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Title

DESIGN AND IMPLEMENTATION OF A DIGITALLY CONTROLLED DC/DC RESONANT CONVERTER

Creator

Bender, Markus

Date

2014, 2014-05

Description

Battery operated electronics with a controller are in need of very precise low voltage direct current [DC] supplies. Often times the supplies...
Show moreBattery operated electronics with a controller are in need of very precise low voltage direct current [DC] supplies. Often times the supplies are realized by a twofold supply. The first stage is called a pre-regulator and has two main tasks. The first one is to produce a stable pre-supply voltage and secondly they are asked to be very efficient as they usually see the biggest voltage difference to the input voltage. The industry is currently regulated to develop more efficient power supplies, which drives them to using resonant converters. The idea of the thesis is to develop a DC to DC resonance pre-converter. To determine the most suitable resonant converter several two-element resonant circuits are analyzed. The analysis includes equations and graphs of input impedance, output impedance, and the circuit’s gain. The design with the best results will then be used for realization. The realization starts with basic simulation profile setups in OrCAD Capture©. At this stage the basic performance of each sub-circuit is realized, before adjusted for the required performance. For controlling the output voltage, a logic level MOSFET driver was implemented to be driven by a microcontroller. The microcontroller allows flexibility when it comes to specific timing requirements on the input pulse generation. For this reason a differential amplifier had to be implemented which gives the controller the feedback from the rectified output voltage. To validate the proposed design it was realized on a manufactured PCB. Since the resonant tank components are crucial for the performance they were realized by hand wound inductor with litz wire, and ceramic capacitors. The controller was realized by a xv microcontroller evaluation board. The control software is based on an example of the processor manufacturer. The measurements are showing that the inverter switches are showing significant switching and conduction losses. The root cause of this behavior was identified to be fast switching causing a big power drop across the transistors for the switching losses. For the conduction losses one major item were found in the high current of the resonant tank causing a significant power drop on the switches. It causes a major power loss of the converter and lead to decreasing the intended output voltage to avoid thermal overstress of the converter. Because the resonant tanks input signal is close to the desired waveform, it was possible to verify the functionality of the main converter stages. The results are proposing to narrow the input voltage range and the load variation to keep the resonant converter operating in a more efficient way. This can be achieved by changing the switching scheme for low load scenarios. Also a transformer could be included to keep the resonant converter in a more efficient range. Both improvements are intended for the next steps. With the microcontroller software in place it is possible to control the converter very accurate. Also performing the necessary controller optimization measurements was conducted with low effort. Noise issues on output voltage measurement caused a limitation in the control parameters. Therefore the control loop was set to a very slow adjustment, to achieve a stable output voltage. This configuration is acceptable because a pre-regulator usually does not see large load variations. The second stage has to have a xvi fast response time as it needs to react to fast load transients and needs to provide a very stable and precise output voltage at the same time.
M.S. in Electrical Engineering, May 2014
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