The stability modeling of ripple-based constant on-time control schemes used in the converters operating in DCM

Ripple-based constant on-time (RBCOT) control schemes have recently received much attention for the DC converters used in many applications mainly due to its high efficiency feature under light-load condition. There are control models reported for such schemes for the continuous conduction mode (CCM) operation but not for the discontinuous mode (DCM). Since the converter is usually operated in DCM under light-load condition, an effort to investigate the RBCOT schemes under DCM is practically important. Two RBCOT schemes are considered in this paper, the basic RBCOT and the RBCOT with virtual inductor current (VIC). A describing function approach and a time-domain approach are used for the investigation of the feedback stability issue of the control schemes in DCM. It is proved that it is always stable for the basic RBCOT converter operating in DCM, but that is not the case for the VICRBCOT converters. Proper VIC signal must be used to maintain stability for both the CCM and the DCM operation. The analysis results are verified with experiments.