Electromagnetic interference analysis of class D amplifiers

Class D amplifiers provide audio amplification with a very high efficiency as they use uses pulse width modulation instead of linear gain, this modulation is easier to amplify using two MOSFETs. The downside of this system is the rapid switching and therefore rapid current change, which induces voltages in parasitic inductors which combined with parasitic capacitors produce oscillations and create electromagnetic interference (EMI). This paper aims to shed light on where the parasitic inductors and capacitors are located so that these oscillations can be reduced in future designs. In order to find these parasitics the class D amplifier circuit was simulated in LTspice and the induced voltage of every inductor was analysed by multiplying the rate of current change with the inductance value. These inductors were tested for their ability to change the frequency. With this information a circuit was built and the effect of the inductors was tested by varying the length of the wires between every component. The parasitic capacitances of the transistors were tested by adding capacitances in parallel to the transistors. The most significant inductors are located in a loop through the powerMOSFETs and the decoupling capacitors. The significant parasitic capacitor was located in the power-NMOS capacitances. With this information the root cause of parasitic oscillations is known and can be used to optimize the power stage for minimum EMI.