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Comment by yourapostasy | Hacker News Reader

Please forgive the ignorant questions of a software guy that probably have blindingly obvious answers to a EE. Does this mean that theoretically, I could attach a 18V/18.5V [0] DC power source (like a solar panel array running into a DC-DC converter) to the prongs and as long as I can supply about 100W/8.5A my MacBook Pro will boot?

[0] http://apple.stackexchange.com/questions/111250/how-to-charg...

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Chargers require a minimum voltage to work. They typically say 85V and might work with lower, but I expect 18V is way too low. I'd also be very nervous about feeding DC into a PFC circuit (which is designed to track the AC waveform) - does anyone know for sure what PFC does if you feed it the wrong frequency? (Someone else asked about 400Hz aircraft power too.)

It probably won't blow anything up, but the active PFC might not be very happy as it tries to track the AC waveform to control the current draw. It might just not work.

You'd be better off getting one of the cheaper clone adapters with passive or no PFC if you want to make a dedicated DC power supply.

Not an ignorant question at all! This is actually a subtle problem in switching regulator design: high duty-cycle operation and efficiency. Basically, if you input a voltage very close to the output voltage, the switch part of a switching regulator must be always or near always closed. And, any inefficiencies such as voltage drops in the power path are visible in the output.

Being Apple, I'm sure it has 100% duty cycle operation. However, it's likely not optimized for hat (uncommon case).

[edit] as the other comment points out, my comments forgot about the PFC circuit. But on switchers without this, near output voltage input is possible.

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