Typical Power Factor for SMPS Built in Late 80’s/Early 90’s - Page 1
Forum section: electronics / projects

author: AJ3G
author_rank: Regular Contributor
timestamp: November 13, 2020, 01:51:50 pm
content: All:I am working with a older SMPS which was made by Farnell. According to the limited documentation I was able to obtain online, the supply was built in the late 80’s early 90’s in the U.K.. I am trying to Calculate some AC Input Current numbers, as I plan to integrate this supply into a project I am working on.I am a complete newb when it comes to SMPS design, as this has never been my area of expertise. I am trying to understand, or guess what kind of Power Factor Correction was typical of a SMPS of this era. I understand PFC has improved dramatically since this supply was made. Anyone have any idea of what a conservative/worse case PFC number might be expected from a supply of this vintage?Rich

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2020, 03:36:20 pm
content: 0.6 or so.Tim

author: AJ3G
author_rank: Regular Contributor
timestamp: November 13, 2020, 03:41:32 pm
content: Hi Tim:That is a reassuring result. I assumed .65, but .6 will be my new worse case number. Any chance it would depart drastically downward from this? If I ever encounter numbers below .55 then I will have a whole other set of problems on my hand.Rich

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2020, 06:33:19 pm
content: SMPS of that age will most likely have no PFC at all.One part of the equation will be phase shift, which is probably not that bad.The other part is crest factor and current distortion which will be horrendous, though this applies to classic transformer/bridge/storage cap supplies as well.

author: AJ3G
author_rank: Regular Contributor
timestamp: November 13, 2020, 07:30:32 pm
content: Thanks for the input! I have managed to secure a Electronic DC Load, which may help me figure out some of this. The load needs to be repaired, as most of the test equipment I use does, but from what I am reading, it may help me gleam some insight into the topic.Rich

author: james_s
author_rank: Super Contributor
timestamp: November 13, 2020, 08:20:02 pm
content: Why don't you just measure the current it draws and then you will know exactly? One of those cheap Kill A Watt units will tell you the power factor with reasonable accuracy.

author: AJ3G
author_rank: Regular Contributor
timestamp: November 13, 2020, 08:34:57 pm
content: That would be a great idea with one problem - I do not have anything at this time which would draw the load I am designing for. This is a 60 - 65Vdc 25A Supply. I am trying to make some assumptions about AC Input protection so when I am able to test it under full specified load or close to it, I don't destroy anything to include myself.I just took the cover off of the FARNELL and I would say this is not a cheaply made switcher by any stretch of the imagination (http://www.farnell.com/datasheets/100779.pdf). To be honest, it rivals that of some of the higher end TDK’s I have seen. The data sheet I have does give efficiency, but no power factor correction specifications. As another posted, it may have to do with a supply of this vintage may not have PFC. I have tried to find more documentation without any luck. Hopefully soon I can learn more from using a DUT or a Electronic Load.

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2020, 08:56:08 pm
content: That's around 1.5 kW. A suggestion for a load would be a 3 kW, 120 V electric heater. As opposed to light bulbs, heaters' resistance vs temperature don't vary that much.Even a couple of cheap toasters could be used.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2020, 09:18:39 pm
content: I mean, it could be 0.5 or less. Depends if it's a voltage doubler configuration, or has excessive capacitance, or is lightly loaded (expect PF to fall to maybe 0.1-0.2 at light load), etc. Not impossible for it to have passive PFC or something approximating that, but uncommon I would guess.Or it's worse for a different reason: phase control. Think those were mostly passe after the 70s though? Should be obvious at a cursory inspection, no heavy iron and SCRs.Tim

author: AJ3G
author_rank: Regular Contributor
timestamp: November 13, 2020, 09:36:17 pm
content: I had anticipated the PF may drop with a light load. I did not imagine that much, but would be ok if the load is <20% of Max Rated. This morning I took a enclosure off, and looked around, but no in depth look at the circuitry. I was impressed with the quality. I have no idea where this supply came from, but by all appearances it was not used. There is no dust on the fan, and even the mounting threads still have paint on them.While not the most modern supply, the MTBF numbers are such that I think it should be put to good use. Hopefully this weekend, I will have a chance to take a closer look at what is going on.Rich

author: james_s
author_rank: Super Contributor
timestamp: November 13, 2020, 11:48:51 pm
content: Quote from: Benta on November 13, 2020, 08:56:08 pmYou won't find a 3kW 120V heater, that's beyond the max power available from a 120V branch circuit and the heaters all top out at 1500W. Larger heaters are available but they will be 240V.Of course nothing stops you from connecting multiple 1500W heaters in parallel, just don't use the kind that have a fan if you're wanting a DC load because the typical shaded pole fan motors won't run on DC. One of those carbon pile car battery testers would probably work too, or in a pinch a bucket of saltwater with some electrodes dipped in it. They will corrode very rapidly with DC but it was a common technique at one time for making cheap theater lighting dimmers and test loads for AC generators.

author: Gregg
author_rank: Super Contributor
timestamp: November 14, 2020, 12:01:50 am
content: Some of my favorite load devices are tungsten-halogen lamps; the bulbs and fixtures have fallen out of favor and are readily available at thrift stores. 500 watt and 350 watt ones are common; lamps can be paralleled to suit your needs.

author: AJ3G
author_rank: Regular Contributor
timestamp: November 14, 2020, 10:47:07 pm
content: All:Thanks for the suggestions. I just got the Electronic Load in, and it looks as though it can be brought back to life. I have ordered a current probe for my scope, so I should be able to get a notional idea of what the PF should look like. If all goes well, I should have the chassis in on Monday, and can start mounting this thing in with the rest of the H/W to get a good working high power supply..Rich