T
treez
Guest
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.)
(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.)