LM3481: Equation on SNVA761A

Hi Brigitte,

Thank you for your correction.

Hello Liu,

Today I will not find the time to go through the formulas. Please give me until end of the week to come back to you.

Best regards,
Brigitte

Hello Liu,

Sorry for the delay. When re-calculating, I am getting the same result. I am in contact with the author to make sure that I did not overlook anything.

Please give me some more days to get back to you.

Best regards,
Brigitte

Hi Brigitte,

Thank you for your response.

So I'll wait for your reply.

Best Regards,
Tom

Hello Liu,

Thank you for your patience. At the moment we have 2 days bank holiday, so it will be somewhat delayed.

Best regards,
Brigitte

Hi Yang,

There are mistakes in Eq.11. We will revise it asap.

Please refer to SNVA866

• 3.4 MOSFET Selection
• 3.5 Diode Selection

Best Regards,

Feng Ji

Hi Feng,

Thank you for your reply.

Could you tell me how the Eq.11 is incorrect?

Best Regards,
Tom

Hi Liu,

I looked into the equations of the app note as well and I agree that the formula and the results do not line up.
I found some minor changes regarding your values, e.g. I_lm should not be the peak current (I_pk_lm), but the average current form equation 6, which is a little bit lower than 9.13A. However, the results are still far away from the app note results.

While I am looking into the origin of the equations to determine the necessary corrections, I recommend to take a look at additional tools to determine the max ratings of the FET and diode.
We provide a calculation tool called Power Stage Designer, which gives a quick overview on the max currents and voltages present in a flyback topology, depending on the input, output and inductor parameters.
The tool can be downloaded from this website:
https://www.ti.com/tool/POWERSTAGE-DESIGNER

I will give you an update on my findings until next week.

Best regards,
Niklas

Hi Niklas,

Thank you very much for your detailed reply.

So I'm waiting for your further updates.

Best Regards,
Tom

Hi Tom,

Here is an update on the FET selection questions.
I still could not find out what parameters were taken to get these results for Irms and Ir_diode current in the MOSFET/diode selection chapter.
The formula in the app note should be correct, but it looks like the input parameters entered in the equation do not align with the specification table mentioned at the beginning of chapter 4 (Vin 5-32V, Vout 12V, Iout 2A).
I would assume the calculation results shown are for different input parameters. That would also explain why the results for all equations in the design section (eq 24, 25, 26) have different results when entering the parameters from the spec table.

The author of this app note is no longer within our application team, therefore getting the source material for these equations is very tedious.
However, I will work on a correction of the app note, so either the equation results fit for the input specs, or the specs are updated to the values that were taken for these calculations.

As there should be no problem with the equations themselves, feel free to use them for the customer design and ideally double check if the ratings line up with the max values within the power stage designer tool.

Best regards,
Niklas

Hi Niklas,

Thank you for your detailed reply.

I have conveyed it to the customer.

Best Regards,
Tom