Trying to design/build my own SMPS, where can I get the flyback transformer? - Page 1
Forum section: electronics / beginners

author: SparkyTD
author_rank: Contributor
timestamp: November 14, 2021, 10:09:45 am
content: I'm designing a product (not commercial of course, just as a hobby) that will be built into a small 71mm wide DIN rail mountable enclosure. The circuitry mainly consists of an STM32 and some other peripherals, so I will need a built-in power supply that can convert the incoming 230VAC to a smooth 3.3V. Due to the enclosure's size, the available space is very limited, so I probably can't just throw an existing power supply in there. This means that I have to design and build my own power supply that will fit on a small custom PCB that is specifically made to fit in the enclosure.I want to use a switched-mode power supply because that's the most efficient, but my problem is that I don't know where I can get an off the shelf flyback transformer that fits all the specs. I'm currently using the WEBENCH Power Designer tool to generate an SMPS schematic that fits my requirements, and below you can see the schematic that I ended up choosing.If I was designing a simple linear PSU, I could go on TME/Farnell/Mouser, and I would know exactly what XFMR to look for. I would just set the primary and secondary voltages that I need in the filters, and I could probably find one that fits in my enclosure. The problem is that in the SMPS the transformer is used in a completely different way that I don't fully understand. I don't even know what parameters to look for in the online stores. In the WEBENCH generator is gives me the core type, the turn ratio and a bunch of other information, but I don't really know what to do with it if I can't find an actual product that I can buy.Schematic:T1 Specs:Keep in mind that I'm a beginner, I only have a very rough idea of how an SMPS works, and I have never designed a power supply circuit before.

author: george.b
author_rank: Frequent Contributor
timestamp: November 14, 2021, 11:31:21 am
content: Normally you'd wind it yourself - WEBENCH gave you all the information you need to do it.Although you can find some off-the-shelf SMPS transformers, you'd then design your SMPS around that transformer, not the way around.

author: geggi1
author_rank: Frequent Contributor
timestamp: November 14, 2021, 11:46:03 am
content: What is you power requirements?If its less than 100mA you might be able to just use a regular transformer and a linear regulator.Another option is to reverse engineer a USB charger and use this in your design. You can even just cannibalize the parts.If you check at electronics supply houses like RS you can find embedded smps that you probably just can add directly to your design. In case you cant find the correct voltage just use a linear regulator to drop the voltage to the 3,3V you need.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 14, 2021, 03:31:08 pm
content: I think I like the Hi-Link power modules such as the "hlk-pm03"https://html.duckduckgo.com/html?q=hlk-pm03I'm not absolutely sure though if these meet all requirements to be used safely as a mains voltage connected power supply.

author: SparkyTD
author_rank: Contributor
timestamp: November 14, 2021, 05:45:27 pm
content: Quote from: Doctorandus_P on November 14, 2021, 03:31:08 pmThis looks like exactly what I need, it would definitely save me a lot of time. And it's currently in stock in one of my local online stores. Thanks a lot for recommending it.I'm still trying to figure out a custom SMPS, but if I give up or run into complications, I will use this module.

author: CaptDon
author_rank: Super Contributor
timestamp: November 14, 2021, 06:11:15 pm
content: I save all kinds of SMPS ferrite core transformers so I could rewind them for specific needs. I also bought up a bunch of 'cup and core' style bare ferrites at local hamfest swap meets. I have even used the two piece cores from old C.R.T. television flyback transformers with the teflon gap spacers. Some of those flyback cores can support nearly 100 watts.

author: dtmouton
author_rank: Contributor
timestamp: November 17, 2021, 08:08:27 am
content: You should have a look at what Coilcraft has.

author: penfold
author_rank: Frequent Contributor
timestamp: November 17, 2021, 08:51:04 am
content: +1 for Coilcraft, they have a few standard items that are cross referenced with TI, AD/Linear/Maxim power ICs for specific volt and current outputs, maybe easier to home in on an optimal solution than trying to cross-reference a custom design to an OTS part.That design table looked a little off to me, maybe that primary side inductance seems a little high. If you continue down the custom route, could be worth getting some validation on the webbench design before settling on a transformer. Meanwell also has some modules like that one suggested above, likely a few other suppliers, XP power probably do too. I doubt you'd get a much smaller device than an OTS potted module, mainly because the potting affords a slightly tighter packing of components that for mains clearance you may struggle to meet with discrete - but yeah for sometimes the manufacturer has other worries than the package size.