Abstract:
A temperature sensing device for improving series resistance cancellation mechanism includes a temperature sensing unit, a signal processing unit, a first current source, a second current source, a third current source, a first switch, a second switch, and a third switch. A control circuit generates a first control signal, a second control signal and a third control signal for controlling the first current source, the second current source and the third current source so as to drive the temperature sensing unit, wherein the first control signal, the second control signal and the third control signal are outputted from the control circuit according to a specific cycle formed by a plurality of switches between the first control signal and the second control signal and a switch between the first control signal and the third control signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a temperature sensing device, and more particularly, to a temperature sensing device for improving series resistance cancellation mechanism. 
         [0003]    2. Description of the Prior Art 
         [0004]    A temperature sensing circuit is widely used in kinds of electronic equipments, such as consumer electronic products, power equipments and industrial instruments, for measuring temperature for the purpose of protection or efficiency enhancement. For a personal computer, a temperature sensing device can help heat dissipation of a power management system of the personal computer, so as to ensure that the personal computer operates in a safety temperature range. 
         [0005]    Please refer  FIG. 1 .  FIG. 1  is a schematic diagram of a temperature sensing device  10  according to the prior art. The temperature sensing device  10  comprises a temperature sensing unit  100 , a signal processing unit  102 , current sources  104  and  106 , and switches  108  and  110 . The temperature sensing unit  100  comprises a temperature sensing component  120  and resistors R B  and R E . The signal processing unit  102  is coupled to the resistors R B  and R E , that is, the signal processing unit  102  is coupled to the two terminals of the temperature sensing unit  100 . The signal processing unit  102  is utilized for generating an output voltage signal V out  for presenting temperature variation according to a difference ΔV BE  between two voltage differences of the two terminals of the temperature sensing unit  100  at different currents. The switch  108  is coupled between the current source  104  and the signal processing unit  102 ; the switch  110  is coupled between the current source  106  and the signal processing unit  102 . A control circuit  12  is utilized for generating control signals for controlling ON/OFF states of the switches  108  and  110  so as to control the current sources  104  and  106  to drive the temperature sensing unit  100 . 
         [0006]    The temperature sensing unit  100  will be described in detail as follows. In the prior art temperature sensing device  10 , the temperature sensing component  120  is usually not located at the place beside the signal processing unit  102 , therefore, the line of the current path between the temperature sensing component  120  and the signal processing unit  102  is equivalent to a series resistor. On the other hand, the temperature sensing component  120  is a non-ideal component with parasitic resistors inside. In  FIG. 1 , the temperature sensing component  120  is a PNP bipolar junction transistor (BJT), wherein the resistors R B  and R E  are regarded as the sum of the parasitic resistors of the temperature sensing component  120  and the series resistors in the lines forming the current path between the temperature sensing component  120  and the signal processing unit  102 . In  FIG. 1 , I C1  and I C2  respectively represent output currents of the current sources  104  and  106 ; ΔV BE  is the difference between two voltage differences of the two terminals of the temperature sensing unit  100  at different currents, I C1  and I C2 , when the current sources  104  and  106  are switched. Let I C2 =N×I C1 , so that V BE1 /V BE2  is the voltage difference between the two terminals of the temperature sensing unit  100  when the current source  104 / 106  drives the temperature sensing unit  100 ; V T  is temperature equivalent voltage; I s  is a saturation current of the temperature sensing component  120 ; β is a characteristic parameter of the temperature sensing component  120 ; r e  is the resistance of the resistor R E ; r b  is the resistance of the resistor R B . According to the series resistor effect, V BE1 , V BE2  and ΔV BE  are given by the following equations: 
         [0000]        V   BE1   =V   T   ×In ( I   c1   /I   s )+ I   c1   ×r   e   +I   c1 /(β+1)× r   b    
         [0000]        V   BE2   =V   T   ×In ( I   c2   /I   s )+ I   c2   &gt;r   e   +I   c2 /(β+1)× r   b    
         [0000]      Δ V   BE   =V   BE2   −V   BE1   =V   T   ×In ( N )+( N− 1)× I ×( r   e +1/ (β+1)× r   b )   (1) 
         [0007]    From the equation (1), it is known that the series resistor effect can be cancelled when N=1, that is, I C1 =I C2 . However, when N=1, ΔV BE =V T ×In(1)=0. In other words, ΔV BE  is always independent of the environment temperature variation of the temperature sensing component  120 . As a result, the temperature sensing device  10  cannot get multiple of ΔV BE  by switching the current sources  104  and  106 , thereby the accuracy of temperature sensing cannot be improved. 
         [0008]    In conclusion, in the prior art temperature sensing device, the effect of current path series resistors and parasitic resistors cannot be cancelled. For improving the accuracy of temperature sensing, there should be a better way to cancel the series resistor effect. 
       SUMMARY OF THE INVENTION  
       [0009]    It is therefore a primary objective of the claimed invention to provide a temperature sensing device for improving series resistance cancellation. 
         [0010]    The present invention discloses a temperature sensing device for improving series resistance cancellation, which includes a temperature sensing unit including a first terminal and a second terminal for generating a plurality of voltage signals, a signal processing unit coupled to the temperature sensing unit for performing a signal process on the plurality of voltage signals for generating an output signal for presenting temperature variation, a first current source for driving the temperature sensing unit, a second current source for driving the temperature sensing unit, a third current source for driving the temperature sensing unit, a first switch coupled between the first current source and the first terminal of the temperature sensing unit for controlling a signal connection between the first current source and the first terminal of the temperature sensing unit according to a first control signal, a second switch coupled between the second current source and the first terminal of the temperature sensing unit for controlling a signal connection between the second current source and the first terminal of the temperature sensing unit according to a second control signal, and a third switch coupled between the third current source and the first terminal of the temperature sensing unit for controlling a signal connection between the third current source and the first terminal of the temperature sensing unit according to a third control signal, wherein the first control signal, the second control signal and the third control signal are generated by a control circuit and are outputted from the control circuit according to a specific cycle formed by a plurality of switches between the first control signal and the second control signal and one switch between the first control signal and the third control signal. 
         [0011]    The present invention further discloses a temperature sensing device for improving series resistance cancellation, which includes a temperature sensing unit including a first terminal and a second terminal for generating a plurality of voltage signals, a signal processing unit coupled to the temperature sensing unit for performing a signal process on the plurality of voltage signals for generating an output signal for presenting temperature variation, a plurality of current sources for driving the temperature sensing unit, and a plurality of switches, each of the plurality of switches being coupled between a corresponding current source of the plurality of current sources and the first terminal of the temperature sensing unit for controlling a signal connection between the corresponding current source of the plurality of current sources and the first terminal of the temperature sensing unit according to one of a plurality of control signals, wherein a number N of the plurality of current sources is greater than or equal to 3 and the plurality of control signals are generated by a control circuit and are outputted from the control circuit according to a specific cycle formed by an output order of a first control signal, a Nth control signal, a second control signal, the Nth control signal, a third control signal, the Nth control signal, . . . , a (N−1)th control signal and the Nth control signal. 
         [0012]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram of a temperature sensing device according to the prior art. 
           [0014]      FIG. 2  is a schematic diagram of a temperature sensing device according to an embodiment of the present invention. 
           [0015]      FIG. 3  is a schematic diagram of a temperature sensing device according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0016]    The prior art temperature sensing device cannot cancel the effect of current path series resistors, therefore, the present invention provides a temperature sensing device, which can cancel the effect of current path series resistors and parasitic resistors according to a specific cycle for switching current sources for improving series resistor cancellation, so as to enhance the accuracy of temperature sensing. 
         [0017]    Please refer to  FIG. 2 .  FIG. 2  is a schematic diagram of a temperature sensing device  20  according to an embodiment of the present invention. The temperature sensing device  20  comprises a temperature sensing unit  200 , a signal processing unit  202 , a first current source  204 , a second current source  206 , a third current source  208 , a first switch  210 , a second switch  212  and a third switch  214 . The signal processing unit  202  is coupled to the temperature sensing unit  200 . The first switch  210  is coupled between the current source  204  and the signal processing unit  202 ; the second switch  212  is coupled between the current source  206  and the signal processing unit  202 ; the third switch  214  is coupled between the current source  208  and the signal processing unit  202 . On the other hand, the temperature sensing unit  200  comprises a temperature sensing component  220  and resistors R B  and R E . In  FIG. 2 , the temperature sensing component  220  is a PNP bipolarjunction transistor (BJT), and the base of the temperature sensing component  220  is coupled to the resistor R B  and the emitter of the temperature sensing component  220  is coupled to the resistor R E . The resistors R B  is a combination representation of a base parasitic resistor of the temperature sensing component  220  and a series resistor in the line forming the current path between the base of the temperature sensing component  220  and the signal processing unit  202 . Similarly, the resistors R E  is a combination representation of an emitter parasitic resistor of the temperature sensing component  220  and a series resistor in the line forming the current path between the emitter of the temperature sensing component  220  and the signal processing unit  202 . 
         [0018]    The operation of the temperature sensing device  20  will be described in detail. The first switch  210  is used to control a signal connection between the first current source  204  and the signal processing unit  202  according to a first control signal S 21 ; the second switch  212  is used to control a signal connection between the second current source  206  and the signal processing unit  202  according to a second control signal S 22 ; the third switch  214  is used to control a signal connection between the third current source  208  and the signal processing unit  202  according to a third control signal S 23 . The first control signal S 21 , the second control signal S 22  and the third control signal S 23  are generated by a control circuit  22 . In addition, let V BE1  be the voltage difference of the two terminals of the temperature sensing unit  200  when the first switch  210  is turned on and the first current source  204  drives the temperature sensing unit  200 . Let V BE2  be the voltage difference of the two terminals of the temperature sensing unit  200  when the second switch  212  is turned on and the second current source  206  drives the temperature sensing unit  200 . Similarly, let V BE3  be the voltage difference of the two terminals of the temperature sensing unit  200  when the third switch  214  is turned on and the third current source  208  drives the temperature sensing unit  200 . 
         [0019]    Note that, the control circuit  22  outputs the first control signal S 21 , the second control signal S 22  and the third control signal S 23  by a specific cycle, so as to respectively control the first switch  210 , the second switch  212  and the third switch  214  for canceling the effect of series resistors. In an embodiment of the present invention, the effect of the resistors R B  and R E  is cancelled by a switch between the first current source  204  and the second current source  206  and a switch between the first current source  204  and the third current source  208 . In other words, the specific cycle describes the output order formed by a switch between the first control signal S 21  and the second control signal S 22  and a switch between the first control signal S 21  and the third control signal S 23 . In addition, ΔV BE  represents a difference between two voltage differences of the two terminals of the temperature sensing unit  200  at different currents. For example, when the current source that drives the temperature sensing unit  200  is switched from the first current source  204  to the second current source  206 , ΔV BE21 =V BE2 −V BE1 , then, the signal processing unit  202  generates an output signal V out  for presenting temperature variation according to ΔV BE . Note that, the temperature sensing unit  200  is an exemplary embodiment of the present invention, and those skilled in the art can make alternations and modifications accordingly. In the present invention, the temperature sensing unit  200  can be any device that can generate ΔV BE  for the signal processing unit  202  for generating the output signal V out . 
         [0020]    Let I, a×I and b×I be the currents of the first current source  204 , the second current source  206  and the third current source  208  respectively. Let M be the number of switches between the first current source  204  and the second current source  206 , and let N be the number of switches between the first current source  204  and the third current source  208 , where a, b, M, N are positive integers; V T  is temperature equivalent voltage; I s  is a saturation current of the temperature sensing component  120 ; β is a characteristic parameter of the temperature sensing component  220 ; r e  is the resistance of the resistor R E ; r b  is the resistance of the resistor R B . According to the series resistor effect, V BE1 , V BE2 , V BE3 , ΔV BE21  and ΔV BE31  are given by the following equations: 
         [0000]        V   BE1   =V   T   ×In ( I/I   s )+ I×r   e +/(β+1)× r   b    
         [0000]        V   BE2   =V   T   ×In ( a×I/I   s )+ a×I×r   e   +a×I /(β+1)× r   b    
         [0000]        V   BE3   =V   T   ×In ( b×I/I   s )+ b×I×r   e   +b×I /(β+1)× r   b    
         [0000]      Δ V   BE21   =V   BE2   −V   BE1   =V   T   ×In ( a )+( a− 1)× I ×( r   e +(1/(β+1)× r   b ) 
         [0000]      Δ V   BE31   =V   BE3   −V   BE1   =V   T   ×In ( b )+( b− 1)× I ×( r   e +(1/(β+1)× r   b ) 
         [0000]        M×ΔV   BE21   −N×ΔV   BE31   =M×V   T   ×In ( a )− N×V   T   ×In ( b )+ M ×( a− 1)× I ×( r   e +(1/(β+1)× r   b )− N ×( b− 1)× I ×( r   e +(1/(β+1)× r   b )   (2) 
         [0021]    From the equation (2), it is known that the series resistor effect can be cancelled when M×(a−1)=N×(b−1), that is, M×ΔV BE21 −N×ΔV BE31 =V T ×In[a M /b N ]. For example, let a=10, b=19, M=2 and N=1, the equation (2) becomes: 
         [0000]      2 ×ΔV   BE21   −ΔV   BE31   =V   T   ×In[ 10 2 /19 1   ]=V   T   ×In (5.26) 
         [0022]    or let a=6, b=16, M=3 and N=1, the equation (2) becomes: 
         [0000]      3 ×ΔV   BE21   −ΔV   BE31   =V   T   ×In [6 3 /16 1   ]=V   T   ×In (13.5) 
         [0023]    If M=2 and N=1, the turning-on order of the current sources is formed by the first current source  204 , the second current source  206 , the first current source  204  and the third current source  208  in order. In other words, the control circuit  22  outputs control signals by the specific cycle formed by the first control signal S 21 , the second control signal S 22 , the first control signal S 21  and the third control signal S 23  in order. Similarly, if M=3 and N=1, the turning-on order of the current sources is the second current source  206 , the first current source  204 , the second current source  206 , the first current source  204  and the third current source  208  in order. In other words, the control circuit  22  outputs control signals by the specific cycle formed by the second control signal S 22 , the first control signal S 21 , the second control signal S 22 , the first control signal S 21  and the third control signal S 23  in order. 
         [0024]    Note that, the temperature sensing device  20  is an embodiment of the present invention, and those skilled in the art can make alternations and modifications accordingly. Please refer to  FIG. 3 .  FIG. 3  is a schematic diagram of a temperature sensing device  30  according to an embodiment of the present invention. The temperature sensing device  30  is similar to the temperature sensing device  20 . The difference is that the temperature sensing device  20  comprises 3 current sources and 3 switches, while the temperature sensing device  30  comprises K current sources and K switches for K≧3. The temperature sensing device  30  comprises a temperature sensing unit  300 , a signal processing unit  302 , K current sources CS 1 -CS k  and K switches SW 1 -SW k . The temperature sensing unit  300  comprises a temperature sensing component  320  and resistors R B  and R E . The operation and the relationships of each unit of the temperature sensing device  30  is similar to the temperature sensing device  20  and is not given here. In addition, a control circuit  32  generates K control signals S 31 -S 3   k . Each control signal of the K control signals controls a signal connection between one corresponding current source of the K current sources and the signal processing unit  302 . Let a 1 ×I, a 2 ×I, a 3 ×I, . . . , a k ×I be the currents of the K current sources CS 1 -CS k  respectively. According to the series resistor effect, the voltage difference of the two terminals of the temperature sensing unit  300  at different current are given by the following equations: 
         [0000]        V   BE1   =V   T   ×In ( a   1   ×I/I   s )+ a   1   ×I×r   e   +a   1   ×I /(β+1) × r   b    
         [0000]        V   BE2   =V   T   ×In ( a   2   ×I/I   s )+ a   2   ×I×r   e   +a   2   ×I /(β+1) × r   b    
         [0000]        V   BE3   =V   T   ×In ( a   3   ×I/I   s )+ a   3   ×I×r   e   +a   3   ×I /(β+1) × r   b    
         [0025]    . . . 
         [0000]        V   BEk   =V   T   ×In ( a   k   ×I/I   s )+ a   k   ×I×r   e   +a   k   ×I /(β+1) × r   b    
         [0026]    In order to cancel the effect of the series resistor effect, the present invention lets a k =(a 1 +a 2 +a 3 + . . . +a k−1 )/(k−1) and then generates the following equation for a specific cycle: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0027]    From the equation (3), it is known that the turning-on order of the K current sources is formed by CS 1 , CS k , CS 2 , CS k , CS 3 , CS k , . . . , CS k−1 , CS k . Therefore, the embodiment of the present invention obtains a regular turning-on order of the current sources. For example, suppose the temperature sensing device  30  comprises 4 current sources CS 1 -CS 4 . Let a 1 ×I, a 2 ×I, a 3 ×I and a 4 ×I are currents of the 4 current source respectively, and let a 4 =(a 1 +a 2 +a 3 )/3, therefore, the regular turning-on order of the 4 current sources is CS 1 , CS 4 , CS 2 , CS 4 , CS 3 , CS 4 , that forms the specific cycle. Note that, the switches of different current sources are controlled by the K control signals S 31 -S 3   k  generated by the control circuit  32 . As to the implementation of the control circuit  32 , it is easier to implement the regular turning-on order, and as a result, the production cost of the embodiment of the present invention is reduced. 
         [0028]    In conclusion, the embodiment of the present invention can preferably cancels the effect of current path series resistors and parasitic resistors. Consequently, the location of temperature sensing component in the temperature sensing device is more flexible, and the production cost is reduced. 
         [0029]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.