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I'm designing an AC-DC based on the UCC28780 active clamp flyback controller Schematic.

I have a question about the primary auxiliary winding that is supposed to power the UCC28780 and the gate driver. My question is, if at instant t0, my controller and my driver are not powered, that means that i have no current flowing across the primary winding. The primary auxiliary winding is nothing but a secondary winding without isolation. So how is my this auxiliary winding is supposed to power my controllers if there is no power flowing through the primary ( because the IC is off, hence the flyback main switch is off).

Thank you in advance for you answers.

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2 Answers 2

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If you check the UCC28780 datasheet, the SWS pin on page four reads: "This sensing input is used to monitor the switch-node voltage as it nears zero volts in normal operation. During start-up, this pin is connected to the VDD pin internally to allow the high voltage sensing network to provide start-up current."

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The clue is in your picture: -

enter image description here

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