Biomedical Engineering Reference
In-Depth Information
markets and are considered to be very reliable. However, their capacitance changes as a
function of use and other factors. For this reason, whenever they are not used for some time,
they require reforming such that they can be made to store the full desired charge. Reforming
is accomplished by charging the capacitors to their full capacity periodically. Recently, high-
voltage tantalum capacitors have been made available for use in de
brillators. These capac-
itors do not require reforming, and their charge acceptance remains more or less constant
despite its use. However, high-voltage tantalum capacitors are very expensive (a few hundred
dollars each), and they are not available in the voltage and capacitance ratings necessary for
our application.
For the prototype, we used two Panasonic TS-HB-series 330-
fi
µ
F capacitors at 450 V in
series for an equivalent capacitance of 165
F at 900 V. As shown in the Figure 8.38, the
capacitors are charged in series directly from the HV power supply. 400-k
µ
bleeder resis-
tors are used to equalize the voltage across the capacitors during charging. In addition, a
0.5-
Ω
resistor was added in the series connection to enable monitoring of the charge-
discharge current. A circuit comprising a blinking LED and a piezo buzzer is powered
directly from the HV line to warn the user (especially during experimentation with the cir-
cuit) that there is energy stored in the capacitor bank.
Ω
Switching Devices
External de
brillators (which deliver up to 400 J) use high-voltage high-current relays to
connect the de
fi
brillation load to the storage capacitor. Although they are simple to con-
trol, these devices are bulky and clearly unsuitable for an implantable device. A common
way of delivering stored energy to the load in implantable de
fi
brillators is through insu-
lated gate bipolar transistors (IGBTs). Noteworthy properties of IGBTs are the ease of
voltage control and the low losses at high voltages. These characteristics are similar to
those of MOSFETs. However, the e
fi
ff
ective ON resistance of IGBTs is signi
fi
cantly lower
than that of MOSFETs.
We selected the IXYS IXGH17N100U1 IGBT for the prototype circuit. A very similar
device in bare-die is used in commercially available implantable de
brillators. This device
features a second-generation HDMOS process with very low V_CE(SAT)
fi
3.5 V for min-
imum ON-state conduction losses. The IXGH17N100U1 is rated for a V_CES of 1,000 V
at 34 A (or 64 A for 1 ms).
The common practice in implantable devices is to drive the IGBTs (or MOSFETs)
using pulse transformers and a driving circuit. For the sake of simplicity, however, this cir-
cuit uses new photovoltaic optocouplers such as the International Recti
er PVI1050. These
devices generate their own dc current at the output, and as such, can be used to implement
much simpler control circuits. Each PVI1050 has two photovoltaic cells driven from a sin-
gle LED source. Each photovoltaic cell produces a maximum of 8 V at 10
fi
A.
As shown in Figure 8.39, the two photovoltaic cells inside each PVI1050 are wired in
series. Two PVI1050s (IC_Enable_1 and IC_Enable_2) are wired in parallel to charge the
IGBT gate capacitance to the saturation voltage. R_Gate_Bleed_IGBT is used to passively
bleed charge accumulated at the gate (e.g., through the Miller e
µ
ff
ect) of the IGBT. This
ensures that the IGBT remains off
until turned on by activating the photovoltaic isolators.
Current for the LEDs of the photovoltaic isolators is switched under the control of the
ff
de
brillation module microcontroller through a small switching FET. Since charge buildup
with constant light output takes some time, this instrument implements a technique to
improve the response time of the photovoltaic isolators [Prutchi and Norris, 2002]. The
current is switched on through the LED through the Q_ENABLE line and is limited to a
safe continuous level through the 39- and 4.7-
fi
s), how-
ever, a very strong current is sent through the LED by way of the Q_KICK line to boost
the output and yield an IGBT turn-on time of under 25
Ω
resistors. For a brief period (100
µ
µ
s at full energy (full saturation).
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