Stability Analysis Of The DC-DC Charging Chip Based On BUCK Topology

In modern society,with the rapid development of portable electronic devices, DC / DC switching converters which own excellent properties have been more and more applications in various types of electronic equipment. Currently, switching power converter used in battery charging is becoming mainstream.This paper focuses on the working principle and stability studies of a single ring charging chip which is in an average current mode based on BUCK topology. First, it makes a presentation of the system working principle in detail. Secondly, we calculate the small signal transfer function of each module in this loop and then complete the error amplifier and compensation network design based on the stability conditions of the system, finally we finish the system modeling and simulate system stability through MATLAB software, the simulation results show that the open-loop DC gain is 76 d B, the bandwidth is 87 KHz, and the phase margin is 56 degrees, so that the entire loop satisfys the conditions of the closed-loop stability. Meanwhile, the system has a relatively high power supply rejection ratio, the DC power supply rejection ratio is 75 d B, and the power supply rejection ratio is 49 d B at the switching frequency, so the system can be stable. Thirdly, we finish the main module circuit design affecting the stability of the system. At last, we complete the module circuits and entire system simulation through HSPICE software.The charging chip uses the three-stage charging method which includes trickle, constant current and constant voltage charging mode, it improves charging efficiency, while increasing the battery life. During the constant and trickle charging process, the system uses a single ring which is in an average current mode based on PWM(Pulse Width Modulation) control mode, and the system works in closed loop, thus it will cause loop stability problems in the charging process, so we need to design the loop of this system carefully, this paper makes a detailed description of the design for the sampling-amplifier-module, the error amplifier, and the PWM comparator, the error amplifier which regulates system stability is an crucial module, and its performance directly affects the stability of the system. The error amplifier is realized herein by a folded-cascade transconductance amplifier structure which has a relatively wide common-mode input range, high output impedance and high gain, but also has better response speed, and it meets the requirements of the system stability. In constant voltagecharging process, the system works in open-loop operation mode, but the process is very short, so as to ensure the accuracy of the charge cut-off voltage requirements.Under 5V input voltage, 800 KHz switching frequency, 0.1Ω sense resistor, trickle charging pin connected to ground conditions, we complete the system simulation, the charging current is 0.1A when the battery voltage is below 2.9V, and when the battery voltage rises above 2.9V, charging current becomes 1A, meanwhile, the charging current with high precision can be achieved and it charges stably. When the battery voltage rises to 4.2V, the system enters into the constant-voltage mode and the magnetic of the inductance is into imbalance mode, the magnetizing is greater than the degaussing, so the inductor current begins to drop until the charging current drops to 0.1A, then the charge-enable signal of the chip shuts down after a little time(this article sets 1.8ms). Finally, the entire charging process is completed.