Abstract:
A lossless inductor current sense technique integrates a matched, tunable complimentary filter with a switch mode power supply (SMPS) controller for accurately measuring current through the power inductor of the SMPS without introducing losses in the power circuit. The complimentary filter can be adjusted in-circuit to significantly reduce the effects of component tolerances, accurately measuring the power inductor current for over current protection and/or closed loop control. The frequency pole and gain of the complimentary integrated filter can be adjusted on the fly in order to adapt to dynamically changing operating conditions of the SMPS system.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to switch mode power supplies, and, more particularly, to lossless inductor current sensing in a switch-mode power supply (SMPS) by utilizing a matching complimentary tunable filter. 
       BACKGROUND 
       [0002]    The synchronous buck switch-mode power converter is a commonly used topology for SMPS applications. Current sensing in this topology can be challenging and must be overcome in design. Knowing or monitoring the current being injected into the load provides protection for the power converter and can improve dynamic performance during closed loop control thereof. 
         [0003]    Some prior technology current sensing techniques are as follows: Series sense resistor in main power path, current sense transformer, sensing voltage drop across the upper MOSFET switch, and an inductor voltage integral measurement by using an auxiliary winding to the power inductor  108 . Referring now to  FIG. 1 , depicted is a prior technology SMPS having a series sense resistor  110  in the main power path. A voltage across the series sense resistor  110  is detected by a differential input operational amplifier  114  and a V SENSE  output therefrom is proportional to the load current being supplied by the SMPS. However, the series sense resistor  110  introduces undesirable power loss. Since high efficiency is usually an overriding requirement in most SMPS applications, resistive circuit elements in the power path should be avoided or minimized. Only on rare occasions and for very specific reasons are power consuming resistances introduced into the main power control path. In auxiliary circuits, such as sequence, monitor, and control electronics of total system, high value resistors are common place, since their loss contributions are usually insignificant. 
         [0004]    Referring to  FIG. 2 , depicted is a prior technology SMPS having a current sense transformer for measuring current to the load. A current sense transformer  214  has a primary connected in series with the power path of the SMPS. A sense diode  216  and sense resistor  218  provide a V SENSE  output proportional to the load current being supplied by the SMPS. The current sense transformer  214  provides current monitoring for cycle-by-cycle peak current limit and peak current mode control. Power loss is minimal for this current monitoring configuration, however, implementation is expensive. 
         [0005]    Referring to  FIG. 3 , depicted is a prior technology SMPS with monitoring of the on-voltage drop across the upper MOSFET switch  104 . Sensing the voltage drop across the upper MOSFET switch  104  when the switch  104  is on provides a lossless signal representative of the current passing through it. A differential input operational amplifier  314  senses the voltage across the MOSFET switch  104  and produces a sense voltage output, V SENSE . However, this voltage drop is an inaccurate representation of the load current and is further subject to inaccuracies due to a high temperature coefficient of the MOSFET R DS-ON . 
         [0006]    Referring to  FIG. 4 , depicted is a prior technology SMPS having an inductor voltage integral measurement by using an auxiliary winding to the power inductor. By adding an auxiliary winding  416  to the power inductor  108 , a substantially lossless signal, V SENSE , representative of the current passing through the power inductor  108  is provided. However, the requirement for a coupled inductor increases the cost of the magnetic components of the SMPS. 
         [0007]    Referring to  FIG. 5 , depicted is a prior technology SMPS having a matching complimentary filter for measuring current through the SMPS inductor. This matching complimentary filter is utilized in combination with the inductor coil resistance, R L , of the power inductor  108  to sense the current therethrough. The matching complimentary filter consists of a resistor  520 , R F , in series with a small value capacitor  522 , C F . This series connected combination is connected in parallel with the inductor  108 . When the complimentary filter impedance is matched to the impedance of the power inductor  108 , i.e., L/R L =R F *C F , the capacitor voltage, V CF , is directly proportional to the current through the inductor  108 . This is readily shown from the following equations: 
         [0000]        V   L   =I   L *( R   L   +s*L ) 
         [0000]        V   L   =I   L   *R   L *(1 +s *( L/R   L )) 
         [0000]        V   CF   =V   L /(1 +s*R   F   *C   F ) 
         [0000]        V   CF   =I   L   *R   L *[(1 +s *( L/R   L ))/(1 +s*R   F   *C   F )] 
         [0000]      if  L/R   L   =R   F   *C   F , then  V   CF   =I   L   *R   L    
         [0000]    Where V L  is the voltage across the inductor  108 , L is the inductance in henrys of the inductor  108 , R L  is the coil resistance in ohms of the inductor  108 , I L  is the current in amperes through the inductor  108 , and s is the complex frequency in the s-domain (i.e., frequency-domain). Where V CF  is the voltage across the matching complimentary filter capacitor  522 , C F  is the capacitance in farads of the capacitor  522 , and R F  is the resistance in ohms of the matching complimentary filter resistor  520 . 
         [0008]    The voltage, V CF , across the capacitor  522 , C F , is applied to the inputs of a differential amplifier  514  and a V SENSE  output therefrom is proportional to the load current, I L , being supplied by the SMPS. Measurement of current through the inductor  108  is lossless since no resistor or impedance has been introduced into the high current path of the SMPS. However, this complimentary filter must be matched to the equivalent inductance, L, and series resistance, R L , of the inductor  108  for accurate and absolute current measurement results. This circuit also suffers from a high temperature coefficient due to the discrete component value changes over an operating temperature range, thereby reducing accuracy over the range of operating conditions of the SMPS. 
       SUMMARY 
       [0009]    What is needed is a system, method and apparatus for accurately measuring current through a SMPS power inductor that does not waste power, is highly accurate over all operating conditions, and is flexible and low in cost to implement in a mixed signal integrated circuit. 
         [0010]    According to a specific example embodiment of this disclosure, a tunable complimentary filter for measuring current through an inductor in a switch-mode power supply (SMPS) comprises: an operational transconductance amplifier (OTA) having a first input coupled to a voltage source side of an inductor in a switch-mode power supply (SMPS), a second input coupled to a load side of the inductor and a current output; an operational amplifier configured as a buffer amplifier and having an input coupled to the current output of the OTA; a first adjustable resistor coupled between the current output of the OTA and a return of the voltage source; a second adjustable resistor having a first end coupled to an output of the operational amplifier; and a tuning capacitor coupled between a second end of the second adjustable resistor and the return of the voltage source; a voltage from the tunable complimentary filter is available at the second end of the second adjustable resistor, wherein the voltage is representative of the current flowing through the inductor of the SMPS. 
         [0011]    According to another specific example embodiment of this disclosure, a tunable complimentary filter for measuring current through an inductor in a switch-mode power supply (SMPS) comprises: an operational transconductance amplifier (OTA) having a first input coupled to a voltage source side of an inductor in a switch-mode power supply (SMPS), a second input coupled to a load side of the inductor and a current output; an adjustable resistor coupled between the current output of the OTA and a return of the voltage source; and a tuning capacitor coupled between the current output of the OTA and the return of the voltage source; a voltage from the tunable complimentary filter is available at the current output of the OTA, wherein the voltage is representative of the current flowing through the inductor of the SMPS. 
         [0012]    According to still another specific example embodiment of this disclosure, a system for controlling a switch-mode power supply (SMPS) comprises: a power inductor; high and low switching power transistors coupled between the power inductor and positive and negative nodes of a voltage source, respectively; a filter capacitor coupled to the power inductor and the negative node of the voltage source; a tunable complimentary filter coupled to the power inductor, wherein the tunable complimentary filter measures current through the power inductor by providing a voltage output from the tunable complimentary filter representative of the current flowing through the power inductor; a SMPS controller having driver outputs coupled to the high and low switching transistors, a first input coupled to the filter capacitor for measuring a regulated output voltage from the SMPS and a second input coupled to the voltage output of the tunable complimentary filter, wherein the SMPS controller uses the regulated output voltage coupled to the first input and the voltage output from the tunable complimentary filter representative of the current flowing through the inductor as SMPS control parameters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    A more complete understanding of the present disclosure thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
           [0014]      FIG. 1  illustrates a schematic diagram of a prior technology SMPS having a series sense resistor in the main power path; 
           [0015]      FIG. 2  illustrates a schematic diagram of a prior technology SMPS having a current sense transformer for measuring current to the load; 
           [0016]      FIG. 3  illustrates a schematic diagram of a prior technology SMPS with monitoring of the on-voltage drop across the upper MOSFET switch; 
           [0017]      FIG. 4  illustrates a schematic diagram of a prior technology SMPS having an inductor voltage integral measurement by using an auxiliary winding to the power inductor; 
           [0018]      FIG. 5  illustrates a schematic diagram of a prior technology SMPS having a matching complimentary filter for measuring current through the SMPS inductor; 
           [0019]      FIG. 6  illustrates a schematic diagram of a circuit for losslessly measuring inductor current of a SMPS, according to a specific example embodiment of this disclosure; 
           [0020]      FIG. 7  illustrates a schematic diagram of a circuit for losslessly measuring inductor current of a SMPS, according to another specific example embodiment of this disclosure; 
           [0021]      FIG. 8  illustrates a graph of pole frequency adjustments for the circuits shown in  FIGS. 6 and 7 ; 
           [0022]      FIG. 9  illustrates a graph of DC gain adjustments for the circuits shown in  FIGS. 6 and 7 ; and 
           [0023]      FIG. 10  illustrates a schematic block diagram of a mixed signal integrated circuit device for controlling a SMPS system using the specific example embodiments of the tunable complimentary filters shown in  FIGS. 6 and 7 . 
       
    
    
       [0024]    While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0025]    Referring now to the drawing, the details of specific example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
         [0026]    The matched filter shown in  FIG. 5  is a basis for the new, novel and non-obvious system, method and apparatus for losslessly measuring SMPS power inductor current, according to the teachings of this disclosure. A matched complimentary filter is integrated into an SMPS controller by utilizing a tunable filter comprising an operational transconductance amplifier (OTA), a variable resistor and a variable capacitor in one specific example embodiment ( FIG. 6 ). In another specific example embodiment, an operational amplifier, configured as a buffer, and a variable resistor have been added, providing independent gain and pole location adjustment ( FIG. 7 ). 
         [0027]    Referring to  FIG. 6 , depicted is a schematic diagram of a circuit for losslessly measuring inductor current of a SMPS, according to a specific example embodiment of this disclosure. A tunable complimentary filter inductor current measuring circuit comprises an operational transconductance amplifier (OTA)  622 , a variable resistor  624 , and a variable capacitor  626 . The OTA  622  is configured as a voltage variable integrator and is used as a first-order low-pass filter (see  FIGS. 8 and 9 ). The transfer function for this integrator is: 
         [0000]        V   O /( V   I1   −V   I2 )= g   m /( s*C   F ) 
         [0028]    The OTA  622  circuit shown in  FIG. 6  has an adjustable pole frequency, and adjustable DC gain. The pole frequency is adjusted by the capacitor  626 , C F , and resistor  624 , R F ; and the DC gain is adjusted by the resistor  624 , R F . The transfer function of the filter shown in  FIG. 6  is represented by: 
         [0000]        V   O /( V   I1   −V   I2 )=( g   m   *R   F )/( s*R   F   *C   F +1) 
         [0000]    As noted from the transfer function, the DC gain is equal to gm*R F ; and the pole frequency is equal to 1/(2π*R F *C F ) Hz. The pole frequency and DC gain can not be adjusted independently. 
         [0029]    Referring to  FIG. 7 , depicted is a schematic diagram of a circuit for losslessly measuring inductor current of a SMPS, according to another specific example embodiment of this disclosure. A tunable complimentary filter inductor current measuring circuit comprises an operational transconductance amplifier (OTA)  622 , a variable resistor  624 , an operational amplifier  728  configured as a buffer, a variable resistor  730 , and a variable capacitor  626 . The OTA  622  is configured as a voltage variable input gain stage with a wide bandwidth. The operational amplifier  728  decouples the input gain stage from the single pole, low pass filter. The pole frequency can be adjusted by changing the resistor  624 , R F , and/or the capacitor  626 , C F , and the DC gain can be subsequently adjusted by changing the variable resistor  730 , R G . The transfer function of the filter shown in  FIG. 7  is represented by: 
         [0000]        V   O /( V   I1   −V   I2 )=( g   m   *R   G )/( s*R   F   *C   F +1) 
         [0000]    As noted from the transfer function, the DC gain is equal to g m *R G ; and the pole frequency is equal to 1/(2π*R F *C F ) Hz. The pole frequency and DC gain can be adjusted independently. 
         [0030]    The tunable complimentary filters shown in  FIGS. 6 and 7  can be adjusted, e.g., tuned, to match the L/RL zero pole, and gain adjusted to amplify the sensed current signal to a desired voltage level. The tunable complimentary filters can further be adjusted in-circuit to significantly reduce the effects of component tolerances. The tunable complimentary filters can be adjusted on the fly in order to adapt to changing operating conditions of the SMPS. The tunable complimentary filters accurately measure the inductor  108  current for over current protection and/or closed loop control of the SMPS. 
         [0031]    Referring to  FIG. 8 , depicted is a graph of pole frequency adjustments for the circuits shown in  FIGS. 6 and 7 . 
         [0032]    Referring to  FIG. 9 , depicted is a graph of DC gain adjustments for the circuits shown in  FIGS. 6 and 7 . 
         [0033]    Referring to  FIG. 10 , depicted is a schematic block diagram of a mixed signal integrated circuit device for controlling a SMPS system using the specific example embodiments of the tunable complimentary filters shown in  FIGS. 6 and 7 . The mixed signal integrated circuit device  1002  comprises a SMPS controller  1004 , power transistor drivers  1006 , a microcontroller  1008  and associated memory  1010 , an OTA  622 , an operational amplifier  728 , a DC gain setting resistor  730 , a pole frequency setting resistor  624 , and a pole frequency setting capacitor  626 . The SMPS controller  1004  may generate a pulse width modulation (PWM), pulse frequency modulation (PFM), pulse density modulation (PDM), etc., signal for controlling the power transistor drivers  1006  that provide the power control signals to the power MOSFET switches  104  and  106  of the SMPS. The SMPS controller  1004  monitors the voltage regulated output voltage, V OUT , and the measured inductor current signal, V O , from the tunable complimentary filter comprising OTA  622 , operational amplifier  728 , variable resistors  624  and  730 , and tuning capacitor  626 . 
         [0034]    The OTA  622 , operational amplifier  728 , variable resistors  624  and  730 , and tuning capacitor  626  are connected and operate as more fully described hereinabove. The microcontroller  1008  controls the variable resistors  624  and  730 , as well as setting parameters for the SMPS controller  1004  (dotted lines represent control signals). It is contemplated and within the scope of this disclosure that the microcontroller  1008  can perform the same functions as and replace the SMPS controller  1004 . The microcontroller  1008  has analog inputs and analog-to-digital conversion circuits (not shown). An operating program for the mixed signal integrated circuit device  1002  may be stored in the memory  1010  associated with the microcontroller  1008 . An additional capacitor  626   a  may be added external to the mixed signal integrated circuit device  1002  and in parallel with the internal capacitor  626 . The microcontroller  1008  may control the capacitance value of the capacitor  626 , and in combination with control of the variable resistors  624  and  730 . Control of the capacitor  626  and/or variable resistors  624  and  730  by the microcontroller  1008  allows dynamic tuning of the gain and/or pole frequency of the tunable complementary filter complimentary filter on the fly for optimal current measurement under changing operating conditions of the SMPS. The tunable complimentary filter implementation(s), according to the teachings of this disclosure can also be applied, but is not limited to, switch-mode power converters, (SMPC), brushless dc motors, etc. 
         [0035]    While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.