Miniature circuit breaker

Abstract

This paper discloses a miniature circuit breaker for providing short-circuit and overload protection. The miniature circuit breaker features a field-effect transistor (FET), which can be a depletion-type metal-oxide-semiconductor FET (D MOSFET), a junction field-effect transistor (JFET), or a silicon carbide JFET. The FET is connected to a bimetallic switch, which acts as a temperature-sensing circuit breaker. The combination of the D MOSFET and the bimetallic switch limits the current flowing to downstream circuit components, thus protecting them from damage.

Description

[Previous Technology]

[0001] Overcurrent or excess current is a situation where a larger current than expected flows through a circuit. Overcurrent can be constant or transient in nature. Voltage transients (short-term surges of electrical energy) are the result of the sudden release of previously stored energy or are caused by other means (such as heavy inductive loads or lightning). Repeatable transients are frequently caused by the operation of motors, generators, or the switching of reactive circuit components. Random transients may be caused by lightning and electrostatic discharge (ESD).

[0002] Component miniaturization has led to increased sensitivity to electrical stresses. For example, microprocessors have structures and conductive paths that cannot handle high currents from ESD transients. Such components operate at very low voltages, so voltage disturbances must be carefully controlled to prevent device interruption and potential or catastrophic failures.

[0003] In view of these and other considerations, the improvements of the present invention may be helpful. [Summary of the Invention]

[0004] This summary is provided to introduce, in a simplified form, a series of concepts that will be further elaborated in the embodiments. This summary is not intended to identify key or essential features of the claimed object, nor is it intended to help determine the scope of the claimed object.

[0005] An exemplary embodiment of a miniature circuit breaker is disclosed, which is operable to provide protection against short-circuit or overload events. The miniature circuit breaker includes a switch that is manually opened or closed by an external lever, but the switch automatically opens in response to a first fault event or a second fault event. The miniature circuit breaker also includes an electromagnetic coil, a bimetallic strip, and a field-effect transistor (FET) connected in series with the bimetallic strip to open the switch in response to the first fault event, wherein the gate terminal and source terminal of the FET are connected to each other. The bimetallic strip and the FET open the switch during the second fault event.

[0006] Another exemplary embodiment of the miniature circuit breaker according to this disclosure may include a bimetallic strip, a field-effect transistor (FET), and a switch. The bimetallic strip includes an elongated metal strip and a metal winding wrapped around the strip. If the rated current of the miniature circuit breaker is exceeded, the elongated metal strip bends. After the metal winding is removed from the elongated metal strip, the FET is connected in series with and thermally bonded to the elongated metal strip. The FET has a gate terminal connected to the source terminal. When a current exceeding the rated current of the miniature circuit breaker enters the miniature circuit breaker, the switch opens due to the bending of the elongated metal strip. The FET provides a current limit above the rated current.

Implementation Method

[0008] This document discloses a circuit for providing overcurrent and overvoltage protection. The circuit is characterized by a depletion-mode metal-oxide-semiconductor field-effect transistor (MOSFET) (D MOSFET) as a current limiter, which is connected to a bimetallic switch that acts as a temperature-sensing circuit breaker. The combination of the D MOSFET and the bimetallic switch limits the current flowing to downstream circuit components, thus protecting them from damage.

[0009] Furthermore, this document discloses a mini circuit breaker (MCB) for providing overcurrent and overvoltage protection. The MCB is characterized by a depletion-type MOSFET (D MOSFET) or junction field-effect transistor (JFET) as a current limiter, connected to a bimetallic strip that acts as a temperature-sensing circuit breaker. The combination of the D MOSFET or JFET and the bimetallic strip limits the current flowing to downstream circuit components, thus protecting them from damage.

[0010] A metal-oxide-semiconductor field-effect transistor (MOSFET) is a semiconductor device used to switch and amplify electronic signals in electronic devices. By adjusting the voltage on the gate of the MOSFET, the width of the channel between the source and drain of the MOSFET is changed. MOSFETs have various configurations, including P-channel devices constructed using an N-type substrate, N-channel devices constructed using a P-type substrate, vertically positioned semiconductors, laterally positioned semiconductors, depletion-type, and enhancement-type.

[0011] Unlike enhancement-mode MOSFETs, which are turned on by applying a voltage across their gates, depletion-mode MOSFETs are called "normally-on" devices when the gate terminals are at zero volts (VGS=0 volts). In addition to a thin gate oxide layer between the source and drain regions, conductive channels are formed using ion implantation beneath the gate oxide layer and between the source and drain regions. The concentration of active dopant in the substrate and channel regions is used to adjust the MOSFET's threshold voltage (VTh) to the desired value. Despite the name, many modern MOSFETs have a polysilicon gate on top of an insulating gate oxide layer, rather than a metal gate.

[0012] A bimetallic switch is a switch consisting of two metal strips joined together (back to back). The bimetallic switch is placed in a circuit between two connection points. The first metal strip has a first coefficient of thermal expansion, and the second metal strip has a different second coefficient of thermal expansion. When heat is applied to the bimetallic switch, once the temperature exceeds the "open" threshold, the switch will temporarily deform or bend based on these two different coefficients of thermal expansion. In the case where the bimetallic switch consists of two back-to-back metal strips of a predetermined length, applying heat will cause the switch to "shorten" or one end to "lift," so that the switch no longer maintains the predetermined length and thus becomes unattached to the two connection points of the circuit, resulting in an open circuit state. In the case where the bimetallic switch is part of a package (e.g., a KSD-01F temperature switch thermostat, which will be discussed further below), the package includes two extending legs that will change their relative positions during heating, thus causing the connection points of the circuit to disconnect, resulting in an open circuit state. In either configuration, once the bimetallic switch cools down again, it will straighten or return to its original shape (generally flat), thus reattaching the switch between the two connection points and closing the circuit.

[0013] FIG1 is a representative diagram of a protection circuit 100 according to an exemplary embodiment. The protection circuit 100 (also referred to herein as the "circuit") consists of a bimetallic switch 102 and a depletion-type MOSFET 104 (hereinafter referred to as "D MOSFET" or "MOSFET") connected in series with each other. The D MOSFET 104 includes a drain (D), a source (S), and a gate (G) through which current flows. Under certain voltage conditions, the gate (G) will affect the current flow between the drain and the source. As a depletion-type device, the D MOSFET 104 is normally "on" even though the gate (G) voltage is 0 volts, where current flows between the drain (D) and the source (S). When the switch is closed, the bimetallic switch 102 provides an electrical path between nodes 110 and 112, and when the switch is not closed, the bimetallic switch 102 causes an open circuit. One end of the bimetallic switch 102 is also connected to the drain (D) of the D MOSFET 104.

[0014] The protection circuit 100 further includes a resistor 106, which is connected at a first end to the source (S) of the MOSFET 104 and at a second end to the gate (G) of the MOSFET. Therefore, the voltage across the resistor 106 is the same as the gate-to-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 downstream of the circuit element already described and is generally shown in FIG. 1.

[0015] 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. The first metal strip has a first coefficient of thermal expansion and the second metal strip has a different second coefficient of thermal expansion. When the temperature sensed by the bimetallic switch exceeds a threshold temperature, this difference causes the switch 102 to undergo temporary deformation (e.g., bending). The temporary 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 disconnect the protection circuit 100, the threshold temperature is also referred to herein as the "disconnection threshold temperature" and the "trip point" or "trip time" temperature.

[0016] In an exemplary embodiment, combining the bimetallic switch 102 with the D MOSFET 104 provides overcurrent protection to the circuit 100. As shown in FIG1, an overcurrent condition can be characterized as 1) a transient overcurrent or 2) a constant overcurrent. Once an overcurrent condition exists, the D MOSFET 104 generates heat, which activates the thermal sensing characteristics of the bimetallic switch 102. A temporary deformation of the bimetallic switch 102 means reaching the open-circuit threshold temperature, preventing the bimetallic switch 102 from connecting to one or more of nodes 110 and 112 of the circuit 100, thereby resulting in an open circuit.

[0017] Once the fault overcurrent condition subsides, the two metal strips of the bimetallic switch 102 will cool down, allowing the switch to return from its temporarily deformed state to its original state. This will cause the bimetallic switch 102 to re-establish a connection between nodes 110 and 112 of the circuit 100, thereby resulting in a closed circuit. The bimetallic switch 102 thus provides an open-circuit protection for the circuit 100, which is a fail-safe environment for protecting other electronic systems or devices within the circuit (indicated as "additional circuit element 108" in FIG. 1).

[0018] Within the protection circuit 100, the D MOSFET 104 provides fast response and blocking capabilities for overcurrent and overvoltage events and can quickly clamp surge current events. The overcurrent clamping capability of the D MOSFET absorbs the energy of fast transient surges, which protects some harmful transient surges from reaching the sensitive electronic device (additional circuit element 108) to be protected.

[0019] In contrast, the bimetallic switch 102 provides high current interruption capability. However, the bimetallic switch 102 cannot respond quickly to prevent rapid transient events. Therefore, in an exemplary embodiment, combining the bimetallic switch 102 with the D MOSFET 104 helps to leverage the advantages of both devices to provide improvements in overcurrent and overvoltage protection.

[0020] In an exemplary embodiment, the D MOSFET 104 is an IXTH16N50D2 depletion MOSFET manufactured by Littelfuse® (where VDSX = 500 volts, ID(on) = 16 amps, RDS(on) = 300 milliohms), and the bimetallic switch 102 is a KSD-01F temperature switch thermostat manufactured by Dongguan Fuyuan Electronics Co., Ltd. (fuyuanfuse.com). As shown in FIG1, the bimetallic switch 102 is connected to the input terminal drain (D) of the D MOSFET 104. A resistor 106 is connected between the GS terminals of the D MOSFET 104. The bimetallic switch 102 acts as a conductive switch. During normal operation, the bimetallic switch 102 allows current to pass through as long as the current does not exceed the disconnect threshold temperature (which can also be considered as the "trigger level" of the bimetallic switch 102).

[0021] The drain current of MOSFET 104, shown as ID, begins to flow through the DS terminal of D MOSFET 104 until the gate-to-source voltage VGS potential difference (ID × R) reaches a level that would cause a more negative VGS to block more current from flowing through D MOSFET 104. In one embodiment, as the applied voltage increases, the current will increase linearly until saturation is reached. An equilibrium is achieved where the combined circuitry (including bimetallic switch 102 and D MOSFET 104) allows the maximum saturation current Isat of D MOSFET to flow through circuit 100. In this state, energy is also dissipated in D MOSFET 104 (where Isat × VDS), as is heat.

[0022] In an exemplary embodiment, the saturation current Isat is the maximum steady-state current through the D MOSFET 104. This means that as long as the saturation current is not exceeded, the D MOSFET will maintain its function without chip failure or failure due to overheating. If the overload current is less than Isat, the D MOSFET 104 will still dissipate energy as heat without any problem. However, if a very high short circuit exists at the input side of the D MOSFET, exceeding its Isat (saturation current) value, the D MOSFET will react quickly and dissipate the excess current as heat more rapidly. This, in turn, raises the temperature of the external package of the D MOSFET 104, causing the bimetallic switch 102 to trip quickly, preventing further current from turning off the D MOSFET. However, the extended current exceeding the MOSFET's saturation current Isat will cause the D MOSFET to overheat beyond its maximum junction temperature, leading to chip failure and MOSFET malfunction. Therefore, the bimetallic switch also helps protect the D MOSFET from overheating failures.

[0023] In an exemplary embodiment, the protection circuit 100 operates using an applied current ranging from 0 amps to the Isat saturation current. Abnormal conditions may cause a sudden increase in the input current ID. For example, abnormal conditions may be caused by a short circuit on the load, load switching, or a sudden overload. These abnormal conditions cause a surge in current through the bimetallic switch 102 and the D MOSFET 104. If this current remains below the saturation current Isat, the D MOSFET will gradually heat up due to power dissipation to the MOSFET. However, if the overcurrent exceeds the saturation current Isat, the input current ID will increase sharply, where heat dissipation to the D MOSFET (ID × VDS) causes the bimetallic switch to trip more quickly to protect the D MOSFET. This will cause the bimetallic switch 102 to reach its disconnection level, thus disconnecting from one or more of nodes 110, 112 at the disconnection threshold temperature. Therefore, the bimetallic switch also provides cut-off protection for the D MOSFET within the safe operating area and thermal limits.

[0024] In one embodiment, when a disconnection occurs at nodes 110 and / or 112, the total current flowing through the protection circuit 100 is cut off, which eliminates the current flowing to the D MOSFET 104, dissipates the heat of the MOSFET, and ultimately prevents the D MOSFET from overheating by eliminating the continuous overcurrent.

[0025] Therefore, 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. Thus, the higher the level of the short-circuit current, the faster the bimetallic switch 102 can cut off the current to the D MOSFET 104, and thus protect the D MOSFET from overheating damage. This feedback occurs because the high heat generated by the D MOSFET 104 under overvoltage / overcurrent conditions causes the bimetallic switch 102 to turn off more quickly, thus preventing more current from flowing through the D MOSFET and eliminating this self-heating, and ultimately protecting the downstream additional circuit elements 108.

[0026] Figures 2, 3, and 4 are illustrations of protection circuits 200, 300, and 400 according to an exemplary embodiment, illustrating the results of continuous current testing. In the exemplary embodiment, the bimetallic switch is a KSD-01 temperature switch thermostat (triggered at 60°C with an operating current of 2 amps and 250 volts), the D MOSFET is an IXTH16N50D2 depletion-type MOSFET (where VDSX = 500 volts, ID(on) = 16 amps, RDS(on) = 300 milliohms), and the resistor 206 is a 0-ohm resistor. In Figure 2, the bimetallic switch 202 is a stand-alone device not coupled to the MOSFET. In Figure 3, the bimetallic switch 302 is connected to the D MOSFET 304 and the resistor 306, wherein the circuit elements are arranged similarly to those in the protection circuit 100 shown in Figure 1. In Figure 4, the bimetallic switch 402 (KSD-01F) is attached back-to-back to the D MOSFET 404 (TO247 package) using screws 412 and bolts (not visible). In one embodiment, the bimetallic switch 402 and the D MOSFET 404 are further thermally linked together using a thermally conductive epoxy adhesive (not shown). In another embodiment, the bimetallic switch 402 and the D MOSFET 404 are further thermally linked together using a conductive epoxy gel. Furthermore, the bimetallic switch 402 and the D MOSFET 404 are electrically connected to each other and to other parts of the circuit via wirings 406, 408, and 410. The KSD-01F bimetallic switch consists of a bimetallic disc, a metal bridge connecting the two legs of the device, metal bridge contacts, and a plastic housing isolated from the heat sink. The bimetallic disc will bend at a specific temperature, causing the metal bridge to connect or disconnect from the legs, thus closing or opening the circuit.

[0027] In Figures 2 and 3, the corresponding arrows 204 and 322 indicate the direction of current flow. In circuit 200, which specifically has a separate bimetallic switch 202, current 204 flows from node 206 to node 208 when the switch is closed. When the bimetallic switch 202 is open, no current flows. In circuit 300, which specifically has 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 of the D MOSFET 304 to node 316 (drain to source), from node 318 of the resistor 306 to node 320, 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 is the same as the gate-to-source voltage VGS of the D MOSFET when current 322 flows. Therefore, the voltages across nodes 314 and 316 (gray) are the drain-to-source voltage VDS of the D MOSFET 304, while the voltages across nodes 318 and 320 (white) are the gate-to-source voltage VGS.

[0028] Circuit 300 can also be a standalone device 300 consisting of bimetallic switch 302, D MOSFET 304 and resistor 306. Overcurrent protection device 300 can therefore be added to any circuit that requires overcurrent protection.

[0029] The conduction overcurrent test current is used to evaluate the tripping time of the bimetallic switch under the following three conditions: ● With the bimetallic switch 202 as a stand-alone device (Figure 2), the test current is 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) of the applied current (below the saturation current Isat). ● With the bimetallic switch 202 connected to a D MOSFET Under the condition of 204 (Figure 3), the test current is 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) of the applied current (below the saturation current Isat). ● The bimetallic switch 202 is thermally connected to the D MOSFET using screws and bolts. Under the condition of 204 (Figure 4), the test current is 100% (2 A), 200% (4 A), 300% (6 A), 400% (8 A), 500% (10 A), 600% (12 A), 700% (14 A), 800% (16 A), 1000% (20 A), and 1200% (24 A) of the applied current (below the saturation current Isat).

[0030] Since the bimetallic switch (KSD-01 temperature switch thermostat) used in these tests has an operating current of 2 amps (at 250 volts), the 2 amp input represents 100% of its operating current. Therefore, these tests are performed to measure the tripping time of the bimetallic switch under different operating conditions. Therefore, in addition to the first test at 2 amps, the device is tested at multiples of its standardized rated current, with the most extreme test occurring at twelve times (24 amps) of the device's rated current.

[0031] Figure 5 is a response waveform 500 of the standalone bimetallic switch 202 shown in Figure 2 according to an exemplary embodiment, wherein a 10-volt source supplies 8 amps of current (400% of the switch's rated current) to circuit 200. Even if the current flowing through the bimetallic switch 202 is rated to be four times the device's normalized rated current, it will still take 48 seconds to reach the device trip point 502. This is not ideal because a prolonged tripping of the bimetallic switch at four times the normalized current would put very high stress on downstream components, thereby increasing the likelihood of failure.

[0032] Figure 6 is a response waveform 600 of a bimetallic switch 302 connected to the D MOSFET 304 shown in Figure 3 according to an exemplary embodiment, wherein a 10-volt source supplies 6 amps (300% of the rated current of the switch) to the circuit 300. As shown in waveform 600, a 10-volt 6-amp overcurrent condition is applied to the protection circuit 300 (Figure 3) to measure its response. The current (ID) (C2) through the D MOSFET 304 and the voltage across the bimetallic switch 302 and the D MOSFET 304 (VDS) (C3) are monitored and captured relative to time. Their scales are marked at the bottom of waveform 600. Trip point 602 is the point where the bimetallic switch 302 begins to disconnect.

[0033] Figure 7 is a response waveform 700 of a bimetallic switch 302 connected to the D MOSFET 304 shown in Figure 3 according to an exemplary embodiment, wherein a 10-volt source supplies 12 amps (600% of the switch's rated current) to circuit 300. In this example, resistor 306 is at 0 ohms. As shown in waveform 700, a 10-volt 12-amp overcurrent condition is applied to protection circuit 300 (Figure 3) to measure its response. At trip point 702, the current ID flowing from D MOSFET 304 drops rapidly from 12 amps to 0 amps, while the voltage VDS across the MOSFET drops from 10 volts to 0 volts. Just before trip point 702, MOSFET 304 dissipates approximately 4.27 volts × 12 amps = 54.24 watts of power (see Table 800 shown in Figure 8 below). A 600% (12 amp) rated current causes heat dissipation in the D MOSFET 304, which raises the temperature of the MOSFET package and reaches the level at 60°C where the bimetallic switch 202 disconnects. In the example shown in Figure 700 (Figure 7), it takes approximately 3.7 seconds to reach the trip point 702 of the bimetallic switch 202. Therefore, in the exemplary embodiment, as shown by these waveforms, the response time for interrupting an overcurrent event depends not only on the magnitude of the overcurrent but also on the inherent heat dissipation caused by the D MOSFET 304.

[0034] Figure 8 includes Table 800 to illustrate the results of overcurrent test current operation according to some embodiments. As shown, further tests were performed with different currents in and out of the presence of the D MOSFET 304. In these examples, the IXTH16N50D2 D MOSFET and a bimetallic switch with a 60°C trigger were tested using a rated current below the saturation current Isat.

[0035] Table 800 provides a summary of tests using different currents and bimetallic switch tripping responses. The top of Table 800 provides tripping time information for the bimetallic switch 302 (Figure 3) connected to the D MOSFET 304 under eight current conditions (6 A, 8 A, 10 A, 12 A, 14 A, 16 A, 20 A, and 24 A); the bottom of Table 800 provides tripping time information for the bimetallic switch 202 (Figure 2) not connected to the D MOSFET under the same eight current conditions. Table 800 shows that the tripping time of the bimetallic switch 202 (Figure 2) alone is 8 seconds (under a current of 12 A). Under the condition of applying the same 12 A to the D MOSFET (Figure 3) combined with the bimetallic switch circuit, the tripping time is reduced to only 3.7 seconds. Therefore, in the exemplary embodiment, for the D MOSFET + bimetallic switch, the tripping time is much faster / improved under a current range of 3X to 6X.

[0036] Table 800 also shows that the bimetallic switch will not trip at 2 amps (100% of the applied current) or 4 amps (200% of the applied current). Instead, the bimetallic switch begins to trip at 6 amps (300%) because there is sufficient energy at a current of 6 amps to thermally activate the bimetallic switch.

[0037] As these experiments demonstrate, in the exemplary embodiment, the presence of the D MOSFET accelerates the tripping of the bimetallic switch at all current ratios. The resistor R determines the maximum permissible current through the D MOSFET in steady state, i.e., the Isat saturation current. In the exemplary embodiment, using a 0-ohm resistor allows for a higher Isat value compared to using a resistor with R = 0.1 ohms. When resistor 306 is zero, the VGS of MOSFET 304 is also zero. However, by slightly increasing the resistance (e.g., R = 0.1 ohms), the gate-to-source voltage VGS of MOSFET 304 will become slightly negative and will begin to pinch off and limit the current flowing through the D MOSFET.

[0038] By increasing the resistance R, both the saturation current Isat and the gate-to-source voltage (VGS) change, thereby altering the power across the D MOSFET and enabling it to dissipate more power. In an exemplary embodiment, these considerations will facilitate the selection of different bimetallic switches to work with circuit breakers of different ratings.

[0039] Figure 9 includes graph 900, which shows improved trip response times for two types of protection circuits: 1) a bimetallic switch without a MOSFET (Figure 2) and 2) a bimetallic switch with a MOSFET (Figure 3). Graph 900 shows the relationship between the trip time (y-axis) in seconds and the current (x-axis) in amperes for the bimetallic switch. Black circles indicate the trip time of a standalone bimetallic switch (e.g., Figure 2), while light circles indicate the trip time of a bimetallic switch with a D MOSFET (e.g., Figure 3). Once the bimetallic switch is combined with a D MOSFET, the trip time in graph 900 shifts from right to left and from top to bottom (meaning a shorter trip time), which is within the safe operating curve of the bimetallic switch trip time. Therefore, adding a D MOSFET with a bimetallic switch increases the total trip time and provides faster protection for downstream electronics.

[0040] Figure 900 illustrates the benefits provided by the D MOSFET. For example, the tripping time of a standalone bimetallic switch at 8 amps, given by black circle 902, is approximately 57 seconds, while the tripping time of a bimetallic switch plus a D MOSFET at the same current, given by light circle 904, is approximately 8 seconds. Similarly, the tripping time of a standalone bimetallic switch at 10 amps, given by black circle 906, is approximately 19 seconds, while the tripping time of the combined circuit (light circle 908) is approximately 6 seconds. It makes sense that the standalone bimetallic switch is better than the combined circuit only at higher currents, since the rated current of the switch is far exceeded. Figure 900 thus illustrates the benefits of a combined circuit consisting of both a bimetallic switch and a D MOSFET working together to prevent overcurrent conditions.

[0041] Returning to the protection circuit 200 shown in Figure 2, according to an exemplary embodiment, the circuit includes a separate bimetallic switch 202 undergoing a surge test. The test is performed at 2 ohms with a peak voltage of 500 volts using a surge current of 1.2 / 50 microseconds. In contrast, Figure 3 shows circuit 300 with bimetallic switch 302 connected to the drain of D MOSFET 304 and resistor 306 connected between the source and gate of the MOSFET. A variation of this circuit 300 does not include the resistor. Again, the test is performed at 2 ohms with a peak voltage of 500 volts using a surge current of 1.2 / 50 microseconds. 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-type MOSFET (where VDSX = 500 volts, ID(on) = 16 amps, and RDS(on) = 300 milliohms).

[0042] Figure 10 includes a response waveform 1000, which illustrates the surge response of a bimetallic switch in a standalone circuit (e.g., circuit 200 shown in Figure 2) according to some embodiments. A 1.2 / 50 microsecond surge waveform exists, with a peak voltage of 500 volts and a virtual impedance of 2 ohms. As shown in waveform 1000, the surge current (C2) through the bimetallic switch has a peak response of 230.7 amps. To convert the voltage waveform, the peak voltage is 230.7 volts × 2 ohms = 461.4 volts. Under conditions where the incoming surge rises slightly, the voltage (C1) across the bimetallic switch remains almost constant at 10 volts. However, the surge does not trigger the bimetallic switch to open. Therefore, in some embodiments, the switch will not be triggered under such surge conditions.

[0043] Figure 11 includes response waveform 1100 to illustrate the surge response of a circuit characterized by a combined D MOSFET and bimetallic switch (e.g., circuit 300 shown in Figure 3) according to some embodiments. As shown in waveform 900, under the same incoming surge conditions, the current through the combination of the D MOSFET and bimetallic switch is down-clamped (C2) and remains "saturated" at a peak current of approximately 21.8 amps at approximately 40 microseconds. The D MOSFET clamps very quickly on the surge and results in a very low current output. This contrasts with the surge test of a separate bimetallic switch described above (Figure 10).

[0044] Therefore, in an exemplary embodiment, a D MOSFET with a bimetallic thermal switch is used, and the switching trigger time is much faster than when independently triggered under the same applied overcurrent conditions. Furthermore, the resulting surge current is at a much lower safety level to protect downstream circuit elements.

[0045] In an exemplary embodiment, the D MOSFET and the bimetallic switch can operate closely together in the circuit to provide mutual protection. In the event of an extended overcurrent protection event, the D MOSFET heats up and triggers the switch at a specified trigger temperature, generating an open-circuit current. This prevents overcurrent from flowing through downstream components of the circuit and also prevents the D MOSFET from overheating. Once the switch housing cools down to the reset level, the switch returns to its normal position from its temporarily deformed position.

[0046] Furthermore, in some embodiments, the combined circuitry described herein can be part of a manually reset circuit breaker. These types of circuit breakers are known to have bimetallic strips; however, once the strip trips, causing an open circuit, the circuit breaker can only be reset with manual intervention. The bimetallic strip combined with a D MOSFET disclosed herein is a suitable alternative to this type of circuit breaker and eliminates the need for manual intervention to reset the circuit breaker. When the two devices (bimetallic switch and D MOSFET) are connected in this manner, they possess self-protection and self-reset characteristics.

[0047] In addition to the bimetallic switch examples given above, the principles described herein can be similarly applied to other types of thermal switches, miniature circuit breakers and relay circuit breakers with internal bimetallic switches, regardless of whether these devices include self-reset or manual reset functions.

[0048] Based on the above test results, the D MOSFET provides an additional heating effect, thereby accelerating the tripping of the bimetallic switch. It has been shown that at all overcurrent levels (e.g., 100%, 200%, and 400% in some embodiments), the bimetallic switch trips faster in the presence of the D MOSFET. Therefore, the bimetallic switch and the D MOSFET work closely together and provide mutual protection.

[0049] In an exemplary embodiment, the above waveforms illustrate placing the bimetallic switch before the D MOSFET while placing the switch together on top of the D MOSFET package (as shown in FIG. 4). This allows the two devices to mutually benefit in preventing overcurrent events. The D MOSFET acts as a current limiter, provided there is (or no) a bias resistor in the gate-to-source terminals of the MOSFET (e.g., resistor 306 in FIG. 3). When the D MOSFET experiences a prolonged current-limiting event, the heat generated by its body (package) heats the bimetallic switch to cause it to open and protect the D MOSFET from overheating (long-term current, I > Isat). In an exemplary embodiment, once the temperature drops to the bimetallic switch's recovery level, the circuit resets to normal. In an exemplary embodiment, the D MOSFET also acts as a surge current limiter to clamp external surges to the circuit to be protected.

[0050] As an example, the principles described herein can be applied to circuits such as miniature circuit breakers (MCBs). MCBs are used, for example, in buildings including offices and homes, 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). A short circuit is an overcurrent caused by a short circuit in the power line or equipment connected to the MCB. The term "overcurrent" includes these short circuit events, but also covers a sudden rise in current caused by external disturbances (e.g., other high-voltage power lines in contact with the power line). MCBs are designed to trip or disconnect the internal circuitry within the MCB, which shuts off current flow and prevents the circuitry from overheating. The tripping action (which occurs very quickly (e.g., less than 3 milliseconds)) also prevents current from being delivered to other devices connected to the MCB. The MCB also utilizes a bimetallic strip that can be connected to a D MOSFET to improve the MCB's response time.

[0051] The MCB described herein is characterized by having a bimetallic strip, namely two metal pieces arranged back-to-back, wherein the two metal pieces are made of different metals with different coefficients of thermal expansion, thereby causing the bimetallic strip to bend during heating. In the protection circuit 300 described and shown above, the bimetallic strip is referred to as a bimetallic switch. This is because, within the circuit, the bimetallic strip acts as a switch to open or close the circuit. In contrast, the bimetallic switch is characterized by the bending of the MCB described below to disconnect a separate circuit component called a main switch, which is a trigger mechanism for disconnecting the circuit. Although the naming is different (strip to switch), the bimetallic strip described below is essentially the same as the bimetallic switch 302 in the protection circuit 300 (Figure 3) described above.

[0052] Figures 12A and 12B are illustrations of an MCB 1200 according to the prior art. The MCB 1200 is a two-terminal device. The MCB 1200 is characterized by having a switch that disconnects in response to short-circuit or overload conditions. The MCB 1200 is characterized by having two different sensing elements that enable a triggering mechanism. The first sensing element is an electromagnetic coil 1208 with a moving valve. The second sensing element is a bimetallic strip 1204. The triggering mechanism is a main switch 1206, which is manually controlled by an external lever 1202 and automatically disconnects in response to a fault condition. The external lever 1202 turns the MCB 1200 on (closing the main switch 1206, as shown in Figure 12A) or turns the MCB off (disconnecting the main switch, as shown in Figure 12B). The MCB 1200 is also characterized by having an arc groove 1210, which is also referred to as an arc absorber.

[0053] Electromagnetic coil 1208 is an electromagnetic sensor that generates an electric field proportional to the current flowing through the coil. The electromagnetic coil 1208 is designed to generate a current up to a thousand times the amplitude of the normal current within milliseconds during a short circuit condition. As the current increases, the magnetic field around the electromagnetic coil 1208 increases. The moving valve of the electromagnetic coil 1208 is positioned close to the main switch 1206. As the magnetic field strengthens, the valve is pushed against the main switch 1206, causing the switch to open (as shown in Figure 12B), thus resulting in an open circuit. The spring tension of the electromagnetic coil 1208 is sufficient to prevent the main switch 1206 from opening during normal current flow, but to trigger the main switch 1206 to open during a short circuit.

[0054] In some situations, when a very high surge overcurrent (5 to 10 times the normal current) enters the MCB 1200, the solenoid coil 1208 saturates and causes the movable solenoid valve inside the coil to very quickly actuate the main switch 1206. This rapid response of the solenoid coil 1208 is crucial for interrupting very dangerously large overcurrents and preventing damage to the circuits and systems connected to the MCB 1200.

[0055] Another sensor in the MCB 1200 is the bimetallic strip 1204. The bimetallic strip 1204 sensor is designed to handle overload conditions and operates more slowly than the solenoid coil 1208. The slower sensor ensures that simply turning on an appliance will not trigger the MCB 1200 to shut it off. For example, a fluorescent lamp has a start-up time of approximately 10 milliseconds. When an overload condition lasts for two seconds or more, the bimetallic strip 1204 trips the MCB 1200.

[0056] As explained above, the bimetallic strip 1204 is composed of two different types of metals, each with a different coefficient of thermal expansion. In an exemplary embodiment, the bimetallic strip 1204 is composed of a first elongated metal strip, which is wrapped with a wiring winding as a second metal strip, the two metal components being connected in series. When heated, due to an overload condition, the elongated metal strip contracts, causing the bimetallic strip 1204 to bend, which in turn causes the main switch 1206 to move and the circuit within the MCB 1200 to disconnect. Alternatively, if the bimetallic strip 1204 is wrapped with a wiring winding, the wiring winding heats up and bends to push the main switch 1206, thereby causing the main switch 1206 to disconnect, thus turning off the MCB from the external power supply. The current value required for the bimetallic strip 1204 to operate can generally be varied within a specific range.

[0057] The arc groove 1210 of the MCB 1200 is designed to dissipate the arc current generated when the main switch 1206 is opened, whether due to the electromagnetic coil 1208 or the bimetallic strip 1204. The arc current flows through the air near the end of the newly opened main switch 1206, causing a significant temperature rise, which could damage the MCB 1200. The arc groove 1210 is therefore positioned above the main switch 1206 because the high-temperature arc current will flow upward. The arc groove 1210 is characterized by having several parallel metal plates (referred to as separators) designed to separate the arc current during upward flow. The metal separators divide the arc current into smaller arc currents, which dissipate more quickly as the arc current continues to flow upward.

[0058] Figure 13 is an illustration of a second MCB 1300 according to prior art. The illustration shows the current path through the MCB 1300. The MCB 1300 includes an external lever 1302 for controlling a main switch 1306, which serves as a trigger mechanism. Like the MCB 1200, a bimetallic strip 1304 and an electromagnetic coil 1308 are the sensing mechanisms of the MCB 1300. Left terminal 1312 and right terminal 1314 are also shown. The MCB 1300 does not have an arc groove, as such a device is optional in some circuit breaker designs.

[0059] The current path is shown as a dashed line in the MCB 1300. Current flows from the right terminal 1314, through the electromagnetic coil 1308, through the bimetallic strip 1304, then through the main switch 1306, and finally out through the left terminal 1312. Current also travels in the other direction, from the left terminal 1312, through the main switch 1306, through the bimetallic strip 1304, through the electromagnetic coil 1308, and out of the right terminal 1314. If the main switch 1306 is opened, the current path in either direction will be interrupted.

[0060] When an overcurrent event with an amplitude of one to four times the normal current occurs, the overcurrent causes the wiring winding wrapped around the bimetallic strip 1304 to heat up and bend, thereby pushing the main switch 1306 until the switch opens, thus shutting off the MCB 1300 from the external power supply. When a very high surge overcurrent event enters the MCB 1300, the increase in the magnetic field causes the movable solenoid valve inside the electromagnetic coil 1308 to push the main switch 1306 very quickly, thereby enabling the MCB 1300 to interrupt a very dangerously large overcurrent.

[0061] Figure 14 is a circuit diagram 1400 of an MCB according to the prior art. Circuit diagram 1400 may represent, for example, an MCB 1200 (Figures 12A and 12B). A bimetallic strip 1404 is shown on the upper left side of the circuit 1400 and an electromagnetic coil 1408 is shown on the upper right side of the circuit 1400, with a main switch 1406 disposed between the two. The bimetallic strip 1404 consists of a first metal wrapped with a second metal (wiring), the second metal being connected in series with the first metal. An arc groove 1410 is located near the main switch 1406 so that the arc current can be dissipated after a fault event (whether short circuit or overload).

[0062] The aforementioned forces (whether from the valve being pushed by the electromagnetic coil 1408 or from the bending of the bimetallic strip 1404) will cause the main switch 1406 to be disconnected, thus interrupting the flow of current. The arc groove 1410 is prepared to absorb excess arc current by transferring the arc to a smaller position inside its metal interface, thus avoiding damage to the inside of the MCB.

[0063] One problem with miniature circuit breakers is the bimetallic strip. As explained above, a bimetallic strip has two different types of metal, each with its own coefficient of thermal expansion. Overcurrent conditions cause the bimetallic strip to bend, thus breaking the circuit. Because the bimetallic strip is designed to handle overload conditions, its operation is slower than that of an electromagnetic coil. However, when the current exceeds its rated current, the bimetallic strip cannot provide current limiting.

[0064] Figures 15A and 15B are circuit diagrams 1500A and 1500B of an MCB with a D MOSFET according to an exemplary embodiment. In Figures 1500A and 1500B (collectively referred to as "Circuit 1500" or "MCB 1500"), a main switch 1506 is disposed between a bimetallic strip 1504 on one side and an electromagnetic coil 1508 on the other side. An arc groove 1510 is located near the main switch 1506 to dissipate arc current. In Circuit 1500A, a D MOSFET 1520 is connected between the bimetallic strip 1504 and the main switch 1506. In Circuit 1500B, a JFET 1522 is connected between the bimetallic strip 1504 and the main switch 1506. Therefore, MOSFET 1520 or JFET 1522 can be used to enhance the novel MCB 1500.

[0065] In an exemplary embodiment, the D MOSFET 1520 is thermally connected to the bimetallic strip 1504, with its gate G connected to its source S (GS terminals short-circuited), and the D MOSFET and bimetallic strip are connected in series (FIG. 15A). In an exemplary embodiment, the JFET 1522 is thermally connected to the bimetallic strip, with its gate G also connected to its source S (GS terminals short-circuited), and the JFET and bimetallic strip are connected in series. In one embodiment, the bimetallic strip 1504 and the D MOSFET 1520 or JFET 1522 are thermally bonded to each other using a thermally conductive epoxy adhesive. In another embodiment, the bimetallic strip 1504 and the D MOSFET 1520 or JFET 1522 are thermally bonded to each other using a conductive epoxy gel. In an exemplary embodiment, when the incoming current exceeds the rated current of the MCB 1500, the bimetallic strip 1504, together with the MOSFET 1520 or JFET 1522, can provide current limiting.

[0066] Similar to MOSFETs described above, junction field-effect transistors (JFETs) are also semiconductor devices used in electronic devices to switch and amplify electronic signals. Both JFETs and MOSFETs are voltage-controlled devices. Since JFETs only exist in depletion-type configurations, they are fully turned on and conducting when there is 0 volts at the gate, just like the depletion-type MOSFETs used in this article. Both have high input impedance and are therefore sensitive to input voltage signals, but the resistance of a MOSFET is greater than that of a JFET. In addition, JFETs are cheaper and simpler to manufacture, while MOSFETs are more fragile due to the presence of metal-oxide-soil insulators in MOSFETs.

[0067] In an exemplary embodiment, the second metal (i.e., the wiring winding) of the bimetallic strip 1504 is removed, and the D MOSFET 1520 or JFET 1522 is tightly attached to the bimetallic strip. In one embodiment, the metal winding is not added back to the elongated metal piece after the D MOSFET 1520 or JFET 1522 is attached. The D MOSFET 1520 or JFET 1522 can directly provide heat to the bimetallic strip 1504. Therefore, removing the metal winding helps to minimize series resistance and ohmic losses in the power line. In an exemplary embodiment, the JFET 1522 is a silicon carbide (SiC) JFET connected between the bimetallic strip 1504 and the main switch 1506.

[0068] In an exemplary embodiment, if a current greater than the rated current of the MCB 1500 is received into the MCB 1500, the D MOSFET 1520 or JFET 1522 enters a current-limiting mode. Therefore, for example, if the MCB 1500 has a rated current of 1 amp, the D MOSFET 1520 or JFET 1522 enters a current-limiting mode when a current greater than 1 amp is received into the MCB 1500. Although the external overcurrent may be as high as 5 amps, the D MOSFET 1520 or JFET 1522, together with the bimetallic strip 1204, helps to quickly "clamp" the current down to 1 amp (the safe operating level of the MCB 1500). Therefore, in an exemplary embodiment, with the assistance of the FET (D MOSFET 1520 or JFET 1522), the MCB 1500 becomes safe by enabling the bimetallic strip 1504 to directly respond to a current exceeding 5 amps. Otherwise, a 5-amp overcurrent could damage electronic devices intended to be protected by the MCB 1500 or the MCB itself.

[0069] FIG16 is an illustration of an MCB 1600 including a D MOSFET 1620 according to an exemplary embodiment. An external lever 1602 manually activates the main switch 1606, which triggers a mechanism to turn on (close) or off (open). As in other MCBs described herein, the solenoid coil 1608 is designed to trip the main switch 1606 in response to a short circuit, and the bimetallic 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 a load and a power supply (not shown). In the exemplary embodiment, the D MOSFET 1620 is thermally connected to the bimetallic strip 1604 by shorting the GS terminal, and the D MOSFET and the bimetallic strip are connected in series. In an exemplary embodiment, the D MOSFET 1620 is tightly attached to the bimetallic strip 1604, and the winding wiring and heat insulation tube of the bimetallic strip are removed before the D MOSFET is attached.

[0070] Figures 17 to 19 are response waveforms of experiments performed on MCBs with and without D MOSFETs connected to a bimetallic strip, according to an exemplary embodiment. For the experiment, a Phoenix Contact UT6-TMC 1A MCB (hereinafter referred to as "PC MCB") (manufactured by Phoenix Contact Corporation) and a Littelfuse IXTY1R6N50D2 D MOSFET device (hereinafter referred to as "LF DMOS") with a trip current TC at 165°C were connected to the bimetallic strip within the MCB. The LF DMOS is a small package that can be assembled into the PC MCB.

[0071] When an overcurrent of 1.5 amps (1.5 times the normal current) is applied, a response waveform of 170° is generated (Figure 17). The PC MCB without LF DMOS trips after 72 seconds, while the PC MCB with LF DMOS trips after 15 seconds. Therefore, the PC MCB trips faster with LF DMOS than without LF DMOS.

[0072] When an overcurrent of 2.0 amps (twice the normal current) is applied, a response waveform of 180° is generated (Figure 18). The PC MCB without LF DMOS trips after 22 seconds, while the PC MCB with LF DMOS trips after 15 seconds. Therefore, the PC MCB trips faster with LF DMOS than without LF DMOS.

[0073] When an overcurrent of 5 amps (5 times the normal current) is applied, a response waveform of 190° is generated (Figure 19). The PC MCB without LF DMOS trips after 3.9 seconds, while the PC MCB with LF DMOS trips after 15 seconds. Therefore, the PC MCB trips slower with LF DMOS than without. Furthermore, for each experiment conducted (1.5 amps, 2 amps, and 5 amps), the PC MCB with LF DMOS trips after 15 seconds.

[0074] The results of these experiments show that adding a D MOSFET to the bimetallic strip helps to ultimately limit the overcurrent to 1 amp (regardless of how high the external overcurrent is). The addition of the D MOSFET has altered the time-trip response curve. The response waveform shows that even when the current exceeds the rated current of the MCB, the bimetallic strip can safely trip and protect the LF DMOS. The combined effect of the LF DMOS and the bimetallic strip makes the combined circuit a current-limiting self-protective switch, which is not possible with a standalone bimetallic strip.

[0075] Therefore, the presence of a FET (D MOSFET or JFET) connected to the bimetallic strip (rather than a simple bimetallic strip temperature sensing structure) forms a current limiting, surge-resistant, and overcurrent protection switch. In addition, during current limiting, the bimetallic strip obtains sufficient heat dissipation from the FET to trigger the main switch of the MCB, thereby causing an open circuit and protecting the FET from overheating or chip failure.

[0076] In an exemplary embodiment, the LF DMOS is an epoxy-encapsulated D MOSFET attached to a bimetallic strip. In other embodiments, bare D MOSFET dies or bare D MOSFET dies attached to a metal plate using a suitable lead frame are used to ensure better heat dissipation for triggering the bimetallic strip. The data from the experiments in Figures 17 through 19 are from a direct-current (DC) operating condition. In some embodiments, when used for an alternating-current (AC) operating condition, two back-to-back D MOSFETs are connected to the bimetallic strip.

[0077] Adding a FET (D MOSFET or JFET) to the MCB is an extension of incorporating a D MOSFET into the bimetallic switch 302 in protection circuit 300 (Figure 3). For example, the MCB represented by circuit diagram 1500 (Figure 15) has a similar but more complex structure compared to protection circuit 300. Adding a FET to the MCB advantageously provides current limiting, surge protection, and overcurrent protection. The bimetallic strip connected to the FET safely disconnects the main switch of the MCB, which helps to provide safe power-off during external hazardous overcurrent events. The FET is therefore an efficient method to improve disconnection speed, thus providing electrical system designers with a safer protection option.

[0078] In an exemplary embodiment, the D MOSFET connected to the bimetallic strip located within the MCB device is further provided with an appropriate drive circuit system for ensuring suitable turn-off and turn-on times, as well as other functions for driving the D MOSFET.

[0079] Elements or steps described herein in the singular and preceded by the indefinite article "a (or an)" should be understood to not exclude a plurality of elements or steps unless such exclusion is expressly stated. Furthermore, references to "one embodiment" of this disclosure should not be construed as excluding the existence of other embodiments that also include the described features.

[0080] Although certain embodiments have been mentioned in this disclosure, many modifications, alterations, and changes may be made to the embodiments without departing from the scope and range of this disclosure as defined in the appended claims. Therefore, this disclosure is intended to be limited to the embodiments described, but to have the full scope defined by the language of the following claims and their equivalents. [Simplified Explanation of the Diagram]

[0007] FIG1 is a diagram illustrating a protection circuit including a bimetallic switch and a D MOSFET according to an exemplary embodiment. FIG2 is a diagram illustrating a protection circuit including a separate bimetallic switch according to an exemplary embodiment. FIG3 is a diagram of an apparatus for providing overcurrent protection to a circuit according to an exemplary embodiment. FIG4 is an example of a bimetallic switch connected to a D MOSFET using screws and bolts according to an exemplary embodiment. FIG5 is a response waveform of an experiment performed using the separate bimetallic switch shown in FIG2 according to an exemplary embodiment. FIG6 and FIG7 are response waveforms of an experiment performed between the bimetallic switch and the D MOSFET shown in FIG3 according to an exemplary embodiment. FIG8 is a table providing the results of an overcurrent test current operation performed on the circuits shown in FIG2 and FIG3 according to an exemplary embodiment. FIG9 is a graph comparing the trip response times between a circuit with a separate bimetallic switch and a circuit with a bimetallic switch with a D MOSFET according to an exemplary embodiment. FIG10 is a response waveform of an experiment performed on the separate bimetallic switch circuit shown in FIG2 according to an exemplary embodiment. FIG11 is a response waveform of an experiment performed on an apparatus including the bimetallic switch and the D MOSFET shown in FIG3 according to an exemplary embodiment. Figures 12A and 12B are illustrations of miniature circuit breakers according to the prior art. Figure 13 is an illustration of a miniature circuit breaker according to the prior art. Figure 14 is a circuit diagram of a miniature circuit breaker according to the prior art. Figures 15A and 15B are circuit diagrams of a miniature circuit breaker with a D MOSFET according to an exemplary embodiment. Figure 16 is an illustration of a miniature circuit breaker with a D MOSFET according to an exemplary embodiment. Figures 17 to 19 are response waveforms of experiments performed on miniature circuit breakers with and without D MOSFETs according to exemplary embodiments.

Claims

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

2. The miniature circuit breaker as claimed in 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, wherein the first coefficient of thermal expansion is different from the second coefficient of thermal expansion.

3. The miniature circuit breaker as claimed in claim 2, wherein the first metal comprises a winding, wherein the winding is removed before the field-effect transistor is thermally connected to the bimetallic strip.

4. The miniature circuit breaker as claimed in claim 1, wherein the field-effect transistor is a depleted metal-oxide-semiconductor field-effect transistor (D MOSFET).

5. The miniature circuit breaker as claimed in claim 1, wherein the field-effect transistor is a junction field-effect transistor (JFET).

6. The miniature circuit breaker as claimed in claim 5, wherein the junction field-effect transistor is a silicon carbide junction field-effect transistor.

7. The miniature circuit breaker as claimed in claim 1, wherein the first fault event is a short circuit.

8. The miniature circuit breaker as described in claim 1, wherein the second fault event is an overload event.

9. The miniature circuit breaker as claimed in claim 1, wherein the bimetallic strip is coupled to the drain of the field-effect transistor.

10. The miniature circuit breaker as claimed in claim 1 further includes an arc slot for absorbing an arc after the first fault event or the second fault event.

11. The miniature circuit breaker as claimed in claim 1, wherein the solenoid coil further includes a movable valve, wherein the movable valve disconnects the switch in response to the first fault event.

12. The miniature circuit breaker as claimed in claim 4, wherein the depletion-type metal-oxide-semiconductor field-effect transistor is an N-channel depletion-type metal-oxide-semiconductor field-effect transistor.

13. The miniature circuit breaker as claimed in claim 1, wherein the bimetallic strip and the field-effect transistor do not cause the switch to disconnect the circuit until the second fault event has occurred for at least two seconds.

14. The miniature circuit breaker as claimed in claim 1, further comprising a rated current, wherein the bimetallic strip and the field-effect transistor provide current limiting when the current received in the miniature circuit breaker is higher than the rated current.

15. A miniature circuit breaker, comprising: A bimetallic strip includes an elongated metal strip and a metal winding wrapped around the elongated metal strip, wherein the elongated metal strip bends in response to the rated current of the miniature circuit breaker being exceeded; a field-effect transistor (FET) is connected in series with and thermally tied to the elongated metal strip after the metal winding is removed from the bimetallic strip, the FET having a gate terminal connected to a source terminal; and a switch is opened in response to an incoming current exceeding the rated current, wherein the switch is opened in response to the bending of the elongated metal strip; wherein the current limit exceeding the rated current is provided by the FET.

16. The miniature circuit breaker as claimed in claim 15, wherein the field-effect transistor is a metal-oxide-semiconductor field-effect transistor (MOSFET).

17. The miniature circuit breaker as claimed in claim 16, wherein the metal oxide semiconductor field-effect transistor is a depleted metal oxide semiconductor field-effect transistor.

18. The miniature circuit breaker as claimed in claim 16, wherein the field-effect transistor is a junction field-effect transistor (JFET).

19. The miniature circuit breaker as claimed in claim 18, wherein the junction field-effect transistor is a silicon carbide junction field-effect transistor.

20. The miniature circuit breaker as claimed in claim 16, wherein the rated current is 1 amp and the current is limited to 5 amps.

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