LM5180EVM-S05: Why no ground pour or traces under transformer?

Matthew Findlow

Part Number: LM5180EVM-S05
Other Parts Discussed in Thread: LM5180-Q1, LM5180

Hi, I'm laying out a virtual copy of the EVM board and wanted to ask why there is no top copper under the transformer? I assume it is to reduce noise coupling into the ground or traces, but there is still a ground on the bottom of the board which is only 1.6mm away, which would pretty much negate any gains made by removing the top copper???

Also the EVM has a 1nF cap bridgeing the primary and secondary grounds. No mention of it in the data sheet. ??

Also BOM says C8 and C10 are 220pF, but schematic shows 22pF. ??

over 4 years ago

0 Arief Hernadi over 4 years ago

TI__Mastermind 23815 points

Hi,

I believe that the no pour under the transformer is for isolation purpose between primary and secondary.

The 1nF bridging capacitor is to make sure that the noise from secondary can be shunted back to the primary source.

I will ask the application engineer on the value of C8 and C10.

Thanks

-Arief

0 Orlando Murray over 4 years ago

TI__Mastermind 22299 points

Hi Matthew,

Arief is right, there is no pour underneath the transformer for isolation.

C8 and C10 are 22pF as shown in the schematic, there is a typo in the BOM.

Hope it helps,

-Orlando

0 Matthew Findlow over 4 years ago in reply to Orlando Murray

Intellectual 880 points

Thanks for the clarification.

Ref LM5180-Q1 Single-Output EVM User's Guide, top of page 14 "For both conducted and radiated emissions measurements, the transformer core is shielded using a copper strap tied to primary GND"

I love to see a photo of that. Should I be making my own provisions to shield the transformer core?

Thanks Matt.

0 Orlando Murray over 4 years ago in reply to Matthew Findlow

TI__Mastermind 22299 points

Hi Matt,

Sorry for the delay, see attached transformer shielding used for EMI testing of LM5180 EVM.

The copper shield reduced radiated magnetic fields and helps with radiated emissions, but also stops the magnetic fields from coupling to the PCB and components resulting in better conducted emissions performance as well.

For EMI we suggest using a shield.

Thanks,

-Orlando Murray

0 Matthew Findlow over 4 years ago in reply to Orlando Murray

Intellectual 880 points

Thanks for the support. Cheers, Matt.

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LM5180EVM-S05: Why no ground pour or traces under transformer?