LM5121 : WEBENCH Transient simulation issue

Yoshwa

Expert 4050 points

Other Parts Discussed in Thread: LM5121

I designed a VIN=5V to 12V and VOUT=20V 2A application using LM5121.

And I had simulated load transient and input transient on WEBENCH.

The attached file is a load transient simulation result.

LM5121 WEBENCH® Design Report.pdf

It seems that VOUT result is not a real behavior.

What is the problem?

over 7 years ago

0 Nikhil Gupta (Product Folder Models) over 7 years ago

TI__Mastermind 39210 points

Yoshwa-san,

For some reason your design is corrupted. If you see, your efficiency is coming out to be negative 4%. Have you made any manual changes to the design? I tried recreating with your design specs and the default design from WEBENCH gave correct results. Please see below.

webench.ti.com/.../SDP.cgi

0 Yoshwa over 7 years ago in reply to Nikhil Gupta (Product Folder Models)

TI__Expert 4050 points

Yes, there is no problem for VOUT=12V 2A application.

WEBENCH designed parameters are the same for VOUT=12V 2A application.

The issue is for the case of VOUT=20V 2A.

Operating values of the FET M3 are abnormal, as you can confirm on the previously attached data.

M3 Pd -3.74W

M3 Rdson 327.542 kohm

As the M3, Infineon BSZ042N04NS G is selected.

It seems M3 model simulation.

0 Nikhil Gupta (Product Folder Models) over 7 years ago in reply to Yoshwa

TI__Mastermind 39210 points

Hi Yoshwa-san,

That is exactly the problem. It seems like the Rdson is not added correctly to our database for this FET. We will look into this and fix it as soon as possible. In the mean time, the solution would be to pick an alternate M3 FET. Just select M3 and click on "Select Alternate Part" Here is an example.

webench.ti.com/.../SDP.cgi

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LM5121 : WEBENCH Transient simulation issue