Patent Publication Number: US-2011050036-A1

Title: Bias Circuit for Electric Field Transducers

Description:
RELATED APPLICATION DATA 
     Priority is hereby claimed to provisional application No. 61/239,658, filed Sep. 3, 2009, the content of which is hereby expressly incorporated herein in its entirety. 
    
    
     COPYRIGHT NOTICE 
     This patent document contains information subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent, as it appears in the US Patent and Trademark Office files or records, but otherwise reserves all copyright rights whatsoever. 
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to circuitry and circuit components for preventing unwanted reverse bias in electric field type transducers, such as ceramic transducers or piezoelectric transducers or polymer film transducers (ferroelectric transducers) or capacitive transducers. 
     BACKGROUND OF THE DISCLOSURE 
     Voltage signals are applied to electric field type transducers in order to impart and transmit the voltage signals in the form of vibrations at frequencies that may range from infrasonic to ultrasonic. More specifically, these types of transducers (e.g. projectors, transmitters, or actuators) convert an applied electric voltage to an electric field in the transducer&#39;s dialectic material, and then convert that electric field to mechanical displacement. Among these electric field type transducers, dielectric type transducers require that a DC bias be supplied from a DC voltage source separate from the input voltage signal to be transmitted. Meanwhile, a large class of ferroelectric type transducers have had their dielectric material “poled”, a process whereby the ceramic material is polarized to create in it a permanent electric field. It is important that, in deployment, the transducer not be reversed biased, i.e., not receive a significant negative voltage value at its positive input. Yet there is a risk that this might happen if the peak-to-peak input voltage level exceeds a certain level or if a reverse biased input voltage signal is mistakenly applied to the transducer. Serious damage to the transducer may result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a background art transducer circuit. 
         FIG. 2  is a block diagram of a transducer circuit in accordance with one embodiment. 
         FIG. 3  is a schematic diagram of a transducer circuit in accordance with another embodiment. 
         FIG. 4  is a schematic diagram of a transducer circuit in accordance with another embodiment. 
         FIG. 5  is a waveform diagram. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified block diagram of a driven transducer system, including AC voltage driver circuitry  10  and an electric field type transducer  12  (ceramic or capacitive or polymer film). The illustrated AC voltage driver circuitry  10  includes circuitry providing a remote (from the transducer) DC bias power source  14 , which may include a blocking capacitor and a blocking resistor. The illustrated transducer  12  may include at least one electric field type transducer, located at the end of a transmission cable driven by an AC voltage from the remote AC driver circuitry  10  simultaneously driven by a DC voltage, from remote DC bias power source  14 . The AC voltage driver may include an AC power amplifier driven by a pulsed AC drive signal. The pulsed AC drive signal may be powered by, for example, 60 Hz AC prime power. A pulsed drive signal may be sent to the power amplifier at some drive signal frequency. The AC power amplifier may include a voltage-source power amplifier, and a DC bias blocking capacitor may be provided between the output of the voltage-source power amplifier and the transmission cable connected to transducer  12 . 
     By way of example referring to the background circuit shown in  FIG. 1 , the AC power amplifier (not shown) and the transducer  12  may each be grounded. The ground connection indicated in all descriptions and Figures, may or may not refer to conventional earth grounding. The voltage V o  of the signal input to transducer  12  is the output of AC voltage driver circuit  10 . This voltage may be represented by the waveform (a) shown in  FIG. 5 , when the level of the AC voltage input by AC voltage driver  10  is in a normal range. As shown, the output voltage V o  oscillates about a DC bias voltage, and stays entirely above the zero voltage level. If the AC voltage provided by AC voltage driver  10  has an excessive peak-to-peak voltage, an undesired reverse bias is formed as shown in waveform (b) in  FIG. 5 . In this case, the DC bias voltage is not high enough to prevent the oscillating AC voltage V o  from becoming a negative voltage value. 
     One embodiment of circuitry of this disclosure is shown in  FIG. 2 . The illustrated circuit includes an AC voltage driver  10 ′ and a transducer  12 ′. The illustrated AC voltage driver  10 ′ may or may not include a DC bias power source  14 ′. At least one blocking capacitor is provided as part of AC voltage driver circuit  10 ′. The illustrated AC voltage driver circuit  10 ′ outputs an output voltage V o , which is input typically via a transmission line, to a positive terminal of transducer  12 ′. The negative terminal of transducer  12 ′ is connected to ground. In the illustrated embodiment, the cathode of a diode  16  is connected to the positive terminal of transducer  12 ′, and the anode of diode  16  is connected to ground. 
     With or without a DC bias power source  14 ′ (including a blocking capacitor), the resulting output waveform V o  across transducer  12 ′ in the circuit of  FIG. 2  will be as shown in waveform (c) in  FIG. 5 . This will be the case, generally, regardless of the peak-to-peak amplitude of voltage V o  applied to transducer  12 ′ by AC voltage driver  10 ′. 
     The AC drive signal voltage V o  may be of a number of different signal formats, for example, single or multiple pulsed sine waves, AM, FM, PM modulated continuous sine waves, or pulsed or continuous pseudorandom noise. By providing diode  16  in the circuit illustrated in  FIG. 2 , a DC bias voltage with an amplitude automatically self-adjusting is added to voltage V o , thereby precluding the applied AC drive signal from approaching a voltage value that goes below a given level, generally zero volts or a small number of volts below zero volts. The diode  16  also prevents reverse-biasing of the transducer  12 ′ even if a negative DC bias were to be mistakenly applied. 
     This results in the prevention of an unwanted reverse biasing of transducer  12 ′, and (optionally) eliminates the need for a DC bias power source. While AC voltage driver circuit  10 ′ may have a DC bias power source  14 ′, a separate and costly (in terms of hardware components and ongoing power consumption) DC power source is not necessary. In addition, the need for an AC signal blocking resistor can be eliminated. In addition, the required level of peak voltage rating of the amplified voltage sent to the transducer cable can be reduced. 
     The circuitry illustrated in  FIG. 2  replaces the equipment and methods found in other circuits for electric field type transducer DC biasing, which, for example, may use a stand-alone, high voltage, DC power supply, adding a settable voltage amplitude and a settable voltage polarity, to produce the correct high voltage DC bias required by electric field type transducers for a proper operation. Conventional circuits, for example, as shown in  FIG. 1 , may also include the use of a high voltage, high capacitance DC bias voltage blocking capacitor (not specifically shown in  FIG. 1 ) and a high resistance DC bias voltage blocking resistor (not specifically shown in  FIG. 1 ). 
     The circuit shown in  FIG. 2  can provide a fail-safe protection function by eliminating the possibility of transducer damage, for example, if the transducer is an electric field type transducer, which could be caused by the inadvertent application of a high-voltage DC bias voltage having an incorrect polarity. 
     In addition, the circuitry as shown in  FIG. 2  can eliminate the possibility of transducer damage and/or improper transducer operation, due to the application of a DC bias voltage of proper polarity, but having a DC bias voltage amplitude which is too small for safe and proper (first quadrant) transducer operation. The described improper combination of levels of DC bias and AC drive voltage can cause the electric field within the ceramic to alternate from a high-level positive value to a moderate-to-high-level negative value, at the signal frequency. These bipolar field alternations can cause heating and destruction of an electric field type transducer. 
     Another benefit of the circuitry shown in  FIG. 2  is that it can eliminate the requirement for an AC blocking resistor, having a high resistance and a high voltage rating. Such resistors are required in certain existing transducer bias configurations, that use a separate DC bias voltage power supply. 
     The circuitry shown in  FIG. 2  can also allow a required high-voltage DC blocking capacitor to be located at the same location as the transducer, rather than being located remotely from the transducer near the power amplifier. This would present an advantage over existing circuits which typically provide a DC blocking capacitor and a stand-alone high voltage DC bias voltage power supply which must be located remotely from the transducer. 
     The circuitry shown in  FIG. 2  also can eliminate the need, found in conventional circuits, to transmit a high voltage DC bias voltage superimposed with a high voltage AC drive signal. The circuit disclosed herein requires a transmission of only an AC drive voltage, through a power-amplifier-to-transducer transmission cable. This can cause a fifty percent (50%) reduction of the transition cable&#39;s peak-voltage rating requirement, between conductors of the amplifier to the transducer transmission cable. 
     Another advantage of the circuitry shown, for example, in  FIG. 2 , is that a high-voltage step-up transformer can be co-located near its associated (for example) in-water transducer, without adding a high voltage conductor to the transmission cable. A conventional approach includes transmitting a DC bias voltage through a transmission cable, to a series blocking resistor, and a DC blocking capacitor, to the secondary of a transformer co-located with a transducer which would require an additional high-voltage-rated conductor in the transmission cable. 
     The circuitry shown, for example, in  FIG. 2 , can further enable the use of low-voltage transmission cable, in conjunction with a transducer or multiple series-connected or parallel connected transducers, with one or more co-located step-up high-voltage transformers. The circuitry disclosed herein may produce a required DC bias at the location of the transducer itself, without requiring a separate and additional high-voltage-rated transmission cable conductor. 
       FIG. 3  shows a circuit in accordance with an alternate embodiment of the present disclosure. The illustrated circuit includes an AC power source  20 , a blocking capacitor  22 , a transducer  24 , and a diode  26 . AC power source  20  is connected between ground  27  and blocking capacitor  22 . The AC power source  20  generates an output voltage V o  at an output side of blocking capacitor  22 , which is applied across transducer  24 . Diode  26  is connected across transducer  24 . 
     In accordance with another embodiment of the disclosure, in  FIG. 4 , an AC driven transducer circuit is provided, which includes an AC power source  20 ′, a blocking capacitor  22 ′, a DC power source  23 , a transducer  24 ′, and a diode  26 ′. DC power source  23  applies a DC bias voltage across transducer  24 ′, to protect, at least partially, against undesired reverse biasing of the voltage across transducer  24 ′. Such a DC power source is not provided in the embodiment shown in  FIG. 3 . 
     In each of the embodiments shown in  FIGS. 3 and 4 , the provision of a diode  26  or  26 ′ provides a number of advantages, including preventing the reverse biasing of the transducer  24  or  24 ′. The diode provided across the transducers in the various embodiments herein may, for example, be a NTE517 silicon high voltage plastic rectifier for industrial and microwave oven use, for example, as provided by Electronics, Inc. at 44 Farrand St., Bloomfield, N.J. 07003. This example of diode includes controlled avalanche characteristics combined with the ability to dissipate reverse power. It includes a low forward voltage drop. The typical reverse leakage current is less than 0.1 micro amps, and the diode includes a high overload surge capacity. 
     Diode  26 ′ may be any semiconductor or other device that has certain characteristics like a diode, such that the shape of the waveform applied by the AC voltage driver remains substantially intact. For this application as shown in  FIG. 4 , the diode has: a sufficiently high reverse breakdown voltage to prevent a reverse over-voltage destruction of the diode due to peak signal voltage plus DC bias voltage; a sufficiently high surge current rating to allow the first cycles of current applied to the blocking capacitor to dissipate to a reasonable range—consistent with the continuous current rating of the diode; and a reverse leakage current that is sufficiently low to not discharge the capacitor. 
     In the embodiment shown in  FIG. 4 , in contrast to conventional circuits, a DC bias blocking resistor between the output of capacitor  22 ′ and the positive terminal of DC power source  23  is not necessary. Typically, a large impedance resistor is placed at this location in conventional circuits. 
     The high impedance resistor that might be provided between capacitor and the positive terminal of DC power source  23  can keep the DC power source from shorting the AC source, and also allow the AC current or most of the AC current to go to the transducer  24 ′. 
     In applications involving sonar, the peak voltage of the AC source can be thousands of volts. 
     While voltage source type drivers are depicted in the above-described embodiments, current source type drivers may be utilized instead. 
     The claims as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.