Abstract:
An inverter system for a vehicle comprising a housing, a primary stage, a secondary stage and a fault detection circuit is provided. The primary stage is configured to receive a first voltage signal from an energy power source to generate a second voltage signal. The secondary stage is configured to generate a third voltage signal in response to the second voltage signal. At least one of the primary and the secondary stages define at least one resistance point for discharging leakage current responsive to generating the third voltage signal. The fault detection circuit is configured to electrically couple the primary stage and the secondary stage to provide the second voltage signal to the secondary stage and to measure a portion of the third voltage signal to determine whether the leakage current being discharged through the at least one resistance point is within a predetermined current range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61/021,472 filed Jan. 16, 2008 which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    One or more embodiments of the present invention generally relate to a system and method for detecting a fault isolation and leakage current for an inverter circuit in a vehicle. 
         [0004]    2. Background Art 
         [0005]    It is known that in order to charge or use electrical devices that are not part of the vehicle (such as, but not limited to, cell phones, laptops, or vacuum cleaners) various aftermarket adapters are needed to be purchased so that such adapters can be plugged into a power outlet of the vehicle and into the electrical device to charge or use the electrical device. To charge and/or use such an electrical device in a vehicle, an aftermarket vehicle adapter is needed that includes a cable and a connector generally shaped in the form of a cylindrical connector to mate with a power outlet (e.g., cigar lighting receptacle) in the vehicle. The connector includes a retractable conductive pin that makes contact with a mating terminal positioned within the power outlet of the vehicle to enable power transmission therebetween. The adapter may include additional circuitry (e.g., inverter circuit) for converting the DC power to AC power so that the electrical device can operate or store power provided by the vehicle. 
         [0006]    Original equipment manufacturers (OEMS) are attempting to obviate the need for vehicle occupants to have to purchase the aftermarket vehicle electrical adapter as described above. For example, OEMs are implementing a female prong connector within the vehicle that is capable of receiving a male prong connector in a similar manner to that invoked when connecting an electrical device to an electrical wall outlet of a home, a building, or other suitable establishment. OEMs are consistently on guard for the need to provide a safe connection for users that may come into contact with the female prong connector or other componentry that is utilized to provide for DC to AC conversion in the vehicle. 
       SUMMARY 
       [0007]    An inverter system for a vehicle comprising a housing, a primary stage, a secondary stage and a fault detection circuit is provided. The primary stage is configured to receive a first voltage signal from an energy power source to generate a second voltage signal. The secondary stage is configured to generate a third voltage signal in response to the second voltage signal. At least one of the primary and the secondary stages define at least one resistance point for discharging leakage current responsive to generating the third voltage signal. The fault detection circuit is configured to electrically couple the primary stage and the secondary stage to provide the second voltage signal to the secondary stage and to measure a portion of the third voltage signal to determine whether the leakage current being discharged through the at least one resistance point is within a predetermined current range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The embodiments of the present invention are pointed out with particularity in the appended claims. However, other features of the various embodiments will become more apparent and will be best understood by referring to the following detailed description in conjunction with the accompany drawings in which: 
           [0009]      FIG. 1  depicts an inverter system for use in a vehicle; 
           [0010]      FIG. 2  depicts various internal resistance points of the inverter system; 
           [0011]      FIG. 3  depicts an inverter system in accordance to one embodiment of the present invention; 
           [0012]      FIG. 4  is a plot depicting a waveform which corresponds to non-isolation fault condition; and 
           [0013]      FIG. 5  is a plot depicting a waveform which corresponds to one isolation fault condition. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale, some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for the claims and/or as a representative basis for teaching one skilled in the art to variously employ the one or more embodiments of the present invention. 
         [0015]    In order to provide for DC to AC conversion in a vehicle so that an electrical device can operate from the high AC based voltage output, various safety measures can be employed to protect users that come into contact with a prong in a connector that is associated with providing the high AC based voltage to the electrical device. In a first measure, any components generally associated with the high AC based voltage (including the connector and the one or more of the prongs) are adequately isolated from the rest of the electrical network of the vehicle (e.g., ground connection to earth via a chassis connection through tires of the vehicle). Such isolation is adequate so long as various isolation resistance points about the DC to AC conversion system (or inverter system) are high. 
         [0016]    A second measure requires that in the event a failure exists with respect to isolating the high AC voltage component from the rest of the vehicle network, such a high AC voltage is disconnected to prevent the risk of shocking the user. One or more embodiments of the present invention are directed toward protecting a user that may come into contact with the prong of the connector that is utilized to transfer the high AC voltage from the vehicle to the electrical device. 
         [0017]      FIG. 1  generally illustrates an inverter system  10  for use in a vehicle. The system  10  comprises an energy source  12  and an inverter module  14 . The inverter module  14  generally includes a first connector  16  having prongs  18   a - 18   n  therein for receiving a second connector (not shown). A user  20  may mate the second connector to the first connector  16 . In general, the inverter module  14  is configured to receive a DC voltage from the energy source  12  and to convert the DC voltage into an AC voltage output. The second connector is coupled to an electrical device. The electrical device may comprise a cell phone, laptop computer, vacuum cleaner, or other suitable device that requires A/C electrical power to operate. 
         [0018]    The AC voltage output may be any one of, but not limited to, 100 Vac/50 Hz, 110 Vac/60 Hz, 200 Vac/60 Hz, 220 Vac/60 Hz, or 230 Vac/50 Hz depending on the country in which the vehicle will be used. In one example, the energy source  12  may be a vehicle battery that generates 12 Vdc. In another example, the energy source  12  may be a DC supply such as a voltage DC/DC stabilizer or converter that converts a high DC-based voltage to a DC voltage level suitable for input to the inverter module  14 . The energy source  12  is recognized to be any such device capable of providing a suitable input to the inverter module  14 . 
         [0019]    The inverter module  14  includes a primary stage  24  and a secondary stage  26 . The inverter module  14  further includes a transformer  22  having a primary coil  22   a  and a secondary coil  22   b.  The primary stage  24  and the secondary stage  26  coact with each other to convert the DC voltage input into the AC voltage output. For example, the primary stage  24  may include a DC/AC converter (not shown) to convert the DC input voltage into a low AC voltage having a high frequency component. The secondary stage  26  converts the low AC voltage back into a high DC voltage (e.g., 300 V or other suitable voltage). The high DC voltage is input to a switching element that includes MOSFET(s), IGBT(s) or other suitable power devices to generate the desired AC voltage output. The above description of converting the DC voltage into the AC voltage output is known in the art and will not be described further. The secondary stage  26  is generally isolated (via various isolation elements) from the primary stage  24  due to the high voltage characteristics of the secondary stage  26 . The isolation elements include, but not limited to, separation mechanisms in printed circuit board (PCB) to separate tracks in primary and secondary stages  24 ,  26 , galvanic isolation in the transformer  22 , optocouplers, or other suitable devices. The primary stage  24  may include the primary coil  22   a  of the transformer  22  and the secondary stage  26  may include the secondary coil  22   b  of the transformer  22 . 
         [0020]    The inverter module  14  includes a housing  25  that is constructed of metal (other electrically conductive materials are contemplated). The primary stage  24  and the secondary stage  26  are generally positioned within the inverter module  14 . The first connector  16  may be positioned in a center stack area of an instrument panel in the vehicle. An insulated wiring cable may be coupled to the first connector  16  and the inverter module  14  to enable electrical communication therebetween. 
         [0021]    The housing  25  may be situated so that contact is made with a surface in the vehicle that is sufficient to establish a suitable ground to earth. In one example, the housing  25  may be coupled to a vehicle chassis (e.g., see chassis connection  28 ). The chassis connection  28  is coupled to earth via wheels (not shown). The wheels may include a small resistance as represented by R wh . 
         [0022]    A connection  27  is made between the housing  25  and a negative feed of the energy source  12 . As noted above, the secondary stage  26  is isolated from the primary stage  24 . Such isolation generally refers to the condition whereby minimal leakage current flows between the primary stage  24  and the secondary stage  26  while the inverter module  14  converts the DC voltage input into the AC voltage output. By isolating the secondary stage  26  from the primary stage  24 , such a condition also isolates the secondary stage  26  from the connection  27  of the negative feed, and the chassis connection  28 . If the secondary stage  26  is not properly isolated from the primary stage  24 , then a large amount of leakage current may flow between the primary stage  24  and the secondary stage  26  if the circuit is closed. Such a condition may harm the user  20  in the event the user  20  contacts the negative prong of the first connector  16 . 
         [0023]      FIG. 2  depicts various internal resistance points (RP 1 , RP 2 , RP 3 , and RP 4 ) that may be present within the inverter system  10 . The ohmic values of the various internal resistance points RP 1 , RP 2 , RP 3  and RP 4  determine the amount of leakage current that flows between the primary stage  24  and the secondary stage  26 . It is to be noted that internal resistance points RP 1 -RP 4  are not to be construed as actual resistors that are implemented within the inverter module  14  for the purpose of converting the DC voltage input into the AC voltage output. Such internal resistance points RP 1 -RP 4  represent locations that may exhibit ohmic values between the primary stage  24 , the secondary stage  26 , and/or the housing  25 . 
         [0024]    The resistance values of the internal resistance points RP 1 -RP 4  under normal operating conditions are high which indicates that the secondary stage  26  is isolated from the primary stage  24 . RP 1  may correspond to an internal resistance between the positive side of the primary coil  22   a  and the positive side of the secondary coil  22   b  of the transformer  22 . RP 2  may correspond to an internal resistance between a negative side of the primary coil  22   a  and a negative side of the secondary coil  22   b.  RP 3  may correspond to an internal resistance between the positive side of the secondary coil RP 3  and the ground (e.g., the metallic housing  25  that is coupled to the chassis connection  28 ). RP 4  may correspond to an internal resistance between the negative side of the secondary coil RP 4  and the ground (e.g., the metallic housing  25  that is coupled to the chassis connection  28 ). 
         [0025]    In the event one or more of the resistance points RP 1 -RP 4  exhibit a low ohmic condition, such a condition may correspond to the secondary stage  26  not being isolated from the primary stage  24 . In such a case, an isolation fault is considered to exist and the leakage current that is passed from the secondary stage  26  to the primary stage  24  is high in the event the first connector  16  is mated with the second connector. Such an isolation fault may be caused due to one or more issues within the electronics in the primary or secondary stage  26 . A low ohmic condition exhibited by any one of the resistance points may correspond to an isolation fault. It is generally recognized that leakage current is passed from the secondary stage  26  to the primary stage  24  (and through the resistance points RP 1 -RP 4 ) while the high AC voltage output is delivered to the electrical device. When an isolation fault is not present, any such leakage current discharged through the resistance points RP 1 -RP 4  is considered to be negligible. 
         [0026]      FIG. 3  depicts an inverter system  50  in accordance to one embodiment of the present invention. The system  50  includes a fault detection circuit  51 . The fault detection circuit  51  is generally configured to determine the amount of leakage current that flows between the secondary stage  26  and the primary stage  24 . The fault detection circuit  51  closes the circuit between the primary stage  24  and the secondary stage  26  to determine whether the leakage current is within a predetermined current range. In one example, the predetermined current range may correspond to a current value that is less than 5 mA. The particular current value(s) used to establish the predetermined current range may vary based on the desired criteria of a particular implementation. The fault detection circuit  51  includes a switching device  52 , a microcontroller  54 , and a voltage divider network (e.g., resistor R 1  and R 2 ). The microcontroller  54  measures the voltage across the resistor R 1  and/or R 2  to determine the amount of leakage current that flows from the secondary stage  26  to the primary stage  24 . 
         [0027]    In operation, the microcontroller  54  is configured to control the switching device  52  (e.g., switch, relay, transistor, or other suitable mechanism) to close for a predetermined amount of time so that the microcontroller  54  measures the voltage across resistor R 2  (or resistor R 1  or both resistor R 1  and R 2 ) to determine if the measured voltage is within a predetermined voltage range. If the measured voltage across resistor R 2  is within the predetermined voltage range, the microcontroller  54  determines that the there are no isolation faults present at one or more of resistance points RP 1 -RP 4 . As such, any such leakage current flowing through the inverter module  14  is generally considered to be negligible (or with the predetermined current range) and may not shock the user  20 . If the measured voltage is not within the predetermined voltage range, then the microcontroller  54  determines that there is at least one isolation fault present and that the leakage current exceeds the predetermined current range. In this case, the microcontroller  54  may shut the inverter module  14  down and cease to convert the DC voltage input into the AC voltage output to remove the potential for the high leakage current to come into contact with the user  20 . 
         [0028]    The microcontroller  54  may control an LED or other suitable mechanism to warn the user in response to detecting an isolation fault. In one example, the microcontroller  24  may be electrically coupled to other controllers about the vehicle via a data communication bus. Such a data communication bus may be implemented as, but not limited to, control area network (CAN), local interconnect network (LIN) or other recognized alternate. The microcontroller  54  may transmit a message over the bus to the other controllers so that the controller notifies the user of the isolation fault (e.g., cluster lighting telltale to warn the user). 
         [0029]    The microcontroller  54  may control the switching device  52  to close and measure the voltage across the resistor R 2  after vehicle engine start-up thereby detecting the presence of a high leakage current condition (or isolation fault) (e.g., one or more of the ohmic values of the resistance points RP 1 -RP 4  is low). The microcontroller  54  shuts the inverter module  14  down in response to detecting the isolation fault before the user may experience a shock condition. It is generally contemplated that the microcontroller  54  may also control the switching device  52  to close and measure the voltage periodically at predefined intervals (e.g., every 10 seconds or suitable time frame) after engine startup. 
         [0030]    In one example, the microcontroller  54  may calculate Vrms across the resistor R 2  with the measured voltage across the resistor R 2 . Vrms may correspond to an AC signal having a 5 V peak-to-peak with a DC offset of 2.5V. The microcontroller  54  may measure the voltage across the resistor R 2  for a period of 20 ms for an A/C output voltage at 50 Hz and a period of 16.6 ms for an A/C output voltage at 60 Hz. After the microcontroller  54  determines the Vrms for the measured voltage, the microcontroller  54  compares the Vrms against the predetermined voltage range. The voltage values in the predetermined voltage range may be in a root mean square format. It is recognized that the time period used by the microcontroller  54  to measure the voltage may include other values than those noted above. 
         [0031]    In one example, the predetermined voltage range may correspond to a range of 0.78 Vrms and 1 Vrms. Meaning, that in the event Vrms for the measured voltage corresponds to a value between 0.78 Vrms and 1 Vrms, such a condition may indicate that there are no isolation faults present and that the leakage current is within the predetermined current range. Such a condition may also indicate that the total resistance of all of the resistance points RP 1 -RP 4  about the inverter module  14  is greater than or equal to 120 Kohms. If, on the other hand, the Vrms for the measured voltage is either less than 0.78 Vrms or greater than 1 Vrms, such a condition may indicate that there is an isolation fault present between the secondary stage  26  and the primary stage  24  and that the leakage current may be outside of the predetermined current range. The microcontroller  54  may shut down the operation of the inverter module  14  in the event such a condition was present. The particular values selected to establish the predetermined voltage range may vary based on the desired criteria of a given implementation. 
         [0032]    The number of resistors which form the voltage divider network in the system  50  may vary and are not intended to be limited to those shown in  FIG. 3 . The voltage divider network (e.g., resistor R 1  and R 2 ) may reduce voltage so that an analog/digital (A/D) converter (not shown) within the microcontroller  54  can read the measured voltage across the resistor R 1  and/or R 2 . It is contemplated that the microcontroller  54  may not include the A/D converter to read the voltage across the resistors R 1  and R 2  and that other suitable methods for reading the voltage may be employed. 
         [0033]    As noted above, the secondary stage  26  receives the low DC voltage from the primary stage  24 . The secondary stage  26  converts the low AC voltage back into a high DC voltage and presents the high DC voltage to power switching element(s) to generate the desired AC voltage output. In another embodiment, the switching device  52  may be coupled to the various DC stages within the secondary stage  26  so that DC-based voltages may be measured across the resistor R 1  and/or R 2  to obtain a DC-based measurement as opposed to the to the root-mean square voltage format as described above. With respect to the root-mean square voltage format as described above, the resistors R 1  and R 2  are generally coupled to various devices within the secondary stage  26  that are associated with the AC voltage component. 
         [0034]      FIG. 4  is a plot  60  depicting a measured waveform  62  which corresponds to non-isolation fault condition in accordance to one example of the present invention. Waveform  62  generally corresponds to the measured voltage across a single resistor (e.g., resistors R 1  or R 2 ). The waveform  62  is shaped in the form of a sinusoidal wave and that such a sinusoidal wave generally indicates that the various internal resistance points RP 1 -RP 4  exhibit a high resistance value (e.g., a measured voltage reading that is within the predetermined voltage range) thereby indicating that the leakage current is within the predetermined current range (e.g., less than 5 mn or other suitable current value). 
         [0035]      FIG. 5  is a plot depicting a measured waveform  72  which corresponds to an isolation fault condition. Waveform  72  is an example of the measured voltage that may be an indicative of a risk of high leakage current (e.g., measured voltage not within predetermined voltage range). In such case, one or more of the internal resistor points RP 1 -RP 4  may exhibit a low resistance state. It is generally contemplated that other such electronics such as capacitor(s), diode(s) and/or inductor(s) included within the inverter module  14  may be combined with the resistors R 1  and/or R 2  to provide an alternate detection circuit to create a signature waveform that corresponds to a high leakage current condition. 
         [0036]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.