Research On Control Strategy For Cascaded Buck-boost Converter

Due to the inherent advantages, including the ability of both voltage step-up and step-down, positive output voltage, the cascaded Buck-Boost converter has been widely used in the applications including electric vehicle, super capacitor storage system, new energy power generation with wide input voltage range.Traditional Constant-Frequency Zero Voltage Switching Modulation(CF-ZVS-M) without adding any auxiliary circuit, allows the MOSFET switches to turn on under zero voltage switching conditions. However the accurate calculation of the switching time, and making three dimensional look-up table increase control complexity. Regarding the issue above, a Modified Constant-Frequency Zero Voltage Switching Modulation(M-CF-ZVSM) is proposed. The inductor current has a negative offset current at the beginning of each pulse period. MOSFET switches can achieve ZVS over full operating range, which significantly improving efficiency and alleviating electromagnetic interference(EMI).Firstly, the basic principle and working process of CF-ZVS modulation are analyzed and the M-CF-ZVS-M applied in the constant voltage control and constant current control is discussed. In the constant voltage control, in order to obtain the minimum inductor current rms, duty cycle and phase angle expression are derived under different mode. Ultimately, getting the minimum inductor current RMS improves the transmission efficiency of the converter. For constant-current control, In order to avoid switching between the three modes, the analysis of the relationship between phase shift angle and the output voltage under the conditions of fixed the duty cycle is shown. Then, large inductor current peak is not conducive to the application of high power, so analysis of the control of multi-module interleaved. In addition, guidelines for parameter design and loss model are given as well. Finally, an experimental prototype with 50~100V input and 100 V output is designed and built to verify the effectiveness and feasibility of all proposals.