The dissertation is divided into two parts. The first part of the thesis focuses on the
implementation and verification of a current driver for a MOSFET. The starting point
is a working and tested system, developed by Rok Vrtovec during his PhD thesis.
The circuit is upgraded from a low side driver to a single-phase (half-bridge) driver,
which is galvanically isolated from the control circuit. In this way, two low side drivers
are combined to control one phase of a three-phase inverter. The implementation of
the new driver is tested by measuring a three-phase motor control test system. The
ability of the driver to control multiple parallelized transistors is tested and its effect
on the shape of the current to each gate of the MOSFET is investigated. The driver is
eventually used to control a MOSFET driving a loaded motor.
The second part of the paper deals with the modelling of the power stage. To test
the behaviour of the power modules under load, an electrical and thermal mathematical
model of the power stage is developed using simulations. The passive and active
electrical circuit elements used are modelled in the SPICE language. An electrical
model of the circuit geometry with the different materials used, originally modelled
using Ansys Q3D Extractor software, is also added in the form of a customised SPICE
model. The thermal properties of the circuit are simulated under test loads. Realistic
thermal conductivity parameters of the used materials and the most accurate possible
layout of the heat sources in the circuit are used. The Ansys Icepak simulator is used
for thermal simulations.
To check for any anomalies in the electrical simulations, the model was first tested
in a half-bridge configuration with pulse tests. After the problems were solved and the
model is updated, it was tested in a full three-phase simulation. A passive model of the
thermal resistance of the module to the heat sink is added to the electrical model of
the transistors. The used load model is simplified to three inductors (series connection
of resistor and inductor) connected in a star configuration.
The power modules are also tested electrically and thermally to verify the correctness
of the simulations. As with the electrical simulations, the modules are first tested
with pulse tests and later in a three-phase configuration. Unlike the simulations, the modules are used to drive the actual motor during testing. Thermal measurements are
also performed but in the linear mode of operation of the transistors. The module is
thermally coupled to an actively cooled heat sink with thermal paste. The transistors
are loaded with a total power of 20 W, 50 W and 100 W. The temperatures on the
module and the heat sink are measured.
Electrical measurements are performed with the modules made of Al2O3 (alumina;
aluminium oxide), while thermal measurements of both Al2O3 and AlN (aluminium
nitride) are performed.
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