Ripple-based Constant On-time Control Research Articles

Accurate Loop Gain Model of Ripple-Based Constant on-time Controlled Buck Converters

Ripple based constant on-time (RBCOT) control has been widely used in DC-DC buck converters over past years. However, there has been no accurate loop gain model to link the property of RBCOT controlled buck converter to existing knowledge about basic feedback control principles like crossover frequency (Fc) and phase margin (PM) so far. The previously reported stability criterion for the output capacitor's ESR, i.e., Ton/(2C), is insufficient to avoid subharmonic oscillations over all duty cycle values. This paper presents a new accurate loop gain model for the RBCOT control. Without simplification, the proposed model can accurately predict the system response even beyond the switching frequency. The proposed modeling approach has been extended to two types of external ramp compensation schemes (for solving the subharmonic oscillation issue) and optimum ranges of ramp compensation amplitudes have been derived accordingly. The modeling results are verified by the SIMPLIS simulations and experimental results.

Digital Hybrid Ripple-Based Constant On-Time Control for Voltage Regulator Modules

This paper presents a digital hybrid ripple-based constant on-time control scheme for voltage regulator modules (VRMs). Due to the sampling effects of the digital implementation, the stability issue becomes worse than the analog ripple-based control schemes, especially when the composite output capacitors are utilized. In order to stabilize the system and to fulfill the output impedance requirement of adaptive voltage positioning (AVP), a hybrid ramp compensation strategy, which includes the external ramp and the estimated current ramp, is proposed in this paper. No complicated compensators are required as the conventional voltage-mode control. The small-signal model of the proposed architecture is derived to provide the design guideline for the ramp compensation gains with different output capacitors. Besides, the oversampling effect of the output voltage by using analog-to-digital converters (ADCs) are also analyzed by using the presented model. Simulation and experimental results show that the proposed digital ripple-based control architecture can achieve fast load transient performance and to fulfill the AVP design requirements of single-phase VRMs.

Subharmonic Stability Limits for the Buck Converter With Ripple-Based Constant On-Time Control and Feedback Filter

A general closed-form subharmonic stability condition is derived for the buck converter with ripple-based constant on-time control and a feedback filter. The turn-on delay is included in the analysis. Three types of filters are considered: low-pass filter (LPF), phase-boost filter (PBF), and inductor current feedback (ICF) which changes the feedback loop frequency response like a filter. With the LPF, the stability region is reduced. With the PBF or ICF, the stability region is enlarged. Stability conditions are determined both for the case of a single output capacitor and for the case of two parallel-connected output capacitors having widely different time constants. The past research results related to the feedback filters become special cases. All theoretical predictions are verified by experiments.

Stability issues and modelling of ripple‐based constant on‐time control schemes operating in discontinuous conduction mode

Ripple-based constant on-time (RBCOT) control schemes have recently received much attention for the DC converters used in many applications, mainly because of the relatively high efficiency under light-load condition while preserving other features such as fast step-load response and small component count. Since the converter is usually operated under discontinuous conduction mode (DCM) under light-load condition, an effort to investigate the RBCOT schemes under DCM is practically important. Rigorous mathematical analyses are used to investigate the feedback stability issues of the buck converters using RBCOT control schemes operating in DCM. Two RBCOT control schemes are considered: the basic RBCOT scheme and the basic RBCOT with virtual-inductor-current signal (VICRBCOT). 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. A proper design procedure to maintain stability for both the continuous conduction mode and the DCM operations in VICRBCOT converters is proposed and verified.

A Ripple-Based Constant On-Time Control With Virtual Inductor Current and Offset Cancellation for DC Power Converters

In recent years, there has been a growing trend of mandating high-power conversion efficiency, for not only the heavy-load but also the light-load conditions. To achieve this purpose, a ripple-based constant on-time (RBCOT) control for dc–dc converters has received wide attentions because of its natural characteristic of switching frequency reduction under the light-load condition. However, a RBCOT control suffers from an output-voltage offset problem and a subharmonic instability problem. In this paper, a modified RBCOT buck converter circuit is proposed to solve both problems. The circuit uses the concept of virtual inductor current to stabilize the feedback, and an offset-cancellation circuit to eliminate the output dc offset. The modified circuit can be fabricated into an integrated circuit (IC) without adding any pin compared to conventional circuits. A control model based on describing function is developed for the modified converter. The small-signal characteristics and design criteria to meet stability are derived. From the model, it is also found out that it is much easier to accomplish adaptive voltage positioning using the proposed modified RBCOT scheme compared to a conventional constant-frequency controller. Simulation and experimental results are given to verify the proposed scheme.