Abstract:
An overcurrent protection circuit for a high side solenoid switch includes a primary bias circuit and a secondary bias circuit. The secondary bias circuit is operable to alter a source to gate bias voltage of the high side solenoid switch during an overcurrent.

Description:
GOVERNMENT RIGHTS 
     This invention was made with government support under Contract No. F33657-99-D-2051 awarded by the United States Air Force. The Government has certain rights in this invention. 
    
    
     BACKGROUND 
     The present disclosure is directed to high side solenoid switch controls, and particularly to an overcurrent limiting circuit for the same. 
     Solenoids for use in engine systems are typically controlled using a high side solenoid switch. The high side solenoid switch is a transistor and is controlled using both software controls and hardware controls. Operation of the solenoid is controlled by the current passed through the high side solenoid switch. In order to protect the solenoid control hardware from overcurrents, a fault sensor is included within the hardware controls of the high side solenoid switch. 
     Typical fault sensors determine that a fault is present when a predefined overcurrent threshold is exceeded for greater than a set period of time. Determining that a fault condition is present only after an overcurrent has existed for greater than the set period of time is referred to as validating the overcurrent condition. Validating an overcurrent condition prevents a power source from being disconnected from the solenoid in the case of isolated transient occurrences. As a consequence of the overcurrent validation, excess current is allowed to flow from the power source during the overcurrent validation period. 
     SUMMARY 
     A high side solenoid switch overcurrent protection circuit includes a high side switch transistor operable to connect a voltage source to a solenoid switch. A sense resistor connects the voltage source to a source node of the high side switch transistor. A bias circuit provides a source to gate bias voltage to the high side switch transistor. A secondary bias circuit alters the source to gate bias voltage during an overcurrent validation. 
     A method for limiting an overcurrent during an overcurrent validation period includes the step of altering a source to gate bias voltage of a high side solenoid switch, thereby limiting a current passing through the high side solenoid switch. 
     A high side solenoid switch overcurrent protection circuit includes a circuit input operable to receive power from a voltage source. A sense resistor connects the circuit input to a source node of a high side solenoid switch. A voltage divider has voltage divider input connected to the circuit input and an output connected to a comparator. An overcurrent transistor has a source connected to the circuit input, a drain connected to a second voltage divider and an anode of a diode, and a gate connected to an output of the sense resistor, such that a current passing through the sense resistor operates as a control current for the overcurrent transistor. The second voltage divider has an output connected to the comparator, which has an overvoltage validation output. A cathode of the diode connects to a capacitor and a secondary bias resistor such that the capacitor and the secondary bias resistor are parallel. A first bias voltage resistor connects to the output of the sense resistor and to a second bias voltage resistor in series. The second bias voltage resistor connects to a gate node of the high side solenoid switch. Each of the capacitor and the secondary bias resistor connects to a node connecting the first bias voltage resistor and the second bias voltage resistor. A bias current transistor has a collector connected to the node connecting the first bias voltage resistor and the second bias voltage resistor. A bias current resistor connects to an emitter of the bias current transistor and to ground. A drain of the high side solenoid switch connects to an output, such that the drain can be connected to a solenoid. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a solenoid control configuration with the solenoid being at least partially controlled via hardware controls. 
         FIG. 2  illustrates a schematic diagram of the hardware controls of the solenoid control configuration of  FIG. 1 . 
         FIG. 3  illustrates a more detailed schematic of the hardware controls illustrated in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example solenoid control configuration  10  with a voltage source  20  providing power to a solenoid  40 . The power flow to the solenoid  40  is controlled at least in part by a hardware control circuit  30 . The hardware control circuit  30  includes a fault detection output  32  that can be connected to a software controller such as a general controller  50 . The fault detection output  32  is a high (non-0 volt) signal when no overcurrent is present, and a low (0 volt) signal when an overcurrent is present. The fault detection output  32  is monitored by the general controller  50 . When the general controller  50  detects an overcurrent that persists for greater than a predefined time period, the general controller  50  determines that a fault is present and disconnects the voltage source  20  from the circuit, thereby preventing excessive current drawn from the voltage source  20 . 
       FIG. 2  illustrates a schematic diagram of the hardware controls  30  of the solenoid control configuration  10  of  FIG. 1 , with like numerals indicating like elements. The voltage source  20  is connected to the solenoid  40  via a sense resistor  102  and a high side solenoid switch  104 . The high side solenoid switch  104  is a transistor. Current flow through the transistor is determined by the source to gate voltage of the transistor. The source to gate voltage of the transistor is referred to as the bias voltage. The bias voltage of the high side solenoid switch is controlled via a bias circuit  110 . The bias circuit  110  is, in turn, controlled by a transistor drive control  112 . The transistor drive control  112  can be part of the general controller  50  or an independent transistor controller. 
     An overcurrent transistor  120  connects the voltage source  20  to a voltage divider  134 . The output of the voltage divider  134  provides an input to a comparator  140 . A second comparator input is provided from a second voltage divider  132 . The second voltage divider  132  receives an input directly from the voltage source  20 . Each of the voltage dividers  132 ,  134  scales the voltage by a known factor prior to passing the voltage to the comparator  140 , thereby conditioning the voltage to be utilized by the comparator  140 . 
     When an overcurrent is present, the voltage from the first voltage divider  134  exceeds the voltage from the second voltage divider  132  and the comparator  140  outputs a low (0 volt) signal. In all other cases, the comparator  140  outputs a high (non-0 volt) signal. Thus, the general controller  50  (illustrated in  FIG. 1 ) can monitor for a fault condition. If the overcurrent persists for longer than a predefined period, the general controller  50  determines that a fault is present and disconnects the voltage source  20 . 
     The overcurrent transistor  120  collector is additionally connected to a secondary bias circuit  150  through a diode  122 . The diode  122  prevents current backflow through the secondary bias circuit  150  when no overcurrent exists. The control input of the overcurrent transistor  120  is the output of the sense resistor  102 , and is configured such that the overcurrent transistor  120  transitions to the on state when current across the sense resistor  102  exceeds a threshold. Thus, the overcurrent transistor  120  remains in the off state unless an overcurrent is present. 
     When an overcurrent exists, and the overcurrent transistor  120  is on, current flows through the diode  122 , through the secondary bias circuit  150 , and into the bias circuit  110 . The current flow from the secondary bias circuit  150  affects the voltage biasing of the bias circuit  110 , thereby altering the source to gate bias voltage of the high side solenoid switch  104 . The altered bias voltage of the high side solenoid switch  104  limits the current flowing into the solenoid  40  to a value set at equal to, or slightly over, the overcurrent threshold. Specific bias voltages for limiting a particular high side solenoid switch  104  to a particular current can be determined by a person of skill in the art using known techniques. In this way, an overcurrent is maintained allowing the overcurrent validation to properly function, while at the same time current drawn from the voltage source  20  is limited, thereby preventing damage to the solenoid control hardware  30 . 
     A more detailed schematic drawing of the hardware controls of  FIG. 2  is illustrated in  FIG. 3  with like numerals indicating like elements. As can be seen in  FIG. 3 , each of the voltage dividers  132 ,  134  is a standard voltage divider constructed of two resistors  230 ,  232 ,  234 ,  236  in series, with an output node between the two resistors. The output voltage of the voltage divider  132 ,  134  is defined by the formula: Vout=Vin(R2/(R1+R2)), with R1 being resistors  230  and  234  in each respective voltage divider  132 ,  134  and R2 being resistors  232  and  236  in each respective voltage divider  132 ,  134 . Using the above formula, a person of skill in the art can determine appropriate resistor values for each voltage divider  132 ,  134  resistor  230 ,  232 ,  234 ,  236  to ensure that the output of the first voltage divider  134  exceeds the output of the second voltage divider  132  only during an overcurrent. 
     The bias circuit  110  includes two bias voltage resistors  212 ,  214  in series connecting the source and gate of the high side solenoid switch  104 , thereby controlling the bias voltage of the high side solenoid switch  104 . A transistor  216  connects a bias current resistor  218  at a node between each of the bias voltage resistors  212 ,  214 . The resistance of the bias current resistor  218  controls the total amount of current drawn through the bias circuit  110 . When no overcurrent is present, the current drawn through the bias circuit  110  is drawn from the bias voltage resistors  212 ,  214  which in turn draw the current from the voltage source  20 . Current passing through the bias voltage resistors  212 ,  214  introduces a bias voltage to the high side solenoid switch  104 . When the bias voltage is sufficiently high, the high side solenoid switch  104  allows for virtually unfettered current flow, whereas when the bias voltage is low the amount of current that can pass through the high side solenoid switch  104  is limited according to known transistor principles. 
     When the current across the sense resistor  102  exceeds a threshold, the overcurrent transistor  120  switches to the on state and allows current to pass. The new current flow path opened by the transistor  120  allows current flowing through the bias current resistor  218  to be drawn from the secondary bias circuit  150  in addition to the bias voltage resistors  212 ,  214 . The secondary bias circuit  150  illustrated in the example of  FIG. 3 , includes a capacitor  252  and a resistor  254  arranged in a parallel configuration. 
     Since the total current drawn by the bias current resistor  218  is constant, the availability of current through the newly opened current flow path reduces the current draw through the bias voltage resistors  212 ,  214 . The reduced current draw causes a corresponding decrease in the voltage drop across the bias voltage resistors  212 ,  214  and a corresponding reduction in the bias voltage of the high side solenoid switch  104 . The decreased bias voltage is sufficiently low to limit the amount of current that can pass through the high side solenoid switch  104 , thus providing a current limit during the overcurrent validation period. 
     Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of this disclosure.