Hysteretic Controlled Dc-dc Converters

Chauhan, Shweta. M.S.Egr, Department of Electrical Engineering, Wright State University, 2014. Hysteretic controlled DC-DC converters. Switched-mode DC-DC converters are widely used in applications requiring stepup and step-down of DC voltages or currents. These converters find their use in portable applications such as laptops and smart phones, radio-frequency power amplifiers, as light emitting diode (LED) drivers, etc. The power converters consist of a switching network, energy storage elements such as inductors and capacitors, and a load resistor. Transformers are used in converters, which require isolation. The switching network comprises of MOSFETs and diodes. With improvement in the VLSI technology, smaller MOSFETs with increased power handling capability are pushing the speed of operation of these power converters to the gigahertz (GHz) range. Operation at such high frequencies not only require energy-efficient semiconductor switches, but also demand for faster control mechanisms. Amongst the various power converter control schemes studied in literature for high-frequency applications, the hysteretic control scheme is given high importance. The hysteretic controller employs a basic operational amplifier (op-amp) and works similar to the Schmitt trigger with hysteresis. Also, the bandwidth of op-amps is theoretically infinite and can be designed with ease for many applications, making the hysteretic controller scheme, simple and widely used method. This thesis focuses on understanding the operation and characteristics of the hysteretic control scheme. Initially, a buck DC-DC pulsewidth modulated (PWM) converter is used as the power stage and the hysteretic controller is designed to ensure proper regulation of the output voltage of the buck converter. Two different types of hysteretic control mechanisms, namely (a) dualcharging mode and (b) capacitive-charging mode are investigated. The equations necessary to design the controller for both modes are derived. Extensive simulations are performed in order to evaluate the load and line regulation with and without iii the controller. Further, similar analysis is performed using a boost DC-DC PWM power converter as the power stage. Various characteristics such as percentage load regulation (LOR), percentage line regulation (LNR), and total harmonic distortion (THD) are estimated for buck converters.