Abstract
An activatable thermal fuse includes a first electrical terminal, a second electrical terminal, and an electrically conductive bridge element having a first electric contact with the first electrical terminal and a second electric contact with the second electrical terminal. At least a part of the bridge element is displaceable from a first position in which the first contact is established to a second position in which the first contact is opened, and a thermally sensitive member releases the part when exposed to a predetermined temperature value. An activating element blocks displacement of the part from the first position, in a first position of the activating element, and enables the displacement of the part in a second position of the activating element. A method of manufacturing a printed circuit board, a method of monitoring, and an electronic circuit including the thermal fuse are also provided.
Claims
1-23. (canceled)
24. An activatable thermal fuse comprising: a first electrical terminal and a second electrical terminal, an electrically conductive bridge element having a first electric contact with said first electrical terminal and having a second electric contact with said second electrical terminal, wherein at least a part of said bridge element is displaceable from a first position in which said first electric contact is established to a second position in which said first electric contact is opened, a thermally sensitive member holding said part in said first position and releasing said part as soon as said thermally sensitive member is exposed to a predetermined temperature value, a biasing member biasing said part towards said second position, and a mechanically displaceable activating element blocking displacement of said part from said first position, in a first position of said activating element, and enabling said displacement of said part in a second position of said activating element.
25. The activatable thermal fuse according to claim 24, comprising a guiding element defining a path of movement of said activating element, the path of movement including at least one of a translation, a rotation, or a screw movement.
26. The activatable thermal fuse according to claim 24, comprising a snapping mechanism preventing a returning of said activating element from the second position of said activating element to the first position of said activating element.
27. The activatable thermal fuse according to claim 24, wherein the predetermined temperature value is 240 C. or less.
28. The activatable thermal fuse according to claim 27, wherein the predetermined temperature value is in a range of from 150 C. to 240 C., inclusive.
29. The activatable thermal fuse according to claim 24, wherein the thermally sensitive member comprises a solder.
30. The activatable thermal fuse according to claim 24, wherein the thermally sensitive member comprises at least one of a bimetal strip, a bimetal disc, or a shape memory alloy.
31. The activatable thermal fuse according to claim 24, comprising a housing having a bottom side on which said first and second electrical terminals are arranged.
32. The activatable thermal fuse according to claim 31, wherein the activating element is integrated into the housing, and arranges as one piece with the housing, and the activating element is accessible from a top side and/or from a bottom side and/or from a lateral side of the housing.
33. The activatable thermal fuse according to claim 32, wherein the position of the activating element is visible from a top side of the housing.
34. The activatable thermal fuse according to claim 24, wherein said biasing member is a compressed, stretched, bended or twisted elastic element.
35. The activatable thermal fuse according to claim 24, wherein the biasing member comprises an elastic section of the bridge element.
36. The activatable thermal fuse according to claim 24, wherein the biasing member comprises a coil spring, a spiral spring or a leaf spring.
37. The activatable thermal fuse according to claim 24, wherein in the second position of said part of the bridge element said first and said second contacts are open, and wherein said bridge element is a substantially rigid element.
38. The activatable thermal fuse according to claim 24, wherein the thermal fuse is adapted to carry electrical current higher than 5 Ampere.
39. The activatable thermal fuse according to claim 38, wherein the thermal fuse is adapted to carry electrical current higher than 30 Ampere and up to 100 Ampere.
40. The activatable thermal fuse according to claim 24, wherein a current-limiting fuse element is arranged in a conducting path between said first and said second electrical terminal.
41. The activatable thermal fuse according to claim 40, wherein said current-limiting fuse element is formed as a constriction in a conducting path of an electrically isolating substrate.
42. The activatable thermal fuse according to claim 24, wherein at least said bridge element comprises an alloy having a temperature coefficient of resistance, and wherein the absolute value of said temperature coefficient of resistance is below 500 parts per million per Kelvin at room temperature.
43. The activatable thermal fuse according to claim 24, wherein the activatable thermal fuse includes further electrical terminals in addition to said first and said second electrical terminals.
44. An electronic circuit comprising an activatable thermal fuse according to claim 24, and further comprising a high power semiconductor device, wherein the thermal fuse is connected in series to a current conducting path of the high power semiconductor device.
45. An electronic circuit according to claim 44, wherein the activatable thermal fuse and the high power semiconductor device are arranged in a common housing.
46. A method of manufacturing a printed circuit board with an activatable thermal fuse, wherein the thermal fuse comprises: a first electrical terminal and a second electrical terminal, an electrically conductive bridge element having a first electric contact with said first electrical terminal and having a second electric contact with said second electrical terminal, wherein at least a part of said bridge element is displaceable from a first position in which said first electric contact is established to a second position in which said first electric contact is opened, a thermally sensitive member holding said part in said first position and releasing said part as soon as said thermally sensitive member is exposed to a predetermined temperature value, a biasing member biasing said part towards said second position, and a mechanically displaceable activating element blocking displacement from said part in said first position, in a first position of said activating element, and enabling said displacement of said part in a second position of said activating element, the method comprising the steps of: covering conductive soldering pads of the printed circuit board with a solder, positioning the first electrical terminal and the second electrical terminal of the thermal fuse on conductive soldering pads covered with said solder, ensuring that the activating element of the thermal fuse is in said first position of the activating element, heating the printed circuit board to a temperature above the melting point of the solder, cooling down the printed circuit board below the melting point of the solder, and moving the activation element of the thermal fuse into the second position of the activation element.
47. A method of monitoring the state of an activatable thermal fuse, wherein the thermal fuse comprises: a first electrical terminal and a second electrical terminal, an electrically conductive bridge element having a first electric contact with said first electrical terminal and having a second electric contact with said second electrical terminal, wherein at least a part of said bridge element is displaceable from a first position in which said first electric contact is established to a second position in which said first electric contact is opened, a thermally sensitive member holding said part in said first position and releasing said part as soon as said thermally sensitive member is exposed to a predetermined temperature value, a biasing member biasing said part towards said second position, and a mechanically displaceable activating element blocking displacement from said part in said first position, in a first position of said activating element, and enabling said displacement of said part in a second position of said activating element, the method comprising: measuring a voltage between two electrical terminals of said activatable thermal fuse; and determining at least one of a triggering state, a current or a temperature of the thermal fuse in response to the measuring of the voltage.
48. A method according to claim 47, comprising: measuring a voltage between said first electrical terminal and said second electrical terminal, and determining a current flowing through the activatable thermal fuse based on the measured voltage and a predetermined resistance value of the activatable thermal fuse.
Description
[0094] FIG. 1 shows schematically and simplified, the activatable thermal fuse according to the invention in four different states.
[0095] FIG. 1.a) shows the activatable thermal fuse in the disabled state. The temperature T of the fuse lies below the predetermined temperature value T.sub.L, which is characteristic for the thermal fuse. T.sub.L could for example be 200 C. An electrical connection is established from a first terminal 1 over an electrically conductive bridge element 3 to a second terminal 2. A part 4 of the bridge element is in a first position, such that the electrical contact to the first terminal 1 is established. A thermally sensitive member 5 holds the part 4 of the bridge element in the first position. A biasing member 6, here symbolically pictured as stretched coil spring, is biasing the part 4 in the direction B indicated by the arrow. An activating element 7 blocks the displaceability of the part 4 of the bridge element.
[0096] FIG. 1.b) shows the activatable thermal fuse still in the disabled state, but the temperature T of the fuse now lies above the predetermined temperature value T.sub.L. The thermally sensitive element 5 releases the part 4. This releasing is indicated by dashed lines. However, the part 4 is not displaced into its second position, because the activating element 7 is blocking the displaceability. The high temperature may occur because the first and second terminal 1, 2 are connected to leads 14, 15 shown on the left and the right by a reflow soldering process.
[0097] FIG. 1.c) shows the activatable thermal fuse in its activated state and operated at a temperature T below T.sub.L. By the application of an activating action in direction A indicated by an arrow, the activating element has been moved to the second position of the activating element. Only the thermally sensitive member 5 holds the part 4 of the bridge element in its first position.
[0098] FIG. 1.d) shows the activatable thermal fuse in its activated state and at a temperature T above T.sub.L. The thermally sensitive member 5 has released the part 4. The part 4 has been displaced into its second position under the biasing effect of the biasing member 6. Thereby, the electrical contact between the part 4 of the bridge element 3 and the first terminal has been opened. The biasing member here is symbolically pictured as contracted coil spring.
[0099] FIG. 2 shows a cross-section through an embodiment of the activatable thermal fuse and through a part of a printed circuit board.
[0100] FIG. 2.a) to 2.c) illustrate steps of the method of manufacturing a printed circuit board with an activatable thermal fuse. FIG. 2.d) shows the state of the same thermal fuse after it was triggered.
[0101] FIG. 2.a) shows the embodiment of the thermal fuse in the disabled state. The temperature T of the fuse lies below the predetermined temperature value T.sub.L. First and second terminals 1, 2 are arranged below a base plate 12. The displaceable part 4 of the bridge element has the form of a hat. It could e.g. be produced by deep-drawing a copper plate. The thermally sensitive member 5 constructed as soldering point, at which the bridge element is soldered to an electrical connection leading to the first terminal 1. A further thermally sensitive member 5 is formed as a similar soldering point and establishes the electrical connection from the part 4 of the bridge element to the second terminal 2. The solder used for the thermally sensitive elements has a melting point at temperature T.sub.L, the predetermined temperature value. A compressed coil spring sits in the hat form of the bridge element and exerts an upward directed biasing force onto the displaceable part 4 of the bridge element. A housing 11 encloses the thermal fuse. The housing e.g. consists of a plastic material resisting a temperature of 260 C., which makes it suitable for a reflow process. In the inside of the housing a guiding element for the activating element 7 is formed, that allows a horizontal translation of the activating element. The activating element is in its first position and blocks the displaceability, in this case an upward movement, of the part 4. The activating element has a through hole 16 in a middle part. The activating element protrudes out of right side of the housing. This way, it is clearly visible from outside, particularly from above, that the thermal fuse is in the disabled state. The thermal fuse is positioned on a printed circuit board 20. Soldering pads 21, 22 are covered with solder 23. The first and second terminals 1, 2 are each placed on top of one of the soldering pads.
[0102] FIG. 2.b) shows the embodiment of the thermal fuse still in the disabled state. The thermal fuse is heated up to a temperature above the melting point of the solder 23. The temperature T of the fuse now lies above the predetermined temperature value T.sub.L. Although the thermally sensitive member 5 melts at this temperature and loses its ability to hold the part 4 in place, the part 4 is blocked by the activating element 7 and does not move.
[0103] FIG. 2.c) shows the thermal fuse after being cooled down to a temperature T below T.sub.L. A soldering contact between soldering pads 21, 22 and the first and second terminal 1, 2 is established. The thermally sensitive member 5 is solid again and has regained its ability to hold part 4 in place. By an activating action in direction A the activating element has been moved into the second position of the activating element. Such an activating action consisting in pushing the protruding part of the activating element 7 into the housing 11 may be performed by a gripper that is as well used for pick and place activity. Now, the hole 16 of the activating element lies above the part 4 of the bridge element enabling an upward movement of the part 4. Only the thermally sensitive member 5 counteracts the biasing of part 4 in direction B. The thermal fuse is in the activated state.
[0104] FIG. 2.d) shows the activated thermal fuse after the temperature T has raised above T.sub.L. The solder forming the thermally sensitive member has melted and the part 4 of the bridge element was displaced into its second position. The coil spring 6 is expanded compared to FIG. 2.c) and holds the part 4 in its second position. The electrical contacts between the bridge element and the first and second terminal and therefore also the electrical contact between the terminals is interrupted.
[0105] FIG. 3 shows a schematic and simplified view of a further embodiment of the activatable thermal fuse in four different states. In this embodiment the displaceable part 4 of the bridge element is formed as electrically conductive section of a movable block 17 being non-conducting apart from the conductive section. Electrical contact to each of the first 1 and second 2 terminal is established via a sliding contact. In this embodiment the thermally sensitive member 5 is a bimetal strip having an interlocking connection to the movable block 17.
[0106] FIG. 3.a) shows the thermal fuse in the disabled state at temperature T<T.sub.L. The bimetal strip is straight such that the interlocking connection to the movable block 17 is established and the part 4 is hold in its first position.
[0107] FIG. 3.b) shows the thermal fuse in the disabled state at T>T.sub.L. The bimetal strip bends upward and the interlocking connection is released. The activating element 7 counteracts the biasing force in direction B of the biasing element 6, which in this embodiment is a compressed coil spring.
[0108] FIG. 3.c) shows the thermal fuse in the activated state at temperature T<T.sub.L. By applying a force in direction A the activating element 7 was pushed into a bore serving as guiding element. The activating element 7 is now in its second position, such that the displaceability of the movable block 17 and therewith the part 4 is enabled. A snapping mechanism 13 prevents a returning of the activating element from its second position into its first position.
[0109] FIG. 3.d) shows the thermal fuse in the activated state at temperature T>T.sub.L. The interlocking connection was released due to the upward bending of the bimetal strip acting as thermally sensitive member 5. The part 4 is in its second position. The sliding contact in connection with the first terminal 1 is only in contact with the non-conductive part of the movable block 17 and the electrically connecting path from the first terminal 1 to the second terminal 2 is interrupted.
[0110] FIG. 4.a) shows an electronical circuit 40 with an activatable thermal fuse according to the present invention. The thermal fuse 10 is mounted in proximity of a high power semiconductor device 41 and is connected in series to a current conducting path of the semiconductor device. The thermal fuse and the semiconductor device are surface mounted to a printed circuit board. The semiconductor device may e.g. be a field effect transistor (FET). The thermal fuse is of the type shown in FIG. 2.a) to 2.d). The thermal fuse is shown in the activated state, corresponding to the cross section shown FIG. 2.c). The activating element is in the second position, which in this embodiment is inside the housing of the activatable thermal fuse and therefore the activating element is not visible. With dashed lines a possible common housing 42 of the thermal fuse and the high power semiconductor device is indicated.
[0111] FIG. 4.b) shows the electronical circuit 40 of FIG. 4.a). Surface temperature is indicated by gray levels corresponding to temperature values. The temperature distribution shown is typical for a normal operating condition, in which the thermal fuse is not triggered.
[0112] FIG. 5 shows an embodiment of an activatable thermal fuse according to the present invention in four different views. In every one of these four views the thermal fuse is in the disabled state.
[0113] FIG. 5.a) is a perspective view from a viewpoint above the thermal fuse. This embodiment is activated by a force applied from the top side in the direction indicated by the arrow A. A groove 51 in the housing 11 guides the activating element 7 along a linear path of movement.
[0114] FIG. 5.b) is a perspective view of the same thermal fuse and from the same viewpoint as FIG. 5.a), but the housing is removed to show the parts inside the housing. The bridge element 4 establishes an electrical contact between the first terminal 1 and the second terminal 2. Thermally sensitive members 5, 5 are formed as thin solder layers at the interfaces between the bridge element 4 and the first and second terminals 1, 2 at the positions indicated by the reference signs.
[0115] FIG. 5.c) is a perspective view from a viewpoint below the thermal fuse allowing a direct view onto the first and second terminal 1, 2. The dash-dotted line C indicates the position of the cross-section shown in FIG. 5.d).
[0116] FIG. 5.d) is a cross-section through the thermal fuse. The cross-section through the thermal fuse is along a plane that lies in the middle between the first and second terminals 1, 2. A nose 52 formed as part of the activating element 7 holds the bridge element 4 in its position. It further provides a snapping mechanism, similar to a snap-fastener. The activating element 7 can be pushed down into its second position, whereby a mechanical resistance is surmounted and whereby a slight deformation of the activating element 7 occurs. With the activating element 7 being in its second position, the bridge element 4 is not hold in position anymore, as there is enough room above the bridge element for the bridge element to be moved out of its first position by the biasing element 6 once the solder forming the thermally sensitive members 5, 5 melts. The biasing element 6 is built as coil spring. It exerts an upwardly directed force onto the bridge element 4. This force is weaker than the force needed to snap the nose 52 across the bridge element 4. For activation of the thermal fuse according to this embodiment a larger force than this force of the spring has to be applied from the top side of the thermal fuse onto the activating element. The activating element 7 is directly accessible and visible from the top side.
[0117] FIG. 6 shows an embodiment of an activatable thermal fuse according to the present invention in two different views FIG. 6.a) and FIG. 6.b). In this embodiment, the bridge element comprises a current-limiting fuse element (61). In both views, the thermal fuse is in the disabled state.
[0118] FIG. 6.a) is a perspective view of the activatable thermal fuse with the housing being removed for better visibility of the components in the interior. In the embodiment shown here, the part 4 of the bridge element is constructed as electrically isolating substrate 63 carrying a conducting path 62, which is soldered to the first 1 and second 2 electrical terminals by the thermally sensitive members 5, 5. A constriction in the conducting path 62 forms a current-limiting fuse element 61. Due to the smaller cross-section in the region of the constriction, the current path melts here, if a current higher than the rated current of the fuse flows through the conducting path. The current-limiting fuse element 61 needs not to be activated. If an activating force is applied onto the activating element 7 in direction of A, the activating element 7 can be moved into the second position of the activation element 7 and thereby enabling an upward displaceability of part 4 of the bridge element. The biasing member 6 in form of a coil spring applies an upward acting biasing force onto the part 4.
[0119] FIG. 6.b) is a perspective view of half the activatable thermal fuse, the other half being cut-away. In FIG. 6.b) the housing 11 is present. The cutting plane cuts through the conducting path 62 at a position, where the current-limiting fuse element 61 is formed by a constriction in the conducting path. The thin layer of the conducting path is carried by an electrically isolating substrate 63. In the first position of the activating element 7 shown in this figure, two noses 52 on the activating element 7 block the displaceability of the part 4 of the bridge element with respect to the housing 11. A groove 51 in the housing guides the activating element 7 when the activating element is moved into its second position. The activating element 7 is deformable to a certain degree, such that the upper nose 52 can be pushed over the obstacle formed by part 4 and the adjacent part of the housing 11. The housing 11 holds the biasing member 6 in place.
[0120] FIG. 7.a) to 7.c) show schematic circuit diagrams of measuring configurations comprising an activatable thermal fuse 10 and a voltmeter 70 used to measure a voltage V as applied in the method of monitoring the state the thermal fuse.
[0121] FIG. 7.a) shows an activatable thermal fuse 10 with a current I flowing into the first electrical terminal 1 and flowing out of the second electrical terminal 2. When triggered, the activatable thermal fuse 10 interrupts the current I. A voltmeter 70 measures a voltage V between the terminal 1 and terminal 2. With the resistance of the thermal fuse known to have a value R, the current I can be calculated as I=V/R.
[0122] FIG. 7.b) shows an activatable thermal fuse 10, which has an additional electrical terminal 71. A voltmeter 70 is arranged to measure the voltage between terminals 2 and 71. This way it is e.g. possible to measure a thermally induced voltage over a thermocouple contact formed along the conducting path of the thermal fuse 10. Another possibility in this measuring configuration is to detect whether the bridge element is still in electrical contact to the terminal 2, i.e. whether the thermal fuse is in a triggered state or not.
[0123] FIG. 7.c) shows an activatable thermal fuse 10 having two additional electrical terminals 71, 72. A voltmeter 70 is arranged to measure the voltage between terminals 71 and 72. In this measurement configuration, e.g. a voltage drop over a defined section of the current conducting path of the thermal fuse is possible.
LIST OF REFERENCE SIGNS
[0124] 1 first terminal [0125] 2 second terminal [0126] 3 bridge element [0127] 4 part of the bridge element [0128] 5 thermally sensitive member [0129] 5 further thermally sensitive member [0130] 6 biasing member [0131] 7 activating element [0132] 10 activatable thermal fuse [0133] 11 housing [0134] 12 base plate [0135] 13 snapping mechanism [0136] 14, 15 leads [0137] 16 hole (in the activating element) [0138] 17 movable block [0139] 20 printed circuit board [0140] 21, 22 soldering pads [0141] 23 solder [0142] 40 electronical circuit with an activatable thermal fuse [0143] 41 high power semiconductor device [0144] 42 common housing (of 10 and 41) [0145] 51 groove (in the housing) [0146] 52 nose [0147] 61 current-limiting fuse element [0148] 62 conducting path [0149] 63 electrically isolating substrate [0150] 70 voltmeter [0151] 71, 72 additional electrical terminals [0152] A direction of activating action [0153] B direction of biasing [0154] I current [0155] T temperature [0156] T.sub.L predetermined temperature value [0157] V voltage
