Steven Thielemans

Improved Natural Balancing With Modified Phase-Shifted PWM for Single-Leg Five-Level Flying-Capacitor Converters

Flying-capacitor converters (FCCs), like most multilevel converter topologies, require a balancing mechanism of the capacitor voltages. FCCs feature natural voltage balancing when a special modulation technique is used. The classic methods, such as phase-shifted pulse width modulation (PS-PWM), result in very slow balancing for some duty-ratio ranges. Previous work has shown that for a single-leg five-level FCC, one time constant is infinite for a zero desired output voltage. In this paper, a modified PS-PWM scheme for a single-leg five-level FCC is presented, which results in faster balancing over the total duty-ratio range. The modified PS-PWM scheme is studied, resulting in an averaged voltage-balancing model. This model is verified using simulations and experiments. The modified PS-PWM scheme solves the slow-balancing problems of the normal PS-PWM method for odd-level FCCs, while maintaining the passive control property, and it provides a self-precharge capability.

Finite-Set Model-Based Predictive Control for Flying-Capacitor Converters: Cost Function Design and Efficient FPGA Implementation

Recently, there has been an increase in the use of finite-set model-based predictive control (FS-MBPC) for power-electronic converters. However, the computational burden for this control scheme is very high and often restrictive for a good implementation. This means that a suitable technology and design approach should be used. In this paper, the implementation of FS-MBPC for flying-capacitor converters in field-programmable gate arrays (FPGAs) is discussed. The control is fully implemented in programmable digital logic by using a high-level design tool. This allows us to obtain very good performances (both in control quality, speed, and hardware utilization) and have a flexible, modular control configuration. The good performance is obtained by exploiting the FPGAs strong points: parallelism and pipelining. Furthermore, an improved cost function for the voltage control of the flying capacitors is proposed in this paper. Typical cost functions result in tracking control for the flying-capacitor voltages, although this does not correspond with the desired system behavior. The improved cost function offers a capacitor voltage control that corresponds more closely with the desired behavior and adds a limitation on the capacitor voltage deviation. Furthermore, the selection of the weight factor in the cost function becomes less critical.

Analysis of some design choices in model based predictive control of flying‐capacitor inverters

Purpose – Flying‐capacitor multilevel converters (FCC) need a passive or active regulation of the capacitor voltages. Recently the trend is towards active control, often implemented separately from the current control. The advantages of a true multi‐variable control sparked the interest to apply Model Based Predictive Control (MBPC) for FCC. In this paper an objective analysis method to evaluate the effects of several design choices is presented. The effects of the weight factor selection, model simplification, and prediction horizon expansion for MBPC of a 3‐level FCC are analyzed in a systematical way.Design/methodology/approach – The analysis is mainly based on the mean square error (MSE) of current and capacitor voltage. The results are analysed for different lengths of the prediction horizon and for a wide range of weight factor values. Similarly the effect of a model simplification, neglecting the neutral point voltage, is studied when implementing MBPC for FCCs while considering the computational a...

Design choices for the prediction and optimization stage of finite-set model based predictive control

The interest in applying model-based predictive control (MBPC) for power-electronic converters has grown tremendously in the past years. This is due to the fact that MBPC allows fast and accurate control of multiple controlled variables for hybrid systems such as a power electronic converter and its load. As MBPC is a family of possible controllers rather than one single controller, several design choices are to be made when implementing MBPC. In this paper several conceptual possibilities are considered and compared for two important parts of online Finite-Set MBPC (FS-MBPC) algorithm: the cost function in the optimizations step and the prediction model in the prediction step. These possibilities are studied for two different applications of FS-MBPC for power electronics. The cost function is studied in the application of output current and capacitor voltage control of a 3-level flying-capacitor inverter. The aspect of the prediction model is studied for the stator flux and torque control of an induction machine with a 2-level inverter. The two different applications illustrate the versatility of FS-MBPC. In the study concerning the cost function firstly the comparison is made between quadratic and absolute value terms in the cost function. Comparable results are obtained, but a lower resource usage is obtained for the absolute value cost function. Secondly a capacitor voltage tracking control is compared to a control where the capacitor voltage may deviate without cost from the reference up to a certain voltage. The relaxed cost function results in better performance. For the prediction model both a classical, parametric machine model and a back propagation artificial neural network are applied. Both are shown to be capable of a good control quality, the neural network version is much more versatile but has a higher computational burden. However, the number of neurons in the hidden layer should be suffciently high. All studied aspects were verified with experimental results and these validate the simulation results. Even more important is the fact that these experiments prove the feasibility of implementing online finite-set MBPC in an FPGA for both applications.

Flying-capacitor multilevel converter voltage balance dynamics for pure resistive load

Multilevel converters need voltage balancing to be able to generate an output voltage with high quality. Flying capacitor converter topology has a natural voltage balancing property. Voltage balance dynamics analytical research methods reported to date are essentially based on a frequency domain analysis using double fourier transform. These complicated methods are not truly analytical, which makes an understanding of parameter influence on time constants difficult. In this paper, a straightforward time domain approach based on stitching of switch intervals piece-wise analytical solutions to a DC modulated H-bridge flying capacitor converter is discussed. This method allows to obtain time-averaged discrete and continuous voltage balance dynamics models. Using small-parameter approximation for pure resistive loads, simple and accurate expressions for voltage balance time constants are deduced, revealing their dependence on load parameters, carrier frequency and duty ratio.

Five-level H-bridge flying capacitor converter voltage balance dynamics analysis

Flying Capacitor (FC) multilevel Pulse Width Modulated (PWM) converters are an attractive choice due to the natural balancing property of the capacitor voltages. A single-leg flying capacitor converter voltage balance dynamics analytical solution may be obtained using switched systems time domain approach based on stitching of switching intervals piece-wise analytical solutions in combination, for inductance dominated load, with a small parameter technique. In this paper, a symmetric five-level H-bridge flying capacitor converter common mode voltage balance dynamics solution is obtained from its single-leg prototype using “mirror hypothesis” formalism. Simple analytical expressions clearly reveal the dependences on load parameters, carrier frequency, and DC PWM normalized voltage command. For AC modulation, the solution obtained by averaging on a fundamental AC period does not depend on a fundamental frequency. The results of the theoretical analysis are confirmed by extensive switched simulations.

Self-precharge in single-leg flying capacitor converters

Flying Capacitor (FC) multilevel pulse width modulated (PWM) converters are an attractive choice due to the natural voltage balance property. During start-up of the converter, care has to be taken that the power switches are not exposed to voltage overstress due to uncharged capacitors. A flying capacitor self-precharge technique is proposed which, by making use of natural balancing and a DC-bus rate control, makes the capacitors balance with a zero average load current. The DC-bus rate control depends on the capacitor voltage balance dynamics. The regular PWM natural balancing technique gives good results for even-level single-leg converter self-precharge, for odd-level converters a special switching pattern is necessary.

Simple time domain analysis of a 4-level H-bridge flying capacitor converter voltage balancing

Flying Capacitor (FC) multilevel Pulse Width Modulated (PWM) converters are an attractive choice due to the natural balancing property of the capacitor voltages. The balancing can be studied in the time domain, which results in easy to interpret expressions regarding parameter influence. Time domain analysis is normally done by time averaging of the system model over a PWM-period. In this paper a new method is presented starting from the “voltage unbalance” excessive energy which has to be dissipated in the converter load by switching harmonics in the load current. This is applied on a four-level H-bridge converter, which has dynamics of a high order and is normally hard to describe in the dime domain. The proposed method results in time domain parameters exactly describing the behaviour and the balancing of the capacitor voltages of the four-level H-bridge converter.

Balancing and harmonic analysis of flying capacitor multilevel converters

To supply medium-voltage AC machines, DC-AC converters working with a high bus voltage are required. By using multilevel converters, these high voltages can be supplied without the need for switches with a high voltage rating. Moreover, the output voltage waveform can be improved considerably. The flying capacitor multilevel converter is a recently developed converter topology assuring a flexible control and modular design. However, the flying capacitor multilevel converter requires a balanced DC voltage distribution. This can be realized by using a special control leading to natural balancing or by measuring the voltages and selecting the appropriate switching state. The balancing is influenced by three factors, namely the harmonic content of the reference waveform, the switching frequency and the load impedance. In addition to the voltage balancing of the flying capacitor multilevel converter, the output voltage must ensure the control of the load, e.g. a three phase AC machine.

Voltage quality analysis of a three-level flying capacitor inverter with model based predictive control

Model based predictive control is an interesting control method for power electronics as it offers many advantages. A known disadvantage compared to optimal pulse-width modulation control is the deteriorated voltage quality, which results in extra losses in the load. In this paper two ways of analyzing the output voltage quality are presented and applied on the model based predictive control of a three-level flying capacitor inverter. The influence of the weight factor and the system model are analyzed. The voltage quality is shown to be good in the same weight factor range as those resulting in good current and capacitor voltage control. No compromises have to be made in this area. A more accurate system model results in improved voltage (and overall control) quality.