This thesis is about the improved design of small size brushless DC fan motor driving circuit. Two main improvements in the new design are increase the stability and decrease the size of motor fan at the same time. To improve the stability, there are two major parts added to the original driving circuit. The delay circuit that protects the H-bridge and the output low current limit circuit. Furthermore, it is believed that the speed control also can improve the stability. With regard to the rotation speed control, two circuits are attached to the motor, 1) speed feedback controller and 2) speed and current feedback controller. Both controllers are attached in the close loop rotation speed control circuit. They are used to increase the efficiency of drive circuit. In order to make the circuit more efficient, they solve problems such as disturbance in miscellaneous noise; also the power dissipation that occurs in open loop rotation speed control circuit. The second improvement in the new design is to reduce the cost and size of system. The design of sensorless control scheme is proposed to replace the Hall sensor to detect rotor position. This sensorless scheme can also supply fan motor voltage to achieve the speed control.

The thesis describes theoretical and experimental studies on the chaotic behaviour of a peak current-mode controlled boost converter, a parallel two-module peak current-mode controlled DC-DC boost converter, and a peak current-mode controlled power factor correction (PFC) boost converter. The research concentrates on converters which do not have voltage control loops, since the main interest is in the intrinsic mechanism of chaotic behaviour. These converters produce sub-harmonics of the clock frequency at certain values of the reference current I[ref] and input voltage V[in], and may behave in a chaotic manner, whereby the frequency spectrum of the inductor becomes continuous. Non-linear maps for each of the converters are derived using discrete time modelling and numerical iteration of the maps produce bifurcation diagrams which indicate the presence of subharmonics and chaotic operation. In order to check the validity of the analysis, MATLAB/SIMULINK models for the converters are developed. A comparison is made between waveforms obtained from experimental converters, with those produced by the MATLAB/SIMULINK models of the converters. The experimental and theoretical results are also compared with the bifurcation points predicted by the bifurcation diagrams. The simulated waveforms show excellent agreement, with both the experimental waveforms and the transitions predicted by the bifurcation diagrams. The thesis presents the first application of a delayed feedback control scheme for eliminating chaotic behaviour in both the DC-DC boost converter and the PFC boost converter. Experimental results and FORTRAN simulations show the effectiveness and robustness of the scheme. FORTRAN simulations are found to be in close agreement with experimental results and the bifurcation diagrams. A theoretical comparison is made between the above converters controlled using delayed feedback control and the popular slope compensation method. It is shown that delayed feedback control is a simpler scheme and has a better performance than that for slope compensation.