Design Of Over-current Protection Circuit With Constant On Time

Any electrical equipment can not work without the power supply. Switching power is widely used because of high conversion efficiency, rich topology, and can achieve a variety of transformation. Constant on-time control mode became the hotspot of switching power supply control mode, which has simple control loop, high response speed, with load changing the operating frequency, high efficiency under light-load. Over-current protection (OCP) circuit is an important part of the power management chip, and it prevents the output current too large to burn the power system and the load when the circuit is shorted, output overload, or other abnormal conditions. In this thesis, an over-current protection circuit of COT architecture Buck converter is designed which has an over-current threshold with temperature compensation, timing adjustment the over-current threshold.First, three kinds of power technology, background and significance of the research, the research situation at home and abroad, as well as the basic principle of the switching power supply are introduced briefly. The basic principles and control mode of the buck converters are summarized and analyzed. The working principle and control mode of COT architecture Buck converter are analyzed in details.Then, the over-current protection of the basic method is introduced, the output current sensing methods commonly used are summarized, and analyzing the advantages and disadvantages of each method; analyzing the traditional OCP architecture of COT Buck converter which through the power MOSFET voltage drop to sample the output current, illustrated the over-current protection circuit, and analyzed their strengths and weaknesses, analyzed the structure of the basic needs of COT Buck Converter over-current protection. Current sensing by the voltage drop of power MOSFET has a large temperature coefficient. It keeps the largest current constant output when the over-current is triggered. The over-current protection circuit is designed which has temperature compensation, timing adjustment threshold current.Finally, the entire circuit is designed by0.5μmBiCMOS process. The sub-modules of the over-current protection circuits include high stability, low offset voltage, high-speed comparator, temperature compensation circuit, operational amplifiers, timing circuits, two kinds of temperature coefficient current bias. Using NPN follower to achieve threshold voltage temperature compensation, by using voltage to current methods to eliminate the impact of the banding wire, using a simple way to eliminate the current temperature coefficient, using PNP follower to achieve the common mode input voltage to zero, three-level pre-amplified, two kinds of temperature coefficient current bias supplied to the differential amplification, achieved maximum delay deviation2nS stable high-speed comparator under-40℃-125℃. Added the over-current protection circuit into the COT structure Buck converter to verify by Hspice, and achieving the over-current threshold temperature compensation, so that the first over-current threshold of output current is5.1A, timing adjustment the over-current threshold, the second over-current threshold is4.3A which can constant current output, reach the system requirements.