Flyback RC RCD Snubber Design Procedure

Original Question

I want to design a flyback converter for my PS. In the design process, I want to design a parallel R&C with a diode [snubber] for the primary of the transformer. But I do not know the relations for this design. Could you please help me to find the relations or provide me with a good reference for this purpose? M. S. 05-14-2008

Answer

"Flyback Converter Snubber Design - How to eliminate the trial-and-error from snubber and RCD clamp designs, by Dr. Ray Ridley."

This article is free to download from the Internet.

  • Go to Ray Ridley's Switching Power Design Resource Center
  • Click on Articles
  • Sign up for the Resource Center and its newsletter by entering your email address and a password. I highly recommend this. The articles are some of the most practical ones you will read on switching-mode power design.
  • Click on Semiconductor
  • Towards the bottom you will find the article Flyback Converter Snubber Design. Click on the title and a PDF file will download.

The article starts with showing you the ringing associated with the unsnubbed flyback. It then describes the design procedure for an RC snubber across the power MOSFET to damp the ringing. The procedure starts with measuring the frequency-dependent leakage inductance of the flyback transformer with a network analyzer. This might be a problem for those without access to a network analyzer.

  1. Measure leakage inductance as a function of frequency
  2. Measure unsnubbed ringing frequency
  3. Calculate the snubber resistor and capacitor using the inductance value at the ringing frequency
  4. Calculate the snubber dissipation
  5. Experimentally verify the design

If an RC snubber does not sufficiently clamp the voltage, then a RCD snubber may have to be used. The design procedure is similar, but with different equations and some subtle differences, like using the measured inductance at the switching frequency, not the ringing frequency (the reasons are clearly given for this and all the design steps.)

Finally, the procedures are given for designing the RC snubbers for the secondary rectifying diodes.

Knowledge Annealing

The goal of this website is to provide the one-best resource answers to questions, preferrable with free access from the Internet. Ray Ridley's Flyback Converter Snubber Design is the best I have found for flyback RC and RCD snubber design. Do you have a better one or any comments? Let me know and help make this a better website for you and your fellow designers. -- Jerrold Foutz



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