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I am trying to design an AC-DC converter using the SM7035P

The pinout is enter image description here

However I am confused with the pins 7,8 as in the reference design they don't connect to ground directly -

enter image description here (Where DC+ is considered GND)

Is this an error in the reference design or am I reading it wrong? Please help.

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  • \$\begingroup\$ Everything is ok, the GND pins are connected to the internal MOSFET Source terminal. \$\endgroup\$
    – G36
    Oct 8, 2021 at 20:52
  • \$\begingroup\$ They call it "chip ground" not "ground " on page 2. \$\endgroup\$
    – Spehro Pefhany
    Oct 8, 2021 at 21:29
  • \$\begingroup\$ @SpehroPefhany They should have called it "Source" or maybe even Vss. GND just confuses people. \$\endgroup\$
    – Edin Fifić
    Oct 8, 2021 at 22:08

1 Answer 1

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It's intentional, this is a known design for an off-line switcher with buck topology.

The internal supply of the chip charges the capacitor on the VDD pin with the voltage differential between the drain and GND (source) pin when the internal MOSFET switch is off. The capacitor will then continue to power the circuit long enough when the MOSFET is switched on and the voltage differential is very low.

Similar parts are for example the UCC28880 or the LNK30X family, with more details about the different topologies in English.

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