In Below SMPS Circuit ( Credit to:- NXP SMPS Design Tool ), in Red mark what is function of this resistor and why mid point of register is connected to Aux biasing of transformer.
Thanks in advance.
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Guessing, so not an answer: this discharges any potential left after unplugging before anyone touches the plug. – Simon Richter Apr 09 '15 at 14:14
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The resistors supply start-up current for the SMPS, after which the auxiliary winding of the transformer takes over. More often a single high value resistor (or two in series) are used from the output of the bridge rectifier (C6 + terminal), but this method will work too.
They also act as bleeder resistors for the 330nF filter capacitor so the user won't get shocked if they touch the plug pins after it has been unplugged.
By using two resistors in the shown configuration they make the former function more reliable and the latter less reliable (start-up resistors going open is a common failure mode of off-line switchers).
Not sure if those are the only reasons, but that's what it looks like to me.
Spehro Pefhany
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Understand those resistor for Start current for the Switcher IC. But one more thing here before Bridge the voltage is sine wave. ( Let suppose 240VAC is input, Due to divider network R1 and R2 the net voltage will be 120Vac. This voltage goes to Vcc pin of Switcher IC. There is zerner of 27Volt that will clamp 120V to 27V if it is DC voltage. But in this case we have 120VAC means half of cycle the zener will act as simple diode that mean VCC pin of Switcher have 120V-diode drop = 119.3V. Isn't is bad for switch IC.) – techover QA Apr 10 '15 at 12:34