Patent ID: 7944190

Claim:
A control circuit of power supply unit for controlling output voltage with electric power supplied from an external power supply, comprising: a monitoring portion monitoring current and voltage outputted from the external power supply and the output power of the external power supply; and a setting portion setting the upper limit value of current outputted from the external power supply based on a result of monitoring by the monitoring portion, wherein the monitoring portion includes: a voltage detecting portion detecting that voltage value outputted from the external power supply is smaller than a lower limit value; a current detecting portion detecting that current value outputted from the external power supply is limited by the upper limit value; an upper limit value changing portion changing the upper limit value set by the setting portion based on a detection result of the voltage detecting portion or the current detecting portion; an electric power calculating portion calculating a first electric power from current value and voltage value outputted from the external power supply based on the upper limit value set by the setting portion or a second electric power from current value and voltage value outputted from the external power supply based on the upper limit value changed by the upper limit value changing portion; an electric power comparing portion determining which is larger the first electric power or the second electric power calculated by the electric power calculating portion; and an adjusting portion, in accordance with a result of determination by the electric power comparing portion, replacing the upper limit value changed by the upper limit value changing portion with an adjustment upper limit value different from the upper limit value.