150W offline LED driver  simply cannot  be 95% efficient? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: December 16, 2017, 03:27:18 pm
content: Hello,A certain major streetlighting company is purporting to have Offline, 220VAC, 150W, dimmable LED streetlights which have efficiency of 95%.This streetlight is dimmable from 5W to 150W.Do you agree that this is not possible with anything other than ridiculously expensive circuitry?I mean, I once made a 150W LED driver which was 95% efficient….but this was from an input voltage of 48VDC , and it consisted of three separate 50W Buck channels feeding into the same LED load. With a 48V input, one’s switching losses are far lower than switching off a 400V post PFC bus. Also, with 48VDC input, one has available some FETs with very low Rdson and low junction capacitances, so as to keep the losses down. Also, the 95% efficient version had a severly undamped drain voltage transistion (to reduce switching losses) and was severely noisy. (when the series gate resistors were reduced from 4R7 to 2R7 , then switching this LED driver on resulted in the lab digital radio going totally quiet when it had just been playing at full volume!)To get 95% efficiency in an offline (mains connected) situation, you are going through a PFC boost converter, and then a downstream LED driver. It would in fact be pretty miraculous to get an offline PFC boost converter to operate with efficiency of 95%, let alone the cascade of PFC Boost/LED driver.To get 92% plus efficiency from a PFC/LED driver cacade combo, then the LED driver would have to be a resonant converter. This would likely mean an LLC converter. However, the LLC converter would definitely not be able to garner 90% plus efficiency in the light load situation of dimmed down operation. This is because the switching frequency of the resonant converter would likely go high at light load, and this would mean more switching loss, since when well above the upper resonant frequency, then the LLC converter gives significantly more switching losses as a proportion of total losses. In order to get zero voltage switching at higher frequencies than f(upper) in an LLC converter, one would have to ensure high enough magnetising current to discharge the Cds’s at the lower switching period…this means more dead time, or decreasing the magnetising inductance sufficiently………but if the dead time was set that high, then this would lead to an efficiency penalty when at the nominal maximum load of 150W (since that’s where the converter is designed to operate at the f(upper) switching frequency…also, a reduction of magnetising inductance means more conduction losses.Not to forget the natural variation in LED Vf, with both tolerance and temperature…this makes for more difficulties for the LLC which mean mitigation and subsequent efficiency penalties for the LLC converter.Therefore, I refute the claim of 95% efficiency, do you agree?

author: Benta
author_rank: Super Contributor
timestamp: December 16, 2017, 03:56:06 pm
content: Not necessarily. The quoted efficiency is very high, I agree.But it's a street lamp, so it doesn't need isolation and could be a much simpler topology than you think.I do however think that the figure is a peak value, obtainable only at one load point. When dimming, efficiency would suffer.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 16, 2017, 03:59:10 pm
content: Meanwhile, in other news, companies sometimes make claims in their marketing which are not entirely accurate.Who'd have thought it?

author: wraper
author_rank: Unknown
timestamp: December 16, 2017, 04:34:18 pm
content: Streetlights or LED driver? It's not impossible for LED driver to be 95% efficient, example for this is that some computer PSUs have such efficiency in mid-full load range. If those are streetlights which contain LEDs in them, it's completely impossible as the best LEDs available don't exceed 30% efficiency.

author: Benta
author_rank: Super Contributor
timestamp: December 16, 2017, 04:38:17 pm
content: Quote from: wraper on December 16, 2017, 04:34:18 pmI would expect the quoted efficiency be input power to LED electrical power. Optical power is of course a lot lower.

author: IanJ
author_rank: Unknown
timestamp: December 16, 2017, 04:42:25 pm
content: Here's some info on part of London's upgrade program......might contain some info:-https://consultations.tfl.gov.uk/streets/daa6f1a7/user_uploads/eeslp-faqs.pdfIan.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 16, 2017, 05:14:57 pm
content: It's believable when talking about the driver/power supply. Since the input V and output V/I are very constrained, designing a highly efficient driver shouldn't be too hard for a skilled EE. The problem comes in when you have to target wide input or output ranges.However, that's in the category of useless but great sounding information. Like the HVAC guys that say things like "this furnace is 20% more efficient" like it's going to save 20% off your bill (it's not, maybe 2%, turning the thermostat down does that).