Robert Priewasser

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.

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.

Battery State Estimation Using Mixed Kalman/Hinfinity, Adaptive Luenberger and Sliding Mode Observer

For electric vehicles, the improvement of the range of miles and with it the utilization of the available cell/battery capacity has become an important research focus in the community. For optimization of the same, an accurate knowledge of internal cell parameters like the state-of-charge (SoC) or the impedance is indispensable. Compared to the state-of-the-art, in this paper discrete-time Kalman and H∞ filtering based SoC estimation schemes - up to now applied to linear battery models - are applied to the nonlinear model of a Li-Ion battery. For that, a linearization method is proposed, which utilizes a prior knowledge about the predominant nonlinearities in the model together with a coarse SOC estimate to obtain a linear state estimation problem. Based on that, a mixed Kalman/H∞ filter-, a discrete-time sliding mode observer-, and an adaptive Luenberger based estimation scheme is furthermore investigated for the nonlinear battery model under test. The above-mentioned methods are compared to the state-of-the-art reduced order SoC observer and the Coulomb counting method. In order to compare the performance, an appropriate battery simulation framework is used, which includes measurement and modeling uncertainties. The evaluation is done with respect to the ability to reduce the impact of error sources present in realistic scenarios. For the simulated load current pattern, best results are achieved by the mixed Kalman/H∞ filtering approach, which achieves an average SoC estimation error of less than 1%.

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).

Non-linear control for energy efficient DC-DC converters supporting DCM operation

Non-linear control is a popular way to regulate the output voltage of switch-mode DC-DC converters. In this paper, a controller based on the non-linear sliding mode theory, but adapted to maintain a constant switching frequency, is proposed. A general definition of the sliding surface is suggested, making it possible to use the same control architecture for different DC-DC converter topologies (e.g. Buck, Boost, Buck-Boost). Moreover, the proposed controller supports the discontinuous conducting mode (DCM) to increase the efficiency of the energy conversion under light loads. Several simulation results are presented in order to prove the performance of the controller. A real-world commercial application, i.e. an efficient power supply for mobile devices, has been used as a reference to accurately choose the simulation parameters.

Trade-off analysis of decoding algorithms and architectures for multi-standard LDPC decoder

Since Low-Density Parity-Check (LDPC) codes deliver excellent decoding performance, they are adopted in several recent communication standards like the IEEE 802.11n, IEEE 802.16e and DVB-S2. This raises the need for multi-standard, multi-mode decoder architectures. In this paper we propose to use the min-sum approximation together with a turbo-like decoding approach for decoding LDPC codes, and afterwards we perform a global trade-off analysis which enables the designer to choose the appropriate decoding algorithm.

Digitally-controlled DC-DC converter with variable switching frequency

An increasing trend to utilize digitally-implemented control laws to regulate Switch-Mode Power Supplies (SMPS) is apparent, because such solutions offer several opportunities for advanced control schemes. This paper presents a digital compensator to regulate a Point-of-Load (PoL) Buck converter. The regulator varies both the duty cycle as well as the switching frequency by controlling not only the on-time, but also the off-time of the switching period. During the modeling stage, the transfer functions relating the on- and the off-time to the output voltage are derived. Subsequently, a digital controller with multiple feedback loops, which achieves tight voltage regulation and an excellent dynamic performance, is proposed. Simulation results are provided to prove the effectiveness of the proposed control structure.