Study and Comparison of various Digital Control Techniques for DC-DC Converters

Abstract

Recent years have seen enormous amount of research and development in designing a highly efficient power supply that has good transient performance. In present scenario portable hand held applications have been dominated by integrated power supply management. In this thesis voltage mode control and current mode control of DC-DC converter has been presented. The overall design of a closed loop DC-DC converter is divided into the following different parts. First of all design of open loop Buck or Boost converter, state space analysis of the converter to find the open loop transfer function, and design of the compensator. State space averaging and then linearization is applied to obtain the small signal model of the converter to derive the various transfer functions. After obtaining the control to output transfer function, a compensator is designed to stabilize the closed loop response of the system. Normally, the compensator is designed in in analog domain and then it is transformed into equivalent discrete domain by using some transformation method like Backward Euler method, Bilinear method or pole-zero matching etc. Z-domain transfer function of the converter and modulator is needed for designing a digital controller directly. In the feedback loop, an analog to digital converter (ADC) is used before the compensator and a digital pulse width modulator (PWM) is used after the compensator. For Zeigler-Nichols tuned PID controllers a new tuning method is introduced for fast transient response and application requiring precise control. The performance of the system having auto tuned PID controller is improved as compared to Z-N tuned PID controllers and robustness and the dead time is controlled by changing the parameter of the controller. Current mode control is used to improve the dynamic response of the system. Normally, there are two control loop in the current mode control, one is voltage control loop and another is current control loop. There are different type of digital current mode control techniques like peak current mode control, average current mode control and valley current mode control. The dynamic response of current mode control is better than voltage mode control.