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Switching Power Converter Techniques for Server and Mobile Applications

Abstract Details

2020, Doctor of Philosophy, Ohio State University, Electrical and Computer Engineering.
With the increasing demand for lowering the power consumption, system cost and size of high-performance electronic devices, power supply designers are being challenged to develop unconventional design techniques so that the efficiency and power density of the power supplies used in these systems could be improved to help meet these demands. On the one hand, linear power supplies can be small and cost-effective as they require no inductors. However, their poor efficiency makes them unattractive in terms of lowering the system power consumption. Moreover, the bulky and expensive heat sinks desired to dissipate the heat resulting from their poor efficiency offset their small size and lower cost advantage. On the other hand, switching power supplies offer much higher efficiency, but require bulky and expensive inductors, unless new design techniques are developed to reduce their impact on the system size and cost. To address these challenges, the work presented in this thesis is focused and performed on the development of new design techniques for switching power supplies to target two distinct applications namely, a) CPUs used in high-performance computer servers, and b) battery-operated mobile devices. Designing switching power converters for CPU’s in high-performance computer server applications is becoming increasingly difficult. This is because higher performance in a CPU necessitates for the following requirements from switching converter that powers it: (a) higher maximum load current rating, (b) high efficiency across a wider load range, and (c) fast Dynamic Voltage Scaling (DVS). Based on this context, the research work documented in this thesis includes two different high-frequency, high-current switching power converter techniques designed to incorporate the above key parameters. The first proposed technique for high performance CPU applications is a 4-phase buck converter design with maximum load of 8 A. The converter switches at 100 MHz to enable fast dynamic response and reduce the required inductance and capacitance to 20 nH (5 nH/phase) and 220 nF. Thus, small foot-print inductors and capacitor can be used and co-packaged with the converter die. To maintain high efficiency across a wide load range, the converter employs PWM control in the 6-8 A load range, and then deactivates phases as the load drops below 6 A while keeping PWM control until only one phase is active when the load is less than 2 A. For loads less than 0.7 A, the converter deactivates the PWM controller and activates a PFM controller to scale switching losses and ensure high efficiency at light loads. The second proposed technique is an imbalanced high current multi-phase buck converter for high performance CPUs. This approach proposes using a different inductor, switching frequency, and share of the total load current for each individual phase, i.e. imbalanced phases. This approach provides additional degrees of freedom through the independent choice of the inductor and the switching frequency of each phase, which enables higher maximum load current rating and higher efficiency across the entire load current range compared to conventional balanced multi-phase designs. Regarding battery-operated applications, increasing demands for extending the run time of battery-operated devices, low-quiescent current, high efficiency at very light load and fast dynamic performance are the critical parameters that a switching power converter must incorporate. Therefore, this research work also addresses two different highly efficient power management solutions to account for the above critical parameters for mobile applications. The first proposed technique is a 1-A 6-MHz buck-boost converter with seamless mode transitions and fast dynamic operation for extending the useable voltage range of Li-Ion batteries in mobile devices, and thereby their battery life. The proposed converter employs a Proportional-Integral (PI) peak-current-mode PWM controller with a digital adaptive slew-rate control scheme that minimizes output undershoots, overshoots, and settling time during mode transitions and dynamic events, such as load and output voltage steps. Additionally, hysteretic mode detection is proposed to prevent falsely triggering the converter to transition between the various operation modes (i.e. buck, boost, and buck-boost) and negatively impacting the performance of the circuit loads. The second proposed technique is a 2-A current-mode hysteretic buck converter with constant switching frequency for battery-operated devices. The converter employs a high-resolution digital frequency correction loop and dual-sided hysteretic band modulation to tune and lock the steady-state switching frequency to a 6-MHz reference clock. As such, the superior dynamic performance of hysteretic control can be leveraged while also maintaining the predictable performance and simplified EMI filter design that result from a constant switching frequency. The dual-sided hysteretic band modulation eliminates any inductor current imbalance due to the continuous tuning of the switching frequency, and thus ensures smooth, transient-free frequency locking. Moreover, a hysteretic loop-suspension filter is proposed to desensitize the steady-state switching frequency of the converter to the 6-MHz reference clock jitter, which relaxes the specifications of the clock generator.
Ayman Fayed, Dr. (Advisor)
Anant Agarwal, Dr. (Committee Member)
Tawfiq Musah, Dr. (Committee Member)
Andrea Sims, Dr. (Committee Member)
108 p.

Recommended Citations

Citations

  • Singh, M. (2020). Switching Power Converter Techniques for Server and Mobile Applications [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu159486698396321

    APA Style (7th edition)

  • Singh, Manmeet. Switching Power Converter Techniques for Server and Mobile Applications. 2020. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu159486698396321.

    MLA Style (8th edition)

  • Singh, Manmeet. "Switching Power Converter Techniques for Server and Mobile Applications." Doctoral dissertation, Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu159486698396321

    Chicago Manual of Style (17th edition)