Design and Analysis of Switched Capacitor Converter Topologies for Low Power Applications

Abstract

DC - DC converter can be regarded as the heart of any electrical or electronic circuit for buck, boost, inverting or conditioning the target voltage from the available source voltage. Switched Mode Power Supplies (SMPS) are the widely used converters in this segment. Any modern SMPS comprises an energy storage element that transfers the energy from source to load. Inductors are the widely accepted storage element in most of the present day SMPS. They are capable of carrying larger currents by virtue of their construction in large power converters. However, when it comes to small converters, bulky and heavy inductors often restrict the application in an on chip miniaturization circuit. Capacitor, which is another energy storage element, because of its high energy density and low equivalent series resistance compared to inductors, is promising for efficient on chip application. Switched capacitor converters (SCC) that are using only switches and capacitors popularly known as flying or charge pump capacitors are gaining popularity in on die power management boards. In this thesis, the concept of generalized Fibonacci single input single output (SISO) SCC is discussed and it has more efficiency and less equivalent resistance value. Generalized Fibonacci SCCs are technologically advanced and operate on fixed conversion ratio. Different target ratios of Fibonacci series have already been carried out by researchers to step-up and step-down configuration. But, 1/6 and 5/6 voltage ratios of Fibonacci SCC have not been proposed in the literature. To solve the unsolved voltage ratios two possible cases are considered: 1) Fourteen switches and four flying capacitors using Fi = Fi−1 + Fi−3 series. 2) Generalized Fibonacci SCC network is used and a new series is developed to solve the voltage ratios 1/6 and 5/6 with 12 switches and 3 flying capacitors. Theoretical results and simulation results are validated. To overcome limited voltage ratios, reconfigured dual input and single output (DISO) SCC is developed. A reconfigurable SCC topology with nine/ten CMOS switches and two flying capacitors is developed. It is capable of accepting two input sources simultaneously with an input voltage in the range of 1-2.5 V and delivers the output for 15 conversion ratios. The proposed SCC iiican drive a load current ranging from 10 µA to 10 mA at an open loop efficiency of >90%. One of the important applications of the proposed converter is the utilisation of photovoltaic (PV) or the combination of PV and other direct current sources. The regulation of the output voltage can be achieved either by changing the voltage ratios or using variable switching frequency. DISO requires more space and more capacitors to develop more voltage ratios. To overcome such issues, the dual input dual output (DIDO) converter is developed. A new dual-input and dual-output SCC is designed to operate with two independent voltage sources that provide two different output voltages and generate 56 voltage ratios. The converter is portable to operate with one or two input sources alternatively and have the ability to vary 56 voltage ratios. An efficient low power SCC is designed for input voltage of 1.5 V to 5 V that gives dual output voltages of 1 V to 10 V. The designed converter can operate in both buck and boost modes. SCC has high drive capability of load current from 10 µA to 25 µA that is adjusted by operating frequency. The algorithm is discussed to solve the coupled case of dual input and dual output converter. Another major contribution in this research is the introduction of R- parameters calculation for the coupled case and is deliberated in detail since it includes all conduction and ohmic losses accounting for coupling effects. To validate the performance of designed SCC, modeling and mathematical analysis has been carried out. Finally, an extended version of reconfigurable SISO SCC is designed for driving the white light emitting diodes (WLEDs). The various voltage ratios are selected to control the WLEDs blacklights using the inverted SCC (ISCC), which helps to save the battery life of electronic devices. The major contribution is developing maximum voltage ratios for power converter driver ICs and equivalent resistance (Req) accurate calculation where it includes all conduction and ohmic losses. Furthermore, ISCC experimental results are obtained from prototype model. Accurate R eq analyses validate the accuracy of the proposed topology. It is designed for low voltage of 10 mV to 0.1 V and it provides the output voltage of -100 mV to -0.5 V. Finally, the accurate Req calculation and the results are verified experimentally, particularly, at the transition reivgion (between slow switching limit (SSL) and fast switching limit (FSL)). Theoretical calculation from the model derived concurs well with simulation and experimental findings. Accurate equivalent resistance calculation and average current calculation are compared with existing Req analysis available in the literature.