A Wide Range LLC Resonant Converter Realized by an Adjustable Turns Ratio Transformer

A transformer winding method that can realize an adjustable turns ratio is proposed. In this article, the LLC resonant converter is taken as an example, where the full-bridge (FB) LLC converter is realized by two split half-bridge (HB) units in parallel. The numbers of the switch and diode are still the same as those of a regular FB LLC converter. By controlling the phase shift angle of the two HB units, the magnetic flux of the central column of the transformer core can be adjusted, so the turns ratio can be changed. In any operating condition, the two HB units always share the same power stress, which is only half of the total. A 500 W prototype with an input voltage range of 160 similar to 480 V and an output voltage of 120 V is built, which shows a frequency range of 67-95 kHz covering a wide load range from 5% to 100% rated power.