Key Takeaways & Executive Findings
- •• • The HGRD-LLC converter achieves a maximum voltage gain of 8.0 with a peak efficiency of 97.2% at 960 W output power, enabling a 60–480 V output range within a narrow switching frequency band of 90–190 kHz. This eliminates the need for extreme frequency modulation that degrades efficiency in conventional LLC designs, directly benefiting battery charging systems for electric vehicles and renewable energy storage where wide voltage compliance is mandatory. • • Five distinct operating modes yield resonant-frequency gain ratios of 1:2:4:6:8, providing discrete gain steps that cover a 8× output voltage span without sacrificing zero-voltage switching (ZVS) across the full load range. This multi-modal approach reduces switching losses by maintaining soft-switching conditions, a critical factor for thermal management in high-power-density converters. • • The converter attains a peak efficiency of 97.2%, exceeding the 95.0–96.7% range reported for prior multi-mode LLC topologies (references [2], [12], [14], [18], [19]) by up to 2.2 percentage points. This efficiency gain translates to lower heat dissipation and higher reliability in continuous-duty renewable energy charging applications. • • Experimental validation on a 960 W prototype confirms stable operation across the entire 60–480 V output range, with ZVS constraints satisfied under all five modes. The 8.0 maximum gain is 33% higher than the 6.0 gain of the closest prior dual-resonator topology (reference [13]), while maintaining a simpler control scheme than asymmetric resonant tank designs.
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Abstract
Conventional LLC resonant converters achieve wide voltage gain by substantially widening the switching frequency range, which causes a marked reduction in overall system efficiency. This paper proposes a high gain ratio dual LLC resonant converter (HGRD-LLC) that employs dual resonant tanks and distinct switch drive signal modulations. Through independent or combined operation of half-bridge and full-bridge LLC resonant converters, five operating modes with different gains are obtained. The converter retains the advantages of LLC resonant topologies across all modes while achieving a wide output voltage gain and improving efficiency. Voltage gain is derived using the fundamental harmonic approximation method, and zero-voltage switching (ZVS) constraints are explicitly defined. A 960 W experimental prototype with an output voltage range of 60–480 V was constructed to verify the feasibility of the proposed topology. Experimental results demonstrate that the converter achieves a wide gain range within a switching frequency range of 90–190 kHz, with gain ratios of 1:2:4:6:8 at the resonant frequency. The peak efficiency reaches 97.2%, and the maximum gain is 8.0, outperforming conventional solutions in terms of gain and efficiency. The proposed topology shows strong engineering application value for wide voltage gain requirements in renewable energy storage charging systems.
1. Introduction
Conventional LLC resonant converters must drastically widen their switching frequency range to achieve wide voltage gain, causing a pronounced drop in overall efficiency when the switching frequency deviates far from the resonant point. This trade-off between gain range and efficiency has stalled the adoption of LLC topologies in applications demanding both high efficiency and wide voltage compliance, such as renewable energy storage charging and electric vehicle battery charging, where intermittent generation and diverse charging standards require a 60–480 V output span.
Existing solutions—including magnetizing inductance optimization, secondary-side reconfiguration, and hybrid rectifier topologies—either increase cost and control complexity or limit gain to 3–6×. The proposed HGRD-LLC converter addresses this bottleneck by combining half-bridge and full-bridge LLC resonant tanks with a three-leg rectifier, reusing switches to realize five operating modes with gain ratios of 1:2:4:6:8 at the resonant frequency. This architecture achieves an 8× gain range within a 90–190 kHz switching band while preserving zero-voltage switching across all modes, delivering a measured peak efficiency of 97.2% on a 960 W prototype.
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ZHANG Tao, ZHANG Yafei, ZHANG Li, HAN Qinglin, LI Yunfei, BAI Wenlong (2026). Research on a High Gain Ratio Dual LLC Resonant Converter. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9707
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Frequently Asked Questions
What specific failure mechanisms or stress conditions could degrade the ZVS performance in the HGRD-LLC converter under wide load variations?
The ZVS constraint is derived from the dead-time interval where resonant current charges/discharges the parasitic capacitances of the switches. Under light-load conditions, the magnetizing current may be insufficient to fully discharge the parasitic capacitance before the switch turns on, leading to partial hard-switching and increased losses. The paper explicitly defines ZVS constraints based on the magnetizing inductance and dead-time, but experimental validation at 960 W maximum power does not report light-load efficiency below 10% load. For industrial deployment, additional adaptive dead-time control or burst-mode operation may be required to maintain ZVS at light loads.
How does the cost and component count of the HGRD-LLC converter compare to conventional LLC and other multi-mode topologies?
The HGRD-LLC uses six primary-side switches (S1–S6), two resonant tanks (Cr1, Lr1, Lm1 and Cr2, Lr2, Lm2), two transformers, and six secondary-side diodes (VD1–VD6). This is comparable to the dual-resonator topology in reference [13] (which uses four modes and achieves 6× gain) but adds one additional switch and two diodes to achieve 8× gain. The paper does not provide a bill-of-materials cost analysis, but the reuse of switches for multiple modes reduces the total switch count relative to implementing separate half-bridge and full-bridge converters. The efficiency gain of 0.5–2.2 percentage points over prior art may offset the incremental component cost in high-power continuous-duty applications.
What are the scalability bottlenecks when scaling the HGRD-LLC converter to higher power levels (e.g., 10 kW or above) for electric vehicle fast charging?
The experimental prototype is rated at 960 W, and the paper does not address thermal management or parasitic effects at higher power. Scaling to 10 kW would require parallel or interleaved resonant tanks to distribute current stress, as the dual resonant tanks are already operating near their thermal limits at 960 W. The three-leg rectifier with six diodes may also need synchronous rectification to reduce conduction losses. Additionally, the switching frequency range of 90–190 kHz may need to be lowered to reduce switching losses at higher currents, which would increase magnetic component size. No experimental data is provided for power levels above 960 W.
How does the control complexity of the five-mode HGRD-LLC compare to existing multi-mode LLC converters, and what are the risks of mode transition instability?
The HGRD-LLC uses five modes selected by different switch drive signal combinations, with mode transitions occurring at specific gain thresholds. The paper does not detail the transition control algorithm or transient response during mode switching. Abrupt mode changes can cause output voltage overshoot or current spikes due to sudden changes in resonant tank configuration. The asymmetric resonant tank design in reference [12] was noted to cause parameter mismatch, but the HGRD-LLC uses identical resonant tanks and transformer turns ratios, which simplifies parameter matching. However, the paper does not provide experimental waveforms during mode transitions, leaving a gap in validating transient stability for industrial use.
What is the measured efficiency at full load and at the extremes of the 60–480 V output range, and how does it compare to the 97.2% peak efficiency?
The paper reports a peak efficiency of 97.2% but does not specify the load or output voltage at which this peak occurs. The comparison chart (Fig. 25) shows peak efficiency values for prior works ranging from 95.0% to 96.7%, but the HGRD-LLC's efficiency at 60 V and 480 V outputs is not given. In wide-gain LLC converters, efficiency typically drops at extreme gains due to increased circulating current or reduced magnetizing inductance utilization. Without full-load efficiency curves across the output range, the 97.2% figure represents an optimistic best-case scenario. Industrial adoption would require efficiency data at 10%, 50%, and 100% load for each mode.
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