| In order to solve the driving problems of power,efficiency,volume and heat dissipation for phased transducers in non-contact object manipulation and its practical applications in medical ultrasound,the class-D power amplifier for the medium and low frequency signals is designed based on the technologies of pulse width modulation(PWM)and spectrum optimization.Firstly,the working principle of the PWM-based class-D power amplifier is analyzed.With theoretical derivations and numerical simulations of the PWM signals,the spectra and the harmonic distributions as well as the frequency response of output power are discussed.The modulation coefficient is optimized to be[0.9 1.2],and the angular frequency?_s of the sinusoidal input should be less than_c/3 of the triangular input with the cutoff frequency in[_s0.3_c]of the low-pass filter.The bandwidth of the output driving MOSFET should be higher than 10_c.Then,based on the practical requirement,the circuit of the PWM-based class-D power amplifier is designed.The performance tests of the PWM modulation,dead time control,power amplify and low-pass filter are conducted.The characteristics of the whole power amplifier is evaluated using a 50Ωresistive load,based on the measurements of the MOS temperature distributions with respect to the sinusoidal frequency and dead-time delay of the circuit,the dependences of the working efficiency on sinusoidal frequency and the load resistance,the nonlinear harmonic distortion and the performance of phase holding.Lastly,in order to enhance the synchronization of phased transducers in ultrasound applications with improved the working efficiency and stability of the PWM-based class-D power amplifier,the ROM table technique of FPGA is employed to generate digital PWM signals to drive the MOSFET directly.A class-D power amplifier based on FPGA is made to drive the ultrasonic horn transducer with the appropriate circuits of low-pass filter and impedance matching.The experimental measurements of the output voltage,the electrical power and the surface vibration velocity of the ultrasonic horn transducer with respect to the voltage supply demonstrate that the PWM-based class-D power amplifier can work at the medium and low frequency in the range from 20kHz to 400kHz with the maximum output power of about 45W and the high efficiency over 79%.The design has the advantages of simple structure,low output impedance,easy impedance matching,high phase keeping linearity,less power loss,and low circuit heating.The PWM-based class-D power amplifier provides a basic circuit of accurate driving for large-scaled phased transducer array,exhibiting prosperous potentials in the fields of ultrasonic detection,nondestructive testing,medical ultrasound,and so on. |
