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UBC Theses and Dissertations

Time delay compensation of digital control for switchmode DC power supplies using prediction techniques Bibian, Stéphane

Abstract

The control of switchmode dc power supplies has been traditionally implemented in analog electronics for its low cost, high bandwidth, and proven technology. However, the emergence of advanced microprocessors and Digital Signal Processors (DSPs) has made it possible for power supply manufacturers to consider digital technology a suitable option. As compared to analog control, digital control provides a number of advantages such as reduced susceptibility to aging and environmental variation (temperature, humidity, etc.), better noise immunity, ability to handle complex control schemes and monitoring functions, possibility to implement communication functions for fault and status information, and easy re-programming for different applications. With the downward trend of microprocessor prices and the versatility of the hardware architecture, digital control offers a cost-effective solution which can compete with analog technology. However, one of the major drawbacks of digital control is the limited bandwidth caused by the inherent time delay required for A/D conversion, computation and PWM generation. Such delay degrades the control loop performance, which makes it difficult to comply with technical specifications in many high-performance products. In this thesis, a simple and straightforward predictive technique based on linear extrapolation is presented to compensate for this delay. Two predictive controllers are derived and applied to a full bridge dc power supply. Simulation and experimental results show that the performances of the converter with respect to dc bus ripple and load disturbances can be significantly improved. Those schemes are characterized by a low computational cost which makes them particularly attractive in the demanding real-time environment due to the ever increasing switching frequency of the converter. A similar prediction concept based on a second order parabolic extrapolation is also presented. This predictor substantially reduces the amount of calculation needed to obtain similar results to a conventional controller, thus freeing valuable processor resources. These resources can be used for less critical tasks such as communication and user interface. As a result, the processor is not solely dedicated to the control of the power supply but can incorporate other functions, thus adding functionality and expandability to the system. Finally, this thesis assesses typical digital control issues such as word length, A/D resolution and fixed point programming.

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