Digital control of high frequency PWM convertors
The thesis begins with a review of presently available analogue and digital control schemes for high frequency PWM converters. Advantages and disadvantages of each scheme are identified, to determine which features would be desirable in a new digital control scheme. An extensive examination of peak and average current mode control is undertaken, using state-space/sampled data modelling, to gain more detailed information on the properties of current mode control. On the basis of this information, a new digital current mode control scheme is put forward. This uses samples of the inductor current, line voltage and output voltage to implement a control strategy in software. Average inductor current is calculated each switching cycle and compared to the current program level, providing true current mode control. This has some advantages over traditional methods. Accurate inductor current tracking of the current program level is achieved and no slope compensation is required for stable operation over the full range of duty ratios. Line voltage feed-forward is possible in buck derived topologies, which provides an effective null in the audio susceptibility transfer function, independent of compensation parameters. Current loop stability is independent of line voltage or load current in the buck topology, allowing operation with optimum loop compensation under all normal operating conditions. Practical implementation of a digital current mode controlled current-fed converter is described. This includes a modular architecture for the hardware and documentation for the software. Effects of component selection on the achievable converter switching frequency and dynamic performance are discussed. A method is put forward for the direct digital measurement of loop gain and phase in digital control systems. This is used to obtain actual loop responses from a test bed digital current mode controlled current-fed converter. Line and load transient response tests are presented which demonstrate the dynamic characteristics of digital current mode control.