Abstract:
An inductor current-sensing circuit for measuring a current in an inductor includes (a) a first RC network coupled between a first terminal of the inductor and a reference voltage source; and (b) a second RC network coupled between a second terminal of the inductor and the reference voltage source. The first RC network and the second RC network each have a time constant substantially equal to the ratio between the inductance and the DC resistance of the inductor. The inductor which current is being measured may be a primary inductor of a four-switch buck boost converter receiving an input voltage and providing an output voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to and claims priority of U.S. provisional patent application (“Copending Provisional Application I”), Ser. No. 62/054,587, entitled “DCR inductor current sensing for 4 switch buck-boost converters,” filed on Sep. 24, 2014. The disclosure of the Copending Provisional Application I is hereby incorporated by reference in its entirety. 
         [0002]    The present application is also related to U.S. provisional patent application (“Copending Provisional Application II”), Ser. No. 62/088,433, entitled “Peak-Buck Peak-Boost Current-Mode Control for Switched Step-up Step-down Regulators,” filed on Dec. 5, 2014. The disclosure of the Copending Provisional Application II is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0003]    The present invention relates to measuring an inductor current in a four-switch buck-boost power converter. In particular, the present invention relates to measuring inductor current using an RC circuit referenced to a virtual ground. 
       2. Discussion of the Related Art 
       [0004]    Four-switch buck-boost power converters are used in many different applications. Such a power converter regulates an output voltage which may be higher than, equal to or lower than the input voltage. A typical four-switch buck-boost power converter has a single inductor and operates synchronously to provide high efficiency over a wide range of load currents. In a power converter, to provide over-current protection, discontinuous-mode operation or current loop regulation, inductor current-sensing is normally required. However, in a four-switch buck-boost power converter, current sensing is made difficult by the high common-mode noise that is often present on both terminals of the inductor. 
         [0005]      FIG. 1  shows a first current-sensing technique using sensing resistors in power converter  100 . Such a technique is used, for example, in the LM5118 and LM25118 circuits available from Texas Instruments, Inc., Dallas, Tex. As shown in  FIG. 1 , power converter circuit  100  includes inductor  101 , diode  104 , sensing resistor  105  and switches  102  and  103 . Sensing resistor  105 , which is connected in series with diode  104  to one terminal of inductor  101 , senses the current in inductor  101  when (and only when) diode  104  is conducting. However, such a configuration cannot sense the peak current in inductor  101 . 
         [0006]      FIG. 2  shows another inductor current-sensing technique in four-switch power converter  200 . Four-switch power converter  200  includes inductor  201 , switches  202 - 205 , output capacitor  206  and sensing resistor  207 . Sensing resistor  207  senses a valley inductor current in “buck” mode (i.e., when switch  205  is maintained in a constant “on” state) and senses a peak inductor current in “boost” mode (i.e., switch  202  is maintained in a constant “on” state). This current-sensing technique is used in the LTC3780, LTC3789, LT3791, LT8705 circuits available from Linear Technology Corporation, Milpitas, Calif. 
         [0007]    The technique of  FIGS. 1 and 2  has two drawbacks. First, both sensing resistor  105  of  FIG. 1  and sensing resistor  207  of  FIG. 2  sense only a portion of their respective inductor currents, as each sensing resistor relies on a switch configuration that allows a current flowing in the respective inductor to flow through the sensing resistor. Second, sensing resistor  105  of  FIG. 1  and sensing resistor  207  of  FIG. 2  both dissipate power, which may lead to thermal issues in the respective circuits. At the same time, using high-power, precision sensing resistors increases system cost and circuit footprint. 
         [0008]    Another current-sensing method, referred to as the “DCR inductor current-sensing scheme” has been widely used in buck or boost converters.  FIG. 3  shows one example of the DCR current sensing scheme in a four-switch buck-boost converter  300 . As shown in  FIG. 3 , four-switch buck-boost converter  300  includes switches  305 - 308 , inductor  303  and output capacitor  309 . The equivalent DC resistance R DCR  of inductor  303  is represented by DCR resistor  304  in  FIG. 3 . The current in inductor  303  is sensed by providing series-connected sensing resistor  301  and sensing capacitor  302  in parallel to inductor  303  (and equivalent DCR resistor  304 ). The DCR inductor current-sensing scheme attempts to match the time constant of inductor current i L , given by the ratio of inductance L of inductor  303  to its equivalent DC resistance R DCR  (i.e., L/R DCR ), by the product of resistance R s  of sensing resistor  301  and capacitance C s  of sensing capacitor  302 . Under this scheme the sensed voltage V sense  across sensing capacitor  302  is proportional to the product of inductor current i L  and DC resistance R DCR  (i.e., V sense =i L *R DCR ). However, as explained in the article “10 MHz Current Mode 4 Switch Buck Boost Converter (4SBBC) for Polar Modulation,” by Park et al., published in the  Proceedings of the  23 rd    Annual Applied Power Electronics Conference , pp-1977-83, the rail-to-rain common mode voltage range and the high common mode noise in the sensed voltage, due to switching in the converter output switches, make the current-sensing circuit complicated and very difficult to implement. 
       SUMMARY 
       [0009]    According to one embodiment of the present invention, an inductor current-sensing circuit for measuring a current in an inductor includes (a) a first RC network coupled between a first terminal of the inductor and a reference voltage source; and (b) a second RC network coupled between a second terminal of the inductor and the reference voltage source. The first RC network and the second RC network each have a time constant substantially equal to the ratio between the inductance and the DC resistance of the inductor. The inductor which current is being measured may be a primary inductor of a four-switch buck boost converter receiving an input voltage and providing an output voltage. 
         [0010]    In one embodiment, the reference voltage source provides a virtual ground reference, which may be connected to a system ground reference through a decoupling capacitor. The virtual ground reference may refer to the output voltage, the input voltage, and an average between the voltages across the inductor, when the four-switch buck boost converter operates in a buck mode, a boost mode and a buck-boost mode, respectively. 
         [0011]    In one embodiment, the inductor current-sensing circuit may further include a third sensing capacitor connected between the first RC network and the second RC network, with the third sensing capacitor having a greater capacitance than each of the effective capacitances of the first and the second RC networks. 
         [0012]    According to another embodiment of the present invention, an inductor current-sensing circuit for measuring a current in an inductor includes: (a) a sensing resistor connected in series with the inductor; (b) a first RC network coupled between a first terminal of the sensing resistor and a reference voltage source; and (c) a second RC network coupled between a second terminal of the sensing resistor and the reference voltage source. The first RC network and the second RC network may each have a time constant substantially equal to the ratio between the inductance of the inductor and the DC resistance of the inductor. The first and second RC network may each include (a) a sensing capacitor; (b) a first resistor coupled between a terminal of the sensing resistor and a first terminal of the sensing capacitor; a blocking capacitor coupled at one terminal to one terminal of the inductor; and a second resistor coupled between the first terminal of the sensing capacitor and the other terminal of the blocking capacitor. The ratio in resistance value between the second resistor of the second RC network and the first resistor of the second resistor network less one may be substantially the ratio in resistance value between the sensing resistor and the equivalent DC resistance of the inductor. The blocking capacitor in each of the first and second RC networks may have a capacitance that is greater than the capacitance of the sensing capacitor in the corresponding one of the first and second RC networks. 
         [0013]    An inductor current sensed using a method of the present invention may be used to control switching in a four-switch buck boost converter. An example of such control may be found, for example, in the Copending Provisional Patent Application II. 
         [0014]    The present invention is better understood upon consideration of the detailed description below in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a first current-sensing technique using sensing resistors in power converter  100 . 
           [0016]      FIG. 2  shows another inductor current-sensing technique in four-switch power converter  200 . 
           [0017]      FIG. 3  shows one example of the DCR current sensing scheme in a four-switch buck-boost converter  300 . 
           [0018]      FIG. 4  shows four-switch buck boost converter  400  implementing a DCR current-sensing method in accordance with one embodiment of the present invention. 
           [0019]      FIG. 5  shows four-switch buck boost converter  500  which eliminates DC bias voltages at sensing capacitors  402 - a  and  402 - b  by referring RC filters  410  and  420  to a virtual ground, in accordance with one embodiment of the present invention. 
           [0020]      FIG. 6  shows four-switch buck boost converter  600  which provides more robust performance than four-switch buck boost converter  500  of  FIG. 5  when operating in buck-boost mode, in accordance with one embodiment of the present invention. 
           [0021]      FIG. 7  shows four-switch buck-boost converter  700  using sensing resistor  701 , rather than the DC resistance of inductor  303 , to sense the current in inductor  303 , in accordance with one embodiment of the present invention. 
           [0022]      FIG. 8  shows four-switch buck-boost converter  800 , which provides sensing capacitor  801  across nodes I sense+  and I sense−  and a virtual ground node  802 , according to one embodiment of the present invention. 
       
    
    
       [0023]    In these figures, like elements are assigned like reference numerals. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]      FIG. 4  shows four-switch buck boost converter  400  implementing a DCR current-sensing method in accordance with one embodiment of the present invention. In contrast with four-switch buck boost converter  300  of  FIG. 3 , which provides sensing resistor  301  and sensing capacitor  302  in parallel to inductor  303 , four-switch buck boost converter  400  provides RC filters  410  and  420 , consisting of sensing resistor  401 - a  and sensing capacitor  402 - a  and sensing resistor  401 - b  and sensing capacitor  402 - b , respectively. The sensed voltage across nodes I sense+  and I sense−  in RC filters  410  and  420 , respectively, represent the differential voltage across switching nodes SW 1  and SW 2 . By matching the time constant L/R DCR  to the time constant R s C s  in each of RC filters  410  and  420 , the sensed voltage V sense  is directly proportional to the inductor current i L  and DC resistance R DCR  of inductor  303  and V sense =I sense+ −I sense− =i L *R DCR  (see  FIG. 4 ). 
         [0025]    In the embodiment of  FIG. 4 , lossless full-inductor current-sensing is achieved without high common-mode noise. However, sensing capacitors  402 - a  and  402 - b  should be kept very well-matched, so as to eliminate any transient differential error. Such matching may be achieved, for example, by fabricating both sensing capacitors on the same silicon substrate. Also, the DC bias voltages at sensing capacitors  402 - a  and  402 - b  vary according to the input and output voltages. Preferably, sensing capacitors  402 - a  and  402 - b  should be implemented by capacitors with low voltage coefficients, so as to maintain the matched time constants over wide voltage range. 
         [0026]      FIG. 5  shows four-switch buck boost converter  500  which eliminates DC bias voltages at sensing capacitors  402 - a  and  402 - b  by referring RC filters  410  and  420  to a virtual ground, in accordance with one embodiment of the present invention. As shown in  FIG. 5 , rather than connecting sensing capacitors  402 - a  and  402 - b  to system ground, sensing capacitors  402 - a  and  402 - b  are coupled to a virtual ground, which may be a different reference voltage, depending on the operation mode. For example, in the buck mode (i.e., the operating mode in which switch  308  is always conducting), the virtual ground may be coupled to output voltage V OUT . In the boost mode (i.e., in the operating mode in which switch  305  is always conducting), the virtual ground may be coupled to input voltage V IN . In the buck-boost mode, the virtual ground may be controlled to track the average voltage of nodes SW 1  and SW 2 . Decoupling capacitor  501  maintains the voltage at the virtual ground during any mode switching, and transient voltage excursions. In four-switch buck boost converter  500  of  FIG. 5 , the resistors  401 - a  and  401 - b  and sensing capacitors  402 - a  and  402 - b  are also designed to match the time constant of inductor current i L  (i.e., L/R DCR =R s C s ; see  FIG. 5 ). The virtual ground is preferably well-maintained during buck-boost mode, so as to avoid any error due to mismatch of sensing capacitors  402 - a  and  402 - b  during transients. Again sensing capacitors  402 - a  and  402 - b  should be well-matched to avoid transient errors in buck-boost mode operations. 
         [0027]      FIG. 6  shows four-switch buck boost converter  600  which provides more robust performance than four-switch buck boost converter  500  of  FIG. 5  when operating in buck-boost mode, in accordance with one embodiment of the present invention. Four-switch buck boost converter  600  improves transient performance by including sensing capacitor  601  (with a capacitance C s ) in addition to sensing capacitors  402 - a  and  402 - b , which are each now provided a capacitance C f ). In four-switch buck boost converter  600 , the time constant of inductor current i L  is matched according to the equation 
         [0000]    
       
         
           
             
               L 
               
                 R 
                 DCR 
               
             
             = 
             
               2 
                
               
                   
               
                
               
                 
                   R 
                   S 
                 
                  
                 
                   ( 
                   
                     
                       C 
                       S 
                     
                     + 
                     
                       
                         C 
                         f 
                       
                       2 
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    (See,  FIG. 6 ). In this equation, the term inside the parentheses may be designed such that capacitance C s  of capacitor  601  is dominant (i.e., capacitance C f  is selected to be much less than capacitance C s ), so as to allow capacitance C s  to match the time constant in the inductor current. Because this approach reduces the sensitivity to sensing capacitors  402 - a  and  402 - b , any effect arising from a mismatch in capacitance between capacitors  402 - a  and  402 - b  is significantly minimized. 
         [0028]    A simulation was performed to investigate the modified DCR current-sensing method in four-switch buck boost converter  600  of  FIG. 6 . In this simulation, inductor  303  is provided an inductance L=4.7 uH, DC resistance R DCR =10 mΩ, sensing resistor  401 - a  and  401 - b  are each provided resistance R S =33.33 kΩ, and sensing capacitor  601  is provided a nominal capacitance C S  of 0.0047 uF, capacitors  402 - a  and  402 - b  are provided capacitances of 0.0037 uF and 0.0057 uF, respectively, to simulate a 10% mismatch of nominal capacitance C f  between capacitors  402 - a  and  402 - b . In this simulation, four-switch buck-boost converter  600  is operated in buck-boost mode. The input voltage is initially ramped up from 0 volts to 10 volts over 0.1 ms, is then maintained at 10 volts for 0.4 ms, and then allowed to rise to 13 volts over 0.15 ms, where it is held until 2.0 ms. During that period, the output voltage is initially at zero, but rises to about 15 volts by 0.7 ms, and is regulated at that level until 1.3 ms, when it is abruptly grounded. The difference between the current in inductor  303  and the measured current based on the voltage drop across capacitor  601  is found to be insignificant throughout the simulation period. The 10% mismatch in capacitors  402 - a  and  402 - b  is estimated to cause a transient voltage of less than 4 mV across sense capacitor  601 . 
         [0029]    For high precision operations, a sensing resistor may be provided in place of DCR  304  (i.e., the DC resistance of inductor  303 ), as the DC resistance of an inductor is less reliable.  FIG. 7  shows four-switch buck-boost converter  700  using sensing resistor  701  to sense the current in inductor  303 , in accordance with one embodiment of the present invention. As shown in  FIG. 7 , sensing resistor  701 , with a resistance R sense , is connected in series with inductor  303  (resistor  701  may be connected to either node SW 1  or node SW 2 , i.e., on either side of inductor  303 ). The terminals of sensing resistor  701  are each respectively coupled to ground through serially connected RC circuits formed by sensing resistors  401 - a  and  401 - b  and sensing capacitors  402 - a  and  402 - b . In addition, resistors  703 - a  and  703 - b  and blocking capacitors  702 - a  and  702 - b  form two RC circuits that respectively connect nodes I sense+  and I sense−  to node SW 2  at the terminal of inductor  303  away from sensing resistor  701 . Blocking capacitors  702 - a  and  702 - b  are each selected to have a capacitance C block  that is much less than the capacitance C S  in each of sensing capacitor  402 - a  and  402 - b . As shown in  FIG. 7 , resistors  401 - a  and  703 - b  both have a resistance R 1  and resistors  401 - b  and  703 - a  both have a resistance R 2 . In this configuration, capacitance C S  and resistor values R 1  and R 2  may be selected such that the RC time constant of resistor  401 - b  and sensing capacitor  402 - b  can be matched to the inductor  303   
         [0000]    
       
         
           
             
               ( 
               
                 
                   i 
                   . 
                   e 
                   . 
                 
                 , 
                 
                   
                     L 
                     DCR 
                   
                   = 
                   
                     
                       R 
                       2 
                     
                      
                     
                       C 
                       S 
                     
                   
                 
               
               ) 
             
             , 
           
         
       
     
         [0000]    while the resistance relationship 
         [0000]    
       
         
           
             
               
                 
                   R 
                   2 
                 
                 
                   R 
                   1 
                 
               
               - 
               1 
             
             = 
             
               
                 R 
                 sense 
               
               DCR 
             
           
         
       
     
         [0000]    holds (see,  FIG. 7 ). The voltage drop V sense  across nodes I sense+  and I sense−  is given by the product of inductor current i L  and sensing resistance R sense . 
         [0030]    The virtual ground technique and the technique of providing a sensing capacitor across I sense+  and I sense−  to avoid the effects of a capacitance mismatch in capacitors  402 - a  and  402 - b , as discussed above with respect to  FIGS. 5 and 6 , may also be applicable to four-switch buck-boost converter  700  of  FIG. 7 .  FIG. 8  shows four-switch buck-boost converter  800 , which provides sensing capacitor  801  across nodes I sense+  and I sense−  and virtual ground node  802 , according to one embodiment of the present invention. As shown in  FIG. 8 , decoupling capacitor (with capacitance C dcouple ) isolates virtual ground node  802  from the true ground reference. The voltage at virtual ground node  802  may be controlled to be equal to the average voltage at node SW 1 , node SW 2 , or their average 
         [0000]    
       
         
           
             
               
                 ( 
                 
                   
                     V 
                     
                       SW 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   + 
                   
                     V 
                     
                       SW 
                        
                       
                           
                       
                        
                       2 
                     
                   
                 
                 ) 
               
               2 
             
             . 
           
         
       
     
         [0000]    In this configuration of four-switch buck-boost converter  800  of  FIG. 8 , by choosing sensing capacitance C S  to be greater than capacitance C f  in each of capacitors  402 - a  and  402 - b  and much less than capacitance C block , the time constant of inductor  303  is matched according to the equation 
         [0000]    
       
         
           
             
               
                 L 
                 DCR 
               
               = 
               
                 
                   R 
                   2 
                 
                  
                 
                   ( 
                   
                     
                       C 
                       S 
                     
                     + 
                     
                       
                         C 
                         f 
                       
                       2 
                     
                   
                   ) 
                 
               
             
             , 
           
         
       
     
         [0000]    while the resistance relationship 
         [0000]    
       
         
           
             
               
                 
                   R 
                   2 
                 
                 
                   R 
                   1 
                 
               
               - 
               1 
             
             = 
             
               
                 R 
                 sense 
               
               DCR 
             
           
         
       
     
         [0000]    holds. The voltage drop V sense  across nodes I sense+  and I sense−  is given by the product of inductor current i L  and sensing resistance R sense  (see,  FIG. 8 ). Of course, as in four-switch buck-boost converter  700  of  FIG. 7 , resistor  701  may be connected to either node SW 1  or node SW 2 , i.e., on either side of inductor  303 . 
         [0031]    The present invention is applicable to any application that requires inductor current-sensing, such as sensing an average inductor current. The methods illustrated in  FIGS. 4-6  may be used to modify an inductor current through high-pass or low-pass filtering, thus providing a lossless method to sense a continuous inductor current without incurring a DC error. Methods of the present invention are suitable for use in both voltage-mode or current-mode control of a four-switch buck boost converter. The present invention may be implemented in an integrated circuit. 
         [0032]    The above-detailed description is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Various modifications and variations within the scope of the present invention are possible. The present invention is set forth in the following claims.