Floating heatsink in offline Flyback SMPS?

[cupoftea]

Hi,
Page 5 of the following shows a metal heat spreader connected right under the
whole (pri and sec) of an offline 65W flyback PCB...

https://www.power.com/sites/default/files/documents/der535.pdf

In this assembly, this metal heat spreader is simply left floating.
This of course is a very bad idea and should never happen due to noise
and EMC reasons.
Do you agree that one of its screwholes should get Y capacitor connected to
secondary ground, and the other of the two screw holes should be y capacitor connected to
the primary side HVDC bus?
Either way, I am sure you would agree that to leave it floating is disastrous?

 

[Easy peasy]

As there is no RF hot tab pressing against the heat spreader ( via insulation ) - there is no RFI issue

a lot easier to pass RFI if the heat spreader is left floating in this case ( as proven by RFI results in document supplied by you )

ToP switches etc are notable for the tabs being at rectified mains 0v.

 

[cupoftea]

Thanks, i saw the EMI results but it didnt say whether or not heat spreader was connected or not for them. Even with the quiet thermal node of the switcher IC, there is still a pair of switching nodes close to that metal and there will be noise coupling to the floating metal. I am surprised they dont tie it down?

On page 82 of this below (another innoswitch cct), the heat spreader is connected to primary side ground.
It makes you wonder why its floating in the aforementioned document.
Floating metal is always bad.

https://www.power.com/sites/default/files/documents/der-712_60w_ultra-wide_input_range_adapter_using_innoswitch3-ep_powigan_mine-cap.pdf

Its often that in testing/board-bring-up for example, one connects say a flying twisted pair scope probe wire to a PCB's nets and
finds that the PCB then doesn't work or malfunctions. Its the same principle.
The flying wire probe acts like an antenna, and so too does any heat spreader etc etc.
They all absolutely have to be tied down, even if that's by a Y capacitor rather than a resistor, or screw etc etc what have you.