Biomedical Engineering Reference
In-Depth Information
FIGURE 2-6
Examples of
electronic switches.
(a) Solid-state relays.
(b) Analog switch.
2.3
POWER SUPPLIES
Medical and biomechatronic devices generally run from mains power if they are static and
from batteries if they need to be portable. In most cases this supply voltage will need to be
converted to one or a number of other voltages to power the various modules within any
device. For example, a prosthetic arm may require 12 V to power the motors,
+
/
−
5 V for
the analog electronics, and 3.3 V for the signal processor.
It is now common to provide power to embedded systems such as cochlear devices
and artificial hearts using electromagnetic induction through the unbroken skin, as this
eliminates a common source of infection. This technology is discussed in more detail later
in the topic. An alternative that is becoming feasible as the efficiency of implants improves
is to scavenge power from flexing muscles or changes in pressure driven by the heartbeat.
2.3.1 Linear Power Supplies
Until recently, most mains powered devices consisted of a step-down transformer followed
by a full-wave rectifier to produce rectified alternating current (AC). This was followed by
large capacitors and chokes to supply the smoothed direct current (DC) voltage required by
the system. If different voltages were required, the transformer would include a number
of secondary windings as shown in Figure 2-7. Unfortunately, transformers are heavy
and expensive, so modern power supplies often rectify the mains directly and then use
switch-mode power supplies to provide the various output voltages.
Irrespective of the method used, good isolation must be maintained between the input
and output voltages, and also the raw DC must be further regulated and filtered to remove
mains hum or switching noise and to maintain a constant and clean output voltage.
Linear regulators are active control systems that compare the output voltage with
a fixed reference and use the error to adjust a series transistor to keep the output con-
stant irrespective of changes in the input voltage or the load, as shown schematically in
Figure 2-8. In this example, a Zener diode is used to generate the reference voltage, which
is compared with the output sense voltage using an operational amplifier (op amp), the
output of which drives the NPN transistor.
It is possible to construct voltage regulators from discrete components, but modern
voltage regulator integrated circuits are cost-effective and extremely sophisticated. They
