PCB fails common mode emissions due to switching node of 1W buck converter

Hello,
Please help us to pass conducted emissions of EMC testing, particularly with reference to common mode noise created by the switching node of our 1W Buck converter
We have a 1W Buck converter (bias supply) on our offline 150W LED driver PCB (which uses linear current regulators).
It firstly failed EMC due to having the Buck’s switching node copper layed out all over the bottom PCB layer, where it was next to the earthed heatsink. (common mode emissions).
We now have removed the switching node copper from the bottom layer. However, do you believe that we should shield the switching node copper (which is now only on the top layer) from the earthed heatsink by putting some “quiet node” copper underneath the switching node of the Buck, in order to shield the switching node from the earthed heatsink on which the PCB sits?
(eg we could put the “quiet node copper” on middle and bottom layers of the 4 layer board. The switching node copper is now only on the top layer. The PCB sits on a thin thermal pad which is on the earthed heatsink)

Hi,

Did we ever see the PCB layout?
Usually the input capacitor should be sufficient.

Klaus

Thanks, the thing is the PCB is on an earthed heatsink, so common mode emissions have reared their heads to us.

Hi,

the answer is as vague as the informations. A common mode choke, a ferrite bead....

Klaus

You basically have increase the impedance (ie decrease capacitance) from the switching node to earth. Shielding it with the buck converter's ground should help (hopefully it is not also earth ground...)

A schematic/photo is needed for further advice.

Thanks, yes the Bucks gnd is not earth ground. some say though that the switching node could induce noise into the buck's ground if the buck's ground is directly beneath the switching node?

A schematic/photo is needed for further advice.

Click to expand...

Thanks
Hello,
Attached please find the PCB area containing the HV DC Buck converter and the switching node.
I wonder if it would be worth adding a small “metal box” kind of like a tent (a shield) above the switching node copper, the inductor and diode? Maybe this would stop the radiation from eminating out from the switching node and going into the earthed heatsink? We could solder the box to a quiet node on the PCB?

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This shielding would reduce common mode emissions?

A shield above the switching node would likely do nothing (or make it worse, if that shield is earthed). You need to consider the return path of the currents caused by capacitance to the switch node (this is where a schematic would be very useful). Those return currents "want" to return to the buck's own gnd, but the electric fields from the switch node pass through the heatsink on the way.

A shield above the switching node would likely do nothing (or make it worse, if that shield is earthed).

Click to expand...

Thanks, if i may ask, then, in that case, why is a shield in layers 2,3,4(bottom) of the PCB, and connected to the Bucks ground (not earth) of use?

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The schem is just as the opto regulated LNK302 schematic...
https://www.mouser.com/ds/2/328/lnk302_304-306-179954.pdf

..page 6 , second schem down.

treez said:

Thanks, if i may ask, then, in that case, why is a shield in layers 2,3,4(bottom) of the PCB, and connected to the Bucks ground (not earth) of use?

Click to expand...

Because you want to prevent electric fields emanating from the switch node from terminating on the earth circuit. Surrounding the switch node with an earthed shield would only increase common mode currents. Shielding with another low impedance node (buck ground) allows the high frequency currents to return through the buck bypass capacitors instead of earth.

The schem is just as the opto regulated LNK302 schematic...
https://www.mouser.com/ds/2/328/lnk302_304-306-179954.pdf

..page 6 , second schem down.

Click to expand...

Not useful, since it doesn't show any chokes, bypass capacitors, etc.