Patent Publication Number: US-8970162-B2

Title: System and method for balancing electrical energy storage devices via differential power bus and capacitive load switched-mode power supply

Description:
RELATED APPLICATION DATA 
     This application is a continuation-in-part of U.S. patent application Ser. No. 13/180,963, filed Jul. 12, 2011. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electronic circuit techniques for shuttling electrical energy across elements of a circuit via differential power buses and, in particular, to reactive energy shuttling using capacitive load switched-mode power supplies for efficient power distribution. 
     BACKGROUND 
     Lithium-ion (including Lithium-ion polymer) batteries are popular in mobile applications due to their high energy density and low self-discharge rate. They have relatively high internal resistance that increases with aging. For high voltage applications, multiple cells are coupled in series into a battery pack. As Lithium-ion batteries cannot be equalized during charging simply by overcharging unlike some other secondary batteries, elaborate cell balancing is essential for such multi-cell Lithium-ion battery systems to cope with cell-to-cell variations such as internal resistance, state-of-charge (SOC), and capacity/energy (C/E) mismatch. 
       FIG. 1  depicts a flying-capacitor charge shuttling scheme for a battery pack in which multiple battery cells B 1 , B 2 , and B 3  are coupled in series. It employs a capacitor C to transfer charge from a high voltage cell to a low voltage cell via a differential power bus (Vbus, Vbus′). Switches S 1 , S 2 , S 3 , S 4 , S 5 , and S 6  are provided to connect the batteries to the power bus, connecting only one cell at a time to the power bus. The ‘flying-capacitor’ C is connected directly across the differential power bus (Vbus, Vbus′). Connecting it to a high voltage cell via the power bus charges the capacitor C; subsequently connecting to a low voltage cell discharges it, thereby transferring energy from the high voltage cell to the low voltage cell, consequently equalizing them. 
     Such flying-capacitor charge shuttling is energy-efficient only when the voltage differences between cells are small because the energy transfer efficiency is the ratio of low cell voltage to high cell voltage. If the voltage differences grow, the energy efficiency of the flying-capacitor charge shuttling plummets. For example, if the high cell voltage is 4V and the low cell is 3V, the energy efficiency of the flying-capacitor charge shuttling is no more than 75%. The rest 25% of the energy is dissipated into heat, as the charge transfer to and from the flying-capacitor is resistive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a flying-capacitor charge shuttling technique for cell balancing. 
         FIG. 2  depicts capacitor-load SMPS energy shuttling technique for cell balancing. 
         FIG. 3  depicts an ideal synchronous buck converter energy shuttle circuit. 
         FIG. 4  depicts an ideal synchronous boost converter energy shuttle circuit. 
         FIG. 5  depicts an ideal synchronous buck-boost converter energy shuttle circuit. 
         FIG. 6  depicts an ideal synchronous forward converter energy shuttle circuit. 
         FIG. 7  depicts a CMOS buck converter energy shuttle circuit. 
         FIG. 8  illustrates simulation results for the circuit of  FIG. 7 . 
         FIG. 9  depicts a CMOS boost converter energy shuttle circuit. 
         FIG. 10  illustrates simulation results for the circuit of  FIG. 9 . 
         FIG. 11  depicts multiple energy shuttles on a common differential power bus. 
         FIG. 12  depicts extended cell balancing with multiple power buses via shared cells. 
         FIG. 13  depicts distributed cell balancing by using transformer-based isolating switched-mode power supplies with shared capacitive load. 
         FIG. 14  depicts a battery management system for balancing multiple cells using multiple energy shuttles via a global differential power bus. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
     Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. 
     As discussed in more detail below, a system and method are provided for transferring electrical energy among multiple electrical energy storage devices via a differential power bus. A capacitive load switched-mode power supply transfers the electrical energy between a load capacitor and the differential power bus to which the electrical energy storage devices (e.g., rechargeable batteries and/or capacitors connected in parallel or series or combinations of both) are connected. As a result, electrical energy is efficiently transferred and distributed among the electrical energy storage devices. 
     Flying-capacitor charge shuttling dissipates substantial energy into heat due to resistive charging and discharging of the flying-capacitor (a reactive load) via a differential power bus. The presently claimed invention provides reactive charging and discharging of the load capacitor by employing a switched-mode power supply (SMPS, or simply switcher) such as a synchronous buck or boost converter for efficient power transfer/distribution via the differential power bus. In other words, the flying-capacitor C of  FIG. 1  is replaced by a capacitor-load SMPS for reactive charging and discharging of the load capacitor C as depicted in  FIG. 2 . The differential power bus (Vbus, Vbus&#39;) multiplexes battery cells B 1 , B 2 , and B 3  by using bus switches S 1 , S 2 , S 3 , S 4 , S 5 , and S 6  to selectively connect them to the switcher SMPS, thereby providing an input power supply (Vin) for the switcher SMPS. As the capacitor-load SMPS (the switcher with load C) ideally does not consume energy, it serves as an efficient energy shuttle between the cells (by picking up energy from a cell and then dumping the energy back to another) thereby equalizing/balancing them. Capacitors C 1 , C 2 , and C 3  may be coupled with battery cells B 1 , B 2 , and B 3 , respectively, to buffer energy (even when a cell is dead with high internal resistance). The SMPS-based energy shuttle can accommodate large voltage differences/changes on the differential power bus without much impacting efficiency of the energy shuttle unlike flying-capacitor charge shuttling. Thus, a single cell or multiple series-cells (even the entire battery pack cells) can be connected to the power bus for reactive energy shuttling. 
       FIG. 3  depicts an idealized synchronous buck converter circuit employing ideal switches. It consists of three branches: a first branch consisting of a voltage source Vbus and a first switch S 1  coupled in series, a second branch consisting of a second switch S 2  that is synchronously operating complementary to the first switch S 1 , and a third branch consisting of an inductor L and a capacitor-load C coupled in series. The three branches are coupled in parallel, one of the two terminal nodes being designated as a ground GND and the other being a switching node w 1  with respect to the ground. The voltage source Vbus represents the differential voltage across the differential power bus (Vbus, Vbus&#39;) of  FIG. 2 . Diodes D 1  and D 2  are coupled in parallel with the switches S 1  and S 2 , respectively, in  FIG. 3 , assuming that the voltage source Vbus is positive, to continuously provide positive current paths for the third branch during dead time that both switches are OFF; the diodes&#39; directions flip if Vbus is negative. Although the order of elements in a branch can be arbitrary, it is assumed that the input voltage source Vbus and the output load are ground referenced. Vin and Vout represent the voltages across the voltage source Vbus, and the load capacitor C, respectively, with respect to the ground GND. For purposes of idealized analyses herein, it is assumed that the states of the ideal switches S 1  and S 2  are complementary at all time (i.e., no dead time for the diodes D 1  and D 2  to function). 
     As the switches S 1  and S 2  are rapidly switching the states with duty cycle D per S 1 &#39;s ON state (and S 2 &#39;s OFF state), current flows in the third branch through the inductor L and the load capacitor C. When the switch S 1  is in the ON state (i.e., closed), the inductor branch current flows from the first branch as the switch S 2  is in the OFF state (i.e., opened). During this time, the current is increasing in the rate proportional to (Vin-Vout). When the switch S 1  is in OFF state (i.e., opened), the inductor branch current continues to flow through the switch S 2  (which is in ON state) of the second branch. This time, the current is decreasing in the rate proportional to Vout. In steady state, the current increment of the ON state and decrement of the OFF state balance out. That is,
 
( V in− V out)* D=V out*(1− D )
 
Thus,
 
 V in* D=V out
 
or
 
 V out/ V in= D  
 
As the duty cycle D is no greater than 1, Vout&lt;=Vin (thus the name “buck” or “step-down” converter). Note that the steady-state relationship of (Vout=Vin*D) is irrespective of the input circuit (voltage source Vbus here) or the output circuit (capacitor C here). In other words, any Vout voltage between 0V and Vin can be obtained by appropriately modulating the duty cycle D. By ramping Vout voltage up (but still under Vin voltage), the load capacitor C picks up energy from Vbus (actually from a cell or cells connected to the differential power bus at the moment); by ramping Vout voltage down, the saved energy in the load capacitor C is dumped back to Vbus (eventually to a cell or cells connected to the differential power bus at the moment).
 
     In practice, changing the states of the switches S 1  and S 2  cannot be instantaneous. It takes a finite time for a control signal to change the state of a switch, and a finite dead time of both switches being OFF safeguards against short-circuiting Vbus. During this dead time, one of the diodes D 1  and D 2  provides a positive path for the inductor current to continue to flow; charging (the load capacitor C) current will flow via the diode D 2 , and discharging current will flow via the diode D 1 . The buck-converter based energy shuttle is rather limited because of the restriction (Vin≧Vout). That is, Vout should be no greater than the lowest cell voltage. A large load capacitor is needed to shuttle a substantial amount of energy; handling the quadratic amount of charge will also slow down the shuttling, needing large transistors for the switches S 1  and S 2 . 
       FIG. 4  depicts an idealized synchronous boost converter circuit, again employing ideal switches. It consists of three branches: a first branch consisting of a load-capacitor C and a first switch S 1  coupled in series, a second branch consisting of a second switch S 2  that is synchronously operating complementary to the first switch S 1 , and a third branch consisting of an inductor L and a voltage source Vbus coupled in series. The three branches are coupled in parallel, one of the two terminal nodes being designated as the ground GND and the other being a switching node w 1  with respect to the ground. The voltage source Vbus represents the differential voltage across the differential power bus (Vbus, Vbus′) of  FIG. 2 . Diodes D 1  and D 2  are coupled in parallel with the switches S 1  and S 2 , respectively, in  FIG. 4 , assuming that the Vin voltage source is positive, to continuously provide positive current paths for the third branch during dead time that both switches are OFF; the diodes&#39; directions flip if Vbus is negative. 
     The boost converter circuit of  FIG. 4  is the same as the buck converter circuit of  FIG. 3  except that Vin and Vout are swapped with each other along with the voltage source Vbus and the load capacitor C, respectively. The buck and boost converters are dual of each other per the perspective of the input (Vin) and output (Vout). Thus, a similar steady-state relationship of (Vin=Vout*D) or (Vout=Vin/D) applies here where the duty cycle D is per S 1 &#39;s ON state (and S 2 &#39;s OFF state). As the duty cycle D is no greater than 1, Vin&lt;=Vout (thus the name “boost” or “step-up” converter). The bi-directional synchronous buck (or boost) converter can be considered as two unidirectional converters of opposite directions superposed, sharing the inductor L and the load capacitor C. For example, the  FIG. 3  circuit without the lower switch S 2  is a forward buck converter; and the  FIG. 3  circuit without the upper switch S 1  is a backward boost converter. Two separate unidirectional converters of opposite directions, sharing the load capacitor C but not sharing the inductor L, coupled in parallel will also do. Note that the steady-state relationship of (Vout=Vin/D) is irrespective of the input circuit (voltage source Vbus here) or the output circuit (capacitor C here). In other words, any Vout voltage greater than or equal to Vin can be obtained by appropriately modulating the duty cycle D. By ramping Vout voltage up, the load capacitor C picks up energy from Vbus (actually from a cell or cells connected to the differential power bus at the moment); by ramping Vout voltage down (but no less than Vin voltage), the saved energy in the load capacitor C is dumped back to Vbus (eventually to a cell or cells connected to the differential power bus at the moment). The boost converter based energy shuttle is more flexible to use with the restriction (Vin≦Vout). That is, it can accommodate even a dead cell (Vin near 0V) or the entire battery pack for Vin as long as Vout is kept high enough. Moreover, the boost controller may be self-powered directly by Vout. 
     There are various other switched-mode power supplies.  FIG. 5  depicts synchronous buck-boost converter based cell balancing circuit. Unlike buck or boost converter based cell balancing circuit, the output voltage Vout is not restrained by the Vbus voltage in this circuit.  FIG. 6  depicts a synchronous forward converter based cell balancing circuit. Transformer provides galvanic isolation of the circuits on both sides of the transformer. Other transformer-based isolating converters (such as flyback converter or push-pull converter) also provide galvanic isolation. 
       FIG. 7  depicts a synchronous buck converter circuit corresponding to the ideal  FIG. 3  circuit by employing MOSFET switches. In accordance with an exemplary embodiment, Vbus represents a piecewise-linear voltage source alternating between 1.75V and 3.5V at a frequency of 6 kHz. P 1  represents a PMOS transistor, implementing the upper switch S 1 , with its n-type substrate biased to a most positive voltage VPLUS. N 2  represents an NMOS transistor, implementing the lower switch S 2 , with its p-type substrate biased to a most negative voltage VMINUS. In accordance with an exemplary embodiment, VPLUS and VMINUS represent 3.5V and −3.5V, respectively, and inductor L and load capacitor C represent 10 uH and 0.1 μF, respectively. VP 1  and VN 2  represent gate terminal voltages of P 1  and N 2  transistors, respectively. Buck_control represents a controller providing the switching signals VP 1  and VN 2  along with the bias signals VPLUS and VMINUS. The signal VP 1  swings between VMINUS and Vbus, whereas the signal VN 2  swings between VPLUS and 0V. The switching frequency of VP 1  and VN 2  is order of magnitude higher (e.g., 1000 times) than the frequency of the Vbus voltage. Vref provides a reference signal to Buck_control. In accordance with an exemplary embodiment, it ramps up to 1.4V when Vbus goes to 3.5V and ramps down to 0.4V when Vbus comes back to 1.75V. Buck_control adjusts the duty cycle D in providing the switching signals VP 1  and VN 2  signals so that the output Vbuck follows the reference Vref in a feedback loop control. 
       FIG. 8  shows simulation results for the circuit of  FIG. 7 . The output Vbuck follows Vref closely, thereby charging the load capacitor C to 1.4V with a constant current when Vbus goes to 3.5V, and discharging down to 0.4V also with a constant current when Vbus comes back to 1.75V. 
       FIG. 9  depicts a synchronous boost converter circuit corresponding to the ideal  FIG. 4  circuit by employing MOSFET switches. In accordance with an exemplary embodiment, Vbus represents a piecewise-linear voltage source alternating between 1.75V and 3.5V at a frequency of 6 kHz. P 1  represents a PMOS transistor, implementing the upper switch S 1 , with its n-type substrate connected to the output Vboost. N 2  represents an NMOS transistor, implementing the lower switch S 2 , with its p-type substrate connected to the ground GND. In accordance with an exemplary embodiment, inductor L and load capacitor C represent 1 uH and 0.2 μF, respectively. VP 1  and VN 2  represent gate terminal voltages of P 1  and N 2  transistors, respectively. Boost_control represents a controller providing the switching signals VP 1  and VN 2 . The signal VP 1  swings between 0V and Vboost, whereas the signal VN 2  swings between 3.5V and 0V. The switching frequency of VP 1  and VN 2  is order of magnitude higher (e.g., 1000 times) than the frequency of the Vbus voltage. Vref provides a reference signal to Boost_control. In accordance with an exemplary embodiment, it ramps up to 5.5V when Vbus goes to 3.5V and ramps down to 3.7V when Vbus comes back to 1.75V. Boost_control adjusts the duty cycle D in providing the switching signals VP 1  and VN 2  signals so that the output Vboost follows the reference Vref in a feedback loop control. 
       FIG. 10  shows simulation results for the circuit of  FIG. 9 . The output Vboost follows Vref closely, thereby charging the load capacitor C to 5.5V with a constant current when Vbus goes to 3.5V, and discharging down to 3.7V also with a constant current when Vbus comes back to 1.75V. 
     In the SMPS-based energy shuttling cell balancing of  FIG. 2 , some cells may only have capacitors without batteries (or such batteries are dead with very high internal resistance). For example, batteries B 1  and B 2  are removed and capacitors C 1  and C 2  are in place. In that case, the capacitor-load SMPS shuttles energy from battery B 3  to capacitors C 1  and C 2 . The series-coupled cells essentially result in an efficient voltage multiplier. Also, the batteries from all cells can be replaced with corresponding capacitors. If external power is applied to the series-coupled capacitive cells stack and output power is drawn from a capacitive cell, the resulting circuit is essentially a voltage divider. Further, not all of the cells need to be coupled in series. The differential power bus makes the SMPS-based energy shuttling independent of the cells&#39; interconnections. 
     We can reduce the number of bus switches as depicted in  FIG. 11  by allowing both polarities of differential power bus voltage. Two separate switched-mode power supplies SMPS 1  and SMPS 2  (with separate load capacitors C 1  and C 2 , respectively) are employed together to support both polarities of differential power bus voltage. They effectively partition battery cells into two parts. A capacitor cell C 3  is provided with double bus switches to connect to the differential power bus in either polarity thereby bridging the two parts in transferring energy across the partition. 
     It is also possible to have multiple differential power buses sharing some or all cells.  FIG. 12  depicts such a case where two differential power buses (Vbus 1 , Vbus 1 ′) and (Vbus 2 , Vbus 2 ′) share a capacitive cell C 3 . (The capacitor C 3  is anchored to the node between cells B 2  and B 4  as indicated by a dotted line.) Each power bus defines a battery management module with its own capacitor-load SMPS energy shuttle. The shared cell C 3  enables energy shuttling across the two modules. 
       FIG. 13  depicts another case where two differential power buses have transformer-based isolating switched-mode power supplies that share the capacitive load bus (Vbank, Vbank′) on which distributed capacitors C 5  and C 6  are coupled across. (The capacitive load bus is anchored to ground as indicated by a dotted line.) Energy is transferred across modules via the shared capacitive load bus. 
     Referring to  FIG. 14 , energy shuttles, such as those discussed above, can be used as part of a battery management system for balancing multiple battery cells via a global differential power bus. In particular, a battery management system can be implemented that eliminates the need for separate module-to-module balancing circuitry through the use of a common power transfer bus (e.g., the global differential power bus) for simultaneous balancing of cells in all modules to an average level. Such a system topology allows for building standard battery modules containing multiple cells and combining such modules using series or parallel (or series-parallel) connections to form a multi-module battery pack. A balancing algorithm can be used for simultaneous balancing of the cells in all modules across the entire battery pack while maintaining a substantially constant voltage on the global differential power bus. The cells can be balanced directly to the battery pack average (as opposed to first balancing cells within a module and then balancing the modules using separate balancing circuitry). A common control communication bus (e.g., using digital or analog communication signals) provides for information exchange across the battery modules on the state of charge (SOC) of battery cells. The SOC of the battery cells can be estimated locally at the module level and communicated to other modules via the global control bus  1034 . Such an arrangement allows for balancing of cells without requiring interaction with a central controller  1042 . Energy is exchanged between the cells and the isolated storage elements (capacitors). The storage elements of all the modules are connected in parallel, thus allowing for energy transfer from any cell in the battery pack to any other cell in the pack, regardless of the module boundary. 
     In accordance with an exemplary embodiment  1000 , multiple modules  1002  containing energy shuttles such as those discussed above, are used to monitor and balance energy among the multiple cells  1016  via respective local differential power buses  1008  and a global differential power bus  1032 . As discuss above, isolating switched-mode power supplies  1004  are coupled via their local differential power buses  1008  to convey the charging and discharging currents  1007  between the battery cells  1016  and storage element  1010 . This storage element  1010 , i.e., the capacitance as discussed above, is coupled to the global differential power bus  1032 . 
     Local control circuitry includes a local controller  1022  (e.g., a microcontroller) and isolation circuitry  1024  (e.g., using transformer coupling for DC isolation). The isolation circuitry  1024  is coupled to a global control bus  1034 . This global control bus  1034  facilitates communications of the multiple modules  1002  between the modules  1002  and with a central controller  1042  which provides for overall system control. Commands and data to and from the central controller  1042  communicated via the global control bus  1034  are isolated by the isolation circuitry  1024  and conveyed to and from the local controller  1022 . The local controller  1022  provides the local control signals to and receives local data from the isolating switched-power supplies  1006  and switch matrix  1014 , which provides the necessary connection between the battery cells  1016  and the local differential power bus  1008  (as discussed above). (The local differential power bus  1008  can be a common bus or a split bus.) 
     As noted above, the modules  1002  can be mutually coupled in series or parallel (or series-parallel) between the electrodes  1017   p ,  1017   n  via which the energy stored within the cells  1016  is to be provided to an external load (not shown). An external load (not shown) can also be coupled across the global differential power bus that provides a substantially constant voltage on it. 
     It should be understood that the particular embodiments of the invention described above have been provided by way of examples and that other modifications may occur to those skilled in the art without departing from the scope and spirit of the invention as express in the appended claims and their equivalents.