Design, Modeling and Control of the Integrated Stacked Resonant Switched Capacitor DC-DC Converter
Mobile, wearable, and internet of things (IOT) electronic systems are typically powered by a single cell Lithium-Ion (Li-ion) battery. Increasing demand for battery life and the constraint of the overall platform size impose an emerging desire for an efficient and com- pact power conversion stage that can directly deliver power from the battery, which ranges from 3.2V to 4.2V, to the supplies of application/communication processors in the range of 1V. The conventional buck converter, dimensioned for the 3.2-4.2 V input range, requires switches that are not typically available in standard sub-micron CMOS technology. Efficient implementation of the conventional buck converter has required a rather bulky off-chip inductor. The switched capacitor (SC) converter, though promising for a fully integrated solution, is inefficient at voltage regulation and is limited in capacitor energy utilization. In addition, with the SC converter, to cover the input range of a Li-ion battery to the processor supply voltage, topology reconfiguration has to be adopted. This additional functionality requires some additional design and implementation complexity.
Given the limitations of either pure inductive or pure capacitive converter topologies, various hybridized power conversion architectures have been proposed recently. The hybrid approaches leverage minimum magnetic components to mitigate the intrinsic charge sharing loss in the standard SC operation while still maintaining a better switch utilization inherent in some SC topologies. This thesis introduces a hybrid dc-dc converter topology named stacked resonant switched capacitor (Stack-ResSC) that eliminates charge sharing losses with a small inductor. Consequently, capacitor energy utilization is improved and lossless voltage regulation is enabled. Since the necessary inductance is small, the magnetic component can still be tightly integrated within the package with a PCB trace or a bondwire. The proposed topology can also adapt to any N-to-1 conversion ratio, which extends the topology for wider applications. Harmonic averaging is used in this thesis to develop the large and small signal model of the converter. A tight load line regulation can be effected with the designed controller according to the developed small signal model.
In addition to the Stack-ResSC, this thesis also introduces the stacked dual active bridge (Stack-DAB) as a derivative of the Stack-ResSC. An integrated circuit example is demonstrated with a nominal 3:1 conversion ratio. It supports a 3.2V-4.2V input voltage with a continuous output regulation of 0.8V-1.5V. The circuit delivers 83.4% efficiency at a power density of 0.46 W/mm2. The converter also shows superior transient response compared to state-of-art integrated implementation. This work demonstrates the advantage of the pro- posed hybrid approach in terms of tight integration and voltage regulation. The scope of the resonant switched capacitor based DC-DC converter is thus further extended.