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| An Improved Two-Phase Series-Capacitor Converter with High Conversion Ratio |
| Liu Guihua, Wang Boxin, Wang We, Jiao Wenbo |
| School of Electrical Engineering and Automation Harbin Institute of Technology Harbin 150001 China |
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Abstract For traditional buck converter, the duty cycle is narrow in high conversion ratio applications. This results in high losses and low efficiency of the converter. At the same time, the output current ripple of the converter becomes larger, and the stability will also deteriorate. In order to solve the above problems, scholars have proposed many methods to improve the voltage conversion ratio of converters. Among them, SC (Series Capacitor) Buck converter is a commonly used high voltage conversion ratio converter, and has the advantage of low voltage stress of switches in steady state. However, the voltage stress of the switches is high during converter start-up. This paper proposes an improved two-phase series-capacitor converter with high conversion ratio. While inheriting the advantages of the SC Buck converter, the converter has a higher voltage conversion ratio and lower output current ripple, and can effectively reduce the input filter capacitor value and the voltage stress of the switches during converter start-up.
The proposed converter adds two capacitors and a switch to the SC Buck converter, which has the following advantages. First, compared to the SC Buck converter, the voltage distribution across the capacitor is changed, which results in a higher voltage conversion ratio and lower switching voltage stress in steady state. Second, the voltage across the inductor of the proposed converter is lower than that of the SC Buck converter, so the inductor current ripple and output current ripple of the proposed converter are smaller. Third, in the steady state, the voltage on the series capacitor satisfies the ampere-second balance, which provides a balanced loop between the two phases. This current sharing mechanism is inherent to the topology and acts like a damped harmonic oscillator. Fourth, the capacitor on the input side of the proposed converter is pre-charged before the converter starts, which reduces the voltage stress of the switches during start-up. Therefore, the proposed converter can use switches with lower voltage rating, which helps to reduce the loss of the switches and improve the efficiency of the converter. Fifth, compared to SC Buck converter, the input voltage ripple of the proposed converter is smaller when the input filter capacitance is the same, so when the input voltage ripple is the same, the input filter capacitance required by the proposed converter is smaller.
A 110 W experimental prototype was built and the characteristics of the converter were verified. The experimental results show that when the converter achieves 48~5 V, the duty cycle is about 30%. In contrast, the SC Buck converter has a duty cycle of only about 20% when the same step-down ratio is reached, proving that the proposed converter has a higher voltage conversion ratio. In steady state, the voltage stress of two switches is 32 V, and the voltage stress of three switches is 16 V. The voltage stress of all switches is less than the input voltage. Before the converter starts, the voltage stress of two switches is 24 V, the voltage stress of three switches is 0 V. The maximum voltage stress of the switches before the converter starts is only half of the input voltage. And the maximum voltage stress of the switches during converter start-up is 32 V, which is less than the input voltage. The single-phase inductor current ripple is 3.5 A, and the current ripple after the superposition of the two phases is about 1.9 A. The two-phase interleaved parallel output structure greatly reduces the output current ripple.
According to theoretical analysis and experimental verification, the following conclusions can be drawn: (1) Compared with SC Buck converter, the proposed converter has a higher voltage conversion ratio and lower output current ripple. (2) Compared with the SC Buck converter, the voltage stress of the switches of the proposed converter in the steady state is lower and the voltage stress of the switches during start-up is reduced. (3) The proposed converter has automatic phase current balancing capability. It can eliminate a control loop that requires high performance circuits to balance phase currents. (4) Compared with the SC Buck converter, when the voltage ripple of the input filter capacitor is the same, the required input filter capacitance is smaller.
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Received: 13 January 2023
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2023, Vol. 38 
