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I have been researching SMPS designs as a hobby recently and finally have the courage to build one for theoretical and practical understanding. I have chosen an isolated design looking at several reference designs from TI, Infineon, ST, etc... My questions is about the isolation. Most reference designs are as follows:

AC In -> AC Mains Filter -> AC/DC Rectification -> PFC -> Isolated DC/DC Buck or Boost -> DC Output Voltage

Why do most of these reference designs implement isolation towards the end of the process and not at the beginning like this:

AC In -> AC Mains Filter -> 1:1 Transformer -> AC/DC Rectification -> PFC -> DC/DC Buck or Boost -> DC Output Voltage

My guess is that the PFC and/ or DC-DC stages also require some sort of inductor (transformer) and therefore you can double up the functionality of that component for isolation, saving one item on the BOM, lowering the cost, increasing efficiency, power density and all that good stuff. Is that correct?

Or is it about safety, where the closer the isolation to the output voltage, the better for the input (AC-In) and output stages (DC-Out)?

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    \$\begingroup\$ Your initial guess is correct. The isolation transformer does double duty and saves cost, weight etc. I'm not aware of any circuit advantages of putting isolation closer to the front end vs the output, other than easier probing with a scope. \$\endgroup\$
    – Aaron
    Oct 30, 2023 at 17:34

2 Answers 2

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AC In -> AC Mains Filter -> 1:1 Transformer -> AC/DC Rectification -> PFC -> DC/DC Buck or Boost -> DC Output Voltage

In this approach the 1:1 transformer has to work at the mains frequency (50 or 60 Hz, depending on the country). A relatively low frequency requires a larger and heavier transformer.

In modern designs, mains frequency transformers are only used in linear power supplies when switching noise has to be avoided.

AC In -> AC Mains Filter -> AC/DC Rectification -> PFC -> Isolated DC/DC Buck or Boost -> DC Output Voltage

In this variant the transformer in the isolated DC/DC works at a high frequency (somewhere in the 50 kHz to 500 kHz range). At a higher frequency a transformer can be much smaller for the same power.

edit:

You can compare the dimensions:
100W mains frequency transformer 100 x 60 x 65 mm
100W high frequency forward transformer 20 x 23 x 9 mm

By the way, the size of flyback transformers grows faster with power than forward, push-pull, bridge. That's why we see flybacks mainly under 100W or so.

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    \$\begingroup\$ a more appropriate comparison would be with a forward converter transformer..The flyback one is larger as it has to store energy. \$\endgroup\$
    – tobalt
    Oct 30, 2023 at 18:07
  • \$\begingroup\$ @tobalt Fixed. You're right. Now the example is a 100W forward transformer. \$\endgroup\$
    – misk94555
    Oct 30, 2023 at 18:35
  • \$\begingroup\$ FWIW, I want to start the design at 1kW, but I understand the comparisons. I am in the US so Mains is 110-120Vac and output voltage will be 48Vdc. Will be using my own mcu in the buck stage to learn about that portion of the project, so will make sure to match all components for high frequency switching. Thank you! \$\endgroup\$ Oct 30, 2023 at 19:09
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Let's assume you have a 100 W power supply. A transformer which can pass 100 watts (+ what's needed to cover the losses) at mains AC 50Hz is a bulky item and needs much more copper when compared to a 100 W (+what's needed to cover the losses) transformer which works with 50 kHz pulses.

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