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Control Circuits
Raymond Mack
CHAPTER 4
We will explore the various forms of controllers available from semiconductor
manufacturers. A large variety of controllers are available, but each part is usually
intended for a narrow application. I will refer to application notes from various
manufacturers. These are available on each manufacturer s website or by contacting the
manufacturer.
The control section is important for the behavior of the switching power supply. It has a direct
effect on output regulation, transient response, efficiency, and its reaction to adverse operating
conditions. There are several methods of controlling a switching power supply, each enhanc-
ing certain aspects of the operation, but also possibly creating difficulties in another area. The
selection of the best control method for your particular application should be based upon a
good understanding of each of the needed operations.
The feedback loop compensation (Chapter 9) is the last aspect of the design of the control sec-
tion. The type of compensation used will determine the stability of the supply, the accuracy and
the transient response of the output.
Even after designing the perfect control section, other unseen factors can affect its proper
operation. I once modified a supply design I had used before. I gave my schematic to the
printed circuit board designer and later built the prototype. I simply could not get the supply to
regulate and it oscillated badly. I then examined the PCB layout. It was electrically correct,
but the control was on one side of a noisy ground node and the voltage sensing circuit was on
the other side. Lots of noise was being injected into the sensitive input of the error amplifier.
I then had to instruct the PCB layout person on the techniques of good layout practices. The
next prototype operated flawlessly. In short, ignorant PCB layout designers and autoroute are
your enemies.
Ray Mack has a very good overview of the various methods of control, their strengths and
shortcomings. There are optimum control methods for each power topology and Ray Mack
makes these associations.
— Marty Brown

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