This paper presents a class D series resonant inverter with a series capacitor and an auxiliary switch. A capacitor and a switch connected in parallel are added to the series resonant circuit in series, which regulates the output power without frequency modulation. The entire impedance is changed by the difference between the switching frequency and the two resonant frequencies. While the auxiliary switch is on, the switching frequency is equal to the resonant frequency of a series circuit composed of a resistor R, an inductor L and a capacitor C. While the auxiliary switch is off, the switching frequency is equal to the resonant frequency of a series circuit composed of R, L, C and an additional capacitor. Applying the sub-optimum class E switching condition to the auxiliary switch, high power conversion efficiency is maintained. State equations are solved with MATLAB for each state. The calculated waveforms were agreed with the simulation results and experiment results. The maximum measured power conversion efficiency was 90.2% at 200 kHz.