Patent Publication Number: US-2016226240-A1

Title: Control circuit and device including control circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-017754, filed on Jan. 30, 2015, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is related to a control circuit and a device including the control circuit. 
     BACKGROUND 
     Feedback control for approximating the air-fuel ratio that is the ratio of air and fuel in an exhaust gas exhausted from an internal combustion engine to an aimed air-fuel ratio for the purpose of improving vehicle fuel efficiency, etc., is well known, where the air-fuel ratio is detected by an air-fuel ratio sensor (A/F sensor). 
     For A/F sensors, an A/F sensor that includes a gas sensor element including a pump cell and a detection cell is known. Such an A/F sensor detects, with the detection cell, the voltage corresponding to the difference between the oxygen concentration in a gas detection chamber and a reference oxygen concentration and supplies a current in a direction and a magnitude corresponding to the difference between the detected voltage and a reference voltage to the pump cell (see, for example, Japanese Laid-open Patent Publication No. 2010-096732) 
     In the above-described A/F sensor, a voltage limitation circuit may be provided in order not to apply an overvoltage to the gas sensor element and a current limitation circuit may be provided to protect the current supply circuit that supplies a current to the gas sensor element. 
     When the current limitation circuit and the voltage limitation circuit run together, however, there is a risk that the current supply circuit that supplies a current to the gas sensor element fails. The same may happen not only to A/F sensors but also to a control circuit including a current limitation circuit and a voltage limitation circuit and a device including the control circuit. 
     SUMMARY 
     According to an aspect of an embodiment, a control circuit includes a current supply, a voltage limitation circuit, a current limitation circuit, and a voltage control stop circuit. The current supply circuit supplies a current to a subject to be controlled. The voltage limitation circuit limits a voltage applied to the subject to be controlled to a predetermined range by controlling the current in the current supply circuit. The current limitation circuit limits the current in the current supply circuit when the current in the current supply circuit is an overcurrent. The voltage control stop circuit stops the voltage limitation circuit to run when the current in the current supply circuit is the overcurrent. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a diagram illustrating an exemplary configuration of an A/F sensor according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating an exemplary structure of the gas sensor element illustrated in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating an exemplary relation among the voltage-current conversion amplifier, the current limitation circuit, and the voltage control circuit illustrated in  FIG. 1 ; and 
         FIG. 4  is a diagram illustrating an exemplary relation between the overcurrent detection circuit and the voltage limitation stop circuit illustrated in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     An embodiment of the control circuit and the device including the control circuit disclosed herein will be described in detail below. The embodiment described below is not to be construed as limiting the invention. For example, a gas sensor element is exemplified below as a subject to be controlled; however, the subject to be controlled is not limited to the gas sensor element, and other sensor elements and other elements may be used. A control circuit including a current limitation circuit and a voltage limitation circuit may be applied to a control circuit capable of precisely protecting a current supply circuit and a device including the control circuit. 
       FIG. 1  is a diagram illustrating an exemplary configuration of an A/F sensor according to an embodiment of the present invention. As illustrated in  FIG. 1 , an A/F sensor  1  includes a gas sensor element  2  and a sensor driver  3  (am exemplary control circuit). 
     As an exemplary gas sensor element, a gas sensor element including two cells will be described below; however, the gas sensor element is not construed as being limited to a gas sensor element including two cells and, for example, a gas sensor element including a cell or a gas sensor element including three or more cells may be used. 
     The gas sensor element  2  illustrated in  FIG. 1  includes a pump cell  4 , a detection cell  5 , an IP terminal T 1 , a COM terminal T 2 , and a VS terminal T 3 . The gas sensor element  2  is a universal A/F ratio gas sensor element that is, for example, disposed in an exhaust pipe of an internal combustion engine of a vehicle (not illustrated) and that detects the oxygen concentration (air -fuel ratio) in the exhaust gas. 
       FIG. 2  is a diagram illustrating an exemplary structure of the gas sensor element  2 . As illustrated in  FIG. 2 , the gas sensor element  2  has a structure obtained by sequentially layering a solid electrolyte  21 , an insulating base  25 , and solid electrolytes  27  and  29 . 
     The solid electrolytes  21 ,  27  and  29  are solid electrolytes having oxygen ion conductivity and are produced by adding yttria (Y2O3) to zirconia (ZrO2). The insulating base  25  is composed of, for example, alumina. 
     A gas detection chamber  30  is formed in the insulating base  25 , and a porous dispersion regulator  24  that regulates the amount of exhaust gas flowing into the gas detection chamber  30  is provided to both ends of the gas detection chamber  30 . 
     The pump cell  4  includes electrodes  22  and  23  formed of, for example, porous platinum and formed on both surfaces of the solid electrolyte  21  and takes in or draws off oxygen into or from the gas detection chamber  30  according to the magnitude and direction of the current supplied between the electrodes  22  and  23 . The electrode  22  is protected with, for example, a porous protective layer  20 . 
     The detection cell  5  includes the solid electrolyte  27  and electrodes  26  and  28  formed of, for example, porous platinum and formed on both surfaces of the solid electrolyte  27 . Supplying a constant current between the electrodes  26  and  28  causes an electromotive force corresponding to the oxygen concentration in the gas detection chamber  30  between the electrodes  26  and  28 . 
     The sensor driver  3  illustrated in  FIG. 1  will be described here. The sensor driver  3  is implemented with an integrated circuit, such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA). 
     As illustrated in  FIG. 1 , the sensor driver  3  includes a constant voltage control circuit  10 , a Vs detection circuit  11 , a current supply circuit  12 , an A/F value detection circuit  13 , an overcurrent detection circuit  14 , and a current limitation circuit  15 , a voltage limitation circuit  16 , and a voltage limitation stop circuit  17 . 
     The constant voltage control circuit  10  outputs a voltage such that the voltage Vcom at the COM terminal T 2  of the gas sensor element  2  is a constant voltage (for example, 3.3 [V]). 
     The Vs detection circuit  11  includes a constant current source that flows a constant current Icp to the detection cell  5 , and flows the constant current Icp from the electrode  28  to the electrode  26  to store oxygen in the constant concentration in the electrode  28 . Accordingly, the electromotive force corresponding to the oxygen concentration between the electrodes  26  and  28  occurs between the electrodes  26  and  28  and appears as the voltage Vs across the COM terminal T 2  and the VS terminal T 3 . 
     The Vs detection circuit  11  detects the voltage Vs across the COM terminal T 2  and the terminal T 3  (hereinafter, “detected voltage Vs”) and compares the detected voltage Vs and a predetermined reference voltage Vref (for example, 450 [mV]). 
     The Vs detection circuit  11  outputs a control voltage to the current supply circuit  12  such that the current Ip in a direction and a magnitude corresponding to the difference between the detected voltage Vs and the reference voltage Vref is supplied from the current supply circuit  12  to the IP terminal T 1  of the gas sensor element  2 . 
     The current supply circuit  12  includes, for example, a voltage-current conversion amplifier OP 1 , resistors R 10  to R 14  and R 20 , and a constant voltage source VA. The voltage-current conversion amplifier OP 1  is, for example, an operational amplifier. The current supply circuit  12  generates a current Ip corresponding to the control voltage that is output from the Vs detection circuit  11  and supplies the current Ip to the IP terminal T 1 . 
     When the detected voltage Vs is larger than the reference voltage Vref, the current supply circuit  12  supplies the current Ip to the IP terminal T 1  to take in oxygen. When the detection voltage Vs is smaller than the reference voltage Vref, the current supply circuit  12  supplies the current Ip to the IP terminal T 1  to draw off oxygen. 
     The current supply circuit  12  is not limited to the circuit illustrated in  FIG. 1 , and it suffices if the current supply circuit  12  has a configuration that enables supply of the current Ip according to the control by the Vs detection circuit  11  to the IP terminal T 1 . 
     The A/F value detection circuit  13  detects the value of the current Ip flowing into the resistor R 1  and calculates the air-fuel ratio (A/F value) based on the value of the current Ip. It suffices if the A/F value detection circuit  13  detects the value of the current Ip and calculates the air-fuel ratio based on the value of the current Ip, i.e., the configuration is not limited to that illustrated in  FIG. 1 . For example, a resistor may be disposed between the IP terminal T 1  and the current supply circuit  12  to detect the current Ip flowing into the resistor. 
     The overcurrent detection circuit  14  detects whether the current Ip in the current supply circuit  12  is an overcurrent. When the current Ip is over a predetermined value I1, the overcurrent detection circuit  14  determines that the current supply circuit  12  is an overcurrent and outputs a current limitation request to the current limitation circuit  15 . According to the example illustrated in  FIG. 1 , the current supply circuit  12  detects the voltage Vb across the terminals of the resistor R 20  disposed between the voltage-current conversion amplifier OP 1  and the IP terminal T 1  to detect the current Ip (=Vb/R 20 ); however, it suffices if a configuration enabling detection of the current Ip is used. 
     When the overcurrent detection circuit  14  detects an overcurrent of the current Ip and outputs a current limitation request, the current limitation circuit  15  limits the current Ip in the current supply circuit  12  to be equal to or smaller than a predetermined value I2 (≦I1). Accordingly, the current supply circuit  12  can be prevented from failing due to the overcurrent of the current Ip. 
     The voltage limitation circuit  16  controls the current Ip in the current supply circuit  12  such that a voltage Vip (=Vp+Vcom) at the IP terminal T 1  does not exceed the pressure resistance of the gas sensor element  2  and such that the voltage Vip is limited to a predetermined range Ra (e.g., 3.3±1.2 [V]). 
       FIG. 3  is a diagram illustrating an exemplary relation among the voltage-current conversion amplifier OP 1 , the current limitation circuit  15 , and the voltage limitation circuit  16 . According to the example illustrated in  FIG. 3 , the voltage-current conversion amplifier OP 1  includes an input stage  31  and an output stage  32 . The input stage  31  controls the current that is output from the output stage  32  on the basis of the voltage that is input via the resistors R 10  and R 11 . 
     When the overcurrent detection circuit  14  outputs the current limitation request, the current limitation circuit  15  limits the current that is output from the output stage  32  such that the current Ip in the current supply circuit  12  is equal to or smaller than the predetermined value I2. On the other hand, the voltage limitation circuit  16  controls the current that is output from the output stage  32  such that the voltage Vip is limited to the predetermined range Ra. In this manner, the current limitation circuit  15  and the voltage limitation circuit  16  can independently control the current output from the output stage  32 . 
       FIG. 1  will be referred back here to continue describing the sensor driver  3 . When the current Ip is an overcurrent, the voltage limitation stop circuit  17  stops the voltage limitation circuit  16  to run. For example, when the overcurrent detection circuit  14  determines that the current Ip in the current supply circuit  12  is an overcurrent, the voltage limitation stop circuit  17  outputs a voltage limitation stop request to the voltage limitation circuit  16  to stop the voltage limitation circuit  16  to run. 
     As illustrated above, because the sensor driver  3  stops the voltage limitation circuit  16  to run when the current Ip is an overcurrent, the current Ip in the current supply circuit  12  can be prevented from being an overcurrent. Accordingly, when the current Ip is an overcurrent, the sensor driver  3  can precisely protect the current supply circuit  12 . The case where the current Ip is an overcurrent is, for example, the case where the IP terminal T 1  and the ground potential (or the power supply potential) are connected with a low resistance. 
     An exemplary case where an overcurrent is supplied from the current supply circuit  12  to the IP terminal T 1  will be described below to describe protection of the current supply circuit  12  that is carried out by stopping the voltage limitation circuit  16  when the current Ip is an overcurrent. 
     First, a case where the voltage limitation stop circuit  17  is not provided to the sensor driver  3  will be assumed. In this case, when the current Ip in the current supply circuit  12  is equal to or larger than the predetermined value I1, the overcurrent detection circuit  14  determines that the current Ip in the current supply circuit  12  is an overcurrent. The current limitation circuit  15  therefore limits the current Ip in the current supply circuit  12  to be equal to or smaller than the predetermined value Ib. 
     When the current limitation circuit  15  limits the current Ip, the voltage Vip of the IP terminal T 1  lowers, so that the voltage Vip may be output of the predetermined range Ra. In that case, the voltage limitation circuit  16  increases the voltage Vip to the predetermined range Ra to control the current Ip in the current supply circuit  12 . Accordingly, the current supply circuit  12  is to supply the current Ip greater than the predetermined value I2, and thus there is a risk that the current Ip in the current supply circuit  12  is an overcurrent and the current supply circuit  12  fails. 
     On the other hand, as illustrated in  FIG. 1 , in the case where the voltage limitation stop circuit  17  is provided to the sensor driver  3 , the voltage limitation circuit  16  stops running when the overcurrent detection circuit  14  detects an overcurrent of the current Ip and the voltage limitation circuit  16  does not control the current Ip in the current supply circuit  12 . Accordingly, the current Ip in the current supply circuit  12  can be prevented from being an overcurrent, which enables precise protection of the current supply circuit  12 . 
     The voltage limitation stop circuit  17  is configured to, when the current Ip is an overcurrent, output a voltage limitation stop request to the voltage limitation circuit  16  before a current limitation request is output from the overcurrent detection circuit  14 . Accordingly, the voltage limitation circuit  16  can be stopped to run before the current limitation circuit  15  limits the current Ip. 
     The relation between the overcurrent detection circuit  14  and the voltage limitation stop circuit  17  will be described here.  FIG. 4  is a diagram illustrating an exemplary relation between the overcurrent detection circuit  14  and the voltage limitation stop circuit  17 . 
     The overcurrent detection circuit  14  illustrated in  FIG. 4  takes the voltage across the terminals of the resistor R 20  as a differential input and, when the current flowing into the resistor R 20  is equal to or larger than the predetermined value I1, carries out an overcurrent detection output (an exemplary current limitation request). 
     The overcurrent detection circuit  14  includes a resistor R 40 , transistors Q 10  to Q 16 , and a constant current sources  40  and  41 . When the voltage across the terminals of the resistor R 20  is larger than the voltage across the terminals of the resistor R 40 , the overcurrent detection circuit  14  carries out an overcurrent detection output. Note that the transistors Q 10  and Q 11  and the constant current source  41  configure, for example, an input stage and the transistors Q 12  to Q 16  configure, for example, an output stage. 
     The voltage limitation stop circuit  17  includes transistors Q 21  to Q 27  and takes the voltage across the terminals of the resistor R 20  as a differential input. When the current flowing into the resistor R 20  is equal to or larger than a predetermined value I3 (&lt;I1), the voltage limitation stop circuit  17  carries out a voltage control stop output (exemplary voltage limitation stop request). 
     The current mirror circuit of the voltage limitation stop circuit  17  illustrated in  FIG. 4  is different from that of the overcurrent detection circuit  14  in that, while there are two parallel connections (transistors Q 13  and Q 14 ) on one side (non-output side) of the current mirror circuit of the overcurrent detection circuit  14 , there are four parallel connections (transistors Q 22  to Q 25 ) on the other side (non-output side). 
     The difference in the number of connections causes a difference in detection sensitivity between the overcurrent detection circuit  14  and the voltage limitation stop circuit  17 , and the voltage limitation stop circuit  17  carries out a voltage control stop output even when the current Ip is lower than the predetermined value I1. Accordingly, when the current flowing into the resistor R 20  is equal to or larger than the predetermined value I1, the voltage limitation stop circuit  17  can carry out a voltage control stop output before the overcurrent detection circuit  14  carries out an overcurrent detection output. 
     As described above, the voltage limitation stop circuit  17  illustrated in  FIG. 4  carries out a voltage control stop output according to the state of the input stage of the overcurrent detection circuit  14 , which enables size reduction. Furthermore, because the voltage limitation stop circuit  17  has a configuration obtained by increasing the number of parallel stages of transistors on one side of the overcurrent detection circuit  14 , a precise voltage control stop output is enabled before an overcurrent detection output. 
     The overcurrent detection circuit  14  may be used as the voltage limitation stop circuit  17 . In this case, the overcurrent detection circuit  14  outputs a voltage limitation stop request upon determining that the current Ip in the current supply circuit  12  is an overcurrent, and the voltage limitation circuit  16  stops running to limit the voltage when the voltage limitation stop request is issued. 
     According to the example illustrated in  FIG. 4 , the difference in detection sensitivity is provided between the overcurrent detection circuit  14  and the voltage limitation stop circuit  17  by the number of parallel connections of transistors. Alternatively, the detection sensitivity difference may be provided by the size of transistor. 
     The embodiment of the invention has been described according to the accompanying drawings; however, they are exemplary only and the invention may be carried out in various modes in which modification and improvement are made according to the knowledge of those skilled in the art, including the mode described in the summary section. 
     The control circuit and the device including the control circuit device according to the embodiment are efficiently used in, for example, controlling combustion and exhaust of vehicle and is suitable for feedback control of fuel supply volume corresponding to the concentration of a specific gas in exhaust gas.