Abstract:
Provided is a heater drive circuit that applies a voltage to a heater resistor in detecting a temperature variation in accordance with the concentration of a predetermined element contained in a liquid, the heater drive circuit including: a booster circuit for boosting a predetermined reference power source voltage to generate a boosted power source voltage; a voltage control circuit including an operational amplifier driven by a boosted power source voltage from the booster circuit; and an emitter follower output circuit having its base terminal connected to an output voltage from the heater resistor, its collector terminal connected to the predetermined reference power source, and its emitter terminal connected to the heater resistor.

Description:
[0001]     This application claims priority based on Japanese Patent application no. 2004-316095, filed Oct. 29, 2004, the contents of which is incorporated herein by reference in its entirety. This priority claim is being made concurrently with the filing of the application.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a heater drive circuit mounted on a vehicle or the like, the circuit driven to detect the thermal resistance of a liquid in a bridge circuit when determining whether urea water for a urea SCR (Selective Catalyst Removing method) catalyst decomposing NOx into water and hydrogen has an appropriate urea concentration.  
         [0003]     In the related art, for example, a urea concentration sensor for an exhaust gas purification system for a diesel is known that amplifies a faint signal of 0.1 mV or below and performs A/D conversion of the resulting signal using a microcomputer to detect urea water having an appropriate concentration. A technology used in such a urea concentration sensor is a high-voltage power source circuit described in Patent Reference 1 that supplies voltage to a heater resistor.  
         [0004]     Patent Reference 1: JP-A-07-107737  
         [0005]     A heater drive circuit or operating the urea concentration sensor is mounted on a diesel engine vehicle. Thus, a power source of a 24V system must be provided to feed a power source voltage to the sensor circuit and apply a predetermined voltage from a low-voltage output circuit to a heater resistor.  
         [0006]     Use of a high voltage from a power source of a 24V system requires serge protection and loss reduction in the heater drive circuit. This results in a larger circuit scale.  
         [0007]     In the related art, as shown in  FIG. 5 , it is necessary to convert a high voltage to a reference voltage of 5V by using a switching regulator  101  as a power source circuit and supply the voltage of 5V to a CPU  102  as well as convert it to 3.45V and supply the voltage of 3.45V to a heater resistor. The problem with this approach is that only a low voltage of 3.3V is supplied to the heater resistor because of a voltage drop of 0.5V caused by an operational amplifier  103  in a voltage control circuit and a voltage drop of 1.2V caused by a transistor  104  in an output circuit. That is, the total voltage drop includes a voltage drop caused by the power source of the operational amplifier  103  minus maximum saturation voltage and a voltage drop caused by Vbe of the transistor  104 .  
       SUMMARY OF THE INVENTION  
       [0008]     The invention is proposed in view of the aforementioned circumstances. An object of the invention is to provide a heater drive circuit capable of supplying a proper voltage to a heater resistor even in the presence of a voltage drop of a circuit element up to the heater resistor.  
         [0009]     In order to solve the problems, according to a first aspect of the invention, there is provided with a heater drive circuit that applies a voltage to a heater resistor in detecting a temperature variation in accordance with the concentration of a predetermined element contained in a liquid, the heater drive circuit including: a booster circuit for boosting a predetermined reference power source voltage to generate a boosted power source voltage; a voltage control circuit including an operational amplifier which is driven by a boosted power source voltage from the booster circuit and compares the output voltage with the predetermined reference power source voltage to control the voltage to the heater resistor; and a switching element connected to the voltage control circuit, the heater resistor and the predetermined reference power source.  
         [0010]     According to a second aspect of the invention, there is provided with the heater drive circuit according to the first aspect, wherein the switching element is an emitter follower output circuit having a base terminal connected to an output voltage from the voltage control circuit, a collector terminal connected to the predetermined reference power source, and an emitter terminal connected to the heater resistor.  
         [0011]     According to a third aspect of the invention, there is provided with the heater drive circuit according to the first aspect, wherein the switching element is an FET having a gate terminal connected to an output voltage from the voltage control circuit, a drain terminal connected to the predetermined reference power source, and a source terminal connected to the heater resistor.  
         [0012]     According to the heater drive circuit of the invention, it is possible to generate a boosted power source voltage in a booster circuit while considering a voltage drop in a voltage control circuit or an output circuit even in case such a control circuit or output circuit is provided, thereby stably supplying a predetermined voltage appropriate for a heater resistor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a block diagram showing an overall configuration of a urea water detection system to which the invention is applied;  
         [0014]      FIG. 2  is a block diagram showing a urea water detection system including a heater drive circuit to which the invention is applied;  
         [0015]      FIG. 3  is a circuit diagram of a booster circuit, a voltage control circuit and an output circuit in the heater drive circuit to which the invention is applied;  
         [0016]      FIG. 4  shows the relationship between a heater output and a temperature variation detected by a temperature sensor;  
         [0017]      FIG. 5  is a block diagram showing the configuration of a related art heater drive circuit; and  
         [0018]      FIG. 6  is a circuit diagram of a booster circuit, a voltage control circuit and an output circuit in the heater drive circuit to which the invention is applied. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     Embodiments of the invention will be described referring to drawings.  
         [0020]     The invention is applied to a heater drive circuit  13  for example in a urea water detection system configured as shown in  FIG. 1 . The urea water detection system mounted for example on a vehicle traveling by way of a diesel engine detects the urea concentration of urea water for a urea SCR catalyst and supplies the detected concentration value to an exhaust gas purification system (not shown).  
         [0000]     [Configuration of Heater Drive Circuit] 
         [0021]     A urea water detection system including a heater drive circuit mainly comprises, as shown in  FIG. 1 , an ECU  1 , an ignition power source  2  activated with an ignition switch of a vehicle and a detection circuit  3  both of which are connected to the ECU  1 . When the ignition switch (IGN) (not shown) of the system is turned on, an ignition voltage is supplied to the ECU  1  from the ignition power source  2  as a power source of a 24V system.  
         [0022]     The ignition voltage is a voltage from a power source of a 24V system, for example. The ignition voltage is first supplied to a power source circuit  11  in the ECU  1 , converted to an operating voltage for a CPU  12 , and supplied to a heater drive circuit  13  from the CPU  12 . The heater drive circuit  13  converts the voltage supplied by the CPU  12  to a predetermined operating voltage for a detection circuit  3  in accordance with the control by the CPU  12 .  
         [0023]     This causes the detection circuit  3  to amplify, on the operational amplifier  16  in the ECU  1 , the detected output varying depending on the presence/absence of urea-containing water and presence of a urea concentration appropriate for the urea-containing water, and supplies the resulting output value to the CPU  12 . The CPU  12  performs arithmetic operation to convert the output value from the operational amplifier  16  to the urea concentration of the urea-containing water and supplies the urea concentration value to an exhaust gas purification system (not shown) via a communications circuit  17 .  
         [0024]     A sensor circuit whose output depends on the urea concentration of urea water includes a bridge circuit including a resistor  14 , a temperature sensor  21 , a resistor  15 , and a temperature sensor  22  as a resistor, all of these components bridge-connected. The bridge circuit has one end connected to a reference voltage IC and the other end connected to a GND terminal. The resistors  14 ,  15  are for example carbon resistors. The temperature sensors  2 l,  22  are for example platinum resistors. A resistance value of a platinum resistor has a large variation in temperature of approximately 3600 ppm/° C. The resistors  14 ,  15  are arranged in the air while the temperature sensors  21 ,  22  are arranged in urea water.  
         [0025]     One resistor among the resistor  14 , temperature sensor  21 , resistor  14  and temperature sensor  22 , for example the temperature sensor  21  is integrated with a heater resistor  23 . The heater resistor  23  is heated and the heat of the same is transmitted. The heater resistor  23  is heated for a predetermined duration by predetermined interval, which causes the resistance value of the temperature sensor  21  to vary periodically. The temperature sensors  21 ,  22  are arranged in the urea water filled in a urea water tank. The higher the urea concentration of the urea water is, the less the dissipation of heat transmitted to the temperature sensor becomes, thus the more slowly the resistance value of the temperature sensor is restored. The lower the urea concentration of the urea water is, the faster the heat at the temperature sensor is dissipated. The ratio of variations at a junction point of the resistor  12  and the temperature sensor  21  in the bridge circuit is proportional to the urea concentration.  
         [0026]     In the sensor circuit, with an operating voltage applied by a reference voltage IC, the heat from the heater resistor  23  integrated with the temperature sensor  21  increases the resistance value of the temperature sensor  21 . The heat from the temperature sensor  21  is dissipated into the urea water to decrease the resistance value. The voltage value divided by the resistor  14  and the temperature sensor  21  and the voltage value divided by the resistor  15  and the temperature sensor  22  are supplied to an operational amplifier  16 . The operational amplifier  16  calculates the difference between the voltage value divided by the resistor  14  and the temperature sensor  21  and the voltage value divided by the resistor  15  and the temperature sensor  22  and amplifies the obtained differential voltage and transmits the corresponding output signal to the CPU  12 .  
         [0027]     The degree of dissipation of urea-containing water is detected in accordance with the urea concentration by the temperature sensor  21 , and the detection output depending on the presence/absence of urea-containing water and presence of a urea concentration appropriate for the urea-containing water is supplied to the CPU  12 .  
         [0028]     As shown in  FIG. 2 , a voltage of 5V generated by the power source circuit  11  is supplied to a booster circuit  31  of the heater drive circuit  13  and supplied as a reference voltage to a voltage control circuit  32  and an output circuit  33  connected to the heater resistor  23 .  
         [0029]     As shown in  FIG. 3 , the booster circuit  31  comprises an inverter circuit  41  connected thereto via the CPU  12  and a resistor R 1  (10 kΩ). The inverter circuit  41  comprises a terminal for inputting a pulse signal from the CPU  12 , a terminal for supplying a 5V reference voltage VCC from the power source circuit  11 , a terminal connected to a GND, and a terminal for outputting a boosted voltage. The inverter circuit  41  outputs a reference voltage of 5V as an 8V power source voltage to a voltage control circuit  32  generated from the pulse signal input from the CPU  12 . The inverter circuit  41  is provided with a capacitor C 1  (0.1 μF) between the terminal connected to the reference voltage and the GND.  
         [0030]     The output pulse signal from the inverter circuit  41  is transmitted by a circuit including a capacitor  2  (1 μF), a resistor R 2 , diodes D 1 , D 2  and a capacitor C 3 , and supplied as a boosted voltage of 8V to the voltage control circuit  32 .  
         [0031]     The output circuit  33  is an emitter follower circuit that has a collector terminal of transistor connected to a reference power source of 5V, a base terminal connected to the voltage control circuit  32 , and an emitter terminal connected to the heater resistor  23 .  
         [0032]     As shown in  FIG. 3 , the voltage control circuit  32  comprises an FET  1  whose gate terminal receives a pulse signal via a resistor R 3  (10 kΩ) and a resistor R 4  (47 kΩ), the voltage control circuit  32  as an operational amplifier, and the output circuit  33  as a transistor. The voltage control circuit  32  compares the reference voltage of 5V with the output voltage of the output circuit  33  to control the output voltage to the output circuit  33 . The voltage control circuit  32  has an output terminal connected to a transistor base terminal as an output circuit  33  via a resistor R 7  (100Ω). The voltage control circuit  32  is connected to a drive voltage of 8V as a drive power source voltage and a reference voltage of 5V via a capacitor C 5  (0.1 μF). Further, the voltage control circuit  32  has a negative terminal grounded via a resistor R 6  (10 kΩ) and a capacitor C 4  (100 pF) and a positive terminal connected to the drain terminal of the FET  1 .  
         [0033]     In case a voltage is supplied to the heater  23  by the heater drive circuit  13 , for example, a voltage of 3.45V is applied to the heater resistor  23  by the heater drive circuit  13  for a predetermined duration by predetermined interval, as shown in  FIG. 4 . The CPU  12  outputs a pulse signal to the booster circuit  31  to convert the reference voltage of 5V to an 8V power source voltage and supplies the 8V voltage to the voltage control circuit  32 . At the same time, the CPU  12  outputs a pulse signal to the FET  1  to generate a voltage of 3.45V dropped by the voltage control circuit  32  and the output circuit  33 .  
         [0034]     This causes the temperature of the urea-containing water to vary. As shown in the dotted lines in  FIG. 4 , a temperature variation is detected by the temperature sensors  21 ,  22  based on the high/low urea concentration. The temperature variation value is converted to a urea concentration by the CPU  12 .  
         [0035]     The heater drive circuit thus configured converts the reference voltage of 5V to a higher 8V voltage on the booster circuit  31  and connect the 8V voltage to the voltage control circuit  32 . The voltage control circuit  32  is capable of supplying a voltage higher than the reference voltage of 5V to the heater resistor  23  via the output circuit  33 . Thus, according to the heater drive circuit  13 , it is possible to reliably supply a predetermined voltage of 3.45V to the heater resistor  23  by using the 8V voltage of the booster circuit  31 , even in the presence of a voltage drop caused by the voltage control circuit  32  and a voltage drop caused by the output circuit  33 .  
         [0036]     The heater drive circuit  13  uses the reference voltage of 5V converted by the power source circuit  11  as a circuit voltage for the booster circuit  31 . This eliminates the need for losses caused by surge protection and voltage drop.  
         [0037]     Further, in  FIG. 6 , the transistor as shown in  FIG. 3  is replace with an FET (Field Effect Transistor).  
         [0038]     The foregoing embodiment is an example of the invention. The invention is not limited to the above embodiment but various changes may be made to the invention depending on the design or the like within the technical philosophy of the invention.