Optimization of Class-D Amplifier Output Stage for Piezoelectric Actuator Control

Design of a high power piezoelectric actuator driver in a small form factor such as Micro Telecommunications Computing Architecture (MicroTCA) introduces a number of challenges due to its size and power limitations. Because of relatively small dimensions and a maximum dissipated heat of only 30 W for the Micro Rear Transmission Module $\pmb{(\mu \mathrm{RTM})}$, low efficiency of a linear amplifier becomes a major drawback. Switching Class-D amplifiers provide exceptional efficiency compared to non-switching solutions, which allows for higher output power without additional heat dissipation. The efficiency, however, comes at a price of high frequency harmonics introducing high Electromagnetic Interference (EMI) and Total Harmonic Distortion (THD). Driving a capacitive load, such as a piezoelectric actuator, at a high voltage and frequency with a LC Low Pass Filter (LPF) output stage is susceptible to signal distortions caused by the resonant frequency of the filter or the parasitic inductance of the cabling connecting the load to the driver. The inductance of the connection together with the capacitance of the load creates a resonant circuit, which must be designed as an integral part of the driver module. This article presents the effects of the connection inductance on the LC LPF output stage and the influence of the parameters of the filter on the signal. The measurements were performed on a custom made driver module for piezoelectric stack actuators with 180 V peak-to-peak voltage, 600 kHz switching frequency and a range of capacitive load from $\pmb{2.2 \ \mu\mathrm{F}}$ to $\pmb{40\ \mu \mathrm{F}}$.