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
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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.