Why are shield windings so common in offline flybacks?....rather than a second CMC?

We all know that with an offline 30-60W, isolated Flyback SMPS project, for the first prototype, the conducted EMC scan often shows a fail at the 28-30MHz point. As we know, this is down to common mode emissions.
(this fail is often there, even with meticulous PCB layout, and correct use of a Y capacitor across the isolation barrier.)

Often we initially try and get away with just one common mode choke, and no shield windings in the transformer. (also, we don’t like to snub our secondary diode too heavily, as this needs bigger snubber resistors. Also, we don’t like to increase the fet gate resistor too much, because although this conveniently reduces drain voltage dv/dt, it also increases switching losses)

So, in many cases, the transformer shield windings, or the second common mode choke with a higher SRF, is needed to clinch the conducted EMC pass and solve the 28-30MHz conducted problem.
You almost always find that people elect to solve it with shield windings in the transformer rather than with a second, smaller common mode choke. ..
Why is this?
..especially since offline flybacks of 30-60w are almost inevitably sandwich wound, so two shield windings are unfortunately needed in order to fully shield the primary from the secondary.

So why do people always elect for shield windings rather than a second common mode choke with a higher SRF?

(After all, shield windings can often mean having to use a bigger, more expensive transformer, in order to fit the shield windings on to the former.)

Shield windings are for shielding, not for stopping power line signals although I agree there can be mechanisms that conduct one to the other. The shields main use is to block capacitive coupling to nearby bodies. Even when the capacitance is small, high voltage, high frequency edges can still cause problems.

Why are you concentrating so much effort in SRF? Self resonance is a measure of the component in isolation (or under specified test conditions) but slap a few uF across it and add some wiring length and the resonance will shift considerably.

If you really have a problem with EMC, have you considered locking the switching frequency and using a tuned filter? I you use an MCU with a crystal oscillator this isn't a hard as you might think and even a simple filter will notch maybe 20 - 30dB from the line.

Brian.

Shield windings are a cheap addition to a Tx, and can be used as the aux on the pri and sec, not uncommon to see 3 wires in hand of 0.25mm dia to complete a layer and give a not too bad shield ( as well as providing an aux) - short answer - they often work and are low cost.

Thanks, and i suppose 3 flat-wound 0.25mm diam wires quickly cover the bobbin width , with just a few turns, and dont have too much height, (so dont take up too much room) and also advantageously have low skin effect problem (due to being trifilar, a bit like Litz)
I suppose being only 0.25mm in height, the shield coil wont decrease pri-sec coupling too much and wont therefore result in big increase in leakage inductance.

I suppose that having an aux coil doubling up as a shield winding is a good use of the (often limited) space in a transformer bobbin. The thing is, I think we would agree that having a dedicated shield winding, that is only connected at one end to a quiet node (ie, not a switching node, say) is best. For example, the shield coil could be connected to circuit ground.
The thought of an auxiliary coil, with its switching node, acting as a shield conjures up images of a switching node actually adding noise aswell as shielding it.
This is why shield windings are best only connected at one end to circuit ground, with the other end being left “floating”?
I mean, I always considered the shield winding was directing the noise back to ground. I suppose the shield winding actually separating the pri and sec coils further apart is also useful in that it reduces the stray capacitance between them…but then again, a simple extra layer of insulating tape could do this.

Using the aux as a shield winding is fine because the volt swings on the terminals of the aux wdgs are very smal compared to the volt swings on the "hot" main pri wdg.

In a telecom Tx design - you would have a pri shield, an earth shield, and the sec shield, such that the total capacitance from in to out is <50pF ( the C is shunted to earth )