Overcurrent protection by depletion mode mosfet or JFET in mini circuit breaker and bimetal temperature-sensitive switch

Abstract

To provide a small circuit breaker for short-circuit protection and overload protection.SOLUTION: In a protection circuit 100, a small circuit breaker has a field effect transistor (FET). It may be a depletion mode metal oxide semiconductor FET (a D MOSFET), a junction field effect transistor (a JFET), or a silicon carbide JFET. The FET is connected to a bimetal switch 102. The bimetal switch acts as a temperature-sensitive circuit breaker. The D MOSFET104 and the bimetallic switch can be combined to limit a current to a downstream circuit component (a circuit element 108) and thus components can be protected from being damaged.SELECTED DRAWING: Figure 1

Description

[Background technology]

[0001] [Cross-reference of related applications] This application claims priority to U.S. Patent Application No. 17 / 018,269, filed on September 11, 2020, and is a continuation-in-part application of the said U.S. Patent Application.

[0002] An overcurrent or excess current is a condition in which a larger current than intended flows through a circuit. Overcurrents can be continuous or transient. Voltage transients, i.e., short-duration surges of electrical energy, are the result of the sudden release of energy that was previously stored or induced by a heavy inductive load or other means such as lightning. Repeatable transients are frequently caused by the operation of motors, generators, or the switching of reactance circuit components. Lightning and electrostatic discharge (ESD) can cause random transients.

[0003] The miniaturization of components has led to increased sensitivity to electrical stress. For example, microprocessors have structures and conductive paths that are unable to cope with the high currents from ESD transients. Since such components operate at very low voltages, controlling voltage disturbances is given high priority to prevent device interruption and potential or fatal failures.

[0004] This improvement may be useful in relation to these and other considerations. [Overview of the project]

[0005] This summary is provided to introduce, in a simplified form, the selected concepts that will be further described below in the detailed description. This summary is not intended to identify any important or essential features of the claimed subject matter, nor is it intended to assist in determining the scope of the claimed subject matter.

[0006] An exemplary embodiment of a miniature circuit breaker capable of operating to provide protection in the event of a short circuit or overload is disclosed. The miniature circuit breaker includes a switch that is manually opened or closed by an external lever but automatically opens in response to either a first fault event or a second fault event. The miniature circuit breaker also includes a magnetic coil that opens the switch in response to the first fault event, a bimetallic strip, and a field-effect transistor (FET) connected in series with the bimetallic strip, the gate and source terminals of the FET being connected to each other. The bimetallic strip and the FET open the switch during the second fault event.

[0007] Another exemplary embodiment of the miniature circuit breaker according to this disclosure may comprise a bimetallic strip, a field-effect transistor (FET), and a switch. The bimetallic strip has an elongated metal strip and a metal winding wound around the strip. The elongated metal strip bends when the current exceeds the rated current of the miniature circuit breaker. After the metal winding is removed from the elongated metal strip, the FET is connected in series with the elongated metal strip and thermally coupled to the elongated metal strip. The FET has a gate terminal connected to a source terminal. When a current exceeding the rated current arrives at the miniature circuit breaker, the switch opens due to the bending of the elongated metal strip. The FET provides current limiting above the rated current. [Brief explanation of the drawing]

[0008] [Figure 1] This figure shows a protection circuit comprising a bimetallic switch and a D MOSFET according to an exemplary embodiment.

[0009] [Figure 2] This figure shows a protective circuit comprising an independent bimetallic switch, according to an exemplary embodiment.

[0010] [Figure 3]A diagram of a device that provides overcurrent protection for a circuit according to an exemplary embodiment.

[0011] [Figure 4] A diagram of a bimetal switch screwed and bolted to a D MOSFET according to an exemplary embodiment.

[0012] [Figure 5] The response waveform of an experiment performed using the stand-alone bimetal switch of FIG. 2 according to an exemplary embodiment.

[0013] [Figure 6] The response waveform of an experiment performed between the bimetal switch of FIG. 3 and a D MOSFET according to an exemplary embodiment. [Figure 7] The response waveform of an experiment performed between the bimetal switch of FIG. 3 and a D MOSFET according to an exemplary embodiment.

[0014] [Figure 8] A table providing the results of an overcurrent test current operation performed on the circuits of FIGS. 2 and 3 according to an exemplary embodiment.

[0015] [Figure 9] A graph comparing the trip response times between a circuit having a stand-alone bimetal switch and a circuit having a bimetal switch with a D MOSFET according to an exemplary embodiment.

[0016] [Figure 10] The response waveform of an experiment performed on the stand-alone bimetal switch circuit of FIG. 2 according to an exemplary embodiment.

[0017] [Figure 11] The response waveform of an experiment performed on a device comprising the bimetal switch of FIG. 3 and a D MOSFET according to an exemplary embodiment.

[0018] [Figure 12A] This is a diagram of a conventional miniature circuit breaker. [Figure 12B] This is a diagram of a conventional miniature circuit breaker.

[0019] [Figure 13] This is a diagram of a conventional miniature circuit breaker.

[0020] [Figure 14] This is a circuit diagram of a conventional miniature circuit breaker.

[0021] [Figure 15A] This is a circuit diagram of a miniature circuit breaker having a D MOSFET according to an exemplary embodiment. [Figure 15B] This is a circuit diagram of a miniature circuit breaker having a D MOSFET according to an exemplary embodiment.

[0022] [Figure 16] This is a diagram of a miniature circuit breaker having a D MOSFET according to an exemplary embodiment.

[0023] [Figure 17] These are response waveforms from experiments performed on a miniature circuit breaker having a D MOSFET and a miniature circuit breaker without a D MOSFET, according to exemplary embodiments. [Figure 18] These are response waveforms from experiments performed on a miniature circuit breaker having a D MOSFET and a miniature circuit breaker without a D MOSFET, according to exemplary embodiments. [Figure 19] These are response waveforms from experiments performed on a miniature circuit breaker having a D MOSFET and a miniature circuit breaker without a D MOSFET, according to exemplary embodiments. [Modes for carrying out the invention]

[0024] A circuit providing overcurrent and overvoltage protection is disclosed herein. The circuit comprises 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 together are capable of limiting the current to downstream circuit components, thereby protecting those components from damage.

[0025] Furthermore, a mini circuit breaker (MCB) that provides overcurrent and overvoltage protection is disclosed herein. The MCB comprises either a depletion-mode MOSFET (D MOSFET) or a junction field-effect transistor (JFET) as a current limiter, the D MOSFET or JFET being connected to a bimetallic strip, the bimetallic strip acting as a temperature-sensing circuit breaker. The D MOSFET or JFET and the bimetallic strip, when combined, are capable of limiting current to downstream circuit components, thereby protecting those components from damage.

[0026] Metal-oxide-semiconductor field-effect transistors (MOSFETs) are semiconductor devices used for switching and amplifying electronic signals in electronic devices. By adjusting the gate voltage, the width of the channel located between the source and drain of the MOSFET is changed. MOSFETs come in a variety of configurations, depending on whether they are P-channel devices built on an N-type substrate or N-channel devices built on a P-type substrate, whether they are longitudinally or transversely oriented semiconductors, and whether they operate in depletion mode or enhancement mode.

[0027] In contrast to enhancement-mode MOSFETs, which are turned on by applying a voltage across the gate terminals, depletion-mode MOSFETs are turned on when the gate terminal is at zero volts (V) GSIt is known as a "normally-on" device when the threshold voltage (V = 0V). In addition to having a thin gate oxide film between the source and drain regions, ion implantation is used to form a conductive channel beneath the gate oxide layer and between the source and drain regions. Th To adjust the value to the desired level, the concentration of the active dopant in the substrate-channel region is used. Despite its name, many modern MOSFETs can be manufactured with a polysilicon gate instead of a metal gate on an insulated gate oxide film.

[0028] 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 will temporarily deform or bend based on these two different coefficients of thermal expansion when the temperature exceeds an "open" threshold. If the bimetallic switch consists of two back-to-back strips of metal of a predefined length, the application of heat will cause the switch to "shorten" or one end to "lift," so that the switch no longer maintains its predefined length and therefore no longer contacts both connection points in the circuit, resulting in an open circuit condition. If the bimetallic switch is part of a manufactured package (for example, the KSD-01F temperature switch thermostat discussed further below), the package has two extension legs that change their relative position during heating, thus resulting in a disconnection from the connection points of the circuit and an open circuit state. In either configuration, when the bimetallic switch cools again, the switch unbends or adapts to its original shape (generally flat), so that the switch makes contact again between the two connection points and the circuit is closed.

[0029] 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 comprises a drain (D) and a source (S) through which current flows, and a gate (G) which, under certain voltage conditions, will affect the current 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) even when the gate (G) voltage is 0V. The bimetallic switch 102 provides an electrical 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 connected to the drain (D) of the D MOSFET 104.

[0030] The protection circuit 100 further includes a resistor 106 connected to the source (S) of MOSFET 104 at its first end and to the gate (G) of MOSFET 104 at its second end. Therefore, the voltage across resistor 106 is the same as the gate-source voltage of MOSFET 104. The second end of resistor 106 (and the gate of MOSFET 104) is connected to a further circuit element 108 to be protected, which is downstream of the circuit elements already described and is shown overall in Figure 1.

[0031] 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 mounted back-to-back with 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 transient deformation (e.g., bending) when the temperature sensed by the bimetallic switch exceeds a threshold temperature. This transient deformation causes the bimetallic switch 102 to no longer connect 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.

[0032] In an exemplary embodiment, the combination of the bimetal switch 102 and the D MOSFET 104 provides overcurrent protection for the circuit 100. The overcurrent condition may be characterized as either 1) transient overcurrent or 2) constant overcurrent, as shown in Figure 1. When an overcurrent condition is present, the D MOSFET 104 generates heat, which activates the thermal sensing properties of the bimetal switch 102. The transient deformation of the bimetal switch 102 reaches the open-circuit threshold temperature, which means that the bimetal switch 102 is no longer connected to one or more of the nodes 110 and 112 of the circuit 100, resulting in an open circuit.

[0033] Once the abnormal 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. The bimetal switch 102 thus provides a circuit-disconnected state for the protection circuit 100, which is a fail-safe environment for the safety protection of other electronic systems or devices in the circuit (shown as “Further Circuit Element 108” in Figure 1).

[0034] Within the protection circuit 100, the D MOSFET 104 is capable of providing fast response and blocking capabilities to overcurrent and overvoltage events, and can rapidly clamp surge current events. The overcurrent clamping capability of the D MOSFET absorbs fast transient surge energy, thereby protecting sensitive electronic equipment (further circuit elements 108) from any harmful transient surges.

[0035] In contrast, the bimetallic switch 102 offers high current interruption capability. However, the bimetallic switch 102 is unable to respond quickly enough to protect against fast transient events. Therefore, the combination of the bimetallic switch 102 and the D MOSFET 104, in exemplary embodiments, helps to leverage the advantages of both devices to provide improvements in overcurrent and overvoltage protection.

[0036] In an exemplary embodiment, D MOSFET104 is an IXTH16N50D2 depletion-mode MOSFET (V) manufactured by Littelfuse®. DSX =500V, I D(on) =16A, R DS(on)(= 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 FIG. 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 G-S terminals of the D MOSFET 104. The bimetal switch 102 acts as a conductive switch. During normal operation, the bimetal switch 102 allows current to pass through it on condition that the current does not exceed the open threshold temperature (which can also be considered the "trigger level" for the bimetal switch 102).

[0037] I D As shown as, the drain current of the MOSFET 104, the voltage between the gate and source V GS The potential difference of (I D ×R) is such that until the negative V GS reaches a level where further current passing through the D MOSFET 104 is blocked as it increases, it begins to flow through the D-S terminals of the D MOSFET 104. In one embodiment, as the applied voltage increases, the current will increase in a linear mode until a saturation state is achieved. A combined circuit (comprising the bimetal switch 102 and the D MOSFET 104) achieves an equilibrium state where the maximum saturation current I sat of the D MOSFET can flow through the circuit 100. In this state, also, energy is dissipated in the D MOSFET 104 as heat (I sat ×V DS ).

[0038] 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 will maintain functionality without being accompanied by chip failure or failure due to overheating. When the overload current is I satIf it is less than [value], the D MOSFET 104 still dissipates energy as heat without any problems. However, if there is a very high short circuit on the input side leading to the D MOSFET, and therefore its I sat If the (saturation current) value is exceeded, the D MOSFET will react quickly, rapidly dissipating the excess current as heat. This, in turn, causes a higher temperature rise on the casing of the D MOSFET 104, which quickly trips the bimetal switch 102, thereby preventing further current from destroying the D MOSFET. Nevertheless, the saturation current I of the MOSFET sat Prolonged current flow exceeding the maximum junction temperature can cause the D MOSFET to overheat, leading to chip failure and loss of MOSFET function. Therefore, a bimetallic switch helps protect the D MOSFET from similar overheating failures.

[0039] In an exemplary embodiment, the protection circuit 100 operates from 0A to a maximum of I sat It operates with applied currents up to the saturation current. An abnormal condition is when the input current I D This can result in a sudden surge in current. Abnormal conditions can be caused, for example, by a short circuit on the load, a load switchover, or a sudden overload condition. These cause a sudden surge in current passing through the bimetal switch 102 and D MOSFET 104. This current can lead to a saturation current I sat If the current remains below the saturation current I, the D MOSFET will gradually heat up due to power dissipation in the MOSFET. However, if the overcurrent exceeds the saturation current I, sat If it exceeds the input current I D The dramatic increase in heat dissipation to the D MOSFET (I D ×V DSThis results in the presence of the bimetal switch, which trips more quickly to protect the D MOSFET. This causes the bimetal switch 102 to reach its disconnection level and therefore disconnect from one or more of nodes 110, 112 at the open threshold temperature. Thus, the bimetal switch also provides cutoff protection to the D MOSFET within a safe operating area and thermal limits.

[0040] 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 to the D MOSFET 104, causing heat dissipation of the MOSFET, and ultimately protecting the D MOSFET from overheating by removing any remaining overcurrent.

[0041] 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 the overvoltage / overcurrent condition causes the bimetal switch 102 to open more quickly, thus preventing further current from passing through the D MOSFET and eliminating this self-heating, and ultimately protecting further downstream circuit elements 108.

[0042] Figures 2, 3, and 4 are diagrams of protection circuits 200, 300, and 400, respectively, used to show the results of a continuous current test according to an exemplary embodiment. In the exemplary embodiment, the bimetallic switch is a KSD-01 temperature switch thermostat (triggered at 60°C and operating current @2A 250V), and the D MOSFET is an IXTH16N50D2 depletion mode MOSFET (V DSX =500V, I D(on) =16A, R DS(on) The resistor 206 is a 0Ω resistor (=300mΩ). 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 those in the protection circuit 100 in Figure 1. In Figure 4, the bimetal switch 402 (KSD-01F) is coupled to the D MOSFET 404 (TO247 package) in a back-to-back manner using screws 412 and bolts (invisible). In one embodiment, the bimetal switch 402 and the D MOSFET 404 are further thermally linked 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 linked 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 bimetallic disc, a metal bridge connecting the two legs of the device, metal bridge contacts, and a plastic case separated from the heatsink. The bimetallic disc bends at a certain temperature, thereby causing the metal bridge to connect to or disconnect from the legs, and thus closing or opening the circuit.

[0043] In Figures 2 and 3, arrows 204 and 322 indicate the direction of current. In circuit 200, which includes an independent bimetallic switch 202, current 204 flows from node 206 to node 208 while the switch is closed. No current flows when the bimetallic switch 202 is open. In circuit 300, which includes 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 of the D MOSFET 304 (drain-source), 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 the following. Therefore, the voltage across nodes 314 and 316 (gray) is the drain-source voltage of MOSFET 304 V DS On the other hand, the voltage across nodes 318 and 320 (white) is the gate-source voltage V GS That is the case.

[0044] The circuit 300 may also be an independent device 300 consisting of a bimetal switch 302, a D MOSFET 304, and a resistor 306. Therefore, the overcurrent protection device 300 may be added to any circuit that requires overcurrent protection.

[0045] Under the following three sets of conditions, an overcurrent test current was applied to evaluate the trip time of the bimetallic switch: • Using a bimetal switch 202 as an independent device (Figure 2), the applied current (saturation current I sat Test currents less than 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) • Using a bimetal switch 202 connected to MOSFET 204 (Figure 3), the applied current (saturation current I sat Test currents less than 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) • Using a bimetallic switch 202 (Figure 4) thermally linked to the D MOSFET 204 with screws and bolts, the applied current (saturation current I sat Test currents less than 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A)

[0046] The bimetallic switch used in these tests, the KSD-01 temperature switch thermostat, 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 first test at 2A, the device is tested at multiples of its normalized rated current, the most extreme test being performed at 12 times the device's rated current (24A).

[0047] Figure 5 shows the response waveform 500 of the standalone bimetallic switch 202 of Figure 2, in an exemplary embodiment, where a 10V power supply provides a current of 8A (400% of the switch's rated current) to the circuit 200. Although the current flowing through the bimetallic switch 202 is rated at four times the device's normalized rated current, it still takes 48 seconds to reach the device's trip point 502. This is not ideal, as a long time-to-trip of the bimetallic switch under four times the normalized current places very high stress on downstream components, increasing the likelihood of failure.

[0048] Figure 6 shows the response waveform 600 of a bimetal switch 302 connected to the D MOSFET 304 in Figure 3, in an exemplary embodiment, where a 10V power supply provides 6A (300% of the switch's rated current) to the circuit 300. As shown in waveform 600, a 10V 6A overcurrent condition was applied to the protection circuit 300 (Figure 3), and its response was measured. The current (I) passing through the D MOSFET 304 D )(C2) and the voltage across bimetal switch 302 and D MOSFET 304 (V DS (C3) was monitored and captured against a time plot. Their scales are marked below waveform 600. Trip point 602 is the point at which the bimetal switch 302 began to disconnect.

[0049] Figure 7 shows the response waveform 700 of a bimetal switch 302 connected to the D MOSFET 304 in Figure 3, in an exemplary embodiment, where a 10V power supply provides 12A (600% of the switch's rated current) to the circuit 300. In this example, the resistor 306 is 0Ω. As shown in waveform 700, a 10V 12A overcurrent condition was applied to the protection circuit 300 (Figure 3), and its response was measured. At the trip point 702, the current I flowing from the D MOSFET 304 was measured. D The current drops rapidly from 12A to 0A, while the voltage across the MOSFET V DS The voltage drops from 10V to 0V. Immediately before the trip point 702, the MOSFET 304 dissipates approximately 4.27V × 12A = 54.24W of power (see Table 800 in Figure 8 below). The 600% rated current of 12A causes thermal dissipation of the D MOSFET 304, which raises the temperature of the MOSFET packaging to the tripping level of the bimetal switch 202 at 60°C. In the example shown in response waveform 700 (Figure 7), it took approximately 3.7 seconds to reach the trip point 702 of the bimetal switch 202. Therefore, in exemplary embodiments, 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 thermal dissipation caused by the D MOSFET 304.

[0050] Figure 8 includes Table 800, which shows the results of overcurrent test current operation according to 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 triggered at 60°C were tested with a saturation current I sat The test was performed using a rated current below a certain limit.

[0051] Table 800 provides an overview of tests using different currents and bimetal switch trip responses. The upper part of Table 800 provides time-to-trip information for the bimetal switch 302 (Figure 3) connected to the D MOSFET 304 for eight current states (6A, 8A, 10A, 12A, 14A, 16A, 20A, and 24A), while the lower part of Table 800 provides time-to-trip information for the bimetal switch 202 (Figure 2) not connected to the D MOSFET for the same eight current states. Table 800 shows that the time-to-trip 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 time-to-trip is reduced to just 3.7s. Therefore, in exemplary embodiments, the time-to-trip is significantly faster / improved in the D MOSFET + bimetal switch case over a current range of 3 to 6 times.

[0052] Table 800 also shows that the bimetallic switch does not trip at applied currents of 2A (100%) or 4A (200%). Instead, the bimetallic switch begins to trip at a current of 6A (300%) because there is sufficient energy at 6A to thermally activate the bimetallic switch.

[0053] As these experiments demonstrate, in exemplary embodiments, the presence of the D MOSFET accelerates the tripping of the bimetallic switch at all current ratios. The resistor R determines the maximum allowable current passing through the D MOSFET in the steady state. This maximum allowable current is I sat This is the saturation current. In an exemplary embodiment, by using a 0Ω resistor, a higher I is obtained compared to when using a resistor R=0.1Ω. 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 MOSFET304 can be increased. GS The voltage becomes slightly negative, pinching off and limiting the current flowing through the D MOSFET.

[0054] By increasing the resistance R, the saturation current I sat and gate-source voltage (V GS Both of these factors change, which in turn causes the power across the D MOSFET to fluctuate, making it possible to provide more power 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.

[0055] Figure 9 includes Graph 900, which shows the improved trip response time of protection circuits of 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. The dark circles represent the trip time of a standalone bimetallic switch (e.g., Figure 2), while the light circles represent the trip time of a bimetallic switch + D MOSFET (e.g., Figure 3). When the bimetallic switch is combined with a D MOSFET, the trip time shifts closer to the right, left, and top, bottom of Graph 900 (meaning the time-to-trip is shorter), which is within the safe operating curve for the trip time of the bimetallic switch. Therefore, adding a D MOSFET to the bimetallic switch improves the overall time-to-trip and provides much faster protection to downstream electronics.

[0056] The benefits provided by the D MOSFET are evident in Graph 900. For example, the time-to-trip of a standalone bimetallic switch at 8A, given by the dark circle 902, is approximately 57 seconds, while the time-to-trip of a bimetallic switch + D MOSFET at the same current, given by the light circle 904, is approximately 8 seconds. Similarly, the time-to-trip of a standalone bimetallic switch at 10A, given by the dark circle 906, is approximately 19 seconds, while the time-to-trip of the combinational circuit (light circle 908) is approximately 6 seconds. Only at higher currents does the standalone bimetallic switch perform comparably to the combinational circuit, which makes sense since these currents far exceed the switch's rated current. Therefore, Graph 900 demonstrates the benefits of having a combinational circuit consisting of both a bimetallic switch and a D MOSFET working together to protect against overcurrent conditions.

[0057] Returning to the protection circuit 200 in Figure 2, the circuit includes an independent bimetallic switch 202 undergoing a surge test, according to an exemplary embodiment. The test is performed with a surge current in 1.2 / 50 μs, having a peak voltage of 500 V at 2 Ω. Figure 3, in contrast, shows circuit 300 in which a 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. Here again, the test is performed with a surge current in 1.2 / 50 μs, having a peak voltage of 500 V at 2 Ω. In both circuits 200 and 300, the bimetallic switch is a KSD-01 temperature switch thermostat, while in circuit 300, the D MOSFET 304 is an IXTH16N50D2 depletion mode MOSFET (V DSX =500V, I D(on) =16A, R DS(on) (=300mΩ).

[0058] Figure 10 includes a response waveform 1000 showing the surge response of a bimetallic switch in an isolated circuit, such as the circuit 200 in Figure 2, according to several embodiments. A surge waveform exists with a peak voltage of 500V and a virtual impedance of 2Ω, with a duration of 1.2 / 50μs. As shown in waveform 1000, the surge current (C2) flowing 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 almost constant at 10V, although there is some slight rise due to the incoming surge. However, the surge does not trigger the opening of the bimetallic switch. Therefore, in some embodiments, the switch does not trigger in this surge condition.

[0059] Figure 11 includes a response waveform 1100 showing the surge response of a circuit comprising a combined D MOSFET and a bimetal switch, such as the circuit 300 in Figure 3, according to several embodiments. As shown in waveform 900, if the conditions of the incoming surge are the same, the current passing through the D MOSFET and bimetal switch combination is clamped and reduced (C2), remaining "saturated" at a peak current of approximately 21.8 A for about 40 μs. The D MOSFET clamps very quickly during a surge, resulting in the output as a very low current output. This is in contrast to the surge test using a standalone bimetal switch (Figure 10) described above.

[0060] Therefore, in exemplary embodiments, when a D MOSFET with a bimetallic thermal switch is deployed, the trigger time of the switch is much faster than when an independent switch is triggered with the same applied overcurrent. Furthermore, the resulting surge current is at a much lower safety level, protecting downstream circuit elements.

[0061] In exemplary embodiments, the D MOSFET and bimetallic switch can work closely together in the circuit to provide mutual protection. Under prolonged overcurrent protection events, the D MOSFET heats up, triggering the switch at a specified trigger temperature, generating an open-circuit current that prevents the overcurrent from passing through downstream components of the circuit and protects the D MOSFET from overheating. The switch resets and returns to its normal position from its temporarily deformed position once its case has cooled to a reset level.

[0062] Furthermore, in some embodiments, the combinational 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. The bimetallic strip + D MOSFET disclosed herein may be a suitable alternative to such circuit breakers and may eliminate the need for human intervention to reset them. Both devices (bimetallic switch and D MOSFET), when connected in this manner, share the characteristics of self-protection and self-resetting.

[0063] In addition to the bimetallic switch examples given above, the principles described herein may also be applied to other types of thermal switches, mini circuit breakers, and relay-type circuit breakers that have a bimetallic switch inside, regardless of whether these devices include a self-resetting function or a manual reset function.

[0064] The test results 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 much 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.

[0065] In an exemplary embodiment, the waveforms above demonstrate that placing the bimetallic switch before the D MOSFET and (as shown in Figure 4) placing the switch together on top of the D MOSFET package provides a mutual benefit of the two devices for protection against overcurrent events. The D MOSFET acts as a current limiter with (or without) a bias resistor (e.g., resistor 306 in Figure 3) at the gate-source terminal of the MOSFET. If the D MOSFET has 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.

[0066] The principles described herein may be applied, as one example, to circuits such as miniature circuit breakers (MCBs). MCBs are used to protect against overcurrent events, such as short circuits (e.g., 10 to 100 times the normal current), very high surge overcurrent events (e.g., 5 to 10 times the normal current), and overload conditions (e.g., 2 to 4 times the normal current), in buildings including offices and homes. A short circuit refers to an overcurrent generated by short-circuiting a power line or equipment connected to the MCB. The term "overcurrent" includes these short-circuit events but also encompasses sudden increases in current caused by disturbances such as other high-power lines coming into contact with the power line in question. MCBs are designed to trip or open the internal circuitry within the MCB, thereby interrupting the current and preventing the circuit from overheating. The tripping action, which occurs very quickly (e.g., in less than 3 ms), also prevents current from being transmitted to other devices to which the MCB is connected. The MCB also utilizes a bimetallic strip, which may be connected to a D MOSFET to improve the MCB's response time.

[0067] The MCB described herein comprises a bimetallic strip, i.e., two metal pieces arranged back-to-back, where the two metal pieces are made of different metals having different coefficients of thermal expansion, thereby causing the bimetallic strip to bend during heating. In the protection circuit 300 described and illustrated above, the bimetallic strip is referred to as a bimetallic switch, because within the circuit, the bimetallic strip acts as a switch to open or close the circuit. In contrast, the bimetallic switch provided in the MCB described below bends to open a separate circuit component called a main switch, which is a trigger mechanism for opening the circuit. Although given a different name (strip versus switch), the bimetallic strip described below is essentially the same as the bimetallic switch 302 (Figure 3) in the protection circuit 300 described above.

[0068] Figures 12A and 12B are diagrams of a prior art MCB1200. The MCB1200 is a two-terminal device. The MCB1200 includes a switch that opens in response to a short circuit or overload condition. The MCB1200 includes two different sensing elements that activate a trigger mechanism. The first sensing element is a magnetic coil 1208, which has a movable valve. The second sensing element is a bimetallic strip 1204. The trigger mechanism is a main switch 1206 that is manually controlled by an external lever 1202 and opens automatically in response to a fault condition. The external lever 1202 turns the MCB1200 on (closes the main switch 1206 as shown in Figure 12A) or turns the MCB off (opens the main switch as shown in Figure 12B). The MCB1200 also includes an arc chute 1210, which is also known as an arc absorber.

[0069] The magnetic coil 1208 is an electromagnetic sensor that generates an electric field proportional to the current passing through the coil. The magnetic coil 1208 is designed for a short-circuit condition that can generate a current up to 1000 times greater than the normal current within a few milliseconds. As the current increases, the magnetic field around the magnetic coil 1208 also increases. The movable valve of the magnetic coil 1208 is positioned close to the main switch 1206. As the magnetic field intensifies, the valve pushes the main switch 1206, thereby opening the switch as shown in Figure 12B, resulting in an open circuit. The spring tension of the magnetic coil 1208 is not sufficient to trigger the opening of the main switch 1206 while a normal current is flowing, but is sufficient to trigger it during a short-circuit condition.

[0070] In some cases, when a very high surge overcurrent (5 to 10 times the normal current) flows within the MCB1200, the magnetic coil 1208 enters a saturated state, causing the movable magnetic valve inside the coil to press the main switch 1206 very quickly. The rapid response of the magnetic coil 1208 is essential for cutting off extremely dangerously large overcurrents and protecting the circuits and systems connected to the MCB1200 from damage.

[0071] Another sensor in the MCB1200 is the bimetal strip 1204. The bimetal strip 1204 sensor is designed to handle overload conditions and has a slower operating speed than the magnetic coil 1208. The slower sensor ensures that appliances simply being turned on do not trigger the MCB1200 to shut them down. Fluorescent lamps, for example, have a rise time of approximately 10 ms. The bimetal strip 1204 causes the MCB1200 to trip if the overload condition persists for more than 2 seconds.

[0072] As described above, the bimetallic strip 1204 consists of two different types of metal, each having a different coefficient of thermal expansion. In an exemplary embodiment, the bimetallic strip 1204 consists of a first elongated strip of metal on which a wire winding is wound, as a second strip of metal, and these two metal pieces are connected together in series. When heated, due to the occurrence of an overload condition, the elongated strip of metal contracts, thereby bending the bimetallic strip 1204, which in turn causes the main switch 1206 to move and the circuit in the MCB 1200 to open. Alternatively, if a wire winding is wound around the bimetallic strip 1204, the wire winding heats up and bends, pushing the main switch 1206, thereby opening the main switch 1206, and thus turning off the MCB from external power. The current value on which the bimetallic strip 1204 should operate can generally vary within a certain range.

[0073] The arc chute 1210 of the MCB1200 is designed to dissipate the arc current generated when the main switch 1206 is opened, whether it originates from the magnetic coil 1208 or the bimetal strip 1204. The arc current flows through the air near the newly opened end of the main switch 1206, causing a significant temperature rise that can damage the MCB1200. Therefore, the arc chute 1210 is positioned above the main switch 1206, as the high-temperature arc current will flow upward. The arc chute 1210 comprises several parallel metal plates known as splitters, which are designed to discretize the arc current while it flows upward. The metal splitters separate the arc current into smaller arc currents that dissipate more quickly while the arc current continues to flow upward.

[0074] Figure 13 is a diagram of a second prior art MCB1300. The diagram shows the current path through the MCB1300. The MCB1300 includes an external lever 1302 that controls the main switch 1306, which is the trigger mechanism. Similar to the MCB1200, the bimetal strip 1304 and magnetic coil 1308 are the sensor mechanism of the MCB1300. The left terminal 1312 and right terminal 1314 are also shown. The MCB1300 does not have an arc chute, because such a device is optional in some circuit breaker designs.

[0075] The current path is shown as a dotted line in the MCB1300. The current flows from the right terminal 1314, through the magnetic coil 1308, then through the bimetal strip 1304, then through the main switch 1306, and finally out through the left terminal 1312. Similarly, in the opposite direction, the current flows from the left terminal 1312, through the main switch 1306, through the bimetal strip 1304, through the magnetic coil 1308, and out through the right terminal 1314. When the main switch 1306 is open, the current path is interrupted in both directions.

[0076] In the event of an overcurrent event having a magnitude of 1 to 4 times that of a normal current, the overcurrent heats and bends the wire windings wrapped around the bimetal strip 1304, pressing the main switch 1306 until the switch opens, and thus the MCB 1300 is turned off from external power. In the event of a very high surge overcurrent event flowing through the MCB 1300, the increase in the magnetic field causes the movable magnetic valve inside the magnetic coil 1308 to press the main switch 1306 very quickly, and thus the MCB 1300 is able to cut off a very dangerously large overcurrent.

[0077] Figure 14 is a circuit diagram 1400 of a prior art MCB. Circuit diagram 1400 may represent, for example, an MCB 1200 (Figures 12A and 12B). A bimetallic strip 1404 is shown on the left side of the circuit 1400, and a magnetic coil 1408 is shown on the right side, with a main switch 1406 positioned between them. The bimetallic strip 1404 consists of a first metal around which a wire, which is a second metal in series with the first metal, is wound. An arc chute 1410 is positioned near the main switch 1406 to dissipate arc current following a fault event (whether short-circuit or overload).

[0078] The force described above, whether from the valve of the magnetic coil 1408 that pushes the valve or from the bending of the bimetal strip 1404, opens the main switch 1406 and therefore interrupts the flow of current. The arc chute 1410 is prepared to absorb the excess arc current by diverting the arc to a smaller level within the metal interface of the arc chute, thus avoiding damage inside the MCB.

[0079] One issue with miniature circuit breakers concerns the bimetallic strip. As explained above, the bimetallic strip has two different types of metal, each with its own coefficient of thermal expansion. An overcurrent condition causes the bimetallic strip to bend, opening the circuit. Because the bimetallic strip is designed to handle overload conditions, it acts more slowly than a magnetic coil. However, the bimetallic strip is unable to provide current limiting when the current exceeds its rated current.

[0080] Figures 15A and 15B are circuit diagrams 1500A and 1500B of an MCB having a D MOSFET, respectively, according to an exemplary embodiment. In both Figures 1500A and 1500B (collectively referred to as "Circuit 1500" or "MCB1500"), the main switch 1506 is positioned between the bimetal strip 1504 on one side and the magnetic coil 1508 on the other side. An arc chute 1510 is positioned near the main switch 1506 to dissipate arc current. In Circuit 1500A, a D MOSFET 1520 is connected between the bimetal strip 1504 and the main switch 1506. In Circuit 1500B, a JFET 1522 is connected between the bimetal strip 1504 and the main switch 1506. Therefore, either the MOSFET 1520 or the JFET 1522 may be used to improve a new MCB1500.

[0081] In an exemplary embodiment, the D MOSFET 1520 is thermally coupled to the bimetal strip 1504 with its gate G and source S connected (GS terminal short-circuited), and the D MOSFET is connected in series with the bimetal strip (Figure 15A). In an exemplary embodiment, the JFET 1522 is similarly thermally coupled to the bimetal strip with its gate G and source S connected (GS terminal short-circuited), and the JFET is connected in series with the bimetal strip. In one embodiment, the bimetal strip 1504 and the D MOSFET 1520 or JFET 1522 are thermally linked to each other using a thermally conductive epoxy adhesive. In another embodiment, the bimetal strip 1504 and the D MOSFET 1520 or JFET 1522 are thermally linked to each other using a conductive epoxy gel. In an exemplary embodiment, the bimetal strip 1504 combined with either the MOSFET 1520 or JFET 1522 can provide current limiting when the incoming current exceeds the rated current of the MCB 1500.

[0082] Similar to the MOSFETs introduced above, junction field-effect transistors (JFETs) are semiconductor devices used to switch and amplify electronic signals in electronic devices. Both JFETs and MOSFETs are voltage-controlled devices. Since JFETs operate only in depletion mode, they are on and fully conduct when 0V is present at the gate, similar to the depletion-mode MOSFETs used herein. Both have high input impedance and are therefore sensitive to input voltage signals, although MOSFETs have higher resistance than JFETs. Furthermore, JFETs are less expensive and less complex to manufacture, while MOSFETs are more fragile due to the presence of metal oxide insulators in them.

[0083] In exemplary embodiments, the wire windings, which are the second metal of the bimetal strip 1504, are removed, and a D MOSFET 1520 or JFET 1522 is mounted in close contact with the bimetal strip. In one embodiment, no metal windings are added back to the elongated metal piece following the mounting of the D MOSFET 1520 or JFET 1522. The D MOSFET 1520 or JFET 1522 can directly supply heat to the bimetal strip 1504. Therefore, the removal of the metal windings can help minimize series resistance and ohmic losses in the power line. In exemplary embodiments, the JFET 1522 is a silicon carbide (SiC) JFET connected between the bimetal strip 1504 and the main switch 1506.

[0084] In an exemplary embodiment, the D MOSFET 1520 or JFET 1522 enters current-limiting mode when a current greater than the MCB 1500's rated current is received within the MCB 1500. Therefore, for example, if the MCB 1500 has a rated current of 1A, the D MOSFET 1520 or JFET 1522 enters current-limiting mode when a current greater than 1A is received within the MCB 1500. While external overcurrents can reach up to 5A, the D MOSFET 1520 or JFET 1522, in conjunction with the bimetal strip 1204, helps to quickly "clamp" the current down to 1A (the safe operating level of the MCB 1500). Therefore, in an exemplary embodiment, the MCB 1500 is made safer by having a bimetal strip 1504 that, with the assistance of the FET (whether D MOSFET 1520 or JFET 1522), is capable of directly responding to a 5A overcurrent. Otherwise, a 5A overcurrent could destroy the electronic equipment that the MCB1500 is supposed to protect, or the MCB itself.

[0085] Figure 16 shows an MCB 1600 comprising a D MOSFET 1620 according to an exemplary embodiment. An external lever 1602 manually activates the trigger mechanism of the main switch 1606 to either on (closed) or off (open). As in other MCBs described herein, a magnetic coil 1608 is designed to trip the main switch 1606 in response to a short circuit, and a bimetal strip 1604 is designed to trip the main switch in response to an overload condition. The left terminal 1612 and the right terminal 1614 connect the MCB 1600 between the load and a power supply (not shown). In the exemplary embodiment, the D MOSFET 1620 is thermally coupled to the bimetal strip 1604 when the GS terminal is short-circuited, and the D MOSFET is connected in series with the bimetal strip. In an exemplary embodiment, the D MOSFET 1620 is tightly fixed to the bimetal strip 1604, with the winding wires and insulation tube of the bimetal strip first removed before the D MOSFET is fixed.

[0086] Figures 17-19 show the response waveforms of experiments performed on an MCB with a D MOSFET connected to a bimetal strip and an MCB without a D MOSFET, according to an exemplary embodiment. For the experiment, a Phoenix Contact UT6-TMC 1A MCB (hereinafter "PC MCB") was used (manufactured by Phoenix Contact), and a Littelfuse IXTY1R6N50D2 D MOSFET device (hereinafter "LF DMOS") was used (with a trip current of T165°C). C The ) is connected to a bimetallic strip within the MCB. LF DMOS is a small package that can be fitted into a PC MCB.

[0087] Response waveform 1700 (Figure 17) shows the result when an overcurrent of 1.5A (1.5 times the normal current) occurs. The PC MCB without LF DMOS tripped after 72 seconds, while the PC MCB with LF DMOS tripped after 15 seconds. Thus, the PC MCB with LF DMOS tripped faster than the one without LF DMOS.

[0088] Response waveform 1800 (Figure 18) shows the result when an overcurrent of 2.0A (twice the normal current) occurs. The PC MCB without LF DMOS tripped after 22 seconds, while the PC MCB with LF DMOS tripped after 15 seconds. Thus, the PC MCB with LF DMOS tripped faster than the one without LF DMOS.

[0089] Response waveform 1900 (Figure 19) shows the result when an overcurrent of 5A (five times the normal current) occurs. The PC MCB without LF DMOS tripped after 3.9 seconds, while the PC MCB with LF DMOS tripped after 15 seconds. Thus, the PC MCB with LF DMOS tripped more slowly than the one without. Furthermore, for each experiment performed (1.5A, 2A, and 5A), the PC MCB with LF DMOS tripped after 15 seconds.

[0090] The results of these experiments demonstrate that adding a D MOSFET to a bimetal strip helps to ultimately limit the overcurrent to 1A, regardless of how high the external overcurrent is. The addition of the D MOSFET alters the time-to-trip response curve. The response waveform shows that the bimetal strip can safely trip and protect the LF DMOS even when the current exceeds the MCB's rated current. The combined effect of the LF DMOS with the bimetal strip causes the combination circuit to become a current-limiting self-protection switch, which a standalone bimetal strip cannot achieve.

[0091] Therefore, instead of having a simple bimetallic strip temperature sensing structure, the presence of an FET (D MOSFET or JFET) connected to the bimetallic strip forms a protective switch that limits current, is surge-resistant, and cuts off overcurrent. Furthermore, the bimetallic strip gains sufficient heat dissipation from the FET during current limiting to trigger the main switch of the MCB, which results in an open circuit, and thus the FET is protected from overheating or chip failure.

[0092] In exemplary embodiments, the LF DMOS is an epoxy-packaged D MOSFET mounted on a bimetallic strip. In other embodiments, a bare D MOSFET die, or a bare D MOSFET die mounted on a metal plate with a suitable lead frame, is used to ensure better heat transfer for triggering the bimetallic strip. The experimental data in Figures 17–19 are derived from DC operating conditions. When used in AC operating conditions, in some embodiments, two back-to-back D MOSFETs are connected to the bimetallic strip.

[0093] The addition of a FET (D MOSFET or JFET) to the MCB is an extension of the D MOSFET combination to the bimetal switch 302 in the protection circuit 300 (Figure 3). The MCB, as shown by circuit diagram 1500 (Figure 15), has a similar but more complex structure to the protection circuit 300. The addition of a FET to the MCB advantageously provides current limiting, surge protection, and overcurrent protection. The bimetal strip connected to the FET safely opens the main switch of the MCB, which helps provide a safe power cutoff during external hazardous overcurrent events. Therefore, the FET is an efficient way to improve cutoff speed and thus provides electrical system designers with a safer protection option.

[0094] In an exemplary embodiment, the D MOSFET connected to the bimetal strip within the MCB device is further provided with appropriate driver circuitry to ensure proper turn-off and turn-on times, as well as other functions for driving the D MOSFET.

[0095] Where used herein, an element or step described in the singular form and followed by the word "a" or "an" should be understood not to exclude multiple elements or steps unless explicitly stated otherwise. Furthermore, any reference to "one embodiment" in this disclosure is not intended to be construed as excluding the existence of further embodiments that similarly incorporate the described features.

[0096] While this disclosure refers to specific embodiments, numerous modifications, alterations, and changes can be made 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 and is intended to have its full scope as defined by the following claims and their equivalents.

Claims

1. a switch that is manually opened or closed by an external lever; a magnetic coil having a movable valve that contacts and opens the switch upon the occurrence of a first fault event; a bimetallic strip that contacts the switch upon the occurrence of a second fault event; a field effect transistor (FET) connected in series with and thermally coupled to the bimetallic strip, the FET having a gate terminal and a source terminal, the gate terminal connected to the source terminal, the bimetallic strip and the FET opening the switch during the second fault event; and 1. A miniature circuit breaker comprising:

2. 10. The miniature circuit breaker of claim 1, wherein the bimetallic strip comprises a first metal having a first coefficient of thermal expansion and a second metal having a second coefficient of thermal expansion, the first coefficient of thermal expansion being different from the second coefficient of thermal expansion.

3. 3. The miniature circuit breaker of claim 2, wherein the first metal includes a winding, the winding being removed before the FET is thermally coupled to the bimetal strip.

4. 4. The miniature circuit breaker of claim 1, wherein the FET is a depletion mode metal oxide semiconductor FET (D MOSFET).

5. 4. The miniature circuit breaker of claim 1, wherein the FET is a junction field effect transistor (JFET).

6. 6. The miniature circuit breaker of claim 5, wherein the JFET is a silicon carbide JFET.

7. 4. The miniature circuit breaker of claim 1, wherein the first fault event is a short circuit.

8. 4. The miniature circuit breaker of claim 1, wherein the second fault event is an overload event.

9. 4. The miniature circuit breaker of claim 1, wherein the bimetallic strip is coupled to the drain of the FET.

10. 4. The miniature circuit breaker of claim 1, further comprising an arc chute to absorb arcing following either the first fault event or the second fault event.

11. 4. The miniature circuit breaker of claim 1, wherein the magnetic coil further comprises a movable valve that causes the switch to open in response to the first fault event.

12. 5. The miniature circuit breaker of claim 4, wherein the MOSFET is an N-channel depletion-mode MOSFET.

13. 4. The miniature circuit breaker of claim 1, wherein the bimetallic strip and the FET do not cause the switch to open the miniature circuit breaker until the second fault event occurs for at least two seconds.

14. 4. The miniature circuit breaker of claim 1, further comprising a current rating, wherein the bimetallic strip and the FET provide current limiting when a current received into the miniature circuit breaker exceeds the current rating.

15. 1. A miniature circuit breaker comprising: a bimetallic strip having an elongated metal strip and a metal winding wrapped around the elongated metal strip, the elongated metal strip bending in response to exceeding the rated current of the miniature circuit breaker; a field effect transistor (FET) connected in series with and thermally coupled to the elongated metal strip after the metal winding is removed from the bi-metal strip, the FET having a gate terminal connected to a source terminal; a switch that opens in response to an incoming current that exceeds the rated current, the switch opening in response to the bending of the elongated metal strip; wherein current limiting above the rated current is provided by the FET.

16. 16. The miniature circuit breaker of claim 15, wherein the FET is a metal oxide semiconductor FET (MOSFET).

17. 17. The miniature circuit breaker of claim 16, wherein the MOSFET is a depletion mode MOSFET.

18. 18. The miniature circuit breaker of claim 16 or 17, wherein the FET is a junction FET (JFET).

19. 20. The miniature circuit breaker of claim 18, wherein the JFET is a silicon carbide JFET.

20. 18. A miniature circuit breaker as claimed in claim 16 or 17, wherein the rated current is 1A and the current limit is 5A.

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