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Power Factor Control with DC input

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ocset:
Some PFCs  RC filter the DC bus for undervoltage input protection…so  if that’s the case you will have to give enough DC input to get above this level.
Virtually all PFCs have a bypass diode, which runs from mains input  diode bridge, straight to the PFC output (this is to prevent inrush ringing overvoltages, since it bypasses the boost inductor)….so that bypass diode could be handy for you… ….but then the downstream converter may have undervoltage protection, so you may need your dc vin to be high enough.
So I guess you have to use the pfc as a booster as already said.
It may have been a “follower boost” pfc stage..in which case its great as  you can just shove in DC of a variety of levels and the downstream converter will still work fine .
If you put in 400vdc then that may be fine, and just go through the described bypass diode.
As you see on the mid right graph on page 4,  some PFC stages may not be able to deliver max power  with a DC input below X volts………………..
https://www.analog.com/media/en/technical-documentation/data-sheets/1248fd.pdf
(IAC represents the instantaneous “half sine  HVDC bus level, which wont be half sine in your case)

TheMG:
I grabbed a random power supply out of the junk bin and played around with it a bit. It's rated 100-240VAC input, output 5V 30A (150W). PFC circuit is based on the LT1248CS.

This is what I found:

Below approx 50VDC, does not start up or operate correctly, though it does try (no apparent input under voltage lock out). At 60VDC it runs fine, output of PFC regulator sits at about 360VDC which is as designed for this particular power supply. Was able to load it up to 50W with what power resistors I have kicking around (yes I know I really do need a nice big beefy electronic load), and with that load it runs just fine.

However, the efficiency at such a low voltage goes down the toilet: 62% efficiency (ouch!!!) at 60VDC, compared to 71% efficiency at 120VDC. This makes sense considering the input currents are more than double at the lower voltage, leading to increased losses in the NTC, bridge rectifier, PFC switching transistors and inductor. Note that server power supplies made specifically for DC-only operation in so-equipped datacenters lack a bridge rectifier and boost converter entirely, the whole point of which of course is to increase efficiency. Using standard AC power supplies on DC, while it technically works just fine, generally defeats the whole purpose because you still have those additional losses.

Efficiency at 120VAC is also 71%, so there seems to be no difference between AC and DC efficiency at the same RMS voltage, at least without modifications to the power supply. Note that efficiency at 240VAC input is 77%, so clearly the higher the input voltage the more efficient the power supply gets. This is definitely not a very efficient supply, but generally speaking all power supplies will exhibit better efficiency at higher input voltage, just it may not be as much of a huge difference with more efficient units.

Speaking of the bridge rectifier... that is one area to be cautious about, because the total current for the power supply will be passing through one half of the bridge rectifier, so its maximum current rating could potentially be exceeded or it could overheat. If you're going to do this, either get rid of the bridge rectifier (and remember to use correct polarity!) or make sure you're operating it within its ratings and it is adequately cooled.

I did grab a few other random power supplies from the stash and although I did not do as much in-depth testing, all seemed to be perfectly happy with 120VDC. Why 120VDC? That's as much as I could get with all my bench power supplies in series. It also happens that the nominal AC voltage in Japan is 100V, so most power supply manufacturers will design them to operate correctly to at least as low as 90VAC. The peak voltage of a 90VAC sinewave... 127V, Close enough.

To summarize:

-most PFC boost regulators will operate fine with DC input
-keep an eye on current through bridge rectifier diodes
-the higher the input voltage the better, I wouldn't go any less than 120VDC
-if possible, use DC-DC power supplies instead, as they will be more efficient

Seekonk:
Thanks for your investigation. The supply should arrive in the next couple of days. That is about what I was thinking after looking into the data sheet. I'm just about as interested in operating it as a boost converter by itself and changing the boost to 160V from 380. These supplies have to be just about considered E-waste for them to be only $8 shipping included.  There has to be some re purpose to them.

schmitt trigger:
Awesome investigation, thanks!

We can all assume that a PFC front end would work with a DC input......but someone had to verify that this particular assumption is correct.

Pawelr98:
Recently I actually converted one PSU which required a separate APFC module to run.
I didn't have the module, just the main board with the switching supply.

Had to modify two things.
1. Undervoltage lockout bypass, normally started at around 370V DC, shorted one resistor in the divider chain, starts at around 220V DC
2. Startup resistors, normally the PFC module also provided some kind of startup circuit, soldered few bypass resistors in parallel for startup

Depending on the design you may either try to make the APFC work with DC input or just bypass the APFC and modify the circuit (undervoltage lock).

Mine is a half-bridge resonant converter, 375V->270V (rectified mains with series choke) is a big hit but it still runs about right.
No abnormal heating or malfunction observed. Just seems to produce more noise (audible noise).

If you can get APFC to run then it will be healthier as the supply was designed for that voltage as the norm.
Another way would be to use an external boost converter.
UC3843 or something like that.
Doesn't take much effort or knowledge to make it run.
Parts salvaged from the APFC circuit may be handy.

Quick check before real circuit may be rectifier + capacitor directly to mains.
Will provide rough DC voltage that should more/less indicate how will the APFC behave.
If on 120V mains just use a voltage doubler for this test if you need 300+V DC.

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