Digital Control Strategy for an LLC converter
Tutor / Supervisor
Student
Cos Martí, Oriol
Document type
Master thesis
Date
2020
rights
Open Access
Publisher
Universitat Politècnica de Catalunya
UPCommons
Abstract
Since the beginning of power electronics studies, the key objective has been to reduce the size of the transformation equipment. The invention of controlled semiconductor devices set up a step forward to this objective, by increasing the switching frequency and hence reducing the volume of reactive elements. However, this new operation strategy came also with an increment of the converter switching losses and setting a constrain due heat dissipation. A new thread in the pursuit of size reduction is set on resonant or quasi-resonant converters which use reactive networks to obtain soft switching topologies. This commutation strategy achieves a drastic reduction of the switching losses (on the switching devices) so the heat constrain is only attached to the conduction losses. In this thesis, a deep analysis of one particular resonant converter has been performed, the LLC topology. LLC structure's main particularity relies on handling the power flow, by using frequency modulation instead of Pulse Width Modulation (PWM) like traditional converters. This intrinsic characteristic eases the control strategy but hinders the model linearization since State Space Averaging Method (SSAM) can no longer be applied. There are two main reasons: The first one is that the method relays on averaging the state variables over a switching period where this period is time invariant. In this particular case the switching period is time dependent, it is our control tool. The second reason is that due converter operation, part of the state variables are sinusoidal zero-centered waveforms. In order to avoid this inconvenience, we will relay on a cycle-by-cycle analysis to define a linear model of the converter state equations so that linear control can be applied. Finally, the designed controllers are discretized with different methods and tested in a 3.3 kW converter through an M4 ARM platform developed by Monolithic Power Systems (MPS).
