. More specifically, an object of the present invention is to 
provide a fuse-less circuit arrangement which is capable of protecting the 
load circuit against excessive current without detecting a load current. 
In order to accomplish above-described and other related objects, a first 
aspect of the present invention provides a novel and excellent load 
actuation circuit comprising a semiconductor relay (10) which controls 
load current supplied to a load (2). The semiconductor relay (10) has a 
current shutoff function working at a predetermined sensitive temperature 
(T.sub.SD) at which the load current supplied to the load (2) is stopped. 
Thus, the semiconductor relay (10) acts as a heat-damage protecting 
device. 
Preferably, the sensitive temperature is set somewhere in a temperature 
range of 70.degree. C. to 140.degree. C. 
Preferably, the sensitive temperature is set to a value corresponding to a 
predetermined allowable load current value. 
Preferably, the semiconductor relay (10) comprises an adjusting device 
(13a) which is used to adjust the sensitive temperature. 
Preferably, a thermal resistive member (24) is provided in a heat radiation 
path for the semiconductor relay (10) to adjust a thermal resistance value 
(R.sub.TEMP) of an entire heat radiation system of the semiconductor relay 
(10). 
A second aspect of the present invention provides a manufacturing method 
for forming a load actuation circuit incorporating a semiconductor relay 
(10) which controls the load current supplied to a load (2). The 
manufacturing method comprises the steps of preparing a plurality of 
semiconductor relays which have various sensitive temperatures, and 
forming the load actuating circuit by selecting a desirable one of the 
prepared semiconductor relays which has a desirable sensitive temperature. 
A third aspect of the present invention provides a mounting structure for a 
load actuation circuit. Specifically, a semiconductor relay (10) is 
electrically connected to and securely fixed on a printed board (21). A 
radiation plate (26) is provided to release heat from the semiconductor 
relay (10). And, the thermal resistive member (24) is interposed between 
the semiconductor relay (10) and the radiation plate (26) so as to adjust 
the thermal resistance value (R.sub.TEMP) of the entire heat radiation 
system of the semiconductor relay (10). 
A fourth aspect of the present invention provides a method for adjusting a 
current-shutoff level for a load actuating circuit, comprising the steps 
of incorporating a semiconductor relay (10) in a load actuating circuit 
for controlling the load current supplied to a load (2), providing the 
thermal resistive member (24) in the heat radiation path for the 
semiconductor relay (10), and adjusting the thermal resistance value of 
the thermal resistive member (24) to set a current-shutoff level at which 
the load current supplied to the load is stopped. 
Preferably, the thermal resistive member (24) is a solder used when the 
semiconductor relay (10) is soldered on a printed board (21), and a 
soldering area is adjusted to set the current-shutoff level. 
Reference numerals in parentheses show the correspondence to the components 
disclosed in preferred embodiments of the present invention described 
later. The reference numerals are thus merely used for the purpose of 
expediting the understanding to the present invention and not used for 
narrowly interpreting the scope of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will be explained 
hereinafter with reference to accompanied drawings. Identical parts are 
denoted by the same reference numerals throughout the drawings. 
FIG. 1 is a circuit diagram showing a load actuation circuit for an 
automotive vehicle in accordance with a first embodiment of the present 
invention. A battery 1 is mounted on an automotive vehicle body (not 
shown) to supply electric power to an electric load 2, such as a lighting 
system and/or a motor driving system, equipped in an automotive vehicle. 
An intelligent power device (abbreviated IPD, hereinafter) 10 is connected 
or interposed between the battery 1 and the load 2. IPD 10 functions as a 
semiconductor relay that actuates the load 2, so as to realize a fuse-less 
load actuation. 
The IPD 10 comprises a P-channel power MOSFET 11 serially connected to the 
load 2. MOSFET 11 has a base terminal connected to a control circuit 3. 
The control circuit 3 sends a control signal to the base terminal of 
MOSFET 11 to control the load current of the load 2. The IPD 10, 
functioning as a temperature-sensible and heat-damage protecting device, 
deactivates the power MOSFET 11 when a sensed temperature exceeds a 
predetermined level. More specifically, IPD 10 comprises a temperature 
sensing circuit 12, a reference voltage generating circuit 13, a 
comparator circuit 14, and a shutoff circuit 15. The temperature sensing 
circuit 12 detects a junction temperature and generates a voltage signal 
representing the detected junction temperature. For example, the 
temperature sensing circuit 12 includes a diode whose output voltage 
varies in response to the detected junction temperature. The reference 
voltage generating circuit 13 generates a reference voltage. The 
comparator circuit 14 has an input terminal connected to the temperature 
sensing circuit 12 and the other input terminal connected to the reference 
voltage generating circuit 13. The comparator circuit 14 compares the 
output voltage generated from the temperature sensing circuit 12 with the 
reference voltage generated from the reference voltage generating circuit 
13. The comparator circuit 14 has an output terminal which generates an 
output signal representing a comparison result. The shutoff circuit 15 is 
connected to the output terminal of the comparator circuit 14 to turn off 
the power MOSFET 11 in response to the comparison result obtained by the 
comparator circuit 14. The above-described components 11 to 15 of IPD 10 
are integrated or united into a semiconductor package. 
When the temperature of IPD 10 increases and reaches its sensitive 
temperature, the output voltage of the temperature sensing circuit 12 
exceeds the reference voltage of the reference voltage generating circuit 
13. In response to the reversal of comparison result, the voltage level of 
the output signal is changed and sent from the comparator circuit 14 to 
the shutoff circuit 15. Thus, the power MOSFET 11 is turned off in 
response to the reversed output signal of the shutoff circuit 15. 
In the above-described circuit arrangement, a consumption power "W" at the 
power MOSFET 11 is expressed by the following equation (1). 
EQU W=I.sup.2 .multidot.R.sub.ON (1) 
where "I" represents an allowable load current value and "R.sub.ON " 
represents an on-resistance value of the power MOSFET 11. 
A thermal resistance value R.sub.TEMP (.degree. C./W) of an entire heat 
radiation system of IPD 10 is defined by the following equation (2). 
EQU R.sub.TEMP =(T.sub.SD -T.sub.ROOM)/W (2) 
where T.sub.SD represents a sensitive temperature of IPD 10 and T.sub.ROOM 
represents an ambient temperature (i.e., operation environmental 
temperature) of IPD 10. 
Accordingly, using the above-described equations (1) and (2), the sensitive 
temperature T.sub.SD is expressed by the following equation (3). 
EQU T.sub.SD =I.sup.2 .times.R.sub.ON .times.R.sub.TEMP +T.sub.ROOM(3) 
As apparent from the equation (3), the sensitive temperature T.sub.SD of 
IPD 10 is proportional to the square of the allowable load current value 
"I." 
Accordingly, the preferred embodiment of the present invention actively 
utilizes the temperature-sensible and heat-damage protecting function of 
IPD 10. In other words, IPD 10 functions as a current shutoff device 
requiring no fuse. When the load current exceeds the allowable level, the 
power MOSFET 11 is turned off at a corresponding sensitive temperature. 
Thus, it becomes possible to prevent the load circuit (including the load 
2 and wire harness between IPD 10 and the earth) from being subjected to 
excessively large current. 
Next, practical settings of the sensitive temperature T.sub.SD will be 
explained. 
Setting Example 1 
The on-resistance value R.sub.ON of the power MOSFET 11 is 0.1 .OMEGA.. The 
ambient temperature T.sub.ROOM is 75.degree.. The thermal resistance value 
R.sub.TEMP is 10.degree. C./W. The allowable load current I is 3 A. When 
these values are entered in the equation (3), the sensitive temperature 
T.sub.SD becomes 3.sup.2 .times.0.1.times.10+75=84.degree. C. 
Setting Example 2 
The on-resistance value R.sub.ON of the power MOSFET 11 is 0.2 .OMEGA.. The 
ambient temperature T.sub.ROOM is 75.degree.. The thermal resistance value 
R.sub.TEMP is 10.degree. C./W. The allowable load current I is 5 A. When 
these values are entered in the equation (3), the sensitive temperature 
T.sub.SD becomes 5.sup.2 .times.0.2.times.10+75=125.degree. C. 
For example, the above-described sensitive temperature T.sub.SD can be set 
by adjusting the reference voltage supplied from the reference voltage 
generating circuit 13. When the reference voltage is given by using a 
variable resistance, the sensitive temperature T.sub.SD can be set to a 
desirable value by adjusting its resistance value. 
The sensitive temperature T.sub.SD of IPD 10 may vary due to manufacturing 
errors. Thus, the sensitive temperature T.sub.SD of IPD 10 of each 
manufactured IPD is measured. Some IPD samples respectively having 
different sensitive temperatures are prepared beforehand. Then, a 
preferable IPD having a desirable sensitive temperature T.sub.SD is 
selected. Thus, the load actuating circuit shown in FIG. 1 is formed. 
Furthermore, IPD 10 shown in FIG. 1 comprises an adjusting resistance 13a 
as one component constituting the reference voltage generating circuit 13. 
According to this circuit arrangement, the reference voltage is changeable 
in accordance with a selected dividing point of the adjusting resistance 
13a. Thus, the desirable sensitive temperature T.sub.SD is obtained by 
trimming the adjusting resistance 13a. 
FIG. 2 shows a detailed mounting structure of IPD 10. 
On a printed board 21, bonding pads 22 and a die pad 23 are formed. A 
thermal resistive member (e.g., metallic member such as molybdenum or 42 
alloy, or ceramic such as aluminum nitride, silicone sheet, mica etc.) 24 
is provided on the die pad 23. IPD 10 is mounted on the thermal resistive 
member 24. IPD 10 is bonded to each bonding pad 22 via an aluminum wire 
25. An aluminum radiation plate 26 is attached to the bottom surface of 
the printed board 21. 
According to the above-described structure, heat of IPD 10 is released from 
the radiation plate 26. The thermal resistive member 24 is interposed in 
the heat radiation path extending from IPD 10 to radiation plate 26. In 
such an arrangement, the entire heat radiation system of IPD 10 includes 
the thermal resistive member 24. Thus, the thermal resistance value 
R.sub.TEMP of the entire heat radiation system of IPD 10 is substantially 
influenced by the thermal resistive member 24. In other words, the 
sensitive temperature T.sub.SD can be set to a desirable value by the 
thermal resistive member 24. 
The thermal resistance value R.sub.TEMP can be modified from the equation 
(3) to the following equation (4). 
EQU R.sub.TEMP =(T.sub.SD -T.sub.ROOM)/R.sub.ON .times.I.sup.2 (4) 
For example, in the above-described setting example 2 of IPD 10 where the 
allowable load current I is set to 5 A and the sensitive temperature 
T.sub.SD is set to 125.degree. C., there may be a requirement that the 
allowable load current I needs to be reduced to 2.5 A. In this case, the 
thermal resistance value R.sub.TEMP needs to be increased to 
(125-75)/0.2.times.2.5.sub.2 =40.degree. C./W from the equation (4). 
However, according to the preferred embodiment of the present invention, 
the required thermal resistance value R.sub.TEMP of 40.degree. C./W can be 
realized by interposing the thermal resistive member 24 of 30.degree. C./W 
between IPD 10 and the die 23 without changing the thermal resistance 
value of IPD 10 itself. 
The thermal resistance value of the thermal resistive member 24 is 
adjustable in the following manner. A first method is to vary a connecting 
area (i.e., soldering area) of the thermal resistive member 24. The 
smaller the connecting area, the larger the thermal resistance value. A 
second method is to vary the thickness of the thermal resistive member 24. 
The larger the thickness, the larger the thermal resistance value. The 
smaller the thickness, the smaller the thermal resistance value. A third 
method is to vary the material (i.e., coefficient of thermal conductivity) 
of the thermal resistive member 24. 
Furthermore, it is possible to increase the thermal resistance value of IPD 
10 by reducing a soldering area of IPD 10. In other words, solder itself 
can be used as the thermal resistive member 24 of the present invention. 
This is advantageous in view of cost as well as simplified manufacturing 
process. 
IPD 10 can be constituted by a resin-molded semiconductor element shown in 
FIG. 3. According to the arrangement shown in FIG. 3, IPD 10 is 
electrically connected to a conductor pattern 28 formed on the printed 
board 21 by means of a lead 27. 
IPD 10 is not limited to a surface mounting type shown in FIG. 2 or 3. For 
example, IPD 10 can adopt a vertical mounting structure shown in FIG. 4. 
According to the arrangement of FIG. 4, IPD 10 is a resin-molded 
semiconductor element which is electrically connected to the printed board 
21 via a rigid lead 29. The lead 29 acts as a mechanical support for 
fixing IPD 10 to the printed board 21. IPD 10 is associated with a 
metallic radiation plate 30. The thermal resistive member 24 and the 
aluminum radiation plate 26 are securely fastened together with the 
metallic radiation plate 30 by a screw bolt 31. 
In both of the mounting structures shown in FIGS. 3 and 4, the thermal 
resistive member 24 is interposed in the heat radiation path extending 
from IPD 10 to the radiation plate 26. Accordingly, by adjusting the 
properties of the thermal resistive member 24, the sensitive temperature 
T.sub.SD can be set to a desirable value corresponding to the allowable 
load current I. 
FIG. 5 shows a relationship between the allowable load current I and the 
sensitive temperature T.sub.SD derived from the above-described equation 
(3). In FIG. 5, a solid curve represents a case where the ambient 
temperature T.sub.ROOM is is set to 70.degree. C., the thermal resistance 
value R.sub.TEMP is set to 10 .degree. C./W, and the on-resistance value 
R.sub.ON of the power MOSFET 11 is set to 0.2 .OMEGA.. 
When the ambient temperature T.sub.ROOM is larger than 70.degree. C., the 
curve representing the relationship between the allowable load current I 
and the sensitive temperature T.sub.SD shifts upward in FIG. 5 as shown by 
a dotted line. When the ambient temperature T.sub.ROOM is lower than 
70.degree. C., the curve representing the relationship between the 
allowable load current I and the sensitive temperature T.sub.SD shifts 
downward in FIG. 5 as shown by an alternate long and short dash line. 
Accordingly, the temperature-sensible and heat-damage protecting function 
of IPD 10 is effected at a lower load current when the ambient temperature 
T.sub.ROOM is higher than a predetermined sensitive temperature T.sub.SD. 
Accordingly, in an actual load actuating circuit, IPD 10 is located at a 
place where the circuit component is subjected to a severe heat 
environment. When an actual ambient temperature T.sub.ROOM exceeds the 
predetermined sensitive temperature T.sub.SD, the heat-damage protecting 
function of IPD 10 is effected at a load current lower than a 
predetermined set value. Thus, it becomes possible to provide a safe and 
reliable load actuating circuit which is capable of surely protecting the 
circuit components from any heat damages. 
The load actuating circuit of the above-described embodiments can be 
applied to any public welfare devices as well as automotive vehicles. In 
this case, it is preferable that the sensitive temperature is set in a 
range of 70.degree. C. to 140.degree. C. considering their operating 
conditions. Thus, IPD 10 can function as a current shutoff device working 
at a desirable sensitive temperature. No fuse is required. 
When the current shutoff function of IPD 10 is effected at the lower 
temperature range of 70.degree. C. to 140.degree. C., an ordinary 
operating temperature of a peripheral circuit element, such as the power 
MOSFET 11, is adequately suppressed below 140.degree. C. Accordingly, 
there is no necessity of guaranteeing performance of each circuit element 
in operation against higher temperatures. Effecting the current shutoff 
function at a lower temperature range makes it possible to arrange IPD 10 
by using a power MOSFET 11 having a smaller on-resistance value. In this 
case, when a plurality of IPDs 10 are mounted on the same circuit 
substrate respectively for corresponding plural electric loads, an overall 
heat generation can be reduced significantly. Thus, thermal design of the 
entire circuit can be simplified. 
According to the above-described embodiment, IPD 10 is disposed adjacent to 
the electric power terminal with respect to the load 2 so that the load 
circuit is formed between IPD 10 and the earth. However, it is possible to 
dispose IPD 10 adjacent to the earth with respect to the load 2 so that 
the load circuit is formed between IPD 10 and the electric power terminal. 
Furthermore, the P-channel power MOSFET 11 disclosed in FIG. 1 can be 
replaced by an n-channel power MOSFET. 
This invention may be embodied in several forms without departing from the 
spirit of essential characteristics thereof. The present embodiments as 
described are therefore intended to be only illustrative and not 
restrictive, since the scope of the invention is defined by the appended 
claims rather than by the description preceding them. And, all changes 
that fall within the metes and bounds of the claims, or equivalents of 
such metes and bounds, are therefore intended to be embraced by the 
claims.