Abstract:
An electrical device includes a package having an array of connections, a thermally conductive, electrically insulative substrate in the package, a plurality of polymeric positive temperature coefficient (PPTC) resistors in the package in thermal contact with the substrate, and, at least one heating element such as a power field effect transistor in thermal contact with the substrate, for indirectly heating the PPTC resistors in response to a control current, thereby to trip the PPTC resistors from a low resistance state to a very high resistance state. A method for controlling a plurality of electrical loads is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a power controlled electrical circuit protection device. More particularly, the present invention provides methods and apparatus for simultaneous control of several low or medium power loads while providing high voltage isolation to the power control element and overcurrent protection of the loads.  
           [0003]    2. Introduction to the Invention  
           [0004]    Many diverse applications exist for delivering low to moderate power simultaneously to a plurality of loads. The loads may be indicators, sensors or actuators such as small electric motors or solenoids for converting electrical energy into motive force. In these applications the power/data/control levels may be provided via a multi-line bus from a central control point, such as a computer or computer-based controller. Good engineering practice dictates that each load needs to be protected from short/overload fault conditions.  
           [0005]    Conventional protection methods include metal-link fuses, circuit breakers, and polymeric positive temperature coefficient (“PPTC”) resistor devices, for example. When an overcurrent condition occurs, fuse links rupture irreversibly, breakers trip and must be manually reset, and PPTC devices switch or trip from a few ohms low resistance to a high impedance state in excess of a Mohm high resistance. Switching occurs as a result of local heating of the PPTC device. Unique to PPTC devices, when the overcurrent condition is corrected and the power to the circuit is removed, the PPTC device cools and automatically resets itself to its low resistance state. This automatic reset function provides a significant advantage over metal-link fuses, which must be manually removed and replaced at a fuse block, or breakers that must be manually reset, by a technician or other user of the apparatus, device or appliance being protected.  
           [0006]    It is known to form an array of thin film or thick film resistors upon a nonconductive ceramic substrate and package the array in a standard configuration such as dual inline package (DIP) or single inline package (SIP). While such devices are widely used to terminate computer buses with fixed impedance loads, they are entirely passive and do not provide overcurrent protection in a manner equivalent to metal-link fuses, breakers or PPTC devices.  
           [0007]    It is also known to provide a single integrated circuit device integrally including load termination resistors and series switches formed of field effect transistor (FET) devices. The active FET devices enable the resistor loads to be collectively disconnected in response to a control signal. Examples of such devices are found in U.S. Pat. No. 5,381,034 issued on Jan. 10, 1995 to Thrower et al., entitled “SCSI Terminator”, and U.S. Pat. No. 5,382,841 issued on Jan. 17, 1995 to Feldbaumer, entitled “Switchable Active Bus Termination Circuit”, the disclosures thereof being incorporated herein by reference thereto. In these patents, resistors are formed and trimmed as part of an integrated circuit fabrication process, but the resistors, while providing desired bus load impedances, do not provide self-resetting protection fuses in the manner provided by PPTC devices.  
           [0008]    It is further known to couple a positive temperature coefficient (“PTC”) or negative temperature coefficient (“NTC”) resistor thermally to an active device, such as a transistor, silicon controlled rectifier (“SCR”), thyristor, FET, etc., in order to control or protect the active device as a function of temperature of the PTC or NTC resistor. Several examples of power line protection devices and methods are described, for example, in the inventor&#39;s commonly assigned, prior U.S. Pat. No. 6,072,681 issued on Jun. 6, 2000 to Cogan et al. and entitled: “Power Line Protection Devices and Methods”, the disclosure thereof being incorporated herein in its entirety. Therein, a PPTC device is thermally coupled to an active element configured as a current switch via a common electrically insulative, thermally conductive substrate. If an overcurrent condition occurred at a load, the PPTC device trips, causing the current switch to open and disconnect power from the line/load. Computer monitoring and reset of the protection circuit is provided in some of the disclosed circuit configurations. Other examples of a PPTC device being thermally coupled to control an active device or circuit are given in commonly assigned U.S. Pat. No. 6,331,763 issued on Dec. 18, 2001 to Thomas et al. (including the present inventor), and entitled: “Devices and Methods for Protection of Rechargeable Elements”, the disclosure thereof being incorporated herein by reference.  
           [0009]    In particular the ′763 patent illustrates in FIGS.  45 - 47  a three-terminal device that includes FET regulator formed upon and thermally coupled to a PPTC substrate element. In these prior approaches, each circuit employed a single PPTC device in conjunction with an active element, and while that approach works well for certain applications, it has a drawback of high cost in circumstances where it is practical or necessary to control multiple lines simultaneously and with a single switch.  
           [0010]    However, for some applications and environments, particularly harsh electrical environments such as motor vehicles, a hitherto unsolved need has remained to provide individual overcurrent protection to plural load lines and also to be able to disconnect all of the loads simultaneously in response to a single control signal. While separate circuits, such as fuses and switches or relays, may be connected in series to carry out these separate functions, a hitherto unsolved need has arisen for simultaneously providing these separate functions in a single device providing the advantages of multiple PPTC devices and characteristics.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    A general object of the present invention is to provide an electrical device providing overcurrent protection to several low or medium power loads while providing simultaneous control of the loads as well as high voltage isolation between the power control element and the loads in a manner overcoming limitations and drawbacks of the prior art.  
           [0012]    Another object of the present invention is to provide a method for protecting plural power loads by a single device which combines individual protection to each load and which may be actuated to disconnect simultaneously all of the loads in response to a control or a sensed condition.  
           [0013]    An electrical device in accordance with principles and aspects of the present invention includes a package having an array of connections. A thermally conductive, electrically insulative substrate is provided in the package. Plural resistive elements are provided in the package. Each resistive element has a predetermined non-zero temperature coefficient, is connected to a different pair of connections of the array, and is in thermal contact with the substrate. At least one heating element is provided in the package and is in thermal contact with the substrate. While a variety of solid state passive and active heating elements may be successfully employed, one particularly preferred heating element enabling thermal autoregulation is a semiconductor device with a control electrode, such as a power field effect transistor (FET). The heating element has electrodes connected to associated ones of the array of connections and responds to a control current by heating the substrate and thereby indirectly heating the resistive elements, thereby to change resistance of the plurality of resistive elements. In one preferred aspect of the invention, the plurality of resistive elements comprises polymeric positive temperature coefficient (PPTC) resistors. In another preferred aspect of the present invention, the heating element comprises a semiconductor device having at least one junction, such as a power diode, a power field effect transistor, a power bipolar transistor, a power thyristor, or a power silicon controlled rectifier. In an alternative preferred aspect of the present invention, the substrate is provided by an integral electrode of the semiconductor device, and the device further includes an electrical insulator layer between the resistive elements and the substrate. In another alternative preferred aspect of the present invention, the substrate comprises a lead frame.  
           [0014]    In one preferred arrangement of the present invention, an electrical device includes a package having an array of connections, a thermally conductive, electrically insulative substrate in the package, a plurality of polymeric positive temperature coefficient (PPTC) resistors in the package in thermal contact with the substrate wherein each PPTC resistor has a predetermined positive temperature coefficient and is connected to a different pair of connections of the array, and, at least one power field effect transistor in the package in thermal contact with the substrate, and having gate, source and drain electrodes connected to associated ones of the array of connections, for heating the substrate and for indirectly heating the PPTC resistors in response to a control current, thereby to trip the PPTC resistors from a low resistance state to a very high resistance state.  
           [0015]    A method for providing separate electrical overload protection to a plurality of electrical loads and providing simultaneous disconnection of the loads in response to a disconnect control signal in accordance with principles of the present invention comprises the following steps:  
           [0016]    providing a common heating substrate,  
           [0017]    mounting or forming a plurality of polymeric positive temperature coefficient (PPTC) resistors to be in thermal contact with the common heating substrate,  
           [0018]    connecting each PPTC resistor in series with one load of the plurality of electrical loads so that the PPTC resistor will trip to a high resistance state in response to an electrical overload at the one load,  
           [0019]    providing a heating element such as a power FET in thermal contact with the common heating substrate, and  
           [0020]    applying the disconnect control signal to a control electrode of the power FET to heat the common heating substrate and thereby trip the plurality of PPTC resistors and simultaneously disconnect the electrical loads.  
           [0021]    These and other objects, advantages, aspects and features of the present invention will be more fully understood and appreciated upon consideration of the detailed description of preferred embodiments presented in conjunction with the following drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is an electric circuit schematic diagram of a multiple load protection and control device in accordance with principles of the present invention.  
         [0023]    [0023]FIG. 2 is a greatly enlarged top plan view of a device embodying principles of the present invention.  
         [0024]    [0024]FIG. 3A is an isometric view illustrating a first presently preferred assembly method of a device embodying principles of the present invention.  
         [0025]    [0025]FIG. 3B is an isometric view illustrating a second presently preferred assembly method of a device embodying principles of the present invention.  
         [0026]    [0026]FIG. 4A is an enlarged, diagrammatic side view of a PPTC resistor arranged in a plane-parallel terminal embodiment of the FIG. 1 device and illustrative of internal current flow and heat applied by the heating element.  
         [0027]    [0027]FIG. 4B is an enlarged, diagrammatic side view of a PPTC resistor arranged in a side-by-side terminal embodiment as exemplified by the FIG. 2 device and illustrative of internal current flow and heat applied by the heating element.  
         [0028]    [0028]FIG. 4C is an enlarged, diagrammatic side view of a PPTC resistor arranged in a flip-chip terminal embodiment of the FIG. 1 device and illustrative of internal current flow and heat applied by the heating element.  
         [0029]    [0029]FIG. 5 is a graph plotting time to trip of a PPTC resistor element against FET heater power in a device in accordance with principles of the present invention.  
         [0030]    [0030]FIG. 6 is graph plotting resistance of a PPTC resistor element of the FIG. 1 device against substrate temperature resulting from FET heating.  
         [0031]    [0031]FIG. 7 is a schematic circuit diagram of circuits including a device in accordance with principles of the present invention.  
         [0032]    [0032]FIG. 8 is another schematic circuit diagram of circuits including a device in accordance with principles of the present invention and an additional active element, which turns off as substrate temperature of the FIG. 1 circuit increases.  
         [0033]    [0033]FIG. 9 is yet another schematic circuit diagram of circuits including a device in accordance with principles of the present invention and a plurality of additional active elements in which an output active element turns on as substrate temperature of the FIG. 1 circuit increases. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    With reference to FIG. 1, a unitary device  10  includes a thermally conductive substrate  12 , an array of PPTC resistor elements  14  formed upon the substrate  12  or otherwise attached thereto in a positive thermal conducting arrangement, and a FET power transistor  16  also formed upon, or forming, the substrate  12 . While six PPTC elements  14 - 1 ,  14 - 2 ,  14 - 3 ,  14 - 4 ,  14 - 5  and  14 -n are shown in FIG. 1, it will be understood by those skilled in the art that the array of PPTC elements  14  may comprise two or more such elements, depending upon the need of a particular application. Each PPTC element  14  may be connected between a source and a load and thereby provide overcurrent protection to the resultant circuit.  
         [0035]    Additionally, a control signal may be applied by an external control circuit  17  to the power FET  16 . When sufficient power is applied to the power FET  16  from an external current source  19 , resultant heat is conducted via the substrate  12  to each PPTC element  14  of the array, causing each PPTC resistor element  14  to trip to a very high resistance state. When power is then removed from FET  16  and resultant heat is dissipated, each PPTC element  14  returns to its low resistance state.  
         [0036]    One PPTC element, such as resistor  14 -n shown in FIG. 1, may be connected in series between a control electrode (gate) of FET  16  and the external control circuit  17 , and the resultant circuit arrangement then provides thermal overload protection for the heating substrate  12  along with automatic thermal regulation in order to maintain the PPTC elements  14  of device  10  in a tripped state at a reduced FET power dissipation level, following a very high initial power level provided to trip the PPTC elements  14 . When control voltage is removed from the gate of FET  16  (assuming an enhancement-mode FET device is employed as power FET  16 ), heating ceases. Thereafter, when the PPTC elements  14  cool below a predetermined trip temperature (see e.g. FIG. 6), they return to a low resistance state, and restore a circuit path between each power source and its associated load.  
         [0037]    While an insulated gate power MOSFET  16  is shown as a preferred heat-generating active element, other types of heating elements may be employed, including active devices such as a diode, a bipolar transistor, a silicon controlled rectifier, a thyristor, or a power integrated circuit, for example. Passive heating devices, such as wire-wound, composition, or film resistors may also be employed for the heating element.  
         [0038]    [0038]FIG. 2 provides a greatly enlarged top plan view of a device  10  in accordance with principles of the present invention. In the FIG. 2 example, the device  10  is housed within a suitable package  20 . A power insulated-gate MOSFET  16  includes a silicon chip substrate forming a FET drain electrode  16 D, a top planar layer forming a FET source electrode  16 S and an insulated gate electrode  16 G. Four PPTC devices  14 - 1 ,  14 - 2 ,  14 - 3  and  14 - 4  are mounted directly to the top planar layer  16 S in direct thermal contact therewith. The package  20  includes an array of internal bonding pads  22  leading to, or forming, external connection pins or pads.  
         [0039]    In the FIG. 2 example electrical interconnect wires  24  extend from the bonding pads  22  to the electrodes of the power MOSFET  16  and to the PPTC elements  14 . Pads  14 A and  14 B of PPTC element  14 - 1  are respectively connected to bonding pads  22 A and  22 B. Pads  14 C and  14 D of element  14 - 2  are connected respectively to bonding pads  22 C and  22 D. Pads  14 E and  14 F of element  14 - 3  are connected respectively to bonding pads  22 E and  22 F. And, pads  14 G and  14 H of element  14 - 4  are connected respectively to bonding pads  22 G and  22 H.  
         [0040]    The drain electrode of FET  16  is connected to three bonding pads  22 J,  22 K and  22 L; and the source electrode  16 S of FET  16  is also connected to three bonding pads  22 M,  22 N and  22 O, in order to carry the electrical current needed to generate an appropriate level of heat. The insulated gate electrode  16 G of FET  16  is connected to a bonding pad  22 I.  
         [0041]    While the PPTC elements  14  of the FIG. 2 device  10  are in direct thermal contact with the source electrode  16 S of FET  16 , they are preferably electrically insulated therefrom by a suitable thermally conductive dielectric layer, not shown in FIG. 2, in order to provide effective electrical isolation between each PPTC element  14  and the FET  16 .  
         [0042]    The FIG. 2 device may be encapsulated or hermetically sealed to prevent unwanted exposure of the FET chip  16  with the ambient environment. Suitable external package pins or pads (not shown) connect to, or extend from, respective ones of the internal bonding pads  22  and, and enable the device  10  to be plugged into a socket, or directly soldered onto a circuit board, as may be desired in a particular application.  
         [0043]    [0043]FIG. 3A illustrates one assembly arrangement of a device  10 A in accordance with principles of the present invention. Therein, a FET chip  16  is mounted to a metal lead frame package  18  and PPTC element chips  14 -n are mounted onto a substrate  12 A formed by the FET chip  16 . FIG. 3B illustrates another assembly arrangement of a device  10 B in accordance with principles of the present invention. Therein, a lead frame package  18  provides the common thermal substrate  12 B. The PPTC elements  14 -n and the FET chip  16  are mounted directly to the lead frame package  18 .  
         [0044]    [0044]FIGS. 4A, 4B and  4 C illustrate several mounting and connection alternatives for the PPTC elements  14 . In FIG. 4A, a PPTC element  14  has two planar electrodes in parallel. One electrode  14 A is connected to a bonding pad  22 , and the other electrode  14 B is connected directly to the electrically conductive substrate  12 C. In this arrangement all of the PPTC elements  14  are commonly connected to a common circuit node, for example provided by the substrate  12 C. In the FIG. 4A example, current flow is shown to be unidirectional, from the electrode  14 A to the electrode  14 B (or vice versa, with a polarity reversal).  
         [0045]    [0045]FIG. 4B shows a PPTC element  14 - 1  having spaced-apart, coplanar side-by-side terminals  14 A and  14 B connecting to respective bonding pads, and a conductive layer  21  providing the thermal bond to the substrate  12 . A dielectric layer (not shown) may be formed between the PPTC element  14 - 1  and the substrate  12  to provide desired electrical isolation therebetween. In the FIG. 4B arrangement current flow through the PPTC element  14 - 1  is bidirectional, from electrode  14 A to electrode  21 , and from electrode  21  to electrode  14 B (or vice versa with a polarity reversal).  
         [0046]    [0046]FIG. 4C shows a PPTC element  14 - 7  arranged in an inverted or “flip-chip” configuration, such that its terminal electrodes  14 X and  14 Y are bonded directly to conductive traces formed on an electrically patterned substrate  12 D. Current flow is bidirectional, as in the FIG. 4B example.  
         [0047]    [0047]FIG. 5 presents a graph illustrating the time in milliseconds required to cause a PPTC element  14  to trip (switch to a very high resistance state) in response to instantaneous heating generated by the FET  16  and applied to the substrate  12  and PPTC element  14 .  
         [0048]    [0048]FIG. 6 presents a graph of resistance of a PPTC element  14  as a function of temperature. The data presented in the FIG. 6 graph were taken with a PPTC resistor mounted directly to a  10 A generic power MOSFET mounted in an SO8 conventional package.  
         [0049]    [0049]FIG. 7 shows a circuit application of the device  10  wherein an external resistor  30  connects to a voltage supply, silicon controlled rectifier and an external load resistor  32  connects to a ground return of the power supply. Resistors  30  and  32  are in series with a first PPTC resistor element  14 - 1  of the device  10 . Resistors  30 ,  14 - 1  and  32  form a voltage divider network. While the load is shown as a resistor  32 , those skilled in the art will appreciate that any suitable load may be used, such as a motor, solenoid, incandescent or solid state lamp, etc. The output voltage at a node between resistors  14 - 1  and  32  will decrease as temperature of the substrate  12  increases (causing resistance of PPTC resistor  14 - 1  to increase). While only a single output resistor is shown in FIG. 7, it should be understood that output resistors or loads are provided to other ones of the PPTC resistors  14  of the device  10  in a particular circuit application/implementation.  
         [0050]    [0050]FIG. 8 shows a circuit including device  10  similar to the circuit of FIG. 7 with the addition of an output FET device  36  and a suitable drain resistor  34  leading to a power supply. An output level at the drain electrode of the FET device  36  will fall as temperature of substrate  12  of the device  10  increases. The output active device  36  can be replaced by an electromagnetic relay, incandescent lamp, light emitting diode indicator, laser diode, comparator, meter, or any other suitable level-sensing or level-indicating device.  
         [0051]    In the FIG. 9 circuit, an additional active device, such as FET  40  having drain resistor  38 , is interposed between the load resistor  32  and the output FET  36 . In this circuit arrangement, the output level at the drain electrode of the output FET device  36  will rise as temperature of substrate  12  of the device  10  increases. The output level thereby provides a positive indication of the “tripped” status of the associated PPTC device  14 .  
         [0052]    These circuit arrangements may be used for connecting and/or disconnecting electrical loads while maintaining electrical isolation (separation) between the driving circuit and the load. These arrangements provide effective methods for power control as well as for circuit protection. Further, the heating substrate  12  may also serve as a protected element, as illustrated for example by the FIG. 1 circuit.  
         [0053]    Having thus described preferred embodiments of the invention, it will now be appreciated that the objects of the invention have been fully achieved, and it will be understood by those skilled in the art that many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. Therefore, the disclosures and descriptions herein are purely illustrative and are not intended to be in any sense limiting.