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High Efficiency CLL Resonant Converter with Synchronous Rectifier

Fig. 1. Circuit of LLC resonant converter.
The CLL resonant dc-dc converter (CLL) is an alternative option in off-line applications. This topology can achieve zero-voltage-switching (ZVS) from zero load to full load range, low circulating energy and zero-current-switching (ZCS) for output rectifiers.

The air gap of the CLL resonant converter transformer is omitted to eliminate fringing effect. Because of very high magnetizing inductance, the transformer primary side current is in phase with the secondary side current. Thus, SR can be easily driven. Moreover, resonant inductance is integrated with the transformer to reduce the passive component volume.

Fig. 2. Circuit of CLL resonant converter.
Fig. 3. LLC transformer with gap
(severe fringing effect).
Fig. 4. CLL transformer without gap
(little fringing effect).

Fig. 5. Primary and Secondary current of LLC transformer (out of phase).
Fig. 6. Waveforms of CLL current driven SR.
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