Abstract

Whether or not a Proportional + Integral + Derivative (PID) controller is suitable for stabilising power supplies has become an emotive debate rather than a scientific discussion. The debate has been compounded by the arrival of many digital power solutions, which seem to come as standard with a PID controller programmed in their software.

People like PID controllers for their simplicity and are reluctant to change what they know and love for something else that performs better, but needs a more detailed understanding of control theory.

In this article we would like to present our reasons as to why, here at Biricha we try to avoid using PID controllers for the majority of the power supplies that we design. This is merely our opinion and is not intended to re-start a debate, and as such, we ask you to decide for yourselves as to whether or not PID is suitable for your application.

Introduction

PID controllers are extremely easy to design and implement. With less than 10 minutes on any search engine you will find many analog and digital realizations; there are even automatic tuning algorithms. Whilst this controller and its close relative (PI) are excellent for stabilizing motor control systems or temperature controllers and the like, it is our opinion that they are unsuitable for compensating most switch mode power supplies. Furthermore, although one may be able to get away with using a PID controller in a current mode converter, there are better alternatives. If you do intend to use a PID controller, we highly recommend reading the excellent analysis by Christophe Basso which was presented at APEC in 2011 and is available for free on Mr Basso’s website [1].

The main attraction of PID is that one does not need to know much about control theory, frequency response analysis, pole-zero placement, phase margin, gain margin, stability criteria, etc etc. Instead, you heuristically (i.e. scientific word for semi-educated trial and error) change the compensator’s 3 gain terms to achieve the desired response. Though this method is extremely effective in millions of applications, we feel that it is not suitable for power supply design.

Most experts agree that switching power supplies should be designed in the frequency domain, and yet PID is a tool whose main advantage is that you don’t design in the frequency domain! This is a contradiction and, therefore, you can see from the onset that PID is not suitable for what we are trying to do, even though it is ideal for many other applications.

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