design power supply -- flyback based converter 220Vac/50Hz or to a 12Vdc, 5V, 4V and 3.3V

Thread Starter

hwgeek1221

Joined Nov 21, 2015
16
Hello, 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
 

dendad

Joined Feb 20, 2016
3,025
What about making your power supply operate on a 12V battery as the main source. Then have a battery volts to 12V, 5V......
Now the mains part is a battery charger. No special switching required. It is an UPS so you will not loose power.
The battery could also be charged from a solar panel.
 

Dodgydave

Joined Jun 22, 2012
8,409
Looking at the datasheet the ic has a single output and uses a Primary Mirror winding to monitor the output voltage, if you want multiple outputs that means more turns on the pulse transformer, and separate diodes and smoothing caps, ideally at the maximum output current capacity.

I would go with the 12V and 5V outputs as per page 42 of the Datasheet..

There maybe one psu already made for this design you can get.
 
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Thread Starter

hwgeek1221

Joined Nov 21, 2015
16
Dear @Dodgydave, thank you for your response. Seperate 12 V winding and 5V winding would work perfectly for my design as described on page 42 however, as I stated in my initial post, I sometimes have to supply 12VDC to my PSU instead of 220Vac. How would that be covered in the design of page 42? And if let's say I used 12V and 5V windings, where would you think is the best place to derive the 4Vdc from (keep in mind I have 2A peaks and 0.5A nominal on the 4V)? Should it be from the 12V along with a buck? Should it be from the 5V along with an LDO? Or should i have a seperate winding from the transformer? And can you please take a look at my question #2 of my initial post?

Thank you very much!
 

Dodgydave

Joined Jun 22, 2012
8,409
Dear @Dodgydave, thank you for your response. Seperate 12 V winding and 5V winding would work perfectly for my design as described on page 42 however, as I stated in my initial post, I sometimes have to supply 12VDC to my PSU instead of 220Vac. How would that be covered in the design of page 42? And if let's say I used 12V and 5V windings, where would you think is the best place to derive the 4Vdc from (keep in mind I have 2A peaks and 0.5A nominal on the 4V)? Should it be from the 12V along with a buck? Should it be from the 5V along with an LDO? Or should i have a seperate winding from the transformer? And can you please take a look at my question #2 of my initial post?

Thank you very much!
You can have multiple outputs, so you can have all four voltages if you wanted, this would save time on buck regulators or Ldo types. If you want just the 12V and 5V outputs, put the 4 and 3V regulators on the the 5V rail.

As for separate grounds it's upto you if they don't need to be connected together to share a common ground.
 

Thread Starter

hwgeek1221

Joined Nov 21, 2015
16
Dear @Dodgydave, I can't imagine how the circuitry would work with multiple output windings on the transformer when 12V supply is used instead of 220VAC. The flyback is designed to work in universal offline mode. Attached is a simplifed drawing of how I see is the only possible way of running the circuitry if I were to supply my PSU with 12VDC and have the other outputs generating the proper voltages. How would you have done it if you used multiple outputs on the transformer?

Thank you
 

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Dodgydave

Joined Jun 22, 2012
8,409
That won't work, you have to use mains input to get the multiple outputs from the transformer.

If your using a pc psu, then you already have 12V, 5V, 3.3V available, you only need to drop the 5V to 4 V.
 

Thread Starter

hwgeek1221

Joined Nov 21, 2015
16
That won't work, you have to use mains input to get the multiple outputs from the transformer.

If your using a pc psu, then you already have 12V, 5V, 3.3V available, you only need to drop the 5V to 4 V.
Dear @Dodgydave I know that supplying 12Vdc instead of 220Vac won't work that's why I was connecting the 12VDC from the external PSU to the 12V output. And no I'm not using a pc PSU I only have 12V and 220VAC. How do you think it is possible to run the circuitry from the external 12VDC and at the same time having multiple windings on the transformers (12V widning and 5V winding)?

Thank you
 

dendad

Joined Feb 20, 2016
3,025
Dear @dendad I don't want a charger as the 12Vdc is not a battery. I don't think your solutions are feasible for my project. Thank you!
The battery does not really need to be there. make the charger a 240V to 12V supply.
Then have all the other supplies running of the 12V. And the 12V can be from the mains OR your other 12V source. There is no need to make a complicated supply. Individual regulators for each voltage can run off the 12V.
 
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