Stability Analysis of Ripple-Based Constant On-Time Control Scheme with Offset-Correction
Date Issued
2012
Date
2012
Author(s)
Chou, Chung-Hsi
Abstract
In recent years, there has been a new mandate to improve light-load efficiency for switching power supplies for many applications. Ripple-based Constant on-time (RBCOT) control for DC-DC power converters scheme has received much attention for this reason. A converter is normally designed to operate in discontinuous conduction mode (DCM) under light-load condition. Under DCM condition, RBCOT scheme naturally reduces converter switching frequency as load current is reduced and therefore increasing the light-load efficiency. This is a constant on-time variable-frequency control scheme, as opposed to conventional constant-frequency variable on-time scheme for DC-DC converters.
A converter with a basic RBCOT control scheme is usually plagued with an output voltage offset. Therefore, an offset correcting circuit is often added to the basic RBCOT scheme to correct the situation. This scheme is abbreviated as OCRBCOT and is the focus of this thesis. There have been publications on the issue of control stability of OCRBCOT under heavy-load condition in which the converters are operated in continuous conduction mode (CCM). In this thesis, efforts were directed toward the investigation of the same issue under DCM operation.
In an OCRBCOT control scheme, switching-frequency ripple of the output voltage is involved in the process of pulse-width modulation and therefore, conventional small-signal model cannot apply. A time-domain approach is taken to address the issue. A DCM stability criterion is established which is practically useful for designers of OCRBCOT converters. Same approach is also used to establish the stability criterion for converter using OCRBCOT scheme along with adaptive voltage positioning (AVP) features. It’s also found out that the addition of AVP feedback loop improves the feedback stability.
Subjects
RBCOT
Discontinuous Conduction Mode (DCM)
Offset Correcting circuit
OCRBCOT
Stability Criterion
Adaptive Voltage Positioning (AVP)
Type
thesis
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