Pulse frequency modulation based tri-mode controller for switched-mode DC-DC converters

• Main Author: Nashit, Salma
• Other Authors: Gwee Bah Hwee
• Format: Theses and Dissertations
• Language: English
• Published: 2016
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Power electronics
• Online Access: http://hdl.handle.net/10356/69021
• Institution: Nanyang Technological University

This thesis pertains to the analysis and design of low-power and low output ripple-voltage multi-mode switched-mode DC-DC converter based on the Pulse Frequency Modulation (PFM). The applications of the DC-DC converter are low-power portable electronic devices, such as smartphones, tablets, notebooks, etc. These devices often operate at the standby condition—where the load current is relatively low—and thus, the power-efficiency at such a condition is paramount to extend the battery lifespan. First, we derive power-loss expressions for the three PFM approaches, namely the Constant On-Time approach, the Constant Off-Time approach, and the Constant Peak-Current approach. The derived expressions provide an insight into the power-loss mechanisms and elucidate the pertinent parameters that affect the power-loss. Based on the simulations for output voltage, Vo = 1.8 V, and duty cycle, D = 0.36, the Constant On-Time PFM is found to be the most power-efficient over the desired range of load-current, Io = 10 mA - 500 mA. We have verified the derived expressions by means of MATLAB and SPICE simulations. Second, on the basis of the aforementioned expressions, we derive the expressions for optimum parameter values that yield the minimum power-loss for each of the PFM approaches. By means of the derived minimum power-loss expressions, we obtain the optimum parameter values for the three PFM approaches at the same test conditions of Vo = 1.8 V, D = 0.36, and Io = 130 mA, and compare the resultant power-loss. The Constant On-Time PFM approach at an optimum on-time of 2.2 µs achieves the lowest power-loss at only 9 mW, whereas the Constant Off-Time PFM approach at an optimum off-time of 3 µs achieves 20 mW power-loss, and the Constant Peak-Current PFM approach at an optimum peak-current of 800 mA achieves 9.8 mW power-loss. Third, we propose a novel tri-mode controller for switched-mode DC-DC converters based on the Constant On-Time PFM approach. The controller allows 3 operating modes: the Continuous Conduction Mode with a PID compensator (CCM-PID) for high load-currents (140 mA – 500 mA), the Discontinuous Conduction Mode with a PID compensator (DCM-PID) for low load-currents (20 mA – 140 mA), and the DCM for extremely low load-currents (<20 mA). The PID compensator reduces the errors during the transient state and at the steady state of the DC-DC converters. Despite the varying switching-frequency, the proposed controller with the PID compensator has an excellent frequency response of a near-constant phase margin of ~67° and a maximum voltage-overshoot of only 5.6%. Simulation results show that, when the proposed tri-mode controller is employed in a switched-mode DC-DC converter design (Vo = 1.8 V, D = 0.36, and an average output switching frequency of 260 kHz), the switched-mode DC-DC converter maintains high power-efficiencies of ~89% at the CCM-PID, ~81% at the DCM-PID, and ~46% at the DCM, and exhibits a low output ripple-voltage of <2% of Vo throughout the desired load-current range.