On the left you can see the AC input common mode choke. On the bottom layer of the PCB you can see that unusual shape for the pads of the choke: they look like some "arrows". To allow an HV to arc in case the choke opens? Why?
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I'll guess the PCB arc points is to protect the magnetic wire lamination from arc damage (punched holes). It's a tradeoff.. Arcs fk up the PCB but not the CM xformer.
That would help maintain the creepage rating of the common mode transformer. (Why that would be important, I dunno yet.) However, the creepage on the PCB is allowed to get worse. Once points get a nice carbon spot, arcing can be achieved at lower voltages. But that might be ok.. Who cares about a loss of CM noise filtering during a freakish line spike anyways..
D from BC myrealaddress(at)comic(dot)com British Columbia Canada
From back in my TV repair days I remember a certain Philips chassis that had a single choke in the mains input circuitry, a common fault was random blowing of the chopper transistor at switch off - the fuse would blow next time it was switched on. The cause was an open circuit damping resistor in parallel with the choke.
Obviously its possible then for a back emf to be generated by a mains input choke, although on a common mode choke I'd expect the spike to also be common mode and probably not particularly dangerous to the PSU.
The deliberate equal spark gaps help ensure the common mode back emfs are clamped at similar peak voltage.
Higher energy CM and DM impulses can break a CM inductor in half, though it's more common with E-core or U-core structures. It's not necessarily a freakish level, either, merely one not often experienced in class 2 household wiring.
Transtectors and Sidacs typically employed to provide safe low-impedance paths for this energy are fairly expensive. Controlled natural (free) breakover methods are hard to configure reliably. While printed wiring is reproducible, it's performance here may not be sufficiently predictable. The assumption is that it did work in physical testing, at some point. I'd read the fine print in the spec before accepting that assuption.
It will attempt to force the coil to 're-orient' itself so as to shorten the magnetic path length of the relevent field. For E-core CM chokes, the core tries to fold in half, joining the distant ends of the wound structure. For toroids, the windings want the core to 'shrink' ~ producing more distributed force.
So's a transformer.
It's typically a 'gas filled tube' or 'gas tube arrestor' with a controlled breakdown voltage that is dV/dT dependant.
If the arc occurs, the energy released locally is minimized. The test waveforms for these surges has a characteristic amplitude, waveshape, duration and characteristic source impedance. The arc occurs across terminals of the circuit that normally do not experience high stress, ie across the coil ( a DC ~short circuit for creepage and cleerance purposes ).
Only testing will demonstrate effectiveness and durability - hence reference to assembly spec sheets. It does no harm, so even failed attempts at design (if it was a serious design effort and not just a whim) may have been left in place, by default.
To be effective, the arc must bypass all components that do not exhibit controlled avalanche, low transient inpedance, or known energy absorption characteristics.
Very often, opportunities for 'probing the limits' of a design iteration are only available to the tightly-scheduled new product developer in this way. If it works ('free' samples for testing!), it's a new value-added feature that can be advertized and re-used; if it doesn't, there's no skin lost and no budget blown - just a gradual accumulation of savvy for the next attempt (if the developer lasts that long).
That may be good electrical isonation, but that "long creepage notch" really reduces the mechanical strength of the board at a bad place, right next to the transformer (probably the heaviest component) in the middle of the board. I think that notch makes the board much more suceptible to breaking if the equipment is dropped.