I would recommend that you use PIXls designer application for doing this design. Use a very low VOR (about 25 V or so). The automated software is designed for slightly higher input voltages... was not optimized for this voltage range...

Also make sure you choose the reduced current limit for the TNY280. This will ensure that the currenmt limit overshoot at high line will n ot trigger an SOA failure...

Thanks PI, I've attached my PIXI design for your comments. I've been trying to set one output winding to a floating type but can't seem to find the correct settings, any help would be appreciated. Could this be squeezed into an EE16 core? How does the design look? Thanks. P.S Is there any easy way to attach a .pxlt3 file

Sorry somehow I missed I had a foil winding !! I've attached the correct files. Thanks

The primary winding wire gauge is oversized. Whie its not a problem and your design will work fine, you can go use thinner wire, go down to 2 primary layers and improve upon your leakage inductance.
About floating outputs, PIXls does not allow you to float an output... PI Expert will allow you to do that.
Once you are happy with the design just replicate it in PI Expert for the transformer construction part and then set the outputs to floating through the stacking dialog. Watch the video on
"How to Define and Force the Software to Design with Floating (Galvanically Isolated) Outputs" from the PI expert download page

Test reply to check email notification of followup is working

Cheers

PI-Chekov

Thank you for your reply, I'll make the changes you suggest.

hi everybody; i m working on a same range(18-265V AC&DC) SPMS design. i checked your App. note di152.pdf too. My transformator design will have 3 outputs too. (5V,5V and 12V) i tried to be carefull about Vor, Cin and Ilimit key points. i have filled the sheet but im not sure is design have any problem. so i attached the documents. i ll be happy if check my design parameters. already thanks for your opinions.

The design looks ok and in general if the spreadsheet did not flag any warnings (other than for low input voltage, which you will separately address) there should be no issues. The only thing I would check on the prototype is the actual current limit at high line. With only 174 uH of inductance, the di/dt at high line is immense and may cause significant current limit overshoot. Although you have configured to use the TinySwitch in reduced current limit I would double check that the actual current limit at high line is below the SOA limit for TNY280 in the datasheet.

Hi Mr Spock,
I've had the transformer made to the specifications pixl suggested with your
recommended changes (the main one being a VOR of 20V) but on testing the
transformer in a circuit designed around DI-152 I've found that the minimum
DC operating voltage is 42V and not the required 20V. I added the current source to your reference design board DAK-91 and the feed back was taken from the 12V winding of my transformer. The CC circuit supplies approx. 580uA at 20V DC input.
The circuit works very well above the 42VDC.

What would be he main cause for this lack of low voltage operation.

Would it be possible for you to give me an EE19 transformer designed to my output specs that would do the job?

Regards
-=Nick Denman

Hmmm. Would you try to increase the supply current to about 1 mA and try again? You are using the reduced current limit mode correct? This is when the BP pin capacitor is set to 1 uF.

Hi Mr. Spock,
Yes I'm using 1uF on the TNY-280 device.
I've increased the current to 1mA and now I'm getting start-up at 30VDC.
I'm no longer convinced that the transformer was made correctly as the 4V
winding is giving me 8V when using the 12V winding as feedback control.

I left a phone message for you today as I'm trying to resolve these issues
promptly and the forum two day turnaround is to slow.

Live long and prosper V/

Nick Denman

That does see to be on the higher side... Under what loading conditions do you see this? Do you have the correct primary inductance?
Can you attach your schematic, I want to make sure that your clamp and feedback circuit is ok.

The circuit will start with full load ~6W at 30VDC as required.
The primary Inductance is as per the transformer design.
I've attached the clamp & feedback circuits I used. The feedback
is taken from the 4.0V output.

Is it mainly primary inductance that effects the start voltage?

Regards
Nick Denman