Capacitive dropper power factor correction - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: February 21, 2019, 01:58:35 am
content: HelloI have seen the classic capacitive dropper, followed by a bridge rectifier used in a ton of consumer electronics (such as led lights that use mains).They also have a TERRIBLE power factor, best ones are in the order of 10%! I was wondering if there is a way to fix that issue with an inductor somehow (the logic being that capacitors are used to correct power factor of inductive loads, so doing the reverse should be possible). I'm not sure if it affects the performance of the circuit or even if it's possible (the capacitive dropper works by creating a phase shift, correcting that probably makes the dropper useless?) I'm not really sure but if anyone knows the answer, please answer down below. Thanks.

author: jbb
author_rank: Super Contributor
timestamp: February 21, 2019, 08:58:03 am
content: Well, the thing with a capacitive dropper is that they are normally used for low power applications. That means that the total VArs developed by the capacitor aren’t very high.Also, many loads are inductive (eg non electronic motors), so having some capacitive VArs about is not a bad thing.In principle, you could calculate the VArs generated by the capacitor during operation, and then fit an inductor between line and neutral which consumes approx that number of VArs. But you would need quite a large inductor.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 21, 2019, 09:18:55 am
content: The idea for such a circuit is the ultimately low cost; the inductor to store the required energy at line frequency would be too large for such small benefit.If high power factor isanykind of priority, using a proper switch-mode active PFC quickly becomes more affordable. This is an already solved problem, with highly integrated cheap and small ICs available.

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2019, 09:34:55 am
content: The inductor would need to be as large as a small transformer, so why just use a transformer based power supply?

author: OM222O
author_rank: Frequent Contributor
timestamp: February 21, 2019, 01:27:38 pm
content: Well what if the mains is rectified first (no smoothing caps), then dropped and smoothed? I'm assuming that would help with the value of the inductor? The problem with switching supplies is that they also require beefy inductors and capacitors, now add in the cost of all the ICs and protection features and I'm not sure it will be more affordable than and simple inductor to correct the power factor?

author: SilverSolder
author_rank: Super Contributor
timestamp: February 21, 2019, 02:02:06 pm
content: If you rectify the mains, you end up with a high DC voltage. How do you plan to drop it down to the level you need?Sadly, no matter which way we turn, our bottoms always point backwards!

author: OM222O
author_rank: Frequent Contributor
timestamp: February 21, 2019, 03:43:11 pm
content: If you rectify mains without smoothing it, it is a high voltage half sine wave AC signal with double the frequency. It will become DC when you smooth it with a capacitor, but as I mentioned in the last post, it's not smoothed before being dropped

author: Siwastaja
author_rank: Super Contributor
timestamp: February 21, 2019, 04:09:53 pm
content: Well, improving the power factor in the 50/60Hz domain would require storing considerable amount of energy (due to the loooooooong period) in the inductor. So, you are throwing away the BOM cost and size benefit.For a real solution, there is no way around of using an SMPS. This way you can minimize the solution size and cost, by:- utilizing the DC link capacitor better (by allowing ripple in the rectified voltage, and still drive the LEDs with constant current, eliminating flicker without massive capacitance)- minimize the inductor size, by using high frequency, so it needs to store much less energy, yet allowing active power factor correction.Granted, this is more complex, and there are more chances of getting it wrong. OTOH, it's not a rocket science, it's a solved problem, and most crappy products are crappy on purpose.Trying to make the cheap, simple, crappy capacitive dropper better by adding expensive parts makes very little sense. Use it, in its simplest form, for its simplicity and low costwhenever applicable.