hi all

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I am a firmware engineer, recently my power adapter for the laptop is broken

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Although I am going to buy a new one out there, i would like to find out how?to build a simple substitute.

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There should be 3 stage, right?? AC-AC, then AC-DC, then DC-DC

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stage 1, use transformer, say turn ratio 5:1 to lower the voltage to 18VAC-48VAC, right?

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stage 2, use bridge rectifier to convert the AC to DC, the outcome will be 24-68VDC, right??

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but is there any combined alternative for stage 1 and 2?? seems some sort of switching technique out there?!

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stage 3, use some sort of IC, just those from national semi. or other MOS switching circuit to convert that to exactly 20VDC3A output, right?? this stage is the most difficult to me, anyone can tell me more detail??

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Thanks much