Overcurrent protection with depletion-mode MOSFET or JFET and bimetal temperature-sensitive switch in mini-circuit breakers.

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 TO RELATED APPLICATIONS] This application claims priority to and is a continuation-in-part of U.S. patent application Ser. No. 17 / 018,269, filed September 11, 2020.

[0002] An overcurrent or excess current is a situation in which more current than intended flows through a circuit. Overcurrents can be continuous or transient in nature. Voltage transients, i.e., short duration surges of electrical energy, are the result of the sudden release of energy that has been previously stored or induced by heavy inductive loads or other means such as lightning. Repetitive transients are frequently caused by the operation of motors, generators, or the switching of reactive circuit components. Random transients can be caused by lightning and electrostatic discharge (ESD).

[0003] The miniaturization of components has led to increased sensitivity to electrical stress. Microprocessors, for example, have structures and conductive paths that are unable to handle the high currents from ESD transients. Because such components operate at very low voltages, controlling voltage disturbances is given high priority to prevent device interruptions and potential or catastrophic failure.

[0004] It is with respect to these and other considerations that the present improvements can be useful. 。 Patent Document 1: U.S. Patent No. 5,875,085 Patent Document 2: U.S. Patent No. 6,055,977 Patent Document 3: U.S. Patent No. 8,203,816 Patent Document 4: U.S. Patent No. 10,446,352 Patent Document 5: U.S. Patent No. 10,985,552 Patent Document 6: U.S. Patent Application Publication No. 2005 / 0103613 Patent Document 7: U.S. Patent Application Publication No. 2009 / 0168273 Patent Document 8: U.S. Patent Application Publication No. 2012 / 0212210 Patent Document 9: U.S. Patent Application Publication No. 2016 / 0174340 Patent Document 10: U.S. Patent Application Publication No. 2016 / 0352318 Patent Document 11: U.S. Patent Application Publication No. 2017 / 0179713 Patent Document 12: U.S. Patent Application Publication No. 2018 / 0159322 Patent Document 13: U.S. Patent Application Publication No. 2019 / 0199340 Patent Document 14: U.S. Patent Application Publication No. 2020 / 0328053 Patent Document 15: U.S. Patent Application Publication No. 2020 / 0366079 Patent Document 16: U.S. Patent Application Publication No. 2002 / 0053066 Summary of the Invention

[0005] This Summary is provided to introduce selected concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter.

[0006] An exemplary embodiment of a miniature circuit breaker operable to provide protection due to a short circuit or overload event 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 a first fault event, a bimetallic strip, and a field effect transistor (FET) connected in series with the bimetallic strip, with the gate and source terminals of the FET connected together. The bimetallic strip and the FET open the switch during a second fault event.

[0007] Another exemplary embodiment of a miniature circuit breaker according to the present 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 wound around the strip. The elongated metal strip bends when the rated current of the miniature circuit breaker is exceeded. After the metal winding is removed from the elongated metal strip, an FET is connected in series with 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 of the miniature circuit breaker reaches 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 drawings]

[0008] [Figure 1] FIG. 1 illustrates a protection circuit comprising a bimetal switch and a D MOSFET according to an exemplary embodiment.

[0009] [Figure 2] FIG. 1 illustrates a protection circuit with a standalone bimetal switch according to an exemplary embodiment.

[0010] [Figure 3] 1 is a diagram of a device for providing overcurrent protection to a circuit according to an exemplary embodiment.

[0011] [Figure 4] FIG. 10 is a diagram of a bimetal switch screwed and bolted to a D MOSFET according to an exemplary embodiment.

[0012] [Figure 5] 3 is a response waveform of an experiment performed with the standalone bimetal switch of FIG. 2 according to an exemplary embodiment.

[0013] [Figure 6]4 is a response waveform of an experiment performed between the bimetal switch of FIG. 3 and a D MOSFET, according to an exemplary embodiment. [Figure 7] 4 is a 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] 4 is a table providing results of overcurrent test current operations performed on the circuits of FIGS. 2 and 3 according to an exemplary embodiment.

[0015] [Figure 9] 10 is a graph comparing trip response times between a circuit having a standalone bimetal switch and a circuit having a bimetal switch with a D MOSFET, according to an exemplary embodiment.

[0016] [Figure 10] 3 is a response waveform of an experiment performed on the stand-alone bimetal switch circuit of FIG. 2 according to an exemplary embodiment.

[0017] [Figure 11] 4 is a response waveform of an experiment performed on a device including the bimetal switch and a D MOSFET of FIG. 3 according to an exemplary embodiment.

[0018] [Figure 12A] 1 is a diagram of a miniature circuit breaker according to the prior art; [Figure 12B] 1 is a diagram of a miniature circuit breaker according to the prior art;

[0019] [Figure 13] 1 is a diagram of a miniature circuit breaker according to the prior art;

[0020] [Figure 14] FIG. 1 is a circuit diagram of a miniature circuit breaker according to the prior art.

[0021] [Figure 15A]FIG. 1 is a circuit diagram of a compact circuit breaker with a D MOSFET in accordance with an exemplary embodiment. [Figure 15B] FIG. 1 is a circuit diagram of a compact circuit breaker with a D MOSFET in accordance with an exemplary embodiment.

[0022] [Figure 16] FIG. 1 is a diagram of a compact circuit breaker with a D MOSFET according to an exemplary embodiment.

[0023] [Figure 17] 10 is a waveform response of an experiment performed on a compact circuit breaker with and without a D MOSFET, according to an exemplary embodiment; [Figure 18] 10 is a waveform response of an experiment performed on a compact circuit breaker with and without a D MOSFET, according to an exemplary embodiment; [Figure 19] 10 is a waveform response of an experiment performed on a compact circuit breaker with and without a D MOSFET, according to an exemplary embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0024] Disclosed herein is a circuit that provides overcurrent and overvoltage protection. The circuit includes a depletion-mode MOSFET (D MOSFET) as a current limiter connected to a bimetal switch, which acts as a temperature-sensitive circuit breaker. In combination, the D MOSFET and bimetal switch are capable of limiting current to downstream circuit components, thereby protecting those components from damage.

[0025] Further disclosed herein is a mini-circuit breaker (MCB) that provides overcurrent and overvoltage protection. The MCB includes either a depletion-mode MOSFET (D MOSFET) or a junction field-effect transistor (JFET) as a current limiter, where the D MOSFET or JFET is connected to a bimetal strip, which acts as a temperature-sensitive circuit breaker. The D MOSFET or JFET and the bimetal strip, in combination, can limit the current to downstream circuit components, thus protecting them from damage.

[0026] A metal-oxide-semiconductor field-effect transistor (MOSFET) is a semiconductor device used to switch and amplify electronic signals in electronic devices. Adjusting the voltage on its gate changes the width of the channel located between the source and drain of the MOSFET. MOSFETs come in a variety of configurations based on whether they are P-channel devices built with an N-type substrate, N-channel devices built with a P-type substrate, vertically or horizontally oriented semiconductors, and depletion or enhancement mode.

[0027] In contrast to enhancement-mode MOSFETs, which are turned on by applying a voltage across the gate, depletion-mode MOSFETs are turned on when the gate terminal is connected to zero volts (V GS = 0V), it is known as a "normally-on" device. In addition to having a thin gate oxide between the source and drain regions, ion implantation is used to form a conductive channel under the gate oxide layer and between the source and drain regions. The threshold voltage (V Th The concentration of active dopants in the substrate-channel region is used to adjust the desired value of . Despite the name, many modern MOSFETs can be fabricated with a polysilicon gate, rather than a metal gate, on an insulating gate oxide.

[0028] A bimetallic switch is a switch consisting of two metal strips joined together (back-to-back). The 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. Upon application of heat to a bimetallic switch, the switch will temporarily deform, or bend, based on these two different thermal expansion coefficients 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 "lift" at one end, so that it no longer maintains the predefined length and therefore no longer makes contact with both connection points in the circuit, resulting in an open circuit state. If the bimetallic switch is part of a manufactured package (e.g., the KSD-01F temperature switch thermostat discussed further below), the package will have two elongated legs that will change their relative position during heating, thus disconnecting them from the connection points in the circuit and creating an open circuit. In either configuration, when the bimetallic switch cools again, the switch will unbend or conform back to its original shape (generally flat) so that it once again makes contact between the two connection points and closes the circuit.

[0029] FIG. 1 is a representative diagram of a protection circuit 100 according to an exemplary embodiment. The protection circuit 100 (also known herein as a “circuit”) comprises a bimetallic switch 102 and a depletion-mode MOSFET 104 (hereinafter “D MOSFET” or “MOSFET”) connected in series. The D MOSFET 104 has a drain (D) and a source (S) through which current flows, and a gate (G) that influences the current between the drain and source under certain voltage conditions. As a depletion-mode device, the D MOSFET 104 is always “on,” allowing current to flow 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 node 110 and node 112 when the switch is closed and presents 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] Protection circuit 100 further comprises a resistor 106 connected at a first end to the source (S) of MOSFET 104 and at a second end to the gate (G) of the MOSFET. The voltage across resistor 106 is therefore the same as the gate-to-source voltage of MOSFET 104. The second end of resistor 106 (and the gate of MOSFET 104) connects to a further circuit element 108 to be protected, which is downstream from the circuit elements already described and is generally shown in FIG. 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 is composed of two dissimilar metal strips attached back-to-back to each other. The first metal strip has a first thermal expansion coefficient, and the second metal strip has a second, different thermal expansion coefficient. This difference causes the switch 102 to exhibit a temporary deformation (e.g., bending) when the temperature sensed by the bimetallic switch exceeds a threshold temperature. The temporary deformation causes the bimetallic switch 102 to no longer connect to both the node 110 and the node 112, resulting in an open circuit. Because 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 bimetallic switch 102 and D MOSFET 104 provides overcurrent protection for circuit 100. An overcurrent condition may be characterized as either 1) a transient overcurrent or 2) a constant overcurrent, as shown in FIG. 1. When an overcurrent condition exists, D MOSFET 104 generates heat, which activates the heat-sensing properties of bimetallic switch 102. A temporary deformation of bimetallic switch 102 reaches an opening threshold temperature, meaning that bimetallic switch 102 no longer connects to one or more of nodes 110 and 112 of circuit 100, resulting in an open circuit.

[0033] Once the abnormal overcurrent condition subsides, the two metal strips of bimetallic switch 102 cool, causing the switch to return from its temporarily deformed state to its original state, causing bimetallic switch 102 to re-establish a connection between both nodes 110 and 112 of circuit 100, resulting in a closed circuit. Bimetallic switch 102 thus provides a circuit-breaking state for protection circuit 100, which is a fail-safe environment for the safeguarding of other electronic systems or devices in the circuit (shown in FIG. 1 as "additional circuit element 108").

[0034] Within the protection circuit 100, the D MOSFET 104 can provide fast response and blocking capabilities to overcurrent and overvoltage events, and can quickly clamp surge current events. The overcurrent clamping capability of the D MOSFET absorbs fast transient surge energy, thereby preventing any harmful transient surges from reaching the sensitive electronic equipment (further circuit elements 108) that must be protected.

[0035] In contrast, the bimetal switch 102 provides high current interruption capability. However, the bimetal switch 102 is unable to respond quickly enough to protect against fast transient events. Therefore, the combination of the bimetal switch 102 and the D MOSFET 104, in an exemplary embodiment, serves to utilize the advantages of both devices to provide improvements in overcurrent and overvoltage protection.

[0036] In an exemplary embodiment, the D MOSFET 104 is a IXTH16N50D2 depletion-mode MOSFET (V 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. A resistor 106 is connected across the GS terminals of the D MOSFET 104. The bimetal switch 102 acts as a conducting switch. During normal operation, the bimetal switch 102 allows current to pass through it, provided that the current does not exceed an opening threshold temperature (which may also be considered the "trigger level" for the bimetal switch 102).

[0037] I D The drain current of the MOSFET 104 is determined by the gate-source voltage V GS potential difference (I D ×R) is negative V GS As the applied voltage increases, the current will begin to flow through the DS terminal of the D MOSFET 104 until it reaches a level where further current is prevented from passing through the D MOSFET 104. In one embodiment, as the applied voltage increases, the current will increase in a linear mode until saturation is achieved. The combinational circuit (comprising the bimetal switch 102 and the D MOSFET 104) limits the maximum saturation current I of the D MOSFET. sat An equilibrium state is reached where energy is allowed to flow through the circuit 100. In this state, energy is also dissipated in the D MOSFET 104 (I sat ×V DS ).

[0038] In an exemplary embodiment, the saturation current I sat is the maximum steady-state current that can pass through the D MOSFET 104. This means that as long as the saturation current is not exceeded, the D MOSFET will remain functional without chip failure or failure due to overheating. satIf the I sat If the saturation current I of the MOSFET is exceeded, the D MOSFET will react quickly and dissipate the excess current as heat. This in turn causes a higher temperature rise in the exterior 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 beyond this will cause the D MOSFET to overheat beyond its maximum junction temperature, resulting in chip failure and loss of MOSFET function. Therefore, the bimetal switch helps protect the D MOSFET from overheating failure as well.

[0039] In an exemplary embodiment, protection circuit 100 operates from 0 A up to I sat It operates with an applied current up to the saturation current. D The fault condition may be caused by, for example, a short circuit on the load, a load switching, or a sudden overload condition, which causes a sudden increase in current through the bimetal switch 102 and the D MOSFET 104. This current increases to the saturation current I sat If the overcurrent stays below the saturation current I, the D MOSFET will gradually heat up due to power dissipation in the MOSFET. sat If the input current I D The dramatic increase in heat dissipation (I D ×V DS), causing the bimetal switch to trip faster to protect the D MOSFET. This causes the bimetal switch 102 to reach its disconnect level and therefore disconnect from one or more of the nodes 110, 112 at the opening threshold temperature. Therefore, the bimetal switch also provides cutoff protection for the D MOSFET within safe operating area and thermal limits.

[0040] In one embodiment, a disconnection at nodes 110 and / or 112 cuts off the entire current flowing through protection circuit 100, thereby eliminating current to D MOSFET 104, causing the MOSFET to dissipate heat, and ultimately protecting the D MOSFET from overheating by eliminating the sustained overcurrent.

[0041] In this way, protection circuit 100 advantageously provides a feedback mechanism that tends to provide faster protection to D MOSFET 104 as the severity of a short-circuit current event increases. Therefore, the higher the level of short-circuit current, the faster bimetallic switch 102 can cut off current to D MOSFET 104, thereby protecting the D MOSFET from damage due to overheating. This feedback occurs because the high heat generated by D MOSFET 104 from an overvoltage / overcurrent condition causes bimetallic switch 102 to open more quickly, thereby preventing further current from passing through the D MOSFET and eliminating this self-heating, and ultimately protecting further downstream circuit elements 108.

[0042] 2, 3, and 4 are diagrams of protection circuits 200, 300, and 400, respectively, used to illustrate the results of continuous current tests, according to exemplary embodiments. In the exemplary embodiment, the bimetal switch is a KSD-01 temperature switch thermostat (triggering at 60°C, operating current @ 2A 250V), and the D MOSFET is a IXTH16N50D2 depletion mode MOSFET (V DSX =500V, I D(on) =16A, R DS(on) =300 mΩ), and resistor 206 is a 0 Ω resistor. In FIG. 2, bimetallic switch 202 is a stand-alone device not coupled to a MOSFET. In FIG. 3, bimetallic switch 302 is connected to D MOSFET 304 and resistor 306, and the circuit elements are arranged similarly to those in protection circuit 100 of FIG. 1. In FIG. 4, bimetallic switch 402 (KSD-01F) is coupled to D MOSFET 404 (TO247 package) in a back-to-back manner using screws 412 and bolts (not visible). In one embodiment, bimetallic switch 402 and D MOSFET 404 are further thermally linked to each other using a thermally conductive epoxy adhesive (not shown). In another embodiment, bimetallic switch 402 and D MOSFET 404 are further thermally linked to each other using a conductive epoxy gel. Furthermore, bimetallic switch 402 and D MOSFET 404 are electrically connected to each other and to the rest of the circuit via wires 406, 408, and 410. The KSD-01F bimetal switch consists of a bimetal disc, a metal bridge connecting the two legs of the device, metal bridge contacts, and a plastic case spaced from a heat sink. The bimetal disc flexes at a specific temperature, causing the metal bridge to connect or disconnect from the legs, thus closing or opening the circuit.

[0043] 2 and 3, arrows 204 and 322, respectively, indicate the direction of current flow. In circuit 200 with standalone bimetallic switch 202, current 204 flows from node 206 to node 208 while the switch is closed. When bimetallic switch 202 is open, no current flows. In circuit 300 with bimetallic switch 302, D MOSFET 304, and resistor 306, current 322 flows from node 308, through the closed bimetallic switch 302, from node 314 to node 316 (drain-to-source) of D MOSFET 304, from node 318 to node 320 of resistor 306, and finally to node 310. Because resistor 306 is connected between the source and gate of D MOSFET 304, the voltage across resistor 306 when current 322 flows is equal to the gate-to-source voltage V of the D MOSFET. GS Therefore, the voltage across nodes 314 and 316 (shaded in gray) is equal to the drain-source voltage V of the DMOSFET 304. DS , while the voltage across nodes 318 and 320 (white) is the gate-source voltage V GS is.

[0044] The circuit 300 may also be a stand-alone 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 requiring overcurrent protection.

[0045] An overcurrent test current was conducted to evaluate the trip time of the bimetallic switch under three sets of conditions: Using the bimetal switch 202 (Fig. 2) as a stand-alone device, the applied current (saturation current I sat Test currents of 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) The applied current (saturation current I sat Test currents of 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A) The applied current (saturation current I) is measured using a bimetal switch 202 (FIG. 4) thermally linked to a D MOSFET 204 using screws and bolts. sat Test currents of 100% (2A), 200% (4A), 300% (6A), 400% (8A), 500% (10A), 600% (12A), 700% (14A), 800% (16A), 1000% (20A), and 1200% (24A)

[0046] The bimetal switch used in these tests, a KSD-01 temperature switch thermostat, has an operating current of 2 A (at 250 V), so a 2 A input represents 100% of its operating current. Therefore, these tests are performed to measure the trip time of the bimetal switch under various operating conditions. Therefore, with the exception of the first test at 2 A, the device is tested at multiples of its normalized rated current, with the most extreme test being at 12 times the device's rated current (24 A).

[0047] 5 is a response waveform 500 of the standalone bimetallic switch 202 of FIG. 2 with a 10V power supply supplying 8 A of current (400% of the switch's rated current) to the circuit 200, according to an exemplary embodiment. 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 because the long time-to-trip of the bimetallic switch under four times the normalized current places very high stresses on downstream components, increasing the likelihood of failure.

[0048] 6 is a response waveform 600 of the bimetal switch 302 connected to the D MOSFET 304 of FIG. 3 with a 10V power supply delivering 6A (300% of the rated current of the switch) to the circuit 300, according to an exemplary embodiment. As shown in waveform 600, a 10V 6A overcurrent condition was applied to the protection circuit 300 (FIG. 3) and its response was measured. The current (I D ) (C2) and the voltage across the bimetal switch 302 and the D MOSFET 304 (V DS ) (C3) were monitored and captured versus time plots, their scales are marked at the bottom of waveform 600. Trip point 602 is the point at which bimetal switch 302 begins to disconnect.

[0049] 7 is a response waveform 700 of the bimetal switch 302 connected to the D MOSFET 304 of FIG. 3 with a 10V power supply delivering 12 A (600% of the rated current of the switch) to the circuit 300, according to an exemplary embodiment. In this example, the resistor 306 is 0 Ω. As shown in waveform 700, a 10V 12A overcurrent condition was applied to the protection circuit 300 (FIG. 3) and its response was measured. At trip point 702, the current I flowing from the D MOSFET 304 D drops quickly from 12A to 0A while the voltage across the MOSFET, V DS drops from 10V to 0V. Just prior to trip point 702, MOSFET 304 dissipates approximately 4.27V x 12A = 54.24W of power (see table 800 in FIG. 8 below). 600% of the 12A rated current results in heat dissipation in D MOSFET 304, which increases the temperature of the MOSFET packaging, reaching the trip level of bimetal switch 202 at 60°C. In the example shown in response waveform 700 (FIG. 7), it took approximately 3.7 seconds to reach trip point 702 of bimetal switch 202. Therefore, in an exemplary embodiment, as illustrated by these waveforms, the response time to cut off an overcurrent event depends not only on the magnitude of the overcurrent, but also on the inherent heat dissipation caused by D MOSFET 304.

[0050] 8 includes a table 800 illustrating the results of overcurrent test current operations, according to some embodiments. As shown, further tests were performed using different currents with and without the D MOSFET 304 present. In these examples, a IXTH16N50D2 D MOSFET and a bimetal switch triggering at 60° C. were tested at a saturation current I sat Tested using less than rated current.

[0051] Table 800 provides a summary of tests using different currents and bimetallic switch trip responses. The top of table 800 provides time-to-trip information for the bimetallic switch 302 ( FIG. 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), while the bottom of table 800 provides time-to-trip information for the bimetallic switch 202 ( FIG. 2 ) not connected to the D MOSFET under the same eight current conditions. Table 800 shows that the time-to-trip for the standalone bimetallic switch 202 ( FIG. 2 ) is 8 seconds (at a current of 12 A). When the same 12 A is applied to a D MOSFET ( FIG. 3 ) combined with a bimetallic switch circuit, the time-to-trip is reduced to only 3.7 seconds. Therefore, in an exemplary embodiment, the time-to-trip is significantly faster / improved in the 3x to 6x current range for the D MOSFET + bimetallic switch.

[0052] Table 800 also shows that the bimetallic switch does not trip at an applied current of 2 A (100%) or 4 A (200%). Instead, the bimetallic switch begins to trip at 6 A (300%) due to the presence of enough energy at 6 A of current to thermally activate the bimetallic switch.

[0053] These experiments demonstrate that, in the exemplary embodiment, the presence of the D MOSFET speeds up tripping of the bimetal switch at all current ratios. The resistance R determines the maximum allowable current through the D MOSFET in steady state. This maximum allowable current is I sat In an exemplary embodiment, using a 0Ω resistor results in a higher I sat When resistor 306 is zero, the V GS However, by slightly increasing the resistance (for example, R=0.1Ω), the gate-source voltage V GS becomes slightly negative and begins to pinch off and limit the current flowing through the D MOSFET.

[0054] By increasing the resistance R, the saturation current I sat and gate-source voltage (V GS ) changes, which varies the power across the D MOSFET, allowing it to dissipate more power. In an exemplary embodiment, these considerations facilitate the selection of different bimetal switches to function with circuit breakers of different ratings.

[0055] FIG. 9 includes a graph 900 illustrating the improved trip response time of both types of protection circuits: 1) a bimetallic switch without a MOSFET ( FIG. 2 ) and 2) a bimetallic switch with a MOSFET ( FIG. 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., FIG. 2 ), while the light circles represent the trip time of a bimetallic switch plus a D MOSFET (e.g., FIG. 3 ). When the bimetallic switch is combined with the D MOSFET, the trip time shifts from right to left and from top to bottom of graph 900 (meaning the time to trip is shorter), which is within the safe operating curve of the trip time of the bimetallic switch. Therefore, adding a D MOSFET to the bimetallic switch improves the overall time to trip, providing much faster protection for downstream electronics.

[0056] The benefits provided by the D MOSFET are evident in graph 900. For example, the time to trip for the standalone bimetallic switch at 8 A, given by dark circle 902, is approximately 57 seconds, while the time to trip for the bimetallic switch plus D MOSFET at the same current, given by light circle 904, is approximately 8 seconds. Similarly, the time to trip for the standalone bimetallic switch at 10 A, given by dark circle 906, is approximately 19 seconds, while the time to trip for the combined circuit (light circle 908) is approximately 6 seconds. Only at higher currents does the standalone bimetallic switch compare favorably to the combined circuit, which makes sense because the rated current of the switch is far exceeded. Therefore, graph 900 illustrates the benefit of having a combined circuit consisting of both a bimetallic switch and a D MOSFET working together to protect against overcurrent conditions.

[0057] Returning to protection circuit 200 of FIG. 2, the circuit includes a standalone bimetallic switch 202 undergoing surge testing, according to an exemplary embodiment. The test is performed with a 1.2 / 50 μsec surge current with a peak voltage of 500 V at 2 Ω. FIG. 3, in contrast, shows circuit 300 in which bimetallic switch 302 is connected to the drain of D MOSFET 304 and resistor 306 is connected between the source and gate of the MOSFET. One variation of this circuit 300 would be without the resistor. Again, the test is performed with a 1.2 / 50 μsec surge current with 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, D MOSFET 304 is a IXTH16N50D2 depletion-mode MOSFET (V DSX =500V, I D(on) =16A, R DS(on) =300mΩ).

[0058] FIG. 10 includes a response waveform 1000 illustrating a surge response of a bimetal switch in a stand-alone circuit, such as circuit 200 of FIG. 2, according to some embodiments. A 1.2 / 50 μsec surge waveform is present, with a peak voltage of 500 V and a virtual impedance of 2 Ω. As shown in waveform 1000, the surge current (C2) flowing through the bimetal switch has a peak response of 230.7 A. To convert the voltage waveform, the peak voltage is 230.7 A x 2 Ω = 461.4 V. The voltage across the bimetal switch (C1) remains approximately constant at 10 V, with some slight increase due to the incoming surge. However, the surge does not trigger the bimetal switch to open. Therefore, in some embodiments, the switch does not trigger under this surge condition.

[0059] FIG. 11 includes a response waveform 1100 illustrating the surge response of a circuit with a combined D MOSFET and bimetal switch, such as circuit 300 of FIG. 3, according to some embodiments. As shown in waveform 900, under the same incoming surge conditions, the current through the D MOSFET and bimetal switch combination clamps down (C2) and remains "saturated" at a peak current of about 21.8 A for about 40 μs. The D MOSFET clamps very quickly during the surge, resulting in a very low current output. This is in contrast to the surge test described above (FIG. 10) using a standalone bimetal switch.

[0060] Therefore, in an exemplary embodiment, when a D MOSFET is deployed with a bimetallic thermal switch, the trigger time of the switch is much faster than if it were to be triggered standalone by the same applied overcurrent. Furthermore, the resulting surge current is at a much lower, safe level to protect downstream circuitry.

[0061] In an exemplary embodiment, the D MOSFET and bimetal switch can work closely together in a circuit to provide mutual protection. Under a prolonged overcurrent protection event, the D MOSFET heats up and triggers the switch at a specified trigger temperature, generating an opening current, thereby preventing excessive current from passing through downstream components in the circuit and protecting the D MOSFET from overheating. The switch resets from its temporarily deformed position back to its normal position when its case cools to a reset level.

[0062] Additionally, in some embodiments, the combination circuits described herein may be part of a manually resettable circuit breaker. These types of circuit breakers are known to have bimetallic strips, but once the strip is tripped, creating an open circuit, the circuit breaker cannot be reset without human intervention. The bimetallic strip + D MOSFET disclosed herein may be a suitable replacement for such circuit breakers and may eliminate the need for human intervention to reset them. Both devices (bimetallic switch and D MOSFET) are both self-protecting and self-resettable when connected in this manner.

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

[0064] From the above test results, the D MOSFET provides an additional heating effect that speeds up the tripping of the bimetal switch. The bimetal switch, in some embodiments, has been shown to be able to trip much faster at all overcurrent levels, such as 100%, 200%, and 400%, when the D MOSFET is present. Therefore, the bimetal switch and the D MOSFET work very closely together and provide mutual protection for each other.

[0065] In an exemplary embodiment, the waveforms above show that placing a bimetallic switch in front of the D MOSFET and placing the switch together on top of the D MOSFET package (as shown in FIG. 4) provides the mutual benefit of the two devices to protect against overcurrent events. The D MOSFET acts as a current limiter, with or without a bias resistor (e.g., resistor 306 in FIG. 3) at the gate-to-source terminals 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 protect 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 bimetal switch. In an exemplary embodiment, the D MOSFET also acts as a surge current limiter, clamping external surges to the circuitry to be protected.

[0066] The principles described herein may be applied to circuits such as miniature circuit breakers (MCBs), as one example. 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 from shorting a power line or equipment connected to the MCB. The term "overcurrent" includes these short-circuit events but also encompasses a sudden rise in current caused by a disturbance, such as another high-power line contacting the power line. MCBs are designed to trip or open internal circuits within the MCB, thereby interrupting the current and preventing overheating of the circuit. 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. MCBs also utilize bimetallic strips, which may be connected to D MOSFETs to improve the response time of the MCB.

[0067] The MCBs described herein comprise a bimetallic strip, i.e., two metal pieces arranged back-to-back, where the two metal pieces are composed of different metals with different thermal expansion coefficients, causing the bimetallic strip to flex 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 that opens or closes the circuit. In contrast, the bimetallic switch included in the MCBs described below flexes to open a separate circuit component, referred to as the main switch, which is the triggering mechanism for opening the circuit. Although named differently (strip vs. switch), the bimetallic strip described below is essentially the same as the bimetallic switch 302 ( FIG. 3 ) in the protection circuit 300 described above.

[0068] 12A and 12B are diagrams of a prior art MCB 1200. The MCB 1200 is a two-terminal device. It includes a switch that opens in response to a short circuit or overload condition. The MCB 1200 includes two different sensing elements that activate a trigger mechanism. The first sensing element is a magnetic coil 1208, which includes 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 automatically opens in response to a fault condition. The external lever 1202 either turns the MCB 1200 on (closing the main switch 1206, as in FIG. 12A) or turns the MCB off (opening the main switch, as in FIG. 12B). The MCB 1200 also includes an arc chute 1210, 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 short-circuit condition for which the magnetic coil 1208 is designed can generate currents up to 1000 times the magnitude of the normal current within a few milliseconds. As the current becomes stronger, the magnetic field around the magnetic coil 1208 increases. The movable valve of the magnetic coil 1208 is positioned in close proximity to the main switch 1206. As the magnetic field increases, the valve pushes against the main switch 1206, causing it to open as in FIG. 12B, creating an open circuit. The spring tension of the magnetic coil 1208 is sufficient to not trigger the opening of the main switch 1206 during normal current flow, but is sufficient to do so during a short-circuit condition.

[0070] In some cases, when there is a very high surge overcurrent (5 to 10 times the normal current) flowing through MCB 1200, magnetic coil 1208 goes into saturation, causing the movable magnetic valve inside the coil to very quickly push main switch 1206. The fast response of magnetic coil 1208 is essential to cut off very dangerously large magnitude overcurrents and to keep circuits and systems connected to MCB 1200 from being damaged.

[0071] The other sensor in the MCB 1200 is the bimetallic strip 1204. The bimetallic strip 1204 sensor is designed to handle overload conditions and is slower acting than the magnetic coil 1208. The slower sensor ensures that appliances that are simply turned on do not trigger the MCB 1200 to shut them off. Fluorescent lights, for example, have a rise time of about 10 ms. The bimetallic strip 1204 causes the MCB 1200 to trip if the overload condition persists for more than two seconds.

[0072] As explained above, the bimetallic strip 1204 is made of two different types of metal, each with a different coefficient of thermal expansion. In an exemplary embodiment, the bimetallic strip 1204 is made of a first elongated strip of metal wrapped with a wire winding as a second strip of metal, with the two pieces of metal connected together in series. When heated, due to the occurrence of an overload condition, the elongated strip of metal contracts, causing the bimetallic strip 1204 to bend, which in turn causes the main switch 1206 to move and open the circuit within the MCB 1200. Alternatively, if a wire winding is wrapped around the bimetallic strip 1204, the wire winding heats up and bends, pushing against the main switch 1206, which opens the main switch 1206 and therefore turns off the MCB from external power. The current value at which the bimetallic strip 1204 should operate can generally vary within a certain range.

[0073] The arc chute 1210 of the MCB 1200 is designed to dissipate the arc current resulting when the main switch 1206 opens, whether caused by the magnetic coil 1208 or the bimetal strip 1204. The arc current flows through the air near the end of the newly opened main switch 1206, causing a significant temperature rise that can damage the MCB 1200. Therefore, the arc chute 1210 is located above the main switch 1206 because the high-temperature arc current will flow upward. The arc chute 1210 comprises several parallel metal plates, known as splitters, which are designed to break up the arc current during its upward flow. The metal splitters separate the arc current into smaller arc currents that dissipate more quickly while the arc current continues to flow upward.

[0074] 13 is a diagram of a second MCB 1300 according to the prior art. The diagram shows the current path through the MCB 1300. The MCB 1300 includes an external lever 1302 that controls a main switch 1306, which is the triggering mechanism. As with the MCB 1200, a bimetallic strip 1304 and a magnetic coil 1308 are the sensor mechanism for the MCB 1300. A left terminal 1312 and a right terminal 1314 are also shown. The MCB 1300 does not include arc chutes because such devices are optional in some circuit breaker designs.

[0075] The current path is shown as a dotted line in MCB 1300. Current travels from the right terminal 1314, through the magnetic coil 1308, similarly through the bimetallic strip 1304, then through the main switch 1306, and finally out through the left terminal 1312. Current also travels in the opposite direction, from the left terminal 1312, through the main switch 1306, through the bimetallic strip 1304, through the magnetic coil 1308, and out the right terminal 1314. If the main switch 1306 is open, the current path will be interrupted in either direction.

[0076] If there is an overcurrent event having a magnitude of 1 to 4 times the normal current, the overcurrent will heat and bend the wire windings wrapped around the bimetal strip 1304, pushing the main switch 1306 until the switch opens, thus turning off the MCB 1300 from external power. If there is an event of a very high surge overcurrent flowing through the MCB 1300, the increasing magnetic field will cause the movable magnetic valve inside the magnetic coil 1308 to push the main switch 1306 very quickly, thus allowing the MCB 1300 to cut off the very dangerously large magnitude overcurrent.

[0077] FIG. 14 is a circuit diagram 1400 of a prior art MCB. The circuit diagram 1400 may represent, for example, the MCB 1200 (FIGS. 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 located between the two. The bimetallic strip 1404 consists of a first metal wrapped with a wire, which is a second metal in series with the first metal. An arc chute 1410 is located near the main switch 1406 to allow for dissipation of arcing current following a fault event (whether a short circuit or an overload).

[0078] The force described above, whether from the valve of the magnetic coil 1408 pushing on the valve or from bending of the bimetallic strip 1404, opens the main switch 1406, thus interrupting the flow of current. The arc chute 1410 is ready to absorb the excess arc current by shunting the arc to a smaller level inside the metal interface of the arc chute, thus avoiding damage inside the MCB.

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

[0080] 15A and 15B are circuit diagrams 1500A and 1500B, respectively, of an MCB with a D MOSFET, according to an exemplary embodiment. In both diagrams 1500A and 1500B (collectively "circuit 1500" or "MCB 1500"), a main switch 1506 is disposed between a bimetallic strip 1504 on one side and a magnetic coil 1508 on the other side. An arc chute 1510 is disposed 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, either the MOSFET 1520 or the JFET 1522 may be used to improve the novel MCB 1500.

[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 shorted), and the D MOSFET is connected in series with the bimetal strip ( FIG. 15A ). In an exemplary embodiment, the JFET 1522 is also thermally coupled to the bimetal strip with its gate G and source S connected (GS terminal shorted), 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 together with either the MOSFET 1520 or the JFET 1522 can provide current limiting when the incoming current exceeds the rated current of the MCB 1500.

[0082] Similar to the MOSFET introduced above, the junction field-effect transistor (JFET) is also a semiconductor device used to switch and amplify electronic signals in electronic devices. Both JFETs and MOSFETs are voltage-controlled devices. Because JFETs function only in the depletion mode, they are on and fully conductive when there is 0 V at the gate, similar to the depletion-mode MOSFETs used herein. Both have high input impedance and are therefore sensitive to input voltage signals, but MOSFETs have higher resistance than JFETs. Furthermore, while JFETs are cheaper and less complex to manufacture, MOSFETs are more vulnerable due to the presence of a metal-oxide insulator in MOSFETs.

[0083] In an exemplary embodiment, the second metal wire winding of the bimetal strip 1504 is removed, and the D MOSFET 1520 or JFET 1522 is closely attached to the bimetal strip. In one embodiment, the metal winding is not re-added to the metal strip following attachment of the D MOSFET 1520 or JFET 1522. The D MOSFET 1520 or JFET 1522 can provide heat directly to the bimetal strip 1504. Therefore, removing the metal winding can help 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 bimetal strip 1504 and the main switch 1506.

[0084] In an exemplary embodiment, D MOSFET 1520 or JFET 1522 enters current-limiting mode when a current greater than the rated current of MCB 1500 is received into MCB 1500. Thus, for example, if MCB 1500 has a rated current of 1 A, D MOSFET 1520 or JFET 1522 enters current-limiting mode when a current greater than 1 A is received into MCB 1500. Although an external overcurrent can reach up to 5 A, D MOSFET 1520 or JFET 1522, in conjunction with bimetal strip 1504, helps to quickly "clamp" the current down to 1 A (the safe operating level of MCB 1500). Thus, in an exemplary embodiment, MCB 1500 is made safe by having bimetal strip 1504 that can respond directly to a 5 A overcurrent with the assistance of a FET (whether D MOSFET 1520 or JFET 1522). Otherwise, a 5A overcurrent may destroy the electronic equipment that is to be protected by the MCB1500 or the MCB itself.

[0085] FIG. 16 is a diagram of an MCB 1600 with a D MOSFET 1620, according to an exemplary embodiment. An external lever 1602 manually activates the trigger mechanism of a main switch 1606 to either turn on (closed) or off (open). As in the other MCBs described herein, a magnetic coil 1608 is designed to trip the main switch 1606 in response to a short circuit, and a bimetallic strip 1604 is designed to trip the main switch in response to an overload condition. A left terminal 1612 and a right terminal 1614 connect the MCB 1600 between a load and a power source (not shown). In the exemplary embodiment, the D MOSFET 1620 is thermally coupled to the bimetallic strip 1604 with the GS terminal shorted, and the D MOSFET is connected in series with the bimetallic strip. In an exemplary embodiment, the D MOSFET 1620 is tightly attached to the bimetal strip 1604, with the winding wire and insulating tube of the bimetal strip first removed prior to attachment of the D MOSFET.

[0086] 17-19 are response waveforms of experiments performed on an MCB with and without a D MOSFET connected to a bimetal strip, according to an exemplary embodiment. For the experiments, 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") (with a trip current T at 165°C). C ) is connected to a bimetal strip inside the MCB. The LF DMOS comes in a small package that can fit into a PC MCB.

[0087] Response waveform 1700 (Figure 17) shows the result when an overcurrent of 1.5 A (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 PC MCB without LF DMOS.

[0088] Response waveform 1800 (Figure 18) shows the result when an overcurrent of 2.0 A (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 PC MCB without LF DMOS.

[0089] Response waveform 1900 (FIG. 19) shows the results when an overcurrent of 5 A (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 slower than the PC MCB without LF DMOS. Furthermore, for each experiment performed (1.5 A, 2 A, and 5 A), the PC MCB with LF DMOS tripped after 15 seconds.

[0090] The results of these experiments show that the addition of the D MOSFET to the bimetal strip ultimately helps to limit the overcurrent to 1A, no matter how high the external overcurrent is. The addition of the D MOSFET modifies the time-to-trip response time curve. The response waveform shows that the bimetal strip can safely trip and protect the LF DMOS even when the current exceeds the rated current of the MCB. The combined effect of the LF DMOS with the bimetal strip makes the combined circuit a current-limiting self-protected switch, something that a standalone bimetal strip cannot do.

[0091] Therefore, rather than having a simple bimetallic strip temperature sensing structure, the presence of a FET (D MOSFET or JFET) connected to the bimetallic strip forms a protection switch that is current limiting, surge tolerant, and cuts off overcurrent. Furthermore, the bimetallic strip captures sufficient heat dissipation from the FET during current limiting for triggering of the MCB's main switch, which results in an open circuit, thus protecting the FET from overheating or chip failure.

[0092] In an exemplary embodiment, the LF DMOS is an epoxy-packaged D MOSFET attached to a bimetal strip. In other embodiments, a bare D MOSFET die, or a bare D MOSFET die attached to a metal plate with a suitable lead frame, is used to ensure better heat transfer for triggering the bimetal 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 bimetal strip.

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

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

[0095] As used herein, elements or steps described in the singular following the word "a" or "an" are to be understood as not excluding a plurality of elements or steps, unless the context explicitly states otherwise. Furthermore, references to "one embodiment" of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

[0096] Although the present disclosure refers to particular embodiments, numerous modifications, alterations, and variations can be made to the described embodiments without departing from the sphere and scope of the present disclosure, as defined in the appended claims. Accordingly, the present disclosure is not intended to be limited to the described embodiments, but rather to have its full scope defined by the language of 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.

Images