Flyback Converter Design Process

Question

I'm kinda new to designing. I have a task at hand which I hope a kind soul is willing to guide me to right direction. I need to design a 2 output flyback converter. First Output would be (12V x 5A = 60 watts). 2nd Output would be (8V x 5A = 40 watts). Vin would be 230V AC, 50Hz. I have sourced through the web but there isnt much help in designing for beginners. Such as how do I calculate Cin, the Snubber, Cout and stuffs like that. I was planning to use the UC3844 as my Controller IC but the datasheet was too complex and only convered based on UC2845. Was planning to add a control feedback circuit from the output to the PWM IC. Thanks in advance.

Answer 1

I would like to warn you that full blown mains converter design, at any reasonable power level is not for beginners, far from it. The mains AC poses high risks during the design and debugging process and there are many practical pitfalls both in schematic design and in the layout of the design, such as parasitics.

But regarding the question the design resources are quite scattered. I do not ever remember running into a "A to Z" article for a multiple output mains converter with all the important aspects covered, but i can tell you that many datasheets and application notes from the various IC giants (Texas Instruments, Analog Devices, Onsemi, Maxim, etc, etc) contain various information in varying quantities. So unless you are willing to buy a good book on converter design (many available from amazon for example, i do not know the best one) you should look at as many PWM controllers datasheets as you can find, you might even run into a controller that makes your life as easy as possible.

You will run into another challenge when you look at the feedback loop design, especially if it requires manual compensation, but you will find some articles online on different compensation schemes and how to use a TL431 for example to create a feedback path through a optocoupler, but it is alot of guesswork and very difficult to verify without a network/spectrum analyzer.

Answer 2

Please mind that you're trying to design an offline SMPS which can turn into an oscillator if not designed carefully and properly.

I've designed a lot of flyback and forward converters but writing down all the steps including transformer design is quite time-consuming for me. Sorry for that. So, for now, the easiest way is providing already-existing materials.

First, please get at least one of the following:

• Switching Power Supply Design by Abraham Pressman

• Switchmode Power Supply Handbook by Keith Billings and Taylor Morey.

Second, googling "UC3843 appnote" and "Offline Flyback Design" can give a lot of design guides and appnotes:

http://www.ti.com/lit/an/slua143/slua143.pdf (Design Guide and Appnote with UC3843/3844/3845)

http://www.onsemi.com/pub_link/Collateral/AN1327-D.PDF (Design Appnote)

https://www.fairchildsemi.com/application-notes/AN/AN-4140.pdf (Transformer Design)

Flyback converter losses (My answer about flyback transformer design on another topic)

https://www.fairchildsemi.com/application-notes/AN/AN-4137.pdf (General Offline Flyback Design but not with UC3843)

http://www.planetanalog.com/author.asp?section_id=3265&doc_id=563351 (RCD Snubber Design)

PS: These materials provide all the necessary information. You cannot get the full background by only reading and calculating. You have to build, measure, see waveforms etc. But if you have questions about specific steps (e.g. air-gap calculation, winding, stabilization network etc.), don't hesitate to ask.