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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