US Patent 7997364 Hybrid vehicle and control method of hybrid vehicle

On condition that a motor temperature is lower than a preset reference temperature and that a vehicle speed is lower than a preset reference speed, a booster circuit is controlled to adjust the voltage level of a high voltage system to be not higher than a certain low voltage. On condition that the motor temperature is lower than the preset reference temperature and that the vehicle speed is not lower than the preset reference speed, the booster circuit is controlled to adjust the voltage level of the high voltage system to be not higher than a maximum input voltage. On condition that the motor temperature is not lower than the preset reference temperature, the booster circuit is controlled to adjust the voltage level of the high voltage system to be not higher than the maximum input voltage, irrespective of the vehicle speed. This arrangement effectively improves the overall energy efficiency in a vehicle where the adjustment of the voltage level of the high voltage system to be not higher than the certain low voltage has the higher energy efficiency than the adjustment of the voltage level of the high voltage system to be not higher than the maximum input voltage.