Fully Integrated Advanced Multiphasing Switched-Capacitor DC-DC Converters

Power converters are an essential part of a global infrastructure upon which much of today's society depends. For DC-DC converters used in electronics, there has been a strong drive towards integration on microchips, ideally together with their load.  In addition to benefits in terms of size and cost, the resulting fully integrated power converters are key in reducing losses related to the power delivery network and in enabling granular dynamic voltage scaling in digital circuits, both of which would lead to substantial improvements in system efficiency. Furthermore, because many of today's central processing units and systems-on-a-chip are limited by their power-density, this efficiency-gain could directly be leveraged for performance. 

Switched-capacitor DC-DC converters have emerged as a promising candidate for integration because both switches and capacitors are readily available in today's integrated technologies. That being said, even for these converters the monolithic context has proven a challenge. Partly due to the large parasitic coupling and the limited capacitance density, the efficiency and power-density that can be obtained is constrained as well. In addition, this type of converter inherently has a very narrow conversion ratio range close to a fixed rational conversion ratio. Naturally, this is problematic for battery-connected devices or dynamic voltage scaling. The main aim of this work is to alleviate these limitations through the concept of advanced multiphasing. Here, multiple converter cores are put in parallel and actively interact with each other over several phases to improve their capabilities. As such this work can also be considered an exploration into the so-far largely unstudied phase-domain.

This dissertation gives an overview of the fundamental concepts behind monolithic switched-capacitor converters and points out that, after optimization, there is distinct low- and high power-density regime at which they can operate. In the former, the converter's efficiency reaches a maximum, while in the latter an efficiency-power-density trade-off can be witnessed. Three figures of merit are introduced that compare a converter's performance to these theoretical regimes, or evaluate a converter to reduce power delivery network related losses. The theory is expanded with the presentation of a voltage-domain analysis that leads to a fundamental law of conventional switched-capacitor converters. Relating several flying capacitor attributes to the conversion ratio, this law indirectly demonstrates the key advantage multiphase converters have over two-phase converters.

With the goal of reducing parasitic coupling losses, a first advanced multiphasing technique, called Scalable Parasitic Charge Redistribution, is proposed where the charge on the parasitic coupling is continuously recycled between out-of-phase converter cores through several charge redistribution buses. Because this technique removes the previously established efficiency limit, the basic loss model is updated with transistor leakage to establish a new fundamental maximum. Thanks to the technique, a converter is realized that reduces its parasitic coupling losses tenfold and achieves a record 94.6% efficiency. Also, because the charge redistribution buses have a DC voltage, they are investigated as a means to supply power to circuitry within the converter itself.

Stage Outphasing and Multiphase Soft-charging are two more techniques that instead focus on the limited capacitance density by spreading charge-transfers between capacitors out over multiple smaller and more efficient steps. The result is that the effective capacitance density of the converter is improved, which is useful for high- and low power-densities. Both techniques are shown to work for several topologies, but are especially beneficial in combination with Dickson converters with larger conversion ratios. An implementation in a baseline integrated technology verifies the working principle of the techniques by obtaining 60% larger effective capacitance density, and a record 82%-efficiency 1.1W/mm²-power density combination.

Conventional switched-capacitor topologies minimize the voltage swing on their capacitors to reduce charge-sharing losses and arrive at an efficient operation, though only for a narrow conversion ratio range. In this dissertation, a fundamentally new type of switched-capacitor converter is introduced with large voltage swing capacitors, that is made efficient using soft-charging enabled by advanced multiphasing. A particular topology is found to behave like a gyrator with a continuously-scalable conversion ratio, which is the first time a purely capacitive DC-DC topology achieves this feat. A realized converter implements this topology and obtains the largest efficient conversion ratio range in the literature.