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I am trying to create a PCB that can operate on 5, 12, or 24 VDC. The PCB will need a 5 V rail for the digital signal to the LED lights and a 3.3 V rail for the MCU.

I would like to make a cost-effective way to accept VIN as 5 V and automatically bypass the DC-DC step-down converter with a P-channel MOSFET. I have no problem with the DC-DC converter that will not be enabled until the threshold is met, so if 12 or 24 V is applied, the converter will step it down to the 5 V rail.

From my research so far I have decided to use an LM358 as it can operate on 24 V DC. It will be low if the voltage is less than 6 V DC and thereby bypass the DC-DC converter and the 5 V rail will be powered by the external power supply.

My main question is: will there be an issue with the MOSFET’s Vgs as the limit is 20 V, will Zener D2 actually do anything to reduce the gate voltage when the gate is pulled low by the LM358? Or do I need another Zener in the circuit somewhere? And is D3 even necessary?

My circuit diagram

ocrdu
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Jimmy James
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  • Nice try but this is not a buck regulator, or even a regulator. Keep reading. – Tony Stewart EE75 Aug 29 '21 at 07:48
  • Good grief you are making life complicated for yourself. Why not an off the shelf integrated buck converter? – winny Aug 29 '21 at 10:32
  • How much load current do you need on the 5V output? On the 3.3V output? A buck-boost converter should solve the problem nicely and will be efficient! Normally, a simple buck converter would do, but you need to go 5V to 5V. – Randy Nuss Aug 30 '21 at 18:33

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This design is a reflection of your starting to learn to make a linear regulator. Follow the design books and webpages to show how series-pass designs should be made. Keep reading please. If you want to make a Buck regulator , search for it after you define your power requirements.

Some comments are marked up for your consideration.

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Tony Stewart EE75
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