Best isolated topology for lowest conducted EMI

Valentinas Pancerovas

Other Parts Discussed in Thread: LMS3655, LM61460, UCC28781

We have Vin 22-32V and Vout 20V 100mA.

Currently we are using LMS3655 in a flybuck mode.

I dont understand why there is LM61460 as a suggestion and described as "lower EMI" when the datasheet shows better EMI with the LMS. Can you explain?

We barely passed our automotive customers conducted emissions with alot of work and filtering components, but i am regretting for not going for some adjustable frequency controller instead, such as or a ZVS ucc28781 which selects frequency based on the inductance which we could have had large. It can operate in audible frequency at low loads and 100mA is still a low load. Only the 10th or so harmonic would start at the 150kHz which is where conducted EMI starts to be measured. Easy pass. Do you agree?

over 2 years ago

0 Aurora Tie over 2 years ago

TI__Genius 17810 points

Hi,

A has the functions of spread spectrum and adjustable SW node rise time, so it can reduce EMI. However, this chip is suitable for high current occasions (6A). If the output current is 100mA, it is not suitable to choose this chip. In addition, I would like to ask, what is the withstand voltage of the input voltage you need? Is 36V ok or still need bigger? Thanks.

Aurora

0 Valentinas Pancerovas over 2 years ago in reply to Aurora Tie

Intellectual 410 points

Hello,

By adjustable SW node rise time, do you mean the Rboot? The datasheet recommends max 10R only so there is not that much impeovement.

I am usung UCC28781 in a new project and its much better than the LMS. Even with 35uH transformer its Fsw is 70kHz at 30mA load. I see no low frequency conducted emissions, just HF.

32V max input handling is fine.

Thank you

0 Aurora Tie over 2 years ago in reply to Valentinas Pancerovas

TI__Genius 17810 points

Hi,

Yes, you can use Rboot pin to adjustable SW node rise time. LM61460 is suitable for high current occasions(6A).

If UCC28781 can meet your requirement, you can use it. If you have other questions about UCC28781, you can submit a new E2E question to contact another product line. Hope this can help you. Thanks.

Aurora

0 Valentinas Pancerovas over 2 years ago in reply to Aurora Tie

Intellectual 410 points

Thank you Aurora,

What happens if Rboot is too high? Will it even start?

0 Aurora Tie over 2 years ago in reply to Valentinas Pancerovas

TI__Genius 17810 points

Hi,

Noise due to stored charge is greatly reduced with higher RBOOT resistance. Switching with slower slew rate also decreases the efficiency. Hope this can help you. Thanks.

Aurora

Power management

Power management forum

Best isolated topology for lowest conducted EMI