Challenge

Beyond Typical LLC Design Challenges in 500W Converters

With a wide operating range, the LLC magnetics could be considered as one of the most complex magnetics to be designed. Its operation at low frequency and high voltage and at high frequency and low voltage involve having two very different operating conditions. The first one highly affects the core losses while the second one has a big impact on the winding losses. On top of this the series resonant inductance and the magnetizing inductance need to be perfectly controlled to ensure the proper control of the converter.

The 500W LLC design did not only have the typical challenges of the LLC converters but also the fact that a center tapped winding was needed. Such a winding could suffer from unbalanced current sharing if it is not properly designed. Furthermore, the design should fit in a maximum space of 40 mm of width and 25 mm of height with natural cooling.

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Solution

Three-Core Structure: Balancing Size and Efficiency in LLC Design

The LLC needs an inductor in series with a transformer. Three approaches can be followed with the magnetic components:

1. Use two separate components.

2. Use a single transformer with poor coupling and high leakage inductance.

3. Use a mergence where the inductor is placed on top of the transformer.

High leakage transformers could exhibit the smallest size but could compromise the efficiency target because poor coupling and large AC winding losses would be required to meet the leakage inductance requirement. Having this in mind a three-core structure was used to minimize the size while maximizing the efficiency of the magnetics. The series inductance was perfectly controlled with the inductor gap and the transformer losses were minimized with an interleaved winding arrangement. The windings were disposed in a way to ensure flux cancellation in the shared core leg between the transformer and the inductor.

Results

A 500W PSU with over 96% efficiency, optimized for 5G telecom applications.

With the support of Frenetic, Infineon was able to design an ultra-compact, fanless power supply that met the demanding specifications of 5G small-cell applications. The final PSU design achieved over 96.5% efficiency at 230V AC, and 95.6% at 115V AC, with only 27 mm of height and without fans.

IDEAL APPLICATIONS: TELECOM, EDGE COMPUTING, SERVER, SMPS.