Startup circuit and method for ac-dc converters

A start-up circuit for a power converter, the start-up circuit comprising:

• an input node, an output node, and a reference node;
• and
• n cascades coupled in series, where n is a positive integer greater than 1, each of the cascades comprising a depletion mode transistor having a first terminal coupled to the input node, a second terminal and a gate terminal, wherein
• the second terminal of the depletion mode transistor in the first cascade is coupled to the output node,
• the second terminal of the depletion mode transistor in the ith cascade is coupled to the gate terminal of the depletion mode transistor in the (i−1)th cascade, where i is a positive integer from 2 to (n−1), and
• the gate terminal of the depletion mode transistor in the nth cascade is coupled to the reference node.

A method of starting-up a power converter, the method comprising:

• generating an AC rectified voltage upon power-up of the power converter;
• generating, by a plurality of depletion mode transistors coupled in cascade, a start-up voltage from the rectified voltage;
• and
• supplying the start-up voltage to a controller of the power converter to provide power for operation of the controller before an output stage of the power converter starts outputting power.