Power Factor Control with DC input - Page 1
Forum section: electronics / projects

author: Seekonk
author_rank: Super Contributor
timestamp: July 02, 2020, 12:09:13 am
content: I just ordered one of those cheap HP server power supplies and pretty sure it has PFC. I've heard PFC don't like a DC input and that is what I will be using. Have never worked with PFC other than repairing some power supplies. Any quick insights on how to fool it or comments in general. I'll probably just bypass it and at worst provide alternate drive. Just something to fool around with and have fun.

author: coppice
author_rank: Super Contributor
timestamp: July 02, 2020, 12:17:05 am
content: I wouldn't trust any modern supply to function correctly from DC unless it is specifically sold as being designed for operation from DC, with the input voltage range properly specified.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2020, 12:33:14 am
content: If it's specified for 100-240V AC, it almost certainly will work on 140-380V or so DC.

author: james_s
author_rank: Super Contributor
timestamp: July 02, 2020, 12:53:01 am
content: I would study the datasheet for the PFC controller chip used and see if you can work out how it will behave on DC.On the other hand, server PSUs can be had for practically nothing, it might just be easier to run one on DC and see what happens.

author: Seekonk
author_rank: Super Contributor
timestamp: July 02, 2020, 03:08:26 pm
content: This is actually a solar application. My first testing will be done with a MSW inverter which is like DC. Someone on the web stated these had only one large electrolytic meaning no voltage doubling. I actually run lots of 120V switching power supplies on only 60V DC from the array with a little modification to the startup circuit and at reduced power. Not actually sure this is PFC, but it is hard to conceive a whole building of power supplies only pulling power at the peak.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 02, 2020, 04:38:28 pm
content: PFC does not make any sense for a DC circuit.On the other hand:All power supplies I know rectify the AC voltage onto a DC bus, and then use some switching topology to get low voltage outputs.PFC is then done between the AC input and the DC bus.It should be fairly easy to disable or short circuit the PFC part of existing power supplies to get it to work on a DC bus.

author: wraper
author_rank: Unknown
timestamp: July 02, 2020, 04:45:56 pm
content: Quote from: Doctorandus_P on July 02, 2020, 04:38:28 pmIt's not that simple. A first non starter is that input voltage get boosted to a certain DC voltage, usually about 400V. You will not have 400V if you disable PFC.QuoteIf you short input-output, magic smoke will escape since PFC is a boost converter.

author: Seekonk
author_rank: Super Contributor
timestamp: July 02, 2020, 05:42:51 pm
content: There is always slicing and dicing involved.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 02, 2020, 06:54:41 pm
content: This has me kind of curious as to what would happen feeding DC (at 127-340V) into typical PFC boost converters. I might do some experimentation on some surplus power supplies myself.In theory it should work, and the PFC regulator will then (hopefully) behave like a simple DC-DC boost converter. I think it's the sharp transients involved with "modified sine wave" inverters that a some PFC controllers have problems with.With any less than about 127VDC (equivalent to 90VAC rectified) there's a chance the power supply will either not start up at all (UVLO) or will be unable to provide full output. Remember that as the the input voltage goes down, the boost regulator must draw more current to deliver the same amount of power to the rest of the power supply.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2020, 10:40:48 pm
content: Quote from: Seekonk on July 02, 2020, 03:08:26 pmModify the inverter for a higher duty cycle (so it outputs ~170V peak at almost a square wave) and you'll get efficiency very close to DC, but without the problems switching DC.If you're thinking of converting server PSUs and telecom rectifiers into MPPT charge controllers, that's possible with a significant control circuit redesign. There's also repurposing Prius inverters if you truly want to be able to deal with a lot of power.

author: ocset
author_rank: Super Contributor
timestamp: July 04, 2020, 10:02:35 am
content: 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)

author: TheMG
author_rank: Frequent Contributor
timestamp: July 05, 2020, 07:48:29 pm
content: 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

author: Seekonk
author_rank: Super Contributor
timestamp: July 05, 2020, 09:37:08 pm
content: 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 05, 2020, 10:57:13 pm
content: 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.

author: Pawelr98
author_rank: Regular Contributor
timestamp: July 06, 2020, 03:21:27 am
content: 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 DC2. Startup resistors, normally the PFC module also provided some kind of startup circuit, soldered few bypass resistors in parallel for startupDepending 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.

author: coppice
author_rank: Super Contributor
timestamp: July 06, 2020, 11:47:26 am
content: Quote from: schmitt trigger on July 05, 2020, 10:57:13 pmWe can all assume everything is OK with all products based on some very limited testing someone did? I hope you don't work professionally as an engineer with that kind of thinking.

author: nuclearcat
author_rank: Unknown
timestamp: July 06, 2020, 11:58:45 am
content: Quote from: coppice on July 06, 2020, 11:47:26 amI think he meant if magic smoke didn't escaped, it is time to test how well it works. If i will manage to find time for this, i might test too, with thermal camera and etc.And quite interesting topic, as country where i am living now plunging in darkness, some % of efficiency saved by using direct DC power to AC power supplies will be helpful (if that possible).Yes it might require some hacks or adjustments, and this is what i am desperate to see here.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 07, 2020, 01:05:50 am
content: Thanks nuclearcat that’s what I intended to mean. That limited testing allows us to be guardedly optimistic.But Coppice has to be its grumpy self and make disparaging comments, otherwise he is not happy.

author: Circlotron
author_rank: Super Contributor
timestamp: July 07, 2020, 01:49:22 am
content: Quote from: NiHaoMike on July 02, 2020, 12:33:14 amOne would think so. I've run my Rigol DS2072 scope many times on DC. It does not have PFC. I have a 12V to 240V "modified sinewave"inverter and the sharp rising edges of it's output cause extra losses in the inverter because they are feeding into X caps in the scope psu. So I made an external bridge rectifier to go between the inverter and scope. Runs for quite a long time on a 12V 7Ah battery. Was using it to probe car electronics while driving around. (I was in the passenger seat).

author: ocset
author_rank: Super Contributor
timestamp: July 07, 2020, 08:34:06 pm
content: With 360VDC output, it sounds as if its a bit low, because PFC output are usually at least 385VDC.With the low efficiency, it could be worth scoping the gate drive to see if a proper gate drive voltage is there.Mind you, old fashioned CCM PFCs were inefficient due to reverse recovery losses in the old slower diodes.At low volts dc input, it could also be worth scoping the sense resistor, because maybe its in some bursting type operation with very high rms current.Changing the diode to a sic type, and then reducing the ohms value of the series gate resistor could result in efficiency improvement....the older pfc's which used slower diodes, used to switch the fet on very slowly so as not to over-excite reverse recovery...this resulted in inefficiency in itself but had to be done to avoid the mega inefficiency of severe reverse recovery of the slow old diodes.If you look at eg HiperPFS series of pfc controllers, they can get 90%+ efficiency with use of more modern diodes.Of course, BCM PFCs dont excite the reverse recovery, but they can have very high fsw at light load...resulting in high switching losses

author: TheMG
author_rank: Frequent Contributor
timestamp: July 08, 2020, 01:04:45 am
content: Quote from: treez on July 07, 2020, 08:34:06 pmThat particular PS is dated 1999, certainly predates modern high-efficiency stuff.360VDC seems to be correct, it only has 400V rated capacitors as opposed to the 450V rating you'd find in power supplies running a 380V PFC.Anyways I don't particularly care about it. Just an old power supply from the junk stash, I picked it as a test victim simply because I don't care if I let the smoke out of it.