Author Topic: Consufed selecting Flyback for LT8302  (Read 678 times)

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Offline Blackgar

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Consufed selecting Flyback for LT8302
« on: May 15, 2022, 12:25:28 pm »

I am going to design a PCB where I want to create an isolated 12V supply. I have already picked the LT8302 but I am a bit confused selecting the correct flyback transformer.

On datasheet, they suggest the SUMIDA 12387-TO79 which I cannot find on common distributors (mouser, farnell, digikey etc).

The input will be a 12V supply and the output must be designed for 12V/1.5A.

The IC switching frequency can be 12 to 400kHz, I struggle to find something that could fit to my design or I misunderstand.

Offline Le_Bassiste

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Re: Consufed selecting Flyback for LT8302
« Reply #1 on: May 18, 2022, 08:41:03 am »
« Last Edit: May 18, 2022, 08:42:59 am by Le_Bassiste »
An assertion ending with a question mark is a brain fart.

Offline Faringdon

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Re: Consufed selecting Flyback for LT8302
« Reply #2 on: May 19, 2022, 10:48:24 pm »
If you want , take a look at the attached, its the complete design stuff for a 12vout, 20W flyback  with vin = 140-400Vdc.

So you could use it on rec'd mains.....I CONFESS IT ONLY HAS 1MM OF CLEARANCE BETWEEN PRI AND SEC, SO YOU COULDNT SHIP THIS....BUT YOU CAN CHANGE THAT IF YOU WANT......damned...sorry about capitals...maybe ill leave it cuzz thats important.

It contains all design data , though not all the resistor values are given, but would be easy to work out.......the transformer design stuff is in there,,,it uses a sandwich wound ETD29 ferrtie core set, gapped.

Its dead easy to do....please ask if you get stuck
Theres also a LTspice sim of it for you to run if you want.

Also attached is the complete schem and layout files in Eagle Pro.

The transformer design page is a bit cryptic i confess...but its two layers of 23 turns of 0.71mm ECW (PRIMARY)....which "sandwich" one layer of TEX-ELZ 7.0.3mm TIW  (secondary)
Core is ETD29 from TDK and its offtheshelf gapped to AL=201.
If you want to cut skin effect a bit more in the primary, then make the 0.71mm say two strands of 0.315mm.

By the way, power integrations website has docs and software which  take you step by step through flyback transformer design.

EDIT...sorry....i just saw your vin is 12v. Still its the same kind of procedure.
« Last Edit: May 20, 2022, 09:54:39 am by Faringdon »

Offline Faringdon

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Re: Consufed selecting Flyback for LT8302
« Reply #3 on: May 21, 2022, 10:38:57 am »
Your spec is 12vin 12vout and 1a5out
I will be back in a few mins with a flyback and transformer deisgn for this...

Offline Faringdon

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Re: Consufed selecting Flyback for LT8302
« Reply #4 on: May 21, 2022, 11:17:26 am »
Attached is your 12vin and 12vout flyback with 1a5 out.
I did it with an offtheshelf  ETD29 ferrite core set, gapped to AL=201.
This was a bit bigger than needed, but does the job fine, and  is easier for you to wind.
It  costs a few pounds to buy the core set and former and clips.
Two parallel secondaries interleave the primary…the only reason to do it this way was because the secondary more  fully covers the bobbin width, so is a better ‘platform’ for the primary middle layer.
So secondary 1 is 16 turns of 0.5mm enamelled copper wire (2 strands together)
Then wind primary as 10 turns of 0.5mm ECW (three strands of)
Then again……another 16 turns of the 0.5mm ecw on top of that (again 2 strands)
…Then remember on the PCB, to parallel the secondaries.
Attached is full design doc and LTspice simulation of it for  you to run
…….If you wish, at some point I can come back and re-do it with the smallest possible transformer.
Also, this uses ECW and no margin tape etc, so cannot be used for full  3500vac isolation…….if you want that, I will come back and do the primary in triple insulated wire for you eg TEX-ELZ….unless you want to do this for yourself?…..the calculation involves simply calculating out which diam goes across the  ETD29 bobbin nicely in 10 turns…then check the conduction loss.

To summarise the transformer is three layers
NP/NS = 10/16

Layer 1 = 16 turns sec
Layer 2 = 10 turn pri
Layer 3 = 16 turn sec

...remember to pllel the secondaries so NP/NS = 10/16

I chose this turns ratio..
1...because it works
2...gets D below 0.5 so you dont need slope compensation

Youll need an inductance meter to check youve phased the windings correctly........(do the fields cancel or not, blah blah, you know the score...)

Youre supposed to wind flyback transformers with each layer nice and  fully of turns.....not a layer with a gap at one end of the bobbin, say..but having said that, if you need to, a little bit of cheating is long as your coupling is good enough for your requirement

« Last Edit: May 21, 2022, 07:43:05 pm by Faringdon »

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