Biomedical Engineering Reference
In-Depth Information
• TOFF: typical turn-off
ff
time
100
µ
s
• IDRM: max. o
ff
-state leakage current
50 mA
In a similar way, a breakaway diode model (DBREAK) is modi
fi
ed to simulate the
behavior of the 40HF100 diodes:
• VRRM: max. peak reverse voltage
1000 V
• IRRM
9 mA
• IF(AV): max. average forward current
40 A
• IF(RMS): max. RMS forward current
62 A
• IFSM: max. peak surge current
570 A
• VF(TO)1: low-level value of threshold voltage
0.65 V
• VF(TO)2: high-level value of threshold voltage
0.70 V
• RF1: low-level value of forward slope resistance
4.29 m
Ω
• RF2: high-level value of forward slope resistance
3.98 m
Ω
• VFM: max. forward voltage drop
1.30 V
find an inductance that keeps the peak current within the SCR's cur-
rent-handling capability (
Now the trick is to
fi
8.3 kA) while conforming to the SCR's switch-on capability
(
dI/dt
s). The PSpice simulation results of Figure 7.26 show that the peak cur-
rent through the coil is approximately 8 kA and
dI/dt
is 270 A/
400 V/
µ
is which are both within
the ST330S16P0's ratings. The stimulating phase of the pulse lasts for approximately
100
µ
s of the current pulse duration doesn't do much as far as stimulat-
ing the tissue but causes considerable heat dissipation in the coil.
The energy delivered to the coil by the stimulator is really divided into Joulean energy
and magnetic
µ
s. The other 900
µ
ow through the sys-
tem's resistance, and is lost as heat. However, some stimulator designs are capable of
recovering the energy stored in the magnetic
fi
field energy. The Joulean energy is from the current
fl
LI
2
max
/2). For exam-
ple, Figure 7.27 shows a magnetic stimulator that uses a nonpolar capacitor bank. As shown
in the PSpice simulation results of Figure 7.28, when the SCR is triggered, the capacitor
discharges through the coil. Selecting
L
and
C
to yield an underdamped response, as the
capacitor voltage reaches zero, the coil current and magnetic
fi
field (given by
E
magnetic
fi
field are at their maximum. As
the magnetic
flow in the same direction until
it charges the capacitor to the opposite polarity. A diode placed antiparallel to the SCR allows
the charge to
fi
field collapses, the coil current continues to
fl
flow back through the coil in the opposite direction, charging the capacitor
to its initial polarity. At this point the current stops because the SCR has turned off.
fl
. If the
loss to Joulean heating is low, most of the energy ends up back in the capacitor bank, and
only a small amount of additional power is needed to restore the capacitor voltage.
ff
Magnetic Stimulator Coil Design
In magnetic stimulation, the design of the coil is in many ways far more important than the
design of the energy source. This is because the coil is placed in very close proximity to
the subject, posing risks of electrocution, burning, and/or mechanical impact to both the
subject and the operator. The following consideration must be made when designing stim-
ulator coils:
1.
The voltage across the coil is typically a few kilovolts. Depending on the coil's con-
struction, the voltage across adjacent turns will typically be in the range 200 to
1000 V. For this reason, insulator materials must be selected such that they can pass
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