Soft-switching condition and control method of boost chopper for DC-DC converter utilizing GaN power devices for automobile applications

This paper presents an isolated step-down DCDC converter using GaN power devices for automobile applications. This converter consists of a LLC converter and a boost chopper converter and supplies power from the high voltage of the motor drive battery at 200V to the low voltage of the sub battery at 13.6V for the auxiliary system in a hybrid electric vehicle. Both converters perform the soft switching operation and achieve high conversion efficiency. However, the boost chopper circuit does not succeed in soft switching and has lower efficiency in the high-power range. In addition, the inrush current flow to the GaN power device can damage the device.Thus, we formulated soft switching conditions and proposed a control method that adjusts the switching frequency to expand the soft-switching operation range. The proposed method achieved a high efficiency of 99 % even in at high power and expanded the output range in the experimental test.