Novel modulation method of a three-level isolated full-bridge LLC resonant DC-DC converter for wide-output voltage application

This paper presents a three-level isolated LLC resonant converter using a novel modulation strategy for wide-voltage range operation. With the proposed modulation strategy, the DC gain of the converter can be changed between V
_out
and ½V
_out
. The output voltage can be then adjusted within the two resulting voltage ranges by either varying the switching frequency of the converter or utilizing a post regulator. In the last option, the three-level converter works as an electronic transformer with variable turns ratio. Therefore, the converter can be designed to operate with a narrow operation condition maintaining high efficiency despite of wide-voltage range variations. A hybrid gate driver which simplifies the implementation of the proposed modulation method is also presented. The novel modulation method is analysed and verified on a 20kW, LLC three-level resonant prototype operating at 110 kHz.