Ching-Jan Chen

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.

Modeling and Design Considerations of a Novel High-Gain Peak Current Control Scheme to Achieve Adaptive Voltage Positioning (AVP) for DC Power Converters

Peak current control (PCC) can be used to achieve adaptive voltage positioning (AVP) in DC power converters for CPU power applications. However, PCC is seldom used because of the problem with output voltage offset from the target load line. A novel high-gain PCC (HGPCC) AVP scheme was recently reported to correct the aforementioned offset problem while retaining the advantages of PCC, such as easy phase-current balancing, inherently cycle-to-cycle protection, and good stability margin. Without an analytical model, however, it is very hard to achieve prescribed characteristics using this complicated scheme. In this paper, a control model and design considerations will be presented for this scheme. Key equations for converter feedback performance and rules for compensating the feedback loop will be presented. The proposed model was experimentally verified.

A novel ripple-based constant on-time control with virtual inductor current ripple for Buck converter with ceramic output capacitors

Ripple-based constant on-time (RBCOT) control has found applications because of its fast load transient response, small component count, and good light-load efficiency. However, this control scheme often encounters instability issue of subharmonic oscillation when ceramic capacitors are used for converter output filter capacitors. In the paper, a novel RBCOT control with virtual inductor current ripple was proposed to alleviate this problem. The novel control scheme improves system stability of RBCOT control without adding extra pins and converter components in IC implementation. Thus resulting in less component count and controller cost. Describing function method was used to derive the model and stability criterion of the proposed scheme, including the effect of delay-time. The scheme was implemented in an IC. Simulation and experimental results also confirm the proposed concept and the accuracy of the stability criterion developed.

A novel constant on-time current-mode control scheme to achieve adaptive voltage positioning for DC power converters

A constant on-time current-mode (COTCM) control features inherent phase-current balancing, good stability margin and improved light-load efficiency which is a new mandate for many CPU power applications nowadays. For a delicate application such as adaptive voltage positioning (AVP) application, however, the output voltage regulation of a COTCM converter is often inadequate when constant converter output impedance feature is also required. In this paper, a modified converter with a DC-offset correction circuit is presented. A small-signal model is also developed based on describing-function approach which is complicated but necessary to uncover the inherent nature of this control scheme. Experimental results will be shown. A comparison with a ripple-based constant on-time (RBCOT) control scheme will also be given.

An Adaptive High-Precision Overpower Protection Scheme for Primary-Side Controlled Flyback Converters

A primary-side controlled universal-line flyback converter is widely used for its low-cost and low standby power advantages; however, conventional overpower protection schemes with primary-side control suffer from inaccurate protection. In this paper, a novel scheme will be reported to achieve a very significant improvement in overpower accuracy. This scheme was implemented successfully in an integrated circuit without adding any extra pin compared to conventional schemes. Experimental and simulation results verified the proposed strategy.

Mix-Voltage Conversion for Single-Inductor Dual-Output Buck Converters

A single-inductor dual-output (SIDO) buck converter has recently found applications in hand-held battery-powered electronic devices. The circuit operation and the functional interdependencies among basic converter parameters such as DC voltage gains, transistor duty cycles, and load current levels are much more complicated than those of the single-output counterpart. In this paper, a rigorous analysis was conducted to develop DC equations for the converters. More importantly, from the analysis results, a possibility of a new mode of operation, dubbed “mix-buck” operation, will be pointed out. In the so-called “mix-buck” operation, the converter is capable of working even when the input voltage is lower than one of the two output voltages. In the past, a SIDO buck converter has been used for providing “pure-buck” outputs, i.e., both output voltages are lower than the input voltage. Therefore, this possibility not only opens up new applications but also may extend the operating battery range in existing applications. Experimental results confirmed the DC equations and the “mix-buck” operation of SIDO converters.

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.

A New Multiple-Frequency Small-Signal Model for High-Bandwidth Computer V-Core Regulator Applications

A new multifrequency small-signal control model for high-bandwidth dc buck regulator is proposed. The describing function approach is taken to model the duty-cycle modulator, incorporating the effects of not only the modulating frequency fm, the switching frequency fs, and the two sideband frequencies fs - fm and fs + fm. This model is significantly more accurate than the conventional average model and the recently proposed two-frequency model for applications with high control bandwidth-to-switching frequency ratio and high input-to-output voltage step down ratio, such as the v-core voltage regulators. The model is verified by simulations and experiments.

Subharmonic Instability Limits for

General closed-form subharmonic oscillation conditions are obtained for V2-controlled buck converters. Both constant switching period and constant on-time operations are analyzed with the outer voltage loop closed or open. Once an arbitrary linear feedback is given, the associated closed-form stability condition of the converter is readily obtained. Past research results based on the describing function technique and the Floquet theory become special cases. This paper provides an alternative and easier method to analyze subharmonic instability.

{\rm V}^2

In recent years, there has been a growing trend of mandating high power conversion efficiency, for both the heavy-load and 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 light-load condition. However, a RBCOT converter suffers from output-voltage offset problem and sub-harmonic instability problem. In this paper, a modified RBCOT buck converter circuit will be proposed to solve both problems using the concept of virtual inductor current to stabilize the feedback and an offset cancellation circuit to cancel out the output DC offset. A control model based on describing function is also developed for the modified converter. From the model, its found out that its much easier to accomplish adaptive-voltage-positioning (AVP) using the proposed modified RBCOT scheme compared to a conventional constant-frequency controller. Simulation and experimental results are also given to verify the proposed scheme.