WEBENCH® Tools/LM5161: Question about the fly-buck solution

Zhengquan Lu

Part Number: LM5161

Tool/software: WEBENCH® Design Tools

Hi team,

My customer would like to use LM5161 device as fly-buck application and get isolated 12V output and non-isolated 12V output. And when they use Webench tool to simulate the fly-buck circuit and find only one output could be set. Could you please help check whether LM5161 could set both outputs (isolated and non-isolated) in the Webench tools? Thanks a lot!

Best Regards

Zhengquan Lu

over 5 years ago

0 Ethan T over 5 years ago

TI__Expert 3835 points

Hi Zhengquan,

The output set is for the isolated side. You can try setting the primary side by changing the feedback resistors.

Regards,

Ethan

0 Zhengquan Lu over 5 years ago in reply to Ethan T

TI__Intellectual 1035 points

Hi Ethan,

When I change the feedback resistors to set the primary side, the isolated side would also change at the same time. How can I set the two outputs with different voltages? Thanks!

Best Regards

Zhengquan Lu

0 Ethan T over 5 years ago in reply to Zhengquan Lu

TI__Expert 3835 points

Hi Zhengquan,

I overlooked the question, my apologies. I will contact a Webench expert regarding this matter and get back to you shortly.

Thanks,

Ethan

0 Orlando Murray over 5 years ago

TI__Mastermind 22301 points

Zhengquan

Please see this appnote explaining the flybuck design equations.

The isolated voltage is set by the primary voltage, the 1:N transformer and the diode drop with the equation:

Vout-iso = (N * Vout-pri) - Vdiode

So to change the primary voltage without the secondary you need to readjust the transformer and/or diode.

If you're doing the same output voltages with a 1:1 then I suggest you design with the lowest diode forward drop so the primary and secondary are closest.

However the diode reverse voltage must be at least ( N*VIN ).

Let me know if you have further questions,

-Orlando

0 Zhengquan Lu over 5 years ago in reply to Orlando Murray

TI__Intellectual 1035 points

Hi Orlando,

Thanks a lot for your reply! Could you please tell me how to set two different outputs? When I try to edit transformer parameters, I cannot find the way to change the turn-ratio. Thanks again!

Best Regards

Zhengquan Lu

0 Orlando Murray over 5 years ago in reply to Zhengquan Lu

TI__Mastermind 22301 points

Zhengquan,

You cannot modify the transformer parameters in WEBENCH.

You will have to select from one of the standard transformers there, most of them have turns ratios of 1, 1.5, 2, etc...

If you have a specific transformer in mind I can review it in the schematic for you.

Hope this helps,

-Orlando

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WEBENCH® Tools/LM5161: Question about the fly-buck solution