Buying flyback core for DCM FBT, 45% dty. max, 50kHz freq, 10Vin, 5V 2A out. - Page 1
Forum section: electronics / beginners

author: debininja
author_rank: Contributor
timestamp: June 02, 2017, 05:13:03 pm
content: Some background: I'm a noob at SMPS and this is my very first SMPS project, so I wanted to start off with something small, but something that would make a good learning experience. I'm trying to make a TL494 driver based, 10Watt (5V @ 2A) step-down, opto-isolated, TL431 feedback regulated SMPS, and for most part, I met most of my original goals. (You can't believe how thrilled I was when it worked).Click here to see what I made.. Unfortunately, the transformer and winding ratios are incorrect due to my trying to repurpose a core from my junkbox.My original goals were:* Working PCB.Check.* Isolated flyback topology.Check.* Working opto-isolated feedback.Check.* Regulated output voltage capability of 5V, 1Amp.Partly successful.--> Drawing anything past 0.7A greatly dropped the voltage <4.5V, and the FBT drew excessive current.I was trying to repurpose a E20/14/5 core, which originally was ungapped, from an ATX PSU's auxillary power supply. I put in a layer of electrical tape to create a gap. Clearly, it wasn't good enough, or the work involved in solving for unknown parameters was too much for a beginner like me. And so, I decided to spend a few bucks and get some known value gapped cores to properly make a FBT.Using this siterecommended to me by a few people, I plugged in the operating voltages, etc, and was recommended the following transformer cores. Note that I used 2Amp output current, because I might as well take the opportunity to upgrade my goal by a bit (2Amps output vs 1Amp original goal; I'll probably have to use a TO-220 Schottky diode now due to the 8A peak current, but oh well). Also note, the given values are for 50% duty cycle, and I'm using 45% (it shouldn't make too much of a difference right?):According to the site, E16/8/5 is the best core, although E20/10/6 would work fine. I want to use a E20/10/6 core however, because I want a bit of headroom for future projects (I'm ordering a bulk of 10 cores x2, so 10 pairs).Now, the question before I buy me some cores is, is the required AL value for apairof gapped cores, or asinglegapped core and a singleungappedcore? Do I buy 10 gapped cores and 10 ungapped cores, or 20 gapped cores?I was not able to find Siemens cores with the same dimensions and specs, but I did find one by Epcos, but with a tiny bit higher AL.See here for datasheet.So I believe I'm supposed to buy 10 gapped cores and 10 ungapped cores?

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 02, 2017, 05:40:25 pm
content: Quote from: debininja on June 02, 2017, 05:13:03 pmI already anticipated and answered this question in the original thread (and why on earth did you start another thread, anyway?), but for the lazy among you I'll repeat myself: the ALvalue given for the gapped core halves assumes a combination of a gapped with an ungapped half.I also covered the duty cycle issue and strongly suggested you stick with DCM for now - that is, current goes to zero for all windings for at least part of each switching cycle. Try manually setting primary inductance to 18E-6 (18uH) and the turns ratio to 2. That will result in a maximum on time of about 9us when the input is at 10V and ensure DCM operation which makes frequency compensation a lot easier.

author: debininja
author_rank: Contributor
timestamp: June 02, 2017, 07:19:38 pm
content: Sorry about that. I figured this would be a better section to ask this kind of question inAnyways, I now have another equation in my arsenal that I can use! Woo!I did it out on paper, solving for the inductance required for the primary, with all else given, and got close to the value you told me to plug in. Got 17.76uH. I had the Iout, Vout, Vf, freq, Vin, and ton (9uS).Now that we got that, we use the AL = L / N^2 formula, right?So N = radical (L / AL), which is N = radical (18E-6 / 100E-9)And I got N = 13.42.Wouldn't let the turns ratio be a little bit lower than 2 (to account for the voltage drop of the diode?). Any reason I wouldn't want to let the program choose the turns ratio? Sorry if I ask something dumb, I'm a SMPS noob after all.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 03, 2017, 11:54:12 am
content: Quote from: debininja on June 02, 2017, 07:19:38 pmUt-oh, I see you found an equation for switch on time in DCM, and one that assumes the converter is not isolated or the transformer has a 1:1 turns ratio. It's also quite a bit messier than the duty cycle equation for CCM and has a bit of a chicken-or-the-egg conundrum in that you need to know the (primary) inductance to figure out the on time.Since the equation for duty cycle in CCM is a lot simpler and it gives a "worst case" duty cycle for the switch if Vin is set to the minimum expected then there is no reason not to use it (the duty [on-time] in DCM goes lower with decreasing loadorincreasing input voltage, but a lower duty cycle is not terribly important; we just need to make sure the duty doesn't exceed what the controller IC can handle).Quote from: debininja on June 02, 2017, 07:19:38 pmUnless you are pushing the core hard (ie - a big flux swing) then round down the number of primary turns when designing a DCM flyback. This lowers the inductance which moves operation further into DCM. Try, for example, 16uH and a 2:1 turns ratio.You may have noticed that different size ferrite cores come pre-gapped to specific ALvalues so, yes, you use the equation N = (L / AL)^0.5 to determine the number of turns. Then check to make sure that number of turns does not take up too high a percentage of the "window area" of the core (usually in the range of 40-60% of the window area is usable, depending on the shape, whether a bobbin [aka coil former] is used, etc.).Quote from: debininja on June 02, 2017, 07:19:38 pmSure, you can lower the turns ratio (or the duty cycle) to account for the voltage drop of the diode and of the switch, force the converter to operate at a lower duty cycle, etc., but similar to using the CCM equation vs. DCM, I left out all those losses to simplify the equations.If you let the Schmidt-Walter website use its default optimizations for the flyback it selects a turns ratio and primary inductance so that the switch operates at a 50% duty cycle, but the TL494 is limited to 45% max duty so you need to manually lower the turns ratio and inductance to ensure it will work.