Optically isolated shock circuit for implantable defibrillator

A defibrillator output circuit utilizes an optically coupled signal for controlling an isolated electronic switch. Variants of the output circuit include coupling via phototransistors or photodiodes for the control of at least one electronic switch. An H-bridge circuit configuration with four switches is connected to a single energy storage capacitor for generating multiphasic shocks across a load. The polarity of the shocks is selectable. Optical coupling methods are employed for driving the high side switches in the H-bridge.

FIELD OF THE INVENTION AND DEFINITIONS 
The present invention relates to delivering high energy shocks in an 
implantable defibrillator, and more particularly to driving electrically 
isolated electronic switches in an implantable defibrillator. The 
invention can be advantageously applied to delivering monophasic or 
multiphasic truncated shocks to a load from a single capacitor. 
In the specification and claims, the following terms are used. A 
"defibrillator" refers to any device intended to revert a tachyarrhythmia 
with electrical energy substantially exceeding the energy provided by 
implantable cardiac pacemakers, including any combination or subset of 
implantable defibrillators, cardioverters, and pacemakers. A "monophasic" 
shock delivers current in one direction. A "multiphasic" shock delivers 
current first in one direction or polarity (called the first phase of the 
shock) and then in the opposite direction or polarity (called the second 
phase), and may provide additional phases which typically alternate 
polarity. A "biphasic" shock provides two phases. A "truncated" shock 
abruptly stops delivering current to the load, either by interrupting the 
current in the load or by rapidly discharging the storage capacitor. The 
"load" represents the impedance of lead wires, defibrillation or shock 
electrodes, the shock electrode-tissue interface, and tissue bulk between 
the shock electrodes. A "single capacitor" and a "capacitor" represent any 
one capacitor or more than one capacitor in a series and/or parallel 
combination of capacitor packages, which results in a single equivalent 
capacitor with two terminals connected to the shock circuit. 
The invention pertains to shock generators for monophasic and multiphasic 
waveforms, including biphasic waveforms. It applies to generators of 
single-capacitor multiphasic waveforms and to generators of 
multiple-capacitor multiphasic waveforms. The single-capacitor waveform 
discharges a single capacitor through the load in a first direction in the 
first phase, interrupts the current, discharges the single capacitor still 
further through the load in the opposite direction in the second phase, 
and then truncates. In consequence, second phase leading edge amplitude 
typically equals first phase trailing edge amplitude. The 
multiple-capacitor waveform discharges a different single capacitor for 
each direction, or even for each phase, so that the leading edge amplitude 
does not necessarily depend on the trailing edge amplitude of the 
preceding phase. 
BACKGROUND OF THE INVENTION 
Early defibrillators provided only monophasic waveforms. USRE27652 to 
Mirowski (priority 09 Feb 70) refers to an automatic defibrillator with a 
monophasic shock circuit which delivered an untruncated shock as soon as 
the storage capacitor charged to a fixed voltage (no isolated control 
signal was needed). FR2257312 to Zacouto (priority 16 Jan 74) refers to 
providing sequential monophasic shocks over multiple electrode pairs, also 
not isolating control. U.S. Pat. No. 4,403,614 to Engle (priority 19 Jul. 
79) and U.S. Pat. No. 4,384,585 to Zipes (priority 06 Mar 81) referred to 
synchronizing shock with detected events, but did not show any details of 
the discharge circuit. U.S. Pat. No. 4,614,192 to Imran (priority 21 Apr. 
82) refers to truncating monophasic shocks by rapidly discharging the 
storage capacitor. The shock switch and driver consisted of a pulse 
transformer driving a silicon controlled rectifier (SCR), a pulse 
transformer controlling a thyristor. 
Following experiments with bidirectional shocks in 1964 and 1980, J. C. 
Schuder et al. described an "Ultrahigh-energy thyratron/SCR bidirectional 
waveform defibrillator", in Med Biol Eng Comput 20:419, 1982, having a 
biphasic generator with one capacitor per phase. SU1149979 to Pekarski 
(priority 08 October 83) also refers to a biphasic truncated shock circuit 
with one capacitor for each phase. 
In 1984, Schuder et al. presented results of a simulated single-capacitor 
truncated biphasic waveform. In their paper entitled "Transthoracic 
Defibrillation of 100 Kg Calves with Bidirectional Truncated Exponential 
Shocks", Vol XXX Trans Am Soc Artif Intern Organs, 1984, the authors 
referred to experiments made with an "asymmetrical truncated exponential 
biphasic waveform . . . which can be implemented in a clinical sized 
apparatus." They showed a waveform where the trailing edge of the first 
phase was equal to the leading edge of the second phase. 
The single capacitor approach simplifies both charging and discharging 
circuits, reducing size, weight, and unreliability in implantable devices. 
As data accumulated showing improved animal and clinical results with 
biphasic truncated shocks, compared to monophasic truncated shocks, there 
have been proposed a variety of single-capacitor multiphasic truncated 
waveform generators. All such circuits include at least four switches in 
an H-bridge configuration (also referred to herein as an "H-bridge 
switch"). 
Designers frequently employ the H-bridge configuration for driving a load 
in two directions from a DC source, for example, driving a stepper or 
servo motor from a battery. In the first phase a first switch connects the 
positive source pole to a first side of the load and a second switch 
connects the negative source pole to the second side of the load. In the 
second phase a third switch connects the positive source pole to the 
second side of the load, and a fourth switch connects the negative source 
pole to the first side of the load. The first and third switches, 
connected to the positive source pole, are called high side switches. The 
second and fourth switches, connected to the negative source pole, are 
called low side switches. 
Prior art implantable discharge circuits employ one or more of three types 
of switches in the H-bridge. Each type of switch has an input, output, and 
control terminal, and responds to a control signal between the control and 
output terminals. Silicon controlled rectifiers (SCRs) turn on in response 
to a pulse on the control terminal, but only turn off when current through 
them falls essentially to zero. Metal-oxide-semiconductor field effect 
transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs) 
remain on while a control voltage appears at the control terminal. 
Depending on how they protect pacing and sensing circuits from 
defibrillation pulses, prior art circuits either isolate the capacitor and 
discharge circuit from pacing and sensing ground, or they connect the 
negative side of the capacitor to ground. In the isolated version they 
must provide isolated switch control signals. In the negative-ground 
version, they must still provide isolated control signals to the high side 
switches. 
Thus, any single capacitor biphasic shock delivery circuit needs: two high 
side switches and two low side switches connected in an H-bridge, and at 
least two isolated switch drivers. The following prior art patents all 
disclosed an H-bridge for generating a single-capacitor multiphasic 
waveform, where the structure for the H-bridge switches and the switch 
control drivers differ in each design. 
U.S. Pat. No. 4,800,883 to Winstrom (priority 02 April 86) refers to an 
isolated discharge circuit with four MOSFET switches, and a transformer 
with an RF carrier, rectification, and rapid shutoff circuits for high and 
low side drivers. A single transformer with two secondaries drives both 
high and low side switches in the same phase. A multilevel capacitor with 
voltage taps is described. 
EP0281219 to Mehra (priority 14 Jan 87) refers to a negative-ground 
discharge circuit with an SCR in series with a MOSFET for each high side 
switch and an SCR for each low side switch. Mehra did not give details of 
the switch drivers. 
EP0280526 to Baker (priority 27 Feb 87) refers to using the Winstrom 
circuit above, with the additional requirement of a first phase duration 
that is longer than the second phase duration (note that in 1984 Jones et 
al. published results for defibrillation pulses with a 5 ms first phase 
and a 1 ms second phase, see Am. J. Physiol. 247 (Heart Circ. Physiol. 
16)). Baker also refers to providing protection against a short-circuited 
load, by opening the H-bridge switches when the load current exceeds a 
preset value. 
EP0324380 to Bach (priority 12 Jan 88) provided another negative-ground 
discharge circuit, with SCRs for high side switches and MOSFETs for low 
side switches. Bach used pulse transformers for high side drivers and 
drove the low side directly. Bach included diodes in series with low side 
switches to protect against external defibrillators. 
EP0326290 to de Coriolis (priority 19 Jan 88) provided yet another 
negative-ground discharge circuit, with two SCRs in series for the first 
phase high side switch, a MOSFET for the first phase low side switch, and 
SCRs for the second phase high and low side switches. de Coriolis 
truncated the second phase by rapidly discharging the storage capacitor 
through the first phase high side switch and the second phase low side 
switch. de Coriolis drove the high side switches with pulse transformers 
and the low side switches with level shifters referred to a positive 
supply. 
U.S. Pat. No. 4,998,531 to Bocchi (priority 28 Mar 90) provided still 
another negative-ground discharge circuit, with four MOSFET switches. Each 
MOSFET switch had a series diode to prevent reverse current during 
external defibrillation. Bocchi used level shifters for low side drivers 
and used a transformer for the high side driver, where a pulse in one 
direction turned the MOSFET on, and a pulse in the other direction turned 
the MOSFET off. 
U.S. Pat. No. 5,111,816 to Pless (priority 22 October 90) provided yet 
another negative-ground discharge circuit, with IGBT or MOSFET switches. 
All Pless variants drive both high and low side switches in the same phase 
from a common transformer with an RF carrier and rectification, and a 
rapid shutoff circuit for at least one switch in each phase. Pless also 
referred the negative battery terminal to ground and inverted this to make 
the pacing voltage. 
All prior art designs either isolate the discharge circuit from pacing and 
sensing ground, or refer the negative pole of the storage capacitor to 
pacing and sensing ground. This requires electrically isolating control 
signals for high side switches. A problem with the prior art designs is 
that they use transformer coupling for isolation. They use pulse 
transformers to drive SCRs, and either pulse transformers or RF 
transformers with rectification and a rapid shutoff circuit to drive 
MOSFETs or IGBTs. 
The disadvantages of such transformer coupling include magnetic coupling 
from other inductors or transformers in the implant, such as the 
transformer which charges the energy storage capacitors; magnetic coupling 
to sensitive circuits elsewhere in the implant, such as current loops in 
the high-gain R-wave sensing circuits; relatively bulky and expensive 
magnetic components which cannot be implemented using integrated circuit 
technology; the possibility of transformer core saturation in a strong 
external DC magnetic field, including fields produced by permanent magnets 
commonly used to test pacemakers and defibrillators; and magnetic coupling 
from strong external AC magnetic fields, such as fields produced by 
industrial heating or welding apparatus. 
In prior art designs the transformer also required complex and power-hungry 
additional radio frequency oscillator or pulse driver circuits. 
There is thus a continuing need for improvement of high-voltage shock 
circuits for use in implantable defibrillators. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of this invention to overcome the 
aforementioned disadvantages in control circuitry for high voltage shock 
switches by providing optically isolated control for the electrically 
isolated electronic switches. 
Optical control in accordance with the present invention is achieved by 
providing a transmitter for converting an electrical signal to an optical 
signal, an electrically isolated optical path, and a receiver for 
converting the optical signal to an electrical signal. 
One variant of the invention provides an optical communication path for the 
control signal for each switch, where the presence of the optical signal 
instructs the receiver to turn on the controlled switch, and its absence 
instructs the receiver to turn it off. Stated otherwise, the optical 
signal may have a first intensity magnitude for a time, relative to a 
selected threshold, that corresponds to the duration the electronic switch 
is to be closed to deliver a shock (the "presence") and otherwise has an 
intensity magnitude that is below the threshold so that the switch is open 
and no shock energy is delivered to the load (the "absence"). Typically, 
in the "present" state, the optical signal is on at some value 
corresponding to a selected logic high level, and in the absent state, the 
optical signal at a logic low level corresponding to an off state where 
there is no optical signal. 
A second variant of the invention provides two optical communication paths 
for controlling each isolated switch, where the presence of a signal on 
the first path instructs the receiver to turn on the controlled switch, 
and the presence of a second optical signal on the second path instructs 
the receiver to turn off the controlled switch. In this variant the 
"presence" of an optical signal may be a pulse of light, or it may be a 
relative change of intensity, i.e., an optical signal present for the 
appropriate "switch on" duration. It is preferred to use short pulses at 
least to open the switch to minimize power consumption. 
It is a further object of this invention to provide photoresponsive optical 
receiver circuits for an isolated switch control. One variant of the 
receiver circuit provides a floating power supply and a photodetector that 
is a phototransistor for applying charging current to the control 
terminals of the controlled isolated switch in response to an optical 
control signal. Another variant provides one or more photodetectors that 
are photodiodes for performing the same function. 
It is yet another object of this invention to provide shutoff circuits to 
turn off the controlled switch rapidly in response to a particular state 
of the optical control signal or signals. 
In one embodiment, the invention is directed to a shock delivery circuit 
for use in an implantable defibrillator, which includes a battery for 
supplying energy, control circuits having electrical signal outputs for 
timing shocks, a shock charging circuit for converting battery energy to 
shock energy, a high voltage capacitor for storing shock energy, an 
electronic switch for connecting the capacitor to a load, and means for 
actuating said electronic switch such that switch control is obtained 
using optical isolation. Preferably, the actuating means is an optical 
isolation circuit that operates the electric switch without direct 
electrical control. One such isolation circuit includes an optical 
transmitter, such as a photoemitter, to convert at least one electrical 
signal from said control circuits to at least one optical signal, an 
optical receiver, such as a photodetector, to convert the at least one 
optical signal to at least one electrical signal for selectively turning 
on and turning off (i.e., closing and opening) the electronic switch, and 
an electrically isolated optical path conveying each optical signal from 
the optical transmitter to the optical receiver. 
According to one embodiment of the present invention, the optical 
transmitter (photoemitter) emits an optical signal whose presence signals 
the optical receiver to turn on the electronic switch, and whose absence 
signals the optical receiver to turn off the electronic switch. More 
preferably, the optical receiver includes a power supply, for providing 
power for charging the control terminals of the electronic switch, a first 
phototransistor switch for selectively conducting a charging current from 
the power supply to the control terminals of the electronic switch, in 
response to the presence of an optical signal, to turn on said electronic 
switch, and a shutoff circuit for selectively discharging the control 
terminals of the electronic switch, in response to the absence of said 
current for charging, to turn off the electronic switches. 
The power supply preferably includes a capacitor for storing energy at low 
voltage, a current-limited path, having at least one high-value resistor, 
for charging the capacitor for storing energy at low voltage from the 
capacitor for storing shock energy, and a voltage limiter to prevent the 
capacitor for storing energy at low voltage from charging to a voltage 
level beyond a preset voltage limit. The voltage limiter is preferably a 
zener diode with a zener voltage of, e.g., approximately 15 V. In this 
embodiment, the shutoff circuit includes a circuit means, such as a 
transistor for discharging the control terminals of the electronic switch, 
when the power supply and phototransistor no longer produce the charging 
current. 
In an alternate embodiment of the optical isolation circuit using a single 
optical signal to control the switch, the optical receiver includes at 
least one photodiode, for selectively providing a charging current to the 
control terminals of the electronic switch in response to the presence of 
the optical signal, to turn on the electronic switch, and a shutoff 
circuit for selectively discharging the control terminals of the 
electronic switch, in response to the absence of said current for charging 
from the one or more photodiodes and turn off said electronic switch. In 
this embodiment, the shutoff circuit preferably includes a transistor for 
discharging the control terminals of said electronic switch when the 
photodiode (or photodiodes) no longer produce the charging current to the 
control terminals of the electronic switch. 
In the alternate embodiment of the optical isolation circuit which uses 
more than one optical signal to control the switch, the optical 
transmitter emits a first optical signal whose presence signals to the 
optical receiver to turn on the electronic switch, and emits a second 
optical signal whose presence signals the optical receiver to turn off the 
electronic switch. In this embodiment of the optical isolation circuit, 
one embodiment of the optical receiver includes a power supply, for 
providing power for charging the control terminals of the electronic 
switch, a first phototransistor switch for selectively conducting power 
from the power supply to the control terminals of the electronic switch, 
in response to the presence of the first optical signal, to turn on the 
electronic switch, and a shutoff circuit for selectively discharging the 
control terminals of the electronic switch, in response to the presence of 
the second optical signal and turn off said electronic switch. 
One such shutoff circuit includes a first phototransistor activated by the 
first optical signal to turn on the electronic switch, as already 
described, and a second phototransistor, actuated by the second optical 
signal, connected to discharge the control terminals of the electronic 
switch when activated. The shutoff circuit also preferably includes a 
resistor across the control terminals of the electronic switch to prevent 
charge buildup on said control terminals when neither optical signal is 
present. 
In a second version of this alternate embodiment, the optical receiver 
includes at least one photodiode, for selectively providing a charging 
current to the control terminals of the electronic switch, in response to 
the presence of the first optical signal, to turn on the electronic switch 
and a shutoff circuit for selectively discharging the control terminals of 
electronic switch, in response to the presence of the second optical 
signal, and turn off said electronic switch. One such shutoff circuit 
includes a phototransistor, actuated by the second optical signal, 
connected to discharge the control terminals of the electronic switch when 
activated, and the aforementioned resistor connected to the electronic 
switch control terminals for preventing charge buildup on said control 
terminals when neither optical signal is present. 
In the case of single-capacitor multiphasic shock systems, the electronic 
switch includes two high side electronic switches and two low side 
electronic switches connected in an H-bridge configuration for connecting 
the capacitor with selective polarity to a load. The H-bridge switch is 
connected at its low side to the low side of the single capacitor and a 
supply voltage at ground. Alternatively, the H-bridge switch may connect 
at its low side to the low side of the single capacitor and a supply 
voltage that is more negative than ground. The latter supply voltage is 
selected to be in the range from -5 to -20 V, more preferably, 
approximately -15 V. 
In the application of the invention to an implantable cardiac defibrillator 
having an H-bridge switch for multiphasic shock delivery, one embodiment 
of the actuating means includes two isolated high side drivers for 
selectively operating each high side switch in response to a corresponding 
signal from the control circuits, and two low side drivers for selectively 
operating each low side switch in response to a corresponding signal from 
the control circuits, such that high side drivers are optically isolated 
in accordance with the present invention. 
The control circuits for timing shocks operate first to actuate one of said 
high side drivers, and then after some preset time, to actuate the 
corresponding one of the two low side drivers, to begin each phase of a 
shock. Then, the control circuits first deactuate the one high side 
driver, and then, after some preset time, deactuate the corresponding low 
side driver, to end each phase of a shock. Alternatively, the control 
circuits may simultaneously deactuate the one high side driver and the 
corresponding one low side driver, to end each phase of a shock. The 
preset time between actuating the high and low side drivers, and 
deactuating the high and low side drivers when used, is preferably on the 
order of several hundred microseconds.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a battery 1 supplies energy to the implantable 
defibrillator circuits, typically at 5 to 15 V. Control circuits 2, for 
timing shocks (i.e., when they occur, their duration, and the number, type 
and sequence of phases), provide control signals for electronic switches 
in the shock generator, in this example for providing single-capacitor 
biphasic shocks. A shock charging circuit 3 converts battery energy to 
shock energy, typically at 0.75 KV, stored on a capacitor at 4, which is 
typically 125 .mu.F. These circuits are well known the art and any such 
circuit may be used. One useful circuit is described in copending and 
commonly assigned U.S. patent application Ser. No. 08/287,834, filed Aug. 
9, 1994 in the name of Peter Jacobson, the disclosure of which is 
incorporated herein by reference. 
FIG. 1 shows an H-bridge configuration shock delivery circuit including an 
H-bridge switch 6, which acts as an electronic switch to connect capacitor 
4 to a load 5 with a selected polarity. Also shown are high side drivers 7 
and 8, and low side drivers 9 and 10, for actuating the individual 
electronic switches 13, 14, 19 and 20 in the H-bridge switch 6. 
Referring to FIGS. 1, 13 and 14, when control circuits 2 assert HF at 11 
and LF at 12, high side driver 7 and low side driver 10 convey these 
control signals to close switches 13 and 14 respectively. As a result, 
current flows from capacitor 4 through load 5 in a first direction, from 
shock electrode 15 to shock electrode 16 (this is the first shock phase). 
Then, control circuits 2 deassert the asserted outputs, more preferably 
all of the control outputs 11, 12, 17, 18, thereby opening all switches. 
This provides a delay between phases, giving time for all switches to 
open. Next, control circuits 2 assert HS at 17 and LS at 18, so that high 
side driver 8 and low side driver 9 close switches 19 and 20 respectively. 
Consequently, current flows from the capacitor 4 through load 5 in a 
second direction, from shock electrode 16 to shock electrode 15 (this is 
the second shock phase). Control circuits 2 next deassert the asserted 
outputs, more preferably all of the control outputs 11, 12, 17, 18, 
thereby opening all switches 13, 14, 19 and 20 and truncating the second 
phase. The control circuits 2 can optionally continue this sequence to 
generate additional phases. 
Switches 13, 14, 19, and 20 illustrated in FIG. 1 can be implemented as 
MOSFETs or IGBTs, as is known to those of ordinary skill in the art. 
MOSFETs or IGBTs should have series diodes, as shown in prior art, to 
prevent external defibrillation from being conducted in the opposite 
direction through the switches. Switches should be rated at approximately 
30 A and 1.0 KV, and should have off-state leakage not exceeding a few 
microamperes. 
Low side drivers 9 and 10 shown in FIG. 1 can be implemented in a 
conventional manner, since they are not isolated. In general MOSFETs with 
an on-resistance of a few hundred ohms are used in a push-pull 
configuration. This gives a rise time and falltime of the control voltage 
on typical switch transistors 14, 20 of approximately 10 .mu.s. A 
capacitor of a few nanofarads (not shown) may optionally be added across 
the output of the switch driver to control risetime and reduce the 
importance of the Miller effect. It is important to maintain a moderate 
slew rate for the shock pulse (on the order of a few amperes per 
microsecond) to reduce inductive and capacitive coupling of the shock 
pulse into other sensitive circuits in the implantable defibrillator, such 
as telemetry circuits and P-wave or R-wave sensing amplifiers. 
The circuit illustrated in FIG. 1 further shows a connection 21 from the 
low side of capacitor 4 and the low side of the H-bridge switch 6, to a 
negative supply that is near ground. This permits operating control 
circuits 2 and low side drivers 9 and 10 between ground and this negative 
supply voltage, simplifying their circuitry, as described in a copending 
and commonly assigned application entitled Shock Generator For Implantable 
Defribillator/Cardiac Stimulator, filed Oct. 11, 1994 in the names of Alan 
H. Ostroff, Peter M. Jacobson, and Daniel P. Kroiss Ser. No. 08/220,854 
(attorney docket 22094.9926), the disclosure of which is incorporated 
herein by reference. However, the invention could equally well be applied 
to shock generators which refer the low side to ground, and which isolate 
the low side. 
Drivers 7, 8, 9, and 10 each comprise an input "in", an output "out" and a 
negative output "out-". References to "HV+" and "HV-" are to the high 
positive and high negative voltages, respectively. 
Numerous variants of the circuit shown in FIG. 1 exist within the scope of 
the invention. For example, additional power supply circuitry could be 
used to provide separate negative voltage supplies for operating control 
circuits 2, pacing and sensing circuitry not shown, and the low side 
drivers 9 and 10, instead of operating these circuits directly from the 
battery. Also, level shifter circuits or voltage multipliers could be used 
to translate logic signals from one supply to the other. This additional 
circuitry is not shown here to preserve clarity. 
FIG. 2 shows the transmitter 22, receiver 23, and optical path 24 for 
driving a single isolated switch. The transmitter 22 has power supply 
inputs "+" at 25 and "-" at 26, and a control signal input "in" at 27. 
Input 27 is a logic level signal operating between ground GND at 25 and 
supply VSS at 26. The receiver 23 has power supply inputs "HV+" at 28 and 
"HV-" at 29. It has an output signal "out" at 30, referred to "out-" at 
31. 
One embodiment of the invention uses only one optical path 24A per driven 
switch, and a second embodiment uses two optical paths 24A and 24B per 
driven switch. 
When control circuits 2 shown in FIG. 1 apply a logic high state signal at 
"in" 27, then the transmitter sends an optical signal via path 24A to the 
optical receiver 23, which responds by asserting the "out" high at 30. 
When the control circuit 2 deasserts the "in" input to an output state, 
then, in the first variant of the invention the transmitter 22 stops 
sending the optical signal, and the receiver 23 responds by deasserting 
"out" at 30 to a low state. In the second variant the transmitter 22 stops 
sending the first optical signal when it is time to open the switch in the 
H-bridge switch and sends at least a brief pulse of a second optical 
signal, via path 24B, to which the receiver 23 responds and shuts off the 
output at 30. 
Referring to FIG. 3, a transmitter circuit for the first variant of 
invention described above is shown, using a single optical path 24A. When 
the control circuits 2 (FIG. 1) assert "in" at 27 to a high level, this 
turns on the N-channel MOSFET at 34, allowing current to flow through 
limiting resistor 32 and light-emitting-diode (LED) 33, until control 
circuits 2 return the "in" input to a low level. Typical LED current is 
approximately 30 to 100 mA. While current flows, the LED 33 sends light 
along the optical path 24A (FIG. 2) to convey control signal information 
to the optical receiver 23 (FIG. 2). The presence of this optical signal 
signals the receiver 23 to turn on the electronic switch, and its absence 
signals the receiver 23 to turn off the electronic switch. 
Referring now to FIG. 4, a transmitter circuit for the second variant of 
the invention described above is shown, using two optical paths 24A and 
24B. Components 25 to 27 and 32 to 34 operate as described in the 
explanation of FIG. 3, except that there are two optical paths 24A, 24B. 
LED 33 emits over path 24A only. Thus the transmitter 22 emits a first 
optical signal whose presence signals the receiver 23 to turn on the 
controlled electronic switch. When the control circuits 2 deassert "in" at 
27 to a low level, this extinguishes the LED at 33 and triggers monostable 
multivibrator 35. The monostable times a period of approximately 0.10 ms 
where its Q output at 36 remains high. The Q output at 36 being high turns 
on N-channel MOSFET 39, allowing current to flow through limiting resistor 
37 and LED 38, again, typically approximately 30 to 100 mA. LED 38 sends 
light along a second optical path 24B shown in FIG. 2. The presence of 
this second optical signal instructs the receiver 23 in FIG. 2 to shut off 
the controlled switch. 
Referring to FIG. 5, a receiver circuit 23 is shown. It includes power 
supply 40 to provide power for charging the control terminals of the 
controlled electronic switch, and a phototransistor switch 41 for 
conducting current from power supply 40 to the control terminals of the 
controlled switch, when activated by an optical signal along optical path 
24A (FIG. 2). Resistor 47, typically 0.47 MOhms, helps shut off 
phototransistor 41 more rapidly. Also shown in FIG. 5 is a shutoff circuit 
42 for discharging the control terminals of the controlled switch when 
either the phototransistor switch 41 ceases to supply a charging current, 
as in the first variant of the invention, or when the shutoff circuit 42 
receives a second optical signal along path 24B (not shown in FIG. 5), as 
in the second variant of the invention. 
The power supply 40 illustrated in FIG. 5 includes a capacitor 43, 
typically 50 nF, for storing energy at low voltage, and a current limited 
path with high value resistors 44 and 45, to connect capacitor 43 to 
charge across the high voltage supply capacitor 4 of the defibrillator 
generator circuit shown in FIG. 1. Since the resistor value is high, 
typically 10 MOhms, only low current flows in the load 5 in FIG. 1 due to 
resistors 44 and 45. (It is evident that when power supply 40 is used in 
conjunction with the discharge circuits shown in FIG. 8, 10, or 12, this 
current can be reduced to a very low value due to blocking diodes 57, 58 
shown in those circuits.) The power supply 40 also has a voltage limiter 
at 46 to prevent overcharging of capacitor 43. Limiter 46 can be, for 
example, a zener diode with a zener voltage of approximately 15 V. 
Referring to FIG. 6, an alternate optical receiver 23 is shown which uses 
at least one photodiode (two are shown at 48 and 49) to replace the power 
supply 40 and the phototransistor switch 41 of the circuit shown in FIG. 
5. In this embodiment, the photodiodes 48, 49 provide the charging current 
to the control terminals of the controlled electronic switch in response 
to the optical signal at path 24A. The photodiodes 48, 49 provide a 
current which is a few percent of the current in the transmitter 23 LED 
33, at a few volts per photodiode, without any other power supply. 
Advantageously, this considerably simplifies circuitry and reduces 
component count. Further, it is possible to place additional photodiodes 
in series to increase the output voltage, or in parallel to increase the 
current for controlling the selected electronic switch. In general it is 
advisable to provide at least 0.15 mA at 15 V to control typical MOSFET or 
IGBT switches. 
Also shown in FIG. 6 is a shutoff circuit 42 to discharge the control 
terminals of the controlled switch, as explained in the circuit shown in 
FIG. 5. 
Referring to FIGS. 7 through 12, different constructions of shutoff 
circuits 42 are shown. The circuit illustrated in FIG. 7 is for use with 
the second variant of the invention described above with two optical 
paths, and the circuits of FIGS. 8-12 are for use with the first variant 
having a single optical path. Each of these circuits connects with inputs 
53 and 54 to the current generators shown in FIGS. 5 and 6. Current enters 
terminal 53 and leaves via 54. Each of these circuits also connects with 
outputs 30 and 31 across the control terminals of the controlled 
electronic switch. When the shutoff circuit 42 is activated, it discharges 
the control terminals by providing a low resistance path from 30 to 31. 
Referring now to the circuit in FIG. 7, it is activated by an optical 
signal on path 24A as described above. This turns on phototransistor 50, 
providing a low resistance path from 30 to 31. Resistor 51, typically 0.47 
MOhms, increases the noise immunity of the phototransistor 50. Resistor 
52, typically 1.0 MOhms, prevents charge buildup on the control terminals 
30 and 31 which could falsely trigger a shock, during the period between 
shocks. 
Illustrated in FIGS. 8 through 12 are shutoff circuits 42 which are 
normally activated, and are deactivated by current entering terminal 53 
and leaving terminal 54. In this manner these circuits normally hold the 
controlled electronic switch (whose control terminals are across 30 and 
31) off, until the power supply and phototransistor or photocoupler 
circuitry shown in FIGS. 5 and 6 supply current to turn on the controlled 
electronic switch. Each circuit uses a transistor 55 to provide a low 
impedance path from terminals 30 to 31. Each circuit has a biasing 
resistor 56, typically 1.5 MOhms, to hold the transistor 55 on when no 
current flows between input terminals 53 and 54. Each circuit uses one or 
two small signal diodes, typically rated at 0.10 A and 15 PIV, at 57 and 
58, which provide a voltage drop to bias the transistor off, when current 
flows to switch control terminals 30, 31. 
The shutoff circuit shown in FIG. 8 is implemented with a depletion-type 
N-channel MOSFET, which is on with a gate-source voltage of zero, and off 
when the gate voltage is a few volts more negative than the source. 
The circuit shown in FIG. 9 uses a P-channel junction field-effect 
transistor (JFET) which is on with a gate-source voltage of zero, and off 
when the gate voltage is a few volts more positive than the source. 
The circuit shown in FIG. 10 uses an N-channel JFET, which operates like 
the depletion-type N-channel MOSFET shown in FIG. 8. 
The circuit shown in FIG. 11 uses a bipolar PNP transistor, which is on 
when the emitter is about 0.7 V more positive than the base. Thus as soon 
as switch control terminal 30 rises above this value with respect to 
terminal 31, and no current flows through diode 57, then transistor 53 is 
on and holds the control voltage to about 0.7 V maximum. When current 
flows through diode 57 to turn on the controlled switch, this turns off 
transistor 53. 
The circuit shown in FIG. 12 uses a bipolar NPN transistor, which is on 
when the base is about 0.7 V more positive than the emitter. The circuit 
operates similarly to the circuit shown in FIG. 11. 
FIGS. 13 and 14 show the timing of the four switch control signals shown in 
FIG. 1. Each signal actuates a switch driver, which in turn actuates a 
switch, as shown in the table below: 
______________________________________ 
signal name 
signal number 
driver number 
switch number 
______________________________________ 
HF 11 7 13 
LF 12 10 14 
HS 17 8 19 
LS 18 9 20 
______________________________________ 
Isolated high side drivers using optical coupling, and especially those 
using photodiodes, can take a relatively long time to turn on or off the 
controlled transistor. This can be on the order of several hundred 
microseconds, depending on the efficiency of the optical transfer and the 
current in the transmitter LED. 
When a switch turns on or off while passing current, this is called hot 
switching. The switch must dissipate power during hot switching. The 
energy the switch must dissipate is the product of the current through the 
switch and the voltage across the switch, integrated over the transition 
time. If hot switching is used, it is necessary to limit the transition 
time to a few tens of microseconds with practical switches for implantable 
shock generators. On the other hand, the transition time should not be too 
short, since high rates of change of current or voltage can couple 
inductively or capacitively to sensitive points elsewhere in the circuit. 
Since there are two switches in series for each phase of the shock in the 
H-bridge configuration shown in FIG. 1, only one switch needs to undergo 
hot switching. Accordingly, the present invention performs hot switching 
on the low side, or where no isolation is used and it is simple to control 
turn-on and turn-off time. 
For this reason, to provide a shock phase, control circuits 2 first assert 
HF at 11, and then wait a preset time period of, for example, several 
hundred microseconds, to allow time for high side driver 7 to turn on 
switch 13, before asserting LF at 12. In this manner, switch 13 is already 
on when switch 14 starts to turn on. There is no hot switching at switch 
13. Thus the rate of turning on switch 14 determines the slew rate of the 
defibrillation shock. Low side driver 10 is constructed as described in 
the explanation of FIG. 1 above to provide controlled shock slew rate. 
At the end of the first phase, the control circuits 2 can first instruct 
the low side driver 10 to turn off switch 14, as shown in FIG. 14. Or, if 
the turn-off time of switch 13 produced by the shutoff circuit 42 in high 
side driver 7 is adequate, the control circuits can simultaneously 
instruct the low side driver and high side driver to turn off their 
switches, as shown in FIG. 13. The control circuits 2 similarly produce 
the second phase. 
One foreseeable modification to the foregoing embodiments within the scope 
of the invention is to configure a floating output stage, as in early 
implantable defibrillators. In this case the optical drivers of the 
invention can be used to drive low side switches as well as high side 
switches, provided a rapid driver is used for at least one series switch 
in each shock phase, to limit transition times as explained above. A 
single transmitter could drive multiple receivers to operate 
simultaneously multiple switches for each phase in a multiphasic 
discharger. 
Another foreseeable modification within the scope of the invention is to 
implement the discharger with high voltage P-channel MOSFETs, P-type 
IGBTs, or PNP bipolar transistors, inverting the version shown above, 
should high-voltage versions of such devices become available in the 
future. 
Although the invention has been described with reference to particular 
embodiments, it is to be understood that these embodiments are merely for 
purposes of illustration, and not of limitation, of the application of the 
principles of the invention. Numerous other modifications may be made and 
other arrangements may be devised without departing from the spirit and 
scope of the present invention.