Research On Control Strategies Of Charger Based On Buck PWM Rectifier

In recent years, batteries and ultra-capacitors are widely used in transportation and communication. Chargers are specially designed for charging batteries and ultra-capacitors.Users and producers are concerned about the size, weight, efficiency and cost of charger.Voltage source PWM rectifier, which is a boost converter, can work under unit power factor.To apply in charger, a DC/DC converter must be linked with its output, which will increase the cost and decrease the efficiency. Buck PWM rectifier can be applied in charger without any converter behind, which makes it more appropriate to be applied in charger. This paper focuses on control strategies of charger based on Buck PWM rectifier.The switch modes of Buck PWM rectifier are discussed and mathematical models in the abc and dq coordinates are established in this paper. The relationship of output voltage and current with input voltage, load, filter inductance and filter capacitance are analyzed. It indicates that increasing input voltage and modulation ratio, or decreasing the phase difference between input voltage and switch function can increase output voltage and current.The transformation of switch functions from 2-value logic to 3-value logic, choice of DC inductance discharge modes, switch modes in area of ??? and the generation method of 3-value logic PWM signals are analyzed. In DC side of Buck PWM rectifier, a freewheeling diode is designed,thus,the DC inductance will discharge via the freewheeling diode which decreases switch power loss, and the rectifier can work in way of seven switch modes. Indirect-current and Direct-current are applied more in control system of Buck PWM rectifier. A voltage out-loop is added in based on strategies above,thus,the rectifier can work in current mode, voltage mode and current-voltage mode.Charger based on Buck PWM rectifier is designed in this paper, the input voltage of this charger is AC220V, the range of output voltage is 30?300V, the range of output current is 10?100A and the power is 30kW. Parameters in the main circuit are analyzed, including filter inductance, filter capacitance, DC inductance, and so on.At last,a model in Matlab/Simulink is established. It indicates the generation of PWM signals in 3-value logic,the working method of seven switch modes is feasible,and that it can work in current mode, voltage mode and current-voltage mode. The output voltage and current, the power factor of input and the THD of input current are compared in both Indirect-current and Direct-current control system in this paper. The compare indicates that Direct-current control system is more steady and can work under unit power factor when the output voltage is very low.