双向 LLC 变换器低压侧零环流的边沿偏移型变导通时间调制

doi:10.19595/j.cnki.1000-6753.tces.251200

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Abstract

The bidirectional LLC converter, valued for its high efficiency and power density, is a prime candidate for low-voltage, high-current applications such as battery optimizers. Traditional modulation, however, is hampered by asymmetrical gain characteristics, difficult bidirectional switching, and significant circulating currents.
To address these problems, this chapter proposes the edge-shifted variable on-time (ESVOT) modulation. This new modulation introduces two additional control degrees of freedom—a phase-shift angle and the on-time of the low-voltage side (LV-side) switches—to optimize the LV-side circulating current and ensure perfectly symmetrical forward and reverse operating characteristics.
Since the ESVOT modulation complicates the converter's operating modes, the conventional fundamental harmonic analysis (FHA) method is no longer adequate for accurately modeling its steady-state characteristics. Therefore, to precisely calculate the phase-shift angle and LV-side switch on-time required to achieve zero circulating current under various load and gain conditions, this paper employs exact time-domain analysis (ETDA) for the steady-state analysis. Based on the characteristics of the resonant tank waveforms, eight operating patterns are identified. The mode analysis and a soft-switching characteristic evaluation are then conducted for each pattern. Among these, four patterns are selected as feasible operating patterns due to their excellent soft-switching performance and their ability to achieve zero circulating current on the LV-side. The ETDA is also used to derive the steady-state voltage gain characteristics and the boundary conditions for each pattern. Building on this analysis, a bidirectional control scheme that integrates the ESVOT modulation with a two-dimensional look-up table (2D-LUT) is developed. This scheme utilizes a 2D-LUT-based feedforward controller to provide the feedforward values for the switching frequency, phase-shift angle, and LV-side switch on-time, thereby ensuring zero circulating current on the transformer's LV-side in the steady state. Concurrently, a closed-loop controller adjusts the switching frequency to regulate the power.
The proposed ESVOT modulation and bidirectional control scheme has been validated on a 2.5 kW prototype. The 2D-LUT based feedforward controller requires a minimum of just 10 kB of memory, offering advantages such as simple implementation and low hardware requirements. Dynamic experimental results demonstrate that the prototype can complete smooth and rapid transitions between forward and reverse operation within 8 ms. Furthermore, steady-state experimental results indicate that, compared to the conventional modulation, the ESVOT modulation successfully eliminates the circulating current on the transformer's LV-side. This boosts the prototype's efficiency under forward and reverse light-load conditions by up to 5.8% and 4.5%, respectively. Finally, the prototype achieved a peak efficiency of 98.67% and a power density of 46 W/in³, while also exhibiting perfectly symmetrical forward and reverse operating characteristics.

Key words： Bidirectional LLC converter modulation control scheme zero circulating current

Received: 08 July 2025

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TM46

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	Li Yong
	Bao Junyang
	Liu Bangyin
	Duan Shanxu

Cite this article:

Li Yong,Bao Junyang,Liu Bangyin等. Edge-Shifted Variable On-Time Modulation for Bidirectional LLC Converter with Zero Circulating Current on the Low-Voltage Side[J]. Transactions of China Electrotechnical Society, 0, (): 20251200-20251200.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.251200 OR https://dgjsxb.ces-transaction.com/EN/Y0/V/I/20251200

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