150W LED streetlight..why is the streetlight superstructure not used as heatsink - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: November 29, 2017, 09:18:41 pm
content: Hello,The following below document (RDR-32) is for a 150W PFC’d LED streetlight.Its noticeable that the schematic on page 8 simply shows the earth connection being simply connected directly to the neutral. Surely this is not wise as RCD’s could be tripped?Surely for the sake of common mode noise mitigation, it would be best to have Y capacitors from live and neutral being connected to the earth connection? The schematic on page 8 shows no Y capacitors in the mains input section. Isn’t this a missed trick?Another point is that in a streetlight, the pole itself is earthed, and the streetlight head itself is inevitably metal and is directly fasted to the earthed pole. In other words, the metal casing of the streetlight head is also earthed. It surely makes sense, to use the metal head of the streetlight as a heatsink. –If one does not do this, then one has to isolate the metal head of the streetlight from the earthed pole, and surely this is a waste of time?Anyway, the RDR-32 document never speaks about using the earthed streetlight metal as a heatsink. Indeed, for the PFC stage, it recommends heatsinking the Boost FET and diode bridge to primary side DC bus ground. –Surely this is missing a trick?......i mean, you have a big chunky metal streetlight head, why haven’t they used it is a heatsink?Is it because they fear getting more common mode noise if they use an earthed heatsink for heatsinking the power switching FETs? Even then, surely it would be best to still use the earthed streetlight enclosure as a heatsink, and then just use a bigger common mode choke in order to mitigate the common mode noise problem?I mean, they have a big metal streetlight..why are they not using the streetlight superstructure itself as the heatsink?...i mean..it’s going to be there anyway. I appreciate that its earthed but earthed heatsinks are not at all uncommon.RDR-32 (150W LED streetlight)http://www.farnell.com/datasheets/2003095.pdf?_ga=2.169092706.278760201.1511984913-287702491.1506211291

author: dmills
author_rank: Super Contributor
timestamp: November 30, 2017, 12:03:57 am
content: If you actually look at the layout that Neutral/Earth bond is almost certainly a drawing cockup, street furniture can be weird but usually if they are doing the TN-C thing it ends at the base of the pillar and is basically treated as TN-C-S from there IIRC. There are standards for the power and earthing of street furniture and I don't recall them being simple.Also remember that app notes are just that, and should always be given a carefully considered eyeballing rather then just used as is, some of them are written by crack monkeys.If you were designing a complete luminaire you might consider using the head casting has the heatsink, but it gets you into an interesting interaction between the electronics mechanical and optical design teams, so given Power Integrations are basically interested in showing a demo board for their chips I can see not going there, besides I bet the LEDs are already heatsunk to it.PI are not in the business of designing streetlights they are in the business of helping someone else design streetlights, and that someone else will have a better handle on what form factor they actually want for the board and what thermal solution makes sense for their design.Another thing to think on, a street light is a big metal pole, and a reasonable number of them are on top of hills..... Connecting things to that nominally 'earthed' metal may want some thinking about.regards, Dan.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2017, 12:45:57 am
content: What about fiberglass poles?Tim

author: ocset
author_rank: Super Contributor
timestamp: November 30, 2017, 06:04:07 am
content: QuoteThanks, do you mean because of lightning?...I was wondering if there is a strategic desire anywhere to avoid using an earthed heatsink for an SMPS because it may increase common mode noise problems?...ie due to the switching node of the SMPS coupling noise into the earthed heatsink.

author: dmills
author_rank: Super Contributor
timestamp: November 30, 2017, 10:18:43 am
content: That sort of thing is more usually seen in crap like cheap laptop supplies, IME the industrial stuff is generally class I with some mixture of earthed and floating heatsinks (Nothing wrong with floating a heatsink at +400V if the warnings are clear, it saves an insulator and gets better thermal performance, but you want it to be done inside an enclosed PSU case).Bear in mind that for something like this, even if you do a class II supply, the LED bank itself will have significant capacitance to ground, doubly so if on a MCPCB heatsunk to the metalwork, and may well even connect one side of the output to the head metalwork (And it might not be the one you expect, for corrosion control reasons positive ground can be favourite).You might find that that design passes conducted emissions on its own just fine, but fails when one side of the output is connected to the lamp head, stranger things have happened.Most street lighting vendors will be buying in the power supply as a module, because there is very little margin in going to the trouble of designing your own, it is not a significant part of the BOM cost, but is a significant source of certification pain.Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: November 30, 2017, 07:46:41 pm
content: QuoteI believe you are correct...its a strange situation though because these street light power supplies always contain electrolytic capacitors.If a country's government dept buys streetlight power supplys from a company of the same country, then no certification is needed, just general safety and general assurance that no harm will be caused by the power supply. After all, the regulations, ultimately , are put in place and made all muddley so that small companies can be kept out of the market....but in your own country....your government will turn a blind eye to home made power supplies which dont follow the letter of the regulations.As an example, as you know, TO220 FETs are illegal according to EN standards...TO220http://www.st.com/content/ccc/resource/technical/document/datasheet/f3/e2/b6/5c/fc/9c/41/16/CD00002501.pdf/files/CD00002501.pdf/jcr:content/translations/en.CD00002501.pdf..This is because the distance between the bare metal of the legs as they attach to the body is just 1.13mm....these FETs get used in offline PSUs...and they are non conformant to EN standards, but no one says anything. It just shows what a gumf the standards have become...the scheme-plan of the large corporations.

author: dmills
author_rank: Super Contributor
timestamp: November 30, 2017, 09:18:34 pm
content: Quote from: treez on November 30, 2017, 07:46:41 pmCite please?That distance would be a problem for a part connected directly across the line or between a live conductor and ground, but neither is generally the case for a switching mosfet.Seems to me that you are into the relm of functional isolation here rather then anything having to do with safety, and I think you need to learn to read standardese better, it is an odd regional dielect I know but seems to be common in Brussels and Whitehall.It has been my general experience that the people who care about standards conformance (Almost the only people who care about it) actually are the various bits of government, you are significantly less likely to sell a street light design to the highways agency if it does not tick all the standards boxes even if you are a local vendor.I can even make a reasonable argument that it is only the standards that keep any profit in selling what would otherwise be a very commodity item.What do electrolytic caps have to do with anything? It is not like you are making your own (hopefully) or have a secret source of special ones, you are buying in from a reputable vendor the same as everyone else in the game.Regards, Dan.

author: ocset
author_rank: Super Contributor
timestamp: December 01, 2017, 06:15:00 am
content: QuoteAs you know, they have lieftime limitations, which makes them unsuitable to be placed up a pole of a lamppost where its expensive to take them down and replace them.

author: capt bullshot
author_rank: Super Contributor
timestamp: December 01, 2017, 07:22:49 am
content: This is clearly marked as a "Reference Design"."Reference Design" is intended to do some benchmark testing, ... but never a real product. This PCB is not intended to be ever mounted in a real streelight.It is made by the IC manufacturer to show off his components, nothing else. It might never had been evaluated or tested for regulations compliance. You may use the schematics as a starting point, but it's your turn to design a proper streetlight according to all your regulations and requirements. And then it's your choice to use the enclosure as heatsink, if I look at common LED streetlights, they do so.