Abstract:
An implantable energy storage arrangement for a medical implant, which is provided with a rechargeable storage for electrical energy and a unit for controlling the charging process via an actuator in the charging path. A control which can be externally activated is provided to bypass the actuator. Furthermore, a corresponding process for operating an implantable energy storage arrangement provides for bypassing of the actuator when storage voltage is overly low.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an implantable energy storage arrangement for a medical implant, the energy storage arrangement comprising a rechargeable storage for electrical energy and a unit for controlling the charging process via an actuator in the charging path. This invention further relates to a process for operating an implantable energy storage arrangement for a medical implant, said energy storage arrangement comprising a rechargeable storage for electrical energy, wherein during normal operation the charging process is controlled by means of a control unit via an actuator in the charging path.  
           [0003]    2. Description of Related Art  
           [0004]    Energy storage arrangements and processes of the aforementioned type are described, for example, in commonly owned, co-pending U.S. patent application Ser. No. 09/311,566 which is hereby incorporated by reference, and U.S. Pat. Nos. 5,411,537, 5,702,431 and 5,713,939. Conventionally, the implantable energy storage arrangements are recharged transcutaneously via an inductive path by means of an external charging device. The charging device conventionally is controlled by measuring the charging current and the voltage of the storage by means of a control unit and by converting the same into the corresponding control pulses for a switch in the charging circuit, wherein a suitable charging program is used.  
           [0005]    When the energy storage is in operation, two undesirable operating states can occur: On the one hand, overcharging of the battery can occur if the charging process is not terminated at the proper time, which may lead to gas evolution with subsequent destruction of the storage. On the other hand, when charging of the storage is not done on time, the storage voltage may drop to values which are below a minimum operating voltage which is necessary for defined operation or optionally for limited function of the implant which is to be supplied by the energy storage. In the latter case, the storage voltage may possibly drop to such an extent that even sufficient voltage supply of the implant-side electronics to control the charging process is no longer ensured. Often, the control electronics comprise a microprocessor system in which, in the case of undervoltage, wrong logic operations can occur or the contents of volatile memories can be lost. Thus, in the case of undervoltage, in these systems proper charge control is no longer ensured; this can lead to the charging path being switched to high resistance by a microprocessor malfunction when it enters the undervoltage range, whereby charging of the storage is permanently prevented.  
         SUMMARY OF THE INVENTION  
         [0006]    A primary object of this invention is to devise an implantable energy storage arrangement for a medical implant and a process for its operation, wherein safe recharging of the energy storage is possible even when the storage is completely discharged or has been discharged at least to such an extent that the storage voltage has dropped below the normal voltage range for the control unit.  
           [0007]    This object is achieved in conformity with the invention by an implantable energy storage arrangement for a medical implant, with a rechargeable storage for electrical energy and a unit for controlling the charging process via an actuator in the charging path, characterized in that there is a means which can be externally activated to bypass the actuator. The above object furthermore is achieved in conformity with the invention by a process for operating an implantable energy storage arrangement for a medical implant, with a rechargeable storage for electrical energy, in normal operation the charging process being controlled by means of a control unit via an actuator in the charging path, wherein when it is not possible to charge the storage via the actuator due to overly low storage voltage, a bypass means provided in the implantable energy storage arrangement is activated from the outside to bypass the actuator.  
           [0008]    In this approach in accordance with the invention, it is advantageous that even when discharge of the storage has progressed far or even the storage has been completely discharged, i.e., at very low storage voltages, the rechargeability of the storage is ensured at any time by preventing blockage of the charging path due to malfunction of the control unit caused by the undervoltage, which blockage is prevented by having the possibility of means of bridging the actuator element in the charging path by external actuation of the bridging unit.  
           [0009]    These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawing which, for purposes of illustration only, shows an embodiment in accordance with the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The single figure is a block diagram of an implantable storage arrangement for a medical implant in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    The sole figure schematically shows the wiring of the essential elements of an implantable storage arrangement  10  which is used as the power supply for a medical implant which can be, for example, a fully implantable hearing system for direct mechanical stimulation of the middle ear or inner ear or electrical stimulation of the inner ear. A charging coil  12  is used to pick up the electrical energy which has been inductively transmitted transcutaneously by an external charging device (not shown) and to feed power into the charging path of a rechargeable energy storage  14  which can be for example a NiMH battery. The voltage induced in the charging coil  12  by the external charging device is rectified and conditioned in a unit  16 . A Zener diode  36  protects the electronic components which are at the charging voltage potential against an excessive charging voltage.  
         [0012]    The charging current flows to the battery  14  through a mechanical switch  18  which is closed in normal operation, through a control and switching element  20  which is preferably essentially a FET and which acts as a controllable resistor in the charging current path, and through a shunt resistor  22 . A monitoring unit  24  is connected directly to the battery  14  and is used to sense and monitor the battery voltage. On the output side the monitoring unit  24  is connected on the one hand to the control and switching element  20  and on the other to the switch  26  via which a microprocessor system  28  is connected to the battery  14  which acts as the voltage supply. The microprocessor system  28  is clocked preferably by a Pierce oscillator (not shown). An A/D converter  30  is used to measure the voltage of the battery  14  and the voltage drop across the shunt resistor  22 , in which manner the two most important parameters when the battery  14  is being charged, specifically the battery voltage and the charging current, can be measured. The A/D converter  30  outputs these values as an input signal to the microprocessor system  28  which depending on the sensed battery voltage and the sensed charging current controls via a D/A converter  32  the control/switching element  20 , and thus, the charging current or the charging voltage, respectively, in conformity with a predetermined charging strategy. In particular, a monitoring function is implemented in a conventional manner, which monitoring function provides for the charging process to be terminated after a predetermined charging termination criterion is reached by switching the control/switching element  20  to high resistance.  
         [0013]    To prevent excess discharging of the battery  14  in operation, the energy storage arrangement  10  is conventionally provided with a function which timely warns the implant wearer of discharge of the battery  14  in order to encourage him to undertake a charging process. If recharging does not take place because, for example, the implant wearer is prevented from doing so, the battery voltage can drop below the allowable lower limit. As soon as the monitoring unit  24  ascertains that the sensed battery voltage has fallen below a predetermined lower threshold, the monitoring unit  24 , on the other hand, turns off the microprocessor  28  by opening the switch  26  interrupting the voltage supply thereof. This ensures that the microprocessor  28  is not operated at power supply voltages which are so low that the microprocessor  28  can execute incorrect logic operations or can lose the contents of volatile memories. This is ensured by correspondingly selecting the lower voltage limit. On the other hand, the monitoring unit  24  sets the control/switching element  20  into a conductive state when the voltage falls below the lower voltage limit to ensure that charging of the battery  14  is also possible at any time in the undervoltage range and blockage of the charging path by malfunction of the microprocessor  28  is prevented. As an additional effect, turning off the microprocessor  28  results in the power consumption of the electronics being reduced to the power consumption of the monitoring unit  24  which is only some 100 nA in practice. This has the benefit that the battery  14  is less loaded and thus the time interval to complete discharge of the battery  14  is lengthened; this reduces the risk of complete discharge.  
         [0014]    In a charging process which is undertaken in the undervoltage range the voltage of the battery  14  gradually increases. After the battery voltage has exceeded the predetermined lower voltage threshold of the monitoring unit  24  by the value of hysteresis, the monitoring unit  24  again turns on the microprocessor system  28  by closing the switch  26 , by which the microprocessor system begins to control and monitor the charging process in the above described manner. In this way, the control of the control/switching element  20  is transferred from the monitoring unit  24  to the microprocessor system  28 . By suddenly turning on the power supply voltage by closing the switch  26 , a steep voltage rise is accomplished which reliably ensures stimulation of oscillations of the Pierce oscillator of the microprocessor system  28 .  
         [0015]    Furthermore, the monitoring unit  24  is designed such that, when the sensed battery voltage exceeds a predetermined maximum value, the control/switching element  20  is switched into a non-conductive state to prevent overcharging of the battery  14  even when the microprocessor system  28  due to an error does not terminate the charging process after reaching the predetermined charging termination criterion.  
         [0016]    The mechanical switch  18  is designed in the conventional manner such that it responds to mechanical expansion of the battery  14  as occurs in case of excess gas evolution which accompanies overcharging, and interrupts the charging path or prevents reception of charging energy to prevent further charging of the battery  14 .  
         [0017]    In this way, three independent monitoring circuits are implemented in the described storage arrangement  10 , which monitoring circuits terminate the charging of the battery at the proper time to prevent damage. It is primarily the microprocessor system  28  which terminates the charging process when a predetermined charging termination criterion is reached. Independent therefrom, the monitoring unit  24  terminates the charging process when the battery voltage sensed by it independently of the microprocessor system  28  exceeds a predetermined maximum value. Finally, when the first two monitoring circuits fail the mechanical switch  18  timely terminates the charging process independently of the electronics such that damage of the battery  14  and hazard to the implant carrier are reliably prevented.  
         [0018]    Furthermore, the monitoring unit  24 , independently of the microprocessor system  28 , detects threatening excess discharging of the battery  14 , and malfunctions of the microprocessor system  28  which may result therefrom are prevented by turning off the microprocessor system. Furthermore, in this case, the monitoring unit  24  assumes control of the charging path, whereby, independently of the microprocessor system  28 , in the undervoltage range, it is always ensured that the charging path is conductive so that recharging of the battery  14  is possible at any time. Besides, by suddenly again turning on the power supply voltage of the microprocessor  28  by the closing of the switch  26  by means of the monitoring unit  24 , start-up problems of the Pierce oscillator of the microprocessor system  28  can be prevented.  
         [0019]    A bypass means  34  is connected in parallel to the control/switching element  20  and can be actuated externally, i.e., from outside the body, in order to bypass or short-circuit the control/switching element  20 . The bypass means  34  is provided for the case in which the storage  14  is discharged such that the storage voltage has dropped to such an extent that it is no longer sufficient for proper operation of the monitoring unit  24 . Here, the case can arise that the control/switching element  20  assumes a resistance which is so high that it does not enable charging of the storage  14  via the normal charging path. In this case, by actuating the bypass means  34  it is possible to intervene from the outside to bypass the control/switching element  20  and thus enable reliable charging of the storage  20  even in case of an extreme undervoltage or complete discharge.  
         [0020]    The bypass means  34  can be formed, for example, by a mechanical switch which can be closed by means of an external magnet (Reed switch), and the magnet can preferably be integrated into the external charging device. In this case, when the charging device is held on the skin of the implant wearer, the switch  34  is closed and charging is possible via the current path through switch  34 . But, as soon as the storage voltage is sufficient again, for reliable operation of the monitoring unit  24 , the switch  34  should be opened again by removing the magnet in order to re-activate the above described normal charging function and especially to prevent overcharging.  
         [0021]    In an alternative embodiment, the bypass means  34  can also be formed by a diode which is poled in the reverse direction, for example a Zener diode. The reverse voltage of the diode is dimensioned such that it is above the charging voltages which occur in the normal charging mode and thus does not influence the normal charging process, but is below the reverse voltage of the protective diode  36 . In an emergency, i.e., when the storage voltage is no longer sufficient for operation of the monitoring unit  24 , a special external emergency charging device is used at the start of recharging, which external emergency charging device differs from the external charging device which is used for the normal charging process essentially in that it provides for a charging voltage to be produced which is much higher than in a normal charging process and above the reverse voltage of the bridging diode  34 , but below the reverse voltage of the protective diode  36 . In this way, bridging of the control/switching element  20  is achieved by the emergency charging device. Instead of using a special emergency charging device, the normal charging device can also be provided with a switchable emergency charging mode. However, when a storage voltage is reached which again is sufficient for reliable operation of the monitoring unit  24 , the emergency charging mode should be terminated to again block the bridging diode  34 , and thus, to re-activate the normal charging function and especially to prevent overcharging.  
         [0022]    By means of the bypass unit  34  it is thus ensured that recharging by external activation is possible even when the storage  14  is completely discharged.