Mains input current to LED driver is oscillatory near zero cross - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: July 22, 2020, 01:31:25 pm
content: Please confirm that the following mains input current waveform to a 17W Offline LED driver is not potentially damaging to the mains supply system, even if millions of these devices operate?(Green = Mains input current;Red = Mains input voltage;Light Blue = HVDC post rectifier bus)Our contractor says it could be damaging.Mains input current average is 111mA, Mains input current RMS is 0.17ALTspice of flyback attached, also schem

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 29, 2020, 04:44:48 am
content: Small LED lamps often have very simple power supplies.This often leads to very high peak currents during turnon.A 17W LED driver can easily draw a peak current of 10A or more during switching.So if you put a million of them on a circuit and switch them with the same switch, you are switching ten million Amps.What those oscillations do with the mains voltage is hard to predict.Much depends on the amount of energy in those oscillations.

author: tom66
author_rank: Super Contributor
timestamp: August 29, 2020, 08:27:10 am
content: Damaging? No. Only thing cables care about is RMS current. Looks like the power factor is pretty close to unity so this is unlikely to be a problem.Compliant with EMC regulations? Hmm... more doubtful. A large number of these installed in a building could cause some issues with sensitive equipment. I'd want to see calmer start up.I have seen a number of cheaper LED drivers which are based on a simple self-oscillating circuit with primary side current monitoring to set the output current. It takes a fair bit of effort to keep such a circuit stable near the edges of operation (low input voltage near zero crossing). Many of these converters have minimal input capacitance to maximise power factor.

author: JohnG
author_rank: Frequent Contributor
timestamp: September 02, 2020, 01:12:55 pm
content: The real circuit noise near the zero crossings will vaguely resemble your simulation, but it will not be the same. It could be better or worse, depending on the parasitics of the circuit, which I do not see much attempt to model here. To be fair, it's far from trivial to do this. Some of these parasitics will make things worse, others better.One big problem is that if you are using the LTspice-supplied models for the RFN5TF8S rectifiers, they do not model reverse recovery properly, and this can have large effects on what you seen near the zero crossings, depending on the rest of the circuit and parasitics. I notice you are using the same diodes everywhere, and they are overkill.Note that people can and do make these small power supplies that meet EMI regulations, so there is nothing inherently wrong. It is just that the devil is in the details, and you haven't got those details in the simulation. I would buy a few, measure them, and take some apart to get some ideas about what to do.John

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 02, 2020, 01:34:57 pm
content: Quote from: tom66 on August 29, 2020, 08:27:10 amWhat frequency is the oscillation? Conducted EMI starts at 150kHz.