Fly back SMPS, please evaluate. - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: December 01, 2021, 07:21:25 am
content: I have tried to make an fly back supply with input protection, EMI reduction for max 3.3V 1.5AEvery resistor is 1W, only missing parts is the resource electrolytes and maybe a inductor.Is the design okay for a low loss, stable PSU?

author: kaindub
author_rank: Regular Contributor
timestamp: December 01, 2021, 08:26:06 am
content: A cursory look says the tl431 will be unstable as it lacks stabilising capacitor.It’s a fairly basic design. I would use one of the dedicated smps chips. Why use a transistor. Mosfets are a better choice.In the end you need to mock-up this circuit and see how it works. Perhaps run it through Spice as a preliminary check.

author: Benta
author_rank: Super Contributor
timestamp: December 01, 2021, 08:32:47 pm
content: I fail to see the purpose of connecting the output capacitor to the TL431 and R9.But that's only one of the dozens of problems I see.Use a dedicated chip, this is really 1970s design.

author: FriedMule
author_rank: Frequent Contributor
timestamp: December 02, 2021, 07:47:12 am
content: Thanks for your help, I'll find a chip instead:-)

author: magic
author_rank: Super Contributor
timestamp: December 02, 2021, 08:43:46 am
content: Are you building this for fun or for actual use?If the latter, what's the reason for not using an off the shelf PSU brick?

author: FriedMule
author_rank: Frequent Contributor
timestamp: December 02, 2021, 10:46:02 am
content: I do in fact plan on using it, my reason for building one is that I want to learn about electronic:-)Can anyone maybe suggest some chips?

author: PKTKS
author_rank: Super Contributor
timestamp: December 02, 2021, 11:08:58 am
content: Quote from: FriedMule on December 02, 2021, 10:46:02 amRather typical simple and fairly seen on "flyback" topologies..( aka offline converters )OnSemi MC44603Paul

author: magic
author_rank: Super Contributor
timestamp: December 02, 2021, 11:30:35 am
content: MC44603 is crap. Its built-in transistor can't switch mains voltage and it lacks several features to conveniently and safely drive external MOSFETs: UVLO, push-pull output, primary peak current limiting.Popular oldschool options are:Viper series from ST: built-in MOSFET, few external components, good enough for a few wattsUC3842~3845 series: very common, needs external FETNCP1207, NCP1377 are another controllers I recently found in off-the-shelf PSUs which appear to be fairly available and have a nice bonus: secondary overvoltage detection and shutdown.There is a lot of newer chips too, mostly from China, but they are more obscure, sometimes without good datasheets, often only in SOT23-6 which you may or may not like.Whatever you do, beware of capacitors charged to 330V and remember that it's difficult/expensive to safely probe primaries of mains-powered PSUs. Such SMPS is not an ideal beginner project. Honestly, I probably wouldn't know how to design one properly, I have only ever repaired them.

author: Benta
author_rank: Super Contributor
timestamp: December 02, 2021, 11:48:54 am
content: The MC44603 is obsolete. Apart from that, it's optimised for TVs, not really a general purpose part.

author: Circlotron
author_rank: Super Contributor
timestamp: December 02, 2021, 12:02:51 pm
content: LED D2 will go into orbit in microseconds. Also, the entire circuit is being fed from the voltage drop across that LED.

author: PKTKS
author_rank: Super Contributor
timestamp: December 02, 2021, 12:26:16 pm
content: Quote from: Benta on December 02, 2021, 11:48:54 amMay be.. I did repair man stuff for decades up to 90s..no surprises my choices and generic stuff is biased..Nevertheless.. it is always handy having classics which can be found today..The rated power and specs of this IC are generic enough for that..But sure we have newer stuff..Paul

author: PKTKS
author_rank: Super Contributor
timestamp: December 02, 2021, 12:41:17 pm
content: Quote from: magic on December 02, 2021, 11:30:35 amUC38xx is more used in generic SMPS PSU units indeed.It can still be seen inside a lot of standard SMPS.Nevertheless even the APPNOTE schema does not use the IC driverdirectly.. it is always a multi stage driver and an external drivercan handle whatever mains needed..Itshouldbe a simple choice..several base examples availablePaul