Matteo Agostinelli

Modeling, Control, and Implementation of DC–DC Converters for Variable Frequency Operation

In this paper, novel small-signal averaged models for dc-dc converters operating at variable switching frequency are derived. This is achieved by separately considering the on-time and the off-time of the switching period. The derivation is shown in detail for a synchronous buck converter and the model for a boost converter is also presented. The model for the buck converter is then used for the design of two digital feedback controllers, which exploit the additional insight in the converter dynamics. First, a digital multiloop PID controller is implemented, where the design is based on loop-shaping of the proposed frequency-domain transfer functions. And second, the design and the implementation of a digital LQG state-feedback controller, based on the proposed time-domain state-space model, is presented for the same converter topology. Experimental results are given for the digital multiloop PID controller integrated on an application-specified integrated circuit in a 0.13 μm CMOS technology, as well as for the state-feedback controller implemented on an FPGA. Tight output voltage regulation and an excellent dynamic performance is achieved, as the dynamics of the converter under variable frequency operation are considered during the design of both implementations.

Leakage–Delay Tradeoff in FinFET Logic Circuits: A Comparative Analysis With Bulk Technology

In this paper, we study the advantages offered by multi-gate fin FETs (FinFETs) over traditional bulk MOSFETs when low standby power circuit techniques are implemented. More precisely, we simulated various vehicle circuits, ranging from ring oscillators to mirror full adders, to investigate the effectiveness of back biasing and transistor-stacking in both FinFETs and bulk MOSFETs. The opportunity to separate the gates of FinFETs and to operate them independently has been systematically analyzed; mixed connected- and independent-gate circuits have also been evaluated. The study spans over the device, the layout, and the circuit level of abstraction and appropriate figures of merit are introduced to quantify the potential advantage of different schemes. Our results show that, thanks to a larger threshold voltage sensitivity to back biasing, the FinFET technology is able to offer a more favorable compromise between standby power consumption and dynamic performance and is well suited for implementing fast and energy-efficient adaptive back-biasing strategies.

A Wireless Charging System Applying Phase-Shift and Amplitude Control to Maximize Efficiency and Extractable Power

Wireless power transfer (WPT) is an emerging technology with an increasing number of potential applications to transfer power from a transmitter to a mobile receiver over a relatively large air gap. However, its widespread application is hampered due to the relatively low efficiency of current Wireless power transfer (WPT) systems. This study presents a concept to maximize the efficiency as well as to increase the amount of extractable power of a WPT system operating in nonresonant operation. The proposed method is based on actively modifying the equivalent secondary-side load impedance by controlling the phase-shift of the active rectifier and its output voltage level. The presented hardware prototype represents a complete wireless charging system, including a dc-dc converter which is used to charge a battery at the output of the system. Experimental results are shown for the proposed concept in comparison to a conventional synchronous rectification approach. The presented optimization method clearly outperforms state-of-the-art solutions in terms of efficiency and extractable power.

Fixed-frequency Pseudo Sliding Mode control for a Buck-Boost DC-DC converter in mobile applications: A comparison with a linear PID controller

Interest is apparently growing in the four-switch noninverting Buck-Boost converter, which can be effectively used as a power supply for mobile devices. A typical application is a dynamic power supply for third-generation (3G) Wideband Code Division Multiple Access (WCDMA) Power Amplifiers (PA). While several control strategies based on the linear control theory have been proposed recently, there is still room for investigation of nonlinear control techniques. In this paper, a fixed-frequency nonlinear controller based on the sliding mode theory is presented and compared to a classical linear Proportional-Integral-Derivative (PID) implementation. The main differences from the design perspective and in terms of performance are then pointed out and commented. The main advantages of the proposed technique generally include an improvement of the dynamic performance and increased energy efficiency of the conversion. Simulation results are provided in order to evaluate and compare the different control strategies.

Comparative study and improvement of battery open-circuit voltage estimation methods

Due to the steadily-increasing demands on more powerful electronic devices, an accumulators operating lifetime plays an essential role for the usability of battery-powered devices. To avoid an insufficient utilization of a cells energy and/or lifetime, a reliable and reasonably accurate knowledge of its internal parameters like the state-of-charge (SoC) is indispensable. The determination of the SoC is often directly related to the estimation of a batterys open-circuit voltage (OCV). In this work, various OCV estimation methods are compared with respect to their inherent accuracy. Additionally, the observability-Gramian-based OCV estimation method is extended to deal with expanded kinds of cell currents. Moreover, interpolation-based methodologies are presented which considerably reduce the average OCV estimation error over the entire SoC range, compared to state-of-the-art implementations.

Constant switching frequency techniques for sliding mode control in DC-DC converters

Sliding mode is a very effective control strategy for switch mode DC-DC converters. However, its widespread adoption has been hampered by some of the drawbacks inherent to this control technique. Among these, the most important is apparently the varying operating frequency, which can cause electromagnetic-interference (EMI) problems. This problem is particularly critical in portable devices (e.g. smartphones, e-readers) where it is highly desirable that the DC-DC converters operate at a constant switching frequency to avoid interferences with other parts of the system. Several methods have been proposed in previous literature to stabilize the operating frequency of sliding mode controllers. In this paper, some of the most promising techniques are evaluated and applied to a specific test-case, i.e. a Buck converter for a wireless system. A detailed model of the DC-DC converter has been developed in order to obtain realistic results. The system parameters have been chosen to reflect an actual commercial application.

Comparative study of linear and non-linear integrated control schemes applied to a Buck converter for mobile applications

Energy-efficient conversion of DC voltages is gaining more and more importance in low-power applications, especially in mobile and wireless systems. Several control techniques can be applied to achieve output regulation of a Buck DC-DC converter. In this paper a linear PID (proportional-integral-derivative) control loop, both in analog and digital domain, is derived and its performance is compared to a non-linear regulation loop based on the sliding-mode theory. A goal of this paper is to point out potential advantages and drawbacks of the different solutions. This exploration forms the starting point for the implementation of the most promising concepts in a 65 nm CMOS technology.

SystemC-AMS modeling and simulation of digitally controlled DC-DC converters

In this paper, an innovative method to model and simulate DC-DC converters with a digital or mixed-signal control loop is proposed using the SystemC-AMS hardware-description language. The proposed method was employed to model a specific test case, consisting of a Buck converter with a digital PID regulator. The reliability of the model was checked by comparing the results with MATLAB/Simulink simulations. The SystemC-AMS approach was found to be well suited to model the proposed system and very efficient from a computational point of view, since the simulation time can be strongly reduced with respect to other solutions (e.g. MATLAB/Simulink).

Design and Evaluation of Mixed 3T-4T FinFET Stacks for Leakage Reduction

In this paper, FinFET stacks consisting of mixed three- (3T) and four-terminal (4T) devices are analyzed in terms of leakage. A novel figure of merit is introduced, and closed-form leakage models are derived. Analytical results are used to derive simple design criteria to minimize the leakage by properly mixing 3T and 4T devices in transistor stacks. The comparison with a bulk technology shows that properly designed FinFET circuits are able to reduce the leakage by one or two orders of magnitude.

Phase-shift and amplitude control for an active rectifier to maximize the efficiency and extracted power of a Wireless Power Transfer system

Wireless Power Transfer (WPT) is an emerging technology with a huge variety of possible applications, including wireless charging of mobile devices like smart-phones or tablet computers. Currently available systems targeting these applications achieve a relatively low efficiency and hence a limited charging capability. To overcome these drawbacks, this work proposes a concept to maximize the efficiency of a WPT system as well as to increase the amount of extracted power. The presented method is based on actively modifying the equivalent secondary-side load impedance by controlling the phase-shift of the active rectifier and its output voltage level. The proposed concept enables WPT systems to actively adjust both the resistive as well as the reactive part of the equivalent load impedance, hence increasing the system performance in terms of efficiency and extracted power. A prototyping system for mobile applications has been developed based on the Qi standard specifications. The acquired experimental results confirm that the proposed concept is able to obtain a higher system efficiency and to extract more power than state-of-the-art solutions.