Offline 60W+ SMPS in totally enclosed plastic has internal ambient >105degC? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: June 27, 2020, 02:14:02 pm
content: I’ve sometimes done 60W+ Offline SMPS that went into Totally enclosed plastic enclosures with no internal fans. I was not allowed to examine the thermal results, and thought this was because…1…it was the companies IP relating to the product …and/or….2…The thermal results may have been higher than would have been liked___________************_______________**************At some places, I have noticed a thermal test being done on a (normally) totally plastic enclosed 60w+ offline SMPS, and there was ventilation getting in to the enclosure as they had not plugged up the dremelled holes which were allowing the thermocouple wires to get in. Also, the plastic enclosure halves were obviously not as one (otherwise it would have been impossible to mount the thermocouples). As such there was a tiny gap opening all round the enclosure, which had been sealed up with a few layers of kapton tape. –A poor test setup.____________****************__________________******************CASE A: However, I did a 60W, 240VAC Offline Flyback LED driver that went into a plastic enclosure that had overall gaps in the plastic of some 30mm by 5mm at each end. This plastic enclosure itself was in a totally enclosed (sealed) metal enclosure. (the plastic enclosure was ~ 16cm x 6 cm x 5cm height) This metal enclosure was part of a single casting to which elsewhere was fixed a LED bank (on MCPCB) with 60W of electrical power going into it. The internal ambient of this plastic enclosure was recorded as71degc(60W,240VAC input) when the ambient outside the containing metal enclosure was recorded as 25degc. The metal casting by the leds (a bit away from the power supply part) was recorded as 37degc.…..since the internal ambient of this was 71degc, then I believe its obvious that the internal ambient of a totally plastic enclosed 60W+ offline SMPS would have an internal ambient of well above 71degC.Would you agree?____________________*********************______________________*************CASE B:I once examined a Test Report for a Totally plastic enclosed Flyback SMPS which was 60W, 240VAC. This SMPS is sold in the 100’000s. (dims = ~11cm by 5cm by 2.5cm height)(this power supply would always be on max power when run, but , in most use cases, was not usually run for very long each day)The report said external ambient was 40degc. It recorded “Top of Case” as being 60degC. When I enquired as to whether this was internal or external they replied only to say that it was “near the main MOSFET, Q1”. Some of the lower power handling components in this enclosure were recorded as having temperatures in the mid 50’s (degc), down to 54degc. As such, I assume that the “top of case” temperature reading must have been on the outside of the case. The bulk input 100uF electrolytic was measured as “69degC”.I believe that the “internal ambient” measurement was not done because it looked too high and may have worried the customer. A very hot component on the thermal cam was the RCD clamp resistor, but the temperature of this was not in the report, and not in the updated report, even after we asked for it.The test report was obviously in error, because its difficult to see how some components (obviously inside the case) had temperatures which were colder than the outside of the plastic case.___________**************________________***************_____________All of this does point to the fact, that the internal ambient for an 60W+ offline SMPS in a totally plastic enclosed enclosure, is quite likely to have an internal ambient above 105degc.Would you agree?As Cornell Dubilier says, operation of an el cap above datasheet quoted temp, even for small periods, can result in significant lifetime reduction of the El Cap.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 27, 2020, 04:21:58 pm
content: Quote from: treez on June 27, 2020, 02:14:02 pmDefinitely not! Some pieces of crap could be that way, but not "quite likely" for any half-decent product. It wouldn't last for long, or would be pain-in-the-ass to design. Semiconductors alone require massive derating, not to speak of the elcaps, although you could try to cope massively derating ripple current rating.Even the transformer winding insulation ratings become a problem at such temperatures, they may be specified only to something like 150-200 degC.Solder melting temperature is close, too.Instead of guessing, why not buy a few 60W+ offline SMPS plastic bricks and and actuallymeasureit? I would be surprised if it gets much over 70-80 degC, except for small hotspots right at the transistors, for example.Try to cover them with blankets and see what the maximum temperature is before shutdown.

author: ocset
author_rank: Super Contributor
timestamp: June 27, 2020, 06:47:35 pm
content: Thanks, i dont have the kit at home where i am now at the moment to do that test.But "CASE A" in the top post showed 71degc internal ambient for a plastic case with vent holes in it, when external ambient was 21degc.......so anything around 70-80 degc would be a big underestimate for a non ventilated plastic case in say 30-40 degc ambient

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2020, 10:20:09 am
content: Obviously that CASE A product was designed with the vent holes, no?If you have another product which is designed with no vent holes and still reported similar power density, the efficiency is likely engineered a tad higher to compensate.It's also completely possible the vent holes are being way less effective than you think they are.For passive vent holes to be effective, you need quite large holes, over a large area or at two spots, preferably one set of holes at bottom, one at top, so that convection can work upwards.If you just have a few holes in a case of a tightly packed (obstructed) SMPS, it's quite likely the air is not moving through said holes. Try to cover them and see if the internal temperature gets any higher, and by how much.Finally, as a general comment, I strongly suggest youmeasuremore, andassumeless. Thermometers are cheap. You can do quite some serious thermal engineering if you invest some $10-$20.

author: ocset
author_rank: Super Contributor
timestamp: June 28, 2020, 11:03:53 am
content: QuoteThanks, it was a 90% efficient, single stage PFC'd flyback, so it was really at the peak efficiency for its kind...(without spending too much extra money).Yes the vent holes were always going top be there.QuoteThe SMPS of CASE A was not really describable as "obstructive". There was decent room for air to move over all the components......there was as you can see from the dimensions, some decent headroom....due to the height of the sideboard and FET and diode heatsinks.I think in CASE A above, the air temperature inside the plastic box would end up being pretty similar to the air temp outside it due to diffusion through the (albeit small) vent holes.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 28, 2020, 07:14:22 pm
content: True diffusive transport is basically negligible over macroscopic distances and at relevant temperatures. If you don't have convection the holes aren't really helping. But as explained: measure. This stuff would all be trivial to measure.