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Let's consider an isolated switching power supply that

  • takes an 85 - 265 Vac input
  • converts it to 400 Vdc using a rectifier and PFC boost converter
  • converts 400 Vdc to a 50 Vdc output using a forward converter

How would the regulation ICs and gate drivers of such a supply be powered? You can't use the existing forward converter, since the only thing that would be present without the control circuitry being powered is a voltage of \$V_{in} \sqrt{2}\$ on the 400Vdc rail.

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2 Answers 2

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Typically, there is a bootstrap power supply, that might look something like this

schematic

simulate this circuit – Schematic created using CircuitLab

The key elements are

  1. a "capacitor dropper" (i.e. C1) is used to lower the voltage to something reasonable

  2. a Zener diode is used as a shunt regulator

Since the power requirements of the bootstrap supply are minimal, this circuit is adequate.

The Zener diode, and possibly other components of the bootstrap supply, may be part of an integrated PSU circuit.

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    \$\begingroup\$ It might even disable the inefficient bootstrap after everything is powered up and switch over to a more efficient switching supply for the logic and control. \$\endgroup\$
    – DKNguyen
    May 2, 2022 at 21:20
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    \$\begingroup\$ Usually just a resistor is used, not a capacitive dropper. A high value resistor charges slowly a supply filter capacitor, and when there is enough voltage, the switch mode power supply chip starts working, and transformer has one secondary for powering mains side circuits. \$\endgroup\$
    – Justme
    May 2, 2022 at 21:29
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    \$\begingroup\$ Regardless of resistor or capacitor, it should be mentioned that this component must have a safe rating to withstand peak mains voltage as well as potential mains overvoltage scenarios. And that it fails "open". \$\endgroup\$
    – tobalt
    May 3, 2022 at 10:17
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Once the SMPS is running there is lots of Switching frequency AC on the transformers and the inductors. Small overwinds sometimes called Aux windings can power low level control circuitry via simple diode rectifiers. If the main control chip has an undervolt lockout where the current consumption is very low when shut down then the system can be started with a high value resistor that is say 220K and does not waste much power. On this scheme the chip usually has a modest say 100 microfarad eltec cap and protective zener across its supply pin.

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