A switched-capacitor power converter with unequal duty cycle for ripple reduction and efficiency improvement

Abstract

This work applies unequal duty cycle to switched-capacitor (SC) dc-dc converters to improve its output ripple as well as the power conversion efficiency. By adjusting the duty cycle, the capacitor charge and discharge states are reasonably balanced. Therefore, we effectively reduce the output ripple and the charge redistribution loss, improving the efficiency. A SC converter verifies this scheme with voltage conversion ratios (VCRs) of 1/4 and 1/3, in a 65 ​nm CMOS low-power process. Operating with a switching frequency of 33 ​MHz, an input voltage of 2.4 ​V, an output range of 0.5–25 ​mA, and a VCR of 1/4, the peak efficiency increases by 1.0% and the output ripple drops by 43.6%, when compared with the equal duty cycle mode. With the input voltage equal to 1.8 ​V and the VCR ​= ​1/3, the peak efficiency increases by 0.7% and the output ripple reduces by 48.0%, compared to the equal duty cycle mode.

Introduction

Fully integrated switched-capacitor dc-dc converters are suitable for low-power Internet-of-Everything (IoE) applications, like wearable and medical devices. In such cases, SC converter has several advantages. Firstly, it has relatively higher power efficiency when comparing to linear regulators, while an SC converter with multiple voltage conversion ratios can achieve high efficiencies over a relatively wide input and/or output voltage ranges [1,2]. In addition, an SC converter can maintain high power efficiency under light load by modulating the switching frequency [3]. Secondly, SC converter shows fast transient load response with its first-order power stage [4,5]. Thirdly, capacitor has lower cost and smaller size, when compared to bulky power inductor in a switching mode power converter. Moreover, SC converter may generate less electromagnetic interference (EMI) to the system [6].

To obtain a clean supply, a linear regulator is usually used in cascade with a switching converter [7], but it reduces the power delivery efficiency. If we can reduce the output ripple of a switching converter, the power supply rejection requirement on the linear regulator stage can be relaxed, or potentially eliminating the post-stage linear regulator and thus increasing the system efficiency.

Fig. 1 shows the circuit techniques that can be used to reduce the output ripple of an SC converter. For on-chip implementations, the multiphase interleaving scheme (Fig. 1(a)) can be easily used to reduce the output ripple with negligible power and area overhead [5]. Alternatively, digital capacitance modulation (Fig. 1(b)), which divides the total flying capacitor into binary-weighted parts that modulate the output voltage and reduces the output ripple [8]. However, for an SC converter with off-chip capacitors, we do not have the luxury to employ interleaving phases with a large number of flying capacitors, which significantly increase the cost and complexity [9]. In such case, switch on-resistance modulation (Fig. 1(c)) and pulse-width modulation (Fig. 1(d)) were introduced to reduce the output ripple. For the switch on-resistance modulation in Ref. [8], dividing the switches into multiple parts equally and controlling them according to the load current can effectively reduce the output ripple. In Ref. [6], a structure with a three-stage switchable amplifier driving the power switch is proposed to modulate the on-resistance through changing the gate-drive voltage of the switch. For pulse-width modulation, the output ripple is usually reduced by partially charging the capacitor [10]. However, the partial charging method does not make full use of the capacitors, as the amount of charge delivered per phase is not equal. In this case, limiting the charging and discharging duration will affect the output voltage and power conversion efficiency. To solve the unbalanced charge delivery problem, multiphase balanced switching scheme was proposed, which reduces the output ripple by distributing the total charge evenly across each conversion phase and by replicating one or more conversion phases in one full conversion cycle [11]. This scheme can also be applied to dual-output SC dc-dc converters for reducing the cross regulation among the two outputs [12]. In short, simple but effective control schemes are favorable.

In this paper, we introduce a new method that uses the unequal duty cycle to reduce the output ripple, and also to increase the power conversion efficiency. Section 2 introduces the working principle of the unequal duty cycle scheme, presents the circuit implementation of the SC converter, discusses, and compares the proposed scheme with the multiphase balanced switching. Section 3 gives the simulation results. Finally, Section 4 draws the conclusions.