Thanks audioguru :) Now i have increase the PWM frequency to 19kHz. But my inverter design at the output there is a bit different. I use ADC to convert a single phase sinewave with peak amplitude of 5V to PWM signals. Then, the PWM is used to switch or chop the 220Vdc. The output that i obtain now is a PWM with amplitude of 220Vdc for positive half cycle and the negative half cycle (in accordance with the ADC signal). Then, my question is how to design the low pass filter circuit (using RC circuit which is easier to control than LC) which able to smoothen this high voltage PWM signals switching at 19kHz? So, how can i design a RC low pass fiter that able to withstand high power(100W) and high frequency. Then, finally output a perfect 220Vac at 50Hz? ???