Constant Switching Frequency Research Articles

Parabolic-Modulated Sliding-Mode Voltage Control of a Buck Converter

Conventional hysteresis-modulation-based sliding-mode (SM) controller exhibits a varying switching frequency. Existing SM controllers for achieving constant-switching-frequency operation often have complicated structures and/or are less straightforward to design to meet different steady state, transient, and loading requirements. In this paper, a new direct SM voltage controller based on parabolic modulation (PM) is proposed and applied to a buck converter. The objective is to achieve constant-switching-frequency operation of a switching converter with simple control structure and good operating performance. Both theoretical study and experimental work on a buck converter show that the proposed PM-based SM control can achieve 1) constant switching frequency in the continuous-conduction mode (CCM) and quasi-constant switching frequency in the discontinuous-conduction mode (DCM) of operation, 2) inherent input-disturbances rejection and superior reference tracking performance, and 3) seamless mode transition between CCM and DCM.

Review of recent advancements of direct torque control in induction motor drives – a decade of progress

Since the invention of direct torque control (DTC) in later 1980s for alternating current (AC) motor drives, it has undergone many modifications to improve the control strategy. This study provides a comprehensive review of recent advancements of DTC of induction motor (IM) for the past one decade. Strategies adopted to improve the performance of DTC based on switching table, constant switching frequency operation, intelligent control, sensorless control and predictive control are extensively discussed with its key results and algorithms.

Terminal sliding mode control-based MPPT for a photovoltaic system with uncertainties

Over the past few decades, the world demand for energy has risen steadily, forcing the world communities to look for alternative sources. Photovoltaic (PV) is seen as the most appropriate solution for this demand. In this context, this paper presents a robust terminal sliding mode control (RTSMC) method for maximum power tracking of stand-alone PV systems. The design method provides good robustness properties in the face of the system uncertainties and change of environment conditions. Starting from the mathematical dependence between the open-circuit voltage (Voc) and the optimal operating voltage (Vop), an MPPT design method is given. To eliminate the tracking voltage error, a RTSMC method is introduced thereafter. A pulse width modulator (PWM) is used to maintain the switching frequency constant. Compared to the P&O algorithm, the proposed methodology reduces the oscillations around the maximum power point (MPP) and provides better performance proprieties. Also, the simulations results prove the robustness qualities of the TSMC-MPPT design method.

Full-order observer for direct torque control of induction motor based on constant V/F control technique

This paper presents another control method for the three-phase induction motor that is direct torque control based on constant voltage per frequency control technique. This method uses the magnitude of stator flux and torque errors to generate the stator voltage and phase angle references for controlling the induction motor by using constant voltage per frequency control method. Instead of hysteresis comparators and optimum switching table, the PI controllers and space vector modulation technique are used to reduce torque and stator-flux ripples and achieve constant switching frequency. Moreover, the coordinate transformations are not required. To implement this control method, a full-order observer is used to estimate stator flux and overcome the problems from drift and saturation in using pure integrator. The feedback gains are designed by simple manner to improve the convergence of stator flux estimation, especially in low speed range. Furthermore, the necessary conditions to maintain the stability for feedback gain design are introduced. The simulation and experimental results show accurate and stable operation of the introduced estimator and good dynamic response of the proposed control method.

Modeling and reliability analysis of three phase z-source AC-AC converter

Abstract This paper presents the small signal modeling using the state space averaging technique and reliability analysis of a three-phase z-source ac-ac converter. By controlling the shoot-through duty ratio, it can operate in buck-boost mode and maintain desired output voltage during voltage sag and surge condition. It has faster dynamic response and higher efficiency as compared to the traditional voltage regulator. Small signal analysis derives different control transfer functions and this leads to design a suitable controller for a closed loop system during supply voltage variation. The closed loop system of the converter with a PID controller eliminates the transients in output voltage and provides steady state regulated output. The proposed model designed in the RT-LAB and executed in a field programming gate array (FPGA)-based real-time digital simulator at a fixedtime step of 10 μs and a constant switching frequency of 10 kHz. The simulator was developed using very high speed integrated circuit hardware description language (VHDL), making it versatile and moveable. Hardware-in-the-loop (HIL) simulation results are presented to justify the MATLAB simulation results during supply voltage variation of the three phase z-source ac-ac converter. The reliability analysis has been applied to the converter to find out the failure rate of its different components.

Analysis and Experiments on a Single-Inductor Half-Bridge LED Driver With Magnetic Control

This paper presents the analysis and experiments of a variable inductor (VI) based LED driver for dc grid lighting applications. The proposed driver requires only a series inductor and a transformer as major components to drive the LED lamp from a half-bridge inverter. By introducing a VI as the series inductor, the LED current can be controlled independently from any other parameter, which makes it possible to drive and regulate several LED branches from the same half-bridge output. Other advantages of the proposed converter include inherent open-circuit and short-circuit protections, zero-voltage switching for the bridge transistor and zero-current switching for the output rectifier diodes, simple dynamics, possibility of analog and pulse width modulation dimming, constant switching frequency operation, and high efficiency. The converter is thoroughly analyzed and modeled for both steady-state and dynamic operation. As another novelty of this paper, the dynamic response of the VI has been studied and taken into account to obtain the complete transfer function of the VI-controlled system. In addition, some housekeeping issues that usually arise when dealing with VI, e.g., how to drive the VI bias winding, are solved in this work. Experimental results provided from a 50 W laboratory prototype demonstrate the correctness of the performed analysis and the good possibilities of the proposed converter.

Constant Switching Frequency Predictive Current Control Technique with Fuzzy Logic Controller for Field Oriented Permanent Magnet Synchronous Motor Drive

Constant Switching Frequency Predictive Current Control Technique with Fuzzy Logic Controller for Field Oriented Permanent Magnet Synchronous Motor Drive

Model-Based Predictive Current Control Method with Constant Switching Frequency for Single-Phase Voltage Source Inverters

Voltage source inverters operated by predictive control methods generally lead to a variable switching frequency, because predictive control methods generate switching operation based on an optimal voltage state selected at every sampling period. Varying switching frequencies make it difficult to design output filters of voltage source inverters. This paper proposes a predictive control algorithm with a constant switching frequency for the load current control of single-phase voltage source inverters. This method selects two future optimal voltage states used in the subsequent sampling period, which are a zero-voltage state and a future optimal voltage state, based on the slope of the reference current at each sampling period. After selecting the two future voltages, the proposed method distributes them to produce a constant switching frequency and symmetric switching pattern. The performance of the proposed method is validated with both simulation and experimental results for single-phase voltage source inverters.

A power quality enhanced grid voltage sensorless predictive direct power control for active front end rectifiers

In this research work, a power quality enhanced grid voltage sensorless deadbeat predictive direct power control (DB-DPC) approach for AC/DC power converters based on latest virtual flux (VF) estimation, DB-DPC and space vector pulse width modulation (SVPWM) is proposed. At first, the grid voltage sensorless technique with switching table based DPC, which is a conventional approach, is discussed and its performance is analyzed under both increase of load as well as decrease of load conditions. Further, the proposed technique performance is also analyzed under the same circumstance and its performance is compared with the traditional method. The proposed technique has numerous advantages over the conventional method such as constant switching frequency, better active and reactive power control, good regulation of dc bus voltage, and enhanced power quality performance. As the proposed approach utilizes VF, DB-PC, and SVPWM techniques, this ensure line voltage sensorless approach, excellent control dynamics and constant switching frequency, respectively. At the end, experimental validation is done to confirm the supremacy of the proposed technique under load varying conditions and has a phenomenal performance under these circumstances.

Dead‐beat predictive direct power control of voltage source inverters with optimised switching patterns

This study presents a novel dead-beat predictive direct power control (PDPC) strategy working at nearly constant switching frequency for the voltage source inverter. The proposed strategy employs two active voltage vectors and one null voltage vector during every control period. To solve the problem of negative predicted duration time of active voltage vectors in the conventional dead-beat PDPC strategy, an automatic updating mechanism is designed. When the predicted duration time is negative, the applied voltage vectors will be updated automatically without the need of reselecting voltage vectors or recalculation of the duration time. Accurate tracking of both active and reactive power can be achieved with small power ripples using the strategy proposed in this study. The proposed PDPC strategy shows excellent steady performance and fast dynamic responses. High sampling frequency can also be avoided and the calculation process is simple enough to be implemented on normal DSPs. Simulation and Experimental results are presented to validate the feasibility of the proposed dead-beat PDPC strategy.

Generalized DTC Strategy for Multilevel Inverter Fed IPMSMs With Constant Inverter Switching Frequency and Reduced Torque Ripples

This paper proposes a generalized direct torque control (DTC) strategy for multilevel inverter fed DTC (MLI-DTC) drives. By using a simple voltage vector decomposition technique, the proposed method is compatible with inverters possessing any number of voltage levels. Consequently, voltage vectors are applied in a similar way as in the case of a two-level inverter fed DTC. A constant inverter switching frequency is imposed in the proposed method by using a torque regulator in place of the traditional torque hysteresis controller, which enables the application of two voltage vectors within one switching cycle. This also contributes to reduce the torque and flux ripples. Detailed design guidelines for the proposed torque regulator are provided. Experimental results obtained with a three-level neutral point clamped inverter show that the proposed MLI-DTC drive operates with low, constant switching frequency while alleviating torque and flux ripples under all operating conditions. Comparative study with a prior art and parametric sensitivity analysis are also presented to verify the effectiveness and robustness of the proposed MLI-DTC method.

Switching Frequency Dynamic Control for DFIG Wind Turbine Performance Improvement Around Synchronous Speed

In a doubly-fed induction generator (DFIG) wind turbine (WT), large thermal and mechanical oscillations occur around synchronous speed due to the nearly zero-frequency current circulating between rotor windings and rotor-side converter (RSC). A switching frequency dynamic control technique is proposed in this paper to overcome the problem. The basic idea is to dynamically reduce the switching frequency during operation so that both the switching losses and dead-time effect of RSC can be reduced around synchronous speed. According to the characteristic that high-frequency pulse width modulation (PWM) harmonics in DFIG decrease significantly with rotor slip, there would be a new space for switching frequency reduction especially around synchronous speed. The reduced of switching frequency is actually determined by a tradeoff between the increase of both the stator current PWM harmonics and speed PWM ripples and the decrease of insulated-gate bipolar transistor temperature. Compared with the conventional technique with constant switching frequency, the new technique can effectively improve the system performance around synchronous speed including not only better thermal behavior and efficiency of RSC but also smaller current total harmonics and speed total ripples in generator. To clarify the method, first, the effects of switching frequency reduction on a DFIG WT system are investigated. Second, the criteria, control scheme, and procedure of the method are presented. Finally, experimental and simulation studies were carried out, and the results validate its feasibility and effectiveness.

Constant Switching Frequency and Torque Ripple Minimization of DTC of Induction Motor Drives with Three-level NPC Inverter

<p>Direct Torque Control (DTC) is widely applied for ac motor drives as it offers high performance torque control with a simple control strategy. However, conventional DTC poses some disadvantages especially in term of variable switching frequency and large torque ripple due to the utilization of torque hysteresis controller. Other than that, performance of conventional DTC fed by two-level inverter is also restricted by the limited numbers of voltage vectors which lead to inappropriate selection of voltage vectors for different speed operations. This research aims to propose a Constant Switching Frequency (CSF) torque controller for DTC of induction motor (IM) fed by three-level Neutral-Point Clamped (NPC) inverter. The proposed torque controller utilizes PI controller which apply different gain for different speed operation. Besides, the utilization of NPC inverter provides greater number of voltage vectors which allow appropriate selection of voltage vectors for different operating condition. Using the proposed method, the improvement of DTC drives in term of producing a constant switching operation and minimizing torque ripple are achieved and validated via experimental results.</p>

PWM–PFM hybrid controlled LCC resonant converter with wide ZVS range and narrow switching frequency variation

Variable frequency controlled LCC resonant converter suffers from wide variation of switching frequency. Although fixed frequency controlled LCC resonant converter benefits from constant switching frequency, it suffers from narrow zero voltage switching (ZVS) range of power switch and high circulating energy. A pulse width modulation – pulse frequency modulation (PWM-PFM) hybrid controlled LCC resonant converter is proposed to achieve wide ZVS range of power switch with narrow variation of switching frequency. Experiment results of a 500 W prototype are provided to verify theoretical analysis of the proposed converter.

A Review on Constant Switching Frequency Techniques for Direct Torque Control of Induction Motor

The Direct Torque Control (DTC) of induction machine has received wide acceptance in many Variable Speed Drive (VSD) applications due to its simple control structure and excellent torque dynamic control performances. However, the conventional DTC which employs a two-level inverter and hysteresis controllers produces two major drawbacks, namely, larger torque ripple and variable switching frequency, which might produce a very high switching frequency (or power loss), particularly at a very low speed operation. This paper highlight the common methods used to provide the constant switching frequency for DTC drives for Induction Motor that able to minimize the power losses and reduced the torque ripple.

A Novel Control Scheme of Three-Phase Single-Switch Quasi-CRM Boost Rectifier

A three phase single switch boost rectifier, operating in a quasi-critical conduction mode of the inductor current, can achieve the power factor correction and meet the IEC 61000-3-2. However, the switching frequency is variable in a line cycle, and the design of the inductor and electromagnetic interference filter as well as the power component selection is deteriorated. In this paper, a novel constant frequency control method is proposed for the converter by modulating the on-time of the switch. Compared with the traditional variable frequency control, the proposed control scheme achieves a constant switching frequency in a line cycle, which simplifies the design of the power devices, especially the magnetic components. Furthermore, the switching frequency and the rms value of the inductor current are also reduced, which contributes to a higher efficiency.

Second‐order sliding‐mode‐based global control scheme for wind turbine‐driven DFIGs subject to unbalanced and distorted grid voltage

Control algorithms for both the rotor- and grid-side power converters of a wind turbine-driven doubly-fed induction generator (DFIG) are detailed, and tuning equations are also provided to assist adjustment of their gains and constants. Those algorithms are based on the second-order sliding-mode control (2-SMC) approach, and they allow the wind turbine to satisfactorily operate under grid voltage non-idealities, such as simultaneously distorted and unbalanced scenarios. The presented solution turns out to be robust against parameter deviations and disturbances, and of high-performance dynamic response. Moreover, it drives the transistors of both power converters at a constant switching frequency, also avoiding decomposition in symmetrical sequences of either the grid voltage or currents. The controllers proposed for the two power converters are validated through experimentation on a 7-kW DFIG test bench subject to a significantly unbalanced and harmonically distorted grid voltage. Their robustness in the presence of both substantial parameter mismatch and disturbances attributable to wind variability is also assessed.

Sensorless Direct Flux Vector Control of Synchronous Reluctance Motors Including Standstill, MTPA, and Flux Weakening

This paper proposes a sensorless direct flux vector control scheme for synchronous reluctance motor drives. Torque is controlled at constant switching frequency, via the closed loop regulation of the stator flux linkage vector and of the current component in quadrature with it, using the stator flux oriented reference frame. A hybrid flux and position observer combines back-electromotive force (EMF) integration with pulsating voltage injection around zero speed. Around zero speed, the position observer takes advantage of injected pulsating voltage. Instead of the commonly used current demodulation, the position error feedback is extracted here at the output of the observer's flux maps, thus resulting in immunity toward the cross-saturation position error. The maximum torque per ampere (MTPA) strategy is used. A detailed analysis puts in evidence the key advantages and disadvantages related to the use of the MTPA in the sensorless control of the synchronous reluctance machine, for both the saliency-based and the back-EMF based sensorless methods. Extensive experimental results are reported for a 2.2 kW synchronous reluctance motor prototype, showing the feasibility of the proposed method. These include speed response to step and sinusoidal load disturbances at standstill, up to 121% of rated torque, and speed response tests covering the flux weakening speed region.

On average real sliding dynamics in linear systems

It is well known that in implementations of sliding mode controllers using hysteresis comparators, when the hysteresis band amplitude tends to zero the real dynamics tends to the ideal sliding dynamics. However, in real systems physical limitations do not allow to effectively lower this value at will, and a steady state error is likely to appear. In this paper we relate this error with a non zero average value of the switching function in each switching period: it is shown that, in linear systems, when the controller has a constant switching frequency and the switching function is periodic, the average value of the difference between real and ideal steady state dynamics is proportional to the average value of the switching function. Hence, when this average value is non zero an average steady state error appears, while a zero average value for the switching function entails no average steady state error. The proof is carried out using a regular form approach, and the result is exemplified in a buck converter. Simulation results show that when the switching function is periodic and shows a piecewise linear behavior within the hysteresis band, thus guaranteeing zero average value, the average state error disappears. In turn, when this piecewise linear character is lost and the switching function has non zero mean value, an average steady state error arises.

Multiple-load series resonant inverter for induction cooking application with pulse density modulation

Multiple-load induction cooking applications are suitable used when multi-output inverters or multi-inverters are needed for multiple-load operation. Some common approaches and modifications are needed in inverter configuration for multiple-load application. This paper presents an inverter configuration with two loads by using pulse density modulation control technique. It allows the output power control of each load independently with constant switching frequency and constant duty ratio. The pulse density modulation control technique is obtained using phase on–off control between two legs of the inverter to reduce acoustic noise. The two-load three-leg inverter configuration provides reduction of the component count for extension of multiple loads. The control technique provides a wide range of output power control. In addition, it can achieve efficient and stable zero voltage switching operation in the whole load range. The proposed control scheme is simulated and experimentally verified with two-load inverter configuration.