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Best and cheapest configuration for PSU

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VSef

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Hello, I need to design a PSU which is connected to the mains 50Hz 220Vrms.
I MUST design a flyback with 24V output and an aux for the flyback IC. I have this part figured out.

However, I need help in the remaining part. I need a 5V @100mA max, 4V for radio IC that works on 0.5A continuous and 2A peak and I need 3.3V for microcontroller. What is the cheapest way to derive these voltages keeping in mind the drawn currents. I was thinking of using 2 buck converters to derive 5V from 24V and the other buck to derive 4V from 24V. The LDO will be used to derive 3.3V from the 5V. Do you think this is the cheapest and most effective way to design such a PSU? What are your thoughts? Please help.

Thank you very much
 

Sounds reasonable, though RFI from the SMPS may be problematic for the radio IC. If you can tolerate some extra power dissipation, I would use a LDO for the 5V rail, and derive the 3.3V from the 4V rail.
 

radio IC that works on 0.5A continuous and 2A peak

When it switches on and off, the change could briefly affect the other supply levels. It's a factor you should take into account. Possibly this supply should be designed so it neither affects nor derives from other supplies.
 

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