Electronics > Power & Renewable Energy

Powering AC Switch Mode Power Supplies with DC

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nbritton:
Does anyone have any insight on whether you can power AC adaptors that have auto-ranging and active PFC directly with high voltage DC? I know you can power traditional full wave bridge rectified SMPS directly with DC, but I'm wondering if you can do the same with modern auto-ranging active PFC SMPS. In particular, I'm wondering if I can power Apple's MacBook AC adaptors directly from a high voltage DC battery bank.

http://www.righto.com/2015/11/macbook-charger-teardown-surprising.html

The Apple SMPS is designed for 100 to 240 volts AC. So given that VDC = VAC * sqrt(2), would this power supply be compatible with a 141 to 339 volt DC battery bank? Also since only half the rectifier diodes would be in use if powered by DC, would it be smart to use a higher voltage so that less current goes through the active diodes? If it were powered by AC, each diode in the bridge would have a duty cycle of 50%. So if I doubled the DC voltage that would drop the current to 50%.

David Hess:

--- Quote from: nbritton on July 21, 2016, 05:33:08 am ---Does anyone have any insight on whether you can power AC adaptors that have auto-ranging and active PFC directly with high voltage DC?
--- End quote ---

Isn't that redundant?  The active PFC either has enough input range to support a universal input or it does not.  I have never seen one with any sort of extra auto-ranging circuit like a doubler.

PFC should not care if the input is DC but I cannot say that there are not some finicky implementations.  Operating at a higher DC voltage will lower the current through the input rectifiers (and increase the current through the boost rectifier do to decreased duty cycle) but I would hope they are derated enough for this not to be a problem.

The only risk I know of is safety.  The input protection fuse and any power switch is not going to be rated to break high voltage DC.

Ian.M:
Just in case there is a dropper resistor based startup circuit, you should *NOT* use a DC input voltage exceeding the max rated RMS AC input voltage.

David Hess:
I would hope that the startup resistor is derated enough for that not to be a problem.

Ian.M:
V2/R

It will certainly be rated to withstand 240V RMS +10%, but if you apply 340V DC it will be dissipating double what the designer expected.  Even if the resistor (or resistor chain) has enough margin on its rating, the excess dissipation is likely to shorten its life and may cause problems for other nearby components.

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