Abstract:
Power management circuitry ( 7 - 2,3,4 ) for converting a harvested voltage (V hrv ) to an output voltage (V BAT ) applied to a battery ( 6 ) includes an inductor (L 0 ) having a first terminal ( 3 ) coupled to receive the harvested voltage (V hrv ) and a second terminal coupled to a first terminal of a first switch (S 0 ). The power management circuitry transfers the current generated by an energy harvester ( 2 ) to the battery if it ( 6 ) is not fully charged, and shunts the current away from the battery ( 6 ) to avoid overcharging if it is fully charged.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to energy harvesters, such as vibration energy harvesters, for scavenging or harvesting very low levels of energy, and more particularly to circuits and methods for protecting batteries or supercapacitors in which the harvested energy is stored. The invention also relates to circuits and methods for preventing surge current damage to inductors and/or other circuit components in DC-DC converter circuits or other power management circuitry that receives energy from the outputs of energy harvesters. 
         [0002]    Various very low power, i.e., “nano-power”, integrated circuits that require extremely low amounts of operating current have been developed which can be powered by very small amounts of power scavenged or harvested from ambient solar, vibrational, thermal, and/or biological energy sources by means of micro-energy harvesting devices and then stored in batteries or supercapacitors. (The term “nano-power” as used herein is intended to encompass circuits and/or circuit components which draw DC current of less than roughly 1 microampere.) The amount of energy available from a harvester usually is small and unpredictable, so intermediate energy storage is often required in these applications to provide for system power needs when energy from the harvester is unavailable or insufficient. Lithium batteries or supercapacitors are commonly used for such intermediate energy storage. 
         [0003]    Prior Art  FIG. 1  shows a circuit  1  including an energy harvester  2  which produces a DC voltage V hrv  on a conductor  3  that is connected to one terminal of a large filter capacitor C 0  and to the input of a conventional boost converter  7 - 1 . Boost converter  7 - 1  includes an inductor L 0  coupled between V hrv  and conductor  4 , which is connected to one terminal of a switch S 0  and to the anode of a diode D 0 . The other terminal of switch S 0  is connected to ground. The cathode of diode D 0  is connected by conductor  5  to the (+) terminal of a battery or supercapacitor  6 . 
         [0004]    Suitable power management circuitry for energy harvester  2  controls switch S 0  so as to provide charging of battery/supercapacitor  6  when energy is available from harvester  2  if battery/supercapacitor  6  is not fully charged to its maximum or fully-charged voltage V BAT(max) . (For a typical lithium battery, V BAT(max)  is 4.5 volts.) 
         [0005]    If battery  6  is fully charged to V BAT(max) , then further charging may permanently damage it. In the unprotected system of Prior Art  FIG. 1  there is nothing to prevent current from harvester  2  from overcharging battery/supercapacitor  6 . However, the output voltage V hrv  generated by harvester  2  should be limited to a value below V BAT(max)  to prevent damage to the battery/supercapacitor. Furthermore, such limiting of V hrv  should prevent surge currents supplied by charged-up filter capacitor C 0  from damaging circuit components such as inductor L 0  and/or other circuit components in the power management circuitry. 
         [0006]    Thus, there is an unmet need for a circuit and method for protecting batteries or supercapacitors in which harvested energy is stored. 
         [0007]    There also is an unmet need for a circuit and method for preventing surge current damage to inductors in DC-DC converter circuits coupled to an energy harvester. 
         [0008]    There also is an unmet need for a circuit and method for both protecting batteries or supercapacitors in which harvested energy is stored and preventing surge current damage to inductors in DC-DC converter circuits coupled to an energy harvester. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the invention to provide a circuit and method for protecting batteries or supercapacitors in which harvested energy is stored. 
         [0010]    It is another object of the invention to provide a circuit and method for preventing surge current damage to inductors and/or other components in DC-DC converter circuits coupled to an energy harvester. 
         [0011]    It is another object of the invention to provide a circuit and method for both protecting batteries or supercapacitors in which harvested energy is stored and preventing surge current damage to inductors and/or other components in DC-DC converter circuits coupled to an energy harvester. 
         [0012]    Briefly described, and in accordance with one embodiment, the present invention provides power management circuitry ( 7 - 2 , 3 , 4 ) for converting a harvested voltage (V hrv ) to an output voltage (V BAT ) applied to an energy storage device ( 6 ) that includes an inductor (L 0 ) having a first terminal ( 3 ) coupled to receive the harvested voltage (V hrv ) and a second terminal coupled to a first terminal of a first switch (S 0 ). The power management circuitry transfers the current generated by an energy harvester ( 2 ) to the energy storage device if it is not fully charged, and shunts the current away from the energy storage device ( 6 ) to avoid overcharging it if it is fully charged. 
         [0013]    In one embodiment, the invention provides an energy harvesting system ( 10 - 1 , 2 , 3 ) including an energy harvester ( 2 ) for generating a harvested voltage (V hrv ) and an energy management circuit ( 7 - 2 , 3 , 4 ) for converting the harvested voltage (V hrv ) to an output voltage (V BAT ). The energy management circuit ( 7 - 2 , 3 , 4 ) includes an inductor (L 0 ) having a first terminal ( 3 ) coupled to receive the harvested voltage (V hrv ) and a second terminal ( 4 ) coupled to a first terminal of a first switch (S 0 ). An energy storage device ( 6 ) is coupled to receive the output voltage (V BAT ). Protection circuitry (S 0   a ,R S  in  FIG. 2 ; S 1 ,R S , 15 - 1 , 2  in FIG.  3 , 4 ) in the energy management circuit ( 7 - 2 , 3 , 4 ) shunts current generated by the energy harvester ( 2 ) away from the energy storage device ( 6 ) if the energy storage device ( 6 ) is fully charged. The energy management circuit ( 7 - 2 , 3 , 4 ) includes a control circuit ( 15 - 1 , 2 ) coupled to a control terminal of the first switch (S 0 ). 
         [0014]    In one embodiment, the energy management circuit ( 7 - 3 , 4 ) includes a second switch (S 1 ) coupled in parallel with the energy harvester ( 2 ) to perform the shunting, and the second switch (S 1 ) has a control terminal coupled to the control circuit ( 15 - 1 , 2 ) to control the shunting. In one embodiment, a first terminal of the second switch (S 1 ) is coupled to a first reference voltage (GND), a second terminal of the second switch (S 1 ) is coupled to a first terminal of a current-limiting resistor (R S ), and a second terminal of the current-limiting resistor (R S ) is coupled to the first terminal ( 3 ) of the inductor (L 0 ). In one embodiment, the energy harvesting system includes a filter capacitor (C 0 ) coupled in parallel with the energy harvester ( 2 ). In one embodiment, the energy management circuit ( 7 - 2 , 3 , 4 ) includes a rectifying element (D 0 ) coupled between the second terminal ( 4 ) of the inductor (L 0 ) and the output voltage (V BAT ). In a described embodiment, the energy management circuit ( 7 - 2 , 3 , 4 ) includes a boost converter. 
         [0015]    In one embodiment, the control circuit ( 15 - 1 , 2 ) is coupled to receive the harvested voltage (V hrv ) and the output voltage (V BAT ) and operates to compare the output voltage (V BAT ) to a maximum energy storage device reference voltage (V BAT(max)  to determine whether the energy storage device ( 6 ) is fully charged, and also operates to maintain the first switch (S 0 ) open and the second switch (S 1 ) closed to shunt the current generated by the energy harvester ( 2 ) away from the inductor (L 0 ) and the energy storage device ( 6 ) if the energy storage device ( 6 ) is fully charged. In one embodiment, the control circuit ( 15 - 1 , 2 ) operates to compare the harvested voltage (V hrv ) to the output voltage (V BAT ), maintain the second switch (S 1 ) open, and operate the first switch (S 0 ) so as to effectuate boosting of the harvested voltage (V hrv ) by the energy management circuit ( 7 - 2 , 3 , 4 ) if the harvested voltage (V hrv ) is less than the output voltage (V BAT ). 
         [0016]    In a described embodiment, the control circuit ( 15 - 1 , 2 ) operates to maintain the first (S 0 ) and second (S 1 ) switches open if the harvested voltage (V hrv ) is less than the output voltage (V BAT ) and the energy storage device ( 6 ) is less than fully charged. 
         [0017]    In a described embodiment, the control circuit ( 15 - 2 ) includes a comparator ( 12 ) having a first input (−) coupled to receive the harvested voltage (V hrv ), a second input (+) coupled to receive a voltage ( 50 ) indicating that the energy storage device ( 6 ) is fully charged, and an output coupled to the control terminal of the second switch (S 1 ), and wherein the control circuit ( 15 - 2 ) includes an amplifier ( 17 ) having a first input (−) coupled to receive the output voltage (V BAT ), a second input (+) coupled to receive the maximum energy storage device reference voltage (V BAT(max) ), and an output coupled by means of a pulse width modulation (PWM) circuit ( 42 ) to the control terminal of the first switch (S 0 ). 
         [0018]    In a described embodiment, the energy management circuit ( 7 - 2 ) includes a second switch (S 0   a ) coupled in parallel with the first switch (S 0 ) to perform the shunting. The second switch (S 0   a ) has a control terminal coupled to the control circuit ( 15 - 1 ) to control the shunting. A first terminal of the second switch (S 0   a ) is coupled to a first reference voltage (GND), and a second terminal of the second switch (S 0   a ) is coupled to the second terminal ( 4 ) of the inductor (L 0 ). 
         [0019]    In one embodiment, a current-limiting resistor (R S ) couples the second terminal of the second switch (S 0   a ) to the second terminal ( 4 ) of the inductor (L 0 ), and the control circuit ( 15 - 1 ) is coupled to receive the harvested voltage (V hrv ) and the output voltage (V BAT ) and operates to compare the output voltage (V BAT ) to a maximum energy storage device reference voltage (V BAT(max) ) to determine whether the energy storage device ( 6 ) is fully charged. The control circuit ( 15 - 1 ) also operates to maintain the first switch (S 0 ) open and the second switch (S 0   a ) closed to shunt the current generated by the energy harvester ( 2 ) away from the energy storage device ( 6 ) if the energy storage device ( 6 ) is fully charged. 
         [0020]    In one embodiment, the control circuit ( 15 - 1 ) operates to compare the harvested voltage (V hrv ) to the output voltage (V BAT ), and maintains the second switch (S 0   a ) open and operates the first switch (S 0 ) so as to effectuate boosting of the harvested voltage (V hrv ) by the energy management circuit ( 7 - 2 ) if the harvested voltage (V hrv ) is less than the output voltage (V BAT ). 
         [0021]    In one embodiment, the invention provides a method for harvesting energy from an energy harvester ( 2 ) to generate a harvested voltage (V hrv ), the method including converting the harvested voltage (V hrv ) to an output voltage (V BAT ) applied to an energy storage device ( 6 ) by means of an energy management circuit ( 7 - 2 , 3 , 4 ) including an inductor (L 0 ) having a first terminal ( 3 ) coupled to receive the harvested voltage (V hrv ) and a second terminal ( 4 ) coupled to a first terminal of a first switch (S 0 ); transferring current generated by the energy harvester ( 2 ) to the energy storage device ( 6 ) by means of the energy management circuit ( 7 - 2 , 3 , 4 ) if the energy storage device ( 6 ) is not fully charged; and shunting the current generated by the energy harvester ( 2 ) away from the energy storage device ( 6 ) to avoid overcharging the energy storage device ( 6 ) if it is fully charged. 
         [0022]    In one embodiment, the method includes shunting the current generated by the energy harvester ( 2 ) away from the inductor (L 0 ). In one embodiment, the method includes coupling the first terminal ( 3 ) of the inductor (L 0 ) through a current-limiting resistor (R S ) and a second switch (S 1 ) coupled in parallel with the energy harvester ( 2 ) to a reference voltage (GND). In one embodiment, the method includes coupling the second terminal ( 4 ) of the inductor (L 0 ) through a current-limiting resistor (R S ) and a second switch (S 0   a ) to a reference voltage (GND). In one embodiment, the method includes operating a control circuit ( 15 - 1 , 2 ) to maintain the first switch (S 0 ) open and the second switch (Sa) closed if the harvested voltage (V hrv ) is greater than the output voltage (V BAT ). 
         [0023]    In one embodiment, the invention provides a system for harvesting from an energy harvester ( 2 ) to generate a DC harvested voltage (V hrv ), including means ( 7 - 1 , 2 ) for converting the harvested voltage (V hrv ) to an output voltage (V BAT ) applied to an energy storage device ( 6 ) by means of an energy management circuit ( 7 - 2 , 3 , 4 ) including an inductor (L 0 ) having a first terminal ( 3 ) coupled to receive the harvested voltage (V hrv ) and a second terminal ( 4 ) coupled to a first terminal of a first switch (S 0 ); means (L 0 , 15 - 1 ,D 0 ) for transferring current generated by the energy harvester ( 2 ) to the energy storage device ( 6 ) by means of the energy management circuit ( 7 - 2 , 3 , 4 ) if the energy storage device ( 6 ) is not fully charged; and means ( 15 - 1 , 2 ; S 1 ; R S ; S 0   a ) for shunting the current generated by the energy harvester ( 2 ) away from the energy storage device ( 6 ) to avoid overcharging the energy storage device ( 6 ) if it is fully charged. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic diagram of a conventional boost converter coupled to an energy harvester, for charging a battery or supercapacitor. 
           [0025]      FIG. 2  is a schematic diagram including a first circuit for preventing overcharging of the battery in  FIG. 1  while also preventing damage to the inductor. 
           [0026]      FIG. 3  is a schematic diagram including a second circuit for preventing overcharging of the battery in  FIG. 1  while also preventing damage to the inductor. 
           [0027]      FIG. 4  is a more detailed diagram of the circuitry shown in  FIG. 3 . 
           [0028]      FIG. 5  is a flowchart of the operation of the boost control circuit in of  FIGS. 3 and 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]      FIG. 2  shows a circuit  10 - 1  including energy harvester  2  which produces a harvested voltage V hrv  on a conductor  3 . (V hrv  either is a DC voltage generated by a suitable rectifier that rectifies AC energy generated by a harvester such as an inductive or piezo electric harvester, or is a DC voltage directly generated by a harvester such as a thermopile harvester or a solar cell harvester.) Conductor  3  is connected to one terminal of filter capacitor C 0  and also to the input of a conventional boost converter  7 - 2 . The other terminal of capacitor C 0  is connected to ground. Boost converter  7 - 2  can be considered to be a power management circuit that controls the flow of harvested energy from conductor  4  to battery/supercapacitor  6  and/or a load. 
         [0030]    Boost converter  7 - 2  includes inductor L 0  coupled between conductor  3  and conductor  4 . As in Prior Art  FIG. 1 , conductor  4  in  FIG. 2  is connected to one terminal of switch S 0  and to the anode of diode D 0 . The other terminal of switch S 0  is connected to ground. The cathode of diode D 0  is connected by output conductor  5  to the (+) terminal of battery or supercapacitor  6 , hereinafter referred to simply as battery  6 . 
         [0031]    In accordance with one embodiment of the present invention, an additional switch S 0   a  is coupled between ground and one terminal of a current-limiting resistor R S . A second terminal of current-limiting resistor R S  is connected to conductor  4 . Current limiting resistor R S  may have a resistance of a few megohms. Switches S 0  and S 0   a  are controlled by a suitable boost control circuit, subsequently described, which compares V BAT  with V BAT(max)  to determine whether battery  6  is fully charged. The boost controller circuit also determines if V hrv  is greater than V SAT . 
         [0032]    It should be appreciated that filter capacitor C 0  typically may have a capacitance of roughly 1 μF. Therefore, even though the power available from harvester  2  is very low, if filter capacitor C 0  is charged up and switch S 0  then is closed, a very large surge of current will be supplied by capacitor C 0  through inductor L 0 . The large surge current would be likely to destroy or seriously damage inductor L 0 . 
         [0033]    Note that switch S 0  in Prior Art  FIG. 1  is a large, low-resistance switch, and it would be possible to provide a gate driver circuit for an MOS transistor implementation of switch S 0 . In this case, the gate driver circuit could limit the gate drive voltage of the transistor switch so as to provide a higher ON resistance of switch S 0 , as an alternative to providing current-limiting resistor R S  and switch S 0   a  as in  FIG. 2 . In this case, switch S 0  could be closed when battery  6  is fully charged and its higher ON resistance would prevent overcharging of battery  6  and also prevent damage to inductor L 0  by a current surge from filter capacitor C 0 . 
         [0034]      FIG. 3  shows another embodiment of the invention. As in  FIG. 2 , circuit  10 - 2  in  FIG. 3  includes energy harvester  2  which produces harvested voltage V hrv  on conductor  3 . Conductor  3  is connected to one terminal of filter capacitor C 0  and also to the input of a conventional boost converter  7 - 3 . The other terminal of capacitor C 0  is connected to ground. Boost converter  7 - 3  can be considered to be a power management circuit that controls the flow of harvested energy from conductor  4  to battery  6  and/or a load. Boost converter  7 - 3  includes inductor L 0  coupled between conductor  3  and conductor  4 . Conductor  4  is connected to one terminal of switch S 0  and to the anode of diode D 0 . The other terminal of switch S 0  is connected to ground. The cathode of diode D 0  is connected by output conductor  5  to the (+) terminal of battery  6 . 
         [0035]    In accordance with another embodiment of the invention, switch S 1  in  FIG. 3  is coupled between ground and one terminal of current-limiting resistor R S . A second terminal of current-limiting resistor R S  is connected to conductor  3 . A booster control circuit  15 - 1  has an output  20  connected to the control terminal of switch S 1  and another output  22  connected to the control terminal of switch S 0 . Booster control circuit  15 - 1  has inputs connected to receive the harvester output voltage V hrv  on conductor  3 , the battery voltage V BAT  on conductor  5 , and a reference voltage V BAT(max)  that represents the fully-charged value of V BAT . 
         [0036]    Switches S 0  and S 1  are controlled by boost control circuit  15 - 1 , which compares V BAT  with V BAT (max) to determine whether battery  6  is fully charged. If battery  6  is fully charged, then boost control circuit  15 - 1  also determines if V hrv  is greater than V BAT . Booster control circuit  15 - 1  operates in accordance with the flow chart of  FIG. 5 . 
         [0037]    Referring to decision block  31  in  FIG. 5 , boost control circuit  15 - 1  determines if both: (1) V hrv  is greater than V BAT , and (2) V BAT  is greater than or equal to V BAT(max) . If this determination is affirmative, then boost control circuit  15 - 1  maintains switch S 0  open, and also maintains switch S 1  closed to prevent harvested current from overcharging the fully-charged battery  6  and to prevent surge currents supplied by filter capacitor C 0  from damaging inductor L 0  or battery  6 . Maintaining switch Si closed has the effect of directing all of the energy stored in filter capacitor C 0  and all of the energy being generated by harvester  2  through current-limiting resistor R S  and switch S 1  as long as battery  6  remains fully charged. (It should be appreciated that in the described energy harvesting applications, the amount of energy stored in filter capacitor C 0  and the amount of energy being generated by harvester  2  are relatively low, so there is little danger of switch S 1  being damaged by current therein. However, if the input of boost converter  7 - 3  is connected to a sufficiently large energy source, switch Si would be destroyed.) The algorithm of  FIG. 5  goes from block  32  to the entry point of decision block  31  and continues to monitor the value of V BAT . 
         [0038]    If the determination of decision block  31  is negative, then boost control circuit  15 - 1  determines whether V BAT  exceeds V BAT(max) , as indicated in decision block  33 . If the determination of decision block  33  is affirmative, boost control circuit  15 - 1  goes to block  34  and keeps switch S 0  open and switch S 1  closed and returns to the entry point of decision block  31 . 
         [0039]    If the determination of decision block  33  is negative, boost control circuit  15 - 1  goes to decision block  35  and determines if V BAT  is nearly equal to V BAT(max) . If this decision is affirmative, then boost control circuit  15 - 1  operates switch S 0  at a reduced duty cycle and keeps switch Si open, to reduce the amount of current through inductor L 0 ; the algorithm then returns to the entry point of decision block  31 . If the determination of decision block  35  is negative, then boost control circuit  15 - 1  operates switch S 0  at a normal duty cycle, and keeps switch S 1  open, as indicated in block  37 , to allow filter capacitor C 0  be charged up to V hrv  and also to allow normal charging of battery  6 . 
         [0040]    Then boost control circuit  15 - 1  goes to decision block  38  and determines whether V hrv  is less than but nearly equal to V BAT . If this determination is affirmative, boost control circuit  15 - 1  maintains switch S 0  open and maintains switch S 1  closed, as indicated in block  39 , to prevent further charging of battery  6 ; boost control circuit  15 - 1  then returns to the entry point of decision block  31 . If the determination of decision block  38  is negative, the algorithm allows normal duty cycle operation of switch S 0  to continue and returns to the entry point of decision block  31 . 
         [0041]    It should be understood that the flowchart of  FIG. 5  is also applicable to the operation of a boost control circuit utilized to control switches S 0  and S 0   a  in  FIG. 2  if switch “S 1 ” in blocks  32 ,  34 , and  35  is replaced by a switch “S 0   a”.    
         [0042]      FIG. 4  shows a circuit  10 - 3  which is the same as circuit  10 - 2  in  FIG. 3 , but with further detail in booster control circuit  15 - 2 . Booster control circuit  15 - 2  includes a comparator  12  having its (−) input connected to receive V hrv  on conductor  3  and its (+) input coupled to receive, via conductor  50 , the output of a comparator  43  having its (+) input coupled to V BAT  and its (−) input coupled to receive V BAT(max) . The output of comparator  12  is connected by conductor  20  to the control terminal of switch S 1 . An amplifier  17  has its (−) input coupled to receive the present battery voltage V BAT  on conductor  5  and its (+) input coupled to receive the reference voltage V BAT(max)  on conductor  16 . The output of amplifier  17  can be connected to the input of a conventional pulse width modulation (PWM) circuit  42 , the output of which is connected by conductor  22  to the control terminal of switch S 0 . PWM circuit  42  controls the duty cycle of switch S 0  in response to the output voltage generated by amplifier  17  so as to decrease the duty cycle of switch S 0  as V BAT  gets closer to V BAT(max) . PWM circuit  42  typically, but not always, is coupled to receive a clock signal (not shown) of a suitable frequency. 
         [0043]    The described invention provides improved reliability of energy harvesting systems by providing simple, economical battery overcharge protection, and also by avoiding damage to inductors and/or other circuit components in power management circuits of the energy harvesting systems. 
         [0044]    While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from its true spirit and scope. It is intended that all elements or steps which are insubstantially different from those recited in the claims but perform substantially the same functions, respectively, in substantially the same way to achieve the same result as what is claimed are within the scope of the invention.