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Power supply design decision

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electrohawk

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I'm trying to design a flyback based converter. The converter I'm trying to make connects either to the utility (50Hz 220Vac) or to a 12Vdc supply (either ac or 12Vdc but not both) and has 12V, 5V, 4V and 3.3V dc outputs.

- My 12 V has light load (60mA max) and is used to power analog sensors.
- My 5 V drives an opamp that interfaces the sensors to the MCU along with power for my character LCD.
- My 3.3V is used to drive my MCU along with light digital loads.
- My 4 Vdc has heavy load with nominal current at 0.5A and peaks of 2A.

I am really confused of how to realize such a converter. My design is a flyback based power supply based on TI's UCC28911. Initially I was thinking of having 2 output windings on the flyback's transformer. One for my 12Vdc and the other for 5Vdc. My 4Vdc and 3.3Vdc would have been derived by 2 separate LDOs. However (this is my personal reasoning and might be wrong), coming up with such a setting would not permit connecting a 12Vdc source (instead of 220Vac 50Hz) to my supply to generate the rest of the voltage. The only permissible way is to have 1 output winding of 12Vdc and deriving 5, 4 and 3.3Vdc by LDOs (for the 3.3V and 4V) and normal linear regulators (for the 5V). I think in such way it is possible to connect the 12Vdc to power up my system. However in doing so I will not have isolation between my analog sensor ground and the other grounds of my circuitry. Now here comes my questions:

1- What do you think is the best way to solve my problem? Should I have 1 output winding on my flyback transof 12Vdc from my transformer and derive the rest from it?
2- This project brought me up to my 2nd question which is when should we have separate and isolated grounds from each other? I know that galvanic isolation ensures that our circuitry is disconnected from the mains protecting the circuit from failures but what about the isolation of the grounds from each other?

Thank you for your help! I would appreciate your feedback
 

To have a 12 V DC supply option, the single 12V output converter is the way to go.

The 4V node (I guess for a GSM radio) would typically use a switch mode buck converter for acceptable efficiency.

If you really need isolation for the sensor circuit, you would use a separate DC/DC converter for its supply. But the isolation is useless without analog and/or digital isolators for the sensor signal. I guess you don't even have a remote idea how to achieve it with reasonable effort and performance.

If not enforced by special application requirements, forget about the isolation. Otherwise explain the requirements in detail.
 

Thank you FvM.

Your guess is correct 4V is for radio. No i don't have special requirements for isolation but i assume i need to design an optoisolator circuitry to have the isolation job done in addition to separate DC output with a separate ground.

I also have another question. What should be my power supply's rating in Watts if my radio IC draws 0.5A in continuous mode and 2A peak during transmission? Should my design assume 2W for the radio part or should it have a higher rating?

I am also having difficulties in designing the flyback's snubber because there are so many different types (RCD only, Zener + diode in series and RCD + Zener) I don't know how to choose which type. It is my first time designing a flyback converter.

Thank you
 

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