power supply heat problem

OK, what happens if you get the efficiency up to 90%? You still got to get rid of the heat

Change the box to metal

martin

Did you look at the Topswitch devices from Power Integrations

Maybe the mosfet isn't right. Choosing the right mosfet is a balance between Rdson and the gate charge (capacitance).

I'm no smps expert and without reading into the post too deeply...here's some bits... For even cooler operation, use parallel mosfets. But, beware of drive issues. Also.. I've read that SiC power diodes can be easily be paralleled for cooler operation. Maybe check your magnetics.. I've heard saturation makes mosfets toasty.

I'm not sure about this but I think if a mosfet does not get proper heatsinking it will be more lossy. The switching and conduction heat increase Rdson which causes a thermal runaway until the mosfet stabilizes at some toasty temperature. Cool makes cooler and hot makes hotter.. I'll get around to doing the math on that someday.. D from BC

Incidentially, what's up with all that nasty ringing?

As for heat, there's a page somewhere where the guy tested a, I think Dell laptop power supply brick. There's no way the UL, CSA, etc. markings on the thing were legitimate in any way. The parts were cooking pretty well at half load; he was afraid to run it at full ratings!

Tim

-- "Librarians are hiding something." - Steven Colbert Website @

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I gather the heat sink is inside the box. I would put it outside, fastened to the rear panel if you can't have ventilation holes. You would have to electrically isolate the heat sink from the components.

Tam

Your efficency should be around 87% with the flyback, that leaves 10W in losses. What is the Real efficency that you measured?

Other than that I second the metal case.

Cheers

Hi to all and thanks for the replies. I'll try and cover all the replies. I can't use a metal box - cost. A big bugbear of mine :0( I didn't think flyback supplies could achieve efficiencies much over about 80%. I'll try and do some measurements and get a real figure. The mosfet I am using (IRFBE30) is one we use elsewhere , that is why I have to use that one. I think it is good for the job though.It still does not answer the question why the QR convertor is causing the fet to get hotter. The ncp1207 has a 500mA output stage and the uc3842 is about half that!!

I can't add more diodes - cost. This eliminates parallel diodes and also synchronous rectification :0( I've checked the current in the transformer( its a flyback arrangement , so it's not strictly a transformer) and it is not going into saturation. Nice linear ramp under all loads.

I can't put the heatsink outside the box. This box fits snugly inside another enclosure. The QR convertor should be more efficient, but is not , any ideas. Waveforms (light , medium and heavy loads) on ABSE

Cheers Rob

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My feeling is that you are asking too much of a 70W SMPS inside a totally enclosed space measuring 90 x 90 x 40 (and tapered).

I can only relate to my experience where I have practical experience of damaged caused by the heat generated by a typical Astec 40W SMPS module measuring 5" x 3" x 1-1/4" operating inside a dumb terminal which has ventilation holes and a heck of a lot more air volume. After several years of 24/7 service the plastic above the module takes on quite a distinct shade of burnt brown and a gentle push eith a finger causes the plastic to disintegrate. Repairing this type of damage is nigh on impossible when it has not been caught early. A small fan added to the enclosure certainly helps get rid of the heat and prevents further damage.

Remember, I am talking only 40W (not 70W) and in a ventilated enclosure of perhaps 50 times the volume of your enclosure using a commercial SMPS. Even assuming it was not as efficient as your SMPS it should still not have been generating heat sufficient to bake plastic. around it.

You problem is getting the heat out of the box. When you say this box fits inside another enclosure, is this enclosure metal? If not you're screwed. What is the temperature rating of the plastic enclosure. I do high temperature electronics design for downhole oil tools that run to 200C ambient. I have designed power eupplis that will run for 5 years at 150C ambient and 160C junction. In a lot of cases the datasheet ratings are very conservative on max temperature.

If the outer enclosure is metal or a theramlly conductive material, you may be in business. Glue an aluminum or copper plate on the bottom of your box to act as a heat spreader, and thermally attach your hot components to it. The heat will spread through the copper and better be able to conduct through the plastic into the outer box.

Remember, in a sealed box you have no convection so all of your heat must be removed by conduction.

Do you have heavy wires or connectors going in and out of the box. Use those as heatsinks too.

On Apr 15, 1:55 pm, wrote:

If you want to understand the sources of dissipation, you need a good current probe such as TCP202 for Tek scopes and a high voltage differential probe. The diff probe goes across power dissipating components, with the current probe simulataneously monitoring current. If your scope allows you to do so, observe the product of current and voltage. If not, use your imagination. Anyhow, you'll quickly find out where the efficiency losses are occurring and whether anything can be done to improve efficiency. Often, the biggest efficiency losses in ACDC switchers with medium output voltages are associated with the primary switch turn-off, but you could be having problems with turn-on, as well. Often, changing to a low gate-charge low Rds on MOSFET will work wonders. Slowly recovering rectifiers can aggravate such problems. If you have low voltage high current outputs, the output rectifier(s) can be the source of the problem. Schottky rectifiers and/or synchronous rectification can help you there. Proximity and skin effect losses in the magnetics can also be a problem....high temperature of these components will be your tell-tale there. If you have the interior enclosure volume, you can consider operating at a lower frequency to improve efficiency for all of the above. Sometimes, lowering the switching frequency allows you to get rid of filter components to save cost and create more room in the enclosure. The filter magnetics dissipate heat, too. Paul Mathews

And ground the heat sink if possible. Advantages in getting safety certs that way.

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It seems to me that you are being asked to come up with a design based on unreasonable and impossible to satisfy demands based solely on cost. SMPS design using readily available components (to keep cost down) will have finite limitations on what is achievable insofar as efficiency and reliability for a given volume. Unless you resort to specialised techniques such as thick film modular construction, planar transformers etc you just can't fit the required power into such a small volume as you are required to. Naturally, you won't get these items for low cost, so that is out of the question. You can't use a metal enclosure because of cost so you are stuck. Tell the client that you can do miracles but the impossible is a little more difficult.