The inverse piezoelectric effect of appropriate crystal materials allows the direct generation of mechanical displacements or forces by electrical voltages. The achievable relative deformation is around 0.2 %. The force generation depends on the stiffness of material. Piezoelectric actuators are able to generate high pressure up to 35 N/mm2, in comparison to approx. 0.02 N/mm2 of electromagnetic actuators. Very high dynamic response is one of the characteristic features of piezo actuators. These actuators are suitable for static as well as for dynamic applications. Fuel injectors, e. g., are one of the important and well known application for quasi-static mode with the piezo actuator opening the injector valve for a very short moment [14]. The ultrasonic vibration or ultrasonic piezoelectric motors are applying the dynamic mode of operation [12], [13].

Therefore, power converters which do not require heavy inductors are of great interest. Size and weight of the magnetic components could be reduced by a carrier-based PWM (CBM) converter with LC-filter (Fig. 1). Investigations show that LC-PWM inverters are more suitable for weakly damped piezoelectric vibration systems such as bond sonotrodes, where none or small reactive power is delivered at the operating point by the inverter [14]. Thanks to the high switching frequencies, the inductivity Ls can be decreased significantly in comparison to those used in resonant filters [12]. This results in smaller and lighter components. However, the high switching frequency of PWM inverters consequently results in higher switching losses and might be in conflict with EMC issues. To overcome these drawbacks, the proposed LLCC-PWM inverter was developed to excite the high-power piezoelectric actuator, where a LLCC-filter circuit is utilized and operated in PWM controlled mode (see Fig. 2) [1], [9], [10]. A PWM method with calculated switching angels eliminates selected number of harmonics (HEM) and influences the frequency spectrum of the converter output voltage u_filter. In conjunction with the band-pass characteristic of the LLCC-filter the HEM method will result in lower total harmonic distortion (THD) of the actuator voltage u_CP as with CBM (cp. Fig. 3).

The novel solution offers significant advantages to improve the performance of the power supply as follows.

• Reactive power of the piezoelectric actuator is compensated locally, by placing the inductor Lp close to the actuator.
• The output filter shows optimized performance at minimized volume and weight, compared to the classical resonant inverters.
• THD of the piezoelectric actuator voltage ucp is reduced without increasing the switching frequency.

The proposed LLCC-PWM inverter (Fig. 4) was employed for driving a high power airborne piezoelectric brake actuator in an EC-funded project PIBRAC [2], [3], [4], [5], [7]. Other potential fields of application of LLCC-PWM inverters are: superimposed sonotrodes assisted ultrasonic drilling, cutting, chiseling, and milling of tooling machines or atomizers for the production of fine granular powder.