A normally closed thermosensitive switch

Abstract

The normally closed thermal switch of the present invention includes a first conductor and a second conductor passing through a cavity in a housing. One end of the first conductor and one end of the second conductor, located cantilevered inside the housing, are in direct conductive contact to form a series circuit. All current flows through the series circuit, or a current-limiting circuit is connected in parallel with the formed series circuit, allowing the current to flow primarily through the series circuit. An expansion bladder is provided in the housing. When the current-limiting circuit is provided, the current-limiting circuit heats the expansion bladder. When the temperature of the expansion bladder reaches a set temperature threshold, the expansion bladder expands towards the first conductor, and during the expansion process, it drives the first conductor to displace, causing the first and second conductors to break at the direct contact point. The series circuit is broken, causing the normally closed thermal switch to open; alternatively, the series circuit is broken, and all current flows through the current-limiting circuit. The normally closed thermal switch of the present invention has a simple structure, fast response speed, and can protect power semiconductor devices.

Description

Technical Field

[0001] This invention relates to the fields of power control and electric vehicles, and in particular to protective devices for power semiconductor devices in the event of fault current. Background Technology

[0002] Power devices are a crucial component of modern power electronic systems. They primarily refer to electronic components designed to handle high voltage and high current, achieving power conversion and control, such as MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), IGBTs (Insulated-Gate Bipolar Transistors), and power diodes. These devices possess high efficiency, low loss, and excellent switching characteristics, and are widely used in motor control, solar photovoltaics, and new energy vehicles. However, in practical applications, these devices may face faults such as overcurrent, overvoltage, and short circuits, leading to serious consequences such as excessively high device temperatures, performance degradation, reduced lifespan, and even damage. Therefore, designing efficient protection methods for power devices is essential to ensuring their reliability and lifespan.

[0003] There are several protection methods currently used for power devices:

[0004] 1. Fuse: A fuse is a traditional protection method. Fuses are typically connected in series at the power input of a circuit to control the total current. Their working principle relies on the increased fault current flowing through the fusible element after a circuit fault, causing it to heat up and melt, thus cutting off the power supply and achieving the protection purpose. They have advantages such as simple implementation, easy maintenance, low cost, and thorough power cut-off during protection, and are therefore widely used in all current electronic circuits and electronic devices.

[0005] The drawback of this method is that the fuse carries the total current of the circuit, and the change in the operating current of a single power device is insufficient to cause the fuse to melt quickly. Therefore, it cannot provide rapid protection for the power device and can only prevent the fault from escalating further.

[0006] 2. The main circuit current detection method or the power device operating current detection method involves inserting a detection element (resistor or current transformer, etc.) in series in the main circuit or power device operating circuit path. By detecting the current signal on the detection element through the detection of the operating current of the main circuit or power device path, and then processing it through the circuit to obtain a fault signal, the current path is controlled to be shut off for protection.

[0007] The drawback of this method is that it only responds after the fault current has formed, resulting in a lag between the detection results and the protection action. It cannot provide rapid protection for power devices and can only prevent the fault from escalating further.

[0008] 3. Parallel voltage drop detection method: This method requires designing a protection circuit connected in parallel with the protected power device. It obtains a signal by detecting the voltage drop of the protected device during operation, and determines whether a fault has occurred based on the voltage reading. The protection method employs a local protection approach, forcibly cutting off the control signal of the protected power device to stop its operation. This method offers relatively accurate measurement of operating status and fault diagnosis, resulting in ideal protection performance.

[0009] The drawbacks of this method are: the protection circuit design is relatively complex, requiring the arrangement of electronic components and the design of certain control logic, high manufacturing requirements, high cost, and poor versatility. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to provide protection for the power semiconductor device by connecting a normally closed thermal switch in series in the circuit where the power semiconductor device is located during a fault current. The thermal energy generated by the power semiconductor device during the fault current triggers the normally closed thermal switch to operate, disconnecting the circuit where the power semiconductor device is located, or limiting the current of the power semiconductor device and forcibly reducing the current in the circuit where the power semiconductor device is located, thereby achieving protection for the power semiconductor device.

[0011] To solve the above-mentioned technical problems, the present invention provides a normally closed thermal switch, comprising a housing with a cavity, in which a first conductor and a second conductor are disposed. The two ends of the first conductor and the second conductor are located inside and outside the housing, respectively. One end of the first conductor and the second conductor located outside the housing serves as the connection end of the thermal switch. The one end of the first conductor and the one end of the second conductor located inside the housing, which are cantilevered, are in direct conductive contact to form a series circuit. All the current flowing through the normally closed thermal switch flows through the series circuit, or a current-limiting circuit is connected in parallel to the series circuit. The current flowing through the normally closed thermal switch mainly flows through the series circuit. An expansion bladder is provided in the housing. When the current limiting circuit is provided, the current limiting circuit heats the expansion bladder. When the temperature of the expansion bladder reaches a set temperature threshold, the expansion bladder can expand towards the first conductor under the action of heat energy. During the expansion process, it drives the end of the first conductor that is in contact with the second conductor to move away from the second conductor, causing the first conductor and the second conductor to break apart from the direct contact point. The series circuit breaks, causing the normally closed thermal switch to open. Alternatively, the series circuit breaks, and all the current flows through the current limiting circuit.

[0012] Preferably, one end of the second conductor located in the cavity of the housing is fixedly disposed on the housing, and the other end of the first conductor located in the cavity of the housing, after being bent and cantilevered, is conductively connected to the second conductor; an accommodating space is provided in the housing, the opening end of the accommodating space faces the cantilevered first conductor, the expansion bladder is located in the accommodating space, and the accommodating space limits the expansion bladder to only expand towards the opening end of the accommodating space, the first conductor cantilevered across the opening end of the accommodating space, located on the expansion path of the expansion bladder and in contact with the expansion bladder; when the expansion bladder expands towards the first conductor, it drives the end of the first conductor that is conductively in contact with the second conductor to displace away from the second conductor, so that the direct contact point between the first conductor and the second conductor is broken.

[0013] Preferably, a rib is provided in the housing, the rib and the inner wall of the housing forming the accommodating space, and the first conductor is laid on the rib.

[0014] Preferably, a positioning groove is provided on the rib, and the first conductor is interlocked in the positioning groove.

[0015] Preferably, the expansion bladder is disposed on a current-limiting component that can be heated, and the two ends of the current-limiting component are electrically connected to the first conductor and the second conductor in the housing to form the current-limiting circuit; when the expansion bladder disconnects the first conductor and the second conductor from the direct conductive contact, all current flows through the first conductor, the current-limiting component and the second conductor connected in series to form the current-limiting circuit.

[0016] Preferably, the current-limiting component is a heating resistor.

[0017] Preferably, an indicator is also provided inside the housing. When the expansion bladder expands and drives the end of the first conductor that is in conductive contact with the second conductor to move, the first conductor drives the indicator to move toward the outside of the housing, so that the end of the indicator facing the outside of the housing extends out of the housing wall.

[0018] Preferably, a through-hole is provided in the shell wall of the housing, the indicator is installed in the mounting hole, and the outer periphery of the indicator is provided with an elastic barb structure, the barb structure elastically abutting against the mounting hole.

[0019] Preferably, one end of the first and second conductors located outside the housing is disposed outside the housing as a connection terminal of the thermal switch in a patch, bent flat-out, or straight quick-connect manner.

[0020] Preferably, the expansion bladder includes an elastic bag and a gas, liquid, or solid with a high coefficient of thermal expansion filled in the bag. When the solid with a high coefficient of thermal expansion is an integral structure and remains an integral structure after expansion, the expansion bladder is composed of a solid with a high coefficient of thermal expansion. Alternatively, the expansion bladder includes a stretchable bag and a gas, liquid, or solid with a high coefficient of thermal expansion filled in the bag. When the expansion bladder is not inflated, it is in a compressed state. When the expansion bladder inflates, the expansion energy can deform from a compressed state to a stretched state.

[0021] Preferably, the outer periphery of the expansion bladder, perpendicular to the direction of the force exerted by the expansion bladder on the first conductor, is configured as a retractable serrated structure.

[0022] Preferably, the housing includes a cover and a base, the base being located at the open end of the cover and closing the open end; a rib is provided on the base, the inner surface of the base, the rib and the side wall of the cover cooperate to form an accommodating space, the first conductor cantilevered over the accommodating space, and the expansion bladder is disposed in the accommodating space and contacts the first conductor.

[0023] Preferably, the cover and the base are connected and fixed by a snap-fit ​​mechanism.

[0024] This invention employs an expansion bladder that can expand under heating. The driving force generated when the expansion bladder expands causes the normally closed thermal switch to open or enter a current-limiting mode. It activates when the temperature of the power semiconductor rises to a certain threshold, providing a rapid response and quick disconnection or current limiting. Moreover, the normally closed thermal switch is a simple device with a simple structure and is easy to connect to the circuit containing the power semiconductor device.

[0025] A normally closed thermal switch expands and disconnects due to an increase in temperature. When the temperature decreases, the first and second conductors can be switched on again.

[0026] Therefore, the normally closed thermal switch of the present invention is low in cost, can effectively protect power semiconductor devices, and improve the service life of power semiconductor devices. Attached Figure Description

[0027] Figure 1 This is a cross-sectional view of a normally closed thermal switch in its initial state.

[0028] Figure 2 yes Figure 1 A side view structural diagram.

[0029] Figure 3 yes Figure 1 A schematic diagram of the structure after the action.

[0030] Figure 4 This is a schematic diagram of a normally closed thermal switch structure with a serrated expansion bladder in its initial state.

[0031] Figure 5 yes Figure 4 A schematic diagram of the structure after the action.

[0032] Figure 6 This is a cross-sectional view of a normally closed thermal switch in its initial state, equipped with an indicator.

[0033] Figure 7 yes Figure 6 The structural diagram after the action.

[0034] Figure 8 This is a cross-sectional view of a normally closed thermal switch when the initial state of the current-limiting components is set.

[0035] Figure 9 yes Figure 8 The structural diagram after the action.

[0036] Figure 10 This is a schematic diagram of a normally closed thermal switch with a bent, flat connection end.

[0037] Figure 11 This is a schematic diagram of a normally closed thermal switch with a quick-connect structure.

[0038] Figure label:

[0039] 1. Cover body, 11. Buffer rib, 2. Base, 3. First conductor, 31. One end of the first conductor located outside the shell, 32. One end of the first conductor located inside the shell, 4. Second conductor, 41. One end of the second conductor located outside the shell, 5. Protruding rib, 6. Expansion bladder, 6a. Serrated structure, 61. Indicator, 7. Heating resistor, 8. Detailed Implementation

[0040] The normally closed thermal switch of the present invention includes a housing with a cavity. A first conductor and a second conductor are disposed within the cavity of the housing. The two ends of the first conductor and the second conductor are located inside and outside the housing, respectively. One end of the first conductor and the second conductor located outside the housing serve as the connection end of the thermal switch. One end of the first conductor and the other end of the second conductor located inside the housing, which are arranged in a cantilever configuration, are in direct conductive contact to form a series circuit. All the current flowing through the normally closed thermal switch flows through the series circuit. Alternatively, a current limiting circuit is connected in parallel to the formed series circuit, so that the current flowing through the normally closed thermal switch mainly flows through the series circuit. An expansion bladder is disposed in the housing. When the current limiting circuit is provided, the current limiting circuit heats the expansion bladder. When the temperature of the expansion bladder reaches a set temperature threshold, the expansion bladder can expand towards the first conductor under the action of heat energy. During the expansion process, the end of the first conductor that is in contact with the second conductor is driven to move away from the second conductor, causing the first conductor and the second conductor to disconnect from the direct contact point. The series circuit is disconnected, causing the normally closed thermal switch to open. Alternatively, the series circuit is disconnected, and all the current flows through the current limiting circuit.

[0041] The following describes preferred embodiments in detail with reference to the accompanying drawings. The directional terms used are for reference only and do not constitute a limitation on the technical solution of this invention.

[0042] See Figures 1 to 3 A normally closed thermal switch mainly includes a housing, a first conductor, a second conductor, and an expansion chamber. The housing is made of insulating material and includes a cover 1 and a base 2. The cover 1 is a cap-like structure with one open end and one closed end. The base 2 is positioned at the open end of the cover 1 and closes the open end, forming a relatively sealed housing with a cavity inside. The side walls of the base 2 and the cover 1 are equipped with snap-fit ​​structures. The base 2 is connected and fixed to the cover 1 by snap-fit. For example, a protruding snap-fit ​​protrusion is provided on the side wall of the base 2 that mates with the cover 1, and a snap-fit ​​hole is provided on the corresponding side wall of the cover 1. During assembly, the snap-fit ​​protrusion of the base 2 is located in the cover 1 and the snap-fit ​​hole, forming a fixed connection. The cover 1 and the base 2 can also be connected and fixed by bolts, rivets, laser welding, or adhesive bonding.

[0043] The first conductor 3 and the second conductor 4 pass through the contact surfaces between the cover 1 and the base 2 on opposite sides of the housing, respectively, so that one end of the first conductor 3 and the second conductor 4 are located in the cavity inside the housing, and the other end (31, 41) is located outside the housing and is bent and attached to the outer surface of the base 2 in a patch manner. The end of the second conductor 4 located inside the housing is attached to the inner surface of the base 2 and is fixed horizontally. The first conductor 3 located inside the housing first passes through the contact surface between the base 1 and the cover 1, attaches to the inner wall of the cover 1 and is a certain distance away from the base 2, and is bent towards the base and cantilevered between opposite sides in the housing. The cantilevered end (32) of the first conductor 3 located inside the housing overlaps the second conductor 4 inside the housing, so that the first conductor 3 and the second conductor 4 are in direct conductive contact to form a series circuit in which the first conductor 3 and the second conductor 4 are directly connected in series. That is, in the initial and normal working state, the first conductor 3 and the second conductor 4 are connected in series and the normally closed thermal switch is in the closed conducting state. In order to improve the reliability of conductive contact, the first conductor 3 after bending inside the housing has a certain feature. One end (32) of the first conductor 3 and the second conductor 4 in conductive contact is set as an arc-shaped structure protruding towards the second conductor 4. The bottom of the arc-shaped structure makes conductive contact with the second conductor 4. The protruding arc-shaped structure is used to shorten the contact stroke between the first conductor and the second conductor, reduce the device volume, and also increase the contact area. It can also be set as a flat shape.

[0044] The structure of the connection terminals of a normally closed thermal switch, such as... Figure 1 As shown, the first conductor 3 and the second conductor 4 are located outside the housing, and one end (31, 41) of the connection terminal of the normally closed thermal switch is set as a surface mount structure. In other embodiments, other structures can also be used, such as... Figure 10 As shown, the ends of the first conductor 3 and the second conductor 4 located outside the housing are bent at a 90-degree angle at the open end face of the cover 1 towards the opposite sides of the cover 1, creating a bent, flat-out connection end for the normally closed thermal switch. A connection hole is provided on the bent, flat-out connection end to facilitate bolting the normally closed thermal switch into the circuit. For example... Figure 11As shown, the ends of the first conductor 3 and the second conductor 4 located outside the housing do not need to be bent. They extend directly from between the cover and the base outside the housing in a straight-out quick-plug structure on the same side outside the housing. The connection end of the normally open thermal switch of this structure facilitates quick plug-in installation with the external mounting base. In this scheme, in the initial state, the expansion bladder is confined inside the housing. Preferably, the expansion bladder can have only one free end, thereby limiting the expansion direction of the expansion bladder. This allows the expansion bladder to expand in the direction that pushes the end of the first conductor located inside the housing away from its initial position when it reaches a temperature threshold, thus separating the first conductor from the second conductor. Different limiting methods can be used for limiting. For example, the bladder material can be used in conjunction with the internal structure of the housing to achieve limiting with only one free end. Alternatively, the following embodiment can be used to limit the expansion bladder within a receiving space with an open end. The receiving space can be integrally formed by the internal structure of the housing or formed by other components installed inside the housing, allowing the expansion bladder to expand or extend in only one direction within the receiving space. When the expansion bladder has only one free end, its expansion direction is definite during expansion, making the product more reliable and utilizing energy more effectively. Specifically, this can be achieved as described in this solution. Figure 1 As shown in the diagram, a rib 5 is provided on the inner surface of the base 2. The rib 5, the inner surface of the base 2, and the inner sidewall of the cover 1 form an accommodating space with one open end. The accommodating space is located on one side of the housing where the first conductor 3 is bent. After being bent, the first conductor 3, located inside the housing, cantilevered over the opening end of the accommodating space and over the rib 5, and then made conductive contact with the second conductor 4. That is, the opening end of the accommodating space is oriented towards the cantilevered first conductor. The first conductor 3, located within the housing, can have an arc-shaped bend at one end to improve the reliability of the conductive contact between the first conductor 3 and the second conductor 4. The bent first conductor 3 within the housing possesses a certain rigidity, ensuring that one end of the first conductor 3 within the housing maintains its initial state in a free state, thus ensuring a stable conductive contact between the first and second conductors. Alternatively, a positioning groove (not shown) can be formed on the top of the rib 5 facing the top of the cover 1. The cantilevered first conductor 3 is slightly press-fitted into the positioning groove on the rib 5, positioning the initial position of the cantilevered first conductor 3 and improving the reliability of the conductive contact between the first conductor 3 and the second conductor 4. To buffer the impact force on the cover 1 caused by the displacement of the first conductor 3 driven by the expansion bladder 6 after expansion, a buffer rib 11 is provided at the top of the cover 1. When the first conductor 3 reaches its final position, it contacts the buffer rib 11, buffering the impact force.

[0045] The expansion bladder 6 is disposed in the accommodating space between the first conductor 3 and the base 2. The accommodating space restricts the expansion direction of the expansion bladder, so that it can only expand along the direction towards the top of the cantilevered first conductor 3 and the cover 1. This structure can ensure that when the temperature of the expansion bladder reaches or exceeds the designed temperature threshold, the expansion bladder 6 expands, and its expansion degree can drive the end of the first conductor 3 that is in conductive contact with the second conductor 4 to move away from the second conductor 4, that is, towards the top of the cover 1, so that the first conductor 3 and the second conductor 4 are separated from conductive contact, and the ends of the first conductor 3 and the second conductor 4 that are in direct conductive contact are insulated, so that the series circuit formed by the direct conductive contact of the first conductor 3 and the second conductor 4 is broken, thereby turning off the normally closed thermal switch.

[0046] The expansion bladder 6 comprises an elastic bladder and a filler with a high coefficient of expansion within the bladder. The filler can be a gas, liquid, or solid with a high coefficient of expansion. Liquids include kerosene, for example. When a solid with a high coefficient of expansion is used as the filler, if the solid is a monolithic structure and maintains its monolithic structure after expansion, the bladder can be omitted, and the solid can be used directly as the expansion bladder. Generally, the outer periphery of the expansion bladder is a regular shape, such as a smooth columnar structure. When the bladder is elastic, the expansion bladder 6 is in a contracted state when not inflated. When the expansion bladder 6 inflates, the bladder of the expansion bladder 6 undergoes elastic deformation under the expansion of the filler, expanding towards the opening end of the accommodating space under the constraint of the accommodating space.

[0047] The bladder 6 can also be a non-elastic bladder, or a non-elastic, stretchable bladder. When the bladder is not inflated, the bladder is in a compressed state. When the bladder inflates, the expanding filler drives the bladder to deform from a compressed state to a stretched state. For example, the bladder could be a columnar steel structure with a wavy or serrated outer circumferential surface, allowing the bladder to be compressed and stretched. When the filler inside the bladder is not inflated, i.e., at the initial position of the bladder 6, it is in a compressed state. When it inflates, the high expansion coefficient of the filler expands, driving the bladder 6 to change from a compressed state to a stretched state, thereby driving the displacement of the first conductor.

[0048] for example Figure 4 and Figure 5 The serrated expansion bladder 6a shown has a serrated outer circumferential surface 61 perpendicular to the direction of expansion from the base 2 toward the top of the cover 1. When the expansion bladder 6 is not inflated, see [reference needed]. Figure 4 The inflatable bladder 6 is in a compressed state. When the inflatable bladder 6 inflates, see... Figure 5 The high expansion coefficient of the filler expands, driving the expansion bladder 6 to change from a compressed state to a stretched state towards the opening of the accommodating cavity. Figure 4 and Figure 5The stretchable and non-elastic bladder shown has its expansion bladder expanding only in the direction of the opening of the accommodating space, which can further constrain the expansion direction of the expansion bladder 6.

[0049] Figures 1 to 3 Working principle:

[0050] When a normally closed thermal switch is connected in series in a circuit near a power semiconductor device, under normal operating conditions, all current flows through the series circuit formed by the first conductor and the second conductor connected in series.

[0051] When a fault current occurs, the heat generated by the normally closed thermal switch itself and the heat generated by nearby power semiconductor devices causes the expansion bladder 6 to expand. During the expansion, the end of the first conductor 3 that is in conductive contact with the second conductor 4 is displaced towards the top of the cover 1, insulating the first conductor 3 and the second conductor 4 and breaking the series circuit formed by the first conductor 3 and the second conductor 4. This causes the normally closed thermal switch to open, disconnecting the circuit containing the power semiconductor device and protecting it. The advantage of the normally closed thermal switch of this invention is that during the transition from normal current to fault current, the expansion bladder gradually expands until the first conductor and the second conductor lose their conductive contact. When the fault current is eliminated or current is limited, the generated heat decreases, and the expansion bladder gradually contracts, allowing the first conductor and the second conductor to make conductive contact again. In other words, the normally closed thermal switch can be reused, unlike fuses, which are non-reusable products.

[0052] In other embodiments, an indicator 7 may also be provided in the normally closed thermal switch to indicate whether the normally closed thermal switch is operational, and also to limit the movement of the first conductor. See below. Figures 6 to 7A mounting hole is provided at the top of the cover 1 at the position corresponding to the contact point between the first conductor 3 and the second conductor 4. An indicator 7 is installed in the mounting hole. One end of the indicator 7 is fixed in the mounting hole at the top of the cover 1, and the other end is located in the cavity of the housing. The indicator 7 is T-shaped, with its larger end located inside the housing, which increases the contact area and facilitates the driving of the indicator 7 when the first conductor 3 is displaced. For the initial position and positioning of the indicator 7, a flexible barb structure (not shown) is provided on the outer periphery of the end of the indicator 7 located at the mounting hole. The flexible barb structure is interference-fitted with the mounting hole. When the expansion bladder 6 drives the first conductor 3 to displace, the end of the first conductor 3 drives the indicator 7 to move towards the outside of the housing. The indicator 7 overcomes the positioning of the barb structure, and the end of the indicator 7 facing the outside of the housing protrudes from the housing wall and is located on the outside of the housing, indicating to the operator that the normally closed thermal switch has been activated. After the indicator 7 is activated, the flexible barb structure moves to the outside of the housing, forming a position limit and preventing the indicator 7 from re-entering the housing. When the temperature drops and the expansion bladder contracts, the first conductor resets, and the indicator can be manually pressed back into the housing, so that the barbed structure of the indicator re-interferences with the mounting hole.

[0053] In other embodiments, to improve the response speed of the normally closed thermal switch, a heating resistor 8 is provided at the expansion bladder 6. See also Figures 8 to 9 A heating resistor 8 is installed on the base 2 at the bottom of the accommodating space inside the housing. One end of the heating resistor 8 is electrically connected to the first conductor 3 located inside the housing. The connection can be made by welding, conductive contact, or other conductive connection methods. The other end of the heating resistor 8 passes through the base 2 and is electrically connected to the end of the second conductor 4 located outside the housing. This is equivalent to having a heating resistor 8 connected in parallel at the conductive connection point of the first conductor 3 and the second conductor 4. Since the resistance of the heating resistor 8 is much greater than the resistance of the first and second conductors, under normal operating conditions, the current mainly flows through the series circuit formed by the direct conductive contact between the first and second conductors, with only a very small current flowing through the heating resistor.

[0054] Figures 8 to 9 Working principle:

[0055] Under normal operating conditions, the current mainly flows through the series circuit formed by the direct conductive contact between the first and second conductors.

[0056] When a fault current occurs, the temperature of the heating resistor rises rapidly, heating the expansion chamber. Combined with the heat energy of the first and second conductors themselves, this causes the expansion chamber to expand rapidly, driving the first conductor 3 away from the second conductor 4. This causes the first conductor 3 and the second conductor 4 to come into direct contact, breaking the series circuit. Since the resistance of the heating resistor is much greater than the resistance of the first and second conductors, the first conductor, the heating resistor, and the second conductor are connected in series to form a current-limiting circuit. All current flows through this current-limiting circuit with the heating resistor in series. Due to the presence of the heating resistor, the current in the current-limiting circuit drops linearly to within the allowable current range, preventing the power semiconductor device from being subjected to fault current and thus protecting the power semiconductor device. In this structure, only by completely disconnecting the circuit and lowering the overall temperature of the normally closed thermal switch can the normally closed thermal switch return to normal operation.

Claims

1. A normally closed thermal switch, characterized in that, The device includes a housing with a cavity. A first conductor and a second conductor are inserted into the cavity of the housing. The two ends of the first and second conductors are located inside and outside the housing, respectively. One end of the first and second conductors located outside the housing serves as the connection end of a thermal switch. One end of the first and second conductors located inside the housing, which are cantilevered, directly contacts each other to form a series circuit. All current flowing through the normally closed thermal switch flows through the series circuit. Alternatively, a current-limiting circuit is connected in parallel to the series circuit, so that the current flowing through the normally closed thermal switch mainly flows through the series circuit. An expansion bladder is provided in the housing. When the current-limiting circuit is provided, the current-limiting circuit heats the expansion bladder. When the temperature of the expansion bladder reaches a set temperature threshold, the expansion bladder can expand towards the first conductor under the action of heat energy. During the expansion process, the end of the first conductor that is in contact with the second conductor is displaced away from the second conductor, causing the first and second conductors to disconnect from the direct contact point. The disconnection of the series circuit causes the normally closed thermal switch to open. Alternatively, the disconnection of the series circuit allows all current to flow through the current-limiting circuit.

2. The normally closed thermal switch according to claim 1, characterized in that, One end of the second conductor located in the cavity of the housing is fixedly disposed on the housing. The first conductor located in the cavity of the housing is bent and cantilevered, with one end conductively connected to the second conductor. An accommodating space is provided in the housing, with the opening of the accommodating space facing the cantilevered first conductor. The expansion bladder is located in the accommodating space, which restricts the expansion bladder to expand only towards the opening of the accommodating space. The first conductor cantilevered across the opening of the accommodating space, located on the expansion path of the expansion bladder and in contact with the expansion bladder. When the expansion bladder expands towards the first conductor, it drives the end of the first conductor that is conductively in contact with the second conductor to displace away from the second conductor, causing the direct contact point between the first conductor and the second conductor to break.

3. The normally closed thermal switch according to claim 2, characterized in that, A rib is provided in the housing, and the rib and the inner wall of the housing form the accommodating space. The first conductor is laid on the rib.

4. The normally closed thermal switch according to claim 3, characterized in that, A positioning groove is provided on the rib, and the first conductor is laid in the positioning groove in an interference fit.

5. The normally closed thermal switch according to claim 1, characterized in that, The expansion bladder is mounted on a current-limiting component that can be heated. The two ends of the current-limiting component are electrically connected to the first conductor and the second conductor in the housing to form the current-limiting circuit. When the expansion bladder disconnects the first conductor and the second conductor from their direct conductive contact, all current flows through the current-limiting circuit formed by the first conductor, the current-limiting component, and the second conductor connected in series.

6. The normally closed thermal switch according to claim 5, characterized in that, The current-limiting component is a heating resistor.

7. The normally closed thermal switch according to claim 1, characterized in that, An indicator is also installed inside the housing. When the expansion bladder expands and drives the end of the first conductor that is in conductive contact with the second conductor to move, the first conductor drives the indicator to move toward the outside of the housing, so that the end of the indicator facing the outside of the housing extends out of the housing wall.

8. The normally closed thermal switch according to claim 7, characterized in that, A through-hole is provided in the shell wall of the housing, and the indicator is installed in the mounting hole. The outer periphery of the indicator is provided with an elastic barb structure, which elastically abuts against the mounting hole.

9. The normally closed thermal switch according to claim 1, characterized in that, One end of the first and second conductors located outside the housing is disposed outside the housing as a connection terminal of the thermal switch in a patch, bent flat-out, or straight quick-connect manner.

10. The normally closed thermal switch according to claim 1, characterized in that, The expansion bladder includes an elastic bag and a gas, liquid, or solid with a high coefficient of thermal expansion filled in the bag. When the solid with a high coefficient of thermal expansion is an integral structure and remains an integral structure after expansion, the expansion bladder is composed of a solid with a high coefficient of thermal expansion. Alternatively, the expansion bladder includes a stretchable bag and a gas, liquid, or solid with a high coefficient of thermal expansion filled in the bag. When the expansion bladder is not expanded, it is in a compressed state. When the expansion bladder expands, the expansion energy can deform from a compressed state to a stretched state.

11. The normally closed thermal switch according to claim 10, characterized in that, The outer periphery of the expansion bladder, perpendicular to the direction of the force exerted by the expansion bladder on the first conductor, is configured with a retractable serrated structure.

12. The normally closed thermal switch according to any one of claims 1 to 11, characterized in that, The housing includes a cover and a base. The base is located at the open end of the cover and closes the open end. A rib is provided on the base. The inner surface of the base, the rib and the side wall of the cover form an accommodating space. The first conductor cantilevered over the accommodating space. The expansion bladder is disposed in the accommodating space and contacts the first conductor.

13. The normally closed thermal switch according to claim 12, characterized in that, The cover and the base are connected and fixed by a snap-fit ​​mechanism.

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