Fast startup control circuit for switch mode power supply

A control circuit for a switch mode power supply (SMPS), the control circuit comprising:

• a power switch for coupling to a primary winding of the power supply;
• a startup resistor coupled to an external input voltage and also coupled to a control terminal of the power switch;
• and
• a controller;
• wherein:
• during a startup phase, the controller is configured to cause the power switch to amplify a startup current from the external input voltage through the startup resistor and provide a startup power to the controller;
• and
• during a normal operating phase, the controller is configured to provide a power switch control signal to turn on and off the power switch for controlling a current flow in the primary winding and regulating an output of the power supply.

A switch mode power supply (SMPS), the power supply comprising:

• a transformer having a primary winding, and a secondary winding, the primary winding for coupling to an external input voltage, the secondary winding for providing an output of the power supply to a load;
• a power switch for coupling to the primary winding of the power supply;
• and
• a startup resistor coupled to the external input voltage, the startup resistor also being coupled to a control terminal of the power switch.