LM7481-Q1: Mosfet SOA margin calculator tool

Alan Xia

Part Number: LM7481-Q1

Tool/software:

Hi team,

The customer found that in this post we recommended the customer to use this tool. I would like to confirm with you whether TI recognizes the authenticity and reliability of this tool.
If it is authentic and reliable, can you explain the judgment basis and logic of the table?

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1370795/lm7481-mosfet-failure

https://www.ti.com/tool/download/FET-INRUSH-SOA-CALC/REV-B

10 months ago

0 Shiven Dhir 10 months ago

TI__Mastermind 48475 points

Hi Alan,

FET SOA stress calculation

A more detailed explanation of SOA calculation is specified in section 2.3 Understanding MOSFET’s Stress Limitations of Robust Hot Swap Design App note.

For your understanding below is a step-by-step procedure (extracted from the app note),

Calculate the power stress on the FET during startup. Refer to section ‘2.3.3 Checking SOA for Non-Square Power Pulses’ of the app note.

Derive t₂. Approximation here is that the FET is stressed for a power loss = P₂ = P_MAX for t₂ duration.

Now, estimate SOA the FET can handle for t₂ This value will be at room temperature. Refer to section ‘2.3.2 Checking SOA for Intermediate Time Intervals’ of the app note.

Remember that the SOA value in the table below needs to be taken at V_DS = V_IN(max) in the SOA curve of the FET.

Now, De-rate the SOA calculated in the above step for the Max operating junction/ case temperature of the FET. Refer to section ‘2.3.1 MOSFET SOA Curve and Thermal Model’ of the app note.

This value can be taken as the temp. of the FET at max load condition,

You can consider that the FET has strong SOA if

SOA(T_C) > 1.2 x (P₂ / V_DS)

Where,

SOA(Tc) is from step 3

P2 is from step 1

VDS = V_IN(max)

The factor 1.2 is taken to include a 20% margin.

Regards,

Shiven Dhir

0 Hao Fu 10 months ago in reply to Shiven Dhir

Prodigy 50 points

Hi Shiven,

I have questions about the two formulas, could you please help explain how they were derived?

Thanks a lot !

Regards,

Hao

0 Shiven Dhir 10 months ago in reply to Hao Fu

TI__Mastermind 48475 points

Hi Hao,

Tstart has been computed from capacitor equation of I =Cdv/dt

Where marked box is average of power limiting and current limit mode during startup.

Second equation is derived as such.

Regards,

Shiven Dhir

0 Hao Fu 10 months ago in reply to Shiven Dhir

Prodigy 50 points

Hi Shiven,

Thanks for your reply!

I am wondering why , How was this formula obtained?

Regards,

Hao

0 Shiven Dhir 10 months ago in reply to Hao Fu

TI__Mastermind 48475 points

Hi Hao,

The SOA curve follows a straight line in a log-log plot. Which can be estimated using a standard power function.

This is used to find SOA at difference time intervals which are not usually mentioned in the FET datasheets.

Regards,

Shiven Dhir

0 Hao Fu 10 months ago in reply to Shiven Dhir

Prodigy 50 points

Hi Shiven,

I have got it. Thank you very much!

0 Hao Fu 9 months ago in reply to Hao Fu

Prodigy 50 points

Hi Shiven,

I am a bit confused about this part.

I think converting the triangle into a rectangle is too conservative.

The equivalent curve assumes that the power limit is the same when VDS changes, but in fact, during the startup process, VDS is decreasing and the corresponding power limit is increasing. Is it because there is no better equivalent solution?

Regards！

Hao

0 Shiven Dhir 9 months ago in reply to Hao Fu

TI__Mastermind 48475 points

Hi Hao,

Yes, triangle has been converted to a rectangle, but the time of stress has also changed. This is the closest equivalent solution.

Regards,

Shiven Dhir

0 Hao Fu 9 months ago in reply to Shiven Dhir

Prodigy 50 points

Hi Shiven,

I have a question about the calculation tool. It seems that the tool only mentions the impact of output capacitance, but in fact, the output is generally π-type filtering. Will this inductor have an impact in the calculation?