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Question on Buck Converters

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Red_Micro:
I've been trying to design a power supply with an ultra wide input range (90-690VAC) and low output power (48V, <10W). As isolation is not required in the application, I was trying to come up with a buck converter. However, I've found out that buck converters are not practical for wide input ranges due to duty cycle limitations, and that flyback is a better alternative for wide input ranges. Has anyone faced this kind of dilemma?

For example, consider this application note from ST. They are designing a buck with 600VAC max input voltage. I'm trying to see if I could adapt this design for 690VAC input maybe adding an external FET in series to increase voltage rating. But again, I've been told from several manufacturers that a buck would not be a good choice for this wide range because the limitations chips have in terms on duty cycle.

ogden:

--- Quote from: Red_Micro on June 03, 2020, 08:22:42 pm ---I've been told from several manufacturers that a buck would not be a good choice for this wide range because the limitations chips have in terms on duty cycle.

--- End quote ---
What's the reason for you to think that several manufacturers are wrong and only way to find right answers is public forum where anybody can tell you virtually anything?

langwadt:
use a tapped inductor to increase the duty cycle?

ogden:
Converter IC used in said appnote has burst mode operation that solves "buck problem". Look at 230VAC input "no load" waveform, check burst frequency. If you are fine with resulting ripple and audible artefacts of burst mode, then your search is basically over.

David Hess:
Duty cycle limitations apply to all switching regulators operating in continuous conduction mode.  Discontinuous conduction mode gives more flexibility but has other drawbacks.

4:1 is typical for wide input range converters but 8:1 is feasible with the typical compromises in efficiency because the power semiconductors have to tolerate both high current and high voltage.

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