THESIS
2013
xvii, 146 pages : illustrations ; 30 cm
Abstract
Digital control is gaining popularity in switching converters because of its flexibility and re-configurability
to implement various control functions without hardware modification. It is also
less susceptible to aging, noise, process and parameter variations. Nonetheless, a major challenge
with digital control is bandwidth limitation which is primarily caused by the phase delay in the
ADC and the digital feedback loop. This research reviews the existing digital control strategies.
A digital controller with adaptive prediction is proposed which increases the unity-gain
bandwidth even with a modest 2x oversampling ratio. By introducing an additional low-frequency
pole-zero pair, the DC gain can be further increased which significantly improves the
line transient dynamics by reduc...[
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Digital control is gaining popularity in switching converters because of its flexibility and re-configurability
to implement various control functions without hardware modification. It is also
less susceptible to aging, noise, process and parameter variations. Nonetheless, a major challenge
with digital control is bandwidth limitation which is primarily caused by the phase delay in the
ADC and the digital feedback loop. This research reviews the existing digital control strategies.
A digital controller with adaptive prediction is proposed which increases the unity-gain
bandwidth even with a modest 2x oversampling ratio. By introducing an additional low-frequency
pole-zero pair, the DC gain can be further increased which significantly improves the
line transient dynamics by reducing the maximum overshoot and undershoot. An FPGA-based
hardware for a digitally-controlled single-output buck converter has been experimentally
verified, demonstrating the effectiveness of the proposed digital control scheme.
Digital control can also be used to regulate single-inductor multiple-output (SIMO) switching
converters. As the LED technology continues to evolve, there is a stronger need for higher power
efficiency and lower cost which motivates us to investigate a new SIMO-based driver topology.
Instead of connecting all the strings to a common output bus as in conventional driver topologies,
the output bus voltage in each LED string can be individually optimized in the proposed driver.
The voltage headroom per string is minimized which reduces the overall power loss. Since each
string is driven independently, same-colored LEDs from neighboring bins with larger forward-voltage
variations can be tolerated, hence lowering the LED cost. Another important design
consideration is the scalability of a SIMO-based driver. The proposed scalability models define
the theoretical maximum achievable number of strings in a general SIMO architecture. They
enable converter designers to quickly determine the maximum possible number of parallel
strings in a SIMO-based driver required for a particular application. A single-inductor dual-string
driver has been implemented in an FPGA-based PCB prototype. The experimental results are
shown to be consistent with the corresponding ones from simulation. Cross-regulation has
always been a major design challenge for SIMO converters. A modified quasi-V
2 pseudohysteretic
control for SIDO driver is proposed in order to reduce the cross-regulation between the
two independently-driven LED channels. The key features include a fast discharge path for the
quasi-V
2 sense node via an additional pull-down switch and a feedback path from the unchanged
output to the first-order RC filter. Similar to the inductor current in DCM, the quasi-V
2 feed-forward
voltage is returned to zero at the end of each switching phase which significantly
suppresses the cross-regulation. The simulation results show that the cross-regulation is
substantially reduced with the modified quasi-V
2 control scheme, compared with the original one. The experimental results demonstrate that no noticeable cross-regulation is observed for a
load current step of 120 mA.
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