poles and zeros in an smps feedback compensation circuit

[eem2am]

poles and zeros of low pass filter

hello,

do you know where to put poles and zeros in an smps feedback compensation circuit?

..say an isolated , offline one transistor forward converter

 

[FvM]

poles and zeros basics

You usually start with determining a time continuous equivalent circuit for your control process and estimating it's transfer function. For a buck converter with LC output filter, it's typically a gain with double pole and a dead time. A PID controller is generally suitable to close the loop. You can draw a bode plot and position the PID pole and zero in a meaningful way to achieve sufficient phase margin and loop bandwidth.

Many PWM IC datasheets and application notes have detailed guides how to design the controller frequency characteristic.

 

[nebisman]

Re: poles and zeros

You are talking about a topic called power electronics system modeling. Transfer function ca be obtained in easy way for a Buck converter, basically the transfer function is the same as the RLC resonant circuit used as low pass filter for the square wave of PWM generation. Other topologies, as Boost converter, require more subtle modeling process and you can find it in the excelent book Power Electronics: Converters, Applications, and Design by Mohan

In this book you will find detailed models for each common topology. Another pretty book with this topic is Eriksson´s fundamentals of power electronics.

When you get the basic learning of the models, yo should use the tool power system blockset of matlab for simulation of these models. Compare the circuits models with inductance, capacitante resistance and transistor with their linear system conterpart.

 

[treez]

...This is an old thread...but of interest.....Few engineering companies will pay engineers to get into all that theory.......just find what the poles are and churn the math and test it with a phase/gain analyser.

EG for flyback in DCM current mode,

you have...

Pole at 1/PI.R.C

..where c = c(out) and R = load

Zero at 1/2.pi.R.C
...where c = c(out) and r = esr

...i doubt many employers will pay somebody and learn up on Maksimovic's book, its great but too time consuming for an employer to pay for that time