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150W LED streetlight....why dont they use the streetlight superstructure as heatsink?

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treez

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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? :bang:

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? :bang:

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)
https://www.farnell.com/datasheets/...706.278760201.1511984913-287702491.1506211291
 

What makes you think that most devices don't use the pole as a thermal spreader? Where else would the heat go?

If your device is fully enclosed, then the enclosure is, by definition, a heatsink for the device. All heat flux must pass to/through it. The real question is how well the device is thermally (and electrically) coupled to that enclosure.
 
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Its noticeable that the schematic on page 8 simply shows the earth connection being simply connected directly to the neutral...

This is certainly not a good idea; the neutral and ground are distinct concepts and the neutral may have significant potential referenced to the ground.

The neutral is supposed to carry unbalanced current from the three phases; the neutral is often rated for a much lower current- the neutral is directly coming from the nearest distribution transformer.

PFC capacitors are often skipped for the neutral because of cost considerations but in a perfectly balanced load, the neutral carries zero current and no PFC cap will be needed.
 

Thanks, we have single phase transmission. The neutral does carry current in our setup.

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. I wondered if powerintegrations may have deliberately avoided using an earthed heatsink for possibly that reason?
 
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we have single phase transmission...

Possible but very unlikely. Almost all modern places have three phase supply and locally single phases are supplied to each domestic customer (depending on the load). The individual phases are distributed randomly but uniformly so that the currents in the individual phases are approximately balanced.

If your domestic supply is fed from a local pole mounted transformer, the primary is fed from one phase of a three phase supply. If you are in an apartment block, the main supply to the block is three phase but individual phases are sent to different apartments in a random but uniform manner. As you are considering an application for the street light, you will notice that three groups of lamps are fed from different phases. And that is also true for electric locomotives: you see that each locomotive is fed single phase but different sections are fed with different phases.

Of course the local neutral is simply the return; they are simply tied together at some point and connected to the distribution transformer somewhere.
 
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Thanks, yes, i appreciate that *ultimately* the phase comes from a three phase transmission.
However, i dont believe that three groups of lamps are fed from different phases....at least not on all runs of streetlights, maybe some. I think in the uk, in most cases, the streetlights dont even have their own dedicated phase, its shared with all kinds of other local domestic and industrial, whatever.
 

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