Grid converter for LED based intelligent light sources

The purpose of this thesis was to investigate the applicability and effects of digital control to line connected switched mode power supplies with power factor correction. The main approach was cost effectiveness with high efficiency. This involved hardware design for increased switching frequency to reduce the size of the magnetic components. A description of different grid regulations was given followed by a set of converter topologies and controllers. Different control techniques were pointed out. Among the listed topologies and control solutions few were selected for further analysis, design and implementation.
Many of different hardware and control solutions available on the market were investigated. Most of the commercial power supplies are controlled by dedicated analog controllers in form an integrated circuit. Thus a survey was conducted to analyze the available state-of-art analog controllers and their implemented control algorithms. As digital control has to be competitive with the existing solutions it was investigated what digital signal processing solutions exist. A performance and cost comparison was also presented.

The chosen converter topologies were thoroughly analyzed. Different converters were chosen for different power levels. At low power simple boost converter as power factor corrector (PFC) and a RCD-clamped forward converter was chosen as DC-DC converter. This with has double output and coupled lter inductor. To design a digital controller with the tools of the classical control theory a small signal linearized model of the converter is needed. Detailed modeling and linearizing of the boost converter is presented.
At high power level interleaving technique is frequently used to reduce the current stress on the switching components. Though the number of magnetic components is increased they became smaller in size resulting in smaller current ripple through them. An interleaved boost converter with two legs is selected as PFC converter. It was shown that the small signal model of the interleaved converter is similar to the simple boost converter. Only the simple inductor has to be replaced by the paralleled inductors of each leg. This statement is valid only if the total inductor current is controlled rather than controlling the current in each leg. As second stage a phase-shifted full-bridge converter with synchronous rectication and current doubler was selected. It was shown that for output current and voltage control this topology can be modeled as a interleaved synchronous buck converter. As it can be seen interleaving technique is also present in this topology. For this topology a fuzzy logic voltage controller is proposed and compared to the traditional PI controller.

After modeling the converters controllers can be designed. The controller design was interconnected with the hardware design and control platform. Thus two dfferent prototypes were designed and built with two dierent digital controllers and the controller design, analysis and implementation was based on these two case studies.

The first prototype was a 70 W two-stage PFC and DC-DC converter with boost and forward converters. Average current mode control was selected, designed, simulated an implemented for the boost PFC converter. The two-loop control structure (fast internal current loop and lower bandwidth external voltage loop) was designed for nominal power but system behavior was also analyzed for low-load conditions. The controller was simulated in Matlab/Simulink using PLECS library and embedded Matlab function. All the parameters were treated and scaled just as they appear in the ADC interrupt of the 16 bit fixed point dsPIC30F1010 microcontroller. Peak current control was implemented for the forward converter, using analog comparator module of the digital-signal controller. The waveforms, eciency and power factor results were compared to the performance of an identical two stage 70 W power supply controlled with an analog PFC/PWM integrated circuit.
The second prototype was a 600 W two-stage PFC and DC-DC converter with interleaved boost and phase-shifted full-bridge (PSFB) converters. Average current mode control was designed simulated and implemented for both converters. The sum of the boost inductor current was controlled to shape the line current and the sum of the lter inductor currents in the PSFB converter was controlled to limit over-currents. Low bandwidth PI controllers control the boost DC and the output DC voltages. A fuzzy logic output voltage controller was also simulated and compared to the performance of the PI controller. All four control loops were implemented in a 16 bitfixed point dsPIC33FJ32GS406 microcontroller driving at the same time 8 PWM channels.

Finally a brief analysis was done on the eect of the grid disturbances, especially voltage sags on the digital controller. Dierent grid codes and compatibility requirements and system behavior through a boost converter with digital control was presented.

In this thesis a deep knowledge of design and digital control of line connected switched mode power supplies with power factor correction was gathered and appropriate solutions were presented. The advancement of this thesis will enable improved design and digital control of high frequency switched mode power supplies in the future. It is concluded that the digital signal processors available today are competitive in performance with the state-of-art analog ICs. The economic reasons for using digital control is not so clear but with falling prices of microcontrollers and increasing demands on the performance of power converters introducing digital control seems to be a reasonable option for the future development of power converters. Advanced control structures can be implemented to improve the performance of switched-mode power-supplies and power factor corrector circuits.