Abstract:
A galvanically isolated charge balance system for a multicell battery includes a balancing circuit associated with each cell; each balancing circuit including a flying capacitor; a variable conductance switch; and a biasing circuit for the variable conductance switch; and a galvanically isolating MEMS switching device for selectively connecting the flying capacitor to a voltage supply to charge it to a predetermined voltage and to the biasing circuit for setting the variable conductance switch to adjust the charge on its associated cell to a preselected level.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a galvanically isolated charge balance system. 
       BACKGROUND OF THE INVENTION 
       [0002]    In one approach charge balancing circuits (passive) discharge one or more cells of a battery to bring all of the cells into balance to improve battery life. Typically this is done by controlling the leakage current from each cell individually. For example, a transistor switch and series resistance are configured in parallel with each cell. The switch is turned on for a predetermined time depending on the over charge of the associated cell, e.g. the greater the over charge the longer the switch is held on to continue the discharge. One shortcoming with this is that, in a multicell battery where only one or a few cells are under charged, it is inefficient to discharge the majority of cells to bring them down to the charge level of the few undercharged. Another approach (active) is to apply a charge, e.g. via a flying capacitor to the one or few cells that are under charged to bring them up to the charge levels of the others. This requires a complex matrix of switches to selectively apply the charge to the cells which are under charged. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    It is therefore an object of this invention to provide an improved galvanically isolated charge balance system. 
         [0004]    It is a further object of this invention to provide such an improved galvanically isolated charge balance system which can employ an on-off and/or linear control for balancing charge. 
         [0005]    The invention results from the realization that an improved, galvanically isolated charge balance system for a multicell battery, which operates either actively or passively (charge or discharge) is achieved using a galvanically isolated MEMS switch for selectively connecting a flying capacitor to a voltage supply to charge an associated cell to a predetermined voltage and to a biasing circuit for setting a variable conductance switch to adjust the charge on the associated cell to a preselected level. 
         [0006]    The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
         [0007]    This invention features a galvanically isolated charge balance system including a balancing circuit associated with each cell of the battery, each balancing circuit including a flying capacitor and a variable conductance switch. There is also a biasing circuit for the variable conductance switch and a galvanically isolating MEMS switching device for selectively connecting the flying capacitor to a voltage supply to charge it to a predetermined voltage and to the biasing circuit for setting the variable conductance switch to adjust the charge on its associated cell to a preselected level. 
         [0008]    In a preferred embodiment the variable conductance switch may include a transistor. The transistor may include a MOSFET. The biasing circuit may include a capacitor. The predetermined voltage may set the variable conductance switch to discharge the cell to the predetermined level. The predetermined voltage may set the variable conductance switch to charge the cell or allow the cell to be charged to the predetermined level. The predetermined voltage may set the variable conductance switch to charge the cell to the predetermined level with a current level which is a predetermined percentage of an external current source. 
         [0009]    The invention also features a galvanically isolated charge balance system including a balancing circuit associated with each cell of a multi-cell battery. Each balancing circuit includes a flying capacitor, a variable conductance switch, and a biasing circuit for the variable conductance switch. A galvanically isolating MEMS switching device selectively connects the flying capacitor to a voltage supply to charge it to a predetermined voltage and to the biasing circuit for setting the variable conductance switch to discharge its associated cell to a preselected level. 
         [0010]    The invention also features a galvanically isolated charge balance system including a balancing circuit associated with each cell of the multi-cell battery. Each balancing circuit includes a flying capacitor, a variable conductance switch, and a biasing circuit for the variable conductance switch. A galvanically isolating MEMS switching device selectively connects the flying capacitor to a voltage supply to charge it to a predetermined voltage and to the biasing circuit for setting the variable conductance switch to charge its associated cell to a preselected level. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic block diagram of a galvanically isolated charge balance system according to this invention; 
           [0013]      FIG. 2  is more detailed view of the system of  FIG. 1 ; 
           [0014]      FIG. 3  is a view similar to  FIG. 2  showing an additional cell and associated circuits; and 
           [0015]      FIG. 4  is a schematic diagram showing the MEMS devices and signal conditioning circuit on adjacent chips or a single chip. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
         [0017]    There is shown in  FIG. 1  a galvanically isolated signal conditioning system  10  including signal conditioning circuit  12  and MEMS device  14  along with flying capacitor  18  and hold capacitor  16  which serves one battery cell  20  of a number of battery cells not shown. Also shown in  FIG. 1  is a galvanically isolated charge balance system  22 , according to this invention, including MEMS device  24 , flying capacitor  26 , bias circuit  28 , and variable conductance switch  30 . Although there is only one cell  20  shown in  FIG. 1 , along with one signal conditioning system  10  and one charge balance system  22  there is typically a number of cells each having associated with it a galvanically isolated signal conditioning system and galvanically isolated charge balance system according to this invention. Signal conditioning circuit includes, among other things, analog to digital converter  32  and digital to analog converter  34 , also shown is a controller  36 . 
         [0018]    In operation MEMS  14  charges flying capacitor  18  to a predetermined voltage, such as representative of the cell voltage from battery cell  20  and then switches flying capacitor  18  to charge hold capacitor  16 . The voltage on hold capacitor  16  is then supplied to ADC  32  and via signal conditioning circuit  12  to a controller such as  36 . As each cell is used to apply a voltage to hold capacitor  16 , controller  36  looks to ADC  32  to determine the value of the charge on that cell. In this way controller  36  monitors the voltage on each of the battery cells and determines the charge state of each. In order to balance the charge on all of the battery cells, controller  36  can either act actively to add charge to the cells that are below a preselected charge level or act in a passive mode to drain or discharge battery cells that are above the preselected level. 
         [0019]    Galvanically isolated charge balance system  22  according to this invention can be applied in either case. For example, assume that controller  36  is operating in a passive mode and that all the battery cells are at a preselected level lower than that of battery cell  20 . Therefore, battery cell  20  is discharged to that lower preselected level. In this case controller  36  commands DAC  34  to provide a predetermined voltage to flying capacitor  26 , which, when switched by MEMS device  24  to be applied to bias circuit  28 , causes variable conductance switch  30  to drain off just enough charge from cell  20  to bring it down to the preselected level. In accordance with the preferred embodiment variable conductance switch  30  is a linear switch i.e., it can have not only on and off states but various conduction levels between on and off. Actually there may be a number of cells that are above the preselected level and they may be at different levels above the preselected level. The adjustment needed and the resultant predetermined voltage required to be applied to flying capacitor  26  is a matter of design and under the supervision of controller  36  which is not a part of this invention. 
         [0020]    Alternatively, in active mode, assuming that battery cell  20  is the only one below a preselected level of charge, DAC  34  is commanded to provide a voltage on flying capacitor  26  which when applied to bias circuit  28  causes variable conductance switch  30  to create a conductive path across battery cell  20  which is less than the conductive paths across each other battery cell in the battery string. This is typically done by controller  36  changing the state of switch  40  to connect it to a current source  42 . This current source may for example provide a one amp charge current on line  44  to battery cell  20  and all of the battery cells in series with it. DAC  34  then, for example, might provide a voltage to flying capacitor  26  such that variable conductance switch  30  is completely off and the entire charging current goes through battery cell  20  while the voltage applied to all of the rest of the battery cells causes their variable conductance switches to shunt 3% of the current through the variable conductance switch and only the remaining 97% through their respective battery cells. In this way battery cell  20  will charge up at a faster rate than the remaining battery cells to reach the required charge level. The predetermined voltage may set the variable conductance switch to charge the cell to a predetermined level which is a predetermined percentage of an external current source. 
         [0021]    The systems of  FIG. 1  are shown in greater detail in  FIG. 2  where it can be seen that MEMS device  14  includes two MEMS switches  15  and  17  and MEMS device  24  includes MEMS switches  23  and  25 . Here biasing circuit  28  includes a capacitor  50  and the variable conductance switch  30  includes a CMOS transistor  52  and a current limiting resistance  54 . 
         [0022]      FIG. 3  illustrates that with a number of battery cells  20 - 20   n  the configuration is simply replicated while using the same holding capacitor  16  to provide the output voltage from the battery cells and the same DAC  34  to provide the input voltages to flying capacitors  26 - 26   n . The physical construction of one embodiment of the invention is shown in  FIG. 4  where the signal conditioning circuit  12  and the MEMS switching devices  14  and  24  are mounted on a single chip  58  such as an SOI chip or there could be two chips  60 ,  62  with signal conditioning circuit  12  formed on CMOS chip  60  and MEMS switches  14 ,  24  formed on SOI chip  62 , adjacent to one another and interconnected by wire bonds. A more detailed disclosure of the galvanically isolated signal conditioning system and the MEMS structure is shown in U.S. patent application Ser. No. 11/005,608, filed Dec. 6, 2004 by the same inventors and having the same assignee as the instant case herein incorporated in its entirety by this reference. 
         [0023]    Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
         [0024]    In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
         [0025]    Other embodiments will occur to those skilled in the art and are within the following claims.