Voltage-reduction-type switch power supply and control circuit thereof

The invention discloses a voltage-reduction-type switch power supply and a control circuit thereof. The voltage-reduction-type switch power supply comprises a voltage-reduction-type basic circuit with inductance. The base circuit has a constant voltage working mode and a constant current working mode; under the constant voltage working mode, during inductance discharging, according to output voltage sampling values of the voltage-reduction-type basic circuit, a conducting state of a power switch tube is controlled, and output voltages of the voltage-reduction-type basic circuit are enabled to keep constant in a first load range; under the constant current working mode, after inductance discharging current reaches zero, the conducting state of the power switch tube is controlled, in addition, during inductance charging, inductance charging currents are detected, according to input voltage sampling values and charging currents of the inductance, a turn-off state of the power switch tube is controlled, so that t