Switching Mode Power Supply half output problem

[ChrisP58]

All of the replacement electrolytic caps I ordered were small and rated for 105°C even though the enclosure doesn't get much above ambient.

You don't choose the caps temperature rating strictly by it's max operating environment in your application. You choose it based on how long you want it to last.

Electrolytic caps are rated in terms of hours per temperature, like 2,000h/105 °C. Which means that it's expected lifetime is not very long if you actually use it at or near 105 °C.

But there is what's known as the 10°C rule which states that, for every 10°C drop in operating temperature, you double the lifetime. So if you take that same 2,000h/105 °C capacitor and use at 35°C, that 2,000 hour scales up to 256,000 hours, which is ~29 years.


For more info see https://en.wikipedia.org/wiki/Electrolytic_capacitor#Lifetime

 

[Nigel Goodwin]

Nigel Goodwin: Thanks for the ESR meter suggestion. It prompted me to look online for one. The last time I used one was so long ago that the only ones available (e.g. Fluke, B&K Precision, etc.) were prohibitively expensive for as infrequently as I’d use one. I ordered a cheap off-brand ESR meter just to have on hand.

unclejed613: All of the replacement electrolytic caps I ordered were small and rated for 105°C even though the enclosure doesn't get much above ambient.

I used to stock nothing else than 105 degree capacitors, with the exception of the high voltage ones - which aren't available normally above 85 degree, and don't matter anyway as they are low frequency.

It's also VERY important to buy good quality capacitors, I mostly bought Panasonic ones.

 

[unclejed613]

it's a trade off between ESR and max temperature. i often found thet 105C caps had 10% higher ESR than the 80C ones, but the 105C caps definitely functioned properly for a lot longer.

I mostly bought Panasonic ones.

also good are Elna and Nichicon. all 3 brands are often counterfeited... i've seen large panasonic marked caps that kind of vibrated when you tapped on the side. turns out there was some off-brand cap inside soldered to the base of the larger case. needless to say, that parts distributor never got an order from me again.

kinda like this picture....

 

[unclejed613]

I ordered a cheap off-brand ESR meter just to have on hand.

you can also use an audio generator and an oscope. most audio generators have a 50 ohm source impedance. if you connect them across a cap, the source impedance of 50 ohms, and the capacitor becomes part of a voltage divider. if you use 100khz or higher as your test signal most of the residual voltage across the cap will be from ESR if the cap is 1uF or larger.