High-Ratio Voltage Conversion in CMOS for Efficient Mains-Connected Standby by Hans Meyvaert & Michiel Steyaert

Author:Hans Meyvaert & Michiel Steyaert
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

5.3.2 Switched-Capacitor Topology Construction

In order to investigate a suitable two-phase switched-capacitor DC–DC topology, it is necessary to have insight into how one is constructed and to what rules it must adhere to. To clarify the following discussion, the naming of the elements that make up an SC topology is now defined. A switched-capacitor DC–DC topology may consist of N multiple capacitor configurations, which are sequentially activated by opening and closing the related power switches. Hence the name switched-capacitor topology. A cycle or switch cycle can be defined as the sequential activation of these N capacitor configurations. As such, a cycle consists of N phases, each representing one of the N capacitor configurations. Hence the origin of an N-phase SC topology. The capacitors transfer charge and consequently are called charge-transfer capacitors. As this charge transfer results from the relocation of these capacitors to a different absolute potential, during which their capacitor bias voltage ideally remains unchanged, they can also be called flying capacitors. Multi-phase operation, a widely popular ripple reduction technique, is not to be mistaken with the N phases of a switch cycle. Instead multi-phase operation consists of, and can be described more uniquely, by the time-interleaved operation of M converter fragments. A converter fragment is readily obtained by dividing an SC converter’s switch and capacitor resources into M equal fragments.

An SC topology consists of an input voltage source, an output voltage source as load, any number of charge-transfer capacitors and switches to configure these building blocks into the capacitor configurations of each phase. For the purpose of SC topology construction, it is only required to consider the voltage sources and the flying capacitors, which are assumed to be ideal and therefore approximate a voltage source. However, they differ in the ability to source or sink charge. A real voltage source is able to source or sink charge indefinitely, i.e., only sourcing charge (input source) or only sinking charge (output load), and this during one or all of the phases of the topology cycle. In contrast, the flying capacitors cannot have an infinite capacitance, and consequently must be charged in at least one phase and discharged in at least one other phase, in order to achieve an equilibrium bias voltage in steady state. In case of a two-phase converter, charging a capacitor in one phase leads to a necessary discharge during the other phase.

An example is given in Fig. 5.6 with a 1–3 step-up converter. During the first phase, the input voltage sources a charge amount of 2q to charge capacitors C 1 and C 2. During the second phase, the flying capacitors are reconfigured and the charge amount is now delivered to the output voltage load. Indeed, a voltage difference V must be present between the steady-state capacitor bias voltage at the end of phase 1 and phase 2, in order for charge flow to occur. The voltage conversion ratio of 3, in Fig. 5.6, is idealized and consequently this V is not shown. But once a non-zero current is consumed

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.