Offline Flyback secondary is Neutral net.

[cupoftea]

Hi,
Got an offline 24W (24V, 1A) flyback design, 4 layer PCB, from an external consultancy for an audio product.....it doesnt need absolute full isolation, as there is plastic casing....therefore, secondary ground is connected to neutral...and the neutral actually forms a pretty complete inner layer over the whole primary and secondary of the PCB.

This actually reduces the common mode noise problem....since its easier for emissions to couple back to the incoming neutral rather than getting "sprayed" out of the SMPS and producing common mode emissions.
So why dont more designs do this?

Its obviosly far safer than having the line connected to secondary ground.
After all, neutral is pretty well at earth potential.

 

[Easy peasy]

All mains equipment must be designed such that the connection ( way round ) of phase and neutral is irrelevant,

still sound safe to you ?

 

[cupoftea]

Thanks, i see your point...in this case they regard the plastic case as sufficient to protect, either way (L or N)....and we all agree that it massively eases common mode emissions.

 

[mtwieg]

You said it's for an audio application, which means it has some I/O. A plastic box won't protect whatever touches those connectors.

 

[cupoftea]

Sorry , i meant it has an audio "thing" in it...its totally enclosed plastic.
So we agree that common mode emissions will be much less with the neutral_to_secondary?

 

[mtwieg]

Ok let's entertain your assumption that this isn't breaking basic safety standards....

Sure, if this PSU and whatever it's connected to has no other connections (USB ports, metal chassis, etc), then by definition there is no path for CM currents to flow in the first place.

 

[cupoftea]

Presumably some CM emissions will still escape though...but far less as you imply..Would you bother with common mode choke at mains input .. and Y cap across the transformer?

The X2 cap means theres no room for the 400v electrolytivcs at the primary side.....so theyve put the input electro's at the secondary side and just wired to them....the secondary side electro's are at the primary side!...its a very small, narrow PCB with connectors that need to be in specific places.

 

[mtwieg]

Presumably some CM emissions will still escape though...but far less as you imply..Would you bother with common mode choke at mains input .. and Y cap across the transformer?

Need to be more specific than this to justify any specific mitigations against EMC.

Also I'm not sure where you would even put a Y capacitor in such a contrived circuit with neither an earth connection or an isolation barrier.

 

[cupoftea]

Thanks, presumably you would put CM filter such as to make the overall CM filter situation look "balanced"? (between live and neutral)