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Why are there no isolated AC/DC buck converters around? For applications < 10W (say 5V/1A output) and mains input (85 - 264VC), is it feasible to use a standard buck circuit but add a 1:1 isolation transformer at the input?

This way you could enjoy the benefits of a buck topology while also having the benefit of isolation.

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There are lots of buck-derived isolated topologies (Google it). Nothing stops you from using a buck after an isolation transformer, but then the isolation transformer is 50 or 60 Hz (i.e. big). The low duty cycle might be an issue as well if you insist on a 1:1 transformer. A buck-derived isolated topology can use a high frequency transformer and will be a much smaller solution. In the example you provide a flyback might be a better solution.

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Why are there no isolated AC/DC buck converters around?

A buck regulator does not isolate. Most low power isolating switching regulators use a flyback circuit. The flyback circuit isolates because it uses a transformer. A buck circuit does not use a transformer. If it did it's name would change from "buck" to "fly back".

It's not economically feasible to add a transformer in the way you say.

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  • \$\begingroup\$ Can you elaborate on the economic feasibility? \$\endgroup\$ – peter.jn Mar 30 '16 at 14:51
  • \$\begingroup\$ A forward or transformer isolated full-bridge are often called "isolated buck" topologies, but I think they are more appropriately called buck-derived. They have no RHP zero in the transfer function like a flyback (e.g. Fig 9): web.eecs.utk.edu/~dcostine/ECE482/Spring2016/materials/… However, for the example given a flyback would be the most appropriate topology. The flyback in this case would use a tiny transformer that would be much cheaper than the big 50/60 Hz transformer you would need to provide isolation in front on a conventional buck. \$\endgroup\$ – John D Mar 30 '16 at 15:01

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