A. Van den Bossche

Digitally controlled boost power-factor-correction converters operating in both continuous and discontinuous conduction mode

Whereas power-factor-correction (PFC) converters for low-power ranges (less than 250 W) are commonly designed for operation in the discontinuous conduction mode, converters for higher power levels are operated in the continuous conduction mode. Nevertheless, when these converters are operated at reduced power, discontinuous conduction mode will appear during parts of the line period, yielding input current distortion. This distortion can be eliminated by employing a dedicated control algorithm, consisting of sample correction and duty-ratio feedforward. The reduction of the harmonic distortion of the input current and the increase of the power factor are demonstrated by experiments on a 1-kW boost PFC converter.

Input-Current Distortion of CCM Boost PFC Converters Operated in DCM

When power-factor correction (PFC) converters designed for operation in continuous-conduction mode (CCM) at full power are operated at reduced load, operation in discontinuous-conduction mode (DCM) occurs in a zone that is close to the crossover of the line voltage. This zone will gradually expand with decreasing load to finally encompass the entire line cycle. Whereas, in CCM, the parasitic capacitances of the switches only cause switching losses, in DCM, they are a source of converter instability, resulting in significant input-current distortion. In this paper, this source of input-current distortion is analyzed, and a solution is proposed. Experimental results are obtained using a digitally controlled boost PFC converter, which is designed to operate in CCM for 1 kW

Small-signal Laplace-domain analysis of uniformly-sampled pulse-width modulators

As the performance of digital signal processors has increased rapidly during the last decade, there is a growing interest to replace the analog controllers in low power switching converters by more complicated and flexible digital control algorithms. Compared to high power converters, the control loop bandwidths for converters in the lower power range are generally much higher. Because of this, the dynamic properties of the uniformly-sampled pulse-width modulators used in low power applications become an important restriction for the maximum achievable bandwidth of control loops. After the discussion of the most commonly used uniformly-sampled pulse-width modulators, small-signal frequency- and Laplace-domain models for the different types of uniformly-sampled pulse-width modulators are derived theoretically. The results obtained are verified by means of experimental data retrieved from a test setup.

Measurement and loss model of ferrites with non-sinusoidal waveforms

The usual data of commercial ferrite grades are given for sinusoidal waveforms, although the voltage in the typical applications in power electronics resembles to square waves. Firstly, an accurate two wire, oscilloscope power measurement is presented. Special wide frequency current and voltage transducers were designed, with a very low phase difference up to 50 MHz. Secondly, a ferrite loss model named natural Steinmetz extension (NSE) is presented. The model is checked with measurements on two different ferrite grades, with square waves with a large variation in duty ratio. The proposed model is compared with modified Steinmetz equation (MSE). The two methods with a different mathematical formulation give comparable but different results.

Segmentation of Magnets to Reduce Losses in Permanent-Magnet Synchronous Machines

This paper studies the losses in the magnets of a permanent-magnet synchronous machine (PMSM) with surface magnets caused by square voltage pulse width modulation (PWM) waveforms. First, the conductivity of the magnets is determined. Second, the effect of segmentation on the losses is calculated by a 3-D time-harmonic finite element model for sinusoidal waveforms. More segmentation of the magnets in axial and circumferential direction results in much lower losses in the magnets, but more losses in the massive rotor yoke. In a third step, the losses are simulated and measured on an experimental PMSM for square waveforms generated by PWM. Superposition of appropriate sinusoidal losses obtained by a time-harmonic FEM seems to be acceptable in order to predict losses in a PMSM for square voltage waveforms.

Input current distortion of CCM boost PFC converters operated in DCM

Power factor correction (PFC) converters for the higher power range are commonly designed for continuous conduction mode (CCM). Nevertheless, operation in the discontinuous conduction mode (DCM) occurs for light load in a zone, close to the crossover of the line voltage. This zone will gradually expand with decreasing load to finally encompass the entire line cycle. Whereas in CCM the parasitic capacitances of the switches only cause switching losses, in DCM they are a source of converter instability, resulting in significant input current distortion. In this paper, this source of input current distortion is analyzed and a solution is proposed. Experimental results are obtained using a digitally controlled boost PFC converter, designed to operate in CCM for 1 kW.

Sample correction for digitally controlled boost PFC converters operating in both CCM and DCM

PFC converters for the higher power range are commonly designed for continuous conduction mode. Nevertheless, at light load, DCM will appear close to the crossover of the line voltage, causing the converter to switch between CCM and DCM. As a result of this switching during a line period, the converter dynamics change abruptly, yielding input current distortion. Moreover, if digital control is applied, another source of input current distortion is posed by the sampling algorithm. After all, the sampling algorithm is only designed to produce samples of the averaged input current in CCM. In this paper, after a study of the input current distortion caused by the sampling algorithm, a correction factor is derived to compensate for the error on the input current samples. The theoretical results are verified experimentally by using a digitally controlled boost PFC converter.

Improved approximation for fringing permeances in gapped inductors

The purpose of this paper is to propose new analytical approximations for fringing flux calculations around the air gaps of inductor cores, including multiple gap cases and different symmetrical cases. Existing 3D techniques using finite element analysis are accurate but require a prohibitive amount of simulation time and special software. We propose an improved analytical approximation for fringing permeance calculation for the most usual field patterns. The approach is extended form 2D to 3D giving analytical solutions for corner effects, thus providing a better accuracy of the approximation. The derived fringing coefficients are used to present all symmetrical cases and cases with multiple air gaps. The accuracy of the proposed equations is sufficient for a normal engineering design. The advantages of analytical approximations are the possibility of generating diagrams, of solving the reverse problems and optimizing more complex problems.

Gate-drive circuit for zero-voltage-switching half-and full-bridge converters

Resonant techniques like zero-voltage switch (ZVS) and zero-current switch (ZCS) are widely applied to raise the switching frequency and efficiency of converters. An important subclass of converters using resonant techniques, is formed by ZVS half- and full-bridge converters. While different varieties of this subclass of converters have been reported, the gate drive circuit is seldom described. In this paper a new gate drive circuit applicable in ZVS half- and full-bridge converters is proposed. The circuit shows excellent features such as transformer isolation, no ringing of the gate-source voltage of the switches, programmable dead-time, no active components beside one low-side driver, insensitivity to component value variation, negative gate-source voltage at switch-off and low drive power. Moreover the circuit can be used in variable frequency applications. The theoretical results obtained, are verified by experiments.

Exact voltage unbalance assessment without phase measurements

In this letter, it is shown that the positive- and negative-sequence components of a three-phase, sinusoidal and unbalanced voltage system can be calculated exactly without the application of the Fortescue transformations in the complex plane, while only the root mean square (RMS) line-to-line voltages are required, and without having to measure the phase relationships between these voltages. This results in a convenient procedure to assess voltage unbalance in the field. The phase relationships between the sequence and line components can also be calculated. Moreover, if the RMS values of the line-to-neutral voltages are known, the zero-sequence component can be calculated exactly as well, again, without the application of complex mathematics.