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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates
generally to isolation devices in electrical circuits, and more particularly to methods using
acoustical isolators in electrical circuits.
BACKGROUND OF THE INVENTION Optical isolators are commonly used to electrically isolate
two separate or independent circuits. A common example where insulation is needed is when one
circuit operates at high voltage (e.g. 110 VAC (110 ac alternating current)) and the other
operates at low voltage (e.g. 5 VDC (5 volt DC)). Another example where isolation is needed is
when the two circuits have different ground potentials, where the isolation device needs to
prevent problems with ground loops. An optical isolator used in such a case would optically
transmit the signal, which would be converted to an electrical signal by the receiving circuit. The
problem with optical isolation devices is that optical coupling is extremely inefficient in power
transfer. For example, a light emitting diode (LED), which is a single optical coupler
(optocoupler), requires 10 milliamperes to generate an optical signal while a sensing diode that
senses the light emitted by that LED. The do uses 10 microamperes. The small output by the
optical coupling device is not sufficient for many products, so additional circuitry will be
In general, isolation devices must face stringent standards due to the potential hazards provided
by high voltage circuits. Such isolation devices need to overcome certain criteria for packaging,
electrical isolation, and internal creepage. Therefore, it would be extremely beneficial if an
isolation device was provided that is efficient in power transfer and still provides sufficient
electrical isolation.
SUMMARY OF THE INVENTION Briefly stated, the present invention comprises a method of
electrically isolating a first circuit from a second circuit. The first and second circuits are not
electrically coupled to one another. The method according to the invention acoustically couples
the first and second circuits together with a poezo-electric device that transmits and receives
mechanical vibrations.
acoustically coupled to a circuit 12 via an acoustic isolator 19. The circuits 11, 12 are electrically
isolated from one another.
While it is necessary to electrically isolate one circuit from another, there are many cases where
it is necessary to provide a non-electrical interface to establish a link between circuits. For
example, between the control circuit and the semiconductor relay. Since semiconductor relays
are generally coupled to potentially unwanted voltages, they need to be electrically isolated from
the control circuitry. A well known circuit isolation device is an optical isolator. Optical isolation
devices are very efficient in providing electrical isolation, but inefficient in transmitting energy.
This inefficiency does not allow optical isolation devices to be applied to devices other than high
impedance loads, limiting its availability to some applications, or additional circuits Create the
need for In general, optical isolators require several forms of amplification to increase drive
output. On the other hand, the acoustic isolation device 19 provides the same electrical isolation
as the optical isolation device and can efficiently transmit energy.
Circuits 11 and 12 are electrically isolated circuits. An acoustic isolator 19 acoustically couples
the circuit 11 and the circuit 12 together with mechanical vibrations corresponding to the
electrical signal. Dynamic vibrations can also be from circuit 11 to circuit 12 or vice versa. For
example, circuit 11 may comprise terminal 17 and terminal 18 to provide an electrical signal to
acoustic isolator 19 and circuit 12 may comprise terminal 21 and terminal 22 to receive an
electrical signal from acoustic isolator 19 It is possible. The acoustic isolation device 19 converts
the electrical signal provided by the circuit 11 into mechanical vibrations, whereby the circuit 11
is electrically separated from the circuit 12.
In the preferred embodiment, the acoustic isolation device 19 has a piezoelectric structure
comprised of a piezoelectric device 13, an acoustic media 16 and a piezoelectric device 14. In the
example described above, the electrical signal provided by the circuit 11 is converted by the
piezoelectric device 13 into mechanical vibrations, which correspond to the electrical signals. In
the preferred embodiment, the electrical signal has a predetermined frequency that matches the
resonant frequency of the piezoelectric device 13 to maximize energy transfer efficiency. The
acoustic medium 16 provides a low loss path through which mechanical vibrations propagate
until coupled to the piezoelectric device 14. The acoustic medium 16 is generally a material that
transmits vibrations such as ceramic, plastic, rubber related compounds and the like. Dynamic
vibrations are converted into electrical signals by the piezoelectric device 14 and received by the
circuit 12. The piezoelectric device 14 is selected to have a resonant frequency that matches the
mechanical vibrational frequency to maximize energy transfer efficiency. The acoustic device can
also be, for example, a piezoelectric ceramic resonance filter in one form.
The acoustic medium 16 provides other benefits as well as mechanical coupling. The acoustic
medium 16 provides physical separation between the piezoelectric devices 13 and 14 and helps
not to violate the limitations of the isolation device, such as the isolation barrier or the internal
cleaning page. The acoustic medium 16 can also be designed to filter unwanted mechanical
vibrational frequencies to reduce noise. In the preferred embodiment, the piezoelectric device 13,
the acoustic medium 16 and the piezoelectric device 14 are formed together as one unit that can
be packaged and re-connected to couple with the circuits 11, 12. It is also possible to have a
When transmitting energy, the piezoelectric device is much more efficient than optical coupling
(from the light emitting diode to the light detector). The transmission efficiency of input and
output power can be as high as 50 percent for a piezoelectric device as compared to optical
coupling, which is 0.1 percent. The high power transfer efficiency allows the acoustic isolation
device 19 to drive the circuit directly (although the circuit is electrically isolated).
FIG. 2 is an example circuit for the acoustic isolator 31. As shown in FIG.
Circuits 32 and 33 are electrically isolated from one another but are acoustically coupled via an
acoustic isolation device 31.
Circuit 32 is an oscillator that provides an electrical signal of a predetermined frequency to
acoustic isolator 31. Circuit 32 operates by applying supply power to input 34 and is enabled by
an enable signal applied to input 36.
The acoustic isolator 31 is similar to the acoustic isolator 19 of FIG. 1 and comprises a
piezoelectric device 37, an acoustic medium 38 and a piezoelectric device 39. The input and
current for the output voltage are determined by the electrode geometry and the area of the
acoustic isolation device 31 (piezoelectric devices 37, 38 described above). This makes the
acoustic isolator 31 compatible with different product specifications. The resonant frequency (or
frequency range) is determined in part by the physical dimensions of the acoustic isolation
device 31. The acoustic isolator 31 optimally transmits energy at its resonant frequency (or
frequency range). In the preferred embodiment, the predetermined frequency provided by the
circuit 32 is the resonant frequency of the acoustic isolator 31. Piezoelectric device 37 generates
mechanical vibrations that couple to acoustic media 38. The mechanical vibrations are then
received by the piezoelectric device 39 and reconverted into electrical signals received by the
circuit 33.
The piezoelectric devices 37, 39 can be formed as a resonant device or tuned circuit. Energy
transfer or energy reception is maximum at the resonant frequency of the piezoelectric devices
37, 39. The acoustic medium 38 can also be formed as a conditioning circuit. The physical
dimensions and physical properties of the acoustic medium 38 determine the frequency at which
the mechanical vibration is optimal. In the preferred embodiment, the acoustic medium 38 is a
narrow band filter and is tuned to the mechanical vibrations generated by the piezoelectric
device 37.
Circuit 33 is a semiconductor relay that couples AC or DC supply voltage 41 to load 42. The
supply voltage 41 generally provides a potentially unwanted voltage, which needs to be isolated
from the circuits 32,33. Supply voltage 41 and load 42 are coupled between terminals 43 and 44
of circuit 33. The load resistor 46 is coupled across the piezoelectric device 39. Depending on the
application of the invention, the need may arise to provide a defined load impedance across the
piezoelectric device 39. The diode 47 is used to rectify the voltage signal generated across the
load resistor 46. The rectified voltage enables the power transistors 48, 49 coupled in series to
couple the supply power 41 to the load 42.
The current capacity of the acoustic isolator 38 is a function of its physical dimensions,
transmission frequency, and power input. The magnitude of the current can be configured to be
sufficient to drive the circuit directly without using a load circuit. For example, the current
generated at 1 to 10 milliamperes can be generated by the piezoelectric device 37, the acoustic
medium 38, and the piezoelectric device 39. Eliminating additional circuitry substantially reduces
cost and complexity. The acoustic isolator 38 has dimensions suitable for being housed in a
semiconductor package. Thus, it is also possible to integrate the acoustic isolation device 38 with
other circuits using integrated circuit fabrication processes.
According to the present invention there is provided a method of electrically isolating a circuit
while the circuit is acoustically coupled.
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