Page 151 - Troubleshooting Analog Circuits

I38 I I. Dealing with References and Regulators

Once I worked on a regulator that ran okay at -55 “C, at room temperature, and at
+I25 “C, but not at some intermediate temperatures. That was a nasty one. Because
some engineers had tested the regulators at hot and cold temperatures and saw no
trouble at these extremes, I had to work very hard to convince them not to ship these
parts. I had to take them by the hand and show them where the trouble was. It’s like a
dumb cartoon I once saw showing three men walking out of a movie-an old man, a
young man, and a middle-aged man. The posters said the movie was “fun for young
and old.” And sure enough, the old man and the young man were smiling, but the
middle-aged man was frowning. Even a dumb cartoon can be instructive if it reminds
you that bad news is not only where you first expect it. It may be lurking in other
places, too.
This story reminds me of a boss who asked me if my new regulator design was
really short-circuit proof. I told him, yes, I had tested it with short bursts and long
pulses and everything in between for days and weeks. With a wry smile, he went over
to a tool cabinet and removed a really big, heavy file.’ He applied this file with
rough, uneven scraping motions to ground and to the output of my regulator. He got
showers of sparks out of the regulator, but he couldn’t kill it. What a “bastard’ of a
test! Then he explained to me that the random, repetitive action of a file sets up pat-
terns of current loads and thermal stresses that can kill a regulator if its short-circuit
protection is marginal. There are many, many tricks you can use to show that a de-
sign really can survive every worst-case condition. Every industry has its own tests,
and most of them have nothing to do with computers. . . .

Switch-Mode Regulators-A Whole New Ball-Game

These simple tips aren’t meant to overshadow the truly difficult areas of regulator
design. You might wonder if it’s possible for a smart, experienced engineer to design
a switch-mode regulator that works well after only minor redesigns and goes into
production without a major yield loss. My answer is: Just barely. The weasel word
here is “smart.” If the engineer forgets some little detail and doesn’t have a contin-
gency plan to test for it, screen it, or repair the regulators that don’t work, then maybe
he or she isn’t very “smart.” Those of us who don’t design switchers all the time
would have a very poor batting average at getting a design to work right off the
drawing board-ven if we’re really good at designing other circuits. After all, a
switcher is a complicated system composed of power transistors, transformers, in-
ductors, one or more control ICs, and lots of other passive components. And, the
circuit’s layout is critical: The layout must guard small signals against electrostatic
interference and cross-talk, and, even more importantly, must control and reject the
electromagnetic strays. I mean, for a switcher to be efficient, the volts per micro-
second and amperes per microsecond get really large, so it doesn’t take many pico-
farads or nanohenries to couple a big noise into the rest of the circuit. The paths for
high currents are important, and the paths for cooling air are even more critical.
So when someone asks me how to design a switcher, I ask, “How many do you
plan to build?” If the answer is just a few dozen and the design is a full-featured high-
power job, I encourage the engineer to buy an existing design. But if large numbers
are involved, an engineer usually has the time to do the design right and spread that
effort over a few thousand circuits.
An alternative to designing your own switcher is using one of the new “Simple
Switchers”-the simple-but-complete switching-regulator ICs. Some of these

1. It was a bastard file.