Overcurrent protection using depletion-mode MOSFETs and bimetallic temperature-sensing switches.

Abstract

To provide a circuit and a device that provide overcurrent and overvoltage protection.SOLUTION: A protection device 100 has a depression mode MOSFET (D MOSFET) as a current limiter, and the D MOSFET is connected to a bimetal switch, which acts as a temperature sensing circuit breaker. The D MOSFET and the bimetal switch can be combined to limit a current to a downstream circuit component, thereby protecting the component from damage.SELECTED DRAWING: Figure 1

Description

[Background technology]

[0001] An overcurrent or excess current is a condition in which more current flows through a circuit than intended. Overcurrents can be constant or transient. A voltage transient is a short-duration surge of electrical energy resulting from the sudden release of energy that was previously stored or induced by a large dielectric load or other means such as lightning. Repeatable transients are often caused by motor operation, generators, or switching of reactance circuit components. Random transients can be caused by lightning and electrostatic discharge (ESD).

[0002] The miniaturization of components has led to increased sensitivity to electrical stress. For example, microprocessors have structures and conductive paths that cannot handle the high currents caused by ESD transients. Since such components operate at very low voltages, controlling voltage disturbances is a priority to prevent device interruption and potential or catastrophic failure.

[0003] With respect to these and other considerations, the improvements of the present invention may be useful. [Overview of the Initiative]

[0004] This summary is provided to introduce a set of concepts in a simplified form, which will be further discussed in a more detailed explanation. This summary is not intended to identify any important or essential features of the claimed subject matter, nor is it intended to help determine the scope of the claimed subject matter.

[0005] An exemplary embodiment of a circuit capable of operating to provide overcurrent protection is disclosed. The circuit includes a metal oxide semiconductor field-effect transistor (MOSFET) connected to a bimetallic switch. The bimetallic switch has first and second terminals, each terminal connected to a node, one of which is connected to the drain of the MOSFET. Together, the bimetallic switch and the MOSFET protect the circuit during an overcurrent event.

[0006] An exemplary embodiment of a device operable to provide overcurrent protection for a circuit is disclosed. The device includes a MOSFET connected to the circuit and connected to a bimetal switch. The bimetal switch has first and second terminals, each terminal is connected to a node, and one of the nodes is connected to the drain of the MOSFET. Both the bimetal switch and the MOSFET protect the circuit during an overcurrent event.

Brief Description of the Drawings

[0007] [Figure 1] FIG. showing a protection circuit including a bimetal switch and a D MOSFET according to an exemplary embodiment.

[0008] [Figure 2] FIG. showing a protection circuit including a stand-alone bimetal switch according to an exemplary embodiment.

[0009] [Figure 3] FIG. showing a device for providing overcurrent protection for a circuit according to an exemplary embodiment.

[0010] [Figure 4] FIG. showing a bimetal switch fixed to a D MOSFET with screws and bolts according to an exemplary embodiment.

[0011] [Figure 5] FIG. showing the response waveform of an experiment performed using the stand-alone bimetal switch of FIG. 2 according to an exemplary embodiment.

[0012] [Figure 6] FIG. showing the response waveform of an experiment performed between the bimetal switch and the D MOSFET of FIG. 3 according to an exemplary embodiment. [Figure 7] FIG. showing the response waveform of an experiment performed between the bimetal switch and the D MOSFET of FIG. 3 according to an exemplary embodiment.

[0013] [Figure 8] This table provides the results of overcurrent test current operation performed on the circuits shown in Figures 2 and 3 according to exemplary embodiments.

[0014] [Figure 9] This graph compares the trip response times between a circuit having an independent bimetal switch and a circuit having a bimetal switch including a D MOSFET, according to an exemplary embodiment.

[0015] [Figure 10] This figure shows the response waveforms from an experiment performed with the independent bimetallic switch circuit shown in Figure 2, according to an exemplary embodiment.

[0016] [Figure 11] This figure shows the response waveforms of an experiment performed with a device including the bimetallic switch and D MOSFET shown in Figure 3, according to an exemplary embodiment. [Modes for carrying out the invention]

[0017] A circuit providing overcurrent and overvoltage protection is disclosed herein. The circuit features a depletion-mode MOSFET (D MOSFET) as a current limiter, the D MOSFET connected to a bimetallic switch, the bimetallic switch acting as a temperature-sensing circuit breaker. The D MOSFET and bimetallic switch can be used together to limit current to downstream circuit components, thereby protecting the components from damage.

[0018] Metal oxide semiconductor field-effect transistors (MOSFETs) are semiconductor devices used for switching and amplifying electronic signals in electronic devices. The width of the channel between the source and drain of a MOSFET is changed by adjusting the voltage at its gate. MOSFETs are available in various configurations, based on whether they are P-channel devices constructed using an N-type substrate, N-channel devices constructed using a P-type substrate, longitudinally or transversely oriented semiconductors, and whether they are depletion-mode or enhancement-mode.

[0019] In contrast to enhancement-mode MOSFETs, which turn on when a voltage is applied across the gate terminals, depletion-mode MOSFETs turn on when the gate terminal is at zero volts (V) GS It is known as a "always-on" device when the threshold voltage (V = 0V). In addition to having a thin gate oxide film between the source and drain regions, a conductive channel is formed below the gate oxide layer and between the source and drain regions using ion implantation. The concentration of the active dopant in the region from the substrate to the channel is determined by the threshold voltage (V = 0V) of the MOSFET. Th It is used to adjust the voltage to a desired value. Many modern MOSFETs, contrary to their name, are sometimes manufactured with a polysilicon gate instead of a metal gate on top of an insulating gate oxide film.

[0020] A bimetallic switch is a switch consisting of two metal strips joined together (back to back). A bimetallic switch is placed between two connection points in a circuit. The first metal strip has a first coefficient of thermal expansion, and the second metal strip has a second, different coefficient of thermal expansion. When heat is applied to a bimetallic switch, the switch temporarily deforms, or bends, based on its two different coefficients of thermal expansion, when the temperature exceeds an "open" threshold. If the bimetallic switch consists of two back-to-back metal strips of a predetermined length, applying heat will result in an open circuit condition because the switch will "shorten" or one end will "lift" so that the switch no longer maintains its predetermined length and will no longer adhere to both connection points in the circuit. If the bimetallic switch is part of a manufacturing package (such as the KSD-01F temperature switch thermostat, which will be discussed further later), the package will include two extension legs, which will result in an open circuit condition because their relative position changes during heating, causing them to detach from the connection points in the circuit. In both configurations, once the bimetallic switch cools down again, it straightens out or returns to its original shape (almost flat), reattaching itself between the two connection points and closing the circuit.

[0021] Figure 1 is a representative diagram of a protection circuit 100 according to an exemplary embodiment. The protection circuit 100 (also known herein as the “circuit”) consists of a bimetallic switch 102 and a depletion-mode MOSFET 104 (hereinafter referred to as “D MOSFET” or “MOSFET”) connected in series with each other. The D MOSFET 104 includes a drain (D) and a source (S) through which current flows, and a gate (G) that, under certain current conditions, affects the current flow between the drain and the source. As a depletion-mode device, the D MOSFET 104 is always “on” and current flows between the drain (D) and the source (S) regardless of the gate (G) voltage being 0V. The bimetallic switch 102 provides a current path between nodes 110 and 112 when the switch is closed, and results in an open circuit when the switch is not closed. One end of the bimetallic switch 102 is also connected to the drain (D) of the D MOSFET 104.

[0022] The protection circuit 100 further includes a resistor 106 connected at its first end to the source (S) of the MOSFET 104 and at its second end to the gate (G) of the MOSFET. Thus, the voltage across the resistor 106 is the same as the gate-source voltage of the MOSFET 104. The second end of the resistor 106 (and the gate of the MOSFET 104) is connected to an additional circuit element 108 to be protected, which is located downstream of the aforementioned circuit elements and is shown in its entirety in Figure 1.

[0023] In an exemplary embodiment, the D MOSFET 104 is a current limiter, and the bimetallic switch 102 acts as a temperature-sensing circuit breaker. The bimetallic switch 102 of the protection circuit 100 consists of two different metal strips attached back-to-back to each other. The first metal strip has a first coefficient of thermal expansion, and the second metal strip has a second different coefficient of thermal expansion. This difference causes the switch 102 to exhibit a temporary deformation (such as bending) when the temperature sensed by the bimetallic switch exceeds a threshold temperature. This temporary deformation causes the bimetallic switch 102 to lose connection to both nodes 110 and 112, resulting in an open circuit. Since the threshold temperature causes the bimetallic switch 102 to open the protection circuit 100, the threshold temperature is also known herein as the “open threshold temperature” and the “trip point” or “time to trip” temperature.

[0024] In an exemplary embodiment, the combination of the bimetal switch 102 and the D MOSFET 104 provides overcurrent protection to the circuit 100. The overcurrent condition may be characterized as 1) transient overcurrent or 2) constant overcurrent, as shown in Figure 1. When an overcurrent condition is present, the D MOSFET 104 generates heat, thereby activating the thermal sensing properties of the bimetal switch 102. The transient deformation of the bimetal switch 102 means that the open threshold temperature has been reached, so the bimetal switch 102 cannot connect to one or more of the nodes 110 and 112 of the circuit 100, resulting in an open circuit.

[0025] Once the overcurrent condition subsides, the two metal strips of the bimetal switch 102 cool down, and the switch returns to its original state from its temporarily deformed state. This allows the bimetal switch 102 to re-establish the connection between both nodes 110 and 112 of the circuit 100, resulting in a closed circuit. In this way, the bimetal switch 102 brings about a circuit break state of the protection circuit 100, which is a fail-safe environment protecting other electronic component systems or devices in the circuit (shown as "additional circuit elements 108" in Figure 1).

[0026] Within the protection circuit 100, the D MOSFET 104 can provide fast response and blocking capability to overcurrent and overvoltage events, and can quickly clamp surge current events. The overcurrent clamping capability of the D MOSFET absorbs the rapid transient surge energy, thereby protecting some harmful transient surges from reaching the highly sensitive electronic components (additional circuit elements 108) that are to be protected.

[0027] In contrast, the bimetallic switch 102 offers high current interruption capability. However, the bimetallic switch 102 cannot respond quickly enough to protect against fast transient events. Therefore, the combination of the bimetallic switch 102 and the D MOSFET 104, in the exemplary embodiment, helps to improve overcurrent and overvoltage protection by leveraging the advantages of both devices.

[0028] In one exemplary embodiment, D MOSFET 104 is a Littelfuse® IXTH16N50D2 depletion mode MOSFET (V DSX =500V, I D(on) =16A, R DS(on) The resistance of the bimetal switch 102 is 300 mΩ, and the bimetal switch 102 is a KSD-01F temperature switch thermostat manufactured by Dongguan Fukuanyuan Electronics Co. Ltd (fuyuanfuse.com). As shown in Figure 1, the bimetal switch 102 is connected to the input terminal drain (D) pin of the D MOSFET 104. The resistor 106 is connected between the GS terminals of the D MOSFET 104. The bimetal switch 102 acts as a conductive switch. During normal operation, current passes through the bimetal switch 102, provided that the current does not exceed the open threshold temperature (which can also be considered the "trigger level" of the bimetal switch 102).

[0029] I D The drain current of MOSFET 104, as shown, is given by the gate-source voltage V GS The potential difference (I D×R) is a more negative V GS until a level is reached at which further current through the D MOSFET 104 is blocked by GS , and current begins to flow through the D-S terminals of the D MOSFET 104. In one embodiment, as the applied voltage increases, the current flow increases in linear mode until saturation is achieved. The combination circuit (including the bimetal switch 102 and the D MOSFET 104) achieves a balance that allows the maximum saturation current I sat of the D MOSFET to flow through the circuit 100. In this state, energy is also dissipated as heat within the D MOSFET 104 at I sat ×V DS .

[0030] In an exemplary embodiment, the saturation current I sat is the maximum steady-state current passing through the D MOSFET 104. This means that as long as the saturation current is not exceeded, the D MOSFET maintains functionality without chip failure or failure due to overheating. If the overload current is less than I sat , the D MOSFET 104 still dissipates energy as heat without problems. However, if a very large short-circuit current that reaches within the D MOSFET and thus exceeds its I sat (saturation current) value exists on the input side, the D MOSFET reacts quickly and dissipates the excess current to heat more quickly. This then causes a higher temperature rise in the package of the D MOSFET 104, quickly trips the bimetal switch 102, thereby stopping further current from being blocked from the D MOSFET. Nevertheless, a continuous current exceeding the saturation current I sat of the MOSFET causes overheating of the D MOSFET beyond its maximum junction temperature, resulting in chip failure and loss of MOSFET functionality. Thus, the bimetal switch helps protect the D MOSFET from damage due to overheating.

[0031] In an exemplary embodiment, the protection circuit 100 ranges from 0 A to the saturation current I satIt is operated with applied currents up to a certain range. An abnormal condition is input current I D This can result in a sudden increase in the current. Abnormal conditions can be caused, for example, by a short circuit in the load, load switching, or sudden overload conditions. These cause a sudden increase in the current passing through the bimetal switch 102 and D MOSFET 104. This current is the saturation current I sat If it remains below the saturation current I, the MOSFET will gradually heat up due to power dissipation to the MOSFET. However, if the overcurrent is less than the saturation current I sat If it exceeds (I D ×V DS ) accompanied by input current I D The voltage increases dramatically, causing the bimetallic switch to trip even faster to protect the D MOSFET. This causes the bimetallic switch 102 to reach its disconnection level, thus disconnecting from one or more nodes 110, 112 at the open threshold temperature. Thus, the bimetallic switch also provides cutoff protection to the D MOSFET within a safe operating range and thermal limits.

[0032] In one embodiment, when a disconnection occurs at nodes 110 and / or 112, the entire current flowing through the protection circuit 100 is cut off, thereby removing the current flow to the D MOSFET 104, causing heat dissipation from the MOSFET, and ultimately protecting the D MOSFET from overheating by eliminating persistent overcurrent.

[0033] Thus, the protection circuit 100 advantageously provides a feedback mechanism that tends to provide faster protection to the D MOSFET 104 as the severity of the short-circuit current event increases. Therefore, the higher the level of the short-circuit current, the faster the bimetal switch 102 can cut off the current to the D MOSFET 104, thereby protecting the D MOSFET from overheating damage. This feedback occurs because the high heat generated by the D MOSFET 104 from an overvoltage / overcurrent condition causes the bimetal switch 102 to open more quickly, thereby preventing further current from passing through the D MOSFET, eliminating this self-heating, and ultimately protecting any additional downstream circuit elements 108.

[0034] Figures 2, 3, and 4 illustrate protection circuits 200, 300, and 400 used to show the results of a continuous current test according to an exemplary embodiment, respectively. In the exemplary embodiment, the bimetallic switch is a KSD-01 temperature switch thermostat (started at 60°C and 2A 250V operating current), and the D MOSFET is an IXTH16N50D2 depletion mode MOSFET (V DSX =500V, I D(on) =16A, R DS(on)The resistor is Ω(=300mΩ) and resistor 206 is a 0Ω resistor. In Figure 2, the bimetal switch 202 is an independent device not coupled to the MOSFET. In Figure 3, the bimetal switch 302 is connected to the D MOSFET 304 and resistor 306, and the circuit elements are arranged similarly to the protection circuit 100 in Figure 1. In Figure 4, the bimetal switch 402 (KSD-01F) is fixed back-to-back to the D MOSFET 404 (TO247 package) using screws 412 and bolts (not shown). In one embodiment, the bimetal switch 402 and the D MOSFET 404 are further thermally coupled to each other using a thermally conductive epoxy adhesive (not shown). In another embodiment, the bimetal switch 402 and the D MOSFET 404 are further thermally coupled to each other using a conductive epoxy gel. Furthermore, the bimetal switch 402 and the D MOSFET 404 are electrically connected to each other and to other parts of the circuit via wires 406, 408, and 410. The KSD-01F bimetal switch consists of a bimetal disk, a metal bridge connecting the two legs of the device, metal bridge contacts, and a plastic case isolated by a heatsink. The bimetal disk bends at a certain temperature, thereby causing the metal bridge to connect or disconnect the legs, closing or opening the circuit.

[0035] In Figures 2 and 3, arrows 204 and 322, respectively, indicate the direction of current flow. In circuit 200, which features an independent bimetallic switch 202, current 204 flows from node 206 to node 208 while the switch is closed. When the bimetallic switch 202 is open, no current flows. In circuit 300, which features a bimetallic switch 302, a D MOSFET 304, and a resistor 306, current 322 flows from node 308, through the closed bimetallic switch 302, from node 314 to node 316 (drain to source) of the D MOSFET 304, from node 318 to node 320 of the resistor 306, and finally to node 310. Since the resistor 306 is connected between the source and gate of the D MOSFET 304, the voltage across the resistor 306 when current 322 flows is the gate-source voltage V of the D MOSFET. GS It is the same as. Therefore, the voltage between nodes 314 and 316 (gray) is the drain-source voltage of MOSFET 304 V DS Therefore, the voltage between nodes 318 and 320 (white) is the gate-source voltage V GS That is the case.

[0036] The circuit 300 may also be a standalone device 300 consisting of a bimetal switch 302, a D MOSFET 304, and a resistor 306. In this way, the overcurrent protection device 300 may be added to any circuit that requires overcurrent protection.

[0037] Overcurrent test currents were performed to evaluate the trip time of the bimetallic switch under three sets of conditions. ·Applied current (saturation current I sat Test currents of 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) (less than 2A), and the bimetal switch 202 as an independent device (Figure 2) ·Applied current (saturation current I satTest currents of 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) (less than 2A) and bimetal switch 202 connected to D MOSFET 204 (Figure 3) ·Applied current (saturation current I sat Test currents of 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) (less than 2A) and a bimetallic switch 202 thermally coupled to the D MOSFET 204 using screws and bolts (Figure 4).

[0038] The bimetallic switch, KSD-01 temperature switch thermostat, used in these tests has an operating current of 2A (at 250V), so a 2A input represents 100% of its operating current. Therefore, these tests are performed to measure the trip time of the bimetallic switch under various operating conditions. Thus, with the exception of the initial test at 2A, the device is tested at multiples of its normalized current rating, with the most extreme test being performed at 12 times the device's current rating (24A).

[0039] Figure 5 shows the response waveform 500 of the standalone bimetallic switch 202 of Figure 2, according to an exemplary embodiment, where a 10V source supplies a current of 8A (400% of the switch's current rating) to circuit 200. Although the current flowing through the bimetallic switch 202 is rated at four times the device's normalized current rating, it still takes 48 seconds to reach the device's trip point 502. This is not ideal, as a long trip time to the bimetallic switch under four times the normalized current places very high stress on downstream components, increasing the probability of failure.

[0040] Figure 6 shows the response waveform 600 of a bimetallic switch 302 connected to the D MOSFET 304 in Figure 3, where a 10V source supplies 6A (300% of the switch's current rating) to the circuit 300, according to an exemplary embodiment. As shown in waveform 600, the response was measured by applying a 10V 6A overcurrent condition to the protection circuit 300 (Figure 3). Current (I D )(C2) and the voltage between the bimetal switch 302 and the D MOSFET 304 (V DS (C3) was monitored and captured for a time plot. These scales were marked below waveform 600. The trip point 602 is the point at which the bimetal switch 302 began to disconnect.

[0041] Figure 7 shows the response waveform 700 of a bimetallic switch 302 connected to a D MOSFET 304 in Figure 3, in an exemplary embodiment, where a 10V source supplies 12A (600% of the switch's current rating) to the circuit 300. In this example, resistor 306 is 0Ω. The response was measured by applying a 10V 12A overcurrent condition to the protection circuit 300 (Figure 3), as shown in waveform 700. At the trip point 702, the current I flowing from the D MOSFET 304 was measured. D The current rapidly drops from 12A to 0A, while the voltage across the MOSFET V DS The voltage is dropping from 10V to 0V. Just before the trip point 702, MOSFET 304 is dissipating power of approximately 4.27V × 12A = 54.24W (see Table 700 in Figure 7 below). The 600% rated current of 12A causes heat dissipation in D MOSFET 304, thereby raising the temperature of the MOSFET package to 60°C, which is the tripping level of the bimetal switch 202. In the example shown in Graph 700 (Figure 7), it took approximately 3.7 seconds to reach the trip point 702 of the bimetal switch 202. Therefore, in the exemplary embodiment, as shown by these waveforms, the response time for cutting off an overcurrent event depends not only on the magnitude of the overcurrent but also on the inherent heat dissipation caused by D MOSFET 304.

[0042] Figure 8 includes Table 800, which shows the results of overcurrent test current operation in several embodiments. As shown, further tests were performed with different currents under conditions with and without the D MOSFET 304. In these examples, the IXTH16N50D2 D MOSFET and a bimetal switch starting at 60°C were subjected to a saturation current I sat The test was performed using a rated current below a certain limit.

[0043] Table 800 provides an overview of tests using different currents and the trip response of the bimetal switch. The upper part of Table 800 provides trip time information for the bimetal switch 302 (Figure 3) connected to the D MOSFET 304 under eight current conditions (6A, 8A, 10A, 12A, 14A, 16A, 20A, and 24A), while the lower part of Table 800 provides trip time information for the bimetal switch 202 (Figure 2) not connected to the D MOSFET under the same eight current conditions. Table 800 shows that the trip time for the standalone bimetal switch 202 (Figure 2) is 8 seconds (at a current of 12A). When the same 12A is applied to the D MOSFET (Figure 3) combined with the bimetal switch circuit, the trip time is reduced to just 3.7 seconds. Thus, in the exemplary embodiment, the trip time is significantly faster / improved in the D MOSFET + bimetal switch case over a current range of 3 to 6 times.

[0044] Table 800 also shows that the bimetallic switch does not trip with applied currents of 2A (100%) or 4A (200%). Instead, the bimetallic switch begins to trip at 6A (300%) due to the fact that there is enough energy at 6A to thermally activate the bimetallic switch.

[0045] These experiments demonstrate that, in exemplary embodiments, the presence of the D MOSFET accelerates the tripping of the bimetallic switch at all current ratios. The resistance R determines the maximum allowable current passing through the D MOSFET in the steady state, which is the saturation current I. satIn one exemplary embodiment, the use of a 0Ω resistor results in a higher I compared to the case where a resistor R=0.1Ω is used. sat The value becomes possible. When resistor 306 is zero, the V of MOSFET 304 GS It is also zero. However, by slightly increasing the resistance (for example, R=0.1Ω), the gate-source voltage V of the MOSFET 304 can be increased. GS The voltage becomes slightly negative, pinching off and limiting the current flowing through the D MOSFET.

[0046] By increasing the resistance R, the saturation current I sat and gate-source voltage (V GS Since both of these change, the power across the D MOSFET fluctuates, which can provide more heat dissipation to the D MOSFET. In exemplary embodiments, these considerations facilitate the selection of different bimetallic switches to work with circuit breakers of different ratings.

[0047] Figure 9 shows Graph 900, illustrating the improved trip response times of protection circuits for both types: 1) a bimetallic switch without a MOSFET (Figure 2), and 2) a bimetallic switch with a MOSFET (Figure 3). Graph 900 shows the trip time in seconds (y-axis) versus the current in amperes (x-axis) for the bimetallic switch. Dark circles represent the trip time for a standalone bimetallic switch (e.g., Figure 2), and light circles represent the trip time for a bimetallic switch with a D MOSFET added (e.g., Figure 3). When a bimetallic switch is combined with a D MOSFET, the trip time shifts from the right to the left and from the top to the bottom of Graph 900 (meaning the trip time is shorter), but this is within the safe operating curve for the bimetallic switch's trip time. Therefore, adding a D MOSFET to a bimetallic switch improves the overall time to trip and provides much faster protection for downstream electronic components.

[0048] The benefits provided by the D MOSFET are evident in Graph 900. For example, the time to trip an isolated bimetallic switch at 8A, given by the dark circle 902, is approximately 57 seconds, while the time to trip an isolated bimetallic switch with a D MOSFET added at the same current, given by the light circle 904, is approximately 8 seconds. Similarly, the time to trip an isolated bimetallic switch at 10A, given by the dark circle 906, is approximately 19 seconds, while the time to trip an isolated bimetallic switch at 10A (light circle 908) is approximately 6 seconds. This makes sense only at higher currents, as the isolated bimetallic switch performs comparably to the isolated circuit, far exceeding the switch's current rating. Thus, Graph 900 demonstrates the benefits of having an isolated circuit consisting of both a bimetallic switch and a D MOSFET working together to protect against overcurrent conditions.

[0049] Returning to the protection circuit 200 in Figure 2, the circuit includes an independent bimetallic switch 202 on which a surge test is being performed, according to an exemplary embodiment. The test is performed with a surge current of 1.2 / 50 microseconds and a peak voltage of 500V with 2Ω. Figure 3, in contrast, shows circuit 300, where the bimetallic switch 302 is connected to the drain of a D MOSFET 304 and a resistor 306 is connected between the source and gate of the MOSFET. One variation of this circuit 300 would be without the resistor. Again, the test is performed with a surge current of 1.2 / 50 microseconds and a peak voltage of 500V with 2Ω. In both circuits 200 and 300, the bimetallic switch is a KSD-01 temperature switch thermostat, and in circuit 300, the D MOSFET 304 is an IXTH16N50D2 depletion mode MOSFET (V DSX =500V, I D(on) =16A, R DS(on) (=300mΩ).

[0050] Figure 10 includes a response waveform 1000 showing the surge response of a bimetallic switch in an isolated circuit, such as circuit 200 in Figure 2, according to several embodiments. A surge waveform of 1.2 / 50 microseconds is given, using a peak voltage of 500V and a virtual impedance of 2Ω. As shown in waveform 1000, the surge current (C2) through the bimetallic switch has a peak response of 230.7A. To convert the voltage waveform, the peak voltage is 230.7A × 2Ω = 461.4V. The voltage across the bimetallic switch (C1) remains nearly constant at 10V, although there is some slight increase due to the incoming surge. However, the surge does not start and open the bimetallic switch. Therefore, in some embodiments, the switch does not start in this surge condition.

[0051] Figure 11 includes response waveforms 1100 showing the surge response of circuits featuring a combination of D MOSFETs and bimetallic switches, such as circuit 300 in Figure 3, according to several embodiments. As shown in waveform 900, given the same incoming surge conditions, the current passing through the D MOSFET and bimetallic switch combination is clamped and reduced (C2), remaining "saturated" at a peak current of approximately 21.8 A for about 40 microseconds. The D MOSFET clamps very quickly during a surge, resulting in a very low-current output. This is in contrast to the surge test using a standalone bimetallic switch described above (Figure 10).

[0052] Therefore, in the exemplary embodiment, when a D MOSFET with a bimetallic thermal switch is deployed, the switch startup time is much faster than starting a standalone type with the same applied overcurrent. Furthermore, the resulting surge current is at a much lower safety level, protecting downstream circuit elements.

[0053] In exemplary embodiments, a D MOSFET with a bimetallic switch can work closely together in the circuit to provide mutual protection. Under a sustained overcurrent protection event, the D MOSFET heats up, initiating the switch at a specified starting temperature and generating an open current that prevents the overcurrent from passing through downstream components of the circuit and protects the D MOSFET from overheating. The switch resets from its temporarily deformed position back to its normal position once its case has cooled to a reset level.

[0054] Furthermore, in some embodiments, the combination circuits described herein may be part of a manually reset circuit breaker. These types of circuit breakers are known to have a bimetallic strip, but once the strip trips, resulting in an open circuit, the circuit breaker cannot be reset without human intervention. A bimetallic strip with a D MOSFET added, as disclosed herein, may be a suitable alternative to such a circuit breaker, eliminating the need for human intervention for resetting. Both devices (the bimetallic switch and the D MOSFET), when connected in this manner, share the characteristics of self-protection and self-resetting.

[0055] In addition to the example of the bimetallic switch described above, the principles described herein may also be applied to other types of thermal switches, miniature circuit breakers, and relay-type circuit breakers that have a bimetallic switch inside, regardless of whether these devices include a self-reset or manual reset function.

[0056] The test results described above indicate that the D MOSFET provides an additional heating effect that accelerates the tripping of the bimetallic switch. In some embodiments, the bimetallic switch has been shown to trip faster at all overcurrent levels, such as 100%, 200%, and 400%, when the D MOSFET is present. Therefore, the bimetallic switch and the D MOSFET work very closely together, providing mutual protection to each other.

[0057] In the exemplary embodiment, the waveforms described above demonstrate that placing the bimetallic switch before the D MOSFET and combining it with the switch placed on top of the D MOSFET package (as shown in Figure 4) provides a mutual benefit of protection between the two devices against overcurrent events. The D MOSFET acts as a current limiter, having (or not having) a bias resistor (e.g., resistor 306 in Figure 3) at the gate-source terminal of the MOSFET. If the D MOSFET experiences a long-term current limiting event, the heat generated from its body (packaging) heats the bimetallic switch, causing it to open and protecting the D MOSFET from overheating (long-term current I > I sat In an exemplary embodiment, the circuit resets and returns to normal when the temperature drops to the recovery level of the bimetallic switch. In an exemplary embodiment, the D MOSFET also acts as a surge current limiter, clamping external surges to the circuit to be protected.

[0058] When used herein, elements or stages described in the singular form and preceded by the word "a" or "an" should be understood as not making any such exclusions unless explicitly stated otherwise. Furthermore, the references to “one embodiment” in this disclosure are not intended to be construed as excluding the existence of additional embodiments that also incorporate the described features.

[0059] While this disclosure refers to specific embodiments, numerous modifications, alterations, and changes are possible to the embodiments described without departing from the scope and realm of this disclosure, as defined in the appended claims. Therefore, this disclosure is not limited to the embodiments described, but encompasses the entire scope defined by the following claims and their equivalents.

Claims

1. A circuit that can operate to provide overcurrent protection, A bimetallic switch having a rated current, wherein the bimetallic switch is A first metal strip having a first coefficient of thermal expansion, A bimetallic switch having a second metal strip having a second coefficient of thermal expansion different from the first coefficient of thermal expansion, wherein the bimetallic switch has a first terminal and a second terminal, the first terminal being connected to a first node and the second terminal being connected to a second node, and A metal oxide semiconductor field-effect transistor (MOSFET) having a drain connected to the second node, wherein the MOSFET includes a MOSFET that absorbs surge energy and generates heat in a surge event, At overcurrent levels of at least 300% to 1200% of the rated current of the bimetal switch, Under the same current, the time it takes for the bimetal switch to reach its trip point is shorter than the time it takes for the bimetal switch alone to reach its trip point compared to the circuit without the MOSFET. circuit.

2. The circuit according to claim 1, further comprising a resistor coupled between the source of the MOSFET and the gate of the MOSFET.

3. The circuit according to claim 1 or 2, wherein the MOSFET is a depletion-mode MOSFET.

4. The circuit according to claim 3, wherein the MOSFET is an N-channel depletion-mode MOSFET.

5. The circuit according to any one of claims 1 to 4, wherein the MOSFET provides a current limiting effect to protect additional circuit elements from damage during the overcurrent event.

6. The circuit according to claim 5, wherein the MOSFET becomes hot during the overcurrent event, causing the bimetal switch to disconnect from either the first node or the second node.

7. The circuit according to any one of claims 1 to 6, wherein the MOSFET absorbs the surge energy in the overcurrent event to protect other elements of the circuit.

8. The circuit according to any one of claims 1 to 7, wherein the bimetallic switch protects the MOSFET from prolonged overheating due to the overcurrent event by disconnecting from either the first node or the second node to create an open circuit.

9. A device coupled to a circuit and capable of operating to provide overcurrent protection to the circuit, A bimetallic switch having a rated current, wherein the bimetallic switch is A first metal strip having a first coefficient of thermal expansion, A bimetallic switch having a second metal strip having a second coefficient of thermal expansion different from the first coefficient of thermal expansion, wherein the bimetallic switch has a first terminal and a second terminal, the first terminal being connected to a first node and the second terminal being connected to a second node, and A metal oxide semiconductor field-effect transistor (MOSFET) having a drain connected to the second node, wherein the MOSFET includes a MOSFET that absorbs surge energy and generates heat in a surge event, At overcurrent levels of at least 300% to 1200% of the rated current of the bimetal switch, A device in which, under the same current, the time it takes for the bimetallic switch to reach its trip point is shorter than that of a circuit with the bimetallic switch alone without the MOSFET.

10. The device according to claim 9, further comprising a resistor coupled between the source and gate of the MOSFET.

11. The device according to claim 9, wherein the MOSFET is a depletion-mode MOSFET.

12. The device according to claim 11, wherein the MOSFET is an N-channel depletion-mode MOSFET.

13. The device according to any one of claims 9 to 12, wherein the MOSFET provides a current limiting effect to protect additional circuit elements from damage during the overcurrent event.

14. The device according to claim 13, wherein the MOSFET becomes hot during the overcurrent event, causing the bimetal switch to disconnect from either the first node or the second node.

15. The device according to any one of claims 9 to 14, wherein the MOSFET absorbs the surge energy in the overcurrent event to protect other elements of the circuit.

16. The device according to any one of claims 9 to 15, wherein the bimetallic switch protects the MOSFET from prolonged overheating due to the overcurrent event by disconnecting from either the first node or the second node and creating an open circuit.

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