Average current mode controlled energy storage in a defibrillator

Techniques are described for charging an energy storage device, such as the high-voltage energy storage capacitors of an external defibrillation device, using an average current mode control technique. By controlling the average current in a transformer, energy may be stored rapidly and at controlled energy levels.

TECHNICAL FIELD

The invention relates generally to energy storage techniques for medical devices, and more particularly techniques employing switching-mode devices to store energy in an energy storage device associated with a defibrillator.

BACKGROUND

Many devices rely on an energy storage device such as a capacitor to store potential energy and supply a voltage to a load. Examples of such devices include photographic flash lamps and flashing warning lights.

A defibrillator is another device that stores energy, typically in one or more high-voltage capacitors, and delivers the stored energy to a patient. In particular, a defibrillator delivers energy to a heart that is undergoing fibrillation and has lost its ability to contract. Ventricular fibrillation is particularly life threatening because activity within the ventricles of the heart is so uncoordinated that virtually no pumping of blood takes place. An electrical pulse delivered to a fibrillating heart may repolarize the heart and cause it to reestablish a normal sinus rhythm.

Although defibrillators may be internally implanted in patients that suffer from chronic fibrillation, an electric pulse may also be applied externally via paddles placed upon the patient's chest. When a switch is closed, the capacitor sends at least a part of the stored energy from paddle to paddle through the patient's chest. The energy transferred may be on the order of several hundred joules. To achieve this level of energy transfer, the power needed to deliver the energy may be on the order of hundreds of kilowatts and the voltage across the capacitor may be on the order of several thousand volts.

A defibrillator such as a portable defibrillator typically includes a battery, which by itself is not capable of providing the high energy, high-voltage electric pulses required for defibrillation therapy. Instead, the battery is used to charge the high-voltage energy storage capacitors. In a flyback charger, the battery supplies energy to the primary coil of a flyback transformer while a control switch is closed. When the control switch is opened, the energy stored in the primary coil is transferred to the secondary coil of the flyback transformer. The energy is then stored on the storage capacitors, which are coupled to the secondary coil by a diode. By opening and closing the control switch, energy is incrementally transferred from the battery to the capacitors, thereby increasing the energy stored in the capacitors and charging the capacitors to a high voltage.

SUMMARY

The invention relates to techniques for charging an energy storage device, such as the high-voltage energy storage capacitors of an external defibrillation device, using average current mode control techniques. The time needed to charge a capacitor to a desired voltage is a function of the current flowing to the capacitor. By controlling the average current, the charge time and the voltage across the capacitor can be more effectively controlled.

Average current mode control represents a highly accurate technique for controlling current in a transformer, and consequently represents a highly accurate technique for storing energy in a storage element such as a capacitor in a short time. In devices such as a defibrillator, rapid and accurate energy storage are especially advantageous. The technique may be extended to a wide range of current levels, charge times and voltage levels.

In one embodiment, the invention is directed to a method for charging an energy storage device associated with a defibrillator. The method comprises applying current to a primary coil in a flyback transformer, sensing an average current through the flyback transformer, controlling the applied current to cause the average current to follow a reference current and transferring energy from the flyback transformer to an energy storage device. By controlling the applied current, the average current may be driven to follow a reference current. The reference current may be a function of one or more parameters, such as energy stored in the energy storage device. When the energy storage device is a capacitor, the stored energy is related to the voltage across the capacitor.

Average current mode control represents an accurate and versatile technique for storing energy in a storage element. By controlling average current, it is possible to control, for example, the time needed to charge a capacitor to a desired voltage level.

In another embodiment, the invention presents a device comprising an energy source and a charging circuit. The device charges an energy storage device associated with a defibrillator. The charging circuit transfers energy from the energy source to an energy storage device, such as a capacitor. The charging circuit includes a flyback transformer, and transfers energy as a function of the average current in the flyback transformer. The device may further comprise electrodes for delivering a defibrillation pulse to a patient and a switch that couples the electrodes to the energy storage device to deliver the defibrillation pulse to the patient.

In an additional embodiment, the invention presents a medical device comprising a transformer, an energy source that supplies energy to the primary coil of the transformer, a switch that regulates the supply of energy to the primary coil, an energy storage device that receives energy from the secondary coil of the transformer and a controller that controls the switch as a function of the average current in the transformer.

In a further embodiment, the invention presents a medical device comprising a difference circuit that generates an error signal as a function of the difference between a reference current and an average current in a transformer that transfers energy to an energy storage device. The device further includes a modulator that modulates the duty cycle of a control signal as a function of the error signal and a switch that regulates the supply of energy to a primary coil of the transformer according to the control signal. The control signal may have a constant period, which advantageously allows for management of the noise spectrum due to the control signal.

DETAILED DESCRIPTION

FIG. 1is a block diagram illustrating components of a defibrillator32formed in accordance with the present invention. Defibrillator32administers therapy to patient8via electrodes12and14, which may be hand-held electrode paddles or adhesive electrode pads placed on the skin of patient8. The body of patient8provides an electrical path between electrodes12and14.

Patient electrodes12and14are coupled to switch20via conductors10and16. Switch20couples patient electrodes12and14to the output of capacitor bank22. Switch20is of conventional design and may be formed, for example, of electrically operated relays. Alternatively, switch20may comprise an arrangement of solid-state devices such as silicon-controlled rectifiers or insulated gate bipolar transistors. In each case, switch20should be capable of carrying relatively high currents.

Capacitor bank28stores the energy to be delivered to patient8. The amount of energy to be delivered may be specified by an operator via energy selector30. Energy selector30supplies energy-setting information to microprocessor26and controls the defibrillation pulse energy to be delivered to patient8. Energy selector30can be set to a specific energy level by an operator, or may be set to one of a series of discrete energy levels. Defibrillator32may present energy selector30a part of a user interface which may take the form of one or more dials or switches and an interactive graphic or text display. In the case of an automated external defibrillator with preprogrammed energy levels, energy selector30may be eliminated.

Before a defibrillation pulse may be delivered to patient8, capacitor bank28must be charged. Microprocessor26directs charging circuit28to charge capacitor bank22to a high voltage level. Charging circuit28delivers energy from energy source24to capacitor bank22. As capacitor bank22stores energy, the voltage across capacitor bank22increases, up to a desired level. Energy source24may be, for example, a series of batteries or a regulated dc source powered by an ac line.

The desired level of voltage across capacitor bank22is a function of the energy to be delivered to patient8. The energy to be delivered is in turn a function of factors such as the energy level selected with energy selector30and the impedance of the body of patient8. Defibrillator32may include instrumentation (not shown inFIG. 1) to measure or estimate the impedance of the body of patient8. Because the energy to be delivered to patient8is a function of the voltage across capacitor bank22, the voltage across capacitor bank22is controlled. As will be described below, average current mode control of a flyback charging circuit is used to quickly charge capacitor bank22to the desired level.

When the voltage across capacitor bank22reaches the desired level, microprocessor26may activate switch20to electrically connect capacitor bank22to patient electrodes12and14, and thereby deliver a defibrillation pulse to patient8. Alternatively, microprocessor26may illuminate a light or activate another indicator that informs the operator that the defibrillator is ready to deliver a defibrillation pulse to patient8. The operator may activate switch20and thereby deliver a defibrillation pulse to patient8.

Before the pulse is administered, an audible warning of the impending pulse is ordinarily given so that no one other than patient8will receive the defibrillation pulse. The warning may be given by the operator, for example, to admonish others to discontinue physical contact with patient8. In the case of an automated external defibrillator, the warning may be an audible alert sounded by defibrillator32.

Activation of switch20closes a circuit in which patient8is a part. Switch20may control whether defibrillation is monophasic or biphasic. The goal of defibrillation is to repolarize the heart with the current and cause the heart to reestablish a normal sinus rhythm. In some patients, one defibrillation treatment is insufficient and one or more additional defibrillation pulses may be administered. Between pulses, capacitor bank22must recharge to a high energy level.

In recharging capacitor bank22, as in the initial charging, time is usually of the essence. Charging circuit28should charge capacitor bank22efficiently, quickly and accurately to a controlled voltage. Charging circuit28may satisfy these objectives using average current mode control of a flyback charging circuit, in accordance with the invention.

In some circumstances, energy stored in capacitor bank22is not to be used to administer a defibrillation pulse. For example, the patient may recover normal sinus rhythm and may not need another shock. In circumstances such as this, the energy in capacitor bank22may be discharged with energy dump18. Energy dump18may include resistive elements that receive energy from capacitor bank22and dissipate the energy as heat. Although energy dump18is shown separate from switch20, some applications combine the functions of switch and energy dump.

FIG. 2is a block diagram illustrating a flyback charging circuit40in cooperation with a capacitor bank, represented by energy-storage capacitor C. Charging circuit40delivers energy from an energy source, represented by battery Vcc, to capacitor C. As more energy is delivered to energy-storage capacitor C, the potential difference Vcacross energy-storage capacitor C increases.

Energy is delivered from battery Vccto energy-storage capacitor C via transformer48. Transformer48is modeled as ideal transformer46, with non-ideal characteristics modeled as additional coils on the primary side. The one additional coil represents the coupling of the primary and secondary coils with coupling coefficient k times inductance L, or k·L. The other coil models the transformer leakage inductance (1−k)·L. Ideally, k=1 and the primary and secondary coils have unity coupling.

Transformer48includes a primary coil, having Npriturns, and a secondary coil, having Nsecturns. Current ipriis supplied by source Vcc, flows through the primary coil and induces current in the secondary coil. The current flowing through the secondary coil is isec.

Current flow is controlled by a switch, modeled as n-channel enhancement MOSFET Q. The state of transistor Q is governed by controller44. When controller44turns transistor Q on, ipriflows into the primary coil of transistor48, causing energy to be stored in the primary coil. When controller44turns transistor Q off, the energy stored in the primary coil transfers to the secondary coil, generating current isec. Current isecflows through diode D and charges energy-storage capacitor C. Diode D prevents energy-storage capacitor C from discharging.

By turning transistor Q on and off, controller44supplies a series of current pulses to energy-storage capacitor C, thereby charging energy-storage capacitor C. Because of the non-ideal characteristics of transformer48, a drain voltage spike may occur when controller turns off transistor Q. This voltage spike, which may affect the performance of transistor Q, is constrained by turn-off clamp circuit42.

Energy-storage capacitor C is charged with a series of pulses. For optimal charging, however, the pulses are not of equal duration. As shown inFIG. 2, controller receives feedback and turns transistor Q on and off as a function of currents ipriand isec. Currents ipriand isec, which may be sensed by current sensors (not shown inFIG. 2) and fed back to controller44, are parameters used in average current mode control. In addition, controller44may use the potential difference Vcacross energy-storage capacitor C as a feedback parameter.

FIG. 3is a block diagram of controller44. Controller44receives currents ipriand isecand provides a summer62that sums ipriand isec, generating current iave. Current iaveis the average current through transformer48and is, by definition, the sum of ipriand isec. Controller44activates switch74to drive iaveto follow a reference current, iref.

In a typical application, reference current irefis a time-varying current and not a constant current. In the embodiment shown inFIG. 3, irefdepends upon the voltage Vc, but irefmay depend upon parameters other than or in addition to Vc. For example, irefmay vary as a function of supply voltage Vcc, to manage power and charge time. A current source66may be embodied as a processor that generates iref, or controls a variable current source that generates iref, as a function of the parameters.

The time needed to charge capacitor C to the desired level of voltage is a function of the charging current, which in turn is a function of iave. By controlling average current iave, therefore, controller44may quickly and efficiently charge capacitor C.

Average current iaveand reference current irefare supplied to differential integrating amplifier64, which generates error signal76as a function of the difference between average current iaveand reference current iref. Amplifier64typically comprises a control feedback loop to perform the differencing function. Amplifier64may also include integrating elements, i.e., reactive elements, that compensate the control loop. In addition, amplifier64may amplify the difference between average current iaveand reference current iref.

Error signal76may be a voltage signal or a current signal. As shown inFIG. 3, error signal76is a voltage signal, which is supplied as one input to comparator70. The other input to comparator70is a clock signal such as periodic ramp signal68. Clock ramp signal68has a fixed period, and therefore a fixed frequency. An advantage of having a fixed frequency for clock ramp signal68is that clock ramp signal68has a known noise spectrum. Consequently, the adverse effect of noise due to clock ramp signal68can be managed, such as by attenuating the noise with a filter or by generating signals at different frequencies to avoid interference with the noise spectrum of clock ramp signal68.

The output of comparator70is a pulse-width modulated (PWM) signal78. Information is encoded within PWM signal78in the duration of the pulses, rather than in the amplitude of the pulses. When the magnitude of error signal76is higher than clock ramp signal68, the output of comparator70is a logically high voltage value such as 3V Otherwise, the output of comparator70is a logically low voltage value such as ground potential. When the magnitude of error signal76is larger, the duration of the high voltage pulse generated by comparator70is longer. Similarly, when the magnitude of error signal76is smaller, the duration of the high voltage pulse generated by comparator70is shorter. In this way, the duty cycle of PWM signal78varies according to error signal76, but the period of PWM signal78remains constant.

PWM signal78opens and closes switch74. Switch74may be, for example, a field effect transistor such as n-channel enhancement mode transistor Q in FIG.2. Switch may also be any of a number of other electronic switches. Driver72drives switch74. Depending on the kind of electronic switch being driven, driver72may, for example, amplify, invert, or clamp PWM signal78to drive switch74.

FIG. 4is a flow diagram illustrating an embodiment of the invention. Charging begins with selection of an initial reference current iref(90), to which controller44will drive average current iave. As mentioned above, reference current irefordinarily varies with time and may depend upon one or a combination of parameters.

Average current iaveis, by definition, the sum of primary current ipriand secondary current isecso ipriand isecare summed to produce iave(92). Current sensors sense ipriand isec, which may be fed back to controller44as currents, and may be summed by application of Kirchhoff's current law. Alternatively, current sensors may sense ipriand isecand convert the currents to voltages, which may be added with the sum being representative of iave.

Amplifier64takes the difference between average current iaveand reference current irefand generates an error signal as a function of the difference (94). Comparator70compares error signal76to clock ramp signal68and generates PWM signal78as a function of the comparison (96). Driver72drives switch74as a function of PWM signal78(98).

As driver72drives switch74, iavechanges to follow iref. Accordingly, ipriand isecchange as well. Current sensors sense ipriand isec, which are fed back to controller44(100) and are summed to produce iave(92). Through feedback, iaveis driven to follow iref.

Reference current irefmay change as a function of one or more feedback parameters. Voltage Vcand/or supply voltage Vcc, for example, may be sensed (102) and fed back to controller44. Reference current irefmay be adjusted as a function of the feedback parameter (104) and may also be adjusted as a function of other parameters.

The invention offers several advantages. Average current mode control represents a highly accurate technique for controlling current in a transformer, and consequently represents a highly accurate technique for storing energy in a storage element such as a capacitor. The accuracy extends over a wide range of current levels, and consequently, the accuracy extends over a wide range of charge times and voltage levels. In addition, the transformer may operate in a continuous mode, in which current in transformer48is nonzero, or a discontinuous mode, in which current in transformer48may reach zero. The average current may be accurately controlled in either mode. Average current control mode using a clock signal with a fixed frequency offers the additional advantage of having a known, and therefore manageable, noise spectrum.

Various embodiments of the invention have been described. These embodiments are illustrative of the practice of the invention. Although described in detail in connection with a defibrillator, the invention may find application with other devices that store energy. The invention is not limited to eternal defibrillators but may be applied to internal medical devices such as implantable cardioverters/defibrillators.

Various modifications to the apparatus or methods may be made without departing from the scope of the invention. For example, signals representative of currents such as ipri, isec, iaveand irefmay be scaled, inverted, converted to voltages or converted to digital values. Clock ramp signal68need not be a ramp, but may be one of a number of other waveforms, such as a triangular wave. Switch74may be driven with a signal that is not a PWM signal. These and other embodiments are within the scope of the following claims.