Research on the Power Stage Model of Flyback Circuit

　　1. Introduction

　　With the rapid development of electronic technology, the application of switching power supplies is becoming more and more extensive, and it is developing rapidly in the direction of integration, intelligence and greening. In the current isolated switching converter, the flyback switching power supply has only one transformer and switching device, which has the advantages of simple structure and small size. It is widely used in various fields such as post and telecommunications, aerospace, instrumentation, and household appliances. Due to the increasing application of flyback switching power supplies, higher requirements are put forward for the accuracy and stability of power supplies. Therefore, higher requirements are also put forward for the control system of flyback switching power supplies, which requires further research on the circuit model of flyback switching power supplies so that the flyback circuit control system can be designed more reasonably. This paper will be based on a single-ended flyback circuit controlled by the UC3843 chip, and compared and verified with the existing five flyback circuit power level models to confirm a power level mathematical model of the single-ended flyback circuit designed in this paper.

　　2. Flyback circuit system model

　　2.1 Flyback Circuit Structure

　　The flyback converter is usually composed of an integrated TOP-Switch power chip or a UC3843 current control chip and a power MOSFET. Figure 1 shows a single-ended flyback circuit controlled by a UC3843 chip. This article will verify and analyze five flyback circuit power level mathematical models for this circuit.

　　The single-ended flyback power converter is evolved from the electrically isolated Buck-Boost power converter, so the model of the single-ended flyback power converter is the same as the model of the Buck-Boost power converter. Therefore, we can combine the power level mathematical model of the Buck-Boost circuit to summarize five flyback circuit power level PA mathematical models.

　　Matlab is then used to calculate the five circuit mathematical models and obtain the step responses of the output in the time domain, as shown in Figures 7 to 11.

　　Now read the time domain parameters of the five circuit models calculated by Matlab, the Psim simulation output waveform, and the output waveform of the single-ended flyback experimental platform. The time domain parameter indicators are summarized in Table 1. Among them, since the time domain output characteristics of model 3 are divergent, the relevant parameter indicators of model 3 are not listed in the table.

　　Since the time domain output characteristics of model 3 are divergent, it is definitely not a suitable model. From the various time domain indicators shown in Table 1, it can be seen that the time domain indicators of model 1, model 2, and model 4 are far from the time domain indicators obtained by experiment and simulation, and are also not suitable for the output characteristics of the single-ended flyback circuit system in this article; the time domain characteristics of model 5, such as overshoot σ%, steady-state value h(∞), rise time tr, peak time tp, and adjustment time ts, are basically consistent with the time domain parameter indicators obtained by experiment and simulation. Considering the influence of device errors in the experimental circuit and calculation errors in the simulation, we can conclude that model 5 is the most suitable model for the single-ended flyback circuit in this article.

　　4. Summary

　　Through the time domain outputs obtained by the above three methods, after comparative analysis, it can be concluded that Model 5 is the most appropriate power level mathematical model of the single-ended flyback circuit in this article.

　　References

　　[1] Zhang Weiping. Modeling and Control of Switching Converters. Beijing: China Electric Power Press, 2005.

　　[2] Zhang Zhansong, Cai Xuansan. Principle and Design of Switching Power Supply[M]. Beijing: Publishing House of Electronics Industry, 2004.

　　[3] Xu Dehong, Shen Xu, Yang Chenglin, et al. Switching Power Supply Design Guide (Second Edition). Machinery Industry Press, 2004.

　　About the Author:

　　Zhang Weiping (1957-), male, PhD, professor, doctoral supervisor, his research interests include power electronics technology, MH lamp electronic ballast, etc.

　　Jiang Zhiliang (1987-), male, master student, his research direction is wide voltage and high efficiency range DC/DC module power supply.