Drive highly capacitive piezo devices using a high-voltage amplifier.
Piezoelectric effect is when certain solid material is compressed under mechanical stress, and it develops a voltage potential by accumulating electrical charges. French physicists Jacques and Pierre Curie discovered the effect in 1880. The inverse piezoelectric effect is when voltage applied to some ceramic materials causes it to expand or contract as shown in Figure 1. Over the next 100 years since the discovery, there have been numerous advancements leading to the discovery of many superior piezo materials (i.e. PZT) and the invention of many practical piezoelectric devices.
Figure 1 Piezo device expand/contract as applied voltage swing positive to negative
Most piezo (sometimes called PZT) devices today use the inverse piezoelectric effect. Examples of inverse piezoelectric effect devices are: piezo actual, motor, speaker/buzzer, high-power ultrasound transducer for cleaning, imaging, levitation, etc. Some of these PZT devices are operating at DC voltages, while some are driven by AC waveforms such as sine, square, and others. Let’s exam the fundamentals of piezo devices and techniques for driving them.
Piezo devices usually require high voltage to operate. Their required voltage ranges from 10V to as high as 200V. For AC devices, the required frequency is as high as 1 MHz. Additionally, piezoelectric devices are generally capacitive (except at resonant). The combination of high capacitance, high frequency, and high voltage requirements makes it difficult to drive these devices.
Usually piezo manufacturers specify their device capacitance at a given frequency. For the first order of approximation, most non-resonant piezo devices can be approximately modeled as a capacitor shown in Figure 2. The impedance is expressed in Equation 1. The current required to drive the PZT device is calculated from Ohm’s law as shown in Equation 2. From Equation 2, the current (I) is proportional to voltage (V), capacitance (C), and frequency (f).
Figure 2 Piezoelectric device approximately equal to a capacitor
Let’s use an example. For a high-frequency piezoelectric actuator whose capacitance is 1.1uF, the required peak voltage is 30V, and the driving frequency is 15kHz; the peak current required from the driver is about 2.5A (5.0A peak-to-peak). The piezo driver must be able to output such high current, high voltage, and high frequency at the same time. Higher frequency requires even more current.
High-current piezoelectric amplifier
As discussed above, the piezo transducer device's operating voltage range can be anywhere from 10V to 200V or more. Furthermore for AC piezo devices that have any combination of high capacitance, high voltage, or high frequency will require a high current driver. Signal and function generators commonly found in labs can output less than 5V into a 50 ohm load. Their output voltage is even lower when the PZT transducer impedance is lower than 50 ohm. Piezoelectric devices often require voltage in the range of 20V or more. Consequently a high output voltage and high output current piezo amplifier driver is required to drive such transducers.
For instance, an actuator/motor specifies 40V peak voltage, but a lab signal generator can only output is 5V or less. To achieve 40V square-wave, a piezo amplifier driver is used to boost the generator signal and outputs high voltage and high current to drive the piezoelectric actuator. Figure 3 shows an example of a piezoelectric transducer driver. Note the driver amplifies a combination of voltage, current, and power.
Figure 3 Piezo amplifier output high voltage and high current to the transducer
Piezo amplifier voltage requirements
It is important to understand the piezoelectric device voltage requirements before selecting the driver. Some piezo transducers require only the peak-to-peak voltage amplitude, while others specify 0-to-peak voltage. Ultrasound transducers for example, only require a peak-to-peak amplitude to produce the ultrasound level. It accepts voltage swing from negative to positive. For example, a sine wave voltage swing of -30V to +30V, which is 60Vpp. An actuator on the other hand, needs 0-to-peak voltage for proper operation; for instance, 0V to +40V square wave. In conclusion, it is crucial to understand the PZT device specifications on voltage and select a driver that meets the voltage range. Accel Instruments offers a wide range of high voltage driver amplifiers voltages.
Understand capacitive piezo power
As discussed before, the two common piezoelectric devices are mostly capacitive and resistive at resonance frequency. The power requirements are different for these two types of devices. Let's look at the capacitive devices first.
Many high-frequency piezoelectric transducers/devices are capacitive in nature. Its impedance is given in Equation 1 above. Recall from basic electrical engineering class that a capacitor’s impedance is equal to its reactant. That means the device’s impedance is imaginary (denoted by j) and there's no real resistance. Reactive devices do not absorb or dissipate power themselves. The power driven into the device is considered reactive power. The reactive power still needs to dissipate somewhere and it is dissipated inside the PZT amplifier driver as opposed to on the device. As a result, the heating is on the piezoelectric driver and not the PZT device itself. The reactive power is expressed in Equation 3, where the impedance (Z) is from Equation 1 and the current (I) is from Equation 2. Use RMS voltage value to calculate the RMS current and power. When selecting a driver, make sure the driver can handle the reactive power.
[Continue reading on EDN US: Increase real impedance]
KC Yang is a product marketing engineer at Accel Instruments.