Abstract:
A fault protection circuit for an alternator is provided for preventing faults such as a prolonged full-field condition in the alternator. The fault protection circuit includes a safety switch that is opened when the alternator output voltage becomes too high, as may occur during a full-field condition caused by an electrical short, or when some other fault is detected within the alternator. The opening of this safety switch disconnects a supply voltage feeding an excitation current control switch. The excitation current control switch normally adjusts an excitation current provided to a rotor in the alternator, in order to regulate a voltage output from the alternator. By providing a safety switch that disconnects the supply voltage for the rotor excitation in the alternator, the alternator output voltage may be prevented from reaching excessive levels that may damage devices in an electrical system and a battery coupled to the alternator.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates to a safety circuit, a brush holder, and a method for preventing or mitigating fault conditions in an alternator. 
       BACKGROUND 
       [0002]    Automotive electrical systems are required to supply electrical power to a variety of devices within a vehicle. These devices typically include an electric starter motor, the engine ignition system, an electronic control unit (computer), headlights, and a variety of accessories. Electrical power is supplied to these devices (loads) by a battery, wherein the most common type is a 12V lead-acid battery. The battery is typically the primary or only source of electrical power when the vehicle&#39;s engine is not running, whereas the battery and an alternator typically both supply electrical power when the engine is running. 
         [0003]    The alternator converts mechanical power generated by the engine into electrical power that is used both for meeting the demands of the various electrical devices and for charging the battery. In order to maintain an adequate charge state in the battery and to avoid damaging the battery (e.g., overcharging) or the various electrical devices, the alternator must generate an output voltage in a fairly narrow range. To support a typical 12V battery and an associated electrical system, the alternator should supply an output in the range of 14.0 to 14.6 V, with an ideal voltage of about 14.2V. The voltage output from the alternator is controlled using some type of voltage regulation. 
         [0004]    The output voltage for most modern alternators is regulated by controlling the amount of current flowing through the windings in the rotor of the alternator. This current, in conjunction with the rotation speed of the rotor, determines the induced current flowing through the windings of the stator which, after rectification, produces the alternator&#39;s output voltage. Voltage regulation is accomplished by sensing the output voltage from the alternator, and using the sensed voltage to determine an appropriate excitation current to provide to the rotor. 
         [0005]    Control of the excitation current provided to the rotor is accomplished via appropriately switching a supply voltage (e.g., the battery voltage) that provides current to the rotor. This switching is typically performed by a power transistor, which is controlled by an excitation control signal. The excitation control signal may be generated by a controller within the alternator, in conjunction with voltage regulation performed by the controller and/or by an electronic control unit (ECU) located outside of the alternator. 
         [0006]    A potential safety issue arises if a fault causes a “full field” condition in the alternator. This occurs when the supply voltage becomes connected directly to the rotor such that the rotor&#39;s excitation current is no longer limited by the controller within the alternator and the associated voltage regulation. Such a fault leads to an uncontrolled alternator voltage output, which is likely to reach excessive levels that may damage the battery (by overcharging it) or other devices connected to the electrical system. 
         [0007]    One solution to the above problem is to, upon detection of a “full-field” condition, disconnect the alternator output from the battery and the other components of the electrical system. However, the alternator outputs relatively high current, e.g., in the range of 50-200 amperes, which makes the switching of this output unfeasible and/or prohibitively expensive. 
         [0008]    Another proposed solution implements a safety function that is implemented by interrupting the control signal, e.g., from the controller of the alternator to the switch controlling the rotor excitation current. Such a solution addresses faults in the controller itself, faults caused by inputs to the controller (e.g., an incorrect sense voltage), or a fault in the signal controlling the excitation current control switch. 
         [0009]    However, the above remedy fails to address the situation in which the full-field condition is caused by a short occurring around the excitation control switch, i.e., the situation wherein the supply voltage is electrically shorted directly to the rotor input. A fault protection circuit and method for addressing a full-field condition caused by such a fault is desirable. 
       SUMMARY 
       [0010]    According to an embodiment of a fault protection circuit, the fault protection circuit comprises an excitation current switch and a safety switch. The excitation current switch controls the amount of current provided to a rotor of an alternator. The amount of current determines the voltage output from the alternator and, thus, can be used to regulate the output voltage. The safety switch is interposed between the excitation current switch and its supply voltage, so that the safety switch can disconnect the supply voltage from the excitation current switch when a fault condition is detected, e.g., during a full-field condition of the alternator. 
         [0011]    According to an embodiment of a brush holder for use in an alternator, the brush holder comprises: a housing that retains a brush, an excitation current switch and a safety switch. The brush is configured to conduct an excitation current to a rotor within the alternator. The excitation current switch provides an excitation current to the rotor via the brush as a function of an excitation current control signal that controls the excitation current switch, so as to regulate an output voltage of the alternator. The safety switch is electrically connected in series between the excitation current switch and its supply voltage, such that the safety switch can disconnect the supply voltage from the excitation current switch, responsive to receiving an indication that a fault condition exists. 
         [0012]    According to a method in an alternator for mitigating a fault condition of the alternator, the method comprises: providing an excitation current to a rotor of the alternator using an excitation current switch; and, responsive to detecting a fault condition in the alternator, disconnecting the supply voltage from the excitation current switch by opening a safety switch electrically connected in series between the excitation current switch and its supply voltage. 
         [0013]    Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0014]    The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. The features of the various illustrated embodiments can be combined unless they exclude each other. Embodiments are depicted in the drawings and are detailed in the description which follows. 
           [0015]      FIG. 1  illustrates a high-level block diagram of a vehicle electrical system including a safety switch disposed within an alternator. 
           [0016]      FIG. 2  illustrates a block diagram of an alternator that includes a safety switch. 
           [0017]      FIG. 3  illustrates a block diagram of an embodiment of a fault protection circuit, including a safety switch, which could be incorporated into an alternator. 
           [0018]      FIG. 4  illustrates a block diagram of an alternate embodiment of a fault protection circuit, including a safety switch, wherein this embodiment includes a supervision circuit. 
           [0019]      FIG. 5  illustrates a brush holder that incorporates a safety switch. 
           [0020]      FIG. 6  illustrates a method for mitigating a fault condition of an alternator, by disconnecting the supply voltage from an excitation current switch that drives a rotor in an alternator. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Embodiments described herein provide for avoiding or mitigating fault conditions such as an uncontrolled full-field condition in an alternator and an associated overvoltage situation at the alternator&#39;s output. This is accomplished without adding a large and expensive switch on the output of the alternator. In order to address a variety of fault conditions that may lead to a full-field condition, including a short between the supply voltage (e.g., the battery voltage) and the input that feeds the alternator rotor, a safety switch is added that is configured to disconnect the supply voltage from an excitation control switch, where the excitation control switch supplies an excitation current to the rotor of the alternator. 
         [0022]      FIG. 1  provides a high-level block diagram of a typical automotive electrical system  100 . The system  100  comprises a variety of vehicle loads  110 , a battery  130 , and an alternator  140 . The vehicle loads  110  include an Electronic Control Unit (ECU)  120 . The alternator  140  includes a safety switch  150 , which is configured to disconnect an excitation current that feeds a rotor of the alternator to prevent fault conditions, such as an overvoltage situation, at the output of the alternator  140 . 
         [0023]      FIG. 2  illustrates a block diagram of the alternator  140 . An alternator controller  210  provides an excitation current to the rotor  230  of the alternator  140 . The rotor  230  is rotated by mechanical means, e.g., a drive belt coupled to a running engine. The rotating rotor  230  creates a magnetic field that induces electrical current in the windings of the stator  240  of the alternator  140 . A typical stator has three windings, as shown in  FIG. 2 , such that three alternating current (AC) outputs are generated. A rectifier  220 , typically constructed using diodes, converts the phased AC current from the windings of the stator  240  into a direct current (DC) output. The DC output should be appropriate both for charging the battery  130  and supplying power to the various vehicle loads  110 . 
         [0024]    Voltage regulation is required to maintain the output voltage (e.g., V bat ) of the alternator  140  at a proper level. The alternator output voltage is determined by the rotation speed of the rotor  230  and the excitation current (EXC) provided to the rotor  230  by the alternator controller  210 . In a first configuration, the alternator controller  210  performs voltage regulation by sensing the alternator output voltage (which is also the battery voltage V bat  in  FIG. 2 ), and setting the excitation current in order to maintain a desired set voltage (V set ). The set voltage V set  may be provided to the alternator controller  210  by an ECU  120  in this first configuration. In a second configuration, the ECU  120  is primarily responsible for voltage regulation. In the second configuration, the ECU  120  senses the voltage V bat  and sends a message to the alternator controller  210  commanding the alternator controller  210  to set a specific excitation current (EXC) for the rotor  230 . 
         [0025]    The alternator  140  also includes a safety switch  150 , which may be used to disconnect the source voltage, e.g., V bat , from the circuitry driving the excitation current to the rotor  230 . The safety switch  150  will be described in more detail below as part of a fault (e.g., overvoltage) protection circuit. 
         [0026]      FIG. 2  shows the alternator controller  210  and the safety switch  150  disposed on a brush holder  200 . While this is a convenient and preferred place for including these elements, it is understood that the alternator controller  210  and the safety switch  150  could be physically located elsewhere, including other locations within the alternator  140  or even outside of the alternator  140 . 
       Fault Protection Circuit Embodiment with Supervision Circuit in ECU 
       [0027]      FIG. 3  illustrates an embodiment of a fault protection circuit  300  that includes the safety switch  150  for interrupting the excitation current that drives the rotor  230 . In more detail, an excitation current controller  310  provides an excitation current signal to an excitation current switch  320 . This signal may be, for example, a pulse width modulated (PWM) waveform as shown in  FIG. 3 , wherein the duty cycle is configured to provide the desired excitation current. As explained previously, the excitation current (EXC) output from the alternator controller  210  is adjusted to maintain a desired set voltage, V set , at the alternator output. The voltage regulation may be performed in the ECU  120 , the alternator controller  210 , or elsewhere in the fault protection circuit  300 . In other words, any of these devices may determine the appropriate excitation current to maintain the desired set voltage, V set , at the output of alternator  140 . 
         [0028]    The excitation current switch  320  may be implemented using many different technologies, but is typically implemented using a power metal-oxide semiconductor field-effect transistor (MOSFET). In one embodiment, the excitation current switch  320  is a double-diffused MOSFET (DMOS). In another embodiment, the excitation current switch  320  is a laterally diffused MOSFET (LDMOS). If the excitation current switch  320  is any type of N-channel MOSFET, the drain terminal of the switch  320  is connected to a voltage supply, the source terminal provides the excitation current (EXC) and is electrically connected to the rotor  230 , and the gate terminal is connected to the excitation current controller  310 . As an alternative to a power MOSFET, the excitation current switch  320  could instead be an insulated-gate bipolar transistor (IGBT) or a high-electron-mobility transistor (HEMT), also known as a heterostructure FET (HFET) or a modulation-doped FET (MODFET). Still other switch types are possible, provided they are capable of providing an adequate excitation current (EXC) for the rotor  230 . The rotor  230  typically requires a maximum excitation current in the range of 5-10 amperes in order to meet the output power requirements of the alternator  140 . 
         [0029]    The alternator controller  210  further includes a communication interface  330 , configured primarily for communicating with the ECU  120 . Communication between the ECU  120  and the communication interface  330  may take place over a serial bus, such as a Local Interconnect Network (LIN) or a Controller Area Network (CAN) bus. The ECU  120  may provide, among other things, commands informing the alternator controller  210  what desired set voltage V set  to use for voltage regulation, or what level of excitation current (EXC) to provide to the rotor  230 . 
         [0030]    The fault protection circuit  300  also includes the safety switch  150  interposed between the excitation current switch  320  and a supply voltage (e.g., V bat ). When the safety switch  150  receives a signal (ACT) indicating a fault such as an overvoltage condition, the safety switch  150  opens thereby disconnecting the supply voltage from the excitation current switch  320 . For faults occurring downstream from the safety switch  150  (e.g., a short  360  between the input and output of the excitation current switch  320 , a short on the control input to the excitation current switch  320 , or a fault in the signal output from the excitation current controller  310 ), disconnecting the supply voltage in this manner serves to mediate the fault. Stated alternatively, the safety switch  150  prevents any prolonged “full-field” condition in the alternator  140 , as caused by a continuous excitation current being provided to the rotor  230 . The safety switch  150  thus prevents excessive voltage in the electrical system  100  from damaging the electrical devices (such as those as associated the vehicle loads  110 ) or the battery  130 . 
         [0031]    Note that the short  360 , as shown in  FIG. 3 , may be a high-impedance short that does not lead to a significant overvoltage situation, as described previously. For example, a highly resistive short of  100  kohms or more might not produce an alternator output voltage that is detected as being excessive. However, such a fault has the potential detrimental consequence that the short  360  could discharge the battery  130  while the vehicle engine rotating the alternator  140  is not running. A fault such as this could also be mitigated using the fault protection circuit  300  described previously. 
         [0032]    As with the excitation current switch  320 , the safety switch  150  may be implemented using many different technologies. The safety switch  150  is preferably implemented using a power metal-oxide semiconductor field-effect transistor (MOSFET), such as a double-diffused MOSFET (DMOS) or laterally diffused MOSFET (LDMOS). If the safety switch  150  is any type of N-channel MOSFET, the drain terminal of the switch  150  is connected to a voltage supply (e.g., V bat ), the source terminal is connected to an input of the excitation current switch  320 , and the gate terminal is connected a control signal (ACT) used for opening the safety switch  150  when a fault condition is detected. Conversely, if the safety switch  150  is a P-channel MOSFET, its source terminal will be connected to the voltage supply and its drain will be connected to the input of the excitation current switch  320 . As an alternative to a power MOSFET, the safety switch  150  could instead be an insulated-gate bipolar transistor (IGBT) or a high-electron-mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET). Still other switch types are possible, provided they are capable of providing an adequate excitation current. 
         [0033]    The current requirement for the safety switch  150  is similar to that of the excitation current switch  320 , since the excitation current for the rotor  230  flows through both of the switches. (As explained previously, the rotor  230  typically requires a maximum current in the range of 5-10 amperes). 
         [0034]    In the embodiment illustrated in  FIG. 3 , the fault condition is detected by a supervision circuit  350 , which is located outside of the alternator controller  210 . Specifically, the supervision circuit  350  is located in the ECU  120 . This configuration is preferred for an arrangement in which the ECU  120  provides the voltage regulation. For such an arrangement, the ECU  120  is already monitoring the alternator output voltage (e.g., V bat ), to maintain an alternator output voltage near the desired set voltage V set . If the alternator output voltage exceeds some threshold V thresh  deemed excessive (e.g., 15V), the supervision circuit  350  determines that a fault has occurred which can lead to an overvoltage condition. The ECU  120  then signals the protection circuit  300  that the safety switch  150  needs to disconnect the power supply from the excitation current switch  320 . 
         [0035]    Two alternative sub-embodiments for controlling the safety switch  150  are illustrated in  FIG. 3  using dashed lines. In a first sub-embodiment, the ECU  120  signals the communication interface  330  that the safety switch  150  should disconnect the supply voltage from the excitation current switch  320 . The alternator controller  210  then directs the safety switch  150  to open, thereby disconnecting the supply voltage (e.g., V bat ) from the excitation current switch  320 . 
         [0036]    In a second sub-embodiment, a communication interface  340  is provided in the fault protection circuit  300 , wherein the communication interface  340  is not within the alternator controller  210 . For example, the communication circuit  340  may be dedicated to controlling the safety switch  150 . The communication circuit  340  may be disposed on the same semiconductor die (chip) as the safety switch  150 . In the second sub-embodiment, the ECU  120  may signal the communication circuit  340  that the safety switch  150  needs to open using a serial bus, such as that described previously regarding the serial interface  330 , or using a dedicated signal line that is separate from any serial bus used by the ECU  120  or the communication interface  330 . 
         [0037]    For either of the sub-embodiments described above, in order to prevent the safety switch  150  from being affected by the same fault as that causing short  360  across the excitation current switch  320 , it may be desirable to physically separate the safety switch  150  from the alternator controller  210 . For example, the safety switch  150  should be disposed on a different die than the alternator controller  210 . 
       Fault Protection Circuit Embodiment with Supervision Circuit in Alternator Controller 
       [0038]      FIG. 4  illustrates an alternative embodiment of a fault protection circuit  300 . In this alternative embodiment, the supervision circuit  350  is part of the fault protection circuit  300 , rather than being located in the ECU  120 . Such a configuration may be preferred for arrangements in which alternator voltage regulation is performed by the alternator controller  210 . For such arrangements, the alternator controller  210  is monitoring the alternator output voltage (e.g., V bat ) already. As previously described herein, if the supervision circuit  350  detects that the alternator output voltage exceeds some threshold deemed excessive (e.g., 15V), the supervision circuit  350  determines that a fault has occurred which can lead to an overvoltage condition. The supervision circuit  350  then commands the safety switch  150  to disconnect the supply voltage from the excitation current switch  320 . 
         [0039]    As shown in  FIG. 4 , the supervision circuit  350  is located within the alternator controller  210 , and could even be disposed on the same semiconductor die as the alternator controller  210 . A sub-embodiment with this configuration has the advantage that circuit components used for the voltage regulation could be reused, e.g., the analog components associated with sensing the alternator output voltage (e.g., V bat ) may also be used to detect when the alternator output voltage is above a predetermined threshold. However, as indicated by the dotted box, the supervision circuit  350  could be disposed elsewhere. 
         [0040]    In a second sub-embodiment (not shown), the supervision circuit is located outside of the alternator controller  210 . For example, the supervision circuit  350  and the safety switch  150  may be grouped together as a safety circuit. These two components may even be disposed on the same semiconductor die, wherein this die is different from that of the alternator controller  210 . This second sub-embodiment has the advantage that a safety circuit including the supervision circuit  350  and the safety switch  150  could be included in the fault protection circuit  300 , without any design changes to an existing alternator controller  210 . 
         [0041]    For either of the above-described sub-embodiments, locating the supervision circuit  350  in the fault protection circuit  300  has the advantage that such an implementation requires no changes to an existing ECU  120 . The changes required to implement the safety features of the fault protection circuit  300  could be implemented entirely within the alternator  140 , meaning that a new alternator design could be included in the electrical system  100  without redesigning the ECU  120  or other components of the electrical system  100 . Similarly, a replacement alternator having these safety features could be installed in an existing electrical system  100 . 
         [0042]      FIG. 5  illustrates a brush holder  500  comprising a housing  510 , the safety switch  150 , and the alternator controller  210 . The brush holder  500  is configured to retain a brush that conducts an excitation current to the rotor  230  of the alternator  140 , as shown in  FIG. 2 . The housing  510  is composed of a suitable material such as plastic, ceramic, or a graphite-based material. The safety switch  150  and the alternator controller  210  may be disposed in an open recess in the housing, embedded within the housing, or attached to an external or internal surface of the housing, e.g., using an adhesive, encapsulation. The safety switch  150  is configured to disconnect a supply voltage from an excitation current switch located within the alternator controller  210  as previously described herein. The brush holder  500  is configured to comprise the first or second embodiments of the fault protection circuit  300  described previously. 
         [0043]      FIG. 6  illustrates a method for preventing a prolonged period in which an alternator is providing excessive voltage at its output. This method may be implemented, e.g., in a fault protection circuit  300  as shown in  FIG. 4 , or in a combination of a fault protection circuit  300  and an ECU as shown in  FIG. 3 . The method begins at step  610  wherein an excitation current is provided to a rotor of an alternator. The excitation current is provided using an excitation current switch that controls the amount of current provided to the rotor. In a step  620 , the alternator output voltage, V alt , is measured. (This voltage may be, for example, the same as a battery voltage V bat .) The voltage V alt  is then compared, in step  630 , against a threshold, V thresh , to determine if the alternator output voltage is excessive. If the alternator output voltage V alt  is greater than the threshold voltage V thresh , then a safety switch is opened at step  640  thereby disconnecting the supply voltage to the excitation current switch. If the alternator output voltage V alt  is not above the threshold voltage V thresh , i.e., the voltage is in a safe range, normal operation continues at step  610 . 
         [0044]    As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
         [0045]    It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise. 
         [0046]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.