Help calculating values for OFF-THE-SHELF transformer to use with Flyback (TOP257)

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jlawley1969

Joined Feb 22, 2021
97
Hi,
So I have seen a few guides from manufacturers(such as https://www.infineon.com/dgdl/an-1024.pdf?fileId=5546d462533600a401535591115e0f6d) which show me how to calculate inductance and core size and such for me to MAKE my own transformer. At this point intime it is much more realistic for me to be choosing off the shelf component.

The issue I am having starts with the fact that transformer manufacturers' datasheets only list inductance/leakage inductance/ saturation current.
Then ontop of this I have read many things about transformers but they all are really surface level. I am at the point where I am wanting to know WHY inductance/leakage inductance/ saturation current matter and how they affect the way a flyback operates. For example the above link show me how to calculate primary inductance but does not explain why it should be that and what would happen if I stray from that value.

I know this is a large question but I feel like I know less the more I look stuff up.
So does anyone have any links to guides/tutorials/application notes that explains why the characteristics (inductance/leakage inductance/ saturation current) matter because that is the most that transformer data sheets provide.
Also is the effects of things like flux density not incorporated into the other transformer characteristics that are listed in data sheets (such as flux density just is what determines leakage inductance or whatever)

Sorry if this sounds dumb it is just not clicking for me and the stuff I have been reading aren't explaining what I need to hear(still looking tho)


(to complete this mess of a post I am using 750871110 with top257/top224 and it is being unreliable but I already made a post about that and need to take a step back yes I have already read https://www.allaboutcircuits.com/te...chpt-9/practical-considerations-transformers/ and I need more depth)


thank you all
 
Last edited:

shortbus

Joined Sep 30, 2009
10,049
In your app note in the other threads it has all of the information your after, look at the schematics it shows, and get the transformer numbers from them for the one that suits your requirements. Google that number and it should find where to buy one. Or you could ask Google for a "cross reference number" for it if it isn't still available.

But since your doing this on a solderless breadboard, your real chances of getting the circuit to work are limited. SMPS need to be built on a PCB, breadboarding is too hit and miss, with contact issues being just one of the problems you can experience.
 

shortbus

Joined Sep 30, 2009
10,049
link Go to the link and start a design. You will need to register. Pick the part TOP257 HS. Fill in the information and the program will give you all the data. Or give me all the information and I can do it.
Will that information from your link also take into consideration that he is just building the circuit on a breadboard, not even a Vero board? Just asking, not arguing. How many times have there been people showing their breadboarded circuit that have long, criss-crossed wires and asking, "why doesn't this work?"
 

jlawley97

Joined Oct 5, 2019
25
Will that information from your link also take into consideration that he is just building the circuit on a breadboard, not even a Vero board? Just asking, not arguing. How many times have there been people showing their breadboarded circuit that have long, criss-crossed wires and asking, "why doesn't this work?"
probably not
I tightened up the the wiring (still on the breadboard tho) and it works reliably now. No more 700V.
the bias winding diode seemed to have an effect on whether it would "fire" or not
1n4148-kept trying to work then shutting down
BYV26C- worked reliably
which is weird bc the only way they differ is 8ns/2pF and 30ns/30pf respectively
 

ronsimpson

Joined Oct 7, 2019
3,204
Will that information from your link also take into consideration that he........
You know the answer to that.

Go the the WWW of the people that make the part. Use their tools. In this case the tools will even help you wind the transformer. This is a wonderful tool. They have many white papers on "25W 110/220in 5V out" supplies.

Vector boards and such can kill the project. So can not having a working schematic. At least get a working design and start there. Many of the examples have PCB layout and transformer drawings and pictures.

CoilCraft and some transformer/coil venders have examples ( use this IC with this transformer ).
----------edited--------
I am not really that smart. I often copy/modify a working design. Creating a supply from nothing is hard! Don't tell my manager.
 

ronsimpson

Joined Oct 7, 2019
3,204
Please don't use this.
1632453563056.png
A little better. I can't find a picture but I have some of these with a ground plane on one side. I also have some with strips of power and ground running through.
1632453632424.png
What I am saying is that off the power line supplies have voltages too high for this and currents that need large traces and thought needs to be put into the placement of the parts. Many of the example power supplies have a layout and for the cost of a 2 sided PCB now days, I get get a board made.
 

Irving

Joined Jan 30, 2016
4,047
Many of the example power supplies have a layout and for the cost of a 2 sided PCB now days, I get get a board made.
Likewise, it saves so much trouble chasing transient problems.

Perf-board as shown works OK for lower current solutions with the addition of tinned copper wire >= #20 & lots of solder to create low-inductance/low-impedance bus-bars.
 
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