Bernardo Cougo

Parallel Three-Phase Inverters: Optimal PWM Method for Flux Reduction in Intercell Transformers

Parallel multilevel converters are now widely used in the industry, particularly in high-current applications such as voltage regulator modules. The reduction of the output current ripple and the increase of its frequency are possible due to the use of interleaving techniques and, as a consequence, the filters associated with the converter may be reduced. The current ripple reduction in each commutation cell of a parallel converter is possible by the use of intercell transformers (ICT). The design of such a special magnetic component depends very strongly on the magnetic flux flowing through their cores. In three-phase systems coupled by ICTs, the injection of zero-sequence signals in the output voltage reference changes this flux. The aim of this paper is to explain the influence of the most popular pulse width modulation (PWM) methods regarding the ICT flux for applications to three-phase loads. An optimal PWM method that minimizes the size of the ICT design is developed. Experimental results verify the analysis presented in this paper and validate the flux reduction provided by the developed optimal zero-sequence signals.

A non-intrusive method for measuring switching losses of GaN power transistors

In modern power electronics, the design of power converters relies on accurate modeling of losses of all components. Losses in the switches are usually the most important point to be considered in a converter design; however, switching losses are not always provided in datasheets. Typical methods for measuring switching losses, such as the double-pulse method, need the insertion of current and voltage probes across the device. These probes add parasitic elements into the power loop, which modifies the switching behavior, especially when modern semiconductor devices, such as Gallium Nitride (GaN) transistors, are considered. Thus, with the purpose of precisely measuring switching losses, an opposition method test bench has been built to characterize, under real working conditions, one GaN power transistor. The measurement method employed can accurately estimate turn-on and turn-off losses, separately. It does not add parasitic elements to the circuit, which ensures operation at the same conditions of the real converter. Results obtained from this method show the switching characteristics of a specific GaN device and measurements include the power dissipation in a reverse conduction mode. These results help to show the best operation points of the device to maximize the efficiency of a given application.

Accurate switching energy estimation of wide bandgap devices used in converters for aircraft applications

This paper describes a method which precisely estimates switching losses of fast power semiconductors. In this method, no transistor current is measured and, consequently the switching behavior of the transistor is not changed by the measurement system. By two different measurement modes, turn-off and turn-on losses can be estimated separately, which is necessary to precisely design isolated DC/DC converters used in aircrafts. The proposed switching loss measurement method is explained and experimental results on Silicon Carbide MOSFETs verify its operation and accuracy.

Design of differential mode filters for two-level and multicell converters

Faster switching devices like WideBandGap semiconductors allow increasing the switching frequency that can be used for a given efficiency, and they are sometimes (often?) presented as the solution to increase power density. However, using these components in standard structures can bring far-from-optimal results since they require bigger filters resulting in poor power density. By contrast, we will show that using these components in multicell converters allows increasing the cutoff frequency of the filters.

Characterization of high power SiC modules for more Electrical Aircrafts

One of the main challenges associated to the More Electric Aircraft is to significantly increase power density of electrical power systems, such as electromechanical chains applied to actuation systems, without compromising on reliability. Nowadays, the best way of considerably increasing power density of power converters is by the use of disruptive technology such as Wide Bandgap (WBG) semiconductors.

Optimal cross section shape of tape wound cores

Tape wound cores are used in different applications where high power density is desired due to their high performance at high frequencies. In high power applications, inductor and transformer cores have large cross sections and consequently wide core legs which may result in different flux densities and temperatures in each layer as experimentally shown. This paper verifies the influence of the core cross section shape in the losses and temperature distribution inside the core. Optimal cross section shapes are developed in order to minimize different criteria such as core losses, heat extraction capabilities and total core weight.