Hamed Latafat

Digital Control of Power Converters—A Survey

Power converters offer a high capability to efficiently manage electrical energy flows. Until a few years ago, their primary use was in supplying motors in industrial applications and in electric traction systems. Nowadays, in addition to those fields they are employed in a very wide range of low, medium, and high power applications including residential applications, renewable energy systems, distributed generation, and automotive. Since digital control represents a key element of modern power converters, this paper presents a review on digital devices [microcontrollers, Field Programmable Gate Arrays (FPGA)], hardware and software design techniques as well as implementation issues useful for designing modern high-performance power converters.

Observer-Based Control of LLC DC/DC Resonant Converter Using Extended Describing Functions

This paper presents theoretical and practical results about dynamic analysis, frequency response, and control of a LLC resonant dc/dc converter operating under wide input voltage and load variations. A nonlinear model for the LLC resonant converter was developed using the extended describing function method; then, based on the derived model, a nonlinear observer-based controller was designed and implemented with a digital signal processor. Transient responses obtained under input voltage and output load variations show that the proposed controller is capable to stabilize the output effectively. Experimental results prove the superiority of the proposed observer-based controller over a conventional PID controller.

Comparative transient response analysis of LLC resonant converter controlled by adaptive PID and fuzzy logic controllers

If properly controlled, LLC resonant converters can achieve zero voltage switching (ZVS) over the entire operating range, thus minimizing switching losses of the power MOSFETs and secondary diodes. For such a reason, this paper presents and compare a half bridge LLC resonant converter controlled by both an adaptive PID and a fuzzy logic controller (FLC). For obtaining the DC voltage transfer function, an analysis tool, the so called Fundamental Harmonic Approximation (FHA) method has been adopted. FHA is also used to determine circuit parameters and for predicting the transient response of the converter during sudden changes of input and load, which is then verified through simulations and laboratory measurements.

A grid-connected PV system with LLC resonant DC-DC converter

This paper presents a grid-connected photovoltaic (PV) system with a three level voltage source converter (VSC) using double closed loop control strategy. The outer DC voltage control loop is to regulate the DC bus voltage, and the inner current control loop is to synchronize the output current with the grid voltage, thus ensuring unity power factor. A LLC resonant DC-DC converter is used in the proposed system to step-up the voltage of the PV array and to extract maximum power. It is intrinsically isolated by a high frequency transformer, then the parasitic capacitance of the PV panels to ground could not be of concern; furthermore, because of soft switching technique, it operates at high frequency with low switching losses. Size and cost of the magnetic components and DC-link capacitor are decreased compared with traditional boost converters. An incremental conductance method integrated within PI controller was used to extract maximum power by PV. Simulation results based on MATLAB/Simulink verify the validity and dynamic performance of the proposed system during fast solar irradiation changes and ensure DC bus voltage stability.

Digital control of a half-bridge LLC resonant converter

Recently, LLC resonant converters are gaining more and more interest in DC/DC conversion due to their high efficiency and power density. Usually, they are used in low power applications, i.e. in the range of few hundred Watt and with constant output load, however they are very appealing in high power and renewable energy applications as well as in many other applications affected by wide variations of the load and/or the source voltage. This paper presents and discusses a novel half bridge DC/DC resonant converter with digital control and fuzzy logic. The proposed system is compared with a more traditional PID controller. A variable frequency control algorithms are implemented using a low cost, high performance Texas Instrument TMS320F28069 DSP for the regulation of the output voltage. Simulation results using MATLAB are presented to evaluate the quality of the obtained result. A laboratory prototype has been realized and experimental results obtained using both PID and Fuzzy controllers are shown and discussed.

Linearization of LLC resonant converter model based on extended describing function concept

LLC resonant DC-DC converters are becoming increasingly popular due to their ability to work with zero voltage switching (ZVS) feature. The reduction in switching power losses, especially in high frequencies, greatly enhances the efficiency of these converters in comparison with conventional PWM boost converters. This paper presents theoretical details for modeling the LLC resonant converter, by which its characteristic and dynamic behavior can be analyzed. To obtain an accurate model, the extended describing function concept is used and the approximate nonlinear model derived. Then the derived model is linearized in vicinity of the steady state operating point and the state space matrices of the system are obtained. The accuracy and validity of the model is then verified using MATLAB/Simulink.

Time domain approach compared with direct method of Lyapunov for transient stability analysis of controlled power system

This paper attempts to study the transient stability of a two machine infinite bus system when affected by large disturbances, by comparison of time domain approach vs. transient energy function. Then the decentralized nonlinear controller is embedded within the power system and simulation results show that the transient stability has been greatly enhanced. Based on the existing transient energy function of uncontrolled power system, the controlled power system has been represented as a forced Hamiltonian system. The Lyapunov function which is suitable for transient stability analysis of this controlled power system has been used for stability assessment. Simulations in different operating points show the enhancement of transient stability of power system with controller in both time domain approach and energy function method.

Mixed harmonic elimination and reduction technique for single phase nine level converters

This paper presents a procedure that works at fundamental frequency to compute switching angles in single phase 9-level converters to eliminate the third and odd triple harmonics and to reduce other low order harmonics, obtaining a low total harmonic distortion (THD). The DC voltages feeding the converter, that are equal, but variable, allow to obtain switching angles independent on modulation index. The proposed procedure consists in a minimization of THD by means of a suitable graphical procedure. Its mathematically demonstrated that the third and all odd multiple of the third harmonic are deleted. Simulation results, obtained by using Matlab/Simulink, are shown.

Fuel Cell power system with LLC resonant DC/DC converter

Fuel Cells (FC) are widely investigated for their potential use in electric transportation due of their capability to provide significant amounts of energy with low-noise and with almost zero-emission. FC-based power sources are non-linear system with low voltage and high current capabilities and with efficiency close to 50%. Several researches are in progress, aiming at increasing overall efficiency through optimised combination of components, mainly the FC and the DC/DC converter necessary for its interface with the vehicle DC bus or the recharging station. In this paper, a novel and efficient power system architecture suitable for fuel cell powered vehicles and for recharging stations is proposed. The proposed system includes a fuel cell stack and a LLC resonant DC/DC converter with step-up capability. The latter due its to soft switching operations ensures enhanced efficiency over conventional hard switching counterpart. The proposed system has been modeled in a mixed PSIM-MATLAB/Simulink environment; design criteria and procedures are discussed for further applications and simulation results are shown to validate the proposed solution.