Switching frequency tuned class-D amplifiers

The most important weakness of the switched-amplifiers is the residual ripple voltage on the load that remains in spite of filtering. It has been shown by examining the theoretical analysis and simulation results of the amplitudes of components comprising the Pulse Width Modulation(PWM) signal that the component having the highest amplitude is again the main component with the switching frequency for each value of modulation depth. A notch filter is proposed to suppress this component very effectively at the switching frequency. The characteristic of the filter is studied with theoretical, simulation and experimental results.Switching frequency of the amplifier should trace notch frequency very precisely since the attenuation characteristic of the proposed filter is a sharp curve around the notch frequency. Otherwise, the filter performance would not be put forward. Because of this reason, an analog control circuit has been designed. Designed frequency control circuit generates a voltage that makes equal the switching frequency to the notch frequency by comparing the phases of the components of filter input and middle node voltages at the switching frequency. This voltage is connected to the leg of the chip that generates PWM signal determining the switching frequency through a series resistor. The performance of the method is studied with realized prototypes and simulation results. For the purpose of comparison, the filter has been transformed into the topology that is known as LC filter for switching amplifiers without changing the component values in the notch filter. Then the comparison of these two filters has been made with the help of experimental and simulation results.Keywords: Switching amplifier, Class-D amplifier, notch filter, ripple steering, frequency tracking