(Editor's note: This article looks at a topic which combines a
pair of subjects which can strike fear into many designers:
power supplies and filters. But love it, fear it, or hate it,
it's an important topic.) Power supplies and Type II
compensators are widely used in the control loops for power
converters. A type II compensator has two poles (one at the
origin) and one zero, and the zero is placed somewhere between
the poles. Designers use this type of compensator to provide a
phase boost to the control loop. It is known that the
compensator reaches its maximum phase boost at the geometric
mean of the zero’s frequency and the second pole’s frequency. To
design this type of compensator, the popular approach is to
place the desired loop crossover frequency at the geometric mean
of the zero and pole. This approach can produce maximum phase
margin from a given compensator. However, it does not take the
other important loop parameter, the gain margin, into
consideration. In general, to make a control loop work p