Fixed Switching Frequency Operation Research Articles

Coupled-Inductor-Based Bidirectional Resonant Converter With Reduced Component Count and Load-Independent Voltage Gain

The bidirectional DC/DC converter (BDC) is a critical part to interface the DC-bus with energy storage system (ESS) in DC microgrid. This paper proposes a coupled-inductor based bidirectional resonant converter with center-aligned pulse width modulation (PWM) modulation method and reduced component count. The structure of dual half-bridges with coupled inductor makes the current analysis more complex than the full-bridge PWM resonant converter. Hence, a method of performance analysis based on the symmetrical pi model is proposed and the corresponding parameter design process is introduced. In addition, fixed switching frequency operation, load-independent voltage gain, full-load range zero-voltage-switching (ZVS), automatic bidirectional power transfer and bidirectional power limitation can be realized in the proposed PWM converter. An experimental prototype with rated power of 1000W is built to demonstrate the operation and performance of the proposed converter.

A simplified dual-stage model predictive controller for modular multilevel converters

The Modular Multi-Level Converter (MMC) topology is suitable for use in medium and high voltage applications. To cater to the multi-objective control requirements of the MMC, several Model-Predictive Control techniques have been developed. However, most of the available MPCs do not sufficiently address the simplification of the apparently complex switching logic of the MMC in regard to its practical implementation. To fill this gap, a modulated dual stage MPC is proposed here that controls the ac output current using a few simple steps, while maintaining sufficient control over the circulating current. The two MPC controller stages are used in a cascaded manner. The first stage calculates the voltage reference for grid current control and the second stage MPC selects the appropriate switching state to ensure circulating current control. The proposed technique uses carrier-based PD-PWM to retain its superior harmonic performance and ensures 2N+1 output pole voltage levels. While avoiding use of any cost function, a generalized fixed step approach is also presented that is independent of N value. The dual stage MPC presented in this paper is executed once in every switching time period to maintain a fixed switching frequency operation. Simulation and experimental studies show satisfactory performance of 25-level and 7-level MMC respectively, in a grid connected mode. Transient case study is also shown to demonstrate fast dynamic performance of the proposed controller.

Quadratic Regression Model-Based Indirect Model Predictive Control of AC Drives

Model predictive control is a promising technique for electric drives as it enables optimization for multiple parameters and offers reliable operation with nonlinear systems. In this article, a novel approach is presented that aims to harness the advantages of both finite and continuous set model predictive methods in converter-fed ac drive control. The proposed method requires the calculation of only seven predicted states. These states are then assigned cost function values. Using a quadratic regression model, the cost function is mapped to the entire modulation region. After solving a constrained optimization problem on this cost function mapping, the optimal voltage vector is obtained, which is then applied via pulsewidth modulation. The presented method can also be applied to multilevel converter structures without the need to calculate predictions for additional voltage vectors. Therefore, the proposed method does not increase in complexity with the utilized converter topology. Furthermore, the method offers a fixed switching frequency operation and an exact noniterative solution to the optimization problem due to the formulation of the regression model. As a case study, simulation and experimental results verify the operation of the predictive torque control for permanent magnet synchronous machines with the proposed method.

A Variable Inductor Controlled Single-Stage AC/DC Converter for Modular Multi-Channel LED Driver

Light-emitting diodes (LEDs) are widely used in street lighting, landscape lighting, liquid crystal display (LCD) backlighting, healthcare, etc. As an important part of LED lighting products, LED driver plays a vital role in maintenance costs reduction, energy saving and lifetime prolongation. In this paper, a single-stage LED driver is proposed, which is integrated by a totem-pole bridgeless power factor correction (PFC) unit and several modular LCL-T resonant rectifiers. The totem-pole bridgeless PFC unit and the LCL-T resonant rectifiers are integrated by sharing switches, which can simplify the circuit and reduce the system cost. The output current can be regulated by applying the variable inductor control. Constant duty cycle and fixed switching frequency operation can be achieved as well. The operating principle and the performance of the proposed converter are analyzed in this paper. In addition, a precise parameter design method is proposed by considering the high-order harmonics. At last, an experimental prototype is established to validate the proposed parameter design and variable inductor control method.

A Novel Dual-Mode Switched-Capacitor Five-Level Inverter With Common-Ground Transformerless Concept

Transformerless (TL) grid-connected photovoltaic (PV) inverters with a common-ground (CG) circuit architecture exhibit some excellent features in removing the leakage current concern and improving the overall efficiency. However, the ability to cope with a wide range of input voltage changes while maintaining the output voltage in a single power conversion stage is a key technological challenge. Considering this, the article at hand proposes a novel dual-mode switched-capacitor five-level (DMSC5L)-TL inverter with a CG feature connected to the grid. The proposed topology is comprised of a single dc source and power diode, three capacitors, four unidirectional, and three bidirectional power switches. Based on the series-parallel switching conversion of the involved switches, the proposed DMSC5L-TL inverter can generate five distinctive output voltage levels during both the boost and buck operation modes with a self-voltage balancing operation for the involved capacitors. A simple dead-beat continuous current controller (DB3C) modulation technique is also used to handle both the active and reactive power exchange while ensuring a fixed switching frequency operation. The proposed circuit description with its DB3C details, the design guidelines with a comparative study, and some experimental results are also given to show the feasibility of the proposed solution for the practical applications.

Implementation and Analysis of a Novel Switched-Boost Common-Ground Five-Level Inverter Modulated With Model Predictive Control Strategy

Common-ground (CG) string inverters with a transformerless (TL) circuit configuration have been broadly popular in grid-connected photovoltaic (PV) applications. The most important feature of a PV string TL inverter with a CG circuit connection is the elimination of leakage current concerns. Having the voltage boosting property within a single power processing stage can also be propitious to further facilitate the integration of low-scale PV string panels with higher efficiency. An efficient topology of such converters is presented in this study, which is able to produce a five-level stair-case output voltage waveform using an integrated switched-boost (SB) cell with only seven power switches. The proposed SB common grounded five-level (SBCG5L)-TL inverter can also be extended for the three-phase CG-based applications with the contribution of the same integrated SB cell. As for the single-phase configuration, it needs two dc-link capacitors and two power diodes along with two small inductors. A quasi-soft charging operation of the involved capacitors is also achieved. To control the injected current under the grid-connected condition, a single-step model predictive control (MPC) technique with a fixed switching frequency operation has also been presented. The proposed circuit description, the theoretical analysis of the applied MPC principles, and the comparative study with associated experimental results are also presented to ascertain the correctness and feasibility of the proposed SBCG5L-TL inverter.

Analysis and design of LLC resonant converter with variable magnetising inductance control

A conventional pulse frequency modulation (PFM) controlled LLC resonant converter requires a wide switching frequency operating range to regulate the output voltage. There are some disadvantages of PFM LLC resonant converters: (i) the design and optimisation of magnetic components and gate driver circuitry are challenging; (ii) due to the variable switching frequency operating range, electro-magnetic interference performance of the converter is degraded; (iii) efficiency degradation; (iv) inability to achieve independent control in multiple-output applications. To address the above-mentioned problems and achieve fixed switching frequency operation, a variable magnetising inductance control (VMIC) strategy is proposed. In the proposed VMIC control strategy, the operating switching frequency of the primary switch is fixed and the duty cycle remains constant at 0.5. At the resonant frequency operating point, the output voltage is independent of both the magnetising inductance and load. In addition, the zero current turn-off operation of the secondary rectifier requires that the switching frequency be designed below the resonant frequency. The operational principles and design considerations of the LLC resonant converter with VMIC are presented. Experimental results from a 200 W prototype are presented to validate the theoretical analysis and the effectiveness of the proposed VMIC control strategy.

A Simple Switching-Frequency-Regulated Sliding-Mode Controller for a VSI With a Full Digital Implementation

This article describes the design of a first-order sliding-mode controller for a voltage source inverter (VSI) operating at a fixed switching frequency. The proposal is simple, as it does not include any integral or resonant action in the switching function, and good results are obtained in terms of robustness, steady-state tracking error, fast transient response, and low total-harmonic distortion (THD). The implementation methodology using a microcontroller preserves all the benefits of a continuous-time implementation. Moreover, the article analyzes the sliding motion dynamics, and stability conditions are derived. Experimental results obtained with a 2.2-kW inverter show that fixed switching-frequency operation and robust output-voltage regulation with good performance indexes are achieved.

Two-Stage Optimization-Based Model Predictive Control of 5L-ANPC Converter-Fed PMSM Drives

Balancing the dc-link voltage and three flying-capacitor (FC) voltages while maintaining very fast control of the motor is always a challenging task in five-level active neutral-point-clamped (5L-ANPC) converter-fed permanent-magnet synchronous motor drives, especially when the FC is small. Besides, the fundamental-frequency operation of the low-frequency cell (LFC) is also required to reduce the losses of the converter. This article proposed a two-stage optimization-based model predictive control (MPC) scheme to achieve these goals. In the first stage, the switching states of the LFC are selected based on the sign of the desired output voltage through a deadbeat approach for its fundamental-frequency operation; in the second stage, the optimal duty cycles for the high-frequency cell are calculated by the multiple-vector MPC and generated by phase-shifted pulsewidth modulation for its fixed-switching frequency operation. The FC and dc-link capacitor voltage balance can be managed by the flexibility of the inherent redundancy in the 5L-ANPC converter. Instead of 512, only six switching vectors must be considered, which significantly simplifies the computational burden of MPC. A thorough experimental evaluation has been conducted to validate the effectiveness of the proposed MPC method.

DSP-based implementation of improved deadbeat control for three-phase shunt active power filters

This paper presents an experimental verification of an all-digital approach based on a deadbeat current control (DBC) algorithm for a three-phase shunt active power filter (shunt-APF), which provides fixed switching frequency operation and eliminates the harmonic and reactive currents drawn by nonlinear loads. The control scheme of the global system is fully digital without any external analog circuit or complex control logic. It mainly integrates the DBC for shunt-APF command current generation and a linear digital controller for DC-bus voltage regulation. The control of the shunt-APF is investigated using MATLAB simulation software and implemented on the dSPACE ds1104 control kit. This hardware calculates the duty cycle (∆T/T) and compares it to a carrier wave to give the control pulses of the inverter switches. The resulting simulation and experimental results verified the validity of the theoretical analysis of the proposed method.

A Current-Mode Digital AOT 4-Phase Buck Voltage Regulator

A synthesizable digital adaptive on-time (AOT) 4-phase buck voltage regulator (VR) for digital systems is presented. A current-mode digital AOT controller with the proposed ripple-injection schemes allow a fast and stable output voltage regulation, a fixed switching frequency operation, and phase synchronization for a multiphase VR. The digital AOT controller is implemented in the area of 0.0054 mm2 using digital standard cells in an automated digital-design flow. The proposed VR achieves an 80-mV droop with a recovery time of 80 ns and greater than 85% power-conversion efficiency over a 50 mA to 1.2-A load current range.

Pareto-based modulated model predictive control strategy for power converter applications

Modulated model predictive control (M2PC) allows fixed switching frequency operation of power converters, producing lower ripple and lower total harmonic distortion (THD) in the output signals than those obtained using regular model predictive control (MPC) algorithms. However, the design of the M2PC algorithm and its performance depend on, among other factors, the tuning of weighting factors, which are required by the cost functions typically used in predictive control algorithms. In this paper, a novel Pareto-based multi-objective M2PC (MO-M2PC) strategy is proposed. In this case, the use of weighting factors is not required, and the optimization problem is solved using a multi-objective approach. The aim of the proposed MO-M2PC strategy is to consider practical rules, such as satisfaction of soft constraints, to calculate the control actions. The proposed Pareto-based MO-M2PC strategy can be applied to any power converter topology. In this paper, experimental validation of the proposed methodology is pursued using a 3-phase 3 kW power converter operating as a shunt active power filter (SAPF). The proposed control is implemented in a control platform based on the dSPACE ds1103 system. The simulation and experimental results demonstrate the advantages and flexibility achieved by the proposed Pareto-based MO-M2PC.

Direct Predictive Current-Error Vector Control for a Direct Matrix Converter

This paper proposes a novel control strategy for matrix converters that is coined “direct predictive current-error vector control.” The proposed control method retains the advantageous features of both a modulation scheme and of a predictive-based controller. The result is a controller that is capable of good dynamic performance and steady-state response with fixed switching frequency operation. Control of load and input currents of a direct matrix converter using the proposed method is demonstrated in this paper by simulation and experimental results.

Novel Current Controller Based on MPC With Fixed Switching Frequency Operation for a Grid-Tied Inverter

In this paper, a novel controller based on the predictive technique has been introduced for a neutral-point-clamped grid-tied inverter. The aim of the developed method is to achieve a constant system switching frequency operation. The proposed method generates the voltage vector by nullifying the derivative of the cost function of a finite-set model predictive controller. The developed controller takes in action of injecting the desired grid current while a closed-loop space vector modulator regulates the capacitors voltages. A comparative study with a similar model predictive algorithm with constant switching frequency (MPC-CSF) is introduced and discussed in terms of computational complexity. Experimental results are provided to verify the performance and robustness of the new method over MPC-CSF with its ability of injecting a very low total harmonic distortion to the grid with almost unity power factor during normal and transient operations.

A Novel Dual Full-Bridge LLC Resonant Converter for CC and CV Charges of Batteries for Electric Vehicles

This paper introduces a novel concept for implementing constant current (CC) and constant voltage (CV) charges of batteries for electric vehicles using a dual full-bridge LLC (FBLLC) resonant converter, which shares its primary switches. In the proposed concept, the CC and CV charges are implemented using the inherent characteristics of an LLC converter as a current source and a voltage source. One FBLLC resonant converter operates with a fixed-resonant network whereas the other operates with a variable resonant network for the CC and CV charge operations. The proposed converter can achieve zero voltage switching (ZVS) and nearly zero current switching (ZCS) for all of the primary switches in the CC charge operation, and ZVS for all of the primary switches in the CV charge operation, which lead to a high efficiency. In addition, fixed switching frequency operation is possible in both the CC and CV charges thanks to the variable resonant network. A 3.3-kW prototype converter is implemented to verify the feasibility and validity of the proposed converter and a maximum efficiency of 98% was achieved.

Variable- and Fixed-Switching-Frequency-Based HCC Methods for Grid-Connected VSI With Active Damping and Zero Steady-State Error

This paper presents variable- and fixed-switching-frequency-based hysteresis current control (HCC) methods for single-phase grid-connected voltage-source inverters with LCL filter. The main feature of the proposed HCC methods is that the reference inverter current is generated through a proportional-resonant (PR) controller for achieving zero steady-state error in the grid current. The consequence of using the PR controller is eliminating the need for using derivative operations in generating the reference inverter current. Furthermore, the active damping method is employed to damp the LCL resonance. An equation is derived for variable-switching frequency. Fixed-switching frequency operation is achieved by modulating the hysteresis band. The performance of both HCC methods has been validated by simulation and experimentally. It is reported that the proposed HCC methods not only preserve the inherent features of the conventional HCC methods, but also damp the LCL resonance using an active damping method and guarantee zero steady-state error in the grid current.

Design of Single Phase Cyclo-Converter Using Cascaded Multilevel Inverter

This paper presents a single phase AC/DC/AC converter based on parallel AC/DC and cascaded multi-level DC/AC Converter. For a two-cell AC/DC/AC converter, two units of full-bridges are connected in parallel and two units of full-bridges are connected in cascade. It is proposed that instead of using two separate control loops of DC link voltages at the AC to DC conversion stage, only a single control loop with only one voltage sensor is proposed in this paper when two cells are equally loaded. It is also shown that by controlling only one cell DC link voltage, the other dc link voltage also gets controlled automatically. Use of simple and unipolar ramp comparison control at both the stages leads to fixed switching frequency operation of all the switches along with the improved quality of input current and output voltage as compared to simple PWM control. The simulation result for the proposed topology has been obtained through the MATLAB simulation studies, for a two-cell based parallel cum cascaded AC/DC/AC converter. The experimental verification of the proposed topology and control strategy has been obtained. Index Terms: AC/DC converter, DC/AC converter, AC/DC/AC Converter, Parallel Converter, Unipolar PWM.

Modeling and design of non-linear hybrid current controller suitable for high dynamic current loops

In this paper, the modeling and study of a new non-linear hybrid current controller is presented. It ensures high dynamic response of the current loop with a fixed switching frequency operation mode, a zero static error and high robustness properties as regard to system parameters variations. To model the proposed non-linear current controller, different tools are developed. In a first step a high frequency average model is proposed. It allows studying the average dynamic properties such as bandwidth, time response and overflow. To investigate the behavior of the current ripple due to the switching effect, a second model, based on the construction of a three-dimensional bifurcation diagram and the definition of a form function is established. This model allows studying the nature of the cycle described by the state trajectory and proving that the system operates with a fixed switching frequency. Design rules of the control parameters of this controller are explained and its robustness properties are tested by numerical simulations and experimental tests.

A ripple control buck regulator with fixed output frequency

This paper presents a ripple control buck regulator for stepping down high-voltage adaptor or battery supplies to low-voltage CPU core supply in notebook computers. Methods and circuits are proposed to achieve fixed switching frequency operation. The control loop adjusts delay of the controller and thus controls the output frequency. This is done by using a phase-frequency detector (PFD) to lock the frequency of switching signal with the input clock. The delay value is determined automatically according to operating conditions. Simulation results showed fast load and line transient responses and fixed frequency operations in steady states. In addition, stability of the regulator is shown under a very low output-to-input voltage ratio condition that is difficult for conventional PWM controllers.