Patent Publication Number: US-9891249-B2

Title: Broad-range current measurement using duty cycling

Description:
In certain circuits, it is useful to be able to accurately sense a large current through a transistor device, such as a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), over a wide current range. High accuracy sensing over a wide current range may be particularly useful in automotive applications because of the high current battery supply used therein. In order to maintain a certain level of efficiency, it is desirable to sense the current without causing substantial losses due to the sensing mechanism. 
     Most known sensing techniques involve providing a sense MOSFET coupled to the main MOSFET device to draw a small sensor current that is proportional to the main current. Generally, the sense MOSFET is smaller than the main MOSFET device by a particular ratio, which defines the proportion of current drawn by the sense MOSFET. In order to achieve accurate sensing, the ratio is typically low. However, the ratio should be high in order to achieve high efficiency. High efficiency typically means that the sense MOSFET causes very low power losses. Thus, there is a difficult design tradeoff between accuracy and efficiency in known current sensor circuits. 
     Current sense MOSFETS have typically only been used as a single point fuse at high current, or for sensing current over a very small range. Moreover, known current sense circuits fail to adequately resolve the heavy tradeoff between accuracy and efficiency. Thus, current sensing has been limited to a narrow range of measurable currents and the resolution available has been very low. 
     Apparatuses, methods and integrated circuits for broad-range current measurement using duty cycling are disclosed. Embodiments of an apparatus for sensing current through a transistor device may include an interface configured to receive a current from the transistor device for sensing. Additionally, embodiments may include a sensor component coupled to the interface and configured to receive the current from the transistor device and to generate a responsive sensor voltage. Embodiments may also include a sense control circuit configured to control a duty cycle of the sensor component. 
     Embodiments of an integrated circuit for sensing current through a transistor device may include an interface configured to receive a current from the transistor device for sensing. Additionally, the integrated circuit may include a sensor component coupled to the interface and configured to receive the current from the transistor device and to generate a responsive sensor voltage. The integrated circuit may also include a sense control circuit configured to control a duty cycle of the sensor component. 
     Embodiments of methods for sensing current through a transistor device may include receiving a current from the transistor device for sensing at an interface configured to receive the current from the transistor device. Additionally, embodiments may include generating a sensor voltage with a sensor component, the sensor voltage being responsive to the current from the transistor device. Embodiments may also include controlling a duty cycle of the sensor component with a sense control circuit. 
    
    
     
       Other aspects in accordance with the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention. 
         FIG. 1  depicts a schematic circuit diagram of one embodiment of a transistor circuit with a current sense transistor. 
         FIG. 2  depicts a schematic block diagram of an embodiment of a system for sensing a current in a transistor device. 
         FIG. 3  depicts a schematic block diagram of an embodiment of a system for broad-range current measurement using duty cycling. 
         FIG. 4  illustrates an embodiment of a duty cycle for broad-range current measurement using duty cycling. 
         FIG. 5  depicts a schematic block diagram of an embodiment of a system for multi-channel current sensing. 
         FIG. 6  depicts a flowchart diagram illustrating one embodiment of a method for broad-range current measurement using duty cycling. 
         FIG. 7  depicts a flowchart diagram illustrating another embodiment of a method for broad-range current measurement using duty cycling. 
     
    
    
     Throughout the description, similar reference numbers may be used to identify similar elements. 
     It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     While many embodiments are described herein, at least some of the described embodiments enable broad-range current measurement using duty cycling for a transistor device, such as a power MOSFET. Advantageously, such embodiments allow accurate and high resolution current measurement. Additionally, such embodiments may be very efficient because the current draw is relatively low. In particular, measurement of the current on a duty cycle may reduce the power consumption due to measurement by the duty cycle ratio and thereby increase efficiency of the system. 
     Embodiments of devices for broad-range current measurement using duty cycling may be implemented in a number of different types of electronic devices. Some examples of electronic devices include semiconductor devices for automotive or marine applications, controllers for factory automation systems, components of alternative power systems, etc. There is no limitation on the general types of applications that might incorporate embodiments of the technology described herein. 
     Advantages of the present embodiments include an ability to provide current sensing in a single sensor circuit that exhibits both high accuracy and high efficiency. Another advantage is that the present embodiments do not require a large circuit footprint, since an Analog to Digital Converter (ADC) may be shared amongst multiple channels. Additionally, the circuit footprint required for digital level shifters between high side domains and low-side domain may be reduced because the level shifters may be functionally replaced by components of the sensor circuitry. 
       FIG. 1  depicts a schematic circuit diagram of one embodiment of a transistor circuit  100 . In an embodiment, the transistor circuit  100  includes a main transistor  102  and a sense transistor  104 . The main transistor  102  may be, for example, a MOSFET device. In a further embodiment, the MOSFET device may be a power MOSFET configured to conduct high currents for automotive applications, for example. The sense transistor  104  may also be a MOSFET device. 
     In an embodiment, the main transistor  102  and the sense transistor  104  are each connected to a drain node (D)  106  and a gate node (G)  108 . In an embodiment, the main transistor  102  includes a Kelvin node (K)  112  for providing an accurate measurement of the potential at a source node  110  of the main transistor  102 . Additionally, the sense transistor  104  may provide a sense current to a sense current node (I SENSE )  114 . 
     The embodiment of  FIG. 1  provides a main transistor  102  configured to conduct a current (I SOURCE ) and a sense transistor  104  configured to provide a measurable sense current (I SENSE ), which may be a portion of the current (I SOURCE ), for external sensing. In the described embodiment, the sense transistor  104  may be configured to provide a sense current that is smaller than the main current by a predetermined ratio. For example, the ratio may be determined by the relative size of the main transistor  102  and the sense transistor  104 . If, for example, the main transistor  102  is n times larger than the sense transistor  104 , the sense transistor  104  may be configured to provide a sense current that is l/n th  of the main current. 
       FIG. 2  depicts a schematic block diagram of a system  200  for sensing a current in the transistor circuit  100 . In one embodiment, the system  200  is implemented in an integrated circuit. In an embodiment, the transistor circuit  100  is a MOSFET configured to conduct a main current (I SOURCE ) as illustrated in  FIG. 1 . The transistor circuit  100  may include both the main transistor  102  and the sense transistor  104  as described in  FIG. 1 . In such an embodiment, the main transistor  102  and the sense transistor  104  may be integrated into a transistor device package as illustrated by the transistor circuit  100  in  FIG. 2 . 
     In an embodiment, the system  200  includes a current sense device  202 . The current sense device  202  may be coupled to the transistor circuit  100 . For example, the current sense device may include a battery interface  220  for coupling to the drain node  106  of the transistor device. Additionally, the current sense device  202  may include a gate interface  222 , a source interface  226 , a Kelvin interface  224 , and a sense current interface  228 , each configured to be respectively coupled to the gate node  108 , the source node  110 , the Kelvin node  112 , and the sense current node  114  of the transistor circuit  100 . In an embodiment, the current sense device  202  may be a stand-alone integrated circuit device having I/O pins corresponding to the gate interface  222 , the source interface  226 , the Kelvin interface  224 , and the sense current interface  228 . 
     In an embodiment, the sense device includes an OPAMP  204  configured to force the voltage on the Kelvin interface  224  and the sense current interface  228  to be equal, such that the main transistor  102  and the sense transistor  104  operate on the same gate-source voltage difference. Because Gate-source voltages of the main transistor  102  and the sense transistor  104  are equal, a current n times smaller than the current that flows through the main transistor  102  will flow through the sense transistor  104 , when the sense transistor  104  is n times smaller than the main device  102 , and is sensed in the current sense device  202 . In an embodiment, the sense current will flow through transistor (M 1 )  218  and generate a large voltage across the sense resistor  216  that can be further processed. Although the illustrated current sense device  202  is shown with certain components and described with certain functionality herein, other embodiments of semiconductor chips and corresponding systems may include fewer or more components to implement the same, less, or more functionality. 
     In an embodiment, the gate driver  214  drives the gate node  108  of the transistor circuit  100 . The Charge Pump (CP)  208  may be configured to provide a voltage supply for the transistor circuit  100 , which may include one or more NMOSFET devices configured to operate in a high-side configuration. In a high-side configuration, the gate voltage may be larger than the supply voltage (battery voltage, VBAT), which is generated by the charge pump  208 . In an embodiment, the Overcurrent Comparator (OC Comp)  212  detects a current larger than the nominal current range. In an embodiment, the overcurrent may be detected by a voltage comparator between the battery terminal  220  and the source terminal  226 . In an embodiment, a current sensing block  210  may include an Analog to Digital Converter (ADC) configured to convert a current sense voltage at the sense resistor  216  into digital sensor data that can be used by external digital components, such as digital controllers. 
     In the embodiment of  FIG. 2 , the voltage across the source resistance (Rsource)  230  may be low (I*Rsource) as compared with the embodiments of  FIGS. 3-4 . For example, the voltage across Rsource  230  can be as low as 5 μV up to about 20 mV in some embodiments. In such embodiment, the voltage across Rsource  230  is not only at the non-inverting input of the OPAMP  204 , but also at the inverting input of OPAMP  204 , which also represents the full headroom available for M 1   218  and the sense resistor Rsense  216 . Therefore, the voltage across the sense resistor  216  is effectively lower than the levels at Rsource  230 . For example, the voltage at the sense resistor  216  may only be half of the voltage at Rsource  230 . Therefore, it may be challenging to design circuitry for sensing voltages between 5 μV and 10 mV with high accuracy and high resolution. 
       FIG. 3  illustrates another embodiment of a system  300  for broad-range current measurement using duty cycling. In system  300 , the OPAMP  204  forces the voltage levels at the K interface  224  and the sense current interface  228  to be equal. The sense current may be n times smaller than the main current (I), and may flow to ground via sense resistors  216   a - n  (Rsense 1 , . . . n) in sensor component  302 . In such an embodiment, the voltage level across the sense resistor(s)  216   a - n  in sensor component  302  may be significantly higher than in the circuit  200  of  FIG. 2 . In such an embodiment, the sensor device  202  may be less sensitive to EMC exposure. 
     In a further embodiment, a sense control circuit  310  may configure the sensor component  302  such that the resistance of the sensor component  302  is adjusted in value depending on the absolute value of the current (I) and the sense current (I/n). In a further embodiment, the sense control circuit  310  may include a reference comparator  304  and an auto-range circuit  306 . Such an embodiment, with adjustable resistance, will allow the measurement over a wide current range with high accuracy and resolution. 
     For example, the reference comparator  304  may be configured to compare the output voltage of the sensor component  302  to a reference voltage and provide feedback to the auto-range circuit  306 . In one embodiment, the voltage may be 1.8 V. One of ordinary skill, however, will recognize other suitable reference voltages that may be used. The auto-range circuit  306  may then cause another sense resistor  216   a - n  in the sensor component  302  to be activated. For example, the auto-range circuit  306  may apply a voltage to a switch transistor  308   a - n  (Mrange 1 , . . . n) causing current to flow through the corresponding sense resistor  216   a - n.    
     In one such example, a sense current may be received at the sense current interface  228  and conducted through transistor  218  (M 1 ) to sensor component  302 . A first switch transistor  308   a  (Mrange 1 ) may be activated causing the current to flow through the corresponding first sense resistor  216   a . The reference comparator  304  may compare the voltage across the first sense resistor  216   a  with the reference voltage and determine that the voltage on the first sense resistor  216   a  is too high. In response, a feedback signal may be sent to sense control circuit  310 , which causes auto-range circuit  306  to deactivate the first switch transistor  308   a  and activate the second switch transistor  308   b . Activating the second switch transistor  308   b  may then cause the current to flow through the second sense resistor  216   b , which may have a different resistance value than the first sense resistor  216   a . Thus, the resistance value of the sensor component  302  may be automatically adjusted or scaled according to the value of the current received at the sense current interface  228 . In an alternative embodiment, the switch transistors  308   a - n  may be activated in parallel, rather than in sequence. 
     Such embodiments may meet high accuracy requirements, but still dissipate unacceptable amounts of power. In an embodiment, the dissipation caused by the sensing circuitry can be expressed as:
 
 P   diss   =I/n*V   BAT   (1)
 
where V BAT  is the voltage level at the battery interface  220 . In certain embodiments, the power dissipation may be high where the value of n in the ratio I/n is low for enhanced accuracy.
 
     In certain embodiments, a duty cycle based measurement, as described with reference to  FIG. 4 , may be implemented that reduces the dissipation of power in the current sense component  302  by the duty cycle ratio according to the following equation:
 
 P   diss =δ*( I/n*V   BAT )  (2)
 
where δ describes a duty cycle ratio, which is a portion of a total measurement period (T) where the sensor is activated (t on ).
 
     As illustrated in  FIG. 4 , the sense component  302  may be activated and deactivated periodically. For example, the graph of  FIG. 4  illustrates the sensor output voltage  402  verses time  404 . The sensor component  302  may generate a sensor output signal  406  which is activated for a portion  408  (t on ) of the period  410  (T) and deactivated for the remainder of the period  410 . In such an embodiment, the auto-range circuit  306  may control the duty cycle of the sensor component  302  by periodically switching the switch transistors  308   a - n  on and off according to a preconfigured duty cycle period. For example, in an embodiment the auto-range circuit  306  may be pre-configured to control the duty cycle of the sensor component  302  according to a predetermined duty cycle function. 
     In an embodiment, the duty cycle based measurement may not detrimentally affect accuracy, because the size ratio n may not be decreased by decreasing the size of the sense MOSFET. Rather, the ratio n may be effectively increased by the duty cycle based measurement. In such an embodiment, both accuracy and resolution parameters may be improved as compared to other systems. 
     In certain embodiments, duty cycle based sensing will decrease the reaction speed of the current sense device  202  on changes in the main current (I SOURCE ). Nonetheless, duty cycle ratios as low as 40% may be suitable for use with the present embodiments. 
       FIG. 5  depicts a system  500  having a four channel high accuracy, high efficiency current measurement device  502 . In an embodiment, the four channel current measurement device  502  may sense the current I 1  . . . I 4  through four individual channels with high accuracy and high efficiency. In an embodiment, each channel is coupled to a separate transistor circuit  100 . For example, a first transistor circuit  100   a  may be coupled to a first current sensing device  202   a , a second transistor circuit  100   b  may be coupled to a second current sensing device  202   b , a third transistor circuit  100   c  may be coupled to a third current sensing device  202   c , and a fourth transistor circuit  100   d  may be coupled to a fourth current sensing device  202   d . Additionally, each channel may include a current sensing device  202   a - d , each being coupled to a shared ADC  504 , which is a more area effective solution compared with a single ADC  210  per channel. 
     In a further embodiment, the digital output of the shared ADC  504  may be communicated to a main digital block  508 . The main digital block  508  may use frequency information generated by the oscillator  506  to reformat the digital sensor signals for communication to an external microprocessor over, for example, a Serial Peripheral Interface (SPI) bus interface  510 , which may include multiple discrete lines in one embodiment. In an embodiment, the current measurement device  502  may be a stand-alone integrated circuit device. 
       FIG. 6  depicts a flowchart diagram of one embodiment of a method  600  for broad-range current measurement using duty cycling. In an embodiment, the method  600  starts at block  602  with receiving a current from the transistor device for sensing at an interface configured to receive the current from the transistor device. At block  604 , the method includes generating a sensor voltage with a sensor component, the sensor voltage being responsive to the current from the transistor device. At block  606 , the method includes controlling a duty cycle of the sensor component with a sense control circuit. 
     For example, gate driver  214  may drive a voltage at the gate node  108  causing current I SOURCE  to flow from the battery interface  220  through the main transistor  102  to the source node  110 , as shown at block  602 . In an embodiment, the sense transistor  104  may simultaneously conduct the sense current I s  to the sense current node  114 . The sense current I SENSE  may then flow through sense current interface  228  to the inverting terminal of OPAMP  204  and through sensor transistor  218 . In an embodiment, the sense current only flows through sensor transistor  218  when the potential energy at the K terminal  224  and the sense current interface  228  is equal. In such an embodiment, the current flows through sensor transistor  218  to sensor component  302 , where a voltage is established over the one or more sensor resistors  216   a - n  (Rsense 1 , . . . n). The voltage is then converted by ADC  210  and provided as digital sensor data to an external component. 
     In a further embodiment, the sensing may be duty cycled as described with reference to  FIG. 4 . For example, the voltage may only be sensed during the t on  portion  408  of the sensing period  410  as controlled by the duty cycle settings of the sense control circuit  310 . In still a further embodiment, multiple currents may flow through multiple transistor circuits  100   a - d  and sensed by a multi-channel current sensor device  502  as described with reference to  FIG. 5 . 
       FIG. 7  illustrates another embodiment of a method  700  for broad-range current measurement using duty cycling. In an embodiment, the method  700  starts at block  702 , by receiving a current from the transistor device for sensing at an interface configured to receive the current from the sensor device. At block  704 , the method includes generating a responsive sensor voltage with a sensor component, the sensor component comprising an adjustable resistance component. The method  700  further includes selecting a value of the adjustable resistance component in response to a level of the current received at the interface, as shown at block  706 . 
     It should also be noted that at least some of the operations for the methods described herein may be implemented using firmware or software instructions stored on a readable storage medium for execution by a programmable logic device, such as an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Programmable Logic Chip (PLC), a processor, or the like. As an example, an embodiment of a program product includes a firmware package stored on a flash memory device and configured to cause an FPGA to perform the operations described herein. 
     In the above description, specific details of various embodiments are provided. However, some embodiments may be practiced with less than all of these specific details. In other instances, certain methods, procedures, components, structures, and/or functions are described in no more detail than to enable the various embodiments of the invention, for the sake of brevity and clarity. 
     Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operations may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be implemented in an intermittent and/or alternating manner. 
     Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.