Poor power factor when 150W LED driver dimmed down to 30W - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: October 02, 2017, 08:40:36 pm
content: Our 150W offline 230VAC, buckboost, single stage PFC’d LED driver has poor power factor (0.85) when dimmed down to 30W.This is due to ringing of current in the input filter near the mains zero crossing. We could put a damped filter in so as to mitigate this, but do we need to bother?....i mean, the power factor is 0.98 when it runs at 150W power. So surely the regulatory standards people will only be interested in what the power factor is like at the maximum power, ie, 150W?

author: jbb
author_rank: Super Contributor
timestamp: October 02, 2017, 08:55:02 pm
content: Depends on the regulatory standard. Often they talk about interesting units.For example, TDD is Total Demand Distortion, which is like THD except instead of dividing the harmonics by the operating fundamental current, you divide by the nominal operating current.Similar things might apply to PF. I wouldn't be surprised to find some metric of VAr / Pnominal in there.The principle is that a) the power supply infrastructure is sized to deliver the nominal power and b) it's hard to get the THD down at low currents.

author: jbb
author_rank: Super Contributor
timestamp: October 02, 2017, 08:57:25 pm
content: Sorry, forgot.You mentioned ringing. Make sure the specific ringing energy doesn't push you over an individual harmonic current or EMC limit. And remember it will likely vary a little from unit to unit.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 06, 2017, 06:57:56 am
content: IMO, power factor has been over-emphasized, leading to designs where it is optimized to 0.99 even at efficiency cost. Which is insane, since the very idea of regulating power factor is to reduce transmission losses and to avoid oversizing cabling and fusing; having a device that draws more power to do the same job is equally bad, even if the power factor was perfect! The regulation was originally needed because power factors were abysmally horrible in some switchers (like 0.5).But, as a whole, a modern higher-efficiency product with smaller BOM footprint, with a power factor of 0.9 is much more "green" as a whole, than a now-traditional 0.99 active PFC device.Also, AFAIK, it is completely normal that active PFC PSUs touting 0.99 power factor fall well behind it with partial loading, so your device is no special. The front page spec is always at 100% load.To me, 0.85 at 20% doesn't sound bad at all - especially, if it's typically ran at higher power levels. If it's sitting there all day at 20% load, the customer is likely to choose a smaller model anyway, correcting the issue.Harmonic content and EMC are much more interesting than the PF itself, over the full 0%-100% load range, over the full input voltage range.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 06, 2017, 07:31:40 am
content: Quote from: Siwastaja on October 06, 2017, 06:57:56 amI totally second this opinion. No need to go for high PF at partial load, better go for total efficiency.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 06, 2017, 09:22:13 am
content: Quote from: Siwastaja on October 06, 2017, 06:57:56 amNot to power companies it isn't. They can't charge reactive power to end customers but I doubt they mind selling some additional active power.

author: jbb
author_rank: Super Contributor
timestamp: October 07, 2017, 10:52:53 pm
content: Quote from: Siwastaja on October 06, 2017, 06:57:56 amYeah, look out for this. There are many pre-built power supply modules and plug packs out there which were only tested at no load and 100% load. Operation at partial load can turn up some nasty spikes (due to oscillation between flyback transformer and MOSFET capacitance in discontinuous conduction mode).