Study On The Digital Current Mode Control Of Switching-Mode Power Supply

Compared with conventional analog control, digital control of switching-mode power supply (SMPS) has got much attention due to its better power management, ability to interface with digital systems, flexibility to implement sophisticated control schemes without changing hardware circuit, and so on.Based on the digital control, digital current control techniques, including digital peak current (DPC) control and digital average current (DAC) control, are studied in this thesis. DPC and DAC control techniques with single edge modulation and dual edge modulation methods are present in the thesis, and they are studied on buck converter. The single edge modulation includes leading edge and trailing edge modulations, while the dual edge modulation includes leading triangle and trailing triangle modulations. The different control laws for each modulation method are derived and compared. Comparison on these shows that DPC control buck converter with leading edge modulation has excellent load transient characteristic among four modulations, and DAC control buck converter with leading triangle and trailing triangle modulations has outstanding load transient characteristic among four modulations.Small-signal s-domain model is derived according to different control algorithms. Small-signal z-domain model is then obtained by using several commonly discretization methods, such as backward rectangular, bilinear, zero-order hold, pole/zero match and so forth. Small-signal analysis and corresponding frequency domain simulation are further performed with comparison among different models.Stability analysis of DPC and DAC with four modulation methods is also performed and studied in the thesis. It is observed that for DPC and DAC, there is a choice of the appropriate modulation method without sub-harmonic oscillation problems. Results show that DPC with leading edge modulation and leading triangle modulation is inherently stable without sub-harmonic oscillation in the whole range of duty ratio. The same is true for DAC with leading triangle modulation and trailing triangle modulation.Fully utilizing the flexibility of the digital control algorithm, digital slope compensation (DSC) technique is proposed to eliminate the subharmonic oscillation, and thus the transient performance of switching dc-dc converters is improved.