Improvement Of Dynamic Performance And System Stability Of Digital Control DC-DC Converter

The increasing performance of load equipment in spacecraft has significantly increased the requirement for high quality power supplies. However, the off-the-shelf power supplies with analog controllers are difficult to meet this requirement. There are mainly two drawbacks: first, the topology of the power supply can not meet the requirement for new load equipment. Topologies such as Forward, Flyback and Push-pull are now widely used in the DC-DC converters for the spacecraft. These topologies are not capable of supplying multiple outputs with precisely regulated voltage and they cannot handle the application with low-voltage high-current as well. Moreover, the feedback methods for these topologies normally use optical coupler feedback or magntic feedback. Optical coupler does not have high reliability, and its current transfer ratio drifts with the changing of the temperature. Magnetic feedback circuit is complicated and the bandwidth is low. Second, analog DC-DC controllers can not realize communication, monitoring and power management functions. They are susceptible to the interference of the working environment, and their parameters are llikely to drift. This will make the converters unstable or even cause them damage. Therefore, power supplies have become the bottleneck that limits the development of the spacecraft platform.One of the main goals of this dessertation is to improve the structure of the current DC-DC converters for the spacecraft in the aspects of topology and controller, so that it can meet the requirement for the future spacecraft. A two stage topology with intermediate DC bus is adopted. Open-loop Push-pull topology is adopted for the first stage. Several parralleled synchronous Buck converters are used for the second stage for the multiple outputs. Since each Buck converter is regulated independently, it eliminates the problems brought by the isolated feedback devices. So this topology can provide mutliple accurate outputs and has low-voltage high-current handling capability. This topology has an optimized structure and can achieve high efficiency. Thus it is suitable for most small and medium rating power supplies in the spacecraft. Furthermore, a digital controller chip for this topology is proposed and developed. A three output DC-DC converter is built here, which aims at providing a platform for the futher research.Digital controllers usually can not drive MOSFETs because they can not provide high trasient currents. An improved transformer coupled gate driver is proposed in this dissertation. It solves the defects caused by the conventional transformer coupled gate driver scheme. When the duty cycle of the input PWM signal changes drastically, the output of the driver can follow this signal rapidly and accuratly. This proposed driver scheme can be widely used in the high frequency switching power supply and motion control applications.An optimized design method for the topology parameters and the phase number of a multi-phase Buck converter is researched. Relationship between the phase number and the input/output current ripple is derived. Based on the proposed method, a multi-phase Buck converter is controlled by a digital controller chip. The steady-state performance and dynamic performance are compared with a traditional single-phase Buck converter.To improve the dynamic performance of the DC-DC converter., an analog compensation circuit is added into the digital control loop. It can improve the control loop phase margin and get the same compensation performance which is normaly achieved by using a high order compensator. So the peak to peak output voltage deviation caused by the changes of load or line voltage can be decreased to achieve a high quality Point-of-Load (POL) converter.The loop gains of DC-DC converters with load impedance is predicted. The influence of the large-capacitance capacitor and its equivalent series resistor (ESR) to the loop gain of the DC-DC converters is analyzed. A method to calculate a new loop gain for the DC-DC converters is derived. The compensator can be revised to improve the loop performance when a large-capacitance output capacitor is used..In this paper small-signal modeling, effect factors and optimized design methods for the output impedance of Buck type DC-DC converters in continuous conduction mode (CCM) are investigated theoretically and verified in experiment. The results will provide references for the standard of setting converters'impedance. A practical impedance measurement approach by using an external small-signal sinusoidal perturbation current is proposed. It can be used to measure the input and output impedance as well as the impedance ratio of cascased converter systems. A simple cascaded converter system is built and the impedance ratio between the source converter and the load converter is measured. According to the concept of the forbidden region, the stability of the system is analyzed.