An investigation of steady-state averaging for the single-inductor dual-output buck converter using Fourier analysis

The SIDO converter is appealing in low-power applications requiring two independent supply voltages. The use of a single inductor significantly reduces the cost and size of the converter, however the complexity of the controller is greatly increased due to the inherent coupling of the two output voltages in continuous conduction mode. The accuracy of the traditional averaging technique is examined for the SIDO converter and it is shown that using a Fourier-based analysis yields a much more accurate steady-state operating point, especially at light loads. The solution can be used to obtain accurate steady-state converter waveforms for a wide range of conditions without requiring lengthy transient simulations. The accurate operating point is also used to obtain the SIDO transfer functions for a synchronous 1 MHz SIDO buck converter. For a fixed duty cycle, the error in output voltage resulting from the standard averaging method can easily reach 25 % at light loads.