A Soft Switching Driving Circuit for Ultrasonic Motor

High driving voltage is required for traveling wave ultrasonic motor (USM for short), and the supply voltage of driving circuit is relatively lower, so driving circuit needs transformer to boost voltage. Push-pull circuit is usually used as the driver of USM for its simple configuration. But in push-pull circuit, the transformer has the risk of dc bias magnet, and there are spikes in the waveform of voltage across switches. These problems don't exist in half-bridge circuit. This study utilizes half-bridge circuit as driving circuit, and analyzes the influence of load characteristic on the driver. By adding an inductor into the driving circuit, load characteristic can be changed from capacitive characteristic to inductive characteristic; zero-voltage-switching (ZVS) can be realized; and switching loss can be reduced. Air gap is added to transformer so as to integrate the additional inductor into the transformer; and to reduce volume and weight of driving circuit. And the design criterion of the air gap is derived. This novel driver is applied to the TRUM-60 type ultrasonic motor, and the experimental results are presented to verify the effectiveness of the proposed circuit.