Design Type II compensation in a systematic way

  (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 properly, it is necessary to have both enough phase margin and gain margin, In this sense, it is important to have a design procedure that can take care of both design parameters.

   In this note, a systematic design procedure is developed to design a type II compensator, which gives the designer an easy way to take both phase margin and gain margin into consideration and to adjust the compensator until the design requirement is satisfied. Due to its length and complexity, "Design type II compensation in a systematic way" is presented as a single Word document, here.

About the author

Liyu Cao is a Sr. Engineer at Ametek Programmable Power, where he is engaged with designing programmable DC and AC power supplies and control loops. Liyu holds a Ph.D. degree in electrical engineering from Tsinghua University, Beijing, China.

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