Design of Resonant Coordinate Circuit for Switch Mode Power Supply

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Overview:
The most troublesome part of designing a switch mode power supply is the RCD buffer. The conventional method of designing an RCD buffer has no detailed description of the off-state transients of the main switch. Therefore, the design equations in the traditional design are not completely correct. This article describes a new approach to designing and analyzing RCD buffers for flyback converters. The resonant coordinates provide an easy way to understand the main switch off transients and help to easily design and analyze the RCD buffer.

1. Introduction Commercially, flyback converters are widely used due to their simple construction, compact size, light weight and low cost. However, its main switch performs a hard switching operation, resulting in higher voltage spikes and oscillations on the main switch. The voltage stress of the main switch increases depending on the magnitude of the voltage spike. To reduce voltage spikes in order to use lower cost, low voltage MOSFETs, the most widely used method is the RCD buffer network. Even if the buffer voltage decreases as the snubber resistance decreases, the power consumption on the buffer network increases, resulting in a decrease in overall system efficiency. Therefore, the RCD snubber network should be optimized to meet both the main switching voltage stress and the overall system efficiency.
This article will first introduce the traditional analysis of voltage spikes caused by the leakage inductance of the main transformer. A simple way of describing the off transient period will be introduced for further analysis. The buffer current will be analyzed in the buffer coordinates to provide a more detailed design equation.

2. RCD buffer design and analysis
2.1 General Method of RCD Buffer Design Figure 1 shows a conventional flyback converter with an RCD buffer.

Figure 1: Traditional flyback converter (please read the PDF for details)