Depletion-mode MOSFET for overcurrent protection

Abstract

To provide a depletion mode mosfet for an over-current protection.SOLUTION: A circuit 700 is a depletion mode mosfet connected to a resistance element that is preferably a positive temperature coefficient (Positive Temperature Coefficient(PTC)) device. A voltage applied to the PTC device 608 is configured to be similar to a gate and source voltage of a MOSFET 606, and a TVS diode 710 for clamping a drain source voltage of the MOSFET is used at an overload current event is used. A heat transfer between the MOSFET and the PTC device progresses an over-current protection. A two-terminal device containing the depletion mode mosfet, the PTC device, and the TVS diode provides the over-current protection to the other circuit. A bilateral circuit containing the two MOSFETs arranged on both sides of the PTC is also considered as an AC voltage over-current protection.SELECTED DRAWING: Figure 7

Description

[Background technology]

[0001] An overcurrent, or excessive current, is a condition in which more current than intended flows through a circuit. Overcurrents can be continuous or transient in nature. Voltage transients, short-duration surges of electrical energy, are the result of a sudden release of energy that has been previously stored or induced by a large inductive load or another means such as lightning. Repetitive transients are often caused by the operation of motors, generators, or switching reactive circuit components. Lightning and electrostatic discharge (ESD) can cause sporadic transients.

[0002] The miniaturization of components has made them more sensitive to electrical stress. Microprocessors, for example, have structures and conductive paths that cannot handle the high currents from ESD transients. Because such components operate at extremely low voltages, voltage disturbance control is a high priority to prevent device interruptions and potential or catastrophic failure.

[0003] It is with respect to these and other considerations that the present improvements may be useful. Summary of the Invention

[0004] This Summary is provided to introduce a selection of 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.

[0005] An exemplary embodiment of a circuit for providing overcurrent protection is disclosed. The circuit includes a metal oxide semiconductor field effect transistor (MOSFET) and a resistive device having two terminals, one terminal connected to a source of the MOSFET and a second terminal connected to a gate of the MOSFET, wherein the MOSFET and resistive device protect the circuit during an overcurrent event. In one embodiment, the resistive device is a Positive Temperature Coefficient (PTC) device. In one embodiment, the MOSFET is a depletion-mode MOSFET. In one embodiment, the MOSFET is an N-channel depletion-mode MOSFET. In one embodiment, the circuit further includes a diode connected between the second terminal of the resistive device and ground, the diode clamping a drain-source voltage of the MOSFET during the overcurrent event. In one embodiment, the diode is a Transient Voltage Suppression (TVS) diode. In one embodiment, the MOSFET is thermally coupled to the resistive device, and in response to the circuit receiving a surge current, the MOSFET heats the resistive device. In one embodiment, in response to the circuit receiving a surge current, the drain-source voltage V of the MOSFET DS The almost instantaneous rise in the gate-source voltage V GS In one embodiment, the PTC clamps down surge current, and the increased resistance of the PTC speeds up the blocking performance of the MOSFET. In one embodiment, the MOSFET has a maximum gate-source voltage, the PTC has a maximum operating voltage, and the maximum operating voltage does not exceed the maximum gate-source voltage.

[0006] Also disclosed is an exemplary embodiment of a device coupled to a circuit for providing overcurrent protection to the circuit. The device comprises a depletion-mode MOSFET and a resistive device coupled between the source of the MOSFET and the gate of the MOSFET, such that current passing through the device generates a voltage across the resistive device equal to the gate-to-source voltage of the MOSFET, whereby the MOSFET and the resistive device protect the circuit during an overcurrent event. In one embodiment, the device further comprises a first terminal for connection to a voltage source of the circuit and a second terminal for connection to a portion of the circuit to be protected from the overcurrent event. In one embodiment, the resistive device is a PTC device. In one embodiment, the device is configured such that the drain-to-source voltage V of the MOSFET decreases during the overcurrent protection. DS In one embodiment, the MOSFET has a maximum gate-source voltage and the PTC device has a maximum operating voltage, the maximum operating voltage not exceeding the maximum gate-source voltage. In one embodiment, the voltage across the PTC resistor is equal to the gate-source voltage V of the MOSFET. GS is equal to.

[0007] Another exemplary embodiment of a circuit for providing overcurrent protection is also disclosed. The circuit includes a MOSFET, a resistive device having two terminals, a first terminal connected to a source of the MOSFET and a second terminal connected to a gate of the MOSFET, and a second MOSFET connected to the resistive device, the gate of the second MOSFET being connected to the first terminal and the source of the second MOSFET being connected to the second terminal. In one embodiment, the resistive device is a PTC device. In one embodiment, the circuit includes a drain-source voltage V across the second MOSFET. DSIn one embodiment, the circuit further comprises a clamping diode for clamping down the first MOSFET. In one embodiment, the circuit further comprises an AC voltage source. In one embodiment, each MOSFET in the circuit has an internal diode such that any of the MOSFETs conducts with its respective internal diode in inversion mode. In one embodiment, the MOSFET and the second MOSFET are N-channel Depletion-mode MOSFETs. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 illustrates a circuit for providing overcurrent protection according to an exemplary embodiment.

[0009] [Figure 2] FIG. 1 illustrates a circuit for providing overcurrent protection according to an exemplary embodiment.

[0010] [Figure 3] 3 is a waveform resulting from testing the circuit of FIGS. 1 and 2, according to an exemplary embodiment. [Figure 4] 3 is a waveform resulting from testing the circuit of FIGS. 1 and 2, according to an exemplary embodiment. [Figure 5] 3 is a waveform resulting from testing the circuit of FIGS. 1 and 2, according to an exemplary embodiment.

[0011] [Figure 6] FIG. 1 illustrates a circuit for providing overcurrent protection according to an exemplary embodiment.

[0012] [Figure 7] FIG. 1 illustrates a circuit for providing overcurrent protection according to an exemplary embodiment.

[0013] [Figure 8] 8 features two tables of test results for the circuit of FIG. 7, according to an example embodiment.

[0014] [Figure 9] 8 is a waveform resulting from testing the circuits of FIGS. 6 and 7, according to an exemplary embodiment. [Figure 10] 8 is a waveform resulting from testing the circuits of FIGS. 6 and 7, according to an exemplary embodiment.

[0015] [Figure 11] 8 is a waveform resulting from testing the circuits of FIGS. 6 and 7, according to an exemplary embodiment. [Figure 12] 8 is a waveform resulting from testing the circuits of FIGS. 6 and 7, according to an exemplary embodiment.

[0016] [Figure 13] FIG. 1 illustrates a two-terminal device for providing overcurrent protection according to an exemplary embodiment.

[0017] [Figure 14] FIG. 1 illustrates a bidirectional MOSFET circuit configuration for providing overcurrent protection, according to an exemplary embodiment.

[0018] [Figure 15] 15 is a waveform resulting from testing the circuit of FIG. 14, according to an exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] Disclosed herein are several circuits for providing overcurrent protection. The circuits feature a depletion-mode MOSFET connected to a resistive element including a positive temperature coefficient (PTC) device, configured so that the voltage across the PTC device is the same as the gate-to-source voltage of the MOSFET. The circuits may further be configured with a transient voltage suppression (TVS) diode to clamp the drain-to-source voltage of the MOSFET during an overcurrent event. Heat transfer between the MOSFET and the PTC device facilitates overcurrent protection. A two-terminal device including a depletion-mode MOSFET, a PTC device, and a TVS diode can provide overcurrent protection to another circuit. A bidirectional circuit including two MOSFETs arranged on either side of a PTC is also contemplated for AC voltage overcurrent protection. In an exemplary embodiment, a combination of a PTC and a depletion-mode MOSFET is included in a circuit to protect each other as a current clamp and stopper.

[0020] 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 at its gate changes the width of the channel located between the MOSFET's source and drain. MOSFETs come in a variety of configurations, depending on whether they are P-channel devices built on an N-type substrate, N-channel devices built on a P-type substrate, vertically or horizontally configured semiconductors, and whether they operate in depletion or enhancement mode.

[0021] Unlike enhancement-mode MOSFETs, which are activated by applying a voltage across the gate, depletion-mode MOSFETs are activated when the gate terminal is at 0 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 creates a conductive channel beneath the gate oxide and between the source and drain regions. The concentration of active dopants in the substrate-channel region is used to determine the threshold voltage (V TH ) to the desired value. Despite their name, many modern MOSFETs can be fabricated with a polysilicon gate rather than a metal gate on an insulating gate oxide.

[0022] Positive temperature coefficient (PTC) devices are made of materials that have an initial resistance that responds to temperature. As the temperature of a PTC device increases, the resistance also increases. When the current passing through the PTC element increases beyond a certain limit, the PTC element heats up, causing its resistance to increase, which can dramatically reduce or prevent the flow of current through the protected device, thereby preventing damage that would otherwise result from unmitigated fault current flowing through the circuit. The PTC returns to its low resistance state once the fault current subsides. Therefore, PTC devices are sometimes referred to as resettable fuses. Polymeric PTC devices are a specific class of devices fabricated using polymers.

[0023] Both transient voltage suppression circuits (TVS) and Zener diodes are used to absorb excess energy when voltage levels spike above the device's clamping voltage. Zener diodes are designed to make voltages more stable, while TVS diodes prevent high-voltage transients such as surges and ESD events. TVS diodes react faster than Zener diodes, on the order of nanoseconds, and can absorb higher surge currents. When a TVS diode is used to protect a circuit, it clamps the transient voltage down to a fixed value to prevent high peak voltages from damaging downstream components in that circuit.

[0024] The exemplary embodiments described herein are considered with these three devices in mind: MOSFET, PTC, and diode.

[0025] A circuit with a resistor connected between the gate and source of a MOSFET 1 is a representative diagram of a circuit 100 for providing overcurrent protection, according to an exemplary embodiment. Circuit 100 includes a voltage source 102, a switch 104, a depletion-mode MOSFET 106, a resistor 108, and a protected circuit designated CKT 110. When switch 104 is closed, current flows from the drain (D) to the source (S) of MOSFET 104 through a channel formed by ion implantation. Being a depletion-mode MOSFET, MOSFET 106 (V GS =0V). In one embodiment, MOSFET 106 is an N-channel depletion-mode MOSFET. In another embodiment, MOSFET 106 is a P-channel depletion-mode MOSFET. A P-channel depletion-mode MOSFET requires a bias V GS It is similar to an N-channel MOSFET, except that the direction of the

[0026] The depletion-mode MOSFET 106 is connected in series with a resistor 108, with one side (terminal) of the resistor connected to the source (S) of the MOSFET and the other side (terminal) of the resistor connected to the gate (G) of the MOSFET. Therefore, the voltage across the resistor 108 is the gate-source voltage (V GS ) CKT 110 is connected in series between resistor 108 and ground.

[0027] 2 is a representative diagram of a circuit 200 for providing overcurrent protection, according to an exemplary embodiment. Similar to circuit 100, circuit 200 includes voltage source 102, switch 104, depletion-mode MOSFET 106, and resistor 108. In circuit 200, a TVS diode 210 is provided instead of or in addition to the protected circuit, CKT 110. In one embodiment, MOSFET 206 is an N-channel depletion-mode MOSFET.

[0028] Being a depletion mode device, MOSFET 106 has a negative threshold voltage, V TH For a depletion-mode MOSFET, the channel is fully conductive and the gate terminal is at 0 volts (V GS = 0 volts), a strong current flows between the drain and source. As the bias at the gate of an N-channel MOSFET becomes increasingly negative, the channel becomes less conductive, eventually reaching -V GS (off) until the device threshold voltage V TH and the MOSFET becomes non-conductive.

[0029] These two periods (V GS = 0 volts period and V GS =-V TH During this period, the resistance through the channel of the MOSFET 106 increases. As the accumulated surge current increases, the MOSFET 106 enters saturation mode very quickly, which results in heat dissipation within the MOSFET.

[0030] Both circuits 100 and 200 are analyzed for two types of overcurrent protection: 1) transient overcurrent, and 2) continuous overcurrent. In an exemplary embodiment, transient and continuous overcurrent tests are performed to determine whether a series connection of resistor 108 between the gate and source of depletion-mode MOSFET 106 can reduce the overcurrent level so as to relieve some of the power dissipation of the depletion-mode MOSFET during an overcurrent condition.

[0031] To test for transient overcurrent, the input voltage is 102 V as X volts for a surge of Y / Z μs. in are specified, where X, Y, and Z are integers. For example, the overvoltage generated by a lightning strike may be characterized by a 1.2 / 50 μs voltage wave, i.e., the voltage reaches a maximum voltage (X volts) within 1.2 μs and decays to 50% of the maximum voltage (0.5X volts) after 50 μs. In one embodiment, circuits 100 and 200 are tested for a 100V transient overcurrent with X=100, Y=1.2, Z=50, or a 1.2 / 50 μs surge. Alternatively, to test for a continuous overcurrent, an input voltage V of W volts may be used. in is supplied to circuits 100 and 200. W is an integer. In one embodiment, W=48 or a 48V input voltage V in , the circuits 100 and 200 are tested for continuous overcurrent.

[0032] For surge current conditions, in the exemplary embodiment, the open circuit voltage is set to 100V and the virtual impedance in the surge generator is 2 ohms. This results in a peak surge current of 100V / 2=50A. In this embodiment, the surge voltage was intentionally kept at 100V. This avoids the use of a separate TVS diode to clamp down the surge. Furthermore, the test is designed to verify the current limiting function of the depletion mode MOSFET, rather than surging the circuit with an extremely high voltage. In this embodiment, a Littelfuse® IXTH16N10D2 depletion mode MOSFET (V DS =100V, I D(on) =16A, R DS(on) = 6.4m ohms).

[0033] The results of empirical testing for the two circuits 100 and 200 are shown in two waveforms, respectively, in Figures 3, 4, and 5, according to an exemplary embodiment. In each of the two waveforms, three parameters are plotted: the input voltage V in As a result of this, the drain-source voltage (V DS) and the gate-source voltage of the MOSFET 106 (V GS ) and surge current I S In other words, the surge current I S The resulting voltage across the MOSFET 106 (V DS ) and the voltage across resistor 108 (V GS Additionally, to test for transient overcurrent, circuits 100 and 200 are each subjected to a 100V input voltage V with a 1.2 / 50µs surge. in As shown in all waveform diagrams disclosed herein, V GS and V DS is the voltage waveform, while the surge current I S is the current waveform. Each waveform shown here is roughly divided into 20 μs intervals.

[0034] In FIG. 3, waveform 300 illustrates the resulting transient surge I on circuit 100 (FIG. 1) where resistor 108 has a resistance of 1 ohm. S First, let's look at waveform 300. The surge current I S occurs at the trigger point T (0 μs). DS The voltage V increases immediately (within 1 μs) and then decays over approximately 80 μs. GS The surge current I immediately decreases, then increases and returns to its original position in about 85 μs. S lasts for approximately 87 μs.

[0035] The second waveform 350 of FIG. 3 illustrates the resulting transient surge I on circuit 200 (FIG. 2) where resistor 108 has a resistance of 1 ohm and TVS diode 210 is a 30KPA30CA diode manufactured by Littelfuse®. S The 30KPA30CA TVS diode has a maximum clamping voltage of V C is 55.2V. Surge current I S starts at the trigger point T (0 μs) and V DS Increase in V GS A decrease in current surge I S This continues for approximately 72 μs. At this time, the drain-source voltage VDS is clamped by the TVS diode 210. The presence of the TVS diode 210 also reduces the surge current I S Affects.

[0036] Continuing with Figure 4, waveform 400 shows the result of a transient surge in circuit 100 where resistor 108 has a resistance of 0.5 ohms. As previously mentioned, the drain-to-source voltage V DS rises immediately, while the gate-source voltage V GS The surge current I S begins at trigger point T and again lasts for approximately 80 μs. Waveform 450 shows the result of a transient surge in circuit 200 where resistor 108 has a resistance of 0.5 ohms and a 30KPA30CA TVS diode 210 is used. Again, TVS diode 210 detects a voltage V DS However, due to the existence of this TVS diode 210, the surge current I S The duration of this effect cannot be reduced.

[0037] 5, resistor 108 has been removed from circuit 100 (R=0 ohms). The resistor in circuit 100 is connected between the source and gate of MOSFET 106. This means that there is no voltage difference between the source and gate. That is, V GS = 0. Therefore, in the waveform 500, the drain-source voltage V DS (the voltage applied to the MOSFET 106) rises, but the gate-source voltage V GS does not change. Surge current I S begins at trigger point T and has a duration of approximately 82 μs. Waveform 550 shows the result of a transient surge in circuit 200 without resistor 108 and using a 30KPA30CA TVS diode 210. Here, surge current I S The duration of the surge is significantly reduced to approximately 62 μs, after which a small artifact surge occurs.

[0038] From the above results, circuits 100 and 200 arein 100V with a 1.2 / 50μs surge. Circuits 100 and 200 are similarly in It may also be tested for continuous overcurrent where V is 48V. The empirical results in Figures 3, 4, and 5 provide guidance for constructing circuits that provide overcurrent suppression.

[0039] A circuit with a PTC connected between the gate and source of a MOSFET 6 and 7 are representative diagrams of circuits 600 and 700, respectively, for providing overcurrent protection, according to exemplary embodiments. Circuit 600 is the same as circuit 100 (FIG. 1), except that resistor 108 has been replaced with a positive temperature coefficient (PTC) device 608. Similarly, circuit 700 is the same as circuit 200 (FIG. 2), except that resistor 108 has been replaced with a PTC device 608. Again, one side (terminal) of PTC device 608 is connected to the source of MOSFET 606, and the other side (terminal) of the PTC device is connected to the gate of the MOSFET. In an exemplary embodiment, PTC device 608 is a polymeric PTC (PPTC) device.

[0040] Similar to circuits 100 and 200, when a surge current is applied to circuits 600 or 700, MOSFET 606 heats up rapidly. Similarly, PTC device 608 is designed to increase in resistance as it heats up. When current passes through PTC device 608, the temperature of the PTC device increases beyond a preset limit known as the trip point or trip condition. This causes the resistance of PTC device 608 to increase significantly, reducing or stopping the flow of current through the PTC.

[0041] Furthermore, placing the MOSFET 606 and PTC device 608 adjacent to one another means that when subjected to a surge current, the MOSFET heats up, heating the PTC device faster than it would without the MOSFET. Thus, in some embodiments, heat transfer occurs from the MOSFET 606 to the PTC 608, thereby accelerating the response of both devices to a surge event.

[0042] When the PTC608 is heated, it becomes more resistive. The PTC608 has a holding current I H and trip current I T The PTC 608 has two parameters: the trip current and the holding current. The PTC 608 will not trip (resistance rises significantly) unless the current flowing through it exceeds the trip current. On the other hand, if the current flowing through the PTC 608 exceeds the trip current value, the resistance of the PTC 608 will rise rapidly to block further current. This rise continues until a steady state is reached where a small current flows to maintain the PTC tripped state. The PTC will only reset to a low resistance state after the fault current has disappeared.

[0043] FIG. 8 includes two tables 800 and 850 used to show the results of performing tests on circuit 700 ( FIG. 7 ), according to an exemplary embodiment. In this example, circuit 700 includes a PTC device 708 connected between the source and gate of MOSFET 706, as well as a TVS diode 710. Both tables provide parameters for two conditions: 1) PTC 708 is a Littelfuse® RXEF375 PTC device (Condition 1), and 2) PTC 708 is removed from circuit 700 (Condition 2), which is equivalent to replacing PTC 708 with a 0-ohm resistor. In other words, the second condition is simply to remove PTC 708 and connect the source of MOSFET 706 to the gate.

[0044] In this embodiment, MOSFET 706 and TVS diode 710 are IXTH16N10D2 D-MOSFET and 30KPA30A TVS diode manufactured by Littelfuse®, respectively. Additionally, circuit 700 operates with an input voltage V of 130V with a 1.2 / 50 μs surge. in Referring to the datasheet for the IXTH16N10D2 MOSFET, V DS and V DG The maximum rated voltage for each of is 100V, and the voltage V GS Its maximum rating for surge current is + / -20V. So for a 1.2 / 50µs transient waveform with a peak voltage of 130V, the MOSFET will absorb the surge energy and allow some of the let-through voltage to flow. A TVS diode is needed to further clamp down the surge energy.

[0045] Table 800 shows the drain-source voltage V of TVS diode 710 for PTC and non-PTC conditions. DS , gate-source voltage V GS , surge current I S , and the maximum clamping voltage V C Table 850 plots the drain-source voltage V for PTC and non-PTC conditions. DS , surge current I S , and MOSFET power. Tables 800 and 850 show that in surge tests with PTC 708 in series with MOSFET 706 and a PTC connected between the source and gate of the MOSFET, the PTC clamps down the surge level and reduces surge power dissipation for the MOSFET (Condition 1) compared to connecting the gate directly to the MOSFET source (Condition 2). Therefore, the PTC contributes to clamping down a portion of the surge level by dissipating heat and further blocks current through the depletion mode MOSFET, thereby reducing the negative gate-source voltage (V GS =-0.5V).

[0046] The results in Figure 8 provide data regarding the operation of circuit 700 when a transient voltage is applied. In an exemplary embodiment, circuit 700 of Figure 7 is also used to test for sustained overcurrent. Tables 800 and 850 show how the clamping voltage level V of TVS diode 710 can be increased or decreased by having PTC 608 in place and replacing the zero-ohm resistor. C Furthermore, the presence of PTC608 contributes to reducing V DS increases, as does the dissipated MOSFET power, while V GS and I S These are encouraging results for providing transient or continuous overcurrent protection.

[0047] The results of additional empirical testing on circuits 600 and 700 with different PTC devices are shown in two waveforms in Figures 9 and 10, respectively, according to an exemplary embodiment. Each circuit 600 and 700 was subjected to an input voltage V of 130V with a 1.2 / 50 μs surge. in Waveform 900 shows the result of applying a transient surge to circuit 600 where PTC 608 is an RXEF375 PTC device, also manufactured by Littelfuse®. The RXEF375 device experiences a trip current I of 7.50 A. T , minimum resistance R of 0.03 ohms MIN , maximum resistance R of 0.05 ohms MAX As mentioned above, the voltage between the drain and source, V DS rises quickly, heating the MOSFET 606, while the gate-source voltage V GS decreases. In this example, the surge current lasts for approximately 50 μs. Waveform 950 shows the results of a transient surge in circuit 700, where PTC 708 is an RXEF375 PTC and a 30KPA30CA TVS diode 710 is used. Again, the surge current duration is approximately 50 μs, and as expected, the drain-source voltage V across the MOSFET channel DSis clamped down by the TVS diode 710. With the contribution of the RXEF375 PTC 708, the current is clamped to approximately 13.57 A. The surge waveform is characterized as a 1.2 / 50 μs wave with a peak voltage of 130 V and a peak current of 130 V / 2 ohms = 65 A (2 ohms is the virtual impedance of the surge generator network for a 1.2 / 50 μs waveform). With the contribution of the RXEF375 PTC 708, the current is clamped to approximately 13.75 A. As previously mentioned, thermal effects between the MOSFETs and each PTC in the two circuits contribute to making the circuits more responsive to surge currents in the exemplary embodiment.

[0048] 10, waveform 1000 shows the result of applying a transient surge to circuit 600 with PTC 608 removed (R=0 ohms). The voltage between the drain and source, V DS rises immediately, while the gate-source voltage V GS does not change. Surge current I S lasts for approximately 40 μs. Waveform 1050 shows the result of a transient surge in circuit 700 with PTC 708 removed and 30KPA30CA TVS diode 710 used. The surge current I S The duration of this is approximately 40 μs, and the drain-source voltage V DS The gate-source voltage V of the MOSFET 706 is clamped down. GS With the gate shorted, the current is clamped at about 26.36 A, which is larger than a PTC in series between the gate and source (Figure 9). The PTC therefore has two effects: first, it increases its resistance in response to the surge, and second, it increases the gate-source voltage V GSto a more negative level, thereby raising the resistance of the MOSFET. This demonstrates the effect of using a PTC between the gate and source of the MOSFET, rather than simply connecting it between the gate and source terminals. Again, the thermal effect between the MOSFET and PTC is observed in both circuits 600 and 700. The above circuits can be tested with other input voltages, MOSFETs, PTC devices, and TVS diodes.

[0049] The results of empirical testing for the two circuits 600 and 700 are shown in two waveforms in Figures 11 and 12, respectively, according to an exemplary embodiment. Three parameters are characterized in each of the two waveforms: the input voltage V in The resulting gate-source voltage (V GS ) and the drain-source voltage of the MOSFET (V DS ) and surge current I S In other words, the surge current I S The resulting voltage across MOSFET 606 (V DS ) and the voltage applied to the PTC608 (V GS ) are measured. Both circuits 600 and 700 are subjected to a 100V input voltage V with a 1.2 / 50µs surge. in receive.

[0050] 11, waveform 1100 shows the result of applying a transient surge to circuit 600 (FIG. 6) where PTC 608 is a Littelfuse® RXEF030 PTC device. The RXEF030 device has a trip current I T , minimum resistance R of 0.88 ohms MIN , maximum resistance R of 1.33 ohms MAX The circuit 600 generates a surge current I at the trigger point T (0 μs). S The voltage between the drain and source, V DS rises immediately and then decays over approximately 60 μs. Meanwhile, the gate-source voltage V GSThis also drops very quickly by about 20V. S lasts for approximately 67 μs, which is significantly shorter than for waveforms 300, 400, and 500.

[0051] A second waveform 1150 in Figure 11 shows the result of applying a transient surge to circuit 700 (Figure 7) where PTC 708 is also an RXEF030 PTC device and TVS diode 710 is a 30KPA30CA diode. Here, TVS diode 710 applies a voltage V across the MOSFET channel. DS The surge current I S The duration of is slightly shorter than that of waveform 1100, at about 65 μs. DS In addition, the TVS diode 612 also generates a surge current I S This has a positive effect by reducing the duration of

[0052] In waveforms 1100 and 1150, V DS The voltage across the MOSFET 606, denoted as I S This almost instantaneous increase in voltage between the drain and source indicates that the MOSFET 606 is heating up. GS The voltage across PTC 608, shown as , drops. However, by the time the current surge ends, PTC device 608 has heated up, as shown in the waveform. Thus, MOSFET 606 has a thermal effect on PTC 608, and that heating allows circuit 600 to respond more quickly to surge currents.

[0053] 12, waveform 1200 shows the result of applying a transient surge to circuit 600 in which PTC 608 is an RXEF065 PTC device, also manufactured by Littelfuse®. The RXEF065 device generates a trip current I of 1.30 A. T, minimum resistance R of 0.31 ohms MIN , maximum resistance R of 0.48 ohms MAX As mentioned above, the voltage between the drain and source, V DS rises immediately, while the gate-source voltage V GS The surge current I S lasts for approximately 65 μs. Waveform 1150 shows the results of a transient surge in circuit 700 where PTC 708 is an RXEF065 PTC and a 30KPA30CA TVS diode 710 is used. This time, the surge current I S The duration of is approximately 65 μs, and the drain-source voltage V DS is clamped down by a TVS diode 710.

[0054] Similar to circuit 600, waveforms 1200 and 1250 represent the waveforms in circuit 700 when MOSFET 706 is driven by voltage V DS The PTC708 is not heated to begin with, but the surge current I S As V decreases GS An increase in indicates that the PTC 708 is heating up due to the thermal effect of its proximity to the MOSFET 706. This thermal effect, in some embodiments, makes the circuit 700 more responsive to surge currents.

[0055] Applications of Two-Terminal Devices 13 shows a representative diagram of a circuit 1300 for providing overcurrent protection, according to an exemplary embodiment. The circuit 1300 features a depletion-mode MOSFET 1304, a Positive Temperature Coefficient (PTC) device 1308, and a TVS diode 1312. Similar to the PTC devices 608 and 708 in circuits 600 and 700 described above, the PTC device 1308 is connected in series with the MOSFET 1304 between its source and gate. Thus, the voltage across the PTC device is the gate-source voltage V of the MOSFET 1304. GSIn one embodiment, MOSFET 1304 is an N-channel Depletion-mode MOSFET. In an exemplary embodiment, PTC device 1308 is a polymeric PTC (PPTC) device.

[0056] Being a depletion mode device, MOSFET 1304 has a negative threshold voltage, V TH For a depletion-mode MOSFET, the channel is fully conductive and the gate terminal is at 0 volts (V GS = 0 volts), a strong current flows between the drain and source. As the bias at the gate of an N-channel MOSFET becomes increasingly negative, the channel becomes less conductive, eventually reaching -V GS (off) until the device threshold voltage V TH and the MOSFET becomes non-conductive.

[0057] These two periods (V GS = 0 volts period and V GS =-V TH During this time, the resistance through the channel of the MOSFET 1304 increases. Therefore, when a surge current is applied to the circuit 1300, the MOSFET 1304 heats up quickly.

[0058] Similarly, the PTC device 1308 is designed to increase in resistance as it heats up. When an electric current is passed through the PTC device 1308, the temperature of the PTC device increases beyond a preset limit, known as the trip point. This causes the resistance of the PTC device 1308 to increase significantly, reducing or stopping the flow of electric current through the PTC.

[0059] Furthermore, placing the MOSFET 1304 and PTC device 1308 adjacent to one another means that when subjected to a surge current, the MOSFET 1304 heats up, heating the PTC device 1308 faster than it would without the MOSFET. Thus, in some embodiments, heat transfer occurs from the MOSFET 1304 to the PTC 1308, thereby accelerating the response of both devices to a surge event.

[0060] As the PTC1308 heats up, it becomes more resistive. A voltage builds up across the PTC1308, eventually reaching the threshold voltage V of the MOSFET 1304. TH exceeds the threshold voltage and turns off the MOSFET.

[0061] Because the gate is connected to one side of PTC 1308, no separate voltage needs to be applied to the gate. In an exemplary embodiment, circuit 1308 is packaged as a two-terminal device for use as overcurrent protection in other circuits. Thus, a first terminal 1314 of device 1300 is connected to a voltage source of the circuit to be protected, while a second terminal 1316 is connected to a portion of the circuit to be protected (e.g., a downstream component of the circuit). In an exemplary embodiment, device 1300 protects against both continuous and transient overcurrents.

[0062] Bidirectional MOSFET circuit 14 is a representative diagram illustrating a circuit 1400 including a pair of MOSFETs providing bidirectional protection disposed between two MOSFETs and connected to a PTC device, according to an exemplary embodiment. The circuit 1400 includes a voltage input 1402, which is an AC input, a switch 1404, a first depletion-mode MOSFET 1406, a PTC device 1408, a second depletion-mode MOSFET 1412, and a CKT 1410. In one embodiment, the MOSFETs 1406 and 1412 are N-channel depletion-mode MOSFETs connected in an anti-parallel configuration. In an exemplary embodiment, the PTC device 1408 is a polymeric PTC (PPTC) device.

[0063] As previously described, the PTC device 1408 has a first terminal connected to the source of the MOSFET 1406 and a second terminal connected to the gate of the same MOSFET. By introducing a second MOSFET, the first terminal is connected to the gate of the MOSFET 1412 and the second terminal is connected to the source of the second MOSFET. When the circuit 1400 receives an AC voltage input, either MOSFET configures with its internal diode. When a positive-cycle AC current flows through the circuit 1400, the first MOSFET 1406 and the PTC 1408 act together as a current limiter, while the other MOSFET 1412 functions in reverse mode, with only the body diode of the MOSFET 1412 conducting. This operation is reversed during the other cycle of AC current flow; that is, the MOSFET 1412 and the PTC 1408 act together as a current limiter, with only the diode of the MOSFET 1406 conducting. Thus, the circuit 1400 provides AC current-limiting protection. In one embodiment, CKT 1410 is replaced with a bidirectional diode to clamp down the voltage across both MOSFET 1412 and MOSFET 1406. In a second embodiment, the diode is a bidirectional TVS diode, thereby protecting against surge currents in both directions. Furthermore, in one embodiment, the RMS voltage multiplied by √2 is the DC peak voltage, so the gate-source voltage V GS The AC RMS voltage is limited to a maximum of 17V to ensure that the input voltage does not exceed 25V (17V x 1.414 = 24V).

[0064] The results of empirical testing on circuit 1400 are shown in two waveforms in Figure 15 according to an example embodiment. Circuit 1400 operates at an input voltage V of 90V with a 1.2 / 50 μs surge. in During long-term overcurrent PTC tripping, the voltage V GS The AC RMS voltage is limited to a maximum of 17V to ensure that the V does not exceed 25V. MOSFETs 1406 and 1412 are both Littelfuse IXTH16N10D2 (V DS =100V, I D(on)= 16A, tightening torque TO-247 R DS(on) = 64 mOhm), and the PTC device 1408 has a maximum resistance R MAX is the 0.05 ohm RXEF375.

[0065] Three parameters are characterized in waveform 1500: input voltage V in As a result, the gate-source voltage (V GS ) and the drain-source voltage of the MOSFET (V DS ) and surge current I S In other words, the surge current I S The resulting voltage across MOSFET 1406 (V DS ) and the voltage applied to the PTC1408 (V GS ) is measured. In waveform 1450, the input voltage V in The resulting gate-source voltage (V GS ) and the drain-source voltage of the MOSFET (V DS ) and surge current I S In other words, the surge current I S The resulting voltage across MOSFET 1412 (V DS ) and the voltage applied to the PTC1408 (V GS ) is measured. Therefore, the voltage across PTC 1408 is the gate-source voltage of both MOSFETs 1406 and 1412.

[0066] Waveforms 1500 and 1550 are nearly identical. Thus, they are nearly symmetrical. This demonstrates that the anti-parallel MOSFET plus PTC configuration of circuit 1400 provides AC power protection.

[0067] The above embodiments discuss, in some cases, particular versions of depletion-mode MOSFETs, PTC devices, and TVS diodes. Nevertheless, other devices may be used in one or more of the above circuits to provide overcurrent protection for a variety of different applications. In some embodiments, VDSX =100V, I D(on) ≥ 16A, and R DS(on) An N-channel depletion mode MOSFET IXTT16N10D2 or IXTH16N10D2 with parameters of ≦64 mOhms is used for the MOSFET. In some embodiments, V DSX =500V, I D(on) ≥ 800mA, and R DS(on An N-channel depletion mode MOSFET IXTY08N50D2, IXTA08N50D2, or IXTP08N50D2 manufactured by Littelfuse® with parameters of ≦4.6 ohms is used for the MOSFET. In some embodiments, V DSX =500V, I D(on) ≥ 16A, and R DS(on) N-channel depletion mode MOSFETs IXTH16N50D2 or IXTT16N50D2 with parameters ≦300 mOhm are used for the MOSFET.

[0068] In some embodiments, a Littelfuse® Poly-Fuse® 72R series radial lead resettable PTC is used for the PTC device. In some embodiments, a Polyswitch® RXEF series radial lead resettable PPPT is used for the PTC device. In some embodiments, a Littelfuse® 30KPA series axial lead transient voltage suppression diode is used for the TVS diode. The use of other devices, whether from Littelfuse® or other device manufacturers, is contemplated without departing from the scope of this disclosure.

[0069] In an exemplary embodiment, the introduction of a PTC device has been shown to work in conjunction with a depletion mode MOSFET connected to a PTC device to limit overcurrent in both transient and sustained conditions. In one embodiment, the MOSFET and PTC combination in a circuit such as that described above functions below 30V. This limits the maximum gate-source voltage V of the MOSFET. GS and is below the maximum PTC operating voltage. In another embodiment, the input voltage range is limited to functional operating voltage levels between 5 and 24 V to prevent damage to the depletion-mode MOSFET. In another embodiment, adding a heat sink to the depletion-mode MOSFET can extend the thermal performance of the MOSFET because the presence of the heat sink improves the overall thermal impedance.

[0070] As used herein, the use of the singular "a" or "an" preceding an element or step does not exclude a plurality of elements or steps, unless the context explicitly states that a plurality of elements or steps is excluded. 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.

[0071] 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 disclosure, as defined by the appended claims. Accordingly, it is intended that the present disclosure not be limited to the described embodiments, but rather have its full scope defined by the language of the following claims and equivalents thereof.

Claims

1. 1. A circuit operable to provide overcurrent protection, the circuit comprising: a first metal oxide semiconductor field effect transistor (MOSFET) having a first internal diode; a resistive device, a positive temperature coefficient (PTC) device having two terminals, a first terminal connected directly to the source of the first MOSFET and a second terminal connected to the gate of the first MOSFET; a second MOSFET directly connected to the PTC device, the second MOSFET having a second internal diode, the gate of the second MOSFET connected to the first terminal, and the source of the second MOSFET directly connected to the second terminal; The drain-source voltage V applied to the first MOSFET and the second MOSFET ＤＳ a bidirectional diode connected directly to the drain of the second MOSFET for clamping down circuit.

2. The circuit of claim 1 further comprising a voltage source, said voltage source being an AC voltage source.

3. 3. The circuit of claim 1, wherein the first MOSFET and the second MOSFET are N-channel Depletion-mode MOSFETs.

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