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Detecting early signs of failure in switch mode power supplies

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I have just experienced a few power supply failures (we use a lot of enclosed AC/DC supplies in the 100W - 500W range) and this got me wondering whether it is possible to spot the early signs of failure to be able to proactively swap a nearly-failed supply out before it goes bang (or in this case, simply stops).

My first throught is to measure the ripple on the output and also look at any changes in its AC current consumption (e.g. high frequency components which may be a sign of input capacitor failure?).

I appreciate there are many failure modes when it comes to PSUs, but most seem to fall into just a few categories (failed electrolytics, rectifier diodes and MOSFETs/drivers for example), so it would be great to be able to spot any degradation from outside the supply if at all possible (i.e. I don't really want to have to modify any supplies).

Any thoughts on whether this is a) feasible, or b) worth doing at all?

It depends on the failure, of course.  However, I would say it's mostly not feasible.

If the semiconductors are going bang the answer is pretty much no.  There is a very slight reduction in efficiency as the main transistors get too hot, but the difference is minimal until the moment of catastrophic failure.

For the output capacitors, you can likely measure ripple at the main switching frequency.  An increase in ripple implies increasing ESR and/or decreasing capacitance.  For the input capacitors, you'll see double your mains frequency (100/120Hz), though these caps going bad can also cause the PFC circuit to self-destruct.   The 100/120Hz primary side ripple will likely only get really bad under high load, whereas the output ripple will likely be present under light load. 

It's worth noting that while bad caps were a frequent issue 10 years ago, nowadays it seems like they are rarely the cause of a failure.  Most power supply failures tend to be "bang and gone", or rare issues you couldn't spot beforehand (control IC suddenly breaking down, for instance).

And the brands of your "failures"  .... would be ?

In most cases not feasible, you would need intelligence inside of them to monitors lots of factors and have some warnings issued

Load(s) applied to them
Over Current(s)
Over Voltage(s)

Many thing(s) to considerate

Comparing the in / out voltages, currents, startup times against a new model could tell you quite a bit about the state of capacitors in general.

Thermal camera and spectrum analysis may tell you couple more things without opening the box, the difficulty being having similar test conditions from one measurement to another (room temperature, surrounding EMI).

Start-up supply resistors going open, aging solders and I.C's failing internally are going to be tough to predict unless you have very well known PSU's with a history of the MTBF and failure modes.

I wouldn't say it's worth the bother... Seems easier to have a backup and repair the defective PSU once it has failed.

IME the most common failure is teh startup supply, and you only find that out when you power down and it won't power up again.
The best thing you can do as prevention is ensure the supply runs as cool as possible, e.g. by under-runnning and ensuring ample ventilation


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