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.

This thesis presents the fundamental issues of the digital controlled DC/DC converter. A lot of challenges exist when you introduce the digital control technique into the control of the DC/DC converter, especially with regards to the voltage regulator module. One issue is the limit cycle oscillation problem caused by the quantization effect from the ADC and DPWM of the digital control chip. Another issue is the delay problem coming from the sample-hold effect. In this thesis, the modeling, analysis and design methodology for the constant frequency voltage-mode control is reviewed. A DPWM (Digital Pulse Width Modulator) model is verified in simulation, which shows what effects the digital control brings to the conventional Pulse Width Modulator. In CPES, the constant on-time control concept is introduced into the digital control of the voltage regulator module. This provides a high resolution of DPWM and allows the digital constant on-time voltage-mode control architecture to be proposed. To limit the oscillation amplitude in the digital control structure, the digital constant on-time current-mode control w/ external ramp is further proposed in CPES. To analyze this structure, a describing function model is proposed for the digital constant on-time current-mode control, which takes both the sample-hold effect and the quantization effect into consideration. This model clearly shows the stability problem caused by the sample-hold effect in the current loop. Using larger ramp's slope values, this stability issue can be alleviated. Based on this model, a design methodology is introduced. By properly designing the current loop's ADC resolution and the voltage loop's ADC resolution, the limit cycle oscillation in this structure can be minimized: the digital constant on-time current-mode control will only have the oscillation coming from the sample-hold effect in the current loop, which can be greatly reduced by adding the large slope's external ramp to this structure. Simulation verification for this design methodology is provided to prove the concepts. Based on the proposed model, the compensator design is performed. The motivation for the compensator design is to push the bandwidth while satisfying the stability condition and the dynamic no-limit-cycle oscillation condition. When analyzing the case of one sample per switching cycle, there is a certain amount of delay, which compromises the phase characteristics. Our design also requires a large external ramp because it will reduce the oscillation amplitude in our system. From our model, it is quite obvious that the external ramp must have a slope larger than one time that of inductor current down slope. A slope that is too larger will weaker the phase and limit the bandwidth. When using the normal current-mode compensator, like the 1-pole 1-zero compensator, the phase is dropped too much and the bandwidth will be limited too low. If we use a 2-pole 2-zero compensator, the phase can be boosted. However, in this case, the gain margin requirement from the dynamic no-limit-cycle oscillation condition will make the further improvements on bandwidth impossible. In our design, the one sixth of the switching frequency is achieved. / Master of Science