Improved capacitance ratio optimization methodology for stacked switched capacitor energy buffers

Abstract

The recently proposed stacked switched capacitor (SSC) energy buffers with film or ceramic capacitors can increase the lifetime of single-phase ac/dc converters by eliminating the relatively short lifetime electrolytic capacitors used for twice-linefrequency energy buffering. This paper presents an improved methodology to minimize the passive volume of SSC energy buffers by optimally selecting their capacitance ratios while taking into account the variation in capacitor energy density with voltage rating. The proposed methodology is compared with the state-ofthe-art approach for both unipolar and bipolar SSC energy buffers. It is shown that for designs with 10% voltage ripple, it reduces the passive volume by up to 43% depending on the capacitor technology and type of SSC energy buffer. The effectiveness of the proposed optimization methodology is validated using a prototype SSC energy buffer designed for an 8-W offline LED driver.Keywords-ac/dc power conversion, dc/ac power conversion, energy buffer, switched capacitor, twice line frequency.

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“…In this regard, the energy buffer, by applying SSC configurations, works on the principle that each capacitor absorbs and delivers energy, maintaining a narrow range of voltage at the buffer port, allowing maximum utilization of the energy storage capability [54]. Another approach to increasing the energy density is to optimize the capacitance ratio of the capacitors used in the energy buffer [55,56].…”

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“…Several implementations of the SSC energy buffer have been proposed, including unipolar and bipolar designs. Each of these have enhanced variants, which can be further optimized by appropriately selecting their capacitance values [10]- [15]. Compared to their bipolar counterparts, enhanced unipolar SSC energy buffers offer reduced switch complexity and are the focus of the rest of this paper.…”

“…The passive volume reduction achieved by an SSC energy buffer relative to a single capacitor depends on its topology and the allowed intermediate dc bus voltage ripple ratio, which is defined as the ratio of the peak voltage ripple amplitude to the nominal value of the intermediate dc bus voltage [15]. For an enhanced unipolar SSC energy buffer, designed for a voltage ripple ratio of 5%, the passive volume of the buffer as a function of the number of switches, relative to a single capacitor (0 switches) is shown in Fig.…”

“…Hence, film and ceramic capacitors can be employed for twice-line-frequency energy buffering, provided a mechanism exists to maintain the bus voltage ripple within prescribed limits. A number of approaches have been presented in literature to increase the energy utilization of capacitors, including the use of bi-directional dc-dc converters [3], [4], energy buffers incorporated in the power stage design [5]- [7], and use of switched capacitor energy buffers [8]- [15]. Amongst the switched capacitor approaches, the stacked switched capacitor (SSC) energy buffer has the least complexity [10]- [15].…”