[Amps] Peter Dahl transformers

[Manfred Mornhinweg]

Peter,

> The downside to the switcher must be reliability. There are far more 
> parts, so the MTBF must be less.

Yes, this part is true, specially when comparing a switcher to a simple, 
non-regulated transformer-type supply. Instead when comparing a switcher 
to a regulated linear supply, the MTBF is often in favor of the 
switcher, because the pass transistors in the linear supply typically 
run very hot and are a common failure point.

With switchers, the most important factor to MTBF are the electrolytic 
capacitors, and specially the small ones, not the big filter caps. A 
great many designers of switching supplies just don't pay any attention 
to the pretty low ripple current rating of small electrolytics, and run 
them far above their rating! The result is that these caps dry out, and 
the supply fails. Probably anyone who fixes elecotronic equipment has 
already run into a power supply using an UC3842 controller, in which the 
small filter cap in the chip's supply has failed, making the beast 
hiccup instead of starting up correctly. I guess this is the most common 
single failure in all of electronics over the last 20 years! But this is 
not an intrinsic problem of switching supplies. It's a problem of 
circuit designers who don't know enough, or aren't careful enough, or 
just plain simply are intentionally designing equipment with a rather 
limited and quite specific life time!

When a switching supply is designed in such a way that the electrolytic 
capacitors are used well within their ripple current and temperature 
ranges to last long enough (20-30 years, at least), and the 
semiconductors are all used well within their SOA, typically the result 
is a very high MTBF. It's not hard to get 300,000 hours MTBF! Some 
quality manufacturer already are around 5 million hours MTBF, but such 
numbers are pretty academic to most users. On the other hand, I have 
suffered my share of badly designed switching power supplies, including 
cellphone chargers, compact fluorescent lamp ballasts, LED drivers, 
laptop computer power supplies, and the like, that hardly reach 500 
hours MTBF!

It's in the hands of the designer.

I don't have MTBF figures on the switching supplies I designed myself. 
When I was starting doing this, at age 18, several of them failed rather 
quickly, and through them I learned to do it right. I'm not aware of any 
failure of any of the power supplies I built as a professional, and some 
of these are now in use for more than 20 years, 24/7, in an 
industrial/scientific environment. The only goof was one multivoltage 
switcher that had to work under extreme conditions, and failed to start 
at a temperature of -25 degrees Celsius. It was supposed to work at that 
temperature, and of course didn't show any weakness while testing it in 
the freezer...  I had to swap an IC for an equivalent by another 
manufacturer, to fix this. I don't consider this a failure, but a "field 
testing result"... :-)   A failure would be something that worked, and 
someday, under the same conditions, stops working.

In short, it's indeed more difficult to achieve a required MTBF in a 
switcher than in a very simple non-regulated linear supply, but it can 
be done, and the cost is often still very much more convenient than that 
of a big transformer!


Manfred

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