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I have to create a power supply that has both a 5V output at 1A and a 3.3V output at 0.4A. I was wondering if I should add another winding to my transformer or just make a 5V 1.5A power supply, and then use a small buck converter to drop down the voltage to 3.3V.

I would think using a buck converter is both cheaper and easier, however I am a little worried about the noise adding a high frequency switching component will induce to the circuit.

So I would like to know, which option is cheaper, and will noise become a significant issue if I go with a buck converter?

Additional information: Flyback topology, switched mode power supply.

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  • \$\begingroup\$ Unless you have an unusual application you should probably not be building a linear power supply (eg with a mains frequency transformer) today at all... \$\endgroup\$ Oct 20 '20 at 16:18
  • \$\begingroup\$ What's your requirement on cross regulation? \$\endgroup\$
    – winny
    Oct 20 '20 at 18:15
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In this answer I'm assuming that the basic circuit is a flyback converter that regulates to produce a 5 volt DC output. A regular old-fashioned power transformer cannot be relied upon to produce a stable 5 volt output due to the transformer's input AC voltage changing and because of its inability to provide decent regulation against load current changes.

The cheaper option is to have an extra winding, rectifier and smoothing capacitor but only by a few cents I suspect. You have to offset the extra few cents you might pay for a buck converter with the extra performance in terms of regulation you get from it when sharing one winding.

This is because only one transformer winding (in a flyback design presumably) can be regulated and the others follow suite. You might find that a heavy load on the 5 volts will increase the DC voltage on the rectified outputs of other windings and this may be a step too far - of course with one winding on the flyback transformer you can use the PWM to regulate it to 5 volts and, if you tee-off to another regulator there's no worries about that producing a stable output because it is a regulator.

It's unlikely that an extra buck converter on the 5 volt DC output from a flyback circuit will cause much of a problem but, I'm generalizing and the devil is in the detail so I can't guarantee this.

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  • \$\begingroup\$ Thank you so much Andy, your answer is well received. You are correct in your assumption that it is coming from a flyback converter. \$\endgroup\$
    – willieb3
    Oct 20 '20 at 17:35
  • \$\begingroup\$ @willieb3 I've rejected your edit suggestion (please don't be sad LOL) because I can't see that it brings anything to the party. \$\endgroup\$
    – Andy aka
    Oct 20 '20 at 18:02
  • \$\begingroup\$ No problem! I was trying to edit my own post and had not realized I accidentally selected yours. \$\endgroup\$
    – willieb3
    Oct 20 '20 at 18:05

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