Abstract:
A method of controlling a slew rate of a MOSFET connected to a battery for supplying an electrical current to an electrical load is provided. A state transition of the MOSFET is provided and a drain-source voltage of the MOSFET is monitored. A variable current is provided through a gate of the MOSFET. A constant current is provided through the gate of the MOSFET, when the drain-source voltage of the MOSFET satisfies a predefined condition that is a function of a battery voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Great Britain Patent Application No. 1507120.0, filed Apr. 23, 2015, which is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure pertains to a method of controlling the slew rate of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and an apparatus thereof used, in particular, to supply and control electrical loads having low resistance such as glow plugs for Diesel engines. 
       BACKGROUND 
       [0003]    It is known that a MOSFET can be used to control pulse width modulation (PWM) processes where the pulse width changes are modulated based upon input provided by the MOSFET. It is also known that Diesel engines are provided with ceramic or metallic glow plugs for allowing cold-start of the engine and for optimizing combustion performance during engine operation. Glow plugs are located in a combustion chamber of the engine and are electrically connected to a voltage power source, for example a battery of the vehicle, by means of an electric switch, for example a MOSFET, controlled by means of PWM processes and driven by an electronic control unit (ECU). 
         [0004]    One of the parameters that define the performance of MOSFET transistors is the “slew rate” of the control signals that are used to control the MOSFET. The slew rate refers to the maximum voltage change allowed per unit time. The slew rate has to be controlled when a MOSFET is used to supply an external load that has a low electrical resistance and therefore absorbs high current, such as in the case of glow plugs, otherwise the fast commutation required by pulse width modulation (PWM) processes generates radiated or conducted noise that may damage the electronic components of the circuit in violation of the Electromagnetic Compatibility (EMC) requirement. 
         [0005]    To address this problem, it is known to use an external series resistor and an external capacitor between gate and drain of the MOSFET. A drawback of this solution, however, is that such components have to be tuned on the specific MOSFET used and its performance changes according to battery voltage. 
         [0006]    Another known solution is to impose a constant current during all the ON-OFF and Off-ON transitions. However, this solution cannot be used for MOSFET with high gate charge, because the propagation delay becomes too long. 
       SUMMARY 
       [0007]    In accordance with the present disclosure, an embodiment is provided, which improves EMC performances and, at the same time, provide a solution that is suitable for a wide set of MOSFETs, also considering that such MOSFETs may be used to control components with a high gate charge. An embodiment of the disclosure provides a method controlling a slew rate of a MOSFET, the MOSFET being connected to a battery for supplying an electrical current to an electrical load. In the method, a state transition of the MOSFET is provided and a drain-source voltage of the MOSFET is monitored. A variable current through a gate of the MOSFET is provided, and a constant current is provided through the gate of the MOSFET, when the drain-source voltage of the MOSFET satisfies a predefined condition that is a function of a battery voltage. An advantage of this embodiment is that it improves EMC performances because its performance is ratiometric with respect to the battery voltage in case of an OFF-ON transition of the MOSFET and also in case of an ON-OFF transition of the MOSFET. 
         [0008]    This embodiment is ratiometric with battery voltage supply because the transition is decided by monitoring the drain-source MOSFET voltage and comparing it with a threshold generated from battery voltage itself. Moreover this solution does not depend on the particular type of MOSFET employed, but on the contrary it is suitable for a wide set of MOSFET&#39;s, also considering components with a high gate charge. 
         [0009]    According to another embodiment of the present disclosure, in an OFF-ON transition of the MOSFET, the predefined condition to be satisfied is that the drain-source voltage of the MOSFET is lower than a first threshold voltage that is a function of the battery voltage. An advantage of this embodiment is that it properly controls the slew rate of the MOSFET in an OFF-ON transition of the MOSFET. 
         [0010]    According to still another embodiment of the present disclosure, the first threshold voltage is equal to VBAT/4 or in words one-fourth of the battery voltage. An advantage of this embodiment is that the transition between the steps of the method is decided by a parameter that is proportional to the battery voltage, allowing this solution to be ratiometric with battery voltage supply. 
         [0011]    According to another embodiment of the present disclosure, the constant current through the gate of the MOSFET is provided, in an OFF-ON transition of the MOSFET, by employing a first current generator connected to the gate of the MOSFET. An advantage of this embodiment is that the first current generator can act as a current mirror to provide a predefined constant current. 
         [0012]    According to another embodiment of the present disclosure, the variable current through the gate of the MOSFET is provided by a first switch bypassing, in a closed position, the first current generator. An advantage of this embodiment is that the maximum current value is limited by a resistor of a pre-driver circuit. 
         [0013]    According to still another embodiment of the present disclosure, in an ON-OFF transition of the MOSFET, the predefined condition to be satisfied is that the drain-source voltage of the MOSFET is greater than a second threshold voltage that is a function of the battery voltage. An advantage of this embodiment is that it properly controls the slew rate of the MOSFET in an ON-OFF transition of the MOSFET. 
         [0014]    According to a further embodiment of the present disclosure, the second threshold voltage is equal to VBAT/6 or in words one-sixth of the battery voltage. An advantage of this embodiment is that the transition between the steps of the method is decided by a parameter that is proportional to the battery voltage, allowing this solution to be ratiometric with battery voltage supply. 
         [0015]    According to still another embodiment of the present disclosure, the constant current through the gate of the MOSFET is provided, in an ON-OFF transition of the MOSFET, by employing a second current generator connected to the gate of the MOSFET. An advantage of this embodiment is that the second current generator can act as a current mirror absorbing a predefined constant current. 
         [0016]    According to a further embodiment of the present disclosure, the variable current through the gate of the MOSFET is provided by a second switch bypassing, in a closed position, the second current generator. An advantage of this embodiment is that the maximum current value is limited by a resistor of a pre-driver circuit. 
         [0017]    The method may be executed with the aid of a computer program including a program-code for carrying out the method described above, and in the form of a computer program product including the computer program. The method can be also embodied as electromagnetic signals, the signal being modulated to carry a sequence of data bits which represent a computer program to carry out all steps of the method. 
         [0018]    According to another aspect of the present disclosure, an apparatus is provided for controlling a slew rate of a MOSFET connected to a battery for supplying an electrical current to an electrical load. The apparatus includes a sensor circuit configured to sense a state transition of the MOSFET and a monitoring circuit configured to monitor a drain-source voltage of the MOSFET. A pre-driver circuit provides a variable current through a gate of the MOSFET and a constant current through the gate of the MOSFET, when the drain-source voltage of the MOSFET satisfies a predefined condition that is a function of a battery voltage. An advantage of this aspect is that it improves EMC performances because its performance is ratiometric with respect to the battery voltage. The performance of the apparatus is ratiometric with battery voltage supply because the transition between is decided by monitoring the drain-source MOSFET voltage and comparing it with a threshold generated from battery voltage itself. Moreover this solution works properly for a wide set of MOSFET&#39;s, also considering components with a high gate charge. The circuits needed for the apparatus can be easily integrated in an ASIC solution with a limited silicon space occupation. 
         [0019]    According to another aspect of the present disclosure, the pre-driver circuit includes a first current generator, arranged between a feeding node and a first intermediate node. An advantage of this aspect that the first current generator can act as a current mirror to provide a predefined constant current. 
         [0020]    According to another aspect of the present disclosure, the pre-driver circuit includes a first switch bypassing, in a closed position, the first current generator. An advantage of this aspect is that the maximum current value is limited by a resistor of a pre-driver circuit. 
         [0021]    According to a further aspect of the present disclosure, the pre-driver circuit further includes a second current generator, arranged between a second intermediate node and a ground node. An advantage of this aspect is that the second current generator can act as a current mirror absorbing a predefined constant current. 
         [0022]    According to still another aspect of the present disclosure, the pre-driver circuit includes a second switch bypassing, in a closed position, the second current generator. An advantage of this aspect is that the maximum current value is limited by a resistor of a pre-driver circuit. 
         [0023]    According to still another aspect of the present disclosure, the electrical load is a glow plug for a Diesel engine. An advantage of this aspect is that it operates a component having a low resistance and, consequently, a high current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present disclosure will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements. 
           [0025]      FIG. 1  shows an automotive system; 
           [0026]      FIG. 2  is a cross-section of an internal combustion engine belonging to the automotive system of  FIG. 1 ; 
           [0027]      FIG. 3  is a schematic of a circuit for a MOSFET used to supply an electrical load in an OFF-ON transition; 
           [0028]      FIG. 4  represents the main variables of the circuit of  FIG. 3  as a function of time in an OFT-ON transition; 
           [0029]      FIG. 5  is a schematic of a circuit for a MOSFET used to supply an electrical load in an ON-OFF transition; 
           [0030]      FIG. 6  represents the main variables of the circuit of  FIG. 5  as a function of time in an ON-OFF transition; 
           [0031]      FIG. 7  represents an OFF-ON transition flowchart, and 
           [0032]      FIG. 8  represents an ON-OFF transition flowchart. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. 
         [0034]    Some embodiments may include an automotive system  100 , as shown in  FIGS. 1 and 2 , that includes an internal combustion engine (ICE)  110  having an engine block  120  defining at least one cylinder  125  having a piston  140  coupled to rotate a crankshaft  145 . A cylinder head  130  cooperates with the piston  140  to define a combustion chamber  150 . A fuel and air mixture (not shown) is disposed in the combustion chamber  150  and ignited, resulting in hot expanding exhaust gasses causing reciprocal movement of the piston  140 . The fuel is provided by at least one fuel injector  160  and the air through at least one intake port  210 . The fuel is provided at high pressure to the fuel injector  160  from a fuel rail  170  in fluid communication with a high pressure fuel pump  180  that increases the pressure of the fuel received from a fuel source  190 . Each of the cylinders  125  has at least two valves  215 , actuated by a camshaft  135  rotating in time with the crankshaft  145 . The valves  215  selectively allow air into the combustion chamber  150  from the port  210  and alternately allow exhaust gases to exit through a port  220 . In some examples, a cam phaser  155  may selectively vary the timing between the camshaft  135  and the crankshaft  145 . 
         [0035]    In the combustion chamber  150  is located a glow plug  360  acting as a heating element which is electrically activated for cold starting of the engine and also for improving the combustion performance within the combustion chamber. The glow plug  360  is electrically connected to a voltage power source, for example a battery of the automotive system, and is controlled to have an on state and an off state. According to a possible embodiment, the on/off states of the glow plug  360  are controlled by an electronic control unit (ECU) intended to control a switch. As it will be disclosed in more detail later, according to a possible embodiment, a switch, such as a MOSFET  560 , can be provided to control the on/off states of the glow plug  360 . 
         [0036]    The air may be distributed to the air intake port(s)  210  through an intake manifold  200 . An air intake duct  205  may provide air from the ambient environment to the intake manifold  200 . In other embodiments, a throttle body  330  may be provided to regulate the flow of air into the manifold  200 . In still other embodiments, a forced air system such as a turbocharger  230 , having a compressor  240  rotationally coupled to a turbine  250 , may be provided. Rotation of the compressor  240  increases the pressure and temperature of the air in the duct  205  and manifold  200 . An intercooler  260  disposed in the duct  205  may reduce the temperature of the air. The turbine  250  rotates by receiving exhaust gases from an exhaust manifold  225  that directs exhaust gases from the exhaust ports  220  and through a series of vanes prior to expansion through the turbine  250 . The exhaust gases exit the turbine  250  and are directed into an exhaust system  270 . This example shows a variable geometry turbine (VGT) with a VGT actuator  290  arranged to move the vanes to alter the flow of the exhaust gases through the turbine  250 . In other embodiments, the turbocharger  230  may be fixed geometry and/or include a waste gate. 
         [0037]    The exhaust gases of the engine are directed into an exhaust system  270 . The exhaust system  270  may include an exhaust pipe  275  having one or more exhaust aftertreatment devices  280 . The aftertreatment devices may be any device configured to change the composition of the exhaust gases. Some examples of aftertreatment devices  280  include, but are not limited to, catalytic converters (two and three way), oxidation catalysts, lean NO x  traps, hydrocarbon adsorbers, selective catalytic reduction (SCR) systems, and particulate filters. Other embodiments may include an exhaust gas recirculation (EGR) system  300  coupled between the exhaust manifold  225  and the intake manifold  200 . The EGR system  300  may include an EGR cooler  310  to reduce the temperature of the exhaust gases in the EGR system  300 . An EGR valve  320  regulates a flow of exhaust gases in the EGR system  300 . 
         [0038]    The automotive system  100  may further include an electronic control unit (ECU)  450  in communication with one or more sensors and/or devices associated with the ICE  110  and with a memory system, or data carrier  460 , and an interface bus. The ECU  450  may receive input signals from various sensors configured to generate the signals in proportion to various physical parameters associated with the ICE  110 . The sensors include, but are not limited to, a mass airflow and temperature sensor  340 , a manifold pressure and temperature sensor  350 , a combustion pressure sensor that may be integral within the glow plugs  360 , coolant and oil temperature and level sensors  380 , a fuel rail pressure sensor  400 , a cam position sensor  410 , a crank position sensor  420 , exhaust pressure and temperature sensors  430 , an EGR temperature sensor  440 , and an accelerator pedal position sensor  445 . Furthermore, the ECU  450  may generate output signals to various control devices that are arranged to control the operation of the ICE  110 , including, but not limited to, the fuel injectors  160 , the throttle body  330 , the EGR Valve  320 , a Variable Geometry Turbine (VGT) actuator  290 , and the cam phaser  155 . Note, dashed lines are used to indicate communication between the ECU  450  and the various sensors and devices, but some are omitted for clarity. 
         [0039]    Referring now to  FIG. 3 , a schematic of a circuit for a MOSFET  560  used to supply an electrical load in an OFF-ON transition is represented. The electrical load may be, for example, a glow plug  360  for a Diesel engine. According to  FIG. 3 , glow plug  360 , a MOSFET  560  and a battery  600  are serially connected in sequence, the MOSFET  560  being part of control circuit  520  of the glow plug  360 , the glow plug  360  being also connected to a ground pole  510 . The MOSFET  560  has three terminals. These three terminals are a drain  530 , a gate  540  and a source  550 . The source  550  is connected to an input terminal of the glow plug  360 , the gate  540  is connected to the pre-driver circuit  500 , which will be better explained in the following description, and the drain  530  is connected to the battery  600 . 
         [0040]    As is known in the art, the glow plug  360  has a resistive heating coil that includes a regulating coil and a heating coil for raising temperature of the fresh air inside the combustion chamber  150 . These coils heat up the fresh air for ignition with injected diesel. The MOSFET  560  functions as an electronic switch according to a PWM process. Electrical connection between the drain  530  and the source  550  is turned ON or OFF by a voltage signal on the gate  540  which comes from the pre-driver circuit  500 . 
         [0041]    The battery  600  serves as an electric energy source for applying electrical voltage and current to the glow plug  360 . The voltage source V_GATE supplies electricity to the pre-driver circuit  500  for its operation. The pre-driver circuit  500  controls the gate  540  of the MOSFET  560  such that electric energy flow from the battery  600  can be shut off or turned on, preferably with an adjustable duty cycle. When the MOSFET  560  is closed, the glow plug  360  is turned on (on state of the glow plug), vice versa when the MOSFET  560  is open, the glow plug  360  is turned off (off state of the glow plug). 
         [0042]    The pre-driver circuit  500  further includes a first switch SW 1  and a second switch SW 2 , wherein the first switch SW 1  may be used to bypass a first current generator  610  and the second switch SW 2  may be used to bypass a second current generator  620 . More specifically, in the pre-driver circuit  500 , the first current generator  610  is arranged between a feeding node  605  and a first intermediate node  615 , wherein the first switch SW 1 , in a closed position, connects the feeding node  605  with the first intermediate node  615 , bypassing the first current generator  610 . In a similar fashion, the second current generator  620  is arranged between a second intermediate node  625  and ground node  635 , wherein the second switch SW 2 , in a closed position, connects the intermediate node  625  with the ground node  635 , bypassing the second current generator  620 . 
         [0043]    The pre-driver circuit  500  further includes a logic unit  630 , a resistor  640  and a ground terminal  650 . The pre-driver circuit  500  can be advantageously implemented on in-house developed electronic control unit (ECU) via an ASIC or a discrete component, using a small silicon area, 
         [0044]    In operation, as stated above, the MOSFET  560  is operated under a PWM process, namely by means of a series of ON-OFF pulses or, in other words, by a series of state transitions, namely OFF-ON and ON-OFF transitions.  FIG. 4  represents the main variables of the circuit of  FIG. 3  as a function of time in an OFF-ON transition and, correspondingly,  FIG. 7  represents an OFF-ON transition flowchart. According to an embodiment of the present disclosure, the OFF-ON transition is performed in the following way. 
         [0045]    A command for a transition from the OFF state of the MOSFET to the ON state is given to the logic unit  630 , as indicated with Command=1 in block  700  of the flowchart of  FIG. 7 . This command has also the effect of closing the first switch SW 1  and applying a variable current to the gate  540  of the MOSFET  560 , the current maximum value being limited only by the resistor  640  in the pre-driver circuit  500 . 
         [0046]    As soon as the drain-source MOSFET voltage reaches a value that is lower than a threshold voltage thereof (block  720 ), the first switch SW 1  is opened and the first current generator  610 , acting as a current mirror, provides a constant current to the gate  540  of the MOSFET  560  (block  730 ). The threshold voltage may be a function of the battery voltage VBAT, for example having the value of VBAT/4. 
         [0047]    The first step employing a variable current ensures a small propagation delay and the second step employing a constant current ensures a slow increase of the voltage provided to the glow plug in order to avoid EMC problems. 
         [0048]      FIG. 6  represents the main variables of the circuit of  FIG. 5  as a function of time in an ON-OFF transition and, correspondingly.  FIG. 8  represents an ON-OFF transition flowchart. According to an embodiment of the present disclosure, the ON-OFF transition is performed in the following way. 
         [0049]    A command for transition from the ON state of the MOSFET to the OFF state is given to the logic unit  630 , as indicated with Command=0 in block  800  of the flowchart of  FIG. 8 . This command has also the effect of closing the first switch SW 2 , providing a variable current to the gate  540  of the MOSFET  560 , the current maximum value being limited only by the resistor  640  in the pre-driver circuit  500 . 
         [0050]    As soon as the drain-source MOSFET voltage reaches a value that is greater than a threshold voltage thereof (block  820 ), the second switch SW 2  is opened and the second current generator  610 , acting as a current mirror, absorbs a constant current from the gate  540  of the MOSFET  560  (block  730 ). In this case, the threshold voltage may also be a function of the battery voltage VBAT, for example having the value of VBAT/6. 
         [0051]    The first step employing a variable current ensures as al propagation delay and the second step employing a constant current ensures a slow reduction of the voltage provided to the glow plug in order to avoid EMC problems. In general, in all embodiments discussed, a constant current through the gate  540  of the MOSFET  560  is provided if the drain-source voltage of the MOSFET satisfies a predefined condition that is a function of a battery voltage. 
         [0052]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.