Patent Description:
Overcurrent or excess current is a situation in which a larger than intended electrical current flows through a circuit. Overcurrent may be constant or transient in nature. Voltage transients, short duration surges of electrical energy, are the result of the sudden release of energy previously stored or induced 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).

Component miniaturization has resulted in increased sensitivity to electrical stresses. Microprocessors for example, have structures and conductive paths which are unable to handle high currents from ESD transients. Such components operate at very low voltages, so a high priority is given to controlling voltage disturbances to prevent device interruption and latent or catastrophic failures. <CIT> discloses a miniature circuit breaker according to the preamble of claim <NUM>.

According to the invention, a miniature circuit breaker with the features of claim <NUM> is provided.

The miniature circuit breaker is operable to provide protection due to short circuits or overload events is disclosed. The miniature circuit breaker includes a switch to be manually opened or closed by an external lever, but which opens automatically in response to either a first fault event or a second fault event. The miniature circuit breaker also includes a magnetic coil to open the switch in response to the first fault event, a bi-metallic strip, and a field effect transistor (FET) connected in series with the bi-metallic strip, where a gate terminal and a source terminal of the FET are connected to one another. The bi-metallic strip and the FET open the switch during the second fault event.

The following drawings depict several exemplary embodiments, of which only the embodiments of <FIG> and <FIG> are part of the invention.

Circuits for providing overcurrent and overvoltage protection are disclosed herein. The circuits feature a depletion mode MOSFET (D MOSFET) as a current limiter, the D MOSFET being connected to a bi-metallic switch, where the bi-metallic switch acts as a temperature sensing circuit breaker. In combination, the D MOSFET and bi-metallic switch are able to limit current to downstream circuit components, thus protecting the components from damage.

Further, Mini Circuit Breakers (MCBs) for providing overcurrent and overvoltage protection are disclosed herein. The MCBs feature either a depletion mode MOSFET (D MOSFET) or a junction field effect transistor (JFET) as a current limiter, the D MOSFET or JFET being connected to a bi-metallic strip, where the bi-metallic strip acts as a temperature sensing circuit breaker. In combination, the D MOSFET or JFET and bi-metallic strip are able to limit current to downstream circuit components, thus protecting the components from damage.

Metal-Oxide Semiconductor Field-Effect Transistor devices, known as MOSFETs, are semiconductor devices used for switching and amplifying electronic signals in electronic devices. By adjusting a voltage on its gate, the width of a channel disposed between the source and drain of the MOSFET is varied. MOSFETs come in a variety of configurations, based on whether they are P-channel devices built with N-type substrates, N-channel devices built with P-type substrates, vertically disposed semiconductors, laterally disposed semiconductors, depletion mode, and enhancement mode.

In contrast to enhancement mode MOSFETs, which are turned on by the application of a voltage across the gate, depletion mode MOSFETs are known as "normally-on" devices when the gate terminal is at zero volts (VGS = <NUM> V). In addition to having a thin gate oxide between the source and drain regions, a conductive channel is formed beneath the gate oxide layer and between the source and drain regions using ion implantation. The concentration of active dopants in the substrate-to-channel region is used to adjust the threshold voltage (VTh) of the MOSFET to a desired value. Despite the name, many modern MOSFETs may be manufactured with a polysilicon gate, rather than metal, atop the insulating gate oxide.

Bi-metallic switches are switches consisting of two metal strips that are joined together (back-to-back). The bi-metallic switch would be placed in a circuit between two connection points. The first metal strip has a first thermal expansion coefficient and the second metal strip has a second, different thermal expansion coefficient. Upon application of heat to the bi-metallic switch, the switch will temporarily deform, or bend, based on these two different thermal expansion coefficients once the temperature exceeds an "open" threshold. Where the bi-metallic switch consists of two back-to-back strips of metal of a predefined length, the application of heat will cause the switch to "shorten" or one end to "lift" such that the switch no longer maintains the predefined length, thus becoming unattached to both connection points of the circuit, resulting in an open circuit condition. Where the bi-metallic switch is part of a manufactured package (such as a KSD-01F Temperature Switch Thermostat, discussed further below), the package includes two extending legs that will change their relative positions during heating, thus resulting in the disconnect from the connection points of the circuit, resulting in the open circuit condition. In either configuration, once the bi-metallic switch cools down again, the switch will unbend or conform back to its original shape (which is generally flat), thus causing the switch to be again attached between the two connection points and closing the circuit.

<FIG> is a representative drawing of a protection circuit <NUM>, according to exemplary embodiments. The protection circuit <NUM> (also known herein as "circuit") consists of a bi-metallic switch <NUM> and a depletion mode MOSFET <NUM> (hereinafter, "D MOSFET" or "MOSFET") connected in series with one another. The D MOSFET <NUM> includes a drain (D) and source (S), through which current flows, and a gate (G) which, under certain voltage conditions, will affect current flow between the drain and source. As a depletion mode device, the D MOSFET <NUM> is normally "on", with current flowing between drain (D) and source (S), despite the gate (G) voltage being <NUM> V. The bi-metallic switch <NUM> provides an electrical path between nodes <NUM> and <NUM> when the switch is closed, and causes an open circuit when the switch is not closed. One end of the bi-metallic switch <NUM> also connects to the drain (D) of the D MOSFET <NUM>.

The protection circuit <NUM> further includes a resistor <NUM>, connected at a first end to the source (S) of the MOSFET <NUM> and at a second end to the gate (G) of the MOSFET. The voltage across the resistor <NUM> is thus the same as the gate-to-source voltage of the MOSFET <NUM>. The second end of the resistor <NUM> (as well as the gate of the MOSFET <NUM>) connect to additional circuit elements to be protected <NUM>, which are downstream of the already described circuit elements and shown generally in <FIG>.

In exemplary embodiments, the D MOSFET <NUM> is a current limiter and the bi-metallic switch <NUM> acts as a temperature sensing circuit breaker. The bi-metallic switch <NUM> of the protection circuit <NUM> consists of two different metal strips attached to one another back-to-back. 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 <NUM> to exhibit a temporary deformation (such as a bend) when the temperature sensed by the bi-metallic switch exceeds a threshold temperature. The temporary deformation causes the bi-metallic switch <NUM> to no longer connect to both nodes <NUM> and <NUM>, resulting in an open circuit. Because the threshold temperature causes the bi-metallic switch <NUM> to open the protection circuit <NUM>, the threshold temperature is also known herein as the "open threshold temperature" and the "trip point" or "time to trip" temperature.

In exemplary embodiments, the combination of the bi-metallic switch <NUM> and D MOSFET <NUM> provide overcurrent protection to the circuit <NUM>. The overcurrent condition may be characterized as either <NUM>) a transient overcurrent or <NUM>) a constant overcurrent, as illustrated in <FIG>. Once there is an overcurrent condition, the D MOSFET <NUM> generates heat, which activates the thermal sensing characteristics of the bi-metallic switch <NUM>. The temporary deformation of the bi-metallic switch <NUM> means that the open threshold temperature was reached, such that the bi-metallic switch <NUM> fails to connect to one or more of nodes <NUM> and <NUM> of the circuit <NUM>, resulting in the open circuit.

Once the fault overcurrent condition subsides, the two metal strips of the bi-metallic switch <NUM> will cool down, causing the switch to return from the temporarily deformed state to its original state. This causes the bi-metallic switch <NUM> to again establish connection between both nodes <NUM> and <NUM> of the circuit <NUM>, resulting in a closed circuit. The bi-metallic switch <NUM> thus provides a circuit breaking condition of the protection circuit <NUM>, which is a fail-safe environment for safe-guarding the other electronics systems or devices within the circuit (indicated in <FIG> as "additional circuit elements <NUM>").

Within the protection circuit <NUM>, the D MOSFET <NUM> is able to provide a fast response and blocking capability to overcurrent and overvoltage events and is able to quickly clamp surge current events. The overcurrent clamping capability of the D MOSFET absorbs fast transient surge energy, which protects some harmful transient surges from reaching into the sensitive electronics (additional circuit elements <NUM>) to be protected.

In contrast, the bi-metallic switch <NUM> provides a high current breaking capability. However, the bi-metallic switch <NUM> is unable to respond quickly to protect against fast transient events. The combination of the bi-metallic switch <NUM> and the D MOSFET <NUM> thus helps utilize the advantages of both devices to provide improvement in overcurrent and overvoltage protection, in exemplary embodiments.

In an exemplary embodiment, the D MOSFET <NUM> is a IXTH16N50D2 depletion mode MOSFET (with VDSX = 500V, ID(on) = 16A, RDS(on) = 300mOhm), manufactured by Littelfuse® and the bi-metallic switch <NUM> is a KSD-01F Temperature Switch Thermostat, manufactured by Dongguan Fukuanyuan Electronics Co. Ltd (fuyuanfuse. As shown in <FIG>, the bi-metallic switch <NUM> is connected to the input terminal drain (D) pin of the D MOSFET <NUM>. The resistor <NUM> is connected between the G-S terminals of the D MOSFET <NUM>. The bi-metallic switch <NUM> acts as a conducting switch. During normal operation, the bi-metallic switch <NUM> allows current to pass through it, providing that the current does not exceed the open threshold temperature (this may also be thought of as a "trigger level" for the bi-metallic switch <NUM>).

A drain current of the MOSFET <NUM>, shown as ID, starts to flow through the D-S terminal of the D MOSFET <NUM> until the gate-to-source voltage, VGS, potential difference (ID x R) reaches a level which causes a more negative VGS to block further current from passing through the D MOSFET <NUM>. In one embodiment, as the applied voltage increases, the current flow will increase in linear mode until a saturation state is achieved. A balance is achieved in which the combination circuit (including the bi-metallic switch <NUM> and the D MOSFET <NUM>) will allow a maximum saturation current, Isat, of the D MOSFET to flow through the circuit <NUM>. In this state, energy is also being dissipated in the D MOSFET <NUM>, with Isat x VDS, as heat as well.

In exemplary embodiments, the saturation current, Isat, is the maximum steady state current passing through the D MOSFET <NUM>. This means that as long as the saturation current is not exceeded, the D MOSFET will maintain functionality without chip failure or failure due to overheating. If the overload current is less than Isat, the D MOSFET <NUM> still dissipates the energy as heat without any problem. However, if there is a very high short circuit at the input side that reaches into the D MOSFET, thus exceeding its Isat (saturation current) value, the D MOSFET will react quickly and dissipate the excessive current into heat more quickly. This, in turn, causes a higher temperature increase of the outer packaging of the D MOSFET <NUM> to quickly trip the bi-metallic switch <NUM>, which stops further current from breaking the D MOSFET. Nevertheless, prolonged current going beyond the MOSFET's saturation current, Isat, will cause over-heating of the D MOSFET to exceed its maximum junction temperature, resulting in chip failure, and loss of MOSFET function. Thus, the bi-metallic switch helps protect the D MOSFET from over-heating failure as well.

In an exemplary embodiment, the protection circuit <NUM> is operated with an applied current ranging from <NUM> A up to the Isat saturation current. An abnormal condition may result in a sudden rise of the input current, ID. The abnormal condition may be caused, for example, by a short circuit on the load, load switching, or a sudden overload condition. These cause an up-surge of current passing through the bi-metallic switch <NUM> and the D MOSFET <NUM>. If this current stays 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, then there will be a dramatic increase in input current, ID, with heat dissipation to the D MOSFET (ID x VDS), resulting in the bi-metallic switch tripping even faster to protect the D MOSFET. This will cause the bi-metallic switch <NUM> to reach its disconnect level, thus disconnecting from one or more of the nodes <NUM>, <NUM>, at the open threshold temperature. Thus, the bi-metallic switch also provides cut-off protection to the D MOSFET within the safe operating area and thermal limit.

In one embodiment, when the disconnection at nodes <NUM> and/or <NUM> occurs, the overall current flowing through the protection circuit <NUM> is cut off, which removes current flow to the D MOSFET <NUM>, causing heat dissipation of the MOSFET, and ultimately saves the D MOSFET from overheating by removing the sustained overcurrent.

The protection circuit <NUM> thus advantageously provides a feedback mechanism that tends to provide quicker protection to the D MOSFET <NUM> as the severity of the short circuit current event increases. Thus, the higher the level of short circuit current, the more quickly the bi-metallic switch <NUM> is able to cut off current to the D MOSFET <NUM>, and thus protect the D MOSFET from overheating damage. This feedback arises because the high heat generated by the D MOSFET <NUM> from the overvoltage/overcurrent condition causes the bi-metallic switch <NUM> to open more quicky, thus preventing further current from passing through the D MOSFET and removing this self-heating, and ultimately protecting the downstream additional circuit elements <NUM>.

<FIG>, <FIG>, and <FIG> are illustrations of protection circuits <NUM>, <NUM>, and <NUM>, respectively, used to illustrate the results of a continuous current test, according to exemplary embodiments. In exemplary embodiments, the bi-metallic switch is a KSD-<NUM> Temperature Switch Thermostat (triggers at <NUM> and operating current @ 2A 250V), the D MOSFET is a IXTH16N50D2 depletion mode MOSFET (with VDSX = 500V, ID(on) = 16A, RDS(on) = 300mOhm), and the resistor <NUM> is a 0Ohm resistor. In <FIG>, the bi-metallic switch <NUM> is a standalone device uncoupled to a MOSFET. In <FIG>, a bi-metallic switch <NUM> is connected to a D MOSFET <NUM> and a resistor <NUM>, with the circuit elements similarly arranged as in the protection circuit <NUM> of <FIG>. In <FIG>, the bi-metallic switch <NUM> (KSD-01F) is tied to the D MOSFET <NUM> (TO247 package) in a back-to-back approach with a screw <NUM> and bolt (not visible). In one embodiment, the bi-metallic switch <NUM> and D MOSFET <NUM> are further thermally linked to one another using thermally conductive epoxy adhesive (not shown). In another embodiment, the bi-metallic switch <NUM> and D MOSFET <NUM> are further thermally linked to one another using conductive epoxy gel. Further, the bi-metallic switch <NUM> and the D MOSFET <NUM> are electrically connected to one another and to other parts of the circuit via wires <NUM>, <NUM>, and <NUM>. The KSD-01F bi-metallic switch consists of a bi-metallic disc, a metal bridge connecting the two legs of the device, a metal bridge contact, and a plastic case which is isolated with a heat sink. The bi-metallic disc will bend at a specific temperature, which makes the metal bridge connect or disconnect the legs, thus closing or opening the circuit.

In <FIG> and <FIG>, respective arrows <NUM> and <NUM> show the direction of the current flow. In the circuit <NUM> featuring the standalone bi-metallic switch <NUM>, current <NUM> flows from node <NUM> to node <NUM> while the switch is closed. When the bi-metallic switch <NUM> is opened, no current flows. In the circuit <NUM> featuring the bi-metallic switch <NUM>, D MOSFET <NUM>, and resistor <NUM>, current <NUM> flows from the node <NUM>, through the closed bi-metallic switch <NUM>, from the node <NUM> to the node <NUM> of the D MOSFET <NUM> (drain-to-source), from the node <NUM> to the node <NUM> of the resistor <NUM>, and finally to the node <NUM>. Because the resistor <NUM> is connected between the source and the gate of the D MOSFET <NUM>, the voltage across the resistor <NUM> as the current <NUM> flows is the same as the gate-to-source voltage, VGS, of the D MOSFET. Thus, the voltage across the nodes <NUM> and <NUM> (in gray) are the drain-to-source voltage, VDS, of the D MOSFET <NUM> while the voltage across the nodes <NUM> and <NUM> (in white) are the gate-to-source voltage, VGS.

The circuit <NUM> may also be a standalone device <NUM> consisting of the bi-metallic switch <NUM>, the D MOSFET <NUM>, and the resistor <NUM>. The overcurrent protection device <NUM> may thus be added to any circuit needing overcurrent protection.

Overcurrent test currents were conducted to evaluate the trip time of the bi-metallic switch under three sets of conditions:.

Because the bi-metallic switch used in these tests, the KSD-<NUM> Temperature Switch Thermostat, has an operating current of 2A (at 250V), a 2A input represents <NUM>% of its operating current. These tests are thus conducted to measure the trip time of the bi-metallic switch at various operating conditions. Thus, with the exception of the first test at 2A, the device is tested at multiples of its normalized current rating, with the most extreme test occurring at twelve times (24A) the current rating of the device.

<FIG> is a response waveform <NUM> for the standalone bi-metallic switch <NUM> of <FIG>, in which a 10V source supplies an 8A current (<NUM>% of the switch's current rating) to the circuit <NUM>, according to exemplary embodiments. Even though the current flowing through the bi-metallic switch <NUM> is rated at four times the normalized current rating of the device, it still takes <NUM> seconds to reach the device trip point <NUM>. This is not ideal, as the long time to trip for a bi-metallic switch under four times normalized current will put a very high stress on the downstream components, increasing the likelihood of failure.

<FIG> is a response waveform <NUM> for the bi-metallic switch <NUM> connected to the D MOSFET <NUM> of <FIG>, in which a 10V source supplies 6A (<NUM>% of the switch's current rating) to the circuit <NUM>, in accordance with exemplary embodiments. As shown in the waveform <NUM>, a 10V 6A overcurrent condition was applied to the protection circuit <NUM> (<FIG>) to measure its response. The current through the D MOSFET <NUM> (ID) (C2) and the voltage across the bi-metallic switch <NUM> and the D MOSFET <NUM> (VDS) (C3), were monitored and captured versus a time plot. Their scales are marked at the bottom of the waveform <NUM>. The trip point <NUM> is the point where the bi-metallic switch <NUM> was starting to disconnect.

<FIG> is a response waveform <NUM> for the bi-metallic switch <NUM> connected to the D MOSFET <NUM> of <FIG>, in which a 10V source supplies 12A (<NUM>% of the switch's current rating) to the circuit <NUM>, in accordance with exemplary embodiments. In this example, the resistance <NUM> is at <NUM> Ohms. As shown in the waveform <NUM>, a 10V 12A overcurrent condition was applied to the protection circuit <NUM> (<FIG>) to measure its response. At the trip point <NUM>, the current, ID, flowing from the D MOSFET <NUM> drops quickly from 12A to 0A while the voltage across the MOSFET, VDS, drops from 10V to 0V. Right before the trip point <NUM>, the MOSFET <NUM> takes around <NUM>. 27V x 12A = <NUM>. 24W of power dissipation (see table <NUM> of <FIG>, below). The <NUM>%, 12A rated current brought about the heat dissipation in the D MOSFET <NUM>, which caused the temperature of the MOSFET packaging to increase and reach a level of disconnection of the bi-metallic switch <NUM> at <NUM>. In the example illustrated in the response waveform <NUM> (<FIG>), it took about <NUM> seconds for the trip point <NUM> of the bi-metallic switch <NUM> to be reached. Thus, in exemplary embodiments, as illustrated by these waveforms, the response time to cut off the overcurrent event depends not only on the overcurrent magnitude but also the intrinsic heat dissipation caused by the D MOSFET <NUM>.

<FIG> includes a table <NUM> to illustrate results of overcurrent test current operations, according to some embodiments. Further tests were conducted with different currents, with and without the presence of the D MOSFET <NUM>, as shown. In these examples, the IXTH16N50D2 D MOSFET and a bi-metallic switch with a <NUM> trigger were tested with rated current below the saturation current, Isat.

Table <NUM> provides a summary of the tests with different current and bi-metallic switch trip response. The top of the table <NUM> provides time to trip information for the bi-metallic switch <NUM> connected to the D MOSFET <NUM> (<FIG>) at eight current conditions (6A, 8A, 10A, 12A, 14A, 16A, 20A and 24A); the bottom of the table <NUM> provides time to trip information for the bi-metallic switch <NUM> without connection to a D MOSFET (<FIG>), at the same eight current conditions. Table <NUM> shows that the time to trip of the standalone bi-metallic switch <NUM> (<FIG>) is <NUM> seconds (with 12A current). With the same 12A applying to the D MOSFET combined with the bi-metallic switch circuit (<FIG>), the time to trip shortens to only <NUM>. Thus, in exemplary embodiments, the time to trip is much faster/improved with 3X to 6X current range for D MOSFET + bi-metallic switch.

Table <NUM> also shows that the bi-metallic switch does not trip at 2A (<NUM>%) or 4A (<NUM>%) applied current. Instead, the bi-metallic switch starts to trip at 6A (<NUM>%) due to the fact that there is sufficient energy to thermally activate the bi-metallic switch at the 6A current.

As these experiments illustrate, in exemplary embodiments, the presence of the D MOSFET speeds up the trip of the bi-metallic switch in all current ratios. The resistance, R, determines the maximum allowable current to passing through the D MOSFET in a steady state, which is the Isat saturation current. Using a <NUM> ohm resistor will allow a high Isat value compared to that using a R=<NUM>. 1ohm resistor, in an exemplary embodiment. When the resistor <NUM> is zero, the VGS of the MOSFET <NUM> is also zero. But, by increasing the resistance slightly (e.g., R=<NUM>. 1ohm), the gate-to-source voltage, VGS, of the MOSFET <NUM> will become slightly negative, and will start to pinch off and limit the current flowing through the D MOSFET.

By increasing the resistance, R, both the saturation current, Isat, and the gate-to-source voltage (VGS) will change, such that power across the D MOSFET varies and is able to provide more power dissipation to the D MOSFET. In exemplary embodiments, these considerations will facilitate the selection of different bi-metallic switches to work with different ratings of circuit breakers.

<FIG> includes a graph <NUM> which illustrates an improved trip response time of both types of protection circuits: <NUM>) bi-metallic switch with no MOSFET (<FIG>) bi-metallic switch with MOSFET (<FIG>). The graph <NUM> shows the trip time in seconds (y-axis) versus current in Amps (x-axis) for the bi-metallic switch. The dark circles show trip times for standalone bi-metallic switches (e.g., <FIG>) while the light circles show trip times for bi-metallic switches plus the D MOSFET (e.g., <FIG>). Once the bi-metallic switch is combined with the D MOSFET, there is a shift of the trip time from right to left in the graph <NUM> and from the top down closer to the bottom (meaning shorter time to trip), which is within the safe operating curve of the bi-metallic switch tripping time. Thus, adding a D MOSFET with a bi-metallic switch will enhance the overall time to trip and provide a much faster protection to the downstream electronics.

The benefit provided by the D MOSFET is evident in the graph <NUM>. For example, the time to trip for a standalone bi-metallic switch at 8A, given by dark circle <NUM>, is about <NUM> seconds whereas the time to trip for a bi-metallic switch plus D MOSFET at the same current, given by light circle <NUM>, is about <NUM> seconds. Similarly, the time to trip for the standalone bi-metallic switch at 10A, given by dark circle <NUM>, is about <NUM> seconds whereas the time to trip for the combination circuit (light circle <NUM>) is about <NUM> seconds. Only at the higher currents does the standalone bi-metallic switch compare favorably to the combination circuit, which makes sense because the current rating of the switch has been far exceeded. The graph <NUM> thus illustrates the benefit of having the combination circuit consisting of both the bi-metallic switch and the D MOSFET working together to protect against overcurrent conditions.

Returning to the protection circuit <NUM> of <FIG>, the circuit includes the standalone bi-metallic switch <NUM> undergoing a surge test, according to exemplary embodiments. The test is performed with a surge current at <NUM>/50µsec, with a peak voltage of 500V at <NUM> Ohms. <FIG>, by contrast, shows a circuit <NUM> with a bi-metallic switch <NUM> connected to the drain of a D MOSFET <NUM> and a resistor <NUM> connected between the source and gate of the MOSFET. One variant of this circuit <NUM> would include no resistor. Again, the test is performed with a surge current at <NUM>/50µsec, with peak voltage of 500V at <NUM> Ohms. In both circuits <NUM> and <NUM>, the bi-metallic switch is a KSD-<NUM> Temperature Switch Thermostat while, in the circuit <NUM>, the D MOSFET <NUM> is a IXTH16N50D2 depletion mode MOSFET (with VDSX = 500V, ID(on) = 16A, RDS(on) = 300mOhm).

<FIG> includes a response waveform <NUM> which illustrates a surge response for the bi-metallic switch in a standalone circuit, such as the circuit <NUM> of <FIG>, according to some embodiments. There is a <NUM>/<NUM>µsec surge waveform, with a peak voltage of 500V and <NUM> Ohms as fictive impedance. As shown in the waveform <NUM>, the surge current going through the bi-metallic switch (C2) has a response of <NUM>. To transform the voltage waveform, the peak voltage is <NUM>. 7A x <NUM> Ohm = <NUM>. The voltage across the bi-metallic switch (C1) stays almost constant at 10V, with some slight rise with the incoming surge. The surge, however, does not trigger the bi-metallic switch to open. Thus, the switch does not trigger with this surge condition, in some embodiments.

<FIG> includes a response waveform <NUM> to illustrate a surge response for a circuit featuring combined D MOSFET and bi-metallic switch, such as the circuit <NUM> of <FIG>, according to some embodiments. As shown in the waveform <NUM>, with the same incoming surge condition, the current passing through the D MOSFET and bi-metallic switch combination is clamped down (C2) and stays "saturated" at a peak current of about <NUM>. 8A at around <NUM>µsec. The D MOSFET clamps very quickly on the surge and causes the resultant output as a very low current output. This contrasts to the above surge test with the standalone bi-metallic switch (<FIG>).

Thus, in exemplary embodiments, with the deployment of a D MOSFET with a bi-metallic thermal switch, the trigger time of the switch is much faster than when it triggers standalone with the same applied overcurrent. Further, the resultant surge current is at a much lower, safe level, so as to protect downstream circuit elements.

A D MOSFET plus a bi-metallic switch are able to work closely together in a circuit to provide mutual protection, in exemplary embodiments. Under prolonged overcurrent protection events, the D MOSFET heats up and triggers the switch at the specified trigger temperature, and creating the open current, which both prevents an overcurrent from passing through the downstream components of the circuit and protects the D MOSFET from overheating. The switch resets back from its temporarily deformed position to its normal position once its case cools down to a reset level.

Further, in some embodiments, the combination circuit described herein may become part of a manually reset circuit breaker. These types of circuit breakers are known to have bi-metallic strips, but, once the strip is tripped causing an open circuit, the circuit breakers can only be reset with human intervention. The bi-metallic strip plus D MOSFET disclosed herein may be a suitable substitute for such circuit breakers and may eliminate the need for human intervention to reset them. Both devices (the bi-metallic switch and the D MOSFET) have a self-protecting and self-resettable feature together when they connect in this fashion.

In addition to the bi-metallic switch examples given above, the principles described herein may similarly be applied to other types of thermal switches, mini circuit breakers, and relay-type circuit breakers with bi-metallic switches inside, whether these devices include either self-reset or manual reset functions.

From the above test results, the D MOSFET provides an additional heating effect to speed up the trip of the bi-metallic switch. It has been shown that the bi-metallic switch, with the presence of the D MOSFET, is able to trip much faster in all the overcurrent levels, such as <NUM>%, <NUM>%, and <NUM>%, in some embodiments. The bi-metallic switch and the D MOSFET thus work very closely together and provide mutual protection to one another.

In exemplary embodiments, the above waveforms show that putting the bi-metallic switch before the D MOSFET while putting the switch on top of the D MOSFET package together (as illustrated in <FIG>) causes mutual benefit of the two devices for protecting against overcurrent events. The D MOSFET acts as a current limiter with (or without) the biasing resistor in the gate-to-source terminal of the MOSFET (e.g., the resistor <NUM> in <FIG>). When the D MOSFET has a long-term current limiting event, heat generated from its body (packaging) will heat up the bi-metallic switch to cause the switch to open and protect the D MOSFET from overheating (long-term current, I > Isat). In exemplary embodiments, the circuit resets back to normal once the temperature drops to the bi-metallic switch restore level. In exemplary embodiments, the D MOSFET also acts as a surge current limiter to clamp external surges to the circuits to be protected.

The principles described herein may be applied to circuits such as miniature circuit breakers (MCBs), as one example. MCBs are used to protect overcurrent events, such as short circuits (e.g., <NUM> to <NUM> times normal current), very high surge overcurrent events (e.g., <NUM> to <NUM> times normal current), and overload conditions (e.g., <NUM> to <NUM> times normal current), such as in buildings including offices and homes. Short circuit refers to overcurrents generated from shorting the power lines or equipment connected to the MCB. The term "overcurrent" includes these short circuit events, but also covers the sudden rise of current caused by external disturbances, such as other high power lines being in contact with the power lines. The MCB is designed to trip or open an internal circuit within the MCB, which shuts off electrical flow and prevents the circuit from overheating. The trip action, which happens very quickly (e.g., under <NUM>), also prevents current from being delivered to the other devices to which the MCB is connected. MCBs also utilize bi-metallic strips, which may be connected to D MOSFETs for improving the response time of the MCB.

The MCBs described herein feature bi-metallic strips, that is, two metal pieces disposed back-to-back, where the two metal pieces are composed of different metals having different coefficients of thermal expansion, causing the bi-metallic strip to bend during heating. In the protection circuit <NUM> described and illustrated above, the bi-metallic strip is called a bi-metallic switch. This is because, within the circuit, the bi-metallic strip acts as a switch, opening or closing a circuit. In contrast, the bi-metallic switches featured in the below-described MCBs bend to cause a separate circuit component, referred to as a main switch, to open, the main switch being the triggering mechanism for opening the circuit. Although named differently (strip vs. switch), the bi-metallic strips described below are essentially the same as the bi-metallic switch <NUM> of the protection circuit <NUM> (<FIG>), above.

<FIG> are illustrations of a MCB <NUM>, according to the prior art. The MCB <NUM> is a two-terminal device. The MCB <NUM> features a switch to open in response to the short circuit or overload condition. The MCB <NUM> features two different sensing elements that activate a triggering mechanism. The first sensing element is a magnetic coil <NUM>, which has a moving valve. The second sensing element is a bi-metallic strip <NUM>. The triggering mechanism is a main switch <NUM> that is manually controlled by an external lever <NUM> and is automatically opened in response to a fault condition. The external lever <NUM> turns on the MCB <NUM> (closes the main switch <NUM>, as in <FIG>) or turns off the MCB (opens the main switch, as in <FIG>). The MCB <NUM> also features an arc chute <NUM>, which is also known as an arc absorber.

The magnetic coil <NUM> is an electromagnetic sensor that produces an electric field proportional to the current passing through the coil. A short circuit condition, for which the magnetic coil <NUM> is designed, may produce a current up to one thousand times the magnitude of a normal current within milliseconds. As the current gets stronger, the magnetic field around the magnetic coil <NUM> gets larger. The moving valve of the magnetic coil <NUM> is positioned close to the main switch <NUM>. As the magnetic field strengthens, the valve pushes against the main switch <NUM>, causing the switch to open, as in <FIG>, resulting in an open circuit. The spring tension of the magnetic coil <NUM> is sufficient to not trigger the main switch <NUM> to open during normal current flow, but instead during the short circuit condition.

In some cases, when there is a very high surge overcurrent (<NUM> to <NUM> times the normal current) going into the MCB <NUM>, the magnetic coil <NUM> goes to a saturated state and causes the moveable magnetic valve inside the coil to push to the main switch <NUM> very quickly. The fast reaction of the magnetic coil <NUM> is critical to cutting off the very hazardous large magnitude of overcurrent and keeping the circuits and systems connected to the MCB <NUM> from being damaged.

The other sensor in the MCB <NUM> is the bi-metallic strip <NUM>. The bi-metallic strip <NUM> sensor is designed to address overload conditions and is slower acting than the magnetic coil <NUM>. A slower sensor ensures that appliances that are simply turned on will not trigger the MCB <NUM> to shut them down. Fluorescent lights have a startup time of about <NUM>, for example. The bi-metallic strip <NUM> causes the MCB <NUM> to trip when the overload condition lasts two seconds or more.

As explained above, the bi-metallic strip <NUM> consists of two different types of metals, with each metal having a different coefficient of thermal expansion. In an exemplary embodiment, the bi-metallic strip <NUM> consists of a first, elongated strip of metal that is wrapped with wire winding as the second strip of metal, with the two metal pieces being connected in series together. When heated up, due to the overload condition occurring, the elongated strip of metal contracts, causing the bi-metallic strip <NUM> to bend, which in turn causes movement of the main switch <NUM> and opening of the circuit within the MCB <NUM>. Or, if the bi-metallic strip <NUM> is wrapped with wire winding, the wire winding heats up and bends to push the main switch <NUM>, causing it to open, thus turning off the MCB from external power. The current value at which the bi-metallic strip <NUM> should operate can generally be varied within a certain range.

The arc chute <NUM> of the MCB <NUM> is designed to dissipate the arc current that results when the main switch <NUM> opens, whether due to the magnetic coil <NUM> or the bi-metallic strip <NUM>. The arc current flows through the air near the ends of the newly opened main switch <NUM>, causing a huge temperature rise that can damage the MCB <NUM>. The arc chute <NUM> is therefore disposed above the main switch <NUM>, as the high-temperature arc current will flow upward. The arc chute <NUM> features several parallel metal plates, known as splitters, which are designed to break up the arc current during the upward flow. The metal splitters separate the arc current into smaller arc currents that more quickly dissipate as the arc current continues to flow upward.

<FIG> is an illustration of a second MCB <NUM>, according to the prior art. The illustration shows a current path through the MCB <NUM>. The MCB <NUM> includes an external lever <NUM> for controlling the main switch <NUM>, which is the triggering mechanism. As with the MCB <NUM>, a bi-metallic strip <NUM> and a magnetic coil <NUM> are the sensor mechanisms of the MCB <NUM>. A left terminal <NUM> and a right terminal <NUM> are also shown. The MCB <NUM> does not feature an arc chute, as such devices are optional in some circuit breaker designs.

The current path is shown as a dotted line in the MCB <NUM>. Current travels from the right terminal <NUM>, through the magnetic coil <NUM>, also through the bi-metallic strip <NUM>, then through the main switch <NUM>, and finally exits via the left terminal <NUM>. The current also travels in the other direction, from the left terminal <NUM>, through the main switch <NUM>, through the bi-metallic strip <NUM>, through the magnetic coil <NUM>, and out the right terminal <NUM>. If the main switch <NUM> is open, the current path, in either direction, would be disrupted.

When there is an overcurrent event with magnitude from one to four times the normal current, the overcurrent causes the wire winding wrapping the bi-metallic strip <NUM> to heat up and bend, pushing the main switch <NUM> until the switch opens, thus turning off the MCB <NUM> from external power. When there is a very high surge overcurrent event going into the MCB <NUM>, the increase in magnetic field causes the movable magnetic valve inside the magnetic coil <NUM> to push the main switch <NUM> very quickly, enabling the MCB <NUM> to thus cut off very hazardous large magnitude of overcurrent.

<FIG> is a circuit diagram <NUM> of an MCB, according to the prior art. The circuit diagram <NUM> may represent, for example, the MCB <NUM> (<FIG>). A bi-metallic strip <NUM> is shown on the left side and a magnetic coil <NUM> is shown on the right side of the circuit <NUM>, with a main switch <NUM> being disposed between the two. The bi-metal strip <NUM> consists of a first metal that is winded with a second metal, a wire, in series with the first metal. An arc chute <NUM> is located near the main switch <NUM>, so that an arc current can be dissipated following the fault event, whether a short circuit or an overload.

The forces described above, whether from the valve of the magnetic coil <NUM> pushing the valve or from the bending of the bi-metallic strip <NUM>, are what cause the main switch <NUM> to be opened, thus disrupting the flow of current. The arc chute <NUM> is ready to absorb the excess arc current by diverting the arc to smaller levels inside its metal interface, thus avoiding damage internal to the MCB.

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

<FIG> and <FIG> are circuit diagrams 1500A and 1500B, respectively, of an MCB with a D MOSFET, according to exemplary embodiments. In both diagrams 1500A and 1500B (collectively, "circuit <NUM>" or "MCB <NUM>"), a main switch <NUM> is disposed between a bi-metallic strip <NUM> on one side and a magnetic coil <NUM> on the other side. An arc chute <NUM> is located near the main switch <NUM> for dissipating arc current. In the circuit 1500A, a D MOSFET <NUM> is connected between the bi-metallic strip <NUM> and the main switch <NUM>. In the circuit 1500B, a JFET <NUM> is connected between the bi-metallic strip <NUM> and the main switch <NUM>. Thus, either the MOSFET <NUM> or the JFET <NUM> may be used to enhance the novel MCB <NUM>.

In an exemplary embodiment, the D MOSFET <NUM> is thermally tied to the bi-metallic strip <NUM> with the gate G and the source S connected (G-S terminal short), with the D MOSFET being connected in series with the bi-metallic strip (<FIG>). In an exemplary embodiment, the JFET <NUM> is thermally tied to the bi-metallic strip, also with the gate G and the source S connected (G-S terminal short), with the JFET being connected in series with the bi-metallic strip. In one embodiment, the bi-metallic strip <NUM> and D MOSFET <NUM> or JFET <NUM> are thermally linked to one another using thermally conductive epoxy adhesive. In another embodiment, the bi-metallic strip <NUM> and D MOSFET <NUM> or JFET <NUM> are thermally linked to one another using conductive epoxy gel. In an exemplary embodiment, the bi-metallic strip <NUM>, in conjunction with either the MOSFET <NUM> or the JFET <NUM>, is able to provide current limiting when the incoming current is above the current rating of the MCB <NUM>.

Like the MOSFETs introduced above, Junction Field Effect Transistors (JFETs) are also semiconductor devices used for switching and amplifying electronic signals in electronic devices. Both JFETs and MOSFETs are voltage-controlled devices. Because JFETs only come in the depletion mode variety, they are on and fully conducting when there is 0V at the gate, as with the depletion mode MOSFETs used herein. Both have high input impedance and are therefore sensitive toward input voltage signals, though MOSFETs are more resistive than JFETs. Further, JFETs are cheaper and less complex to manufacture, while MOSFETs are more fragile, due to the presence of the metal oxide insulator in the MOSFETs.

In an exemplary embodiment, the second metal of the bi-metallic strip <NUM>, the wire winding, is removed, and the D MOSFET <NUM> or the JFET <NUM> is attached closely to the bi-metallic strip. In one embodiment, the metal winding is not added back to the elongated metal piece following the attachment of the DMOSFET <NUM> or the JFET <NUM>. The D MOSFET <NUM> or JFET <NUM> are able to provide heat directly to the bi-metallic strip <NUM>. Thus, removing the metal winding can help minimize the series resistance and ohmic loss in the power line. In an exemplary embodiment, the JFET <NUM> is a silicon carbide (SiC) JFET is connected between the bi-metallic strip <NUM> and the main switch <NUM>.

In an exemplary embodiment, the D MOSFET <NUM> or the JFET <NUM> enters the current limiting mode if current more than the current rating of the MCB <NUM> is received into the MCB <NUM>. Thus, for example, if the MCB <NUM> has a current rating of 1A, the D MOSFET <NUM> or JFET <NUM> enters the current limiting mode if current greater than 1A is received into the MCB <NUM>. Although the external overcurrent may reach up to 5A, the D MOSFET <NUM> or the JFET <NUM>, in conjunction with the bi-metallic strip <NUM>, helps to quickly "clamp" down the current to 1A (the safe operating level for the MCB <NUM>). Thus, in exemplary embodiments, the MCB <NUM> is made safe by having the bi-metallic strip <NUM> be able to respond to the 5A overcurrent directly, with the assistance of the FET (whether D MOSFET <NUM> or JFET <NUM>). Otherwise, the 5A overcurrent may destroy the electronics to be protected by the MCB <NUM> or the MCB itself.

<FIG> is an illustration of an MCB <NUM> which includes a D MOSFET <NUM>, according to exemplary embodiments. An external lever <NUM> manually activates the main switch <NUM> triggering mechanism to be either on (closed) or off (open). As in the other MCBs described herein, a magnetic coil <NUM> is designed to trip the main switch <NUM> in response to a short circuit, and a bi-metallic strip <NUM> is designed to trip the main switch in response to an overload condition. Left terminal <NUM> and right terminal <NUM> connect the MCB <NUM> between a load and power source (not shown). In an exemplary embodiment, the D MOSFET <NUM> is thermally tied to the bi-metallic strip <NUM> with a G-S terminal short, with the D MOSFET being connected in series with the bi-metallic strip. In an exemplary embodiment, the D MOSFET <NUM> is tightly affixed to the bi-metallic strip <NUM> with the winding wire and heat insulating tube of the bi-metallic strip being first removed before affixation of the D MOSFET.

<FIG> are response waveforms for experiments performed on an MCB with and without a D MOSFET connected to a bi-metallic strip, according to exemplary embodiments. For the experiments, a Phoenix Contact UT6-TMC 1A MCB (hereinafter, "PC MCB") is used (manufactured by Phoenix Contact) and a Littelfuse IXTY1R6N50D2 D MOSFET device (hereinafter, "LF DMOS"), with a trip current, TC, of <NUM>, is connected to the bi-metallic strip within the MCB. The LF DMOS is a small package that can fit into the PC MCB.

The response waveform <NUM> (<FIG>) results when an overcurrent of <NUM>. 5A (<NUM> times normal current) is invoked. The PC MCB without the LF DMOS tripped after <NUM> seconds while the PC MCB with the LF DMOS tripped after <NUM> seconds. Thus, the PC MCB tripped faster with the LF DMOS than without the LF DMOS.

The response waveform <NUM> (<FIG>) results when an overcurrent of <NUM>. 0A (twice the normal current) is invoked. The PC MCB without the LF DMOS tripped after <NUM> seconds while the PC MCB with the LF DMOS tripped after <NUM> seconds. Thus, the PC MCB tripped faster with the LF DMOS than without the LF DMOS.

The response waveform <NUM> (<FIG>) results when an overcurrent of 5A (<NUM> times normal current) is invoked. The PC MCB without the LF DMOS tripped after <NUM> seconds while the PC MCB with the LF DMOS tripped after <NUM> seconds. Thus, the PC MCB tripped slower with the LF DMOS than without the LF DMOS. Further, for each experiment conducted (<NUM>. 5A, 2A, and 5A), the PC MCB with the LF DMOS tripped after <NUM> seconds.

The result of these experiments shows that the addition of a D MOSFET to the bi-metallic strip helps to limit the overcurrent to 1A eventually, no matter how high the external overcurrent is. The addition of the D MOSFET has altered the time-to-trip response time curve. The response waveforms show that the bi-metallic strip can trip and protect the LF DMOS safely even when the current exceeds a current rating of the MCB. The combined effect of the LF DMOS with the bi-metallic strip makes the combined circuit a current limiting self-protecting switch, which a standalone bi-metallic strip cannot perform.

Thus, rather than having a simple bi-metallic strip temperature sensing structure, the presence of the FET (D MOSFET or JFET) connected to the bi-metallic strip forms a current limiting, surge tolerant, and cut-off overcurrent protection switch. Further, the bi-metallic strip acquires sufficient heat dissipation from the FET during current limiting to trigger the main switch of the MCB, resulting in an open circuit, thus protecting the FET from overheating or chip failure.

In exemplary embodiments, the LF DMOS is an epoxy packaged D MOSFET attached to the bi-metallic 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 thermal propagation to trigger the bi-metallic strip. The data from the experiments of <FIG> 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 bi-metallic strip.

The addition of the FET (D MOSFET or JFET) to the MCB is an extension of combining the D MOSFET to the bi-metallic switch <NUM> in the protection circuit <NUM> (<FIG>). The MCB, such as is represented by the circuit diagram <NUM> (<FIG>), has a similar but more complicated structure than the protection circuit <NUM>. The addition of the FET to the MCB advantageously provides current limiting, surge protection, and overcurrent protection. The bi-metallic strip connected with the FET causes the main switch of the MCB to open safely, which helps to provide a safe power cutoff during external hazardous overcurrent events. The FET is thus an efficient way to improve cut-off speed, thus providing a safer protection option for electrical system designers.

In exemplary embodiments, the D MOSFET connected to the bi-metallic strip within the MCB device is further provided with proper driver circuitry to ensure an appropriate turn-off and turn-on time, as well as other functions to drive the D MOSFET.

Claim 1:
A miniature circuit breaker, comprising:
a switch (<NUM>) to be manually opened or closed by an external lever;
a magnetic coil (<NUM>) comprising a moving valve, the moving valve to make contact with the switch upon occurrence of a first fault event, opening the switch;
a bi-metallic strip (<NUM>) to make contact with the switch upon occurrence of a second fault event: and
a field effect transistor FET (<NUM>), characterized in that the field effect transistor is connected in series with and thermally tied to the bi-metallic strip, the FET having a gate terminal (G) and a source terminal (S), wherein the gate terminal is connected to the source terminal, wherein the bi-metallic strip and the FET open the switch during the second fault event,
wherein the FET is a junction field-effect transistor JFET (<NUM>).