How to choose Inductors and Transformers for Flyback Converters? - Page 1
Forum section: electronics / projects

author: wholder
author_rank: Regular Contributor
timestamp: June 28, 2017, 09:13:27 am
content: I'be been using and designing with switching power supply ICs for a few years, but I always start by adapting from a design published in data sheet or app notes for the particular IC, which includes using one of the suggested inductors of transformers. However, quite frequently the parts specified are hard to get, or are no longer available and I need to find replacements. For simple, low voltage boost/buck converters I can usually find parts that work, although I often wonder if if my choice works as efficiently as another part might. However, I recently needed to design a circuit that could boost 5 volts to around 240 volts. This is for use in an electrostatic application, so current requirements are small.I looked at several ICs that were designed to work as "photoflash" boost converters, but most of these now seem to be obsolete or are hard wired to produce 300 volts, not 240. I decided to see if I could work out a circuit of my own but soon realized that, while I understand the concepts behind a transformer-based flyback converter (the basic circuit is quite simple), I don't really understand how one goes about selecting a suitable transformer for this application. I've read a lot of stuff online, such as articles about building power supplies for nixie tubes, and such, but I've yet to find one that talks about how to select the right transformer for the job. So, if anyone here can help educate me, I'd greatly appreciate it.For example, in the circuits shown for the photoflash ICs I mentioned, one used a transformer with a 1:10 turns ratio, while another used one with a 1:30 turns ratio, yet both claimed to boost from around 5 volts to 300 volts. So, what is the relationship between the turns ratio and the amount of voltage one can get from a transformer used in a flyback converter? Does it matter? As a counter example, I saw a geiger counter circuit that used an audio transformer with a 1:1 turns ratio that boosted to several hundred volts and then drove a diode/capacitor ladder to get the 900 volts, or so, needed for the geiger tube. This is counterintuitive to me.Wayne

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 28, 2017, 10:02:18 am
content: Hmm... this seems to be a popular subject these days. Please refer to the thread below:https://www.eevblog.com/forum/projects/(please-check-my-design)-tl494-controller-opto-isolated-regulated-flyback/msg1223717/#msg1223717

author: Dave
author_rank: Super Contributor
timestamp: June 28, 2017, 10:07:04 am
content: I think you're missing some fundamental understanding of magnetics.Here's the kicker: The 'transformer' in a flyback converter isn't really a transformer as such, but it is actually a coupled inductor. One winding is used to build up the magnetic field and the other is used to collapse it, there is no real transformer action going on in the circuit. As a result, the input to output voltage ratio isn't firmly tied to the turns ratio of the coils.If you really want to understand these things, I think you need to start from the basic topologies and build your knowledge and understanding from there. A flyback transformer can be fairly easily explained as a variation of the inverting (buck-boost) converter.

author: wholder
author_rank: Regular Contributor
timestamp: June 28, 2017, 07:02:22 pm
content: Thanks for the reply. I understand about the "transformer" actually acting as coupled inductors in a flyback converter, but it seems like the turns ratio must have some effect. I also read frequently about the need to for a flyback transformer to have an air gap to avoid saturation of the magnetic flux, but I've yet to see a data sheet for a transformer intended for use in a flyback converter mention anything about an air gap.Wayne

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 28, 2017, 07:40:34 pm
content: Quote from: wholder on June 28, 2017, 07:02:22 pmI cover that extensively in the post I linked to above. Ahem. Turns ratio is not terribly critical in a flyback, but it does, indeed, have some effect. One way to think about a step-up flyback using a 1:n turns ratio is to treat it like a boost converter that has to step up the input voltage to Vout / n. There is some merit to the argument for selecting a turns duty cycle that results in a duty cycle around 50% - which would imply a Vout /n that is twice Vin - but it really isn't critical. But again, read the post I linked to previously.Quote from: wholder on June 28, 2017, 07:02:22 pmYou aren't looking hard enough? See, for example, this pre-sorted list of all the gapped and ungapped ferrite cores in stock at Mouser in the ever popular N87 material:http://www.mouser.com/EPCOS-TDK/Passive-Components/Ferrites/Ferrite-Cores-Accessories/_/N-fb8t6Zscv7?P=1yxgwk1Z1ytdx60&FS=TrueAlthough it is true that most of the coupled inductors (which implies the presence of a gap) do a poor job of specifying their components. I suppose they don't want you to try to duplicate them yourself too easily.http://www.mouser.com/Passive-Components/Inductors-Chokes-Coils/Coupled-Inductors/_/N-afrbk?Keyword=flyback&FS=True

author: wholder
author_rank: Regular Contributor
timestamp: June 28, 2017, 07:47:57 pm
content: Sorry, I was still processing your post when I replied to Dave's reply. I appreciate your help.Wayne