Application Note
AN-1109
Overlapping Stitching Capacitor
An example of a floating stitching capacitance is shown in
Figure 8. The reference planes are shown in blue and green, and
the floating coupling plane is shown in yellow. The capacitance
of this structure creates two capacitive regions (shown with
shading) linked by the nonoverlapping portion of the structure.
To ensure that there is no dc voltage accumulated on the
coupling plane, the area on the primary and secondary should
be approximately equal.
A simple method of achieving a good stitching capacitance is to
extend a reference plane from the primary and secondary sides
into the area that is used for creepage on the PCB surface.
W
I
W
W
2
1
I
d
Figure 7. Overlapping Plane Stitching Capacitance
The capacitive coupling of the structure in Figure 7 is calculated
with the following basic relationships for parallel plate capacitors:
d
Aε
d
Figure 8. Floating Stitching Capacitance
C =
and ε = ε0 × εr
The capacitive coupling of the structure in Figure 8 is calculated
with the following basic relationships for parallel plate capacitors:
where:
C is the total stitching capacitance.
Axε
d
c1 ×c2
c1 +c2
Cx =
, ε = ε0 x εr, C =
A is the overlap area of the stitching capacitance.
ε0 is the permittivity of free space, 8.854 × 10−12 F/m.
εr is the relative permittivity of the PCB insulation material,
which is about 4.5 for FR4, as shown in Table 2.
where:
C is the total stitching capacitance.
A is the overlap area of the stitching capacitance.
ε0 is the permittivity of free space, 8.854 × 10−12 F/m.
εr is the relative permittivity of the PCB insulation material,
which is about 4.5 for FR4, as shown in Table 2.
lwε
C =
(1)
d
where w, d, and l are the dimensions of the overlapping portion
of the primary and secondary reference planes as shown in
Figure 7.
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
w1 ×w2
w1 + w2
lε
d
C =
×
(2)
(3)
The major advantage of this structure is that the capacitance is
created in the gap beneath the isolator, where the top and
bottom layers must remain clear for creepage and clearance
reasons. This board area is not utilized in most designs. The
capacitance created is also twice as efficient per unit area as the
floating plane.
where w1, w2, d, and l are the dimensions of the overlapping
portions of the floating plane and the primary and secondary
reference planes as shown in Figure 8.
If w1 = w2, the equation simplifies to
lw1ε
C =
This architecture has only a single cemented joint and a single
layer of FR4 between the primary and secondary reference
planes. It is well suited to smaller boards where only basic
insulation is required.
2d
There are advantages and disadvantages to this structure in real
applications. The major advantage is that there are two isolation
gaps, one at the primary and one at the secondary. These gaps
are referred to as cemented joints, where the bonding between
layers of FR4 provides the isolation.
Table 2. Electrical Properties
Dielectric Constant
at 1 MHz
Dielectric Strength
(V/mil)
Type
FR4
There are also two sequential paths through the thickness of the
PCB material. The presence of these gaps and thicknesses is
advantageous when creating a reinforced isolation barrier under
some isolation standards. The disadvantage of this type of
structure is that the capacitance is formed under the active circuit
area so there can be via penetrations and traces that run across
the gaps. Equation 2 also shows that the net capacitance result-
ing from two capacitors in series is only half the value that
results from using the same PCB area to form a single capacitor.
Therefore, this technique is less efficient from a capacitance per
unit area perspective. Overall, it is best suited to applications
4.5
1000 to 1500
1000 to 1200
750
GETEK
3.6 to 4.2
BT-Epoxy 4.0
Floating Stitching Capacitor
A good option is to use a floating metal structure on an interior
layer of the board to bridge between the primary and secondary
power planes. Note that planes dedicated to ground or power
are referred to as reference planes in this application note
because, from an ac noise perspective, they behave the same
and can be used interchangeably for stitching capacitance.
Rev. 0 | Page 7 of 20
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