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I'm currently developing a project, where I have to design SMPS using a digital controller.

The system is supposed to supply the microcontroller during operation. But the microcontroller still needs a supply to start up the system. I was told there were methods to supply the microcontroller during start up, which later gets turned off when the SMPS is fully operational.

Right now I am having a hard time finding any sources or applications notes that describes these methods.

Edit 1:

The microcontroller used is STM32l4R5-p with an input range of 1.71 to 3.6 V_DD when using internal LDO. But the whole SMPS project has an input range of 115 to 230V RMS ±10%, which I know is a bit of a challenge.

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  • \$\begingroup\$ What is the voltage range of your controller? What is your power source? You could start at a lower voltage using an LDO to kick in your higher voltage SMPS (which intrinsically shuts down the LDO by raising the voltage). \$\endgroup\$ Mar 3 '20 at 14:44
  • \$\begingroup\$ Draw a block diagram and label all the voltages and currents. I see this because your words are ambiguous and I'm not going down the route of trying to extract meaning so that you can fix your words when a block diagram would be better by miles. \$\endgroup\$
    – Andy aka
    Mar 3 '20 at 15:11
  • \$\begingroup\$ You need any old supply that'll make the microprocessor work, than can be switched over to the main supply once it's up. You probably want to go simple and cheap, because it'll be off most of the time. \$\endgroup\$
    – TimWescott
    Mar 3 '20 at 15:23
  • \$\begingroup\$ @CristobolPolychronopolis, TimWescott: The output voltage cannot "intrinsically shut down" the LDO because these voltages are on the different sides of the main transformer. Unless you add another coil to draw small amount of power for rectifier/chopper side \$\endgroup\$
    – Maple
    Mar 3 '20 at 16:40
  • \$\begingroup\$ And why on Earth was this question marked for close as off-topic? This is as engineering design question as it could possibly be \$\endgroup\$
    – Maple
    Mar 3 '20 at 16:43
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the NCP785A from On Semiconductor might be a solution for you. It is a Linear Regulator with an extremely wide input voltage rage of up to 450V. A simple diode and capacitor at the input is sufficient for rectification of AC. The drawback is: it delivers only 10mA. But if you calculate the amount of energy needed by the micro-controller for the start-up sequence, then you can calculate the necessary value of the output capacitor to hold the voltage during that time. Because the operating rage of the µC is quite large (1.71 to 3.6 V), this should be feasible.

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  • \$\begingroup\$ "only 10mA" is not a drawback in this case, it is almost a requirement. it would defeat the point of building SMPS if its control logic draws too much power. Good find, I did not know such LDOs even existed. \$\endgroup\$
    – Maple
    Mar 3 '20 at 16:53
  • \$\begingroup\$ @maple they require expensive high voltage caps though (when running at 170V peak anyways). I used some for a cheap AC-DC gate drive supply. \$\endgroup\$
    – DKNguyen
    Mar 3 '20 at 17:04
  • \$\begingroup\$ @DKNguyen 2.2uF 400V electrolitic caps are $0.17 in bulk at digikey. \$\endgroup\$
    – Maple
    Mar 3 '20 at 18:09
  • \$\begingroup\$ @Maple Well, I was using ceramic caps. Because electroytics were too tall. \$\endgroup\$
    – DKNguyen
    Mar 3 '20 at 20:59
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You can use an linear regulator. SMPS ICs have one inside to start themselves. Just look at their block diagrams.

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