PSpice Application Notes

Designing for Efficiency 8 www.cadence.com 7. Transformer Design Optimization In the reference converter shown in Figure 1, transformer XFMRAUX forms the electrical boundary between the push-pull switching stage and the secondary rectifier network. As semiconductor switching losses decrease with SiC devices, the transformer often becomes the dominant remaining source of inefficiency. In many high-frequency converters, magnetics determine whether the theoretical efficiency improvement of wide bandgap devices is realized in practice. Transformer design is therefore not an isolated magnetic problem — it directly interacts with switching behavior, EMI perfor- mance, and thermal limits. The following subsections describe the key design considerations and how they influence overall converter performance. 7.1 Core Loss Optimization Transformer core loss arises from hysteresis and eddy current effects in the magnetic material. These losses increase rapidly with switching frequency and flux density. Core loss is commonly approximated by the Steinmetz relationship: This relationship highlights an important design tradeoff: increasing switching frequency reduces passive component size but raises core loss nonlinearly. In practical designs, engineers select a flux density that balances efficiency and size. Operating too close to saturation increases temperature and reduces long-term reliability. Wide bandgap devices enable higher switching frequency, but the transformer must be designed so that increased frequency does not negate semiconductor efficiency gains. 7.2 Avoiding Core Saturation Push-pull converters rely on symmetrical drive to prevent DC bias in the transformer. Even small timing imbalances introduce flux offset that can accumulate over time. The peak flux swing is approximately: This relationship shows that higher input voltage or longer on time increases magnetic stress. Designers maintain margin below saturation by selecting sufficient primary turns and appropriate core material. Saturation is particularly dangerous because it causes rapid current rise, which increases switching stress and can damage semiconductors. Accurate control timing and conservative magnetic design prevent this failure mode. 7.3 Copper Loss and High-Frequency Effects Winding loss is proportional to the AC resistance of the conductor: At high frequency, current concentrates near the conductor surface due to skin effect. The effective penetration depth is:

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