Switched-mode power supply, power supply circuit thereof, and power supply method

A power supply method of a switched-mode power supply, wherein the switched-mode power supply comprises a first switch transistor, wherein a drain of the first switch transistor receives an input voltage on a direct current input bus of the switched-mode power supply, and a source of the first switch transistor is connected to a reference ground;

• and the power supply method comprises:
• adopting a junction field-effect transistor (JFET), wherein a drain of the JFET receives the input voltage, a gate of the JFET is connected to the reference ground, and a source of the JFET outputs a supply voltage or a supply current;
• and
• during each switch cycle, controlling a drain voltage to be greater than or equal to a first threshold voltage when the first switch transistor is turned on, such that the supply voltage or the supply current satisfies a drive voltage of the first switch transistor and an operating voltage of a to-be-powered circuit of the switched-mode power supply.