Page 152 - Troubleshooting Analog Circuits

Switch-Mode Regulators I39

chips-LM2575, LM2576, LM2577, LM2578-are about as foolproof-for a
switcher-as you can get. The data sheets of these parts explain that. You may need a
couple resistors, a few capacitors, an inductor, and a fast rectifier, and then it’s done.
You’ll have a cookbook circuit that really does work. And if you want to get the
component selection information from a program on a floppy disk, I am told that
works quite well, too, and is considered pretty ‘‘user-friendly.’’ If you only need to
supply a few hundred milliamps to your circuit, you may not even need a power
transistor or a heat sink. Even in the last couple years there have been advances in
designs that really do work, as opposed to “paper designs” that have no chance of
surviving a short on the output or of working under worst-case conditions. Although
a few of these useless paper designs still pop up from time to time, thank heaven
most of them have been driven out.
One of the stories that keeps rattling around the industry is about a group of engi-
neers who decide to band together and start a new computer company. The smartest
one is assigned to do the main processor board. Another smart engineer does all the
interfaces. And the smart but green “kid engineer” is assigned to do the switch-mode
power supply because, of course, that’s the easiest part to do. (Anybody who has
worked on a big switcher will probably speak up right away: The switcher is not as
easy as it seems.) In the end, the power-supply design takes a lot longer than every-
body expects.
One day, the young engineer opens up the compartment where the balky power
supply resides, and it blows up in his face. After his co-workers take the poor fellow
to the hospital, they ask around and find a consulting engineer who makes a living
out of fixing exactly this kind of switcher problem. The switcher design was slightly
off-course and needed the hand of an expert before it would work correctly. So re-
member, designing switchers is no simple task. Don’t hesitate to call in an expert.
Note, if this story were not substantially true, the consulting engineer would have
starved to death, long ago. I rest my case.

Regulators Suit Different Power Levels

There are several different configurations of switch-mode regulators. At low power
levels, capacitively coupled switcher designs are simple, but don’t provide much
choice of Vout: 1.9 X Vi,, -0.9 X Vi,, and 0.45 X Vi” are almost the only choices.
Flyback regulators are the simplest and cheapest magnetically coupled regulators.
However, at power levels above 100 W, their disadvantages become objectionable,
and forward or push-pull schemes are more appropriate. At the highest power levels,
bridge-type designs are best. If you try to use a configuration at an inappropriate
power level, you may have to struggle to get it working. Likewise, the use of current-
mode regulation may help you get faster loop response, but the concept is difficult to
understand, let alone execute.
Current limiting is always a problem with switchers. The choice of a sense resistor
is not easy because the resistor must have low inductance. As with most aspects of
switch-mode regulation, to achieve good reliability and to avoid trouble, you have to
spend the time to design and test the current-limiting circuit carefully. Some newer
switch-mode controller ICs have been engineered to make it reasonably easy. Older
ones like the LM3524 haven’t been, usually.
Similarly, a soft-start circuit is important for a large switcher, especially when the
switcher strains to put out a lot of current to quickly charge up the output filter capac-
itors, and especially for a boost configuration, where the inductor’s current might
saturate and refuse to pull the output high enough. For a large supply, this current