4.17.1 Effective Layers
The effective layers referred to in Figs. 3.4.5 and 3.4.6 are the number of layers between a
plane of zero mmf and a plane of maximum mmf. Hence, in the case of the secondary wind-
ing in the sandwiched construction of Fig. 3.4.8b, the effective layers are only half the total
number of secondary layers, because a zero mmf occurs in the center of the winding.
Since the maximum mmf and proximity effects are lower in the sandwiched construc-
tion, the copper losses and leakage inductance are reduced. If the secondary is a single
layer, giving an effective half layer, the wire or strip may be twice as thick as the optimum
given for a single layer in Fig. 3.4.5 or 3.4.6.
In sandwiched construction, the normal design approach is to split the primary into two
halves with the secondary windings sandwiched between them, as shown in Fig. 3.4.8c.
In some multiple-output applications, particularly those in which the secondary wind-
ings have low-voltage, high-current, relatively constant loads, there is some advantage in
splitting the secondary windings into two sections, with the primary in the middle. In this
second arrangement, the winding carrying the highest secondary current would be placed
close to the core, as the mean turn length will be smaller, resulting in lower copper losses.
A second advantage of this topology is that windings that are close to the core will now
have lower ac voltages, reducing the RFI coupling from primary to core, secondary, and
case. However, this approach should be used with caution, because the secondary wind-
ings must be selected for equal ampere-turns loading above and below the primary and the
loads must be constant; otherwise large leakage inductances can occur.
In this example the primary has been split into two equal parts, one above and one
below the secondary windings. Four screens have been incorporated between the primary
and secondary windings. The screens adjacent to the primary, S1 and S4, are Faraday
screens, fitted to reduce RFI coupling between the high-voltage primary windings and
the safety screens S2 and S3. The Faraday screens are connected to the primary common
line, to return capacitively coupled RFI currents to the primary circuit. The safety screens
S2 and S3 are connected to the chassis or ground line, to isolate secondary outputs from
the primary circuit in the event of an insulation failure. These screens, although necessary
to meet safety and emission requirements, occupy considerable space and increase the
primary-to-secondary leakage inductance. (See Sec. 4.22.)
After deciding on the winding topology, calculate the space occupied by screens and
insulation. The remaining space is then available for primary and secondary windings.
Some further constraints are placed upon the winding design.
It is preferable that windings occupy an integer number of layers. In the case of the
split primary winding, the number of layers should be even to allow equal splitting of
the half primaries. Partly wound layers are both inefficient in the use of the winding
space and promote insulation breakdown where the terminating wire is brought across
the top of the underlying layers. Since the volts per turn can be quite large in high-
frequency transformers, a terminating wire that spans several turns will be subjected to a
higher breakdown voltage stress. Further, the terminating wire is subject to considerable
mechanical stress, because it forms a discontinuity or “bump” in the winding, and the
remaining layers apply considerable pressure to this bump. Most insulation breakdown
failures in switching transformers can be traced to this type of winding discontinuity or
to bad termination practices in which wires are crossed over each other. Good winding
practice dictates that all layers should be complete, that terminating wires be brought out
with additional insulation, and that terminating wires not cross over other windings or
terminating wires wherever practical.
To meet the creepage distance and spacing requirements demanded by VDE, UL, IEC,
and CSA specifications, it is necessary to leave a creepage distance of up to 8 mm between
the primary and secondary windings. (See Fig. 3.4.9a, b, and c.)