Electronics > Power/Renewable Energy/EV's

Powering AC Switch Mode Power Supplies with DC

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Someone:

--- Quote from: David Hess on July 23, 2016, 07:59:44 am ---But what simplification could they make which would prevent operation with DC?  Is there an application note discussing this or a reference design which has this problem?
--- End quote ---
You are right to ask for references, I dont know of any and to be accurate  we could look at the chips used for the specific apple power supplies and see they all use a multiplier based approach which will work fine with a DC supply. T3sl4co1l seems to like saying lots of things and feeling important while not listening to the actual requests made in posts.

T3sl4co1l:

--- Quote from: Someone on July 24, 2016, 12:19:09 am ---You are right to ask for references, I dont know of any and to be accurate  we could look at the chips used for the specific apple power supplies and see they all use a multiplier based approach which will work fine with a DC supply. T3sl4co1l seems to like saying lots of things and feeling important while not listening to the actual requests made in posts.

--- End quote ---

If you can't use your imagination, I'm sorry for that...

I'm afraid I don't have any specific examples that I've studied and shown they do/not work with DC.  If you'd like, you can send me a broad selection of examples and I can check if they work or not; I'll need somewhere to send the bill, though.

A quick search returns few ICs; this one says so in the description, surprisingly enough: http://www.mouser.com/ds/2/149/fairchild%20semiconductor_fan9612-320438.pdf

Yes, bypassing the PFC altogether will most likely work; the usual output is around 400VDC.  If anything is powered from the PFC controller's AUX (LV DC) supply, or enables/power-good signals are used with another controller, you may need to fill in for those as well.

Tim

David Hess:

--- Quote from: T3sl4co1l on July 24, 2016, 04:26:04 pm ---If you can't use your imagination, I'm sorry for that...

I'm afraid I don't have any specific examples that I've studied and shown they do/not work with DC.  If you'd like, you can send me a broad selection of examples and I can check if they work or not; I'll need somewhere to send the bill, though.
--- End quote ---

I do not have any examples either despite looking.  All I have is anecdotal reports occurring under unusual conditions (like UPS operation do to power disturbances) which by themselves could destroy any power supply.


--- Quote ---A quick search returns few ICs; this one says so in the description, surprisingly enough: http://www.mouser.com/ds/2/149/fairchild%20semiconductor_fan9612-320438.pdf

--- End quote ---

I must have missed it in the datasheet or misunderstood the context of your statement.  What part of the description says that the Fairchild FAN9612 does not work with DC?

Features:
...
Works with DC and 50 Hz to 400 Hz AC Inputs

The measured peak value is then used in the following
half-line cycle while a new measurement is executed to
be used in the next half line cycle. This operation is
synchronized to the zero crossing of the line waveform.
Since the input voltage measurement is held steady
during the line half periods, this technique does not feed
any AC ripple into the control loop. If line zero crossing
detection is missing, the FAN9612 measures the input
voltage in every 32 ms; it can operate from a DC input
as well. ...

I am dubious of this scheme using peak detection instead of RMS measurement (Is Fairchild avoiding a patent?) however it looks like they went out of their way to make sure it works with DC and modified SIN inputs.  I suspect their peak detection method fails with high crest factor waveforms but that would be a very unusual operating condition.

T3sl4co1l:

--- Quote from: David Hess on July 26, 2016, 04:30:38 pm ---I must have missed it in the datasheet or misunderstood the context of your statement.  What part of the description says that the Fairchild FAN9612 does not work with DC?
--- End quote ---

Just to clarify, I was looking for examples that do or do not (definite rather than undefined) work on DC.  Most are undefined; that was one which actually states it (positive definite). :)


--- Quote ---I am dubious of this scheme using peak detection instead of RMS measurement (Is Fairchild avoiding a patent?) however it looks like they went out of their way to make sure it works with DC and modified SIN inputs.  I suspect their peak detection method fails with high crest factor waveforms but that would be a very unusual operating condition.

--- End quote ---

AC, DC, funky waveforms, who knows... you can't win. :-DD

It would be pretty reasonable, at least, to expect that very quickly changing waveforms will have poor response (since the current loop can only react so quickly), but given that limitation -- a general approach should work with any waveform, frequency* and crest factor.

*Limited further by the voltage regulator loop, so you should avoid frequencies from almost-DC to 43Hz or whatever (else you get large output ripple, poor PFC, and poor regulation, because, what do you expect?).

So, these are the basic limitations we should expect from a general PFC.  Anything that's further limited probably has a reason (be it patent evasion, design shortcuts, or good old fashioned laziness).

Tim

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